Stable and radiogenic Sr isotope fractionation across the Paleocene-Eocene Thermal Maximum event as recorded in marine barite

Abstract

Radiogenic strontium (Sr) ratios (87/86Sr) have historically been used to investigate weathering and hydrothermal inputs into the ocean. However, the application of 87/86Sr is limited, because it does not fractionate between seawater and Sr sinks, hence these ratios do not record the complete Sr cycle. Stable Sr ratios (δ88/86Sr) in seawater fractionate between seawater and carbonate, the main sink of Sr in the ocean. Therefore, δ88/86Sr measurements can be combined with 87/86Sr ratios to characterize the complete Sr cycle. Moreover, δ88/86Sr in seawater are positively correlated to carbonate burial rates. The aim of this study is to investigate δ88/86Sr and 87/86Sr in seawater over the Paleocene-Eocene Thermal Maximum (PETM) event using the Double Spike-Thermal Ionization Mass Spectrometry (DS-TIMS) method to investigate carbonate dissolution during the PETM. Marine barite samples from a period between 56.22-56.37 Ma were analysed. Preliminary data suggests that δ88/86Sr in seawater was lower (0.3‰) during the PETM than it was 75 Ma before the PETM (0.4‰). No isotopic excursion is visible in the data. Rather, seawater δ88/86Sr appears to reach a stable lower value during the PETM. Changes in the CCD and ocean pH at the PETM indicate that carbonate sediments have dissolved. Hence the lack of a clear signal in the δ88/86Sr data implies that either the excursion is not captured in the current resolution, or the seawater Sr concentrations were too high for carbonate dissolution to make a quantifiable impact. Further work to improve TIMS analysis of Sr in marine barite and increasing the temporal resolution is necessary to produce more accurate and precise data, which is necessary to be able to provide a definitive conclusion.