Surface ocean circulation and organic carbon export across the mid-Pleistocene climate transition

Abstract

The mid-Pleistocene climate transition (MPT) marks a change in the nature of the climate system response to external forcing by insolation. This is represented first by the expansion of the northern hemisphere ice-sheets ca. 920 ka, and the emergence and dominance of glacial-interglacial oscillations with a 100-kyr period by 640 ka. The principal aim of this thesis is to understand the role of surface ocean circulation systems in the Atlantic and tropical Pacific and the strength of the biological carbon pump as drivers of the MPT. These two systems are examined as they exert important influences over global climate change through their impacts on heat and moisture transports, and carbon cycling and atmospheric CO2 concentrations. The robustness of the alkenone-derived U(^K)(_37') index for reconstructing past sea-surface temperatures (SSTs) during the mid-Pleistocene is confirmed. It is shown that the index is insensitive to changes in the alkenone source organisms, the Prymnesiophytes, has minimal impact on the relationship between and SST. Reconstructions of SSTs from the mid-high latitudes of the Atlantic Ocean and the tropical Pacific Ocean between 1500-500 ka reveal that a synchronous and global drop in surface ocean temperatures of more than 1oC occurred ca.1145 ka (from Marine Isotope Stage, MIS, 34). The global cooling is coincidental with a shift toward a 'La Nina'-state of atmospheric circulation within the tropical Pacific. By analogy with the modem climate system, this intensification of atmospheric circulation in the tropical Pacific would have reduced the flux of heat to the high northern latitudes. It is proposed that this development of a cooler global climate system eventually drove the mid-Pleistocene expansion of the northern hemisphere ice-sheets. However, the global cooling was accompanied by asynchronous expansion of sea-ice cover in the North Atlantic and North Pacific that reached its maximum extent ca. 990 ka. It is proposed that this initially limited the supply of moisture to the ice-sheet source regions. Thus, ice-sheet response lags the onset of the global cooling by nearly 300-kyr, occurring between 920 and 880 ka (MIS 22). The driving mechanism for the global cooling remains unclear, but may be related to a long-term decline in deep-water temperatures. It is accompanied by an