Phosphorus in belemnites: Extraction, quantification, and variability

Phosphorus is generally considered the ultimate limiting nutrient for marine primary productivity over geological timescales and plays a key role in modulating several biogeochemical cycles. Most established methods for investigating P cycling do not provide direct evidence for water-column P concentrations, but recent work on carbonate associated phosphorus (CAP) has shown there is a potential to record ancient dissolved P concentrations. We present a method to extend the application of carbonate associated P measurements to belemnites, and we quantify variability in P contents within and between belemnite specimens from two stratigraphic levels in Jurassic rocks from the Yorkshire coast, UK, as well as in modern analogues of belemnites. We show that there is little difference in P measurements between different preparative methods in uncontaminated belemnite calcite samples. In samples with higher levels of contaminant phases of P (P other than CAP), or a greater extent of diagenetic alteration, cleaning with non-oxidative methods and dissolution in weak acids (acetic) was found to minimise the impact of contamination on the measured P contents. P concentrations vary within and between specimens, but variations are not a result of taxonomic differences, and overall P measurements are reproducible between replicate samples, within individual belemnites and within stratigraphic levels. There were statistically significant differences in belemnite P concentrations between the stratigraphic levels studied here, indicating the potential for this technique to be used to measure changes in belemnite CAP through time. •Most phosphorus in belemnites is authigenic, likely carbonate associated P.•Non-oxidative cleaning and acetic acid dissolution reduce contaminant P dissolution.•No significant taxonomic differences in belemnite P concentration.•Significant differences in P between stratigraphic levels.