EOCENE PALEOCEANOGRAPHY

The middle and upper Eocene on Blake Nose consist of thick sequences of light green and yellow siliceous chalk and ooze that accumulated at 2–6 cm/k.y. There has been considerable pore-water exchange through the sediment pile, but several hiatuses and lithologic breaks act as aquicludes, channeling and distributing the flow of pore waters across Blake Nose (Ussler et al., Chap. 2, this volume). Like other parts of the Blake Nose succession, the sediments preserve a well-defined magnetostratigraphy (Ogg and Bardot, Chap. 9, this volume) and a clear cyclicity in physical property records that can be used together with the biostratigraphy to create detailed chronologies. Indeed, well-defined biostratigraphies for calcareous nannofossils (Mita, Chap. 7, this volume), dinoflagellates (van Mourik and Brinkhuis, Chap. 6, this volume; van Mourik et al., 2001) and radiolarians (Sanfilippo and Blome, 2001) are available for correlation to magnetostratigraphic and cyclostratigraphic time scales. Eocene sediments also preserve numerous volcanic ash beds that can be used for intersite correlation on Blake Nose (Pletsch and Reicherter, Chap. 8, this volume). Impact ejecta derived from the Chesapeake impact event is present in the upper Eocene sequence at Site 1053 and should in due course provide a well-resolved chronology for that event that can be applied worldwide.

To date, most work on the middle and upper Eocene record has focused on isotope paleoceanography. Wade et al. (2001a; Chap. 5, this volume) have shown that there are large amplitude (2–2.5) shifts in ¹⁸O of near surface water species within the middle Eocene that suggest large changes in SST or surface-water salinity on precessional and eccentricity time scales. Indeed, development of orbital time scales based on physical property records suggests that the rate and magnitude of paleoceanographic change in the middle Eocene was not all that different from that observed in stable isotope time series from the Pleistocene (e.g., Kroon et al., 2000). Hence, it appears that variability of Eocene SST may have been much larger than has been previously predicted from low temporal resolution analyses of Eocene sediments.

Currently, there are several as yet unfinished studies of Eocene strata drilled during ODP Leg 171B. Norris and Nishi (unpubl. data) are producing a detailed planktonic foraminifer biostratigraphy for the middle and upper Eocene in Sites 1051, 1052, and 1053 that will be tied to the magnetostratigraphy and cyclostratigraphy in these sites. H. Nishi (unpubl. data) has produced faunal counts of planktonic foraminifer assemblages across the middle Eocene/late Eocene boundary to document faunal turnover associated with the extinction of the morozovellid and acarininid planktonic foraminifers. He and his students are extending their faunal work across the late Eocene impact horizon at Site 1053. Smit and Kroon (unpubl. data) are producing the stable isotope record of planktonic foraminifers across the same interval at Site 1053. A long benthic foraminifer time series record has been produced by N. Shackleton (unpubl. data) for the upper Eocene sequence at Site 1052, and future additions to this record will include isotopic analyses of surface-water planktonic foraminifers by B. Wade and P. Wilson.