Phosphorus in caves : Oxygen isotopes in phosphate as a novel speleothem palaeothermometer

Morgan, Alistair and Wynn, Peter and Surridge, Ben and Smith, Andrew (2022) Phosphorus in caves : Oxygen isotopes in phosphate as a novel speleothem palaeothermometer. Masters thesis, Lancaster University.

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Abstract

Speleothems are key archives for palaeoclimatological study, yet current methods for palaeotemperature records are often affected by processes independent to temperature. Phosphorus is a ubiquitous component of speleothem calcite in caves, yet its efficacy as a palaeothermometer has not yet been fully explored. The fractionation of phosphate-oxygen-isotopes (δ18OPO4) to temperature by pyrophosphatase (PPase) enzymes is frequently mentioned throughout the literature as a chemical thermometer but has yet to be tested in speleothems. This dissertation therefore aimed to evaluate the efficacy of δ18OPO4-thermometry in a contemporary cave monitoring and palaeoclimatic context. This was accomplished through δ18OPO4-temperature-equilibration experiments of drip-waters and speleothem material collected from Poole’s Cavern, Buxton. Also tested were two speleothems grown over the last 100-years from Ethiopia and the UK, and two palaeo-δ18OPO4-archives from Australian speleothem YB-F1 (99-37 ka) and the Archean Ocean (3.2-3.5 Ga). Results show that a PPase fractionation equation for Poole’s Cavern of 1000퐿푛∝(푃푂4−퐻2O)=15.801∙(1000푇)−29.106 (R2 = 0.69 P <0.01) describes a viable δ18OPO4-temperature relationship between 7 and 30 ◦C. This has a similar overall trend to current literature (-0.2‰/°C) but is demonstrates a positive shift compared to previously determined relationships due to the hyperalkaline pH of Poole’s Cavern drip-waters. δ18OPO4 of calcite material shows mixed results, partly due to acid hydrolysis during late sample prep, but can be mitigated stoichiometrically in the future. Nonetheless, natural δ18OPO4 in Poole’s cavern waters translated into in-situ calcite with minimal (±1‰) fractionation, and δ18OPO4-thermometry correctly predicted temperatures for Rukiesa Cave, Ethiopia. Moreover, δ18OPO4 data from YB-F1 shows good correlation to expected temperature change 99-37 Ka but with exaggerated temperature readings. In all, this marks a critical ‘first-step’ in showcasing promise for δ18OPO4-thermometry in speleothem palaeoclimatology.