25th International Meeting on Organic Geochemistry IMOG 2011

P-464
Effects of within-catchment provenance on terrestrial climate
parameters in marine sediments off large rivers
Enno Schefuß 1 , Gesine Mollenhauer 1,2 , Holger Kuhlmann 1 , Matthias Prange 1 , Jürgen
Pätzold 1
1 MARUM - Center for Marine Environmental Sciences and University of Bremen, Bremen, Germany, 2 Alfred
Wegener Institute for Polar and Marine Research, Bremerhaven, Germany (corresponding
author:schefuss@uni-bremen.de)
Reconstructions of terrestrial climate conditions are
often applied by using lipid biomarker proxies in
marine sediment cores off large rivers to assess
continental climate changes in the catchments (e.g.,
(Schefuß et al., 2005; Weijers et al., 2007). The
assumption to reconstruct catchment-integrated
climate changes is challenged by studies reporting
overprint of organic particulate material compositions
during fluvial transport (e.g., (Bouillon et al., 2009).
Here, we present a multi-proxy comparison of climatic
changes recorded in a sediment core taken offshore
the Zambezi River in southeast Africa. Compoundspecific
hydrogen isotope analyses of long-chain nalkanes
reveal profound changes in continental
hydrology. Depleted hydrogen isotope compositions
are found for time intervals corresponding to northern
high-latitude cold events such as the Younger Dryas
suggesting higher rainfall in the catchment. Parallel
depletions of soil pH estimates based on
compositional changes in terrestrial GDGTs (CBT
index) support this finding. Lower soil pH values are
caused by higher rainfall amounts due to the removal
of cations from surface soils (Johnson et al., 1998).
Additionally, synchronous increased sedimentary
terrigenous elemental and biomarker concentrations
as well as elevated relative contributions of terrestrial
GDGTs (high BIT index) suggest strong fluvial
discharge during these events. Taken together, these
proxies would indicate higher rainfall in the catchment
leading to lowered soil pH and increased contributions
of terrestrial elements, plant lipids and soil organic
matter to the marine sediments.
In contrast to these observations suggesting wetter
conditions, stable carbon isotope analyses of longchain
alkanes reveal higher contributions of C4 plants
during these times. Under present-day conditions, C4
plants (tropical grasses) prevail over C3 plants in
Africa when rainfall amounts are too low to permit C3
trees to grow or when the rainy season is too short.
For past climate conditions, also atmospheric CO2
levels and growing-season temperatures potentially
affect the relative abundance of C4 plants in the
vegetation cover. The elevated C4 plant contributions
during the discharge events would thus suggest drier
conditions, lower atmospheric CO2 concentrations or
higher temperatures. As atmospheric CO2 levels rose
during the Younger Dryas changing atmospheric CO2
concentrations are ruled out as potential driver of the
observed C3/C4 changes. Parallel to the increase in
C4 plant contribution, estimates of mean annual
temperatures based on terrestrial branched GDGT
compositions (MBT index) suggest slightly warmer
temperatures. The relatively small amplitude of the
reconstructed temperature changes, however, cannot
explain the observed strong C3/C4 plant changes.
Therefore, we infer that catchment-internal shifts of
the main rainfall area towards the headwaters of the
Zambezi River currently vegetated by C4 grasslands
and experiencing warmer temperatures caused the
observed coherent, but apparently contradicting
climate reconstructions.
When considered alone, the terrestrial proxy
parameters would lead to deviating climate
interpretations. However, when judging their
significance for each specific setting and interpreted
in conjunction, they may provide insights into withincatchment
provenance changes.
Bouillon, S. et al. (2009) Biogeosciences 6, 2475-
2493.
Johnson, D.W. et al. (1998) Water Air Soil Pollut. 105,
251-262.
Schefuß, E. et al. (2005) Nature 437, 1003-1006.
Weijers, J.W.H, et al. (2007) Science 315, 1701-1704.
589