25th International Meeting on Organic Geochemistry IMOG 2011

P-225
The change of land plant derived n-alkane characteristics in
ocean margin sediments in relation to the distance from the
continent
Tanja Badewien, Jürgen Rullkötter
Institute of Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University of
Oldenburg, P.O. Box 2503, D-26111 Oldenburg, Germany
(corresponding author: t.badewien@icbm.de)
The ocean is an important sink for terrigenous organic
matter transported off shore by wind and rivers. The
composition of the land plant derived organic material
varies due to changing environmental conditions on
the continent. Thus, marine sediments preserve a
record of continental vegetation and climate for
instance by the incorporation of land plant derived
biomarkers (e.g. n-alkanes). But it was postulated that
the biomarker characteristics may vary as a function
of the distance to the continent [1]. This possibly
obscuring effect on continental climate assessment
from sediment analyses is in the focus of this study.
The seven surface sediment samples investigated
were recovered off southwest Africa (approximately
26°S) from water depths between 100 and 4700 m
during RV Meteor cruise M76/1. The chain lengths
and isotopic compositions of long chain n-alkanes
were analyzed. In the course of the transect the
average chain length (ACL) of the odd carbon
numbered n-C27 to n-C33 alkanes is fairly uniform for
water depths from 100 to 3000 m but decreases
toward greater depths (Fig. 1). With ACL27-33
endmember data for C3 and C4 plants [2, 3] the
contribution by C4 plants was estimated. The results
indicate that the abundance of C4 plants decreases
from an exclusive C4 signal to 77% with increasing
water depth. These results are substantiated by
decreasing δ 13 C values of the n-alkanes.
The samples are located off a C4 plant dominated
area on the adjacent continent (Fig. 1). From the
continent the plant material is transported into the
ocean by roughly latitudinal winds during austral
winter. However, not only short-range but also longrange
eolian contributions have to be taken into
account. A higher proportion of C3 material at greater
water depths may be caused by a higher significance
of long-range transport from the hinterland with higher
C3 plant abundance. Thus, the differences in the
observed contributions by C4 plants are possibly due
to different catchment areas.
Fig. 1: Simplified present-day land cover map of
southwest Africa (based on [4]) with sampling
locations (rectangles), ACL27-33 of n-alkanes and the
calculated contribution by C4 plants to the sediment.
Arrows indicate generalised wind directions according
to [5].
However, the main statement that the plant material in
the sediments is derived from a C4 dominated area
remains valid whatever sample is considered.
References
[1] Rommerskirchen et al., 2003, Geochem. Geophys.
Geosyst. 4, 1101.
[2] Vogts et al., 2009, Org. Geochem. 40, 1037-1054.
[3] Rommerskirchen et al., 2006, Org. Geochem. 37,
1303-1332.
[4] Mayaux et al., 2004, J. Biogeogr. 31, 861-877.
[5] Dupont, L.M., Wyputta, U., 2003, Quatern. Sci.
Rev. 22, 157-174.
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