Temperature Calibration of Elevated Mg/Ca in Planktic Foraminifera Shells From the Hypersaline Gulf of Aqaba

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2023-07, Vol.24 (7), p.n/a
• Main Authors: Levy, N., Torfstein, A., Schiebel, R., Chernihovsky, N., Jochum, K. P., Weis, U., Stoll, B., Haug, G. H.
• Format: Article
• Language: English
• Subjects: Blooms; Calcium; Calibration; Diagenesis; Environmental conditions; Foraminifera; Fossil Foraminifera; Globigerinoides ruber; High temperature; Magnesium; Marine environment; Mixed layer; Phytoplankton; Salinity; Sea surface; Sea surface temperature; Seawater; Sediment; Sediment traps; Shells; Surface temperature; Surface water; Temperature; Temperature changes; Trends; Uptake; Water column
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2022GC010742

The Mg/Ca of marine calcareous Planktic Foraminifera (PF) shells is commonly used for sea surface temperature reconstructions. However, compared to open marine environments, hypersaline (>40) oligotrophic seas have been shown to accommodate PF with higher Mg/Ca and divergent temperature to Mg/Ca relationships. To investigate influencing factors of PF Mg uptake in hypersaline regions, we measured the Mg/Ca of two flux‐dominating PF species, Globigerinoides ruber albus and Turborotalita clarkei, derived from a monthly resolved time series of sediment traps in the Gulf of Aqaba, northern Red Sea as well as the corresponding temperature, salinity, and pH values. The PF exhibit elevated Mg/Ca which cannot be explained by post‐deposition or interstitial sediment diagenetic processes. G. ruber albus displays Mg/Ca trends that strongly follow seasonal mixed layer temperature changes. Conversely, T. clarkei Mg/Ca trends do not follow temperature but rather show significant Mg/Ca enrichment following mixing of the surface water column. We present a framework for incorporating elevated Mg/Ca into global Mg/Ca‐T calibrations for G. ruber albus and present a new Mg/Ca‐T calibration suitable for hypersaline marine environments. Plain Language Summary Past seawater temperature is reconstructed from the magnesium‐to‐calcium ratio (Mg/Ca) in the calcareous shells of a group of marine microplankton called foraminifera. Two foraminifer species, Globigerinoides ruber albus and Turborotalita clarkei, are abundant in the Gulf of Aqaba, northern Red Sea, at year‐round high temperatures and salinities. The shells of these foraminifera have elevated Mg/Ca relative to other marine regions, and here, we explore the factors causing this. The Mg/Ca values of both G. ruber albus and T. clarkei reflect the environmental conditions of the water column. For G. ruber albus, temperature and salinity appear to be factors responsible for the Mg/Ca trends and elevated values. We incorporate the new Mg/Ca data for G. ruber albus to calibrate elevated Mg/Ca with temperature for high‐salinity (>40) marine environments. The Mg/Ca of the deeper dwelling T. clarkei show higher ratios following deep mixing of the surface water column and may indicate annually recurring phytoplankton blooms caused by nutrient input into the sunlit ocean surface. Key Points A new Mg/Ca temperature calibration for high salinity environments is presented for Globigerinoides ruber albus A framework for incorporating high