Heterogeneous seawater phosphorus concentrations during the Sturtian glaciation: Evidence from P/Fe ratios of Fulu Formation ironstone in South China

Abstract

Phosphorus (P) is one of the key bio-limiting elements in the modern ocean, and controls marine primary productivity over the geological time scales. It is proposed that the Ediacaran atmosphere oxygenation and animal evolution might have been triggered by high seawater P concentration in Cryogenian oceans. Seawater P concentration can be reconstructed by the ratio of phosphorus to iron [expressed as (P/Fe)100] in ironstones. Here we measured the P/Fe ratios of the banded iron formation (BIF) from the Cryogenian Fulu Formation in South China. The (P/Fe)100 ratios of the Fulu BIF vary between 0.08 and 1.81 (mean = 0.45, n = 22), considerably lower than the values of coeval BIF deposited in other continents (mean = 1.24, n = 74). Variation of P/Fe ratio of ironstones may suggest a large seawater P concentration gradient during the Sturtian glaciation, which is comparable to, or even higher than, that in the modern ocean (~1.2 μM in North Atlantic and ~2.6 μM in North Pacific). Though being explained in different ways, the heterogeneous seawater P concentration requires active organic matter production in the surface ocean and/or decomposition in the ocean interior as well as possible onset of thermohaline circulation during the Sturtian glaciation. Finally, our study suggests that precipitation of BIF in the completely ice-covered ocean was unlikely.