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  • 1.
    Ampel, Linda
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Bigler, Christian
    Department of Ecology and Environmental Science, Umeå University.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Lotter, André F.
    Institute of Environmental Biology, Laboratory of Palaeobotany and Palynology, Utrecht University.
    Veres, Daniel
    “Emil Racovita” Speleological Institute, Clinicilor 5, 400006 Cluj-Napoca, Romania.
    Modest summer temperature variability during DO cycles in western Europe2010In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 29, no 11-12, p. 1322-1327Article in journal (Refereed)
    Abstract [en]

    Abrupt climatic shifts between cold stadials and warm interstadials, termed Dansgaard-Oeschger (DO) cycles, occurred frequently during the Last Glacial. Their imprint is registered in paleorecords worldwide, but little is known about the actual temperature change both annually and seasonally in different regions. A recent hypothesis based on modelling studies, suggests that DO cycles were characterised by distinct changes in seasonality in the Northern Hemisphere. The largest temperature change between stadial and interstadial phases would have occurred during the winter and spring seasons, whereas the summer seasons would have experienced a rather muted temperature shift. Here we present a temporally high-resolved reconstruction of summer temperatures for eastern France during a sequence of DO cycles between 36 and 18 thousand years before present. The reconstruction is based on fossil diatom assemblages from the paleolake Les Echets and indicates summer temperature changes of ca 0.5–2 °C between stadials and interstadials. This study is the first to reconstruct temperatures with a sufficient time resolution to investigate DO climate variability in continental Europe. It is therefore also the first proxy record that can test and support the hypothesis that temperature changes during DO cycles were modest during the summer season.

  • 2.
    Ampel, Linda
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Veres, Daniel
    Paleolimnological response to millennial and centennial scale climate variability during MIS 3 and 2 as suggested by the diatom record in Les Echets, France2008In: Quaternary Science Reviews, ISSN 0277-3791, Vol. 27, no 15-16, p. 1493-1504Article in journal (Refereed)
    Abstract [en]

    A 27 m long sediment sequence retrieved from the central part of the Les Echets basin in France has been analysed in sub-centennial resolution for biogenic silica and fossil diatom remains. The sequence corresponds to the later part of Marine Isotope Stage (MIS) 3 and to most of MIS 2. Distinct changes in diatom productivity, diversity and taxonomic composition between 36.2 and 31.7 kyr BP appear to relate to Dansgaard–Oeschger (DO) climate variability. Intervals characterized by low diversity, productivity and small-sized benthic diatom taxa are most likely a response to colder conditions in relation to DO stadials. In contrast, higher diversity, productivity and a high abundance of planktonic taxa indicate a response to warmer temperatures during DO interstadials. The time interval between 30.3 and 15.7 kyr BP is characterized by continuous low diatom productivity and a benthic dominated community with intermediate species richness, suggesting a transition to more stable conditions. Three time intervals with extremely low concentrations of diatom valves (46.1–36.2, 31.7–30.3 and 26.3–23.6 kyr BP) overlap with ages reported for Heinrich (H) events 4, 3, and 2. We speculate that the lake at Les Echets suffered from severe ecological stress as a response to H events. This is the first detailed study exemplifying the response of a lake, based on diatoms, to climate variability during late part of MIS 3 and most of MIS 2 in Europe.

  • 3.
    Ampel, Linda
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wohlfarth, Barbara
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Veres, Daniel
    Leng, Melanie
    Kaislahti Tillman, Päivi
    Diatom assemblage dynamics during abrupt climate change: The response oflacustrine diatoms to Dansgaard-Oeschger cycles during the last glacialperiod2010In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 44, no 2, p. 397-404Article in journal (Refereed)
    Abstract [en]

    The sedimentary record from the paleolake at Les Echets in eastern France allowed a reconstruction of the lacustrine response to several abrupt climate shifts during the last glacial period referred to as Dansgaard-Oeschger (DO) cycles. The high-resolution diatom stratigraphy has revealed distinct species turnover events and large fluctuations in stable oxygen isotope values in diatom frustules, as a response to DO climate variability. More or less identical species compositions became re-established during each DO stadial and interstadial phases, respectively. However, the relative abundance of the most dominant species within these assemblages varies and might indicate differences in climatic conditions. Interstadial phases are characterized by identical species successions. Transitions from stadial to interstadial conditions show a distinct Fragilaria-Cyclotella succession, which resembles the diatom regime shifts that have been recognized in some lakes in the Northern Hemisphere since the mid-nineteenth century.

  • 4.
    Berntsson, Annika
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kaislahti Tillman, Päivi
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Ling, Johan
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Shore displacement in the world heritage area Tanum on the Swedish west coast2007Conference paper (Other (popular science, discussion, etc.))
  • 5. Carlie, Anne
    et al.
    Arcini, Caroline
    Druid, Henrik
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Archaeology, forensics and the death of a child in Late Neolithic Sweden2014In: Antiquity, ISSN 0003-598X, E-ISSN 1745-1744, Vol. 88, no 342, p. 1148-1163Article in journal (Refereed)
    Abstract [en]

    The discovery of a child's skeleton in a Late Neolithic well in Sweden raises again the issue of watery rituals and human sacrifice in prehistoric societies. Analysis of diatoms from the right humerus and from the surrounding sediment indicated that the child died by drowning and had not simply been disposed of in the well after death. The scenarios of accidental drowning and murder are examined to account for this discovery. The preferred hypothesis, based on a comparative study of similar finds from north-western Europe, interprets this instead as a ritual sacrifice. The use of diatom analysis to establish drowning as the cause of death adds a new weapon into the armoury of forensic archaeology.

  • 6. Ekblom, Anneli
    et al.
    Gillson, Lindsey
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Chidoub, Zara
    Rainfall variability and vegetation dynamics of the lower Limpopo Valley, Southern Africa, 500 AD to present2012In: Palaeogeography, Palaeoclimatology, Palaeoecology, ISSN 0031-0182, E-ISSN 1872-616X, Vol. 363, p. 69-78Article in journal (Refereed)
    Abstract [en]

    The long-term responses of vegetation to climate variability are of relevance for predicting present and future vegetation change, and have implications for the management of savanna and riparian ecosystems. This paper explores the links between regional rainfall, hydrology and vegetation dynamics in the savannas and riverine forests of the lower Limpopo Valley, southern Africa, from 800 AD to the present, reviewing palaeoecological data (fossil pollen, spores, diatoms and lithology) from several hydrological systems in Kruger National Park (KNP), South Africa and Limpopo National Park (PNL), Mozambique. The PNL-KNP records show that riverine arboreal taxa expanded during wetter periods, including 800-1400 AD and after 1800 AD. Between 1400 and 1800 AD, grasses, savanna taxa and generalist taxa were favored over riparian taxa, a change that is linked with the onset of dry spells in the region (corresponding to the so-called Little Ice Age). The most extreme drought events around 1700 AD resulted in a marked decline of riparian forest taxa near Lake Mapimbi, KNP. In contrast, many water-scarce sequences away from the riverine environment, such as Radio Pan, Mafayeni Pan, Malahlapanga Pan and Lake Makwadzi show stable grassland vegetation throughout the last 1200 years. The results demonstrate the resilience of the grassland-savanna ecosystems to projected climate change with warmer and overall drier climate. The riverine forests are predicted to be more vulnerable especially as more extreme weather events are projected.

  • 7. Ekblom, Anneli
    et al.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Coastal forest and Miombo woodland history of the Vilankulo region, Mozambique2014In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 24, no 3, p. 284-294Article in journal (Refereed)
    Abstract [en]

    The present day distribution of Miombo savanna-woodland in Mozambique has been attributed to an expansion due to the clearing of original coastal forests through agriculture and use of fire. Here, we test this hypothesis using palaeoecological data from Lake Nhauhache, situated in the Vilankulo region. Our analysis shows that Brachystegia, one of the main constituents of the Miombo, has varied over time, and its variability seems to be driven by hydrological changes related to climatic variability rather than by land-use changes. The analyses show that Brachystegia was most common during ad 200-700 when a marshy forest/shrub community was dominant. After ad 700, this community changes to a dominance of Syzygium and Fagara linked to gradually rising water levels. Brachystegia remains in low abundance and fluctuating over time. From ad 1000, a general decline in trees/shrubs in favour of grasses concurs with an increase in grass pollen (possibly cereal) and charcoal, most probably as a result of farming activities. The decline in tree taxa was probably exacerbated by periodic droughts after c. ad 1200 as indicated by the diatom assemblage. In the period ad 1700 to late 1800, arboreal pollen is well represented, and this is concurrent with the diatom record suggesting high lake levels.

