The evolution of the marine phosphate reservoir

Abstract

Phosphorus is a biolimiting nutrient that has an important role in regulating the burial of organic matter and the redox state of the ocean-atmosphere system. The ratio of phosphorus to iron in iron-oxide-rich sedimentary rocks can be used to track dissolved phosphate concentrations if the dissolved silica concentration of sea water is estimated. Here we present iron and phosphorus concentration ratios from distal hydrothermal sediments and iron formations through time to study the evolution of the marine phosphate reservoir. The data suggest that phosphate concentrations have been relatively constant over the Phanerozoic eon, the past 542 million years (Myr) of Earth's history. In contrast, phosphate concentrations seem to have been elevated in Precambrian oceans. Specifically, there is a peak in phosphorus-to-iron ratios in Neoproterozoic iron formations dating from ∼750 to ∼635 Myr ago, indicating unusually high dissolved phosphate concentrations in the aftermath of widespread, low-latitude 'snowball Earth' glaciations. An enhanced postglacial phosphate flux would have caused high rates of primary productivity and organic carbon burial and a transition to more oxidizing conditions in the ocean and atmosphere. The snowball Earth glaciations and Neoproterozoic oxidation are both suggested as triggers for the evolution and radiation of metazoans. We propose that these two factors are intimately linked; a glacially induced nutrient surplus could have led to an increase in atmospheric oxygen, paving the way for the rise of metazoan life.

Geochemical constraints on the origin of Doushantuo cap carbonates in the Yangtze Gorges area, South China

“…This means that there should be a large reservoir of dissolved P and Fe in the anoxic basin. This is consistent with earlier studies which proposed that anoxic P-and Fe-rich marine deep waters were a general feature of the later Neoproterozoic according to Fe speciation and P/Fe ratios in Neoproterozoic sediments (Canfield et al, 2008;Planavsky et al, 2010). In order for the P and Fe in the anoxic bottom water to reach the shallow platform, deep basin brines would have to have experienced overturn and upwelling creating ideal conditions for the Group-F carbonate precipitation.…”

A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia

“…Rhenium enrichments in the Black River Dolomite are consistent with increased ocean oxygenation in the wake of the Sturtian glaciation, as suggested by Planavsky et al (2010). While Re data from the Upper Velkerri and Touirist formations may potentially reflect transiently oxygenated marine conditions, the extent to which these data represent spatiotemporal variations in the midProterozoic atmosphere-ocean redox state remains to be tested with higher-resolution Re data for stage 3.…”

Iron formations: A global record of Neoarchaean to Palaeoproterozoic environmental history

“…This model was based on partitioning coefficients (K D ) derived for P to Fe(III) oxyhydroxides associated with modern hydrothermal plumes. However, because the K D value for P sorption to Fe(III) oxyhydroxides diminishes in the presence of dissolved silica, later workers suggested that IF P/Fe ratios suggest that Archaean phosphate concentrations may not have been limiting (Konhauser et al, 2007a;Planavsky et al, 2010b).…”