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  • 1.
    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.

  • 2.
    Ryner, Maria
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Bonnefille, Raymonde
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Muzuka, Alfred
    Vegetation changes in Empakaai Crater, northern Tanzania, at 14,800–9300 cal yr BP2006In: Review of Palaeobotany and Palynology, ISSN 0034-6667, Vol. 140, p. 163-174Article in journal (Refereed)
    Abstract [en]

    Vegetation changes are documented from a well-dated pollen record from Lake Emakat, Empakaai Crater, northern Tanzania.

    This pollen record includes the time interval covering the Pleistocene/Holocene transition, analysed at a resolution interval

    averaging 200 yr. Around the crater lake, an Hagenia-forest development starting at 14,500 cal yr BP lasted until 13,000 cal yr BP.

    A change in vegetation, indicated by an increased proportion of Nuxia congesta in the forest and Artemisia in the afro alpine

    grassland after 13,000 cal yr BP, corresponds in time to the Northern Hemisphere's Younger Dryas cooling. Grasses and sedges

    increased at ∼10,100 cal yr BP, indicating a significant increase in local pollen possibly attributed to lowered lake level, related to

    drier conditions. Although the Empakaai pollen record documents continuous forest conditions, from 14,500 to 10,100 cal yr BP,

    the variation in the proportion of forest components seem to respond to environmental changes at the millennium scale

  • 3.
    Ryner, Maria
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Gasse, F
    Verschuren, D
    Rumes, B
    Climatic and hydrological instability in semi-arid equatorial East Africa during the late Glacial to Holocene transition:: A multi-proxy reconstruction of aquatic ecosystem response in northern Tanzania2007In: Palaeogeography, Palaeoclimatology, Palaeoecology, ISSN ISSN: 0031-0182, Vol. 248, p. 440–458-Article in journal (Refereed)
    Abstract [sv]

    This paper reports new multi-proxy palaeoenvironmental data on the late Glacial-Holocene transition (c. 14.8–9.3 ka) in equatorial

    East Africa, in the form of microfossil assemblages (chironomids, diatoms and ostracods) recovered from the sediment record of Lake

    Emakat, Empakaai Crater, northern Tanzania. In the context of available palaeoclimatic and palaeoecological information from the

    region and previously published fossil pollen and carbon and nitrogen isotopic data for the same sediment sequence, we here

    reconstruct local lake-system response to regional climatic and hydrological instability during the period of post-glacial warming. The

    aquatic biological proxy indicators suggest that the water level and chemistry of Lake Emakat evolved, first from a shallow freshwater

    body at 14.8 ka to a deeper freshwater phase between c.14.4 and 10.3 ka and then to a markedly shallower, alkaline-saline environment

    after c.10.3 ka. The lake appears to have been deepest between 13.2 and 12.0 ka, at a time of climatic drying when moist montane

    forest vegetation within the lake's crater catchment was being replaced by open wood-and scrubland. Some palaeohydrological

    changes reconstructed for Lake Emakat are in phase with lake evolution elsewhere in the region and thus apparently track broad-scale

    climate changes, but some are not. Collectively these multi-proxy paleolimnological data indicate a complex adjustment of the local

    aquatic ecosystem to temporal variations both in total annual effective precipitation and its seasonal distribution. The lake's

    hydrological response was further conditioned by local factors, notably its geological and topographic setting.

  • 4.
    Ryner, Maria
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Gasse, Francoise
    Rumes, Bob
    Verschuren, Dirk
    Climatic and hydrological instability in semi-arid equatorial eastern Africa during the late Glacial to Holocene transition:  a multiproxy reconstruction of aquatic ecosystem response in northern Tanzania2007In: Palaeogeography, Palaeoclimatology, Palaeoecology, ISSN 0031-0182, Vol. 248, no 3-4, p. 440-458Article in journal (Refereed)
    Abstract [en]

    This paper reports new multi-proxy palaeoenvironmental data on the late Glacial-Holocene transition (c. 14.8–9.3 ka) in equatorial East Africa, in the form of microfossil assemblages (chironomids, diatoms and ostracods) recovered from the sediment record of Lake Emakat, Empakaai Crater, northern Tanzania. In the context of available palaeoclimatic and palaeoecological information from the region and previously published fossil pollen and carbon and nitrogen isotopic data for the same sediment sequence, we here reconstruct local lake-system response to regional climatic and hydrological instability during the period of post-glacial warming. The aquatic biological proxy indicators suggest that the water level and chemistry of Lake Emakat evolved, first from a shallow freshwater body at 14.8 ka to a deeper freshwater phase between c.14.4 and 10.3 ka and then to a markedly shallower, alkaline-saline environment after c.10.3 ka. The lake appears to have been deepest between 13.2 and 12.0 ka, at a time of climatic drying when moist montane forest vegetation within the lake's crater catchment was being replaced by open wood-and scrubland. Some palaeohydrological changes reconstructed for Lake Emakat are in phase with lake evolution elsewhere in the region and thus apparently track broad-scale climate changes, but some are not. Collectively these multi-proxy paleolimnological data indicate a complex adjustment of the local aquatic ecosystem to temporal variations both in total annual effective precipitation and its seasonal distribution. The lake's hydrological response was further conditioned by local factors, notably its geological and topographic setting.