  • 8. Emmanouilidis, Alexandros
    et al.
    Katrantsiotis, Christos
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Kylander, Malin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sheik, Taariq Ali
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Iliopoulos, George
    Avramidis, Pavlos
    Middle to late Holocene palaeoenvironmental study of Gialova Lagoon, SW Peloponnese, Greece2018In: Quaternary International, ISSN 1040-6182, E-ISSN 1873-4553, Vol. 476, p. 46-62Article in journal (Refereed)
    Abstract [en]

    The coastal areas of Eastern Mediterranean have long been the subject of research, due to their rapid geomorphological changes, but also because of their archaeological interest. Our study is focused on a shallow coastal lagoon of Peloponnese, Gialova Lagoon, which for several years has attracted the scientific interest of archaeologists, geomorphologists as well as sedimentologists. Gialova lagoon is located near the ancient city of Pylos, the kingdom of king Nestor during the Mycenaean period (1600-1100 BC). The objectives of this study are: (a) to reconstruct the middle to late Holocene depositional environments of the lagoon and (b) to correlate our data to already existing publications, in order to shed new light on the Holocene evolution of the lagoon and the associated coastal palaeoenvironmental changes. An 8m deep vibracore was drilled and a multi proxy analysis was carried out on the sediment sequence, including sedimentological (grain size analysis and moment measures, total organic carbon - TOC, total nitrogen e TN and total phosphorus - TP), high resolution geochemical (XRF-scanning) and palaeontological (micro-and macro faunal) analysis. The chronological framework is based on five C-14 datings forming the basis for an age depth model, calculated using the OxCal software. The radiocarbon dates from previous studies (6 cores, similar to 20 dates) were also taken into account. The data synthesis and interpretation provided robust and coherent indications regarding the palaeoenvironment, shoreline changes and the rate of geomorphological changes of the coastal area of Gialova Lagoon, as well as useful information about the palaeonvironmental and palaeoclimatic conditions that prevailed during the Mycenaean period. The interpretation, reveal a transition from a shallow marine environment (65005800 yr B.P.) to a brackish/lagoonal (5800-3300 yr B.P.), followed by a shift towards a freshwater/marsh environment (3300 yr B.P. to present).

  • 9. Field, M. H.
    et al.
    Ntinou, M.
    Tsartsidou, G.
    Henegouwen, D. van Berge
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Tourloukis, V.
    Thompson, N.
    Karkanas, P.
    Panagopoulou, E.
    Harvati, K.
    A palaeoenvironmental reconstruction (based on palaeobotanical data and diatoms) of the Middle Pleistocene elephant (Palaeoloxodon antiquus) butchery site at Marathousa, Megalopolis, Greece2018In: Quaternary International, ISSN 1040-6182, E-ISSN 1873-4553, Vol. 497, p. 108-122Article in journal (Refereed)
    Abstract [en]

    Exposures of Middle Pleistocene lacustrine sediments at the margins of an open-cast lignite mine at Marathousa near Megalopolis, western Arcadia, Greece yielded the partial remains of a Palaeoloxodon antiquus skeleton which exhibited signs of being butchered. Sedimentation occurred between ca. 400 and 480 ka. Lithic artefacts were found in close spatial and stratigraphic association with the elephant remains. A palaeobotanical investigation (involving carpological, phytolith and wood remains) as well as diatom analysis led to a detailed reconstruction of the local environment at the time of sediment deposition. The results of this study enabled the environmental context of the butchering of the Palaeoloxodon antiquus carcass to be established. Palaeobotanical data show that sediment deposition at the Palaeoloxodon antiquus site occurred in shallow water in front of a reed swamp with trees in the immediate surroundings (particularly Alnus and Salix) on a flat plain where the water table was at or just below the ground surface. Warm conditions prevailed at the time of sediment deposition allowing aquatic plants typically recorded in the climatic optima of European interglacial assemblages to occur (e.g. Brasenia schreberi) and palms (Palmae) to live close-by. This suggests that the organic-rich sediments that contain the archaeological finds represent a transition between the underlying clastic sediments deposited during colder conditions and the overlying lignite that was deposited during warm climatic conditions as reported in previous studies.

  • 10.
    Finné, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Siliceous microfossils as Late Quaternary paleo-environmental indicators at Braamhoek wetland, South Africa2010In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 20, no 5, p. 747-760Article in journal (Refereed)
    Abstract [en]

    A peat-sequence covering the last 16 ka (16 000 cal. yr BP) from Braamhoek wetland, eastern South Africa, was analysed in terms of phytolith and diatom composition. The fossil peat was rich in phytoliths, while diatoms were less prominent, probably as a result of degradation during wetland plant growth associated with silica uptake. With this study we present the first continuous phytolith and diatom record from South Africa covering the Late Pleistocene and Holocene period. The phytolith assemblages indicate a clear dominance of C

    3-grasses within the wetland throughout the sequence. The fossil diatom record infer changes in past moisture conditions. Unlike the modern wetland, which is dominated by benthic and aerophilic diatoms, the Late Pleistocene– early Holocene wetland favoured growth of planktonic species. Abundance of planktonic diatoms suggests three main phases when water depth was deeper than today; at c.13.6 ka, 11.3 ka and 10.4–10.0 ka. These indications of past fluctuations in humidity mostly provide confirmation of previously published indications of pollen, charcoal fragments and isotopes in the same core, but the siliceous microfossil data also help to refine the paleo-environmental interpretation of the sequence.

  • 11.
    Helmens, Karin F.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Katrantsiotis, Christos
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Salonen, J. Sakari
    Shala, Shyhrete
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Bos, Johanna A. A.
    Engels, Stefan
    Kuosmanen, Niina
    Luoto, Tomi P.
    Väliranta, Minna
    Luoto, Miska
    Ojala, Antti
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Weckström, Jan
    Warm summers and rich biotic communities during N-Hemisphere deglaciation2018In: Global and Planetary Change, ISSN 0921-8181, E-ISSN 1872-6364, Vol. 167, p. 61-73Article in journal (Refereed)
    Abstract [en]

    Detailed studies on fossil remains of plants or animals in glacial lake sediments are rare. As a result, environmental conditions right at the moment of deglaciation of the large N-Hemisphere ice-sheets remain largely unknown. Here we study three deglacial phases of the Fennoscandian Ice Sheet as a unique, repeated element in a long sediment record preserved at Soldl in northern Finland. We summarize extensive multi-proxy data (diatoms, phytoliths, chironomids, pollen, spores, non-pollen palynomorphs, macrofossils, lithology, loss-on-ignition, C/N) obtained on glacial lake sediments dated to the early Holocene (ca. 10 kyr BP), early MIS 3 (ca. 50 kyr BP) and early MIS 5a (ca. 80 kyr BP). In contrast to the common view of an unproductive ice-marginal environment, our study reconstructs rich ecosystems both in the glacial lake and along the shores with forest on recently deglaciated land. Higher than present-day summer temperatures are reconstructed based on a large variety of aquatic taxa. Rich biota developed due to the insolation-induced postglacial warming and high nutrient levels, the latter resulting from erosion of fresh bedrock and sediment, leaching of surface soils, decay of plant material under shallow water conditions, and sudden decreases in lake volume. Aquatic communities responded quickly to deglaciation and warm summers and reflect boreal conditions, in contrast to the terrestrial ecosystem which responded with some delay probably due to time required for slow soil formation processes. Birch forest is reconstructed upon deglaciation of the large LGM ice-sheet and shrub tundra following the probably faster melting smaller MIS 4 and MIS 5b ice-sheets. Our study shows that glacial lake sediments can provide valuable palaeo-environmental data, that aquatic biota and terrestrial vegetation rapidly accommodated to new environmental conditions during deglaciation, and that glacial lake ecosystems, and the carbon stored in their sediments, should be included in earth system modeling.

