25th International Meeting on Organic Geochemistry IMOG 2011

P-240
Evidence for transport mechanisms of soil derived branched
GDGTs
Hendrik Grotheer 1 , Sabine Kasten 2 , Gesine Mollenhauer 1,2
1 Department of Geosciences, University of Bremen, Klagenfurter Str., 28359 Bremen, Germany, 2 Alfred-
Wegener-Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany
(corresponding author:h.grotheer@freenet.de)
Input of terrigenous material to the oceans is an
important process in global element cycles, including
the carbon cycle. Therefore, many studies in the past
have used either organic or inorganic proxies to
quantify the terrigenous contribution and paleoclimatic
interpretations were made based on these proxy
records. Among the more recently introduced proxies
for export of soil-derived organic matter and the
reconstruction of environmental conditions on land
like mean air temperature and soil pH, respectively,
are the so-called BIT, MBT and CBT indices [1, 2] , using
abundance ratios of branched tetraether lipids
(GDGTs) believed to derive from unknown soildwelling
anaerobic bacteria. The habitat of these soilorganisms,
however, is not well constrained, nor is
the association of these lipids with mineral grains
exported to the ocean.
We therefore analysed abundance of these branched
GDGTs using HPLC-APCI-MS and measured
concentrations of the elements iron, titanium,
aluminium, and potassium, typically taken as
indicators for terrigenous material, by ICP-OES on
sediment core (GeoB3910-2). The core was
recovered 100 km north-east of the Brazilian coast at
4°14.7‘S and 36°20.7‘W in 2362 m water depth in the
deposition area of the Piranha River (Açu River). We
investigated a segment deposited between 10 and 20
ka B.P. containing records of climate response to the
Younger Dryas (YD) and the Heinrich Event 1 (H1)
cold events in NE Brazil.
Our results confirm previous data suggesting a
southward shift of the ITCZ during YD and H1 leading
to an increase in precipitation in NE Brazil [3] and
therefore to enhanced erosion of organic and
lithogenic material. The calculated pH-values also
support previous findings of a two-step vegetation
response to the enhanced precipitation [4] during H1.
Furthermore the combination of organic and inorganic
analyses allows a partial reconstruction of transport
pathways for GDGTs from anaerobic soil bacteria.
A clear correlation between branched GDGT
concentration and aluminium content suggests the
formation of insoluble organo-metal complexes [5] in
anaerobic and acidic podsol horizons. These
complexes are then leached out and transported
towards the river system. Additionally, the titanium
and iron content show a similar correlation to the
GDGT concentration implying that besides aluminium
the formation of organo-metal complexes with other
elements can serve as a transport host as well.
Figure 1: Correlation between terrigenous elements and
sum of branched GDGTs. Squares (H1) and diamonds
(YD) indicating periods with increased precipitation.
1. Hopmans, E.C., et al., Earth and Planetary Science
Letters, 2004. 224(1-2): p. 107-116.
2. Weijers, J.W.H., et al., Geochimica et Cosmochimica
Acta 2007. 71: p. 703-713.
3. Jaeschke, A., et al., Paleoceanography, 2007. 22(4): p.
PA4206.
4. Dupont, L.M., et al., Global Change Biology, 2009.
5. Bardy, M., et al., Geochimica et Cosmochimica Acta,
2007. 71(13).
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