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Environmental history in southern Mozambique: Reconstruction of flooding events, hydroclimate and sea-level dynamics since mid-Holocene
Stockholm University, Faculty of Science, Department of Physical Geography. (Climate and Quaternary)
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of this thesis has been to reconstruct paleoenvironment, paleohydrology and paleoclimate in coastal southern Mozambique, with emphasis on tracing past flooding events on the lower Limpopo River floodplain. In order to extend flood chronologies beyond periods covered by instrumental data, sediments from lakes on the floodplain were studied (Lake Lungué, Coassane Oxbow, Lake Magandane and Lake Soane). Past sea-level variations and climate changes were deduced by analyzing sediments from coastal sites north of the floodplain area (Lake Chilau, Lake Nhauhache and Macassa Bay). To achieve the established objectives, a multi-proxy approach was applied on most of the retrieved sediment cores, involving analysis of mineral magnetic parameters, grain-size and organic carbon in combination with analysis of microfossils such as diatoms and/or phytoliths. Chronologies for the constructed time-series analysis were obtained by radiocarbon dating and age-depth modelling. The synthesized data from the sampled sites on the Limpopo River floodplain suggest that the area was affected by at least 16 flooding events of variable magnitudes during the studied period. These are dated to c. AD 940, 980, 1040, 1100, 1250, 1300, 1370, 1580, 1665, 1730, 1755, 1855, 1920, 1945, 1970 and 2000. In calibrated years BP these ages correspond to 1010, 970, 910, 850, 700, 650, 580, 370, 285, 220, 195, 95, 30, and 5 cal yrs BP. The two youngest are dated to 20 and 50 years AP (After Present being 1950). Proxy data further suggest that southern Africa was subject to two periods of sea-level highstands, at c. 5000–4200 BC (6950–6150 cal yrs BP) and AD 300–950 (1650–1000 cal yrs BP). The former represents the middle part of the postglacial climatic optimum. The wettest period in the Limpopo River floodplain was reported between AD 1360 and 1560 (590 and 390 cal yrs BP) in the Lake Lungué record, while Lake Chilau experienced wet conditions between AD 1200 and 1400 (750 and 550 cal yrs BP), then returning to drier conditions that prevailed until c. AD 1600. In Lake Nhauhache, however, drier conditions prevailed from c. AD 1200–1700 (750–250 cal yrs BP), shifting towards wetter at c. AD 1900 (50 cal yrs BP). The deviating signals between records can partly be explained by Lake Lungué basin being located on the Limpopo River floodplain, responding to flooding events associated with precipitation upstream the drainage area. Therefore, wet and dry periods in floodplain lakes (e.g. Lake Lungué) are not expected to correlate with precipitation changes on a local scale, as indicated by e.g. Lake Nhauhache. This is supported by a relatively weak agreement between Lake Lungué record and other nearby records (outside the floodplain), but a better correlation with records from the upper catchment, where a more regional climate signal is provided of the southern African summer rainfall region.

Abstract [sv]

Denna avhandling behandlar studier av paleomiljöer i södra Mozambique med fokus på översvämningshistorik i Limpopo-flodens nedre lopp. Tidsspannet är mitt- och sen-holocen och inkluderar även klimat och havsnivåvariationer. Följande analysmetoder har applicerats på ett flertal sedimentkärnor för att belysa ovanstående frågeställningar: mineralmagnetiska parameterar, kornstorlek, organiskt kol samt kiselmikrofossil (diatoméeter och fytoliter). Åldrar har bestämts med 14C-metoden. 

