The modern Eastern Equatorial Pacific (EEP) Ocean is a large oceanic source of carbon to the atmosphere1. 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 ocean2,3 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 sediments4,5, but lower opal accumulation rates cast doubts on the importance of the EEP as an oceanic region for significant glacial carbon dioxide drawdown6,7. 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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We thank R. Francois, S. Kienast, W. Geibert, E. Galbraith and G. Leduc for discussions. Funding for this project was provided by the Scottish Alliance for Geoscience Environment Society (SAGES), the Natural Environment Research Council (NERC) through a Standard NERC grant (to R.S.G. and L.E.P.) and a Marie Curie Intra-European fellowship (to L.E.P.). I.C. and L.P. acknowledge funding from the ROMIAT and GRACCIE-CONSOLIDER projects.
Author Contributions L.E.P. and R.S.G. initiated the project. L.E.P. and B.C.R. measured silicon isotopes in ETH Zurich, L.E.P. measured elemental composition and nitrogen isotopes, L.E.P., K.K. and R.M.E. made the 230Th measurements. I.C. and L.P. provided the age model. L.E.P. and R.S.G. wrote the paper with the participation of B.R.C. All authors participated in the discussions on the results and commented on the manuscript.
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Pichevin, L., Reynolds, B., Ganeshram, R. et al. Enhanced carbon pump inferred from relaxation of nutrient limitation in the glacial ocean. Nature 459, 1114–1117 (2009). https://doi.org/10.1038/nature08101
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