Calibrating stable Sr isotope proxy for paleo-oceanographic studies: Insights from δ88/86Sr variability in modern and Holocene coastal marine system in South Australia 

Abstract

Ocean water yields an integrated global signal of geological and biological processes operating on our planet, which in turn control the marine C cycle, oceanic alkalinity budget and atmospheric CO2 levels. Therefore, reconstructing the chemical and isotope composition of seawater and/or coastal waters through time represents one of the main research objectives of earth system evolution studies. Here we present stable and radiogenic Sr isotope variations (δ88/86Sr and 87Sr/86Sr) measured in waters and carbonates from modern and Holocene coastal marine system in South Australia (Coorong Lagoon/Murray River Estuary) that is connected to the Southern Ocean, thus exhibiting large gradients in water chemistry, salinity and carbonate saturation (Mosley et al. 2023; Shao et al. 2021). The studied hydrological system shows a large salinity range from brackish (<20 psu) to normal marine (~35 psu) and hypersaline (~110 psu), with the salinity changes being tightly linked to DIC and Alkalinity, and thus CaCO3 saturation state (SI values) of local waters, calculated via PHREEQC. The primary research aim was to assess how spatial and temporal changes in salinity, carbonate chemistry and CaCO3 saturation (dissolution vs precipitation of carbonates), monitored throughout the year (in spring, summer, fall and winter), impact the Sr isotope composition in the present-day coastal marine system. Such knowledge is, in turn, important for a better calibration and assessment of the δ88/86Sr proxy for paleo-oceanographic and environmental applications including past marine alkalinisation/acidification events and/or paleo-salinity reconstructions (Farkas et al. 2024; Shao, 2022). Importantly, our results from seasonal sampling and monitoring showed that the δ88/86Sr in waters is positively correlated with their SI values (carbonate saturation) and salinity, with the heaviest or most positively fractionated stable Sr isotope signatures of +0.48 ± 0.03‰ (thus above ‘normal seawater’ of +0.39 ‰) measured in summer season (hot and dry period) in hypersaline (>70 psu) and oversaturated (SI ~1) waters. In contrast, the isotopically light and systematically lower stable Sr isotope signatures (< 0.35‰) are documented in brackish waters that are also undersaturated with respect to CaCO3 minerals (SI < 0). Overall, these results point to the primary control of carbonate dissolution versus precipitation phenomena, and thus CaCO3 saturation, on the δ88/86Sr proxy in the modern coastal marine system. Finally, we will also illustrate how a coupled δ88/86Sr and 87Sr/86Sr approach can be applied for paleo-salinity reconstructions of the coastal marine systems, such as the Coorong Lagoon, based on the Sr isotope analysis of recent and fossil carbonate archives (bivalve shells) recovered from local sediment cores (Shao, 2022). Briefly, available results and geochemical modeling of the Sr isotope data from Holocene fossil shells suggest that over the last ~2400 years the Coorong Lagoon become progressively more evaporitic, exhibiting a temporal shift from a purported brackish paleo-lagoon to the present-day hypersaline carbonate producing system.      References Farkas et al. (2024) Treatise on Geochemistry, Third Edition. Elsevier (Book Chapter 00086) Mosley et al. (2023) Marine Pollution Bulletin, 188, 1-16.   Shao (2022) PhD Thesis, University of Adelaide  Shao et al. (2021) GCA, 293, 461-476.