25th International Meeting on Organic Geochemistry IMOG 2011

P-401
Hydrogen isotope ratios of leaf wax lipids illustrate temporal and
spatial variations in the hydrologic cycle – results from two
calibration studies off NW Africa
Britta Beckmann 1 , Eva Niedermeyer 2 , Alex Sessions 2 , Enno Schefuss 1
1 MARUM – Center for Marine Environmental Sciences - University of Bremen, Bremen, Germany,
2 California Institute of Technology - Division of Geological & Planetary Sciences, Pasadena, United States of
America (corresponding author:bbeckman@uni-bremen.de)
We worked on marine sediments deposited off NW
Africa to explore how continental (paleo)-hydrologic
conditions preserved in the hydrogen isotopic
composition (δD, in ‰ relative to VSMOW) of
terrestrial plant waxes are reflected in marine
sediments. In the first study, δD values of long-chain
n-alkanes extracted from surface sediments along a
transect paralleling the arid sub-Saharan to the moist
subhumid Sudanian climate zones on land were
compared to the isotopic composition of precipitation
over the continent. To address temporal variations on
short time-scales, we examined in a second study the
δD values in n-alkanes from a short sediment core
(GeoB9501-4) covering the past 100 years (Mulitza et
al., 2010) and compared them with instrumental data
before, during and after the Sahel drought.
n-alkane distributions and concentrations in surface
sediments indicate considerable contribution of
terrestrial organic matter at all sites. Values of the
carbon preference index (CPI, n-C25 to n-C33) vary
between 3.4 and 4.6. Distribution patterns of n-C29
and n-C31 alkanes illustrate the dominance of grasses
in the northern part of the investigated region. Shrubs
and trees appear to prevail in the southern part of the
study area as indicated by a dominance of n-C29
alkanes (e.g., Rommerskirchen et al., 2006; Vogts et
al., 2009), which is in accordance with the vegetation
distribution on the continent. δD values of the n-C31
alkanes vary between -125‰ and -150‰ in the
surface samples. In general, δD values are higher in
n-alkanes from the northern part of the study area (~
21°N) and decrease southward. Lowest δD values
were measured in samples from the southernmost
part of the study area around 13°N. The observed
spatial distribution of δD values from n-C31 alkanes
resembles the distribution of growing season
precipitation δD on the continent (Bowen et al., 2005)
which shows highest values along the coast between
21° and 28°N and lowest values between 9° and
14°N.
Information on short time-scales come from sediment
core GeoB9501-4 taken north of the Senegal rivermouth
and sampled in high temporal resolution (each
sample covering approx. 4 years). Here, δD values of
the n-C31 alkane (which was the most cleanly
separated compound of adequate abundance) reveal
temporal variations that can be linked to hydrologic
conditions over the western Sahel as captured by the
instrumental record. Among other features, the onset
and persistence of the Sahel drought in the late 1960s
to the mid 1990s can be traced in a positive 12‰ shift
of the δD signal. In a further step, this shift will be
related to instrumental data of continental runoff and
average precipitation amounts (annual and growing
season) in NW African sub-Saharan countries.
References
[1] Bowen, G.J. et al. (2005) Oecologia. 143, 337-
348.
[2] Mulitza, S. et al. (2010) Nature 466, 226-228.
[3] Rommerskirchen, F. et al. (2006) Org. Geochem.
37, 1303-1332.
[4] Vogts, A. et al. (2009) Org. Geochem. 40, 1037-
1054.
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