25th International Meeting on Organic Geochemistry IMOG 2011

O-34
Hydrogen isotopic composition of long chain n-alkanes from a
marine sediment core transect off Africa: implications for the
tropical African rainbelt
James Collins, Enno Schefuß, Stefan Mulitza, Matthias Prange, Gerold Wefer
MARUM, Bremen, Germany (corresponding author:jcollins@marum.de)
The tropical African rainbelt, which oscillates on a
seasonal basis, forms an important component of the
climate system. However, millennial-timescale
changes in the distribution of rainfall are poorly
constrained, owing to the relative scarcity or poor
quality of terrestrial proxy data from this vast
continent.
Isotopic analyses of terrestrial lipid biomarkers are
proving to be useful tools for palaeoclimate
reconstructions. In particular, the hydrogen isotopic
composition of long-chain n-alkanes from marine
sediment cores has been used to infer changes in the
hydrological cycle (Schefuß et al, 2005, Niedermeyer
et al, 2010). More negative δD values are interpreted
to reflect more rainfall (‗amount effect‘) and/or
reduced evapotranspiration from plant leaves or soils.
However, the exact processes that are recorded by
hydrogen isotopic composition of sedimentary nalkanes
are not yet well constrained. For example,
precipitation δD may also be affected by changes in
moisture source area or extent of continental
recycling whilst any evapotranspirational enrichment
processes in the plant may be modulated by
vegetation type or photosynthetic pathway.
We have analysed the hydrogen isotopic composition
of n-alkanes from 9 marine sediment cores off
western Africa, covering the full range of the tropical
rainbelt. Samples were taken from the Last Glacial
Maximum (LGM; 19-23ka), Heinrich Stadial 1 (HS1;
16-19ka) and the mid-Holocene (6-8ka) and
compared to the late Holocene (last 2000yrs).
For the late Holocene (present day) timeslice, δD
values of the C29, C31 and C33 n-alkanes are more
negative in the regions at the Northern and Southern
rims of the rainbelt (-160 ‰) and are less negative in
the equatorial regions (-150 ‰). Although
instrumental data are sparse, this seems to reflect the
spatial pattern in the δD of precipitation, where most
negative values are found in areas experiencing a
brief but intense rainy season. However, the late
Holocene pattern in the n-alkanes may also be due to
the dominance of C4 vegetation in peripheral regions
and the dominance of C3 vegetation in the equatorial
regions.
For the LGM, HS1 and mid-Holocene, however, the
spatial pattern of δD values compared to the late
Holocene do not reflect the spatial pattern of
vegetation type as suggested by δ 13 C values from
these timeslices (Collins et al, 2011). As such, we
can rule out a dominant effect of vegetation type on
δD values. Instead, during the mid-Holocene, δD
values are more negative (by ~10 ‰) than the late
Holocene for all cores of the transect. We interpret
this to represent an increase in rainfall intensity
across the full range of the rainbelt and, in
combination with increased wet season length
(Collins et al, 2011), reflects increased rainfall
amount.
LGM and HS1 δD values are broadly similar to each
other for all cores. When compared to the late
Holocene, the six northernmost cores for the LGM
and HS1 display similar or slightly less negative δD
values, suggesting similar or reduced intensity relative
to the late Holocene. However, the three
southernmost cores of the transect display more
negative δD values (~5 ‰) relative to the late
Holocene. This may reflect a brief but intense wet
season, as characterises the modern day peripheral
regions. Alternatively, however, this may also reflect
a more distal moisture source. Overall, our results
show that when used together, δ 13 C and δD may
have potential for disseminating both the wet season
length and intensity.
Collins et al, 2011, Nature Geoscience 4, 42-45.
Schefuß et al, 2005, Nature 437, 1003-1006.
Niedermeyer et al, 2010, Quaternary Science
Reviews 29, 2996-3005.
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