HR: 0830h
AN: PP51B-0926 [PDF]
TI: Upwelling Dynamics and Nitrogen Cycling on the Namibian Shelf and Slope over the last two Climatic
Cycles
AU: * Pichevin, L E
EM: l.pichevin@epoc.u-bordeaux1.fr
AF: Departement de Geologie et Oceanographie, Universite Bordeaux 1
Avenue des Facultes, Talence, 33405
France
AU: Martinez, P
EM: p.martinez@epoc.u-bordeaux1.fr
AF: Departement de Geologie et Oceanographie, Universite Bordeaux 1
Avenue des Facultes, Talence, 33405
France
AU: Bertrand, P
EM: bertrand@epoc.u-bordeaux1.fr
AF: Departement de Geologie et Oceanographie, Universite Bordeaux 1
Avenue des Facultes, Talence, 33405
France
AB:
The Benguela upwelling system is the world's most productive eastern boundary current. Whether local paleoproductivity was
mainly supported by new or recycled nutrients is a tricky issue. Solving this question is, however, an essential prerequisite
for estimates of the net CO2 pumping attributable to this upwelling through the past and, ultimately, for forecasts of the
future climate changes. In this regard, the mechanisms that control the local nutrient budget and their variations with
climate changes must be addressed. We interpret the $\delta^{15}$N signals of three cores distributed from the upper to the
lower continental slope of Namibia (25$\deg$6S) as reflecting an interplay of changes in local nitrogen cycling and regional
denitrification during the last 250 kyrs. Detrital grain-size distributions and SST records were used as proxies for wind
stress and upwelling dynamics, respectively, and TOC and MAR Corg values as indicators of paleoproduction. The upper slope
core displays a noisy $\delta^{15}$N signal without obvious glacial-interglacial variability, whereas the lower slope cores
display low $\delta^{15}$N during cold periods and high $\delta^{15}$N during climatic optima. This dissimilarity results
from the segregation of the upwelling structure in two cells, decoupling nutrient dynamics of the shelf from those beyond the
shelf-edge. We hypothesize that, for the coastal cell, denitrification and nutrient recycling change little over time,
though regenerated production may be more intense during sea level high-stands. The coastal cell d15N seems insensitive to
wind stress variations. Changes in nutrient availability in the shelf-edge cell waters, however, may be partly mediated by
the strength and zonality of the wind, impacting off-shore productivity, though this cannot explain the full range of
variability. Positive $\delta^{15}$N peaks during full interglacial periods, namely stages 7.5, 5.5 and 1, are concurrent
with $\delta^{15}$N maxima from other upwelling systems adjoining OMZs. We suggest that, during full interglacial periods,
some degree of water column denitrification occurs in the Benguela region: globally lower oxygen concentrations in
intermediate waters at these times, possibily in combination with sporadic intrusions from a poleward, oxygen depleted
undercurrent, led to suboxia in the OMZ.
DE: 1615 Biogeochemical processes (4805)
DE: 4267 Paleoceanography
DE: 4279 Upwelling and convergences
DE: 4516 Eastern boundary currents
DE: 4546 Nearshore processes
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting