BIOTIC RESPONSE TO PALEOCEANOGRAPHIC CHANGES IN THE MIDDLE EOCENE OF BLAKE NOSE, WESTERN NORTH ATLANTIC

• Other Authors: Thomas, Deborah, Firth, John, Thomas, Olszewski, Wade, Bridget, Slowey, Niall
• Format: Book
• Language: English
• Published: 2009
• Subjects: Nannofossil abundance; North Atlantic; Planktonic foraminifera
• Online Access: http://hdl.handle.net/1969.1/ETD-TAMU-2009-05-420

Geochemical proxy records of sea surface environmental conditions at Blake Nose indicate short-term high amplitude variations. In order to investigate the response of calcareous nannoplankton to these short-term sea-surface environmental changes, we developed a high-resolution Eocene planktonic foraminifera Mg/Ca record and calcareous nannofossil absolute abundance record for ODP Site 1052 spanning the late middle Eocene (37.85 to 37.45 Ma). These data enable direct comparison of the absolute nannofossil abundance data to geochemical paleoenvironmental proxies (Mg/Ca, ?18O and ?13C) to determine the phytoplankton response to paleoenvironmental change and whether temperature or nutrient levels exerted primary control. The nannofossil absolute abundances were estimated using a combination of the random settling technique (RST) and spiking method (SM), which are two independent methods. Both methods yielded similar and reproducible results. The calculated Mg/Ca paleotemperature record indicates a decrease in SSTs from ~33 to 28?C, while the ?18Osw values calculated from the Mg/Ca paleotemperature also decreased from ~3? at 37.83 Ma to ~2? at 37.6 Ma. The combined trends of the SST and ?18Osw suggest that continental ice did not have a major influence on the climate during the study interval. Variations in the Gulf Stream (e.g. track of proto-Gulf stream, number and transit of eddies, and Gulf Stream related upwelling), may have contributed to the reconstructed sea surface hydrographical changes at Blake Nose. However, overall weakening of the hydrological cycle as global climate transitioned from a greenhouse to an icehouse could be the major factor controlling the hydrographical changes at Blake Nose. Approximately 48 nannofossil taxa were identified in this study. These taxa have been grouped into major, minor and rare taxa based on their abundance in any given sample. Results of the statistical analyses (canonical correspondence analysis CCA, Cluster analysis, and correspondence analysis CA) used to relate the nannofossil abundance to environmental parameters did not show expected relationships between the nannofossil absolute abundance and environmental parameters. There are several explanations for this which includes dominance of biotic control over abiotic control, difference in depth habitats between the planktonic foraminifera used for the geochemical proxies and the calcareous nannoplankton, and changing combinations of environmental controls operating during the ~400 ky interval. This suggests that these factors should be taken into account when making conclusions about nannofossil environmental preferences.