| 1. |
- Baker, Andrea, et al.
(författare)
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Biomarker records of palaeoenvironmental variations in subtropical Southern Africa since the late Pleistocene : Evidences from a coastal peatland
- 2016
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Ingår i: Palaeogeography, Palaeoclimatology, Palaeoecology. - : Elsevier. - 0031-0182 .- 1872-616X. ; 451:1, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- Southern Africa's unique global position has given rise to a dynamic climate influenced by large sea surface temperature gradients and seasonal fluctuations in the Inter Tropical Convergence Zone. Due to the semi-arid climate of the region, terrestrial palaeorecords are rare and our understanding of the long-term sensitivity of Southern African terrestrial ecosystems to climatic drivers is ambiguous. A 810 cm continuous peat core was extracted from the Mfabeni peatland with a 14C basal age of c. 47 thousand years calibrated before present (kcal yr BP), positioning it as one of the oldest known sub-tropical coastal peatlands in Southern Africa. This peat core provides an opportunity to investigate palaeoenvironmental changes in subtropical Southern Africa since the late Pleistocene. Biomarker (n-alkane, n-alkanoic acid and n-alkanol) analysis, in conjunction with previously published bulk geochemical data, was employed to reconstruct organic matter (OM) sources, rates of OM remineralisation and peatland hydrology. Our results showed that the principal OM source into the peatland was emergent and terrestrial plants with exception of shallow lake conditions when submerged macrophytes dominated (c. 44.5–42.6, 29.7, 26.1–23.1, 16.7–7.1 and 2.2 kcal yr BP). n-Alkane proxies suggest that local plant assemblages were predominantly influenced by peatland hydrology. By incorporating temperature sensitive n-alkanoic acid and n-alkanol proxies, it was possible to disentangle the local temperature and precipitation changes. We report large variations in precipitation intensities, but subdued temperature fluctuations during the late Pleistocene. The Holocene period was characterised by overall elevated temperatures and precipitation compared to the preceding glacial period, interspersed with a millennial scale cooling event. A close link between the Mfabeni archive and adjacent Indian Ocean marine core records was observed, suggesting the regional ocean surface temperatures to be the dominant climate driver in this region since the late Pleistocene.
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| 2. |
- Baker, Andrea, et al.
(författare)
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Climatic variability in Mfabeni peatlands (South Africa) since the late Pleistocene
- 2017
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Ingår i: Quaternary Science Reviews. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0277-3791 .- 1873-457X. ; 160, s. 57-66
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Tidskriftsartikel (refereegranskat)abstract
- It has been postulated that a bipolar seesaw interhemispheric mechanism dominated the relationship between the Northern and Southern hemisphere climates since the late Pleistocene. A key test for this proposition would be to undertake palaeoenvironmental studies on terrestrial archives in climatically sensitive regions. Southern Africas contemporary C-3 and C-4 terrestrial plant distributions display a definitive geographical pattern dictated by different growing season rainfall and temperature zones; however, the region is generally archive poor due to its overall semi-arid climate and high relief topography. The Mfabeni peatland, with a basal age of c. 47 k yrs calibrated before present (kcal yr BP), is one of the oldest continuous coastal peat deposits in Southern Africa. Molecular leaf wax isotopes (delta C-13(wax)) were generated for a 810 cm long core, and combined with previously published bulk geochemical (delta C-13(bulk), %TOC), palynological, and stratigraphic data, to reconstruct the late Pleistocene and Holocene palaeoenvironments. We interpreted environmental shifts associated with the Heinrich 4, Last Glacial Maximum, deglacial and Holocene periods, which are consistent with adjacent Indian Ocean sea surface temperature records. However, the other shorter climate perturbations during the Heinrich 5, 3, 2, 1, Antarctic cold reversal and Younger Dryas, were muted, most likely due to local hydrological overprinting on the Mfabeni record. A general anti-phase sequence was observed between the Mfabeni record and better established Northern Hemisphere events, underpinning the bipolar seesaw interhemispheric mechanism proposed for global climate forcing since the Late Pleistocene. (C) 2017 Elsevier Ltd. All rights reserved.
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| 3. |
- Baker, Andrea, et al.
