25th International Meeting on Organic Geochemistry IMOG 2011

P-482
Meter-scale oxygen gradients and selective organic matter
degradation: implications for proxy interpretation
Kara Bogus 1 , Karin Zonneveld 1,2 , David Fischer 2 , Sabine Kasten 3 , Gerard Versteegh 2
1 Universität Bremen, Bremen, Germany, 2 Marum — Center For Marine Environmental Sciences, Bremen,
Germany, 3 Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
(corresponding author:ka_bo@uni-bremen.de)
Organic matter (OM) degradation processes
selectively preserve the more recalcitrant fractions in
sediments. For environmental reconstructions through
the application of OM-based proxies, this degradation
bias has to be accounted for. The oxidation state of
the sediments is an important variable influencing OM
degradation whereby low oxygen levels result in much
less degradation. This is demonstrated by the
elevated carbon contents in black shales, sediments
from oxygen minimum zones (OMZ) and anoxic
basins as well as at oxidation fronts like the Madeira
Abyssal Plain and Eastern Mediterranean sapropels.
Most studies investigating the impact of the oxidation
state on OM degradation do not involve samples with
identical initial OM composition since the samples
compared have been deposited at different locations
or differ in age. Others compare pre-aged material,
thus missing the early and most rapid degradation
process. This degree of uncertainty makes it difficult
to separate initial environmentally-induced changes
from those induced by selective aerobic degradation.
In order to constrain these problems and evaluate the
extent of early selective aerobic degradation on
proxies in surface sediments, we restricted sampling
distance to a meter-scale oxygen gradient associated
with a cold seep in the northeastern Arabian Sea.
Samples taken along a similar seep within the OMZ
and on a larger transect through the OMZ served as
controls. Here, we report on the impact of selective
oxic degradation at the sediment-water interface on
the relative lipid and palynomorph proxy composition.
Biomarker-based proxies used to represent sediment
alteration, including the diol/keto-ol (DOXI) ratio and
alcohol preservation index (API), are predicated on
selective degradation of the more labile components.
They indicated increased sediment alteration with
increasing oxygen concentrations at the sedimentwater
interface. Additionally, proxies used to
reconstruct productivity, such as Brigantedinium spp.
dinoflagellate cysts, phytol, cholesterol, and dinosterol
were highly labile in the oxygen-rich surface
sediments. However, the development of indices
based on the abovementioned biomarkers as well as
TEX86, and their degradation products exhibited a
more complex picture. It appears that for the
productivity proxies, factors additional to the oxygen
concentration, most likely sorptive protection of the
molecular proxies, to some extent protect these labile
components from early diagenesis. In contrast, the
seep environment and not the oxygen state of the
sediments influence the TEX86. Overall, the
differences between OM composition in relation to
oxygen availability on very small spatial scales clearly
demonstrate the importance of oxygen in the
degradation of OM. Therefore, care must be taken
when using these sensitive proxies for oceanographic
reconstructions in areas where changing redox
conditions at the sediment-water interface could be
expected.
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