Weathering of phosphorus in black shales

Rock weathering is the ultimate source of phosphorus (P) to the oceans, where P can be a limiting nutrient for biological production. In this paper, P weathering is examined in soil chronosequences formed in weathering profiles on the organic-rich Woodford Shale, New Albany Shale and Green River Shale. At all sites, organic P and inorganic P concentrations reveal that P weathering is far from complete, prior to erosion. Carbon (C)/P ratios decrease significantly from unweathered shale to the weathered shale at all sites, which is driven by loss of total organic C with weathering. Here we characterize organic phosphorus across a weathering profile from the Woodford Shale using solid-state CPMAS P-31 NMR spectroscopy techniques, revealing that P esters are the dominant forms of P during all stages of weathering. Certain P esters appear to be resistant to chemical weathering during the millions of years between deposition, uplift and erosion, possibly representing a significant long-term global sink for P.