  • 12.
    Higgins, Lindsey
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Koutsouris, Alexander J.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Westerberg, Lars-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Surface Area Variability of a North-Central Tanzanian Crater Lake2016In: Geosciences, ISSN 2076-3263, Vol. 6, no 2, article id 27Article in journal (Refereed)
    Abstract [en]

    A history of modern (1973–2015) surface area variability for Lake Basotu in north-central Tanzania has been reconstructed using archived Landsat images from the dry season between June and October. This record was compared to local weather data as well as larger scale weather patterns. The lake has been in a state of decline interrupted by major flood events since the beginning of the satellite record. From 1973 to 1997, the lake area was between 0.97 km2 and 4.28 km2. Lake extent abruptly increased to 13.86 km2 in 1998, when a co-occurrence of El Niño and a positive Indian Ocean Dipole led to extensive flooding. It is hypothesized that local agricultural practices leading to soil erosion and subsequent basin sedimentation have most likely increased the sensitivity of Lake Basotuto climatic fluctuations.

  • 13.
    Higgins, Lindsey
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Westerberg, Lars-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Snowball, Ian
    Andersen, Thorbjørn J.
    Sediment Based Reconstruction of Water-Level Changes in Basotu, TanzaniaManuscript (preprint) (Other academic)
  • 14.
    Holmgren, Karin
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Börjeson, Lowe
    Stockholm University, Faculty of Social Sciences, Department of Human Geography.
    Dahlberg, Annika
    Stockholm University, Faculty of Social Sciences, Department of Human Geography.
    Muzuka, Alfred
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Öberg, Helena
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Ryner, Maria
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Sitoe, Sandra
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Eduardo Mondlane University, Mozambique.
    Westerberg, Lars-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Widgren, Mats
    Stockholm University, Faculty of Social Sciences, Department of Human Geography.
    Yanda, Pius
    The vulnerable continent (PLATINA): Historical perspectives on Africa´s climate, environment and societies2009In: Meeting global challenges in research cooperation: Proceedings of a conference and workshop in Uppsala, May 27-29, 2008 / [ed] Ingrid Karlsson, Kristina Röing de Nowina, Uppsala: Uppsala University, 2009, p. 585-596Conference paper (Refereed)
    Abstract [en]

    Our research, based on studies of different climate archives from Tanzania, Mozambique and South Africa, contributes information on changes in climate and vegetation over the past 24000 years. This time perspective, reaching beyond the information available from instrumental records is needed for a better understanding of regional global climate dynamics and issues surrounding environmental change, throughout Africa, and is a prerequisite for increasing climate forecasting capabilities for the region. We argue that African people have vast experience from living in a variable climate and research on past interactions between climate and societies demonstrate the significance of lessons learnt for present situations. Our findings, underline the complex interactions between climate/environment and societies that may lead to different developments in time and space. Considering the so called vulnerable continent, extended investigations of how African communities cope with and adapt to climatically driven changes is needed to increase the capability to realise the potential as well as the limitations, of modern African communities to adapt to future climate change.

  • 15.
    Holmgren, Karin
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Freudendahl, Johan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Achimo, Mussa
    Ekblom, Anneli
    Mugabe, Joao
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Sitoe, Sandra
    University of Eduardo Mondlane, Mozambique.
    Water-level variations in Lake Nhauhache, Mozambique, during the last 2,300 years2012In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 48, no 2, p. 311-322Article in journal (Refereed)
    Abstract [en]

    Stratigraphic variations in diatom composition and phytolith abundance in a sediment core from a small, hydrologically isolated waterbody, Lake Nhauhache, Mozambique, provide evidence of water-level fluctuations over the past 2,300 years. Ten AMS radiocarbon dates on bulk sediment samples show that the lake came into existence about 2,300 years ago and that it has dried out since then, but only for brief time periods. Changes in the diatom assemblage composition indicate that lake level fluctuated in response to shifting humidity conditions. The changes reflect wetter conditions ca. 300 BC-AD 800, more variable conditions between AD 800 and 1150, a distinct dry phase within the time span AD 1150-1700 and a return thereafter to more humid conditions until present. There is general agreement between the Lake Nhauhache record and other records from the Southern Hemisphere summer rainfall region. This suggests that sediments from small interdunal lakes, which are abundant along the coast of southern Africa, provide reliable, regional paleoenvironmental information about an area from which more such data are needed.

  • 16.
    Kaislahti Tillman, Päivi
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Berntsson, Annika
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Jansson, Krister
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Helmens, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Diatom stratigraphy of the MIS 3 deposits at Sokli, Northern Finland2007In: Nordic Diatomists´ Meeting, 2007Conference paper (Other academic)
  • 17.
    Katrantsiotis, Christos
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Physical Geography. Stockholm University, Faculty of Science, Department of Geological Sciences.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Skelton, Alasdair
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    High-resolution environmental reconstruction in SW Peloponnese, Greece, covering the last c. 6000years: Evidence from Agios Floros fen, Messenian plain2016In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 26, no 2, p. 188-204Article in journal (Refereed)
    Abstract [en]

    A paleolimnological record from the central Messenian plain (southwestern Peloponnese, southern Greece) indicates rapid changes in the water level and chemistry of a transient lake on the flanks of the Taygetos Mountains during the last c. 6000years. The analyses are based on diatoms as well as carbon and nitrogen isotopes from bulk sediments in a 7.5-m-long sediment core retrieved from the drained fen of Agios Floros, at the northern banks of the ancient River Pamisos. The sequence consists of fen peat in the uppermost section underlain by lacustrine sediments, which are punctuated by two layers of clay with diatomaceous silt bands. High accumulation rate is recorded in the oldest part of the section (up to 23mm/yr), particularly during two decadal-long periods centered at c. 5700 and c. 5300 cal. BP. The diatom record reveals pronounced peaks in the planktonic taxon Cyclotella distinguenda, which correspond to the laminated sequences, reflecting the rapid development of a deep lake with an open water environment during these two time periods. Another two events with intermediate water levels are inferred at c. 5200 and c. 4600 cal. BP. These short-lived phases were probably, to a large extent, caused by local tectonic processes and the consequent hydrological anomalies of the nearby karst springs, although abrupt climatic changes with enhanced precipitation might have also played a role. At c. 4500 cal. BP, our data suggest the development of terrestrial conditions in this area, which can be attributed to the decreasing activity/dry up of springs, probably associated with more arid climate. After c. 2500 cal. BP, the diatom record infers a return to wetter conditions, probably as a response to more humid climate with marked seasonality and human activities, developing the present-day environment with cultivated and seasonally semi-flooded fields.

  • 18.
    Katrantsiotis, Christos
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Morphological study of Cyclotella distinguenda with a description of a new fossil species Cyclotella paradistinguenda sp nov from the Agios Floros fen, SW Peloponnese, Greece in relation to other Cyclotella species2016In: Diatom Research, ISSN 0269-249X, E-ISSN 2159-8347, Vol. 31, no 3, p. 243-267Article in journal (Refereed)
    Abstract [en]

    In a previous palaeoenvironmental study based on a high-resolution diatom record from a core sampled at the Agios Floros fen, SW Peloponnese, Greece, Cyclotella distinguenda was reported as exhibiting two morphs with distinct central area and stria arrangement, as well as a specific distribution throughout the sequence. In the present paper, we examine this morphological variability through detailed observations using light and scanning electron microscope combined with a simple statistical approach. Our new data suggest that the two morphs present substantial and constant differences in the structure and size of their central areas, the structure and number of their striae, the arrangement of their marginal fultoportulae/density of costae between fultoportulae, the shape of their rimoportulae and their alveolar chambers. On the basis of these morphological and stratigraphic variations one morph is described as a new species, Cyclotella paradistinguenda sp. nov., while the other is assigned to C. distinguenda and is also consistent with the original description of this taxon. The two species share well-defined central areas without fultoportulae, almost equal length striae and one rimoportula situated on a costa within the ring of marginal fultoportulae. Cyclotella paradistinguenda sp. nov. can be distinguished by a combination of the following characteristics: (1) large central area (1/3 of valve diameter), smooth or decorated with puncta and depressions, flat or concentrically undulated, (2) distinct, narrow striae (12-15/10 mu m) consisting of one or two short rows of areolae of the same size expanding into three rows at the mantle, (3) marginal fultoportulae on each first to third costae (4) poorly developed alveolar chambers and rimoportula. Based on the stratigraphic distribution and the associated flora in the Agios Floros sequence, it can be inferred that C. paradistinguenda sp. nov. is tolerant of shallower water conditions with lower nutrient availability and/or higher pH than C. distinguenda.

