Trace element and stable isotope variations in larger benthic foraminifer Baculogypsina sphaerulata: A potential proxy of seawater temperature

Abstract

Larger benthic foraminifera (LBF) contribute significantly to the modern reef carbonates. They were also the major producers of carbonate in the shallow marine environments of the Paleogene. It is thus imperative to investigate their potential as a paleotemperature proxy. It is common to calculate past seawater temperatures using trace elements and stable isotope proxies on foraminiferal calcite. The various LBF genera comprise tests with different MgCO3% and exhibit vital effects, making it important to carry out genus-specific temperature calibration studies. For the present study, live specimens of Baculogypsina sphaerulata were collected over seven months from reef flats of Akajima (Okinawa, Japan). The whole-shells of B. sphaerulata from each of the seven sampling months were analyzed using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) and IRMS (Isotope Ratio Mass Spectrometer) techniques. The relationship between Mg/Ca and δ18O with seawater temperature is established, and thus its potentiality for temperature calibration is discussed. The following proxy-temperature calibrations are proposed in this study: (i) Mg/Ca (mmol/mol) = 0.0267e0.3389T and (ii) T (oC) = 19.0–2.94 (δ18Of−δ18Ow).