Fine-fraction carbonate stable isotopes as indicators of seasonal shallow mixed-layer paleohydrography

We have measured stable isotopic compositions of Miocene pelagic fine-fraction (<63 μm) carbonates from oligotrophic deep-sea sites in the Pacific and Atlantic oceans and compared them with those of coexisting foraminifers to test their utility as near sea-surface indicators. Fine-fraction carbonates (primarily polyspecific nannofossils) and surface-dwelling planktic foraminiferal (Globigerinoides) stable isotopes both have been considered to reflect surface-water hydrographic conditions. However, our results indicate that fine-fraction stable isotopes are offset from and do not correlate well with those of Globigerinoides. In contrast, stable isotopic records of the deep-dwelling planktic foraminifer Globoquadrina are in good correspondence with the fine-fraction records in terms of long-term (ca. >1 m.y.) trends and temporal variability. On the basis of a time-series hydrography and flux study site in the oligotrophic subtropical North Atlantic, we interpret the isotopic discrepancies between fine-fraction and Globigerinoides as resulting primarily from season of calcification, as well as possible vital effects. We suggest that fine-fraction stable isotope values from oligotrophic waters reflect late winter–early spring relatively cool, nutrient-rich shallow mixed-layer conditions during the time of deep mixing (i.e., spring bloom), whereas Globigerinoides stable isotope values record conditions that prevailed in the stratified surface waters in the warmer late spring–fall. This implies that paired analyses of fine-fraction and surface-dwelling planktic foraminiferal δ¹⁸O could be applied to reconstruct paleoseasonality of the open oceans. However, because the fine-fraction δ¹³C values are not representative of the annual mean surface-water δ¹³C, we recommend use of near surface-dwelling planktic foraminiferal δ¹³C as a proxy for δ¹³C of stratified surface waters that are more or less in equilibrium with the atmosphere with respect to pCO₂.