  • 19.
    Massuanganhe, Elidio A.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. Universidade Eduardo Mondlane, Moçambique.
    Berntsson, Annika
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Westerberg, Lars-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Christiansson, Marie
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Preusser, Frank
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Bjursäter, Stefan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Achimo, Mussa
    Palaeogeography and dynamics of the deltaic wetland of Save River, Mozambique2018In: Palaeogeography, Palaeoclimatology, Palaeoecology, ISSN 0031-0182, E-ISSN 1872-616X, Vol. 489, p. 64-73Article in journal (Refereed)
    Abstract [en]

    Many mangrove wetland systems in deltaic environments are negatively affected by massive sedimentation fromriver inflows. In this paper we use the example of the Save River delta to assess the palaeogeographic distributionof mangrove wetlands and to analyze their dynamics. To track past occurrences of mangrove wetlands in thestudy area we have integrated sedimentological data with siliceous microfossil analysis combined with AMSradiocarbon and OSL dating. The results show a fine-grained deposit with an approximate thickness of 2 m,present at different sampling sites. In the upper deltaic plain, the deposit is interbedded between sand layers,while in the lower deltaic plain the deposit occupies the uppermost stratigraphic position. In most of thesampling sites the deposit shows a succession with brackish-marine diatoms at the bottom of the sequence whilethe upper part shows only scattered occurrences. Based on sedimentological and microfossil characteristics wehave interpreted the layer to represent a mangrove wetland deposit. The development of the deposit in the studyarea is suggested to have been initiated around 3100 cal. yr BP, induced by sea-level rise. Thereafter, the developmentfollowed the combined effect of a sea-level fall and delta progradation processes. In some areas,particularly in the proximal part of the delta, the mangrove deposit has developed progressively on top of thedelta-front. From around 1300 years ago (OSL) onwards, massive alluvial sedimentation impacted the mangroveecosystem. However, the retreat of mangrove wetland coincided with a regional fall of sea level. At the edges ofthe alluvial deposit, the current mangrove ecosystem has reclaimed the habitat in some sectors where gullyerosion has exposed the once extinct mangrove habitat.

  • 20.
    Massuanganhe, Elidio
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. Universidade Eduardo Mondlane, Moçambique.
    Westerberg, Lars-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Preusser, Frank
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Bjursäter, Stefan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Achimo, Mussa
    Geomorphology and landscape evolution of Save River delta, South-central MozambiqueManuscript (preprint) (Other (popular science, discussion, etc.))
    Abstract [en]

    The Save River delta has well-preserved geomorphological and sedimentological features with potential to reveal its evolution as well as past regional environmental changes, which are anticipated to integrate into other data at a global scale. In this study we mapped and integrated these features to understand the environmental processes that drove the late evolution of the delta. The results suggest that the present configuration of the delta was primarily dominated by a delta-front protruding into the sea, in a faulted coast section that may have favoured the formation of the depocenter. The development of the delta-front was initiated prior to c. 3100 cal. yrs BP, and around this age, fine-grained sediments started to accumulate on it forming a mangrove wetland deposit that expanded from the proximal sector of the delta to the current position shown in a geomorphological map. The results show successive dune ridges in the southern sector of the delta which indicate a series of consecutive shorelines over time as the delta prograded towards the current position. These dunes have created favourable conditions for accumulation of fine-grained sediments in the back-barrier sectors, contributing to the development of the mangrove wetland. An alluvial floodplain developed during the last millennium, probably a result of high precipitation the catchment area.

  • 21.
    Massuanganhe, Elídio A.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. Eduardo Mondlane University, Mozambique.
    Westerberg, Lars-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Morphodynamics of deltaic wetlands and implications for coastal ecosystems – A case study of Save River Delta, Mozambique2018In: Geomorphology, ISSN 0169-555X, E-ISSN 1872-695X, Vol. 322, p. 107-116Article in journal (Refereed)
    Abstract [en]

    Deltaic wetlands experience changes over time, with substantial impacts on the coastal ecosystems. These changes, whether they are natural or human-induced, are caused by multiple factors through complex links and interdependences, and constitute challenges for coastal management aiming to set up practical adaptation measures. In this study, we investigate a case study of Save River Delta to interpret the typical morphodynamic pattern on the deltaic plain over an interdecadal timescale and the implications of geomorphological changes for the coastal ecosystems, with emphasis on mangroves. Our results reveal the pattern of the geomorphological changes on the deltaic wetland in river and back-barrier sectors. In both sectors, erosion and accretion are mutually adjusting processes, and they result in geomorphological settings characterized by a distinctive interaction with the ecosystem; on the one hand, mangrove trees colonize new favorable settings; on the other hand, the existing mangrove trees undergo degradation related to the morphodynamic processes. Notwithstanding current episodic events that affect the deltaic wetlands (e.g. cyclones and floods), the changes observed in the study area are part of interdecadal timescale morphodynamics. These changes were consistent for the 50-year time period analyzed. If, on the one hand, some of the episodic and high magnitude weather events such as floods undermine the status of the deltaic ecosystem, on the other hand these events contribute to develop the same ecosystem over a longer timescale. Within interdecadal timescales, biogeomorphological changes in deltaic wetlands are a critical reference frame for understanding future scenarios of environmental changes caused by climate change.

  • 22.
    Miller, Urve
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, JanStockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.Wastegård, StefanStockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Quaternary stratigraphy, vegetation dynamics and human impact - current research projects and new results. Studies in honour of Ann-Marie Robertsson2007Conference proceedings (editor) (Refereed)
  • 23.
    Miller, Urve
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Wastegård, Stefan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Quaternary stratigraphy, vegetation dynamics and human impact - current research projects and new results. Studies in honour of Ann-Marie Robertsson2007In: GFF, Vol. 129, no 4, p. 273-276Article in journal (Other academic)
  • 24.
    Norström, Elin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Navarino Environmental Observatory, Greece.
    Katrantsiotis, Christos
    Stockholm University, Faculty of Science, Department of Physical Geography. Navarino Environmental Observatory, Greece.
    Finné, Martin
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography. Navarino Environmental Observatory, Greece.
    Smittenberg, Rienk H.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bjursäter, Stefan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Biomarker hydrogen isotope composition (D) as proxy for Holocene hydroclimatic change and seismic activity in SW Peloponnese, Greece2018In: Journal of Quaternary Science, ISSN 0267-8179, E-ISSN 1099-1417, Vol. 33, no 5, p. 563-574Article in journal (Refereed)
    Abstract [en]

    We present a 6000-year-long record tracing hydroclimate changes in SW Greece, based on hydrogen isotope composition of aquatic plant-derived n-C-23 alkanes (D-C23) in a sediment core from the Messenian plain, Peloponnese. The D-C23 record co-varies with other eastern Mediterranean records, suggesting relatively wetter conditions c. 6-4.5ka, followed by progressively drier conditions leading up to maximum aridity c. 2.8 ka. This arid phase was interrupted by a shift in D-C23 between 3.3 and 3.1ka inferring wetter conditions and/or tentative responses to anthropogenic water regulating activities during the Late Bronze Age. After 2.7ka, a return to more humid conditions was followed by increased dryness and stronger seasonality contrasts from c. 2.0ka. The D-C23 record shows three short-lived excursions (5.7, 5.3, 2.8ka), where isotope values dropped by >20 parts per thousand and immediately stabilized again. The events were paralleled by abrupt increases in sedimentation rates. We hypothesize that the isotopic shifts represent a response to mixing of ground water systems during tectonic events, followed by sealing of seismically derived cracks in the active fault. The outcome of the study is promising for future expansion of isotope-based proxies on sediments in the region, to reconstruct both hydroclimate and past seismic activity.

