25th International Meeting on Organic Geochemistry IMOG 2011

P-483
Selective aerobic and anaerobic degradation of lipids and
palynomorphs in the Eastern Mediterranean since the onset of
sapropel S1 deposition
Gerard J.M. Versteegh 1 , Karin A.F. Zonneveld 1 , Gert J. de Lange 2
1 MARUM Bremen University, Bremen, Germany, 2 Faculty of Earth Sciences, Utrecht University, Utrecht,
Netherlands (corresponding author:versteegh@uni-bremen.de)
Selective degradation of organic matter (OM) in
sediments is important for reconstructing past
environments and understanding the carbon cycle.
We present changes between and within lipid classes
and kerogen types (as palynomorph groups) in
relation to the OM flux to the sea floor and oxidation
state of the sediments since the early Holocene for
Eastern Mediterranean site ABC26. This includes the
initially oxic but now anoxic presapropel, the still
unoxidised lower part of the organic rich S1 sapropel,
its postdepositionally oxidised and now organic-poor
upper part as well as the overlying postsapropelic
sediments, which have always been oxic. A general
~2.3 times increase in terrestrial and marine input
during sapropel formation is estimated on the basis of
the total organic carbon (TOC), pollen, spore,
dinoflagellate cyst, n-alkane, n-alkanol and n-alkanoic
acid concentration changes in the unoxidised part of
the sapropel (Fig. 1). The long-chain alkenones, 1,15
diols and keto-ols, loliolides and sterols indicate that
some plankton groups, notably dinoflagellates, may
have increased much more. Apart from the terrestrial
and surface water contributions to the sedimentary
OM, anomalous distributions and preservation of
some C23–C27 alkanes, alkanols and alkanoic acids
have been observed, which we consider as a
contribution by organisms living in situ.
Comparison of the unoxidised S1 sapropel with the
overlying oxidised sapropel and the organic matter
concentration profiles in the oxidised postsapropelic
sediments demonstrate strong and highly selective
aerobic degradation of lipids and palynomorphs.
There seems to be a fundamental difference in
degradation kinetics between lipids and pollen which
may be possibly related with the absence of sorptive
preservation as a protective mechanism for palynomorph
degradation. The n-alkanes, Impagidinium,
and Nematosphaeropsis are clearly more resistant
than TOC. The n-alkanols and n-carboxylic acids are
about equally resistant whereas the pollen, all other
dinoflagellate cysts and other lipids appear to degrade
considerably faster, which questions the practice of
normalising to TOC without taking diagenesis into
account. Selective degradation also modifies the
relative distributions within lipid classes, whereby the
longer-chain alkanes, alcohols and fatty acids
disappear faster than their shorter-chain equivalents.
Accordingly, interpretation of lipid and palynomorph
assemblages in terms of pre- or syndepositional
environmental change should be done carefully when
proper knowledge of the postdepositional
preservation history is absent. Two lipid-based
preservation proxies are tested the diol-keto-ol
oxidation index based on the 1,15C30 diol and keto
ols and the alcohol preservation index (API) whereby
the former seems to be the most promising.
Fig. 1 Concentration changes over the oxidation front
and within the visible sapropel. Each open circle
represents a lipid or palynomorph taxon. Closed
circles represent group averages OS, oxidised
sapropel; VS, visible sapropel.
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