25th International Meeting on Organic Geochemistry IMOG 2011

P-408
Sea surface salinity reconstructions of the Agulhas current
based on alkenone hydrogen isotope ratios
Sebastian Kasper 1 , Marcel T.J. van der Meer 1 , Geert-Jan Brummer 2 , Isla S. Castañeda 1 ,
Rainer Zahn 3 , Jaap S. Sinninghe Damsté 1 , Stefan Schouten 1
1 Royal Netherlands Institute for Sea Research, Department of Marine Organic Biochemistry, 1790 AB Den
Burg (Texel), Netherlands, 2 Royal Netherlands Institute for Sea Research, Department of Marine Geology,
1790 AB Den Burg (Texel), Netherlands, 3 Universitat Autònoma de Barcelona, Institut de Ciència i
Tecnologia Ambientals, 08193 Bellaterra, Spain (corresponding author:sebastian.kasper@nioz.nl)
The South Equatorial Current transports
water masses with relatively high temperatures and
salinity across the Indian Ocean from Indonesia to
Africa. In front of Madagascar it branches into the
Northern Boundary Current and the East Madagascar
Current going south. Northwest of Madagascar the
Northern Boundary Current divides into the East
African Coastal Current and the Mozambique Current.
The latter is dominated by strong eddies which form
the Mozambique Channel. The Mozambique current
and the East Madagascar Current unite at 27°S to
form the Agulhas Current. At the southern tip of the
African shelf the Agulhas Current retroflects back into
the Indian Ocean whereby so called ‗Agulhas rings‘
spin off and release warmer and more saline water
into the South Atlantic Ocean. The magnitude of the
Agulhas leakage, therefore, has a strong impact on
the Atlantic Meridional Overturning Circulation and
hence the entire meridional heat flux. Thus, glacial –
interglacial fluctuations of the Agulhas leakage into
the Atlantic Ocean can affect global temperatures (1).
Reconstructions of the dynamics of the Agulhas
current based on geochemical proxies for sea surface
temperatures and salinity could help to understand
the link between the magnitude of the Agulhas current
and global temperatures.
Although there are a number of proxies
available for reconstructing sea surface temperatures,
few tools are available for salinity reconstructions. It
has been suggested that alkenone-derived hydrogen
isotope ratios are linearly related to salinity and
therefore could be used as a tool for sea surface
salinity reconstructions (2, 3). To investigate the
possibility of using deuterium isotopes of alkenones
as a proxy for salinity reconstructions of the Agulhas
current we are currently performing a core top study
from the Mozambique Channel and the Agulhas
current. In addition, we are generating long term
alkenone hydrogen isotope records of selected
sediment cores to examine temporal variations in
salinity in the Agulhas system with focus on the
transition from marine isotope stage (MIS) 6 to 5.
During MIS 6 the subtropical front is thought to have
moved northwards (4) weakening the exchange
between the Indian and Atlantic Ocean and a build-up
of warm saline water at the tip of South Africa. During
early MIS 5 the exchange is thought to increase again
resulting in a decrease in SST and salinity. This drop
in salinity is reflected in a decrease in Δδ 18 Osw in the
same core from the Agulhas corridor (Fig. 1) (4).
Initial results show that there is a decrease of
approximately 15 per mill in the alkenone hydrogen
isotope ratio from MIS 6 to 5 in agreement with the
suggested drop in salinity, suggesting that it can be a
useful tracer for salinity changes in the Agulhas
region.
1. C. S. M. Turney, R. T. Jones, Journal of
Quaternary Science 25, 839 (2010).
2. S. Schouten et al., Biogeosciences 3, 113
(2006).
3. M. T. J. van der Meer et al., Earth and
Planetary Science Letters 262, 594 (2007).
4. G. Martinez-Mendez et al., Paleoceanography
25, (2010).
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