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Reduced nitrogen fixation in the glacial ocean inferred from changes in marine nitrogen and phosphorus inventories


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 oceans1,2. 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 fixation4,5, we suggest therefore that marine productivity did not increase in glacial periods in response to either increased nutrient inventories or greater iron supply.

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Figure 1: Water column and sediment properties across the continental shelf and slope off northwest Mexico.
Figure 2: Records of δ18O of benthic foraminifera, weight per cent phosphorus and organic carbon, and δ15N of bulk sediments in Core NH15P (425 m water depth; 22° 41.0′ N, 106° 28.8′ W).


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We thank A. Devol and J. Murray for the opportunity to collect sediment cores during the NSF-supported voyage 90-5 of RV New Horizon to the Mexican margin. We thank M. Soon and B. Nielsen for carrying out elemental and isotopic determinations, and J. Brandes for permission to use his water column nitrogen isotope data. This work was supported by the Natural Sciences and Engineering Research Council of Canada and the Natural Environment Research Council of the UK.

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Correspondence to Raja S. Ganeshram.

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Ganeshram, R., Pedersen, T., Calvert, S. et al. Reduced nitrogen fixation in the glacial ocean inferred from changes in marine nitrogen and phosphorus inventories. Nature 415, 156–159 (2002).

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