A novel method for analysing phosphorus-bound stable oxygen isotope ratios in groundwater ecosystems
Mineralogy, Petrology and Geochemistry
Final Report Abstract
Our aim was to develop a methodological approach for reliably determining and interpreting stable oxygen isotope ratios of phosphate (δ18OPO4) within operationally defined phosphate (PO4) pools in aquifers, encompassing both groundwater and the solid phase. Examining the δ18OPO4 signals of PO4 pools in aquifers offers valuable insights into the phosphorus (P) cycling in groundwater systems, which has been relatively poorly understood until now. In this project, we have refined and validated two sequential extraction procedures using a selection of seven pre-defined minerals and their mixtures, which we each labelled with 18 O- enriched PO4 via surface adsorption and co-precipitation, respectively. Furthermore, we applied the most promising procedure to analyze a set of 20 natural samples derived from three distinct aquifer systems, representing a diverse range of geochemical properties. Additionally, we explored a method to extract δ18OPO4 signals from dissolved PO4 in groundwater. Our approach facilitated the identification of the strengths and limitations of the proposed method for assessing δ18OPO4 signals across various PO4 pools within aquifers. Our findings indicate that δ18OPO4 analysis holds promise as a valuable tool, albeit with inherent uncertainties in the interpretation. Notably, we observed a lack of sensitivity in the sequential extraction solutions. Consequently, we recommend acknowledging this limitation during data interpretation and refraining from attempts to differentiate between detrital and authigenic apatite. Similarly, we advise against specifically targeting PO4 associated with Fe(II) minerals. In terms of isotope signals, we found that the extractable PO4 quantity significantly influences sample preparation, currently restricting application mainly to the HCl-extractable PO4 pool, primarily comprising apatite and CaCO3-associated PO4. Moreover, we noted potential impacts on isotopic signals resulting from the hydrolyzation of organic P, necessitating cautious interpretation for samples rich in organic matter. Additionally, we demonstrated the robustness of the applied approach for assessing δ18OPO4 values in dissolved PO4 in groundwater. Overall, the methodological approach we recommend, despite its current limitations, enables the determination of δ18OPO4 values across various PO4 pools within aquifers. These findings lay the foundation for the application of the method to further advance our understanding of the biogeochemical P cycle in groundwater systems.
