Environmental influences on the Mg/Ca ratios of an intertidal foraminifer Rotalidium annectans in the West coast of India

Foraminifera (forams) are shell secreting marine protists. They incorporate calcium carbonate as well as trace elements into their shells. The trace elements incorporated in foraminiferal shells are commonly used as paleoproxies. The ratio of magnesium to calcium (Mg/Ca) in CaCO3 shells of foraminifera is widely used to determine paleotemperatures. However, Mg/Ca is highly variable within and between species, suggesting a strong physiological influence on the incorporation of Mg2+ into the shells. The present work aims to study the influence of environmental variables on the incorporation of Mg into the shells of an intertidal foraminifer, Rotalidium annectans. The relationship between environmental factors (temperature, pH, salinity and Mg/Ca ratio of seawater) and elemental composition (Mg and Mg/Ca ratio of foraminifera) were analyzed for 17 months in shells of R. annectans, which were collected from coastal sediments of Payyannur (the west coast of India). Results show that incorporation of Mg was mainly controlled by sea water temperature as Mg content of foraminifera was positively correlated with seawater temperature. An inverse correlation between seawater temperature and Mg/Ca ratio of R. annectans showed a strong physiological influence on the Mg partitioning in R. annectans as it is exposed to rapid temperature fluctuations in intertidal zone. An increase in the Mg/Ca ratio with a decrease in pH presumes a decrease in calcification rate, consistent with observations of ocean acidification due to higher atmospheric CO2. The positive correlation between Mg/Ca ratio of seawater and Mg/Ca ratio of foraminifera showed a moderate significant Mg/Ca-seawater effect on Mg/Ca-foraminifera. Therefore variations in Mg/Ca ratios of seawater should be taken into account when evaluating the role of foraminifera in the past oceanic carbon cycle.