(författare)
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n-Alkan-2-one biomarkers as a proxy for palaeoclimate reconstruction in the Mfabeni fen, South Africa
- 2018
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Ingår i: Organic Geochemistry. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0146-6380 .- 1873-5290. ; 120, s. 75-85
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Tidskriftsartikel (refereegranskat)abstract
- The sub-tropical Mfabeni fen is the only continuous coastal peat deposit that documents glacial and interglacial palaeoenvironmental conditions since the late Pleistocene (ca. 47 cal kyr BP) in southern Africa. Published bulk geochemical, biomarker and leaf wax delta C-13 data, along with palynology and stratigraphic studies of the Mfabeni peat sequence, render it an ideal record for testing new palaeoreconstruction proxies. In this study, we aimed to establish the proxy potential of n-alkan-2-one (n-ket) compounds by tracing their source/origin and post-depositional diagenetic change, and if they preserve or not a robust palaeoenvironment signal that complements our understanding of palaeoclimatic variations. In the Mfabeni archive the most likely source for n-kets is via microbial decarboxylation of n + 1-alkanoic acids (n-FAs) and, to a lesser degree, oxidation of same chain length n-alkanes (n-alks). The n-ket average chain length (ACL(ket)) and n-C-23 and C(25)ket/precursor ratios displayed a statistical significant negative relationship with the n-alk aquatic plant proxy (P-aq), suggesting the source of n-kets to be submerged aquatic plants during waterlogged conditions that suppressed microbial activity during the ensuing anoxic conditions. Both the mid-chain and long chain n-ket/precursor ratios displayed predominant water level fluctuation controls, with temperature as a secondary regulator. By comparing the n-ket data with published environmental and climate reconstructions from the same core, and with geomorphology and palynological studies of the Mfabeni basin, we conclude that the n-kets show promise as a palaeoclimate proxy and can be used in conjunction with other biomarker proxies to reconstruct ancient hydrological changes in sub-tropical peatlands. (C) 2018 Elsevier Ltd. All rights reserved.
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| 4. |
- Das, Supriyo Kumar, et al.
(författare)
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Biomarker evidence of macrophyte and plankton community changes in Zeekoevlei, a shallow lake in South Africa
- 2009
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Ingår i: Journal of Paleolimnology. - : Springer Science and Business Media LLC. - 0921-2728 .- 1573-0417. ; 41:3, s. 507-521
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Tidskriftsartikel (refereegranskat)abstract
- Zeekoevlei is the largest freshwater lake in South Africa and has a century-long history of anthropogenic impact that caused hyper-eutrophic conditions. We used biomarkers (alkanes and pigments), stable isotopes (delta C-13 and delta N-15), rates of primary palaeoproduction and total inorganic carbon (TIC) accumulation rates in the lake sediments to investigate changes in plankton and macrophyte communities in response to anthropogenic activities in this shallow lake. Specific alkanes (I C-15 pound,C-17,C-19, pristane, phytane and n-C-29/n-C-17 ratio) and pigment (chlorophyll a, beta,beta-carotene, echinenone, fucoxanthin and zeaxanthin) concentrations in lake waters indicated the present-day hyper-eutrophic condition and seasonal fluctuations of cyanobacteria, zooplankton and diatom populations. Eutrophic conditions were initiated in the lake with the start of recreational activities and construction of a sewage treatment plant in the early 1920s. The lake transformed from a eutrophic to a hyper-eutrophic waterbody following damming, pondweed eradication and accelerated catchment-derived nutrient input. The change in lake trophic state was recorded by a sharp decline in the terrestrial to aquatic ratio (TAR) of specific n-alkanes, low carbon preference index (CPI) and increased delta C-13 values in the sediment core. In addition, the aquatic macrophyte n-alkane proxy (P-aq) values (similar to 1) indicated a slow takeover by floating macrophytes after the eradication of submerged pondweeds in 1951. Elevated n-alkane (I C-15 pound,C-17,C-19), total alkane and pigment (chlorophyll a, beta,beta-carotene, zeaxanthin and zeaxanthin to beta,beta-carotene ratio) concentrations, low delta N-15 values and low TIC accumulation rates in the upper middle section of the core indicated the beginning of intense cyanobacterial blooms after the dredging in 1983. Although the cyanobacterial population has decreased in recent years, hyper-eutrophic conditions are reflected by low CPI < 0.04 and TAR < 1 values at the top of the sediment core.
