Marine Isotope Stage 11 in the Pacific Southern Ocean; a coccolithosphore perspective
Final Report Abstract
This project provided new insights into thepalaeoproductivity of the late Pleistocene Marine Isotope Stage 11 (MIS 11; between 424 and 374 kyr) in the pacific sector of the Southern Ocean (SO). MIS 11 is a good analogue of what the Holocene would have been without anthropogenic interference. A multi-proxy coccolithophore dataset was generated on five sediment cores retrieved from different water depths (between 2984 and 4480 m) and within an area from 54.3 to 57.5 ºS and 102.7 to 157.2 ºW. The studied interval is characterized by highest CaCO3 content representing periods of massive coccolith ooze deposition. Coccolithophore assemblage counts from three of the cores (59-2, 76-2 and 79-2) indicated that the coccolith fraction (CF; <20m) was dominated by Gephyrocapsa caribbeanica and small Gephyrocapsa during MIS 11. A relative abundance increase of heavily calcified G. caribbeanica was noted during Termination V at 79-2 coincident with an increase in coccolith diversity. Numbers of coccoliths, CF Sr/Ca data and productivity (i.e., temperature-corrected CF Sr/Ca) records were consistent and showed a clear increase in coccolithophore productivity during Termination V, highest productivity throughout MIS 11 which decreased during late MIS 11-early MIS 10 in all the cores. We explain the high coccolithophore production during MIS 11 by changes in sea surface temperature and nutrient regimes, due to a re-organisation of the surface circulation patterns and a latitudinal migration of the frontal systems. Furthermore, the immense carbonate production of the coccolithophores may have contributed to increased atmospheric CO2 contents, causing a drawdown of the carbonate saturation and an increase in dissolution at the seafloor. However the atmospheric CO2 did not reach higher values probably due to the effective ballasting of organic matter by coccoliths. The coccolith carbonate mass flux estimates in sediment core 59-2 and the amount of CO2 outgassed due to coccolithophore calcification calculated for the SO during MIS 11 suggests a rather limited contribution of the coccolithophores to the atmospheric CO2 when compared to the global fossil fuel emissions. We consider that future studies should include G. caribbeanica morphometries and systematic coccolith mass estimates during Termination V in order to find out the origin of these anomalously thick specimens and to explain the existent discrepancies observed.
