Genetic Evidence Reveals Species Dependent δ18O Shift in Neogloboquadrina pachyderma (dextral) Between Glacial and Interglacial Times

Genetic evidence shows that apparent "single species records" used in paleoceanographic reconstructions most likely contain an alternation in species concurrent with environmental change. In this study the distribution pattern of small subunit (SSU) ribosomal (r)RNA genotypes of N. pachyderma (sin.) and (dex.) in the Nordic Seas are compared with surface sediment coiling ratios and isotopic values of N. pachyderma (sin.) and (dex.) for the same regions. The genetic evidence for N. pachyderma show that the morphological distinction of coiling direction is not sufficient to distinguish between genotypes and species. We have found morphologically right coiling specimens which are genotypically identical to N. pachyderma Type I (sin). These right coiling N. pachyderma Type I (sin) genotypes are specifically found in regions where δ¹⁸O isotopes of N. pachyderma (dex.) and N. pachyderma (sin.) are identical in surface sediments; and they occur at a constant percentage of approximately 2-3% relative to the left coiling N. pachyderma morphotypes. On the other hand, morphologically and genetically right coiling specimens of N. pachyderma Type I (dex.) only occur in regions where δ¹⁸O isotopes of N. pachyderma (dex.) and N. pachyderma (sin.) in surface sediments are different. In these regions the relative abundance of N. pachyderma (dex.) is always above the "threshold" value of 2-3%. We then examine and discuss the potential alternation of genotypes of N. pachyderma (sin.) and (dex.) between glacial and interglacial conditions and its effect on the isotopic signature of paleo-records. Our analyses clearly show that the glacial-interglacial δ¹⁸O signature in N. pachyderma (dex.) in the Nordic Seas contains a species-dependent isotopic shift of about 0.5‰ when relative abundances cross a certain "threshold" value. This finding not only enhances the calibrated use of δ¹⁸O records of N. pachyderma (dex.), but also highlights the general need to genetically investigate other paleoceanographically important morphospecies in order to refine their use as paleoproxies.