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| 5. |
- Das, Supriyo K., et al.
(författare)
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Elemental (C, N, H and P) and stable isotope (del15 N and del13C) signatures in sediments from Zeekoevlei, South Africa : a record of human intervention in the lake
- 2007
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Ingår i: Journal of Paleolimnology. - : Springer. - 0921-2728 .- 1573-0417. ; 39:3, s. 349-360
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Tidskriftsartikel (refereegranskat)abstract
- We used elemental carbon, nitrogen, phosphorus and hydrogen ratios (C/N, N/P and H/C) with total organic carbon (TOC) and total phosphorus (TP) as well as stable carbon and nitrogen isotopes (δ13C and δ15N) to investigate the source and depositional conditions of organic matter in sediments from Zeekoevlei, the largest freshwater lake in South Africa. Typical C/N (10–12), H/C ratios (≥1.7) and δ13Corganic values (−22 to −19‰) together with the increase in TOC concentration indicate elevated primary productivity in lower middle (18–22 cm) and top (0–8 cm) sections of the sediment cores. Seepage of nutrients from a nearby waste water treatment plant, rapid urbanization and heavily fertilized farming in the catchments are responsible for the increased productivity. Consistent with this, measured δ15Norganic values (∼11‰) indicate increased raw sewage input towards the top-section of the core. Although cyanobacterial blooms are evident from the low δ15N values (∼3‰) in mid-section of the core, they did not outnumber the phytoplankton population. Low N/P ratio (∼0) and high TP (100–2,200 mg l−1) support cyanobacterial growth under N limited condition, and insignificant input of macrophytes towards the organic matter pool. Dredging in 1983, caused sub-aerial exposure of the suspended and surface sediments, and affected organic matter preservation in the upper mid-section (12–14 cm) of the core.
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| 6. |
- Das, Supriyo Kumar, et al.
(författare)
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Elemental (C, N, H and P) and stable isotope (delta N-15 and delta C-13) signatures in sediments from Zeekoevlei, South Africa : a record of human intervention in the lake
- 2008
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Ingår i: Journal of Paleolimnology. - : Springer Science and Business Media LLC. - 0921-2728 .- 1573-0417. ; 39:3, s. 349-360
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Tidskriftsartikel (refereegranskat)abstract
- We used elemental carbon, nitrogen, phosphorus and hydrogen ratios (C/N, N/P and H/C) with total organic carbon (TOC) and total phosphorus (TP) as well as stable carbon and nitrogen isotopes (delta C-13 and delta N-15) to investigate the source and depositional conditions of organic matter in sediments from Zeekoevlei, the largest freshwater lake in South Africa. Typical C/N (10-12), H/C ratios (>= 1.7) and delta C-13(organic) values (-22 to -19 parts per thousand) together with the increase in TOC concentration indicate elevated primary productivity in lower middle (18-22 cm) and top (0-8 cm) sections of the sediment cores. Seepage of nutrients from a nearby waste water treatment plant, rapid urbanization and heavily fertilized farming in the catchments are responsible for the increased productivity. Consistent with this, measured delta(15) N-organic values (similar to 11) indicate increased raw sewage input towards the top-section of the core. Although cyanobacterial blooms are evident from the low delta N-15 values (similar to 3 parts per thousand) in mid-section of the core, they did not outnumber the phytoplankton population. Low N/P ratio (similar to 0) and high TP (100-2,200 mg l(-1)) support cyanobacterial growth under N limited condition, and insignificant input of macrophytes towards the organic matter pool. Dredging in 1983, caused sub-aerial exposure of the suspended and surface sediments, and affected organic matter preservation in the upper mid-section (12-14 cm) of the core.
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| 7. |
- Das, Supriyo Kumar, et al.
