25th International Meeting on Organic Geochemistry IMOG 2011

P-447
The vertical niche of Thaumarchaota in Lake Malawi;
implications for the TEX86 temperature signal in the sediment
Martijn Woltering 1 , Josef Werne 2 , Melissa Berke 1 , Ellen Hopmans 3 , Jaap Sinninghe
Damsté 3,4 , Stefan Schouten 3,4
1 University of Minnesota, Duluth, United States of America, 2 University of Minnesota Duluth, Duluth, United
States of America, 3 Royal Netherlands Institute for Sea Research, Texel, Netherlands, 4 Utrecht University,
Utrecht, Netherlands (corresponding author:wolte082@umn.edu)
Previous studies applied the TEX86 paleo temperature
proxy to sediment archives from Lake Malawi to
produce records of past temperature variability that
spanned the Holocene to middle Pleistocene time
periods (Powers et al., 2005 Science; Powers et al.,
p 3 in press ;Woltering et al., p 3 in press). Based on the
observation that core top sample yielded
temperatures in the range of annual mean lake
surface temperatures (LST) and that trends of TEX86
values down core appear to capture well documented
global climate events, these previous studies
interpreted TEX86 as reflecting annual mean surface
water temperature. However, there is no information
on the ecology of the Group 1 Crenarchaeota (now
named Thaumarchaeota) in the water column of Lake
Malawi to test the hypothesis that TEX86 values from
the sediments represent an annual mean LST. Here
we present data from a study of suspended
particulate matter (SPM) in the water column of the
north, central and south basins of Lake Malawi during
the austral summer (wet season) that investigated the
vertical niche of Thaumarchaeotal production. SPM
was extracted and analysed to produce vertical
profiles of core and specific intact Thaumarchaeotal
glycerol dialkyl glycerol tetraether (GDGT) membrane
lipid abundances. The intact crenarchaeol-hexosephosphohexose
lipid (HPH crenarchaeol) has low
abundances in the warm surface water layer and a
maximum in abundance at 50m depth, beneath the
chlorophyll maximum at the base of the thermocline,
and declining abundance with depth. Profiles of other
intact crenarchaeol lipids: crenarchaeol di-hexose,
crenarchaeol hexose and crenarchaeol hexose ‗180‘
differ from the profile of HPH crenarchaeol with
maxima at greater depths suggesting that these lipids
may be degradation products of HPH crenarchaeol
and crenarchaeol di-hexose and thus may not be
ideal markers for living Thaumarchaeota (cf.
Schouten et al., 2010).
TEX86 derived temperatures from SPM from the
epilimnion are significantly lower than the actual
surface water temperature. At 50m depth there is
good agreement between TEX86 temperature and the
in situ water temperature. Between 50-150m there is
a distinct trend (5-6 o C) of increasing TEX86
temperature with increasing depth, while between
depths of 150-300 TEX86 temperatures show a similar
decrease in TEX86 temperature to 300m, where
values were close to those observed at 50m depth.
This pattern of increase and subsequent decrease of
TEX86 temperatures may be caused by a potential
difference in degradation rate as well as in situ
production of different intact GDGTs in the water
column yielding different ratios of core GDGTs, thus
affecting the TEX86. Our observation that at the time
of sampling Thaumarchaeota predominantly live at
~50m water depth would mean that the TEX86 signal
produced does not capture the epilimnetic water
temperatures during this warmest part of the year,
and therefore the TEX86 signal in the sediments may
underestimate the temperature relative to the annual
mean LST. However, our sampling took place during
the austral summer which is characterized by a
shallow thermocline due to diminished wind activity
over the lake during this season. The dry season,
however, is characterized by windy conditions with
strong winds coming from the north, which can mix
surface waters down to 200m. The deepening of the
thermocline during the windy season likely results in
that isoprenoid GDGTs at this time are being
produced in the epilimnion and are therefore could
reflect surface water temperature. This would explain
why the TEX86 derived temperature from a core top
sediment from the study site yields temperatures that
are lower than the summer surface water
temperature, but higher that the hypolimnetic water
temperature. This may also explain why Powers et al.
(p 3 in press) observed that although absolute TEX86
derived temperatures from a shallow sediment core
from Lake Malawi agree well with instrumental
summer water temperatures, the TEX86 trend down
core corresponds more with the trend of the winter
surface water instrumental record.
An investigation of sinking particles over an annual
cycle for TEX86 in Lake Malawi may provide a more
definitive insight on actual temperature reflected by
the TEX86 proxy in the sediments of Lake Malawi.
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