Analyserna visar att mineralmagnetiska parametrar är lämpliga för att identifiera översvämningar, speciellt magnetisk susceptibilitet och SIRM. Dessa parametrar visar på ett sediments magnetiska kornstorlerkar och koncentrationen av finkorniga magnetiska mineral. Även kornstorleksanalyser fungerade som ett bra komplement till dessa metoder. Sammantaget har ett minimum av 16 översvämningar registrerats i Limpopo/flodens nedre lopp under de senaste 1100 åren. Särskilt stora översvämningar har daterats till AD 1250 (700 kal år BP), AD 1370 (580 kal år BP), AD 1580 (370 kal år BP), AD 1855 (95 kal år BP), AD 1920 (30 kal år BP), AD 1970 (20 kal år AP) och AD 2000 (50 kal år AP), där AP betyder ”after present”, vilket är 1950. Mindre kraftiga översvämningar har daterats till AD 940 (1010 kal år BP), AD 980 (970 kal år BP), AD 1040 (910 kal år BP), AD 1100 (850 kal år BP), AD 1300 (650 kal år BP), AD 1665 (285 kal år BP), AD 1730 (220 kal år BP), AD 1755 (195 kal år BP) och AD 1945 (5 kal år BP). Skriftliga källor visar på att åtminstone åtta översvämningar har ägt rum under de senaste 62 åren. Under samma period har denna undersökning registrerat två händelser, vilket indikerar att endast ett fåtal har registrerats med ovan nämnda metoder. Det är sannolikt att sedimentationsmönstret under en översvämning är komplicerat och inte heller likartad från gång till gång. Detta innebär att ett flertal sedimentkärnor behöver analyseras och dateras väl. Ytterligare en försvårande faktor är flodens mycket aktiva meandringsaktivitet.

Analys av diatoméer har visats vara värdefulla när det gäller översvämningshistorik, klimatförändringar och havsytevariationer. Sedimenten i den något till flodslättens nedre lopp perifert belägna sjön Lungué indikerar fuktiga förhållanden mellan AD 1360 och 1560 (590 till 390 kal år BP). Kombinerade fytolit- och diatoméanalyser av sedimenten i sjön Chilau indikerar fuktiga förhållanden från AD 1200 till 1400 (750 till 550 kal år BP), varefter torrare förhållanden rådde till ca AD 1600 (350 kal år BP). Diatoméanalys av sedimenten i sjön Nhauhache indikerar generellt sett torrare förhållanden mellan AD 1200 och AD 1700 (750 till 250 kal år BP) då klimatet blev fuktigare. De fuktiga och torra förhållanden som dokumenterats i sjön Lungué och översvämningshistoriken korrelerar inte helt med lokala nederbördsförhållanden. Däremot stämmer erhållna data bättre med förhållanden i den övre delen av Limpopoflodens dräneringsområde. Även sjöar och fyllda meanderbågar på själva flodslätten verkar reagera tydligare på översvämningar orsakade av nederbörd i den övre delen av flodloppet.

Undersökningen indikerar två faser av en förhöjd havsyta längs kusten i södra Mozambique. En äldre fas är daterad till ca 5000–4200 f.Kr. (6950–6150 kal år BP), vilken representerar den mellersta delen av det postglaciala värmeoptimet. Denna tolkning stöds även av andra undersökningar. En yngre fas har daterats till AD 300–950 (1650–1000 kal år BP), vilken korresponderar till en period med något förhöjda temperaturer i ett globalt perspektiv.

Abstract [pt]

O presente projecto teve como objectivo a reconstrução do paleoambiente, paleohidrologia e paleoclima da costa Sul de Moçambique, com ênfase na identificação de vestígios de eventos de cheias que tenham, no passado, afectado a planície de inundação do Rio Limpopo, com vista a fazer a extensão das datações de cheias para o período anterior aos instrumentos de medição. Deste modo, foram estudados sedimentos recuperados através de testemunhos de sondagem em lagos dentro da planície de inundação (Lago Magandane, Lago Lungué, Lago Soane e o canal abandonado Coassane). As variações do nível do mar e mudanças climáticas do passado foram deduzidas a partir da análise de sedimentos de áreas costeiras a norte da planície de inundação (Lago Chilau, Lago Nhauhache e a Baía de Macassa). Para o alcance destes objectivos, os sedimentos dos testemunhos de sondagem recuperados foram submetidos a várias análises laboratoriais que incluem propriedades de minerais magnéticos, granulometria, teor de matéria orgânica e de microfósseis (diatomáceas e fitólitos). Para o estabelecimento do modelo cronológico foram usadas datações pelo método 14C feitas em matéria orgânica, assim como conchas de gastrópodes e bivalves.

