Changes in calcareous nannofossil assemblages across the Paleocene/Eocene transition from the paleo-equatorial Pacific Ocean

Quantitative analyses of selected calcareous nannofossils in deep-sea sections recovered from the paleo-equatorial Pacific (ODP Leg 199) provide new information about biostratigraphy, biochronology and the evolutionary history of calcareous nannofossils across the Paleocene/Eocene transition interval. The sediment cores from ODP Leg 199 represent the first continuous Paleocene/Eocene boundary sections ever to be sampled in the central equatorial Pacific Ocean. Calcareous nannofossil assemblages are studied to document the distribution of biostratigraphically useful taxa such as Ericsonia, Discoaster, Fasciculithus, Rhomboaster and Tribrachiatus. Focus is given to the evolution of the Rhomboaster-Tribrachiatus lineage in the lower Eocene interval at Site 1215, and on the stratigraphic relationship of these taxa relative to species in the genus Fasciculithus. Critical intervals of North Atlantic DSDP Site 550 have also been re-examined. The Tribrachiatus digitalis morphotype was described at Site 550 from an interval affected by down-hole contamination, partly originating from within the Tribrachiatus orthostylus range. The T. digitalis morphotype represents an evolutionary transitional form between T. contortus and T. orthostylus, entering the stratigraphic record within the range of the former species and disappearing within the lower part of the range of the latter species. The subzonal subdivision of Zone NP10 hence collapses. Lithological and colour variability reflecting orbital cyclicity occur in the lower Eocene of Site 1215, permitting a relative astronomical age calibration of the Tribrachiatus taxa. The distinct Rhomboaster spp.-Discoaster araneus association also occurs in the paleo-equatorial Pacific Ocean, together with a marked decrease in diversity of Fasciculithus spp. Site 1220 reveals a short peak abundance of Thoracosphaera spp. just above the P/E boundary interval, which probably reflects a stressed surface water environment.

Vertical distribution of phytoplankton pigments in the equatorial Pacific

Abundance distribution and cellular characteristics of picophytoplankton were studied in two distinct regions of the equatorial Pacific: the western warm pool (0°, 167°E), where oligotrophic conditions prevail, and the equatorial upwelling at 150°W characterized by high-nutrient low-chlorophyll (HNLC) conditions. The study was done in September-October 1994 during abnormally warm conditions. Populations of Prochlorococcus, orange fluorescing Synechococcus and picoeukaryotes were enumerated by flow cytometry. Pigment concentrations were studied by spectrofluorometry. In the warm pool, Prochlorococcus were clearly the dominant organisms in terms of cell abundance, estimated carbon biomass and measured pigment concentration. Integrated concentrations of Prochlorococcus, Synechococcus and picoeukaryotes were 1.5x10**13, 1.3x10**11 and 1.5x10**11 cells/m**2, respectively. Integrated estimated carbon biomass of picophytoplankton was 1 g/m**2, and the respective contributions of each group to the biomass were 69, 3 and 28%. In the HNLC waters, Prochlorococcus cells were slightly less numerous than in the warm pool, whereas the other groups were several times more abundant (from 3 to 5 times). Abundance of Prochlorococcus, Synechococcus and picoeukaryotes were 1.2x10**13, 6.2x10**11 and 5.1x10**11 cells/m**2, respectively. The integrated biomass was 1.9 g C/m**2. Prochlorococcus was again the dominant group in terms of abundance and biomass (chlorophyll, carbon); the respective contributions of each group to the carbon biomass were 58, 7 and 35%. In the warm pool the total chlorophyll biomass was 28 mg/m**2, 57% of which was divinyl chlorophyll a. In the HNLC waters, the total chlorophyll biomass was 38 mg/m**2, 44% of which was divinyl chlorophyll a. Estimates of Prochlorococcus, Synechococcus and picoeukaryotes cell size were made in both hydrological conditions.

(Table 2) Similarity Indexes (Pearson's correlation coefficient) of the silicoflagellate assemblages of IODP Exp302

The silicoflagellate and ebridian assemblages in early middle Eocene Arctic cores obtained by IODP Expedition 302 (ACEX) were studied in order to decipher the paleoceanography of the upper water column. The assemblages in Lithologic Unit 2 (49.7-45.1 Ma), one of the biosiliceous intervals, were usually endemic as compared to the assemblages that occurred outside of the Arctic Ocean. The presence of these endemic assemblages is probably due to a unique environmental setting, controlled by the degree of mixing between the low-salinity Arctic waters and relatively high salinity waters supplied from outside the Arctic Ocean, such as the Atlantic and possibly the Western Siberian Sea. Using the basin-to-basin fractionation model, the early middle Eocene Arctic Ocean corresponds to an estuarine circulation type, which includes the modern-day Black Sea. The abundant down-core occurrence of ebridians strongly suggests the past presence of low-salinity waters, and may indicate that low oxygen concentrations prevailed in the euphotic layer, on the basis of the ecology of the modern ebridian Hermesinum adriaticum.