  • 25.
    Norström, Elin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Neumann, F. H.
    Scott, L.
    Smittenberg, Rienk
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Holmstrand, Henry
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Lundqvist, S.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Snowball, I.
    Sundqvist, Hanna
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Bamford, M.
    Late Quaternary vegetation dynamics and hydro-climate in the Drakensberg, South Africa2014In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 105, p. 48-65Article in journal (Refereed)
    Abstract [en]

    Abstract A multi-proxy study of a sediment sequence from Braamhoek wetland, covering the last c. 16,000 years, reveals a record of regional climate and vegetation dynamics in the Drakensberg region, South Africa, including signals from both the organic sediment fraction (fossil pollen, charcoal, n-alkane abundance, n-alkane δ13C, TOC) and the inorganic fraction (mineral magnetic properties). The reconstruction, supported by a robust chronology, indicates two major periods of increased regional wetness during the late Pleistocene to early Holocene phase (c. 13,800–12,600 cal yr BP; c. 10,200–8500 cal yr BP) and one during the late Holocene (c. 2000 cal yr BP to present). Drier conditions are recorded during the Younger Dryas (c. 12,600–11,300 cal yr BP) and mid-Holocene (c. 7000–2000 cal yr BP). A major decline in fynbos vegetation during the early Holocene suggests a shift towards warmer temperatures and possibly towards less pronounced winter rains in eastern South Africa from c. 8500 cal yr BP. Comparison with records from interior of South Africa show relatively high inter-site variability, however, the Braamhoek moisture proxies do co-vary with the speleothem isotope records from Makapansgat, suggesting a similar hydro-climate evolution in eastern and interior parts of the summer rainfall region during the studied period. On multi-millennial time scales, an inverse hydro-climatological pattern is evident between these two South African records and reconstructions from tropical locations in southeast Africa. Such a rainfall dipole between eastern tropical and southern Africa, has previously been identified on shorter time scales, i.e. on inter-annual to millennial scales. The Braamhoek study suggests that a similar dipole pattern is acting also on a multi-millennial perspective. These long-term precipitation anomalies are tentatively coupled to teleconnections from multi-millennial changes in the dynamics of the Indian Ocean Dipole (IOD) and El Niño-Southern Oscillation (ENSO).

  • 26.
    Norström, Elin
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Gröndahl, Helena
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Snowball, Ian
    Mugabe, João Alberto
    Raúl Sitoe, Sandra
    University of Eduardo Mondlane, Mozambique.
    Coastal paleo-environment and sea-level change at Macassa Bay, southern Mozambique, since c. 6600 cal BP2012In: Quaternary International, ISSN 1040-6182, E-ISSN 1873-4553, Vol. 260, p. 153-163Article in journal (Refereed)
    Abstract [en]

    Changes in Holocene coastal paleo-environment and sea-level variations have been recorded in estuary sediments from Macassa Bay, southern Mozambique. Methods include analysis of fossil diatoms, sediment stratigraphy, mineral magnetic properties, stable carbon and nitrogen isotope composition and carbon and nitrogen content. The chronology, based on six AMS 14C dates, suggests a basal age of the sediment core of c 6600 cal BP (calibrated years Before Present). The multi-proxy dataset implies two phases when the site was experiencing marine conditions and tentative sea-level high-stands: 1) between c 6600–6300 cal BP which is contemporary with the last phase of the global transgression and the Holocene climatic optimum, and 2) between c 4700–1000 cal BP, a period when sea level curves from the region suggest occasional minor sea level high-stands. Between these phases (c 6300–4700 cal BP), the environment within the Macassa Bay estuary was less influenced by marine processes, and studied proxies indicate a freshwater phase associated with relatively low sea levels. After c 1000 cal BP, a terrestrial environment prevailed at the site, probably as a result of a combined effect of sea level lowering and high accumulation of organic peat within the estuary.

  • 27.
    Norström, Elin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, Department of Physical Geography.
    Öberg, Helena
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Sitoe, Sandra R.
    Stockholm University, Faculty of Science, Department of Physical Geography. University of Eduardo Mondlane, Mozambique.
    Ekblom, Anneli
    Westerberg, Lars-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Vegetation dynamics within the savanna biome in southern Mozambique during the late Holocene2018In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 28, no 2, p. 277-292Article in journal (Refereed)
    Abstract [en]

    This study explores temporal dynamics within grassland and Miombo woodland ecosystems in southern Mozambique and their potential coupling to hydro-climate change during the late-Holocene period. Palaeo-reconstructions are based on phytolith and diatom assemblages and mineral magnetic properties in fossil sediments from Lake Chilau, southern Mozambique. Phytolith interpretation was aided by previous ecological studies on modern plants and soils. The Lake Chilau record suggests high abundance of Panicoideae and other mesophytic grasses during the AD 1200s and 1300s, followed by an increase in Chloridoideae and grasses of more xerophytic affinity between ca. AD 1400 and 1550. This vegetation transition takes place during the early phase of the so-called ‘Little Ice Age’ (LIA), when regional palaeoclimate records report a shift from warmer and wetter towards drier and cooler conditions in southern Africa. Concurrent to these shifts within the grassland biome, the Chilau record reports an increase in phytoliths associated with arboreal vegetation (ca. AD 1400–1550), probably associated with the woody component of the Miombo savanna ecosystem. This supports previous studies hypothesizing that the forest component of the Miombo savanna was favoured by LIA dryness, although at Chilau, this expansion may have been amplified by a decline in fire disturbance. These tentative responses in the woody components of the savanna biome to shifts in moisture availability in the past have implications for future management and sustainability of the Miombo ecosystem in southern Mozambique under a changing climate.

  • 28.
    Plikk, Anna
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Helmens, Karin F.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Fernández-Fernández, María
    Kylander, Malin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Löwemark, Ludvig
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Salonen, J. Sakari
    Väliranta, Minna
    Weckström, Jan
    Development of an Eemian (MIS 5e) Interglacial palaeolake at Sokli (N Finland) inferred using multiple proxies2016In: Palaeogeography, Palaeoclimatology, Palaeoecology, ISSN 0031-0182, E-ISSN 1872-616X, Vol. 463, p. 11-26Article in journal (Refereed)
    Abstract [en]

    A 12 m long lacustrine record from Sokli, N Finland, was analyzed for diatoms, non-pollen palynomorphs, macrofossils, pollen and geochemistry in order to reconstruct the development of a high-latitude Eemian lake and investigate the influence of climatic and environmental changes on the lake ecosystem. Based on this multi-proxy dataset we distinguished five major lake phases in the lake's evolution. An initial minerogenic, glacio-lacustrine phase was followed by an organic-rich early Eemian lake phase characterized by anoxic bottom waters, high seasonality and rising nutrient levels. A long open water season, pronounced summer stratification and high productivity characterized the following early mid-Eemian lake phase, corresponding to the Eemian thermal maximum. During the late mid-Eemian lake phase decreasing water depths due to infilling and extensive mixing of the water column resulted in less stable summer stratification and decreased anoxia. The late-Eemian lake phase was characterized by shallow and dynamic conditions and a cooling climate. Superimposed on these general trends are two events characterized by colder and more arid conditions, that possibly match cold and arid events registered in palaeolimnological records on the European continent. In general, the multi-proxy record reflects a nutrient rich lake, where changes in mixing regime associated with climatic forcing and lake level changes asserted a major impact on the aquatic assemblages. The changes in the aquatic assemblages reflect the major patterns of climate change that took place during the Eemian in northern Europe; i.e. a rapid warming and high seasonality during the early Eemian, decreased seasonality during the mid Eemian and a cooling late Eemian with increased seasonality. The high latitude Sokli Eemian palaeolake record lengthens the latitudinal extent of Eemian terrestrial records across Europe, adding to the understanding of climatic gradients and drivers over Europe.