A combinação dos resultados obtidos para os pontos de amostragem localizados dentro da planície de inundação do Rio Limpopo, sugerem que esta área foi afectada por um mínimo de 16 eventos de cheias de variada magnitude nos últimos 1100 anos. As cheias de alta magnitude tiveram lugar nos anos 1250, 1370, 1580, 1855, 1920, 1970 e 2000. Por seu turno, as cheias de magnitude moderada ocorrerram nos anos 940, 980, 1040, 1100, 1300, 1665, 1730, 1755 e 1945.. Os anos a negrito são indicativos de eventos de cheias de alta magnitude. O número total de eventos identificados no presente estudo é um mínimo que poderá ter afectado a planície de inundação, tendo em consideração que fontes escritas reportam a ocorrência de pelo menos oito eventos nos últimos 62 anos, tendo o presente estudo revelado somente dois. Isto indica que apenas eventos de cheias intensos podem ser revelados pelos métodos aplicados nesta investigação. Adicionalmente, este estudo mostra a necessidade de aumentar o número de pontos de amostragem para permitir a identificação de mais eventos de cheias ocorridos no passado, devido à diferenciada resposta sedimentológica e geomagnética às cheias, a qual se encontra directamente relacionada à distância do rio meandrante.

As análises de microfósseis de diatomáceas permitiram reconstruir o paleoclima e as influências do nível do mar nas áreas de estudo deste projecto. Os registros de diatomáceas do Lago Lungué indicam um período húmido entre AD 1360 e 1560. No Lago Chilau, dados de diatomáceas e fitólitos sugerem condições de humidade entre AD 1200 e 1400. Por seu turno, no Lago Nhauhache, condições de seca dominam entre AD 1200 e 1700. As condições de seca e humidade documentadas no Lago Lungué e os eventos de cheias revelados pelos sedimentos analisados da planície de inundação do Rio Limpopo não mostram total correlação com períodos de baixa e alta precipitação a nível local, mas apresentam boa correlação com evidências a montante na área de drenagem do Rio Limpopo. Em geral, lagos activos ou preenchidos por sedimentos mostrarão ser mais susceptíveis a cheias com origem a montante do que a nível local.

As investigações conduzidas permitiram identificar dois períodos de alto nível do mar na região sul de Moçambique. A fase mais antiga teve lugar há cerca de 5000–4200 BC (6950–6150 anos do calendário BP), representando a fase mais recente do pós-glaciar climático óptimo (postglacial climatic optimum, PCA). A fase recente é datada de AD 300–950 (1650–1000 anos do calendário BP), correspondendo a um período de temperaturas relativamente altas a nível global.

Place, publisher, year, edition, pages
Stockholm: Department of Physical Geography, Stockholm University , 2017. , p. 58
Series
Dissertations from the Department of Physical Geography, ISSN 1653-7211 ; 69
Keywords [en]
Southern Africa, Limpopo River floodplain, flooding event, sea-level change, climate change, diatoms, phytoliths, mineral magnetics, grain-size, radiocarbon dating
National Category
Earth and Related Environmental Sciences
Research subject
Quaternary Geology
Identifiers
URN: urn:nbn:se:su:diva-148465ISBN: 978-91-7797-024-8 (print)ISBN: 978-91-7797-025-5 (electronic)OAI: oai:DiVA.org:su-148465DiVA, id: diva2:1152747
Public defence
2017-12-14, Nordenskiöldsalen, Geovetenskapens hus, Svante Arrhenius väg 12, Stockholm, 13:00 (English)
Opponent
Supervisors
Projects
Climate and Environmental Research
Funder
Sida - Swedish International Development Cooperation Agency, 2011-002102Available from: 2017-11-21 Created: 2017-10-26 Last updated: 2022-02-28Bibliographically approved
List of papers
1. Paleo-environment and flooding of the Limpopo River-plain, Mozambique, between c. AD 1200-2000
Open this publication in new window or tab >>Paleo-environment and flooding of the Limpopo River-plain, Mozambique, between c. AD 1200-2000
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2015 (English)In: Catena (Cremlingen. Print), ISSN 0341-8162, E-ISSN 1872-6887, Vol. 126, p. 105-116Article in journal (Refereed) Published
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.

