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Enhanced carbon pump inferred from relaxation of nutrient limitation in the glacial ocean
Nature
Vol 459, No. 7250, pp. 1114-1117, 25 June 2009.
doi:10.1038/nature08101
L.E. Pichevin1, B.C. Reynolds2, R.S. Ganeshram1,
I. Cacho3, L. Pena3, K. Keefe4 and R.M. Ellam4
1 School of Geosciences, Grant Institute, University of Edinburgh, West Main Road, EH10 3JW, Edinburgh, UK.
2 IGMR, ETH Zürich, Clausiusstrasse 25, CH-8092 Zürich, Switzerland
3 GRC Geociències Marines, Facultat de Geologia, Universitat de Barcelona C/ Martí Franques s/n 08028 Barcelona, Spain
4 Scottish Universities Environment Research Centre, Rankine Avenue, Scottish Enterprise Technology Park, East Kilbride, G75 0QF, UK.
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ABSTRACT:
The modern Eastern Equatorial Pacific (EEP) Ocean is a large
oceanic source of carbon to the atmosphere. Primary productivity
over large areas of the EEP is limited by silicic acid and iron
availability, and because of this constraint the organic carbon
export to the deep ocean is unable to compensate for the
outgassing of carbon dioxide that occurs through upwelling
of deep waters. It has been suggested that the delivery of
dust-borne iron to the glacial ocean could have increased
primary productivity and enhanced deep-sea carbon export
in this region, lowering atmospheric carbon dioxide
concentrations during glacial periods. Such a role
for the EEP is supported by higher organic carbon burial
rates documented in underlying glacial sediments, but lower
opal accumulation rates cast doubts on the importance of the
EEP as an oceanic region for significant glacial carbon dioxide
drawdown. Here we present a new silicon isotope record that
suggests the paradoxical decline in opal accumulation rate
in the glacial EEP results from a decrease in the silicon
to carbon uptake ratio of diatoms under conditions of increased
iron availability from enhanced dust input. Consequently, our
study supports the idea of an invigorated biological pump in
this region during the last glacial period that could have
contributed to glacial carbon dioxide drawdown. Additionally,
using evidence from silicon and nitrogen isotope changes,
we infer that, in contrast to the modern situation, the
biological productivity in this region is not constrained by
the availability of iron, silicon and nitrogen during the
glacial period. We hypothesize that an invigorated biological
carbon dioxide pump constrained perhaps only by phosphorus
limitation was a more common occurrence in low-latitude areas
of the glacial ocean.
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