(författare)
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Major and trace element geochemistry in Zeekoevlei, South Africa : A lacustrine record of present and past processes
- 2008
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Ingår i: Applied Geochemistry. - : Elsevier BV. - 0883-2927 .- 1872-9134. ; 23:8, s. 2496-2511
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Tidskriftsartikel (refereegranskat)abstract
- This study reports a multi-parameter geochemcial investigation in water and sediments of a shallow hyper-eutrophic urban freshwater coastal lake, Zeekoevlei, in South Africa. Zeekoevlei receives it greater fraction of dissolved major and trace elements from natural Sources (e.g., chemical weathering and sea salt). Fertilizers, agricultural wastes, raw sewage effluents and road runoff in contrast, constitute the predominant anthropogenic sources, which supply As, Cd, Cu, Pb and Zn in this lake. The overall low dissolved metal load results from negligible industrial Pollution, high pH and elevated metal Uptake by phytoplankton. However, the surface sediments are highly polluted with Pb, Cd and Zn. Wind-induced sediment resuspension results in increased particulate and dissolved element concentrations in bottom waters. Low C/N ratio (10) indicates primarily an algal Source for the sedimentary organic matter. Variation in sedimentary organic C content with depth indicates a change in primary productivity in response to historical events (e.g., seepage from wastewater treatment plant, dredging and urbanization). Primary productivity controls the enrichment of most of the metals ill sediments, and elevated productivity with higher accumulation of planktonic debris (and siltation) results in increased element concentration in surface and deeper sediments. Aluminium, Fe and/or Mn oxy-hydroxides, clay minerals and calcareous sediments also play an important role in adsorbing metals in Zeekoevlei Sediments.
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| 8. |
- Das, Supriyo Kumar, et al.
(författare)
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Phosphorus dynamics in shallow eutrophic lakes : an example from Zeekoevlei, South Africa
- 2009
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Ingår i: Hydrobiologia. - : Springer Science and Business Media LLC. - 0018-8158 .- 1573-5117. ; 619, s. 55-66
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Tidskriftsartikel (refereegranskat)abstract
- Zeekoevlei is the largest freshwater lake in South Africa and has been suffering from hyper-eutrophic conditions since last few decades. We have used total P (TP), dissolved phosphate (PO4 (3-)), organic P (OP), calcium (Ca) and iron (Fe) bound P fractions to investigate the relevant physical, chemical and biological processes responsible for sedimentation and retention of P and to study phosphorus (P) dynamics in this shallow lake. In addition, redox proxies (V/Cr and Th/U ratios) are used to study the prevailing redox conditions in sediments. Adsorption by CaCO3 and planktonic assimilation of P are found to control P sedimentation in Zeekoevlei. Low concentration of the labile OP fraction in surface sediments restricts the release of P by bacterial remineralisation. Low molar Ca/P and Fe/P ratios indicate low P retention capacity of sediments, and P is most likely released by desorption from wind-induced resuspended sediments and mixing of pore water with the overlying water column.
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| 9. |
- Das, Supriyo Kumar, et al.
(författare)
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Sources and historic changes in polycyclic aromatic hydrocarbon input in a shallow lake, Zeekoevlei, South Africa
- 2008
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Ingår i: Organic Geochemistry. - : Elsevier. - 0146-6380 .- 1873-5290. ; 39:8, s. 1109-1112
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the source, seasonality and historical changes in polycyclic aromatic hydrocarbons (PAHs) in Zeekoevlei, a shallow freshwater lake in South Africa. PAH input follows historical changes in land use pattern and recreational activities in Zeekoevlei since the early 1990s. Because anthropogenic activity in the catchment is low, this has resulted in an overall low PAH concentration in the water column and sediments. PAHs of mainly petrogenic origin dominate.
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| 10. |
- Fietz, Susanne, et al.
