Phosphorus sources for phosphatic Cambrian carbonates

J. R. Creveling,D. T. Johnston,S. W. Poulton,C. Marz,B. Kotrc,A. H. Knoll,D. P. Schrag

doi:10.1130/b30819.1

J. R. Creveling, D. T. Johnston + Show 5 more

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https://doi.org/10.1130/b30819.1

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Abstract

The fossilization of organic remains and shell material by calcium phosphate minerals provides an illuminating, but time-bounded, window into Ediacaran–Cambrian animal evolution. For reasons that remain unknown, phosphatic fossil preservation declined significantly through Cambrian Series 2. Here, we investigate the phosphorus (P) sources for phosphatic Cambrian carbonates, presenting sedimentological, petrographic, and geochemical data from the Cambrian Series 2–3 Thorntonia Limestone, Australia, some of the youngest Cambrian strata to display exceptional phosphatic preservation of small shelly fossils. We find that within Thorntonia sediments, phosphate was remobilized by organic decay and bacterial iron reduction, with subsequent reprecipitation largely as apatite within the interiors of small shelly fossils. We discuss the merits of bioclastic-derived, organic matter–bound, or iron-bound P as potential sources to these strata. Petrographic observations suggest that the dissolution of phosphatic skeletal material did not provide the P for fossil preservation. In contrast, high organic carbon contents imply significant organic fluxes of P to Thorntonia sediments. Sedimentology and iron-speciation data indicate that phosphorus enrichment occurred during times of expanded anoxic, ferruginous conditions in subsurface water masses, suggesting that phosphorus adsorption to iron minerals precipitating from the water column provided a second significant P source to Thorntonia sediments. Simple stoichiometric models suggest that, by themselves, neither organic carbon burial nor an iron shuttle can account for the observed phosphorus enrichment. Thus, we infer that both processes were necessary for the observed phosphorus enrichment and subsequent fossil preservation in the Thorntonia Limestone.

Highlights

Phosphorite and phosphatic carbonate define a spectrum of sedimentary lithologies enriched in the authigenic calcium phosphate mineral apatite (Kazakov, 1937; Baturin and Bezrukov, 1979; Riggs, 1986; Cook and Shergold, 1986; Cook et al, 1990; Föllmi, 1996; Trappe, 2001)
We find that while the high organic carbon content of the Thorntonia Limestone suggests that organic-bound P contributed significantly to authigenic apatite formation, C to P ratios indicate that organic-bound P was insufficient to account entirely for the observed phosphorus enrichment
Within drill core NTGS 99/1, phosphorus enrichment is confined to the middle and upper members of the Thorntonia Limestone, and petrographic observations reveal that this enrichment reflects authigenic apatite mineral nucleation primarily associated with the interior of bioclasts and, more rarely, as cement in bioclastic grainstone

Summary

Introduction

Phosphorite and phosphatic carbonate define a spectrum of sedimentary lithologies enriched in the authigenic calcium phosphate mineral apatite (Kazakov, 1937; Baturin and Bezrukov, 1979; Riggs, 1986; Cook and Shergold, 1986; Cook et al, 1990; Föllmi, 1996; Trappe, 2001). The punctuated temporal distribution (Cook and McElhinny, 1979; Cook and Shergold, 1984, 1986) and evolving spatial distribution (Brasier and Callow, 2007) of phosphatic lithologies through Earth history suggest that unique and restrictive physical (Filippelli and Delaney, 1992) and/or chemical (e.g., Föllmi, 1996) conditions govern phosphate deposition in time and space. Perhaps foremost is the practical concern for understanding how ore-grade sedimentary phosphorites form (e.g., Cook and Shergold, 1986). Biogeochemists increasingly invoke perturbations to the ancient phosphorus cycle to explain inferred fluctuations in biological productivity, organic carbon burial and oxidant accumulation over geological time-scales (Tyrrell, 1999; Bjerrum and Canfield, 2002; Saltzman, 2005; Holland et al, 2006; Konhauser et al., 2007; Algeo and Ingall, 2007; Planavsky et al, 2010; Swanson-Hysell et al, 2012)

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