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. 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 ratio, and that N/P ratios greater than the Redfield ratio are likely to suppress nitrogen fixation, we suggest therefore that marine productivity did not increase in glacial periods in response to either increased nutrient inventories or greater iron supply.
journal_name
Naturejournal_title
Natureauthors
Ganeshram RS,Pedersen TF,Calvert S,François Rdoi
10.1038/415156akeywords:
subject
Has Abstractpub_date
2002-01-10 00:00:00pages
156-9issue
6868eissn
0028-0836issn
1476-4687pii
415156ajournal_volume
415pub_type
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