Abstract:
Geologic records of past global warming events are garnering increased attention as scientists strive to constrain the potential consequences of human activities. Of particular interest, is an ancient (ca. 55 Ma) global warming event referred to as the Paleocene-Eocene Thermal Maximum (PETM). In deep-sea cores, hallmarks of the PETM are: (1) a negative carbon isotope excursion (CIE) on the order of approximately 3 per mil, (2) intensified carbonate dissolution, and (3) an extinction event that eliminated nearly half of all deep-sea benthic foraminiferal species. Collectively, these lines of evidence indicate that a rapid release (<10 kyr) of vast quantities of isotopically light carbon into the exogenic carbon cycle that exacerbated climatic warming and acidified the global ocean. Although the cause of the PETM and the source of the liberated carbon are still debated, models invoking the thermal destabilization of marine sedimentary gas hydrates have received the most attention. This scenarid dictates that oceanic warmth preceded the carbon release. To test this hypothesis, we have generated a high-resolution stable isotope record across a PETM record (ODP Site 738) recovered from the Kerguelen Plateau in the Indian sector of the Southern Ocean. The resulting chemostratigraphies differ from conventional stable isotope records in that most isotopic analyses were performed on single, individual foraminiferal shells. This approach permitted the distinction of "non-excursion" shells with background delta (super 13) C values from "excursion" shells with CIE delta (super 13) C values, a distinction that enabled us to gauge the degree to which the Site 738 PETM record was altered by sediment-mixing processes. Although the stratigraphy of this PETM section is punctuated by hiatuses, our research has generated a high-resolution profile of the isotopic changes in delta (super 13) C and delta (super 18) O among the surface, thermocline, and benthic dwelling foraminifera across the PETM interval. This record indicates that the surface ocean did indeed warm prior to the CIE at ODP Site 738. In fact, the warming was significant enough to have fostered the migration of warm-water planktonic species from the subtropics into this polar region prior to the CIE. Pre-excursion delta (super 13) C values of subtropical species at any high-latitude locality have not been previously documented. Significant warming prior to the CIE and the subsequent oceanographic changes associated with dramatic climate shifts provide the most plausible mechanism for the destabilization of methane hydrates from the sea floor. Additionally, low-resolution analysis of genus level changes within planktic foraminiferal assemblages at Site 738 provides insight into the response of the calcareous microplankton to the changing environmental conditions in the Indian Sector of the Southern Ocean. Comparison of the events at Site 738 and the reference Site 690, in the Southern Atlantic Ocean, 1) has elucidated likely hiatuses within the record at Site 738, 2) highlighted apparent differences in local sedimentation rates between the two Southern Ocean cores, 3) documented the isotopic and faunal events that transcend local effects and represent dramatic global ocean events during the Paleocene-Eocene transition, and 4) provided the first conclusive evidence of a significant climate and oceanographic shift prior to the CIE.