  • 29.
    Plikk, Anna
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Helmens, Karin F.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Diatom assemblages from an Eemian palaeolake in northern Europe with morphological observations of rare Aulacoseira sp. resting sporesManuscript (preprint) (Other academic)
    Abstract [en]

    An overview of the diatom flora of the last interglacial (Eemian) lacustrine gyttja deposit at Sokli, northeastern Finland is presented together with detailed descriptions including photomicrographs of resting spores similar to the Aulacoseira islandica-skvortzowii group, not found in recent material from Europe. Comparisons are made with literature data on similar taxa. The morphology of the resting spores (e.g. the number of striae and areolae in 10 µm) attributes them to the A. islandica-skvortzowii group but the lack of vegetative cells in the Sokli record makes declaration of true synonymy difficult. The presence of spores similar/related to the A. islandica-skvortzowii group in the Sokli Eemian palaeolake may reflect a more widespread  distribution of this group in the past.

  • 30.
    Raúl Sitoe, Sandra
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. University of Eduardo Mondlane, Mozambique.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Snowball, Ian
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Achimo, Mussa
    Mugabe, João
    Paleo-environment and flooding of the Limpopo River-plain, Mozambique, between c. AD 1200-20002015In: Catena (Cremlingen. Print), ISSN 0341-8162, E-ISSN 1872-6887, Vol. 126, p. 105-116Article in journal (Refereed)
    Abstract [en]

    Multi-proxy analysis was performed on a radiocarbon-dated core, collected from a relic oxbow lake in theLimpopo River-plain, Mozambique, with the aim to reconstruct paleo-environment and past flooding of thelower river system over the past c. 800 years. An additional objective was to evaluate and investigate the potentialuse of different proxies as recorders of paleo-flooding events and paleo-environmental variability within thefloodplain. The proxies applied in this study were: mineral magnetic properties, grain-size distribution, organiccarbon content and diatom microfossil assemblages.We found that sediment grain-size and mineral magneticproperties of the minerogenic fraction were the most sensitive proxies in terms of detecting signals from highintensityriver-discharge events. In the 800 year long sequence, variations in sand content, magnetic susceptibilityand saturation isothermal remnant magnetization suggest at least four major flooding events at the siteduring the reconstructed period; in the mid-1200, late-1300, mid-1500 AD and during the last century. Thediatom proxy reflects the development of the site from an open oxbow lake to a mainly terrestrial area. Thediatom assemblage indicates that open lake conditions prevailed at the site between c. AD 1200–1400, withperiodic inundation by marine water, most likely due to late Holocene sea-level changes. From c. AD 1400 andonwards, diatoms were rarely deposited at the site, which indicates drier conditions. This was a result of soilformation and gradual in-filling of the lake, a processwhich possiblywas accentuated by a regionally dry climatesituation. Our study shows that oxbow lakes and the proxies used here have great potential for reconstructingflooding events, a knowledge that is crucial for potential prediction and mitigation of flooding events inMozambique in the future. Although chronological uncertainties limit comparisons to other paleoenvironmentalrecords, it seems that the flooding events recorded at our site occurred both during regionallywet and dry periods. Our data infer however, that flooding was probably more clearly recorded during thelake-stages than during infilled stage, probably as the terrestrial environment was more exposed to erosion.

  • 31.
    Raúl Sitoe, Sandra
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. University of Eduardo Mondlane, Mozambique.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Westerberg, Lars-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Late Holocene sea-level changes and paleoclimate recorded in Lake Lungué, southern Mozambique2017In: Palaeogeography, Palaeoclimatology, Palaeoecology, ISSN 0031-0182, E-ISSN 1872-616X, Vol. 485, p. 305-315Article in journal (Refereed)
    Abstract [en]

    We report on late Holocene paleoenvironmental and sea-level changes in southern Mozambique, based on analysis of diatom stratigraphy, mineral magnetic susceptibility, Saturation Isothermal Remnant Magnetization (SIRM) and organic carbon content in a sediment core from Lake Lungué, located ca. 30 km north of Xai Xai City in the Limpopo River floodplain. Eleven radiocarbon dates performed on terrestrial shells allowed establishment of an age-depth model. High content of brackish-marine taxa, especially Diploneis suborbicularis and Navicula yarrensis, suggests that the Lake Lungué basin was part of the Indian Ocean coastal zone between ca. 740 and 910 CE, suggesting higher relative sea-level during this phase. A similar diatom distribution in older parts of the sequence, of unknown age, indicates that the site was connected to the Indian Ocean also at some stage prior to 740 CE. Between ca. 910 and 1130 CE the basin was under a combined effect of sporadic marine water influx and Limpopo River actions. A freshwater lake was established sometime between 1130 and 1360 CE, when the basin became less affected by marine influences due to lower sea-levels. Instead, the lake system was more influenced by Limpopo River meandering dynamics and flooding events, and eventually isolated into a more or less independent lake ecosystem where diatom diversity most likely responded to lake levels fluctuations driven by shifts in relative humidity and rainfall amounts within the floodplain and also from upstream sources. During this phase, high abundance of freshwater planktonic species, i.e. Aulacoseira granulata and A. ambigua, indicates high lake levels and wetter conditions dated to ca. 1360–1560 CE. From 1560 CE until present, the lake has been subject to drier conditions and higher evaporation as indicated by lower lake levels, reported by a decline in freshwater planktonic taxa and increase in brackish taxa, e.g. Amphora robusta.

  • 32.
    Risberg, Jan
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Alm, Göran
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Björck, Niclas
    Riksantikvarieämbetet, UV GAL.
    Guinard, Michel
    Societas Archaeologica Upsaliensis.
    Synkrona paleokustlinjer 7000 - 4000 kal.BP i mellersta och norra Uppland2007In: Stenåldern i Uppland, Vol 1 - Arkeologi E4 Uppland - studier: Uppdragsarkeologi och eftertanke, Societas Archaeologica Upsaliensis , 2007, p. 99-135Chapter in book (Other (popular science, discussion, etc.))
  • 33.
    Risberg, Jan
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Berntsson, Annika
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kaislahti Tillman, Päivi
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Strandförskjutning under mesolitikum på centrala Södertörn, östra Mellansverige: Kvartärgeologiska undersökningar längs väg 73, Överfors - Västnora2007Report (Other academic)
    Abstract [sv]

    The aim with this study was to reconstruct the shore displacement on central Södertörn, eastern middle Sweden. The achieved curve is supposed to serve as a base for archaeological discussions regarding dating and settlement location.

    By determining the isolation ages of local bogs and fens at different altitudes, a new shore displacement curve was compiled. The thresholds of five ancient lakes (Borgbergetmossen 50.1 m a.s.l., Långmossen 45.3 m a.s.l., Millingsmossen 44.8 m a.s.l., Vimossen 43.0 m a.s.l., and Malmbergamossen 38.7 m a.s.l.) were levelled and their isolations were determined on the basis of lithology, diatom analysis, organic carbon analysis and AMS radiocarbon dating of terrestrial macrofossils.

    At about 6800 BC, the Litorina Sea shore laid at a level that is now uplifted to c. 50 m above sea level. The shore displacement was regressive during the early Litorina Sea, e.g. the isostatic uplift was greater than the eustatic sea level rise, but a transgressive Litorina stage (L1) stopped the process of land gaining between 6800–5950 BC. Analyses of cores from Borgbergetmossen indicate that the basin had begun to isolate from sea due to regression and that this process disrupted by an increase in water depth. The transgression is also observed as a layer of clay at Vimossen, indicating greater water depth. During the following regressive period, c. 5950–5600 BC, with a relative uplift of c. 13 mm/yr, Borgbergetmossen, Långmossen and Millingsmossen became isolated lakes. A second transgression (L2) is recorded in the following century, when gyttja clay and clay was accumulated in Vimossen and Malmbergamossen, respectively. After c. 5500 BC, the shore displacement shows a regressive trend at a rate of c. 6mm/yr. Malmbergamossen became isolated from the sea at c. 4500 BC. This basin is now uplifted to c. 39 m a.s.l.