Keywords
Flooding events, Mineral magnetism, Limpopo River, Diatoms, Mozambique
National Category
Earth and Related Environmental Sciences
Research subject
Quaternary Geology
Identifiers
urn:nbn:se:su:diva-148426 (URN)10.1016/j.catena.2014.10.038 (DOI)000348956700014 ()
Available from: 2017-10-24 Created: 2017-10-24 Last updated: 2022-02-28Bibliographically approved
2. Late Holocene sea-level changes and paleoclimate recorded in Lake Lungué, southern Mozambique
Open this publication in new window or tab >>Late Holocene sea-level changes and paleoclimate recorded in Lake Lungué, southern Mozambique
2017 (English)In: Palaeogeography, Palaeoclimatology, Palaeoecology, ISSN 0031-0182, E-ISSN 1872-616X, Vol. 485, p. 305-315Article in journal (Refereed) Published
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.

Keywords
Limpopo River, Lake level changes, Human impact, Diatom, Mineral magnetic properties
National Category
Earth and Related Environmental Sciences
Research subject
Quaternary Geology
Identifiers
urn:nbn:se:su:diva-144400 (URN)10.1016/j.palaeo.2017.06.022 (DOI)000419747400023 ()
Available from: 2017-06-20 Created: 2017-06-20 Last updated: 2022-02-28Bibliographically approved
3. Vegetation dynamics within the savanna biome in southern Mozambique during the late Holocene
Open this publication in new window or tab >>Vegetation dynamics within the savanna biome in southern Mozambique during the late Holocene
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2018 (English)In: The Holocene, ISSN 0959-6836, E-ISSN 1477-0911, Vol. 28, no 2, p. 277-292Article in journal (Refereed) Published
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.

Keywords
diatoms, Miombo, Mozambique, palaeoclimate, palaeoecology, phytoliths
National Category
Earth and Related Environmental Sciences
Research subject
Quaternary Geology
Identifiers
urn:nbn:se:su:diva-145627 (URN)10.1177/0959683617721327 (DOI)000424957000009 ()
Available from: 2017-08-14 Created: 2017-08-14 Last updated: 2022-02-28Bibliographically approved
4. Coastal paleo-environment and sea-level change at Macassa Bay, southern Mozambique, since c. 6600 cal BP
Open this publication in new window or tab >>Coastal paleo-environment and sea-level change at Macassa Bay, southern Mozambique, since c. 6600 cal BP
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2012 (English)In: Quaternary International, ISSN 1040-6182, E-ISSN 1873-4553, Vol. 260, p. 153-163Article in journal (Refereed) Published
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.

National Category
Geology
Research subject
Quaternary Geology
Identifiers
urn:nbn:se:su:diva-148450 (URN)10.1016/j.quaint.2011.11.032 (DOI)000304722900014 ()
Available from: 2017-10-25 Created: 2017-10-25 Last updated: 2024-02-23Bibliographically approved
5. Water-level variations in Lake Nhauhache, Mozambique, during the last 2,300 years
Open this publication in new window or tab >>Water-level variations in Lake Nhauhache, Mozambique, during the last 2,300 years
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2012 (English)In: Journal of Paleolimnology, ISSN 0921-2728, E-ISSN 1573-0417, Vol. 48, no 2, p. 311-322Article in journal (Refereed) Published
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.

Keywords
Southern Africa, Lake-level changes, Paleohydrology, Paleoclimatology, Microfossils
National Category
Earth and Related Environmental Sciences
Research subject
Quaternary Geology
Identifiers
urn:nbn:se:su:diva-80595 (URN)10.1007/s10933-012-9596-3 (DOI)000305959000003 ()
Note

AuthorCount:8;

Available from: 2012-09-28 Created: 2012-09-25 Last updated: 2022-02-24Bibliographically approved

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