Evaluating phosphorus incorporation into belemnite calcite as an environmental proxy

Phosphorus is the ultimate limiting nutrient for marine primary productivity over geological
timescales and plays a key role in modulating several geochemical cycles. Established methods
for investigating phosphorus do not provide direct evidence of water-column phosphorus
cycling, but recent work on carbonate associated phosphorus has shown potential to record
dissolved phosphorus concentrations. This project developed a method to extend the
application of carbonate associated phosphorus measurements to belemnites and quantify the
variability of phosphorus and other elements within and between belemnites. This technique
was applied to samples from across the Early Jurassic, in particular the Pliensbachian and
Toarcian. Samples analysed were from three locations in the Laurasian Seaway, the Moesian
Basin (Bulgaria), the Cleveland Basin (Yorkshire, UK) and the Cardigan Bay Basin (North Wales,
UK).
Carbonate associated phosphorus was found to vary coherently through time, with similarities
between the records generated for each study location. When carbonate associated
phosphorus records were compared to different established geochemical proxies, different
regional controls on P/Ca values were identified including redox conditions and temperature.
This suggests that carbonate associated phosphorus can record water column phosphorus
concentrations, but the controls on dissolved phosphorus concentrations are complex.
Correlation between P/Ca and Mg/Ca was also identified, which was unrelated to temperature
variations, and suggested potential mineralogical control on phosphorus incorporation. Na/Ca
records were also investigated and were found to show clear stratigraphic variation. Na/Ca
changes were shown to be unrelated to temperature or salinity changes, but also showed
evidence of mineralogical control on sodium incorporation.
This work indicates that belemnites may be a promising record of carbonate associated
phosphorus, but also emphasises the importance of holistic analysis of belemnite calcite in the
interpretation of geochemical proxies.