Differing oxygen isotopic signals of two Globigerinoides ruber (white) morphotypes in the East China Sea: Implications for paleoenvironmental reconstructions

The stable isotope composition of planktonic foraminifera, Globigerinoides ruber (G. ruber), is a commonly used paleoenvironmental proxy. Globigerinoides ruber has two primary morphotypes, sensu stricto and sensu lato, which are differentiated by taxonomic criteria and, in some instances, stable isotopic values. Previous studies have reached differing conclusions surrounding the potential bias caused by the use of non-selective mixtures of morphotypes in paleoenvironmental reconstructions involving stable isotopes. Here, G. ruber s.s. and G. ruber s.l. from five time intervals within the last 85 ka, were analyzed from IODP Site U1429 sediments in the East China Sea. The oxygen isotopic compositions of these two morphotypes show statistically significant differences within all five intervals, with smaller s.s. values relative to s.l. values. The largest differences (0.77‰ to 0.82‰) are observed during interglacials (Holocene and Marine Isotopic Stage 5.1) with smaller differences during Marine Isotopic Stage 3 (0.45‰ to 0.56‰) and the Last Glacial Maximum (0.21‰). The isotopic offset between s.s. and s.l. is consistent with a deeper (colder) calcification depth of the s.l. morphotypes in the East China Sea. The decreased offset between the two morphotypes with increased glacial conditions may reflect decreased water column stratification during glacial intervals relative to interglacial intervals. According to these results, using a non-selective mixture of G. ruber morphotypes in paleoenvironmental reconstructions is inadvisable in this region.