Low level of phosphorous concentration in terminal Paleoproterozoic shallow seawater: Evidence from Chuanlinggou ironstone on North China Platform

Abstract

Phosphorus (P) is a bio-limiting nutrient element, which through modulating marine primary productivity can influence ocean redox conditions. Precambrian, especially the mid-Proterozoic (1.8–0.8 Ga), seawater phosphorous concentrations have been poorly constrained. The well-preserved ∼1.65 Ga ironstones in North China provide a nice opportunity to explore the phosphorous concentrations of shallow seawater at that time. To avoid potential contamination we performed fabric-specific analysis of dense laminasets in ooid cortices without cracks, and in stromatolitic columns. These data revealed a low P/Fe molar ratio of 0.0007 ± 0.0003 (n = 12), pointing to an extremely low phosphorus concentration (<0.002–0.016 μM) in ∼1.65 Ga shallow seawater. If such a low phosphorus concentration is an universal phenomenon during terminal Paleoproterozoic, this is likely the significant reason for limited primary productivity and therefore a low atmospheric oxygen level during that time.