25th International Meeting on Organic Geochemistry IMOG 2011

P-479
Assessment of the 1,14 diol SST proxy based on sediment trap
samples from the NW African upwelling area
Eleonora Uliana 1 , Gerard Versteegh 1 , Gesine Mollenhauer 1,2 , Enno Schefuss 1
1 MARUM, Bremen, Germany, 2 Alfred Wegener Institut, Bremerhaven, Germany (corresponding
author:euliana@marum.de)
The most commonly applied organic proxies for sea
surface temperature (SST) so far are the alkenonebased
U K 37 index and TEX86 based on glyceroldialkyl-glycerol-tetraethers
from marine
crenarchaeota. However, both proxies are not always
reflecting precisely the environmental signal recorded
in surface waters. In areas with input of upwelled,
nutrient-rich waters siliceous phytoplankton
suppresses coccolithophorid productivity by
competition as long as silica is available. Therefore,
U K 37-based SST estimates are often too high and
don‘t reflect the cooling during full upwelling
conditions. They record mean annual SSTs rather
than upwelling SSTs (Müller et al. 1998). In contrast
diatoms bloom especially in times of high nutrient
availability, making their biomarkers a potentially ideal
SST-proxy for paleo-reconstructions during upwelling
conditions. Recently, a new biomarker specific for
upwelling diatoms has been proposed. Rampen, et al.
(2009) have introduced a new index (1,14 diol chain
length index: 1,14DI) which combines long chain 1,14
diols obtained from diatom cultures of the genus
Proboscia and those from surface sediments from the
eastern South Atlantic. The new index has shown a
clear correlation between chain length and
temperature.
The presented study is based on measurements of a
series of sediment trap samples collected off Cape
Blanc in the upwelling area off Mauritania (NW
Africa). Sediment traps were analysed to investigate
the seasonal variations in diol index-based SST
estimates. Seasonal changes in both Proboscia sp.
frustules as well as long chain diols (1,14 C28 and C30)
have been recorded. Calculation of the 1,14 DI
following the method of Rampen et al. (2009) results
in two well defined groups of samples. The first group
(open dots in Fig. 1) coincides with the period of
stratification of the water column (non-upwelling
season) and has a 1,14DI value range between 0.25
and 0.35.The second group of samples with much
higher 1,14DI values (0.43 - 0.65, filled dots in Fig. 1)
corresponds to the upwelling season. While the DI
values of upwelling samples (filled dots) correspond
well with the core top SST calibration (filled triangles,
Rampen et al. (2009), the non-upwelling samples
C30/(C28+ C30) 1,14-diols
(open dots) reveal lower than expected 1,14DI values
resulting in too low SST estimates compared to actual
SSTs based on satellite measurements. These values
seem to fall more on the temperature calibration
derived for cultures of Proboscia sp. (open triangles,
Rampen et al.,2009) .
1
0.8
0.6
0.4
0.2
0
y = 0.028x - 0.023
R 2 = 0.52
0 10 20 30
Temperature [°C]
Fig. 1. Correlation of Diol Index values from sediment trap
samples from Cape Blanc (dots, filled: upwelling season, open:
non-upwelling season, our study) and core tops from the eastern
south Atlantic (filled triangles) plus cultures (open triangles) with
temperature. Core-tops, culture data and correlation from Rampen
et al. (2009).
Rampen, et al. 2009. Org. Geochem., 40, 1124-1131.
Mueller et al. 1998. Geochim. Cosmochim. Acta, 62,
1722–1757
Proxies to assess past sea surface temperature
(SST) variations are a powerful tool to infer past
changes in ocean circulation and upwelling.
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