Ontogenetic isotopic variation in foraminifera – Implications for palaeo proxy

The oxygen isotopic composition (δ18O) of Marginopora kudakajimaensis (Foraminiferida) is found to differ by 0.5‰ among the individuals living under similar physico-chemical conditions. The δ18O variation along the growth direction (ontogeny) is much higher, ranging between 1.5 and 2.5‰. In comparison, the ontogenetic variation in carbon isotopes (δ13C) is relatively small. The range and standard deviation of ontogenetic δ18O decrease progressively for deeper water species. There is no such systematic change in δ13C values. The study suggests that as palaeo proxy, the isotopic data not only require correct species discrimination but also a good understanding of the palaeobiology of the species.