A field‐based study on variation in Mg/Ca and Sr/Ca in larger benthic foraminifera

Abstract

The Mg/Ca and Sr/Ca ratios of foraminifera are used as proxies of paleotemperature. Although the Mg/Ca is found to be related with temperature in planktonic foraminifera, the reliability of this proxy in benthic foraminifera has yet to be established. It is essential therefore to understand the minor elements' composition and their relation to ambient water parameters in different benthic foraminiferal species. In this study, a systematic field‐based study was carried out on larger benthic foraminifera (LBF) of Okinawa (Japan) to assess if Mg/Ca and Sr/Ca in their tests could be used as proxies of seawater temperature. Five seasonal samples were collected from the reef flat to pick live specimens of Amphistegina lessonii, Neorotalia calcar, Calcarina hispida, Calcarina gaudichaudii, Baculogypsina sphaerulata, and Marginopora kudakajimaensis. The Mg/Ca in A. lessonii was ∼60 mmol/mol, while in the remaining five species the values were >200 mmol/mol, substantially higher than that recorded in planktonic and smaller benthic foraminifera. Interspecies and intraspecies Mg/Ca variations were fairly large even in samples of the same season. The Sr/Ca were quite uniform (∼2 mmol/mol) across the species and seasons. The correlations between test Mg/Ca and Sr/Ca and ambient water temperature and salinity were found to be poor. The possible reasons are (1) the influence of biological activities, particularly the photosynthesis‐respiration cycle of algal symbionts, and (2) masking of the true relationship due to an “averaging” effect in whole‐test analysis of species having longer a life span. In view of this it is suggested that the whole‐shell Mg/Ca and Sr/Ca of LBF is not an apt proxy of paleotemperature.