25th International Meeting on Organic Geochemistry IMOG 2011

P-495
Testing branched GDGT-derived proxy parameters in the Arctic
Ocean
Gesine Mollenhauer 1,2 , Konstanze Schipper 2 , Kirsten Fahl 1 , Rüdigel Stein 1
1 Alfred-Wegener-Institute, Bremerhaven, Germany, 2 University of Bremen, Bremen, Germany
(corresponding author:gesine.mollenhauer@awi.de)
The Arctic land masses are covered by vast areas of
permafrost soils, in which high amounts of fossil
organic matter are freeze-locked accounting for up to
50% of the estimated global soil carbon pool
(Tarnocai et al., 2007). Climate warming is expected
to affect the Arctic region unproportionally strongly
resulting in mobilization and transport of this fossil
organic matter to the ocean, where it becomes bioavailable
and can be respired to the atmosphere.
Therefore, quantifying export of soil organic carbon
and characterizing the conditions in the source region
is of great interest.
Several organic-geochemical proxy parameters to
quantify terrigenous organic matter input to ocean
sediments have been used, including bulk
measurements like C/N ratios and � 13 C values as well
as more specific biomarker based proxies (e.g.,
abundance of leaf-wax derived lipids or sterols
specific for terrestrial plants). In the past decade, the
BIT index proxy based on soil-derived branched
GDGTs has been suggested to provide an estimate of
soil organic matter input (Hopmans et al., 2004).
Furthermore, two proxy indices based on abundance
ratios of the same compounds have been suggested
to provide estimates of soil-pH and mean air
temperature (MAT) in the source regions of the soilderived
particles (Weijers et al., 2007). These proxy
parameters have been successfully applied in
temperate and tropical regions.
We analyzed the GDGT-based indices BIT, MBT, and
CBT in core-top sediments collected along four
transects reaching from the mouths of the Russian
Arctic Rivers Ob, Yenisei, Olenek, and Lena, to the
open ocean. The BIT index values are compared with
existing and published bulk parameters and terrestrial
biomarker proxy data (Fahl & Stein, 2007). Generally,
a good agreement is observed between these
indicators for terrestrial organic matter contribution.
Using the MBT index to derive estimates of MAT in
the source-regions of the branched GDGTs yields
temperatures around the freezing point in samples
collected near the river mouths with high terrestrial
organic matter contribution, while values at the more
offshore positions are significantly higher, suggesting
that either the proxy parameter is not reliable when
BIT index values are lower than 0.5, or that
sedimentological conditions favor deposition of
material derived from more inland regions at the
offshore sites. TEX86 estimates for SST are
consistently too high for the Arctic Ocean, including
the offshore sites where BIT index values are low.
Our data show that branched GDGT-derived proxy
parameters hold strong potential for reconstructing
export of permafrost soil organic matter and, to some
extent, of environmental conditions prevailing in the
source regions.
BIT index
1
0.8
0.6
0.4
TOC (%)
4
3
2
1
0
0 200 400 600
Distance from mouth of Yenisei (km)
400
300
200
100
0
concentration (�g/g)
long-chain n-alkanes
�-sitosterol
Figure 1: Proxy data from of the core-top transect off
the Yenisei River. Note inverted scale for MAT
estimates.
Fahl & Stein, 2007. Geo-Marine Letters 27, 13-25.
Hopmans, E.C., et al., 2004. Earth and Planetary Science Letters 224,
107-116.
Tarnocai, C., et al., 2009. Global Biogeochem. Cycles 23, GB2023.
Weijers, J.W.H., et al., 2007. Geochimica et Cosmochimica Acta 71,
703.
-4
-2
0
2
4
6
8
MAT estimate (°C)
617