Oxygen isotope studies of biogeochemical cycling of phosphorus

Phosphorus (P) is essential for function and growth of all living organisms and limits primary productivity in some ocean regions today. Thus, it is important to understand the processes of P utilization in aquatic systems. P occurs in nature primarily in one oxidation state (+5) and in one form, orthophosphate (PO4). Although P has only one stable isotope (31P), during most biogeochemical cycling reactions P is bonded to oxygen (O), an element with three stable isotopes. Thus isotopic ratios of oxygen bonded to P provide an opportunity for stable isotope studies of reactions of P in nature. The overall goal of the studies in this thesis is to characterize the O isotope effects of the reactions that take place during biogeochemical cycling of P, and to apply these O isotopic signatures to trace the sources and the reaction pathways of P in natural environments.; Distinctive and distinguishable O isotopic signatures are produced by different P cycling pathways. In experiments mimicking natural apatite formation, only a small fractionation (∼1‰) was observed between solid and aqueous Pi phases. Degradation of organic matter by UV photooxidation retains unaltered delta18O values of PO4 derived from phosphomonoesters; and microbial turnover of Pi---uptake and subsequent release by cells---will drive Pi toward equilibrium with ambient water (Blake et al., 2001). Pi regenerated from phosphomonoesters incorporates one O atom from ambient water, whereas Pi derived from phosphodiesters incorporates two. Incorporation of water O causes Pi regenerated from enzymatic degradation of Porg to shift away from equilibrium delta18O values.; Pidelta18O was applied to study P cycling in organic-rich sediments with high microbial activities at the Peru Margin. The measured Pidelta18O values together with interstitial water chemistry show that in addition to a strong microbial turnover signature, two different processes (enzymatic Porg degradation and redox controlled Pi absorption/desorption) affect P cycling, and that microbial activity was the primary force behind post-deposition P redistribution. This successful application of Pidelta 18O values to tracing P cycling processes in natural environments strongly supports the use of Pidelta18O as a novel tool for studying reaction pathways of P in aquatic systems.