25th International Meeting on Organic Geochemistry IMOG 2011

P-462
Branched tetraether lipid derived air temperature and soil pH in
lake sediments from Zeekoevlei, South Africa
Supriyo Das 1,2 , James Bendle 2 , Joyanto Routh 1,3
1 MTM, Örebro University, Örebro, Sweden, 2 Department of Geographical and Earth Sciences, Glasgow
University, Glasgow, United Kingdom, 3 Dept. of Earth Sciences, IISER-Kolkata, Mohanpur, India
(corresponding author:supriyo.das@oru.se)
Here, we apply branched glycerol dialkyl glycerol
tetraethers (br-GDGTs) based methylation index of
branched tetraethers/cyclisation ratio of branched
tetraethers (MBT/CBT) proxy to reconstruct decadal
scale changes in terrestrial temperature from an
urban shallow (mean depth < 5m) oxic lake sediment.
We estimate the mean air temperature (MAT) and
catchment soil pH from 1946 to 2004 in a sediment
core collected from Zeekoevlei, the largest freshwater
lake in South Africa, which is located 20 km south of
Cape Town. The lake falls in the sub-tropical
Mediterranean winter rainfall climate zone, and is a
warm-water lake. The annual lake surface
temperature ranges between 10.3°C and 28.5°C. We
compare the reconstructed br-GDGT derived MAT
(TMBT) with the historical air temperature data from
1946 to 2004, which is available in Global Historical
Climatology Network (GHCN-monthly) Version 2
database (www.ncdc.noaa.gov/oa/climate/ghcnmonthly/index.php).
The monthly temperature data
were recorded at the World Meteorological
Organization (WMO) station in a United States of
America Navy site in Cape Town. We have used a
yearly mean of the GHCN-monthly temperature
record in this abstract.
TMBT closely follows the mean annual GHCN-monthly
temperature record (TGHCN; Fig. 1). However, the TMBT
values are ~2ºC warmer than TGHCN. This indicates
that br-GDGT bacterial production is biased towards
the summer, and yields temperature estimates that
fall between summer (Austral summer) and TGHCN.
Our finding lies in line with similar findings in
Greenland 1 that suggests bias of br-GDGT derived
MBT/CBT proxy towards warmer temperature. This
finding may also indicate an in-situ production of br-
GDGTs 2,3,4 .
We infer the variation in TMBT (Fig. 1) in Zeekoevlei
with regard to El Niño events and post-apartheid rapid
urbanization. We also find a reflection of
socioeconomic changes in the lake‘s catchment in the
reconstructed soil pH. Our results provide an example
of the application of the MBT/CBT-MAT proxy to lake
sediments in catchment settings impacted by
agricultural and urban pollution.
Fig. 1 Variation in GHCN-monthly summer, winter and
yearly mean temperature (TGHCN), TMBT and CBT
derived catchment soil pH from 1946 to 2004. Bold
line in TGHCN represents smoothening of data in
CBT
Soil pH
±1 σ
9.5
8.5
1
GHCN-Monthly
Summer temperature
T MBT in Zeekoevlei
±1 ζ
T TGHCN GHCN
GHCN-Monthly
Winter temperature
SigmaPlot 8.02
References:
8
1946 1954 1962 1970 1978 1986 1994 2002
Year
1 Schouten, S. et al. Geology 36, 147-150
(2008).
2 Peterse, F. et al. Organic Geochemistry 40,
692-699 (2009).
3 Tierney, J. E. et al. in AGU 2010 Fall
<strong>Meeting</strong> (2010).
4 Buckles, L. K. et al. in AGU 2010 fall <strong>Meeting</strong>
(2010).
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Air Air Temperature Temperature (ºC) (ºC)
587