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 mum) 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 delta(18)O could be applied to reconstruct paleoseasonality of the open oceans. However, because the fine-fraction delta(13)C values are not representative of the annual mean surface-water delta(13)C, we recommend use of near surface-dwelling planktic foraminiferal delta(13)C as a proxy for delta(13)C of stratified surface waters that are more or less in equilibrium with the atmosphere with respect to pCO(2). (C) 2002 Elsevier Science B.V. All rights reserved.