Salinity information from hydro-sensitive, biogenic marine carbonate δ18O records

Stable oxygen isotope ratios in marine biogenic carbonates (δ18Ocarb, e.g., from corals, coralline algae, bivalves, sclerosponges) have greatly extended the instrumental record, providing invaluable information about climate variability and change from the tropics to high latitudes. These δ18Ocarb records reflect seawater temperature and δ18O (δ18Osw) at the time of calcification, which occurs sufficiently rapidly to permit reconstruction of sub-annual to annual variability over the lifetime of the organism. δ18Osw is strongly related to salinity, as both are similarly impacted by hydroclimate processes such as precipitation, evaporation, and advection. Many studies have leveraged δ18Ocarb and the tendency for temperature and salinity to covary constructively in the tropics (e.g., warm with wet or low salinity, and vice versa) to reconstruct changes in major modes of climate variability and responses to forcings. However, with limited networks of in situ seawater temperature, salinity, and δ18O measurements, quantifying the relative contribution of temperature and δ18Osw to δ18Ocarb variability remains a large source of uncertainty. Here we use ‘pseudo-carbonate’ δ18O records modeled from temperature and salinity to identify sites where salinity contributes substantially to δ18Ocarb variance. These ‘hydro-sensitive’ δ18Ocarb records are located predominantly in the Indo-Pacific Warm Pool, Caribbean Sea, Gulf of Mexico, and Aleutian Archipelago. Notably, we find that temperature and salinity vary destructively in δ18Ocarb (i.e., warm with drier or higher salinity) in many regions for which high-resolution paleoclimate reconstructions have provided key constraints on ocean circulation—the eastern equatorial Pacific, the North Atlantic, and at depth. However, the inferred salinity variability at these sites is sensitive to uncertainties in the δ18Osw-salinity relationship, emphasizing the need for a coordinated network of in situ salinity and δ18Osw measurements. Plain-language Summary Ocean salinity integrates valuable information about changes in the global water cycle, including changes in precipitation, evaporation, and ocean currents. However, our understanding of recent salinity changes are hampered by the limited coverage of direct salinity measurements. Many marine organisms incorporate information about their local environment in their skeleton as they grow, providing indirect evidence of past ocean salinity. The interpretation of these records is dependent on the relative magnitude of temperature and salinity signal recorded, and in turn, how these variables relate to one another. We show that samples from the western Pacific Ocean, eastern Indian Ocean, Caribbean Sea, Gulf of Mexico, and Aleutian Archipelago are most likely to provide reliable salinity information across seasons, years, and decades, as the organisms in these regions primarily record salinity at all timescales. Finally, we show that direct seawater observations are critical to improve the salinity information obtained from the skeletons of marine organisms.

Keywords

Salinity, Stable isotopes, Carbonates, Corals, Algae, Climates, Latitude effect, Tropical regions, Seawater, Indian Ocean, Gulf of Mexico, Caribbean Sea, Pacific Ocean

Citation

Thompson, D. M., Conroy, J. L., Williams, B., Konecky, B. L., Stevenson, S., DeLong, K. L., McKay, N., Dassie, E. P., Fischer, M., Jonkers, L., Martrat, B., & Pages Iso2K Project Members. (2021). Salinity information from hydro-sensitive, biogenic marine carbonate δ18O records. Invited presentation to the AGU Fall Meeting 2021, New Orleans, Los Angeles and Online, 13-17 December 2021. In AGU Fall Meeting Abstracts (Vol. 2021, PP14B-01). Retrieved from: https://agu.confex.com/agu/fm21/meetingapp.cgi/Paper/989425