A review of phosphate oxygen isotope values in global bedrocks: characterising a critical endmember to the soil phosphorus system

Abstract

Understanding the phosphate oxygen isotope (δ18O‐PO4) composition of bedrock phosphate sources is becoming ever more important, especially in areas of soil research which use this isotope signature as a proxy for biological cycling of phosphorus (P). For many of these studies, obtaining a sample of the source bedrock or applied mineral fertiliser for isotope analysis is impossible; meaning there is now a demand for a comprehensive characterisation of global bedrock δ18O‐PO4 to support this work. Here we compile δ18O‐PO4 data from a wide range of global bedrocks, including 56 new values produced as part of this study and a comprehensive overview of those within the previously existing literature. We present δ18O‐PO4 data from the range of major phosphatic lithologies alongside as much metadata for the samples as could be gathered. Much of the data comes from bedrocks of marine sedimentary origin (< 1 Ma = > +22‰, > 540 Ma = ≈ +12‰), but we also present data from bedrocks associated with guano (range: +19.5 to +15‰) and igneous deposits (range: +12 to –0.8‰), both of which have distinct δ18O‐PO4 signatures due to their formation mechanisms. We show that where repeat measurements of the same formation have been undertaken, regardless of method or exact sample location, there is an average within formation error of ± 1.25‰. This is important, as is constitutes a reasonable level of uncertainty for phosphorus cycling studies which need to estimate bedrock δ18O‐PO4 composition based on the literature. In combination, this data set presents 284 δ18O‐PO4 values from 56 countries; a comprehensive starting point for researchers interested in understanding bedrock end member δ18O‐PO4.