25th International Meeting on Organic Geochemistry IMOG 2011

P-465
Hydrological and biogeochemical variations in the hypersaline
Lake Tswaing (South Africa) during the last 84 ka BP
Frauke Schmidt, Hedi Oberhänsli, Heinz Wilkes
Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam, Germany
(corresponding author:frauke.schmidt@uni-bremen.de)
Organic matter in lacustrine sediments of endorheic
lakes has generally a great potential of recording
changes in the regional climate and the environmental
conditions during deposition. Lake Tswaing is a small
impact crater lake in the NE of South Africa; its
sediments represent the longest continuous record of
environmental change on the southern African
subcontinent. The present regional climate is
characterized by an unbalanced hydrology with
elevated lake water evaporation resulting in a highly
alkaline, saline and anoxic lake body which is
primarily inhabited by cyanobacteria and bacteria. A
recent study investigated the modern carbon cycle in
Lake Tswaing via stable carbon isotope analyses of
total organic carbon (TOC) and lipid biomarkers [1].
Here, we analysed lipid biomarkers and their stable
carbon and hydrogen isotopic signatures in a 35 m
long sediment core for a comprehensive
understanding of the climatic influence on the lake
evolution and for insights into the adaption of the lake
biocoenosis to the extreme and changing
environmental conditions during the last 84 ka BP.
The hydrogen isotopic composition of higher plantderived
n-alkanes (δDwax) varied strongly between
−155 and −106‰ indicating highly variable
hydrological conditions which partly controlled TOC
content in the sediments. Periods with low TOC
contents corresponded to heavy δDwax values, i.e., dry
climate conditions which lowered the allochthonous
input and the productivity in the lake. On the other
hand, high amounts of fresh organic matter from land
plants and aquatic organisms were deposited under
humid climate conditions resulting in sedimentary
TOC contents of up to 9.8%. Low values of the higher
plant n-alkane index (HPA [2]) suggested that the
increased rainfall during these periods reduced the
vertical salinity gradient resulting in water column
mixing and oxygenation of the bottom water. The
biocoenosis in the lake consisted primarily of algae
and bacteria. It showed strong variations in the
abundance of the different organisms and a temporal
succession in response to the changing nutrient
conditions in the lake during specific intervals. For
example, dinoflagellates were most prominent in the
time before 66 ka BP followed by a period with higher
bacterial activity (Fig.1). δ 13 C of dinosterol varied
strongly and showed partly very heavy values of up to
−0.5‰ pointing to a strong limitation in dissolved CO2
during times of high aquatic productivity and/or high
lake alkalinity. The occurrence of 4,4-dimethylcholest-
8(14)-enol, a specific marker for methanotrophic
bacteria, and light δ 13 C values of bacteria-derived
moretene provided evidence for methanotrophic
activity in certain time intervals.
Fig. 1. TOC content, lipid biomarker concentrations
and their δ 13 C values in the time from 71 to 57 ka BP.
13 C-depleted values for 4,4-dimethylcholest-8(14)enol
and moretene indicate methanotrophic activity in
Lake Tswaing.
References:
[1] Kristen, I., Wilkes, H., Vieth, A., Zink, K.-G.,
Plessen, B., Thorpe, J., Partridge, T., Oberhänsli, H.,
2010. J. Paleolimnol. 44, 143 – 160.
[2] Poynter, J., Eglinton, G., 1990. Proc. ODP Sci.
Results 116, 155 – 161.
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