25th International Meeting on Organic Geochemistry IMOG 2011

P-163
Fluxes and distributions of core and intact tetraether membrane
lipids in the water column of Lake Challa, East Africa
Laura Buckles 1 , Johan Weijers 1 , Gert-Jan Reichart 1 , Dirk Verschuren 2 , Jaap Sinninghe
Damsté 1,3
1 Utrecht University, Utrecht, Netherlands, 2 Ghent University, Ghent, Belgium, 3 NIOZ Royal Netherlands
Institute for Sea Research, ’t Horntje, Texel, Netherlands (corresponding author:l.buckles@geo.uu.nl)
The methylation index of branched tetraethers/
cyclisation ratio of branched tetraethers (MBT/CBT)
palaeotemperature proxy measures the distribution of
branched glycerol dialkyl glycerol tetraether bacterial
membrane lipids (brGDGTs) to generate a
quantitative mean annual air temperature (MAAT)
estimate [1]. This proxy has been applied mostly to
marine sediment records that contain a large
proportion of terrestrial organic matter, for example
near river outflows. The prospective use of the
MBT/CBT in lake sediment archives would vastly
extend the proxy‘s area of application; however, some
recent studies have discovered a mismatch between
the distributions of these brGDGTs in catchment area
soils versus those found in lake sediments. These
suggest that brGDGTs may be produced in-situ in
lakes, with obvious implications for MBT/CBT
application [2,3]. In order to investigate the potential
and limitations of the MBT/CBT proxy in lakes, it is
necessary to look at modern fluxes of GDGTs in lake
systems to resolve the exact sources and distributions
of these compounds.
The bacterial source of brGDGTs remains unknown,
making it difficult to use microbiological techniques to
trace their sources within the lake and its catchment
area. Using a novel separation method, GDGTs can
be split into intact polar tetraether membrane lipids
(IPLs) and core tetraether membrane lipids (CLs) [4].
IPLs are commonly believed to degrade rapidly upon
cell lysis when the labile polar head group is
hydrolysed, thereby converting the ‗living‘ IPLs to the
more stable ‗fossil‘ CLs [5]. This makes it possible to
use IPLs as a tracer for living, GDGT-synthesising
bacteria.
This study concentrates on Lake Challa, a
permanently stratified crater lake in equatorial East
Africa. Thirty-three contiguous monthly sediment trap
samples (December ‗07 to August ‗10) from 35
metres depth, as well as lake surface sediments and
catchment soils, were analysed for both brGDGT IPLs
and CLs. Increased fluxes of CL and IPL brGDGTs in
the water column do not correspond to precipitation
events, while brGDGT distributions in sediment trap
material differ from those of the surrounding soils
(Fig.), arguing against these soils as the dominant
source. The African lake calibration of the MBT/CBT
proxy was applied to each monthly collection of the
sediment trap material [3], yielding estimated MAATs
that show a strong seasonal signal. This signal is
offset by 5-6 months against measured mean monthly
air temperatures. The amplitude of variation in
estimated MAAT over the season is also much larger
than the measured temperature range. Not only do
brGDGT distributions differ between catchment soils,
sedimenting matter and surface sediments (Fig.), but
catchment soils are also characterised by a much
lower proportion of IPLs (compared to CLs) than
either sedimenting matter or sediments. This sum of
evidence shows significant in-situ production of
brGDGTs occurring in the water column of Lake
Challa.
Figure: Plot of MBT vs. degree of cyclisation of
brGDGT CLs in and around Lake Challa.
This includes lake surface sediments (0-1cm
depth), catchment soils and sediment traps.
[1] Weijers et al. (2007) Geochim. Cosmochim.
Acta, 71, 703-713.
[2] Sinninghe Damsté et al. (2009) Geochim.
Cosmochim. Acta, 73, 4232-4249.
[3] Tierney et al. (2010) Geochim. Cosmochim.
Acta, 74, 4902-4918.
[4] Pitcher et al. (2009), Org. Geochem., 40, 12-19.
[5] Lipp et al. (2008) Nature, 454, 991-994.
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