THE EVOLUTION OF OCEAN STOICHIOMETRY AND DIVERSIFICATION OF THE MARINE BIOSPHERE

The emergence and diversification of the marine biosphere have depended on the evolving stoichiometry of the oceans in response to rising oxygen levels and the coupling of nutrient availability to primary productivity. The Phanerozoic fossil record indicates that the “energetics” of the marine biosphere--biomass, metabolic rates, physical activity such as predation, and food requirements necessary to sustain such activity--have generally increased through geologic time. These trends are mirrored by the sequence of appearance of the dominant fossilized phytoplankton taxa and their biomarkers, the increasing nutrient demands of modern representatives of fossil phytoplankton taxa, and increasing primary productivity and carbon burial. However, the wide range of carbon isotope values and total organic carbon (TOC) has been inferred to indicate relatively constant primary productivity.

A metaanalysis of the TOC (n=2613) and P (n=849) composition of sedimentary rocks of the last 3 billion years indicates that the ratio of carbon to phosphorus of sediments is a more revealing indicator of the geochemical behavior of phosphorus and carbon in relation to nutrient availability, primary productivity and diversification of the marine biosphere. Micronutrient and phosphorus input from rising oxygen levels and orogeny during the middle-to-late Proterozoic stimulated eukaryotic primary production, which in turn increasingly sequestered nutrients in biomass. Nutrient sequestration culminated in the initiation of positive feedback on biosphere growth during the Neoproterozoic-Phanerozoic transition, when nutrient recycling via metazoan bioturbation and mesozooplankton grazing began to augment primary productivity. During the Phanerozoic, increasing rates of nutrient recycling paralleled increasing oxygenation and bioturbation and may have been necessary to sustain further expansion of the biosphere.