The Paleocene-Eocene transition in the subtropical Pacific Ocean (Shatsky Rise, Leg 198) : inference from planktonic foraminifera

The Paleocene-Eocene interval was cored during Leg 198 at four sites (1209 to 1212) in a depth range of 2387 m to 2907 m, and corresponds to an 8- to 23-cm-thick layer of clayey nannofossil ooze with a sharp base and a gradational upper contact. Detailed analysis on two selected holes (1209B and 1210B), were performed with high-resolution cm-scale sampling extended below and above the clay-rich layer for few centimetres. A total of 80 samples were analysed for planktonic foraminifera. At these sites the Paleocene yellowish brown calcareous ooze is overlain by a thin dark brown clay seam (~ 2 mm) which corresponds to the base of the Paleocene-Eocene Thermal Maximum (PETM) and to the Paleocene/Eocene boundary. At Shatsky Rise, the onset of the PETM is marked by the abrupt onset of the negative carbon isotope excursion (CIE) which lies just below the sharp lithologic contact between more carbonate-rich ooze overlying clay-rich ooze. In the upper part of the PETM interval, the clay-rich ooze gradually becomes more carbonate rich and carbon isotope values gradually increase. A high-resolution centimeter-scale quantitative analysis of the planktonic foraminiferal assemblages was performed in order to provide a high resolution bio-chemostratigraphic correlation across the Paleocene/Eocene boundary and to document the composition of the planktonic foraminiferal assemblages. Quantitative analyses well document the similarity in composition of the planktonic foraminiferal assemblages between the two sites. In general, Morozovella dominates the assemblages and its maximum relative abundance is coincident with the carbon isotope excursion. Subbotinids show an opposite trend and are absent in the interval of maximum abundance of Morozovella. A decrease in abundance of about 10% in average is observed within the Acarinina group from the base to the top of the studied sections. The excursion taxa first appear at the CIE and rich their maxima abundance (10%) at the beginning of the morozovellids decline. A minimum in species diversity values is observed in the carbonate-rich ooze below the CIE. This diversity minimum is followed by a general increase in the number of species, reflecting the progressive appearance of igorinids and globanomalinids, and of the excursion taxa. The maximum diameter of largest specimens of surface-dwelling planktonic foraminifera (Morozovella spp. and Acarinina soldadoensis), and deep-dwelling planktonic foraminifera (Subbotina spp.) were measured in each sample. During the warming event the test size of deep-dwelling planktonic foraminifera, Subbotina, has slight minimum, whereas that of the surface-dwelling Morozovella shows a maximum. The other surface-dwelling planktonic foraminifer, A. soldadoensis, shows no significant change in size throughout the studied interval. Within the morozovellids several specimens of M. velascoensis show diameter close to 800 μm in the interval containing the negative carbon isotope excursion. The anomalous planktonic foraminiferal test size is discussed and correlated with planktonic foraminiferal distribution and abundances, benthic foraminifera tests size, lithology, calcareous plankton and benthic events, and with the negative carbon isotope excursion.