25th International Meeting on Organic Geochemistry IMOG 2011

P-157
Can laterally advected intermediate nepheloid layers affect the
efficiency of the biological pump? – a biomarker study of subsurface
alterations of marine snow aggregates off Cape Blanc,
NW Africa
Andreas Basse 1,2 , Gesine Mollenhauer 1,2 , Gerd Fischer 2 , Morten Iversen 2 , Gerard
Versteegh 2 , Gökay Karakas 1
1 Alfred-Wegener-Institute for Polar and Marine Research (AWI), Bremerhaven, Germany, 2 Universiti of
Bremen, Bremen, Germany (corresponding author:abasse@marum.de)
Marine particulate organic matter (POM) is an
important part of the global carbon cycle, since its
production in the surface waters and subsequent
export to the deep ocean and sediments causes a net
removal of CO2 from the atmosphere. This process is
called the ‗Biological Pump‘ (Volk & Hoffert 1985).
The NW-African upwelling system is the second most
productive of the four eastern-boundary upwelling
systems and is characterized by high export of POM.
Previous studies have shown that the transport of
POM to the deep sea is not only controlled by the
vertical flux, but also by lateral advection from the
shelf area. Field observations and models show
intermediate and near-bottom nepheloid layers (NLs).
(Karrakas et al. 2006, Fischer et al. 2007). It has been
shown that biological alterations are controlling POM
export from the surface ocean (Iversen et al. 2010),
and an equally important alteration of the POM may
occur as it sinks through the NLs by scavenging of
laterally advected particles.
We analyzed samples collected with in-situ pumps,
core top sediment samples, and surface water
filtrations along an EW-transect off Cape Blank.
We found the lipid composition of the POM to change
significantly with water depth. This suggests that
degradation processes at all depths alter the POM as
it sinks, and/or that the NLs contain distinct lipids
which are scavenged by the POM aggregates during
their vertical flux through the NLs.
We measured UK‘37, TEX86 and bulk radiocarbon age.
The UK‘37 showed no significant changes throughout
the water column, indicating that the UK‘37-Index is
not affected by lateral transport and alteration. The
TEX86, however, showed significant increases
correlating with the nepheloid layers. This implies that
the material in the NLs has either 1) a different source
of POM compared to the water layers above and
below the NLs, or 2) that the POM in the NLs
undergoes different alteration processes than the rest
of the POM in the water column. The age
determinations via 14 C indicated that the NLs
contained a mixture of fresh and very old material
compared to the water layer above and below. to the
old material was probably laterally advected from the
shelf area (Karakas et al. 2006), while the fresh
material likely derived from surface produced POM
aggregates. Hence, we have a complex system
where biological and physical processes influence the
efficiency of the biological pump at different depths
through the water column.
References
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