    Comparison with archaeological data indicates that the nearby Mesolithic sites were established along the former shore lines. Even though the general trend of the shore displacement was regressive, the inhabitants continued to live at the sites long after direct contact with the shore had ceased. This is explained by the topographically pronounced fissure-valley landscape; long vertical distance does not result in long horizontal distance.

    We have used the chronological timing of L1 and L2 for correlation with periods of increased summer temperatures in western Fennoscandia. Melting of glaciers resulted in a eustatic sea level rise that neutralized the isostatic uplift.

  • 34.
    Risberg, Jan
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Isaksson, Mikael
    Arkeologikonsult.
    Alm, Göran
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Site location in the Stone Age landscape on the Södertörn peninsula, eastern middle Sweden: an ecological and economical approach2011In: Skrifter från Arkeologikonsult, ISSN ISBN 978-91-979352-0-3, no 1, p. 5-44Article in journal (Other academic)
    Abstract [en]

    This paper aims to use GIS applications to interpret ecological and economical presumptions for the Stone Age habitation on the south-eastern part of the peninsula Södertörn, south of Stockholm, southeast Sweden. Specific questions concern how characteristic geographical features of the landscape have influenced the settlement sites before, during and after usage and how periods of continuity and discontinuity may be explained based on economical and ecological presumptions. The used data have been achieved from both geological and archaeological sources, such as the age of isolated lake basins and dates of sealed features like hearths and cooking pits. These have served as basis for the establishment of paleogeographical maps of the land/sea configuration at 6250, 5500, 4700 and 4000 BC constructed as tilted terrain models.

    It is concluded that the long-term occupation of the sites can be explained by their strategic location, which due to the steep topography was retained over extensive time periods. Their boundary positions have allowed a variety of resources to be exploited. Their location along waterways was convenient and efficient. If the results would render valid on a general scale, sites with a comparable function and patterns of use should be located along waterways in similar boundary zones. Such sites would probably have been settled as soon as suitable areas emerged from the sea and abandoned when the connectivity along the waterways seized.

  • 35. Rosenberg, Thomas M.
    et al.
    Preusser, Frank
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Plikk, Anna
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kadi, Khalid A.
    Matter, Albert
    Fleitmann, Dominik
    Middle and Late Pleistocene humid periods recorded in palaeolake deposits of the Nafud desert, Saudi Arabia2013In: Quaternary Science Reviews, ISSN 0277-3791, E-ISSN 1873-457X, Vol. 70, p. 109-123Article in journal (Refereed)
    Abstract [en]

    Present climate in the Nafud desert of northern Saudi Arabia is hyper-arid and moisture brought by north-westerly winds scarcely reaches the region. The existence of abundant palaeolake sediments provides evidence for a considerably wetter climate in the past. However, the existing chronological framework of these deposits is solely based on radiocarbon dating of questionable reliability, due to potential post-depositional contamination with younger C-14. By using luminescence dating, we show that the lake deposits were not formed between 40 and 20 ka as suggested previously, but approximately ca 410 ka, 320 ka, 200 ka, 125 ka, and 100 ka ago. All of these humid phases are in good agreement with those recorded in lake sediments and speleothems from southern Arabia. Surprisingly, no Holocene lake deposits were identified. Geological characteristics of the deposits and diatom analysis suggest that a single, perennial lake covered the entire south-western Nafud ca 320 ka ago. In contrast, lakes of the 200 ka, 125 ka, and 100 ka humid intervals were smaller and restricted to interdune depressions of a preexisting dune relief. The concurrent occurrence of humid phases in the Nafud, southern Arabia and the eastern Mediterranean suggests that moisture in northern Arabia originated either from the Mediterranean due to more frequent frontal depression systems or from stronger Indian monsoon circulation, respectively. However, based on previously published climate model simulations and palaecolimate evidence from central Arabia and the Negev desert, we argue that humid climate conditions in the Nafud were probably caused by a stronger African monsoon and a distinct change in zonal atmospheric circulation.

  • 36.
    Shala, Shyhrete
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Helmens, Karin F.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Jansson, Krister N.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kylander, Malin E.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Löwemark, Ludvig
    Palaeoenvironmental record of glacial lake evolution during the early Holocene at Sokli, NE Finland2014In: Boreas, ISSN 0300-9483, E-ISSN 1502-3885, Vol. 43, no 2, p. 362-376Article in journal (Refereed)
    Abstract [en]

    The development of a glacial lake impounded along the retreating, northeastern ice margin of the Fennoscandian Ice Sheet during the last deglaciation and environmental conditions directly following the early Holocene deglaciation have been studied in NE Finland. This so-called Sokli Ice Lake has been reconstructed previously using topographic and geomorphologic evidence. In this paper a multiproxy approach is employed to study a 3-m-thick sediment succession consisting of laminated silts grading into gyttja cored in Lake Loitsana, a remnant of the Sokli Ice Lake. Variations in the sediment and siliceous microfossil records indicate distinct changes in water depth and lake size in the Loitsana basin as the Sokli Ice Lake was drained through various spillways opening up along the retreating ice front. Geochemical data (XRF core-scanning) show changes in the influence of regional catchment geochemistry (Precambrian crystalline rocks) in the glacial lake drainage area versus local catchment geochemistry (Sokli Carbonatite Massif) within the Lake Loitsana drainage area during the lake evolution. Principal component analysis on the geochemical data further suggests that grain-size is an additional factor responsible for the variability of the sediment geochemistry record. The trophic state of the lake changed drastically as a result of morphometric eutrophication once the glacial lake developed into Lake Loitsana. The AMS radiocarbon dating on tree birch seeds found in the glaciolacustrine sediment indicates that Lake Loitsana was deglaciated sometime prior to 10 700 cal. a BP showing that tree Betula was present on the deglaciated land surrounding the glacial lake. Although glacial lakes covered large areas of northern Finland during the last deglaciation, only few glaciolacustrine sediment successions have been studied in any detail. Our study shows the potential of these sediments for multiproxy analysis and contributes to the reconstruction of environmental conditions in NE Finland directly following deglaciation in the early Holocene.

  • 37.
    Sjöström, Jenny
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography. Stockholm University, Faculty of Science, Department of Geological Sciences.
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Physical Geography. Stockholm University, Faculty of Science, Department of Geological Sciences.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Schoeman, Maria H.
    Late Holocene palaeoenvironmental reconstruction from Mpumalanga Province (South Africa) inferred from geochemical and biogenic proxies2017In: Review of Palaeobotany and Palynology, ISSN 0034-6667, E-ISSN 1879-0615, Vol. 246, p. 264-277Article in journal (Refereed)
    Abstract [en]

    Here we present a palaecological reconstruction covering the last 1700 yr from Lydenburg fen, located in the north-eastern grassland biome, Mpumalange, South Africa. A 300 cm peat sequence was analysed for biogenic (grass phytoliths, diatoms) and geochemical proxies (delta C-13, delta N-15, carbon/nitrogen content) to infer past grassland dynamics and hydro-climatic changes. The Lydenburg record reports a C-4 dominated grassland throughout the studied period, with more or less pronounced fluxes between C-4-Chloridoideae and C-4-Panicoideae grass subfamilies. The record reflects moderate to dry conditions from AD 400 to 1000; more mesic conditions until around AD 1250; followed by a significantly drier period between c. AD 1250 and c. AD 1350, when Chloridoideae grasses expand at the expense of Panicoideae grasses. During this phase, the delta C-13-record reports more enriched values indicating higher influx of C-4 grasses. Furthermore, lithological evidence indicates highly erosive conditions, with significant gravel input from the surrounding hills. After AD 1350, proxy indications suggest a shift towards more mesic conditions. During this increasingly mesic but also unstable period, farming communities using specialized agricultural practices (e.g. the people in Bokoni) expanded their settlements into new regions (Delius et al., 2008). This expansion was also coupled to population growth, suggesting these communities applied techniques that enabled improved food production under environmentally challenging conditions. Over the last century, Lydenburg delta C-13-values indicate increased input of C-3 taxa. The phytolith record shows that this increase is not coupled to an increase in Pooideae (C-3) grasses, suggesting that the C-3 input may be related to woody encroachment.