(författare)
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Terrestrial temperature evolution of southern Africa during the late Pleistocene and Holocene: Evidence from the Mfabeni Peatland
- 2023
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Ingår i: Quaternary Science Reviews. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0277-3791 .- 1873-457X. ; 299
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Tidskriftsartikel (refereegranskat)abstract
- The scarcity of suitable high-resolution archives, such as ancient natural lakes, that span beyond the Holocene, hinders long-term late Quaternary temperature reconstructions in southern Africa. Here we target two cores from Mfabeni Peatland, one of the few long continuous terrestrial archives in South Africa that reaches into the Pleistocene, to generate a composite temperature record spanning the last similar to 43 kyr. The Mfabeni Peatland has previously been proven suitable for temperature and hydrological reconstructions based on pollen and geochemical proxies. Here we use branched glycerol dialkyl glycerol tetraethers (brGDGTs) preserved in the Mfabeni peatland to derive a new quantitative air temperature record for south-east Africa. Our temperature record generally follows global trends in temperature and atmospheric CO2 concentrations, but is decoupled at times. Annual air temperatures during Marine Isotope Stage (MIS) 3 were moderately high (c. 20.5 degrees C), but dropped by c. 5 degrees C during the Last Glacial Maximum, reaching a minimum at c.16-15 ka. Asynchronous with local insolation, this cooling may have resulted from reduced sea surface temperatures linked to a northward shift in the Southern Hemisphere westerly winds. Concurrent with the southward retreat of the westerlies, and increasing sea surface temperatures offshore, warming from minimum temperatures (c. 15.0 degrees C) to average Holocene temperatures (c. 20.0 degrees C) occurred across the deglaciation. This warming was briefly but prominently interrupted by a millennial-scale cooling event of c. 3 degrees C at c. 2.4 ka, concurrent with a sudden change in hydrological conditions. The average Holocene temperatures of c. 20.0 degrees C were similar to those reconstructed for MIS 3, but after the 2.4 ka cooling period, air temperatures in the Mfabeni peat recovered and steadily increased towards the present. In summary, our record demonstrates that land temperature in eastern South Africa is highly sensitive to global drivers as well as nearby sea surface temperatures. (C) 2022 Elsevier Ltd. All rights reserved.
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| 11. |
- Kumar Das, Supriyo, et al.
(författare)
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Biogeochemistry of shallow lake sediments: a case study from Verlorenvlei, South Africa
- 2015
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Ingår i: Current Science. - : INDIAN ACAD SCIENCES. - 0011-3891. ; 109:8, s. 1486-1491
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Tidskriftsartikel (refereegranskat)abstract
- Studying the biogeochemistry of shallow lake sediments, especially the source of sedimentary organic matter (OM), is challenging because of the low preservation of OM in shallow lake sediments. Here we report the source of sedimentary OM in a shallow freshwater lake, Verlorenvlei, in South Africa using a number of biogeochemical proxies. Elemental carbon and nitrogen ratio (C/N), and stable C and N isotopes (delta C-13 and delta N-15) indicate algal source of the sedimentary OM. Total organic and inorganic C, different phosphorus fractions, delta C-13 and delta N-15 values indicate repetitive presence of non-N-fixing cyanobacteria under moderate N-limited conditions. Cyanobacterial population in Verlorenvlei is likely influenced by the availability of dissolved inorganic C. Cyanobacterial proliferation in the lake has ceased with accelerated N input as recorded at the top of the core.
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| 12. |
- Kumar Das, Supriyo, et al.
(författare)
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Connecting pigment composition and dissolved trace elements to phytoplankton population in the southern Benguela Upwelling zone (St. Helena Bay)
- 2017
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Ingår i: Journal of Marine Systems. - : ELSEVIER SCIENCE BV. - 0924-7963 .- 1879-1573. ; 176, s. 13-23
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Tidskriftsartikel (refereegranskat)abstract
- Rich in upwelled nutrients, the Southern Benguela is one of the most productive ecosystems in the world ocean. However, despite its ecological significance the role of trace elements influencing phytoplankton population in the Southern Benguela Upwelling System (SBUS) has not been thoroughly investigated. Here, we report pigment composition, macronutrients (nitrate, phosphate and silicate) and concentrations of dissolved Cd, Co, Fe and Zn during late austral summer and winter seasons in 2004 to understand the relationship between the selected trace elements and phytoplankton biomass in St. Helena Bay (SHB), which falls within the southern boundary of the SBUS. Chlorophyll a concentrations indicate higher phytoplankton biomass associated with high primary production during late summer in SHB where high diatom population is inferred from the presence of fucoxanthin. Diminished phytoplankton biomass and a shift from diatoms to dinoflagellates as the dominant phytoplankton taxa are indicated by diagnostic pigments during late winter. Dissolved trace elements (Cd, Co and Zn) and macronutrients play a significant role in phytoplankton biomass, and their distribution is affected by biological uptake and export of trace elements. Continuous uptake of Zn by diatoms may cause an onset of Zn depletion leading to a period of extended diatom proliferation during late summer. Furthermore, the transition from diatom to dinoflagellate dominated phytoplankton population is most likely facilitated by depletion of trace elements (Cd and Co) in the water column.
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