  • 5.
    Ryner, Maria
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Holmgren, Karin
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology (INK).
    Taylor, David
    A record of vegetation dynamics and lake level changes from Lake Emakat, northern Tanzania, during the last c. 1200 years2007In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 40, no 2, p. 583-601Article in journal (Refereed)
    Abstract [en]

    Analyses of down-core variations in pollen and charcoal in two short cores of lake sediment and wood samples taken from the in situ remains of Nuxia congesta from Lake Emakat, a hydrologically-closed volcanic crater lake occupying the Empakaai Crater in northern Tanzania, have generated evidence of past vegetation change and lake level fluctuations. Eight AMS radiocarbon (14C) dates on bulk samples of lake sediment provide a chronological framework for the two cores and indicate that the sediment record analysed incorporates the last c. 1200 years. The in situ remains of a Nuxia congesta tree, now standing in deep water, were dated with three additional AMS 14C dates, suggesting tree growth within the interval ∼1500–1670 AD. Down-core variations in pollen from terrestrial taxa, particularly the montane forest trees Hagenia abyssinica and Nuxia congesta, indicate a broad period of generally more arid conditions in the catchment to c. 1200 AD and at a prolonged period between c. 1420 and 1680 AD. Variations in pollen from plants in lake margin vegetation indicate low lake levels, presumably as a result of reduced effective precipitation, contemporary with indications of relatively dry conditions mentioned above, but also during the late 18th and the late 19th centuries. The presence of charcoal throughout both cores indicates the frequent occurrence of vegetation fires. An increase in burning, evident in the charcoal data and dated to the early to mid second millennium AD, could relate to an expansion of human population levels and agricultural activity in the region.

  • 6.
    Ryner, Maria
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. 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. Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Taylor, David
    A record of vegetation dynamics and lake level changesfrom Lake Emakat, northern Tanzania, during the last c 1200 years2008In: Journal of Paleolimnology, ISSN 0921-2728, Vol. 40, no 2, p. 583-601Article in journal (Refereed)
    Abstract [en]

    Analyses of down-core variations in pollen and charcoal in two short cores of lake sediment and wood samples taken from the in situ remains of Nuxia congesta from Lake Emakat, a hydrologically-closed volcanic crater lake occupying the Empakaai Crater in northern Tanzania, have generated evidence of past vegetation change and lake level fluctuations. Eight AMS radiocarbon (C-14) dates on bulk samples of lake sediment provide a chronological framework for the two cores and indicate that the sediment record analysed incorporates the last c. 1200 years. The in situ remains of a Nuxia congesta tree, now standing in deep water, were dated with three additional AMS C-14 dates, suggesting tree growth within the interval similar to 1500-1670 AD. Down-core variations in pollen from terrestrial taxa, particularly the montane forest trees Hagenia abyssinica and Nuxia congesta, indicate a broad period of generally more arid conditions in the catchment to c. 1200 AD and at a prolonged period between c. 1420 and 1680 AD. Variations in pollen from plants in lake margin vegetation indicate low lake levels, presumably as a result of reduced effective precipitation, contemporary with indications of relatively dry conditions mentioned above, but also during the late 18th and the late 19th centuries. The presence of charcoal throughout both cores indicates the frequent occurrence of vegetation fires. An increase in burning, evident in the charcoal data and dated to the early to mid second millennium AD, could relate to an expansion of human population levels and agricultural activity in the region.

  • 7.
    Westerberg, Lars-Ove
    et al.
    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.
    Börjeson, Lowe
    Stockholm University, Faculty of Social Sciences, Department of Human Geography.
    Håkansson, Thomas
    Laulumaa, Vesa
    Ryner, Maria
    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.
    The development of the ancient irrigation system at Engaruka, northern Tanzania: Physical and societal factors2010In: Geographical Journal, ISSN 0016-7398, E-ISSN 1475-4959, Vol. 176, no 4, p. 304-318Article in journal (Refereed)
    Abstract [en]

    Climate data from Empakaai Crater in northern Tanzania, covering the last 1200 years, are related to the establishment, development and decline of the ancient irrigation system at Engaruka. New dates for the system are linked to reconstructed climatic variations and historical data on long-distance and regional trade and migration patterns. A shift from a comparatively humid climate to drier conditions in the 1400s prompted the establishment of irrigated agriculture at Engaruka, and a flourishing long-distance trade increased its value as a water and food source for passing caravans. Once established, the land-use system at Engaruka was sufficiently resilient to survive and even intensify during much drier climate from c. 1500 to 1670 CE (Common Era) and during the decline of caravan trade between c. 1550 and 1750. The ancient land-use system probably reached its maximum extension during the humid conditions between 1670 and 1740, and was deserted in the early to mid 1800s, presumably as a result of the added effects of climate deterioration, the Maasai expansion, and change of livelihood strategies as agriculturalists became pastoralists. Towards the end of the 1800s irrigated agriculture was again established at Engaruka, in part driven by the transfer from pastoral to agricultural livelihoods caused by the Rinderpest.

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