Evaluation of oxygen isotope and Mg/Ca ratios in high-magnesium calcite from benthic foraminifera as a proxy for water temperature

Oxygen isotope ratios (δ¹⁸O) and Mg/Ca ratios in organically produced calcium carbonate (calcite and aragonite) by marine species have been used as proxies for paleotemperature. The relationship between the components of large benthic foraminiferal tests (high-magnesium calcite) and seawater temperature has been little investigated yet. We conducted culture experiments using two species of reef-dwelling large benthic foraminifera under six temperature conditions (21–30°C). We selected the perforate species Calcarina gaudichaudii, and the imperforate species Amphisorus kudakajimensis, both of which are host to algal symbionts, for the experiments. The results indicated the significant correlation of Mg/Ca ratios (C. gaudichaudii and A. kudakajimensis) and δ¹⁸O (C. gaudichaudii) with temperature. The test weight of both species showed temperature dependency, with maxima at 27°C or 29°C and subsequent decline at 30°C. After correction for the contribution of the initial tests, which can be calculated from the weight ratios of the initial and cultured tests, the relationships of the Mg/Ca ratios and δ¹⁸O to temperature were obtained with the least squares best fit method as follows: C. gaudichaudii: T = 18.5–4.52 (δ¹⁸O_c–δ¹⁸O_sw), A. kudakajimensis: T = 25.0–4.96 (δ¹⁸O_c–δ¹⁸O_sw), and C. gaudichaudii and A. kudakajimensis: Mg/Ca = 2.85 T + 81.20. We concluded that species-specific calibration is unnecessary for Mg/Ca-temperature relationship, although it is necessary for δ¹⁸O related to the different calcification mechanisms of imperforate (A. kudakajimensis) and perforate (C. gaudichaudii) species.