Fossil indicators of nutrient levels. 1: Eutrophication and climate change

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Abstract
Both phosphorus and nitrogen are elements essential to life. Availability of these biolimiting nutrients therefore exerts a major control on the carbon cycle through ocean productivity, bringing about changes in ocean-atmosphere chemistry and climate. This review begins with a brief account of the biological importance of phosphorus and nitrate to the cell, and introduces current terminology relating to the interlinked carbon-nutrient cycles in the oceans. Nitrate-limited, eutrophic ecosystems are associated with modern upwelling zones, characterized by high rates of delivery of new phosphorus, high rates of new primary production and export production. Fossil indicators include high accumulation rates of biogenic silica, apatite, baryte and organic matter, plus non-spinose, smaller planktonic foraminifera. Examples from biomineral chemistry include indications of high Ba/Ca, high Cd/Ca and increased differences between δ13C in planktonic and benthic foraminiferal test calcite. Such eutrophic indicators are shown to have peaked during glacial phases in the Quaternary, supporting a model in which Milankovitch-related reductions in solar isolation influenced the availability of nutrients, increasing primary productivity and lowering the levels of atmospheric carbon dioxide.

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