1. Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK
2. Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, V6T 1Z4, Canada
3. Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02540, USA

Correspondence to: Raja S. Ganeshram¹ Correspondence and requests for materials should be addressed to R.S.G. (e-mail: Email: Raja.Ganeshram@glg.ed.ac.uk).

Abstract

To explain the lower atmospheric CO₂ concentrations during glacial periods, it has been suggested that the productivity of marine phytoplankton was stimulated by an increased flux of iron-bearing dust to the oceans^{1, 2}. One component of this theory is that iron—an essential element/nutrient for nitrogen-fixing organisms—will increase the rate of marine nitrogen fixation, fuelling the growth of other marine phytoplankton and increasing CO₂ uptake. Here we present data that questions this hypothesis. From a sediment core off the northwestern continental margin of Mexico, we show that denitrification and phosphorite formation—processes that occur in oxygen-deficient upwelling regions, removing respectively nitrogen and phosphorus from the ocean—declined in glacial periods, thus increasing marine inventories of nitrogen and phosphorus. But increases in phosphorus were smaller and less rapid, leading to increased N/P ratios in the oceans. Acknowledging that phytoplankton require nitrogen and phosphorus in constant proportions, the Redfield ratio³, and that N/P ratios greater than the Redfield ratio are likely to suppress nitrogen fixation^{4, 5}, we suggest therefore that marine productivity did not increase in glacial periods in response to either increased nutrient inventories or greater iron supply.

1. Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK
2. Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, V6T 1Z4, Canada
3. Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02540, USA

Correspondence to: Raja S. Ganeshram¹ Correspondence and requests for materials should be addressed to R.S.G. (e-mail: Email: Raja.Ganeshram@glg.ed.ac.uk).