  • 38. Tsugai, Alina
    et al.
    Plado, Jueri
    Joeleht, Argo
    Kriiska, Aivar
    Mustasaar, Mario
    Raig, Hanna
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Rosentau, Alar
    Ground-penetrating Radar and Geological Study of the Kudrukula Stone Age Archaeological Site, Northeast Estonia2014In: Archaeological Prospection, ISSN 1075-2196, E-ISSN 1099-0763, Vol. 21, no 3, p. 225-234Article in journal (Refereed)
    Abstract [en]

    A combined ground-penetrating radar (GPR), drilling and diatom survey was carried out in order to characterize formation of the Kudrukula Stone Age Comb Ware archaeological site, northeast Estonia. A few decades ago a cultural layer was discovered, located within fine-grained sands of the right bank of the Kudrukula Stream an altitude of 1.25-1.60 m above sea level. The layer is 15-35 cm thick and composed of poorly sorted sand of intensive reddish colour with abundant arte- and ecofacts, human bones, burned hearthstones and charcoal pieces. Artefacts, in particular pottery, are well preserved and frequently present as large pieces. The Kudrukula site holds an exceptional position among other Stone Age coastal settlements in that it is not located directly on top of beach ridges, but is buried under similar to 3 m of sand. When the Kudrukula settlement existed the level of the Litorina Sea was similar to 6 m above present sea level, which is similar to 4.5 m above the present location of the cultural layer. The current research is used to describe the geological structure of the area to provide a perspective on the history of the cultural material, and reveals that the cultural material is not restricted to one layer/lens only because it is irregularly and widely distributed within the sands of Kudrukula. The GPR and diatom analyses show that the Kudrukula cultural material is redeposited, because it occurs within an ancient point-bar succession of a (Narva) river meander. Originally, the dwelling site had been located on top of the Narva-Joesuu sandy coastal ridges, which subsequently suffered erosion and the sand resedimented at the Kudrukula location. Good preservation of the cultural items shows that, after redeposition, they were buried fast, probably within a year. The present Kudrukula stream has cut into the floodplain of the ancient (Narva) river to expose the redeposited layer/lens.

  • 39.
    Widgren, Mats
    et al.
    Stockholm University, Faculty of Social Sciences, Department of Human Geography.
    Maggs, Tim
    Plikk, Anna
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Schoeman, Maria H.
    Westerberg, Lars-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Precolonial agricultural terracing in Bokoni, South Africa: Typology and an exploratory excavation2016In: Journal of African Archaeology, ISSN 1612-1651, E-ISSN 2191-5784, Vol. 14, no 1, p. 33-53Article in journal (Refereed)
    Abstract [en]

    Earlier work on the terraced settlements of the Bokoni area (16th to 19th century, Mpumalanga province, South Africa) focussed on the homesteads, their contents, layout and chronology. This paper suggests a terminology and typology of agrarian structures in Bokoni in order to improve comparative approaches in Africa and beyond. The typology and an excavation of the terracing have made possible preliminary conclusions relevant for the further analysis of the terracing and stone-walling in Bokoni. The terracing developed incrementally, whereby cultivation, stone-clearing and terracing were intermixed processes. This is supported both by the organic content in a section of a terrace and by a phytolith analysis. The phytolith analysis furthermore indicated that maize was cultivated on the terraces, but this should be seen as a pilot study only, and presence of maize in Bokoni must be tested with other archaeobotanical methods.

  • 40.
    Wohlfarth, Barbara
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Veres, Daniel
    Ampel, Linda
    Lacourse, Terri
    Blaauw, Maarten
    Preusser, Frank
    Andrieu-Ponel, Valerie
    Keravis, Didier
    Lallier-Verges, Elisabeth
    Björck, Svante
    Davies, Siwan
    de Beaulieu, J. L.
    Risberg, Jan
    Hormes, Anne
    Kasper, H. U.
    Possnert, Göran
    Reille, Maurice
    Thouveny, Nicolas
    Zander, Anja
    Rapid ecosystem response to abrupt climate changes during the last glacial period in western Europe, 40-16 ka2008In: Geology, Vol. 36, no 5, p. 407-410Article in journal (Refereed)
  • 41.
    Öberg, Helena
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Andersen, Thorbjørn J.
    Westerberg, Lars-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    A diatom record of recent environmental change in Lake Duluti, northern Tanzania2012In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 48, no 2, p. 401-416Article in journal (Refereed)
    Abstract [en]

    Lake Duluti is a small, topographically closed crater lake located on the flanks of Mt Meru, northern Tanzania. Analyses of diatoms in three short sediment cores and four modern samples from Lake Duluti were used to infer past environmental changes. 210Pb and 137Cs activity profiles combined with AMS 14C dates provide the chronological framework. Weak agreement between the 210Pb and 14C records, together with dating uncertainty, precludes construction of precise age models. The modern diatom flora, from plankton and three periphytic habitats, is dominated by Aulacoseira ambigua (Grunow) Simonsen, Gomphonema parvulum (Kützing) Grunow and Nitzschia amphibia Grunow. All three cores display similar stratigraphic succession, but the relative ratio of habitats represented by the diatoms varies substantially between cores. Diatoms indicate that the oldest part of the record is characterized by relatively low lake level and swampy vegetation. In the late nineteenth or early twentieth century there was a rapid lake level rise and the swamp turned into an open-water lake. High lake levels have prevailed since that time.

  • 42.
    Öberg, Helena
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Norström, Elin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Ryner, Maria Malmström
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Westerberg, Lars-Ove
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Eddudottir, Sigrun Dogg
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Andersen, Thorbjörn J.
    Muzuka, Alfred
    Environmental variability in northern Tanzania from AD 1000 to 1800, as inferred from diatoms and pollen in Lake Duluti2013In: Palaeogeography, Palaeoclimatology, Palaeoecology, ISSN 0031-0182, E-ISSN 1872-616X, Vol. 374, p. 230-241Article in journal (Refereed)
    Abstract [en]

    Fossil pollen and diatoms have been analyzed in a sediment sequence from a topographically closed crater lake in northern Tanzania (Lake Duluti), with the aim to reconstruct past changes in lake level and vegetation dynamics. The results provide a new paleoenvironmental record from equatorial Africa covering the period c. AD 1000 to AD 1800. Overall, the pollen and diatom records generate comparable stories of dry and wet periods. Dry conditions are inferred at c. AD 1040-1470, c. AD 1510-1640 and C. AD 1650-1670 with the lowest lake levels at c. AD 1260-1290 and AD 1600-1640. Wetter conditions occurred c. AD 1640-1650 and c. AD 1670-1790. The chronology is based on combined analyses of Pb-210 activity and AMS C-14 on bulk sediment, and a Bayesian model was applied to establish the age-depth relationship. The hydroclimatic record from Lake Duluti shows good correlation with several East African lakes in a centennial time perspective, although comparison of high frequency variability in the region is hampered by dating uncertainties.

  • 43.
    Öberg, Helena
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Risberg, Jan
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Stabell, Bjørg
    Department of Geosciences, University of Oslo.
    Morphology, valve ultrastructure and stratigraphical variability of Discostella taxa in a tropical crater lake, northern Tanzania2009In: Diatom Research, ISSN 0269-249X, E-ISSN 2159-8347, Vol. 24, no 2, p. 341-356Article in journal (Refereed)
    Abstract [en]

    Morphological and ultrastructural variability of Discostella Houk et Klee (Bacillariophyceae) valves from a small crater lake in northern Tanzania have been examined under scanning electron microscope (SEM) and light microscope (LM). Three types of Discostella are described; Discostella woltereckii, as well as a small variety of D. woltereckii denoted Discostella woltereckii var. minor Öberg, Risberg et Stabell and an unidentified species, Discostella sp. The diatom record during the last ~100 years is dominated by D. woltereckii and D. woltereckii var. minor. The lithology and diatom stratigraphy of a 36 cm sediment surface core show a period of low lake level with extensive aquatic vegetation from around AD 1600. Sometime around AD 1900 there is a substantial rise in the lake water level indicated by an increase in euplanktonic Aulacoseira ambigua and facultative planktonic Discostella spp.

1 - 43 of 43
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