25th International Meeting on Organic Geochemistry IMOG 2011

P-489
Branched GDGTs as biomarkers for temperature
reconstructions from paleosols. Three case studies and
potential complications
Roland Zech 1 , Li Gao 2 , Rafael Tarozo 2 , Yongsong Huang 2
1 Geological Institute, ETH Zurich, Zurich, Switzerland, 2 Geological Institute, Brown University, Providence,
United States of America (corresponding author:godotz@gmx.de)
Branched Glycerol Dialkyl Glycerol Tetraethers
(GDGTs) are membrane lipids derived from yet
unknown soil bacteria. Empirical studies have shown
that the composition of branched GDGTs in topsoils
varies in their degree of cyclisation and methylation,
which is expressed as CBT and MBT (i.e. cyclisation
and methylation index of branched tetraethers)
depending on soil pH and mean annual air
temperature [1]. Can we use MBT and CBT to
reconstruct past pH and temperatures from loesspaleosols-sequences?
First applications on the
Chinese Loess Plateau seem to be promising [2,3].
Here we present results from three case studies –
three well-studied loess-paleosol sequences, in which
we measured branched GDGTs in order to empirically
test the applicability of the newly discovered
biomarkers for paleoenvironmental reconstruction and
to possibly obtain quantitative records for regional
paleotemperatures [4]. The results show that
significant disagreements exist between
interpretations based on available stratigraphic,
pedological and geochemical data on the one hand,
and GDGT-derived reconstructions on the other hand.
The case study from the ~150 ka loess section
‗Crvenka‘ in Serbia, for example, shows an almost
monotonous increase in GDGT-derived temperatures
from the bottom of the profile to the top (Fig. 1).
Reconstructed temperatures for the loess V L1L1 are
thus much higher than one would expect from its
correlation with Marine Isotope Stage (MIS) 2, while
the paleosol V S1 shows the lowest reconstructed
temperatures of the whole record, which seems to be
at odds with its correlation with the last interglacial
(MIS 5).
Another case study, the ~6.5 m deep and ~80 ka
paleosol sequence ‗Maundi‘ from Mt Kilimanjaro
shows GDGT-derived temperatures that are
monotonously increasing with depth by more than
10°C. This contradicts the fact that most of the
sediments below the uppermost Holocene soils were
deposited during MIS 2 to 4, i.e. under presumably
cold, last glacial conditions.
Given that very little is known about the GDGTproducing
organisms so far, we speculate that there
might be several reasons that are currently
complicating the straight-forward interpretation of the
respective indices: (i) Some of the target peaks in the
HPLC chromatograms consist of multiple, yet
unidentified compounds. Further research is
necessary to improve compound separation,
identification and, accordingly, calibration. (ii) GDGT
patterns may not only depend on soil temperature and
pH. Other potential factors, such as redox conditions,
nutrient availability, seasonality, changes in the
bacterial communities etc, need to be tested. (iii)
GDGT-producing bacteria do not only live in the
topsoils, and a non-negligible subsurface production
of GDGTs could thus result in an overprint of ‗older‘
paleo-environmental signals at depth, a concern that
could be coined the ―growth depth effect‖. (iv) Eolian
transport might contribute non-negligibly to the
amounts of GDGTs in some loess-paleosols. This
might overprint the in-situ signal of the proxies.
Fig. 1. The loess-paleosol-sequence Crvenka, in Serbia,
and results of the GDGT measurements.
References
[1] Weijers, J.W.H. et al. (2007), Geochim. Cosmochim.
Acta, 71, 703–713.
[2] Peterse, F. et al. (2011), Earth Plan. Sci. Lett., 301, 256-
264.
[3] Gao, L. et al. (2011), Palaeo3, submitted.
[4] Zech, R., Gao, L., Tarozo, R. and Huang, Y. (2011), Org.
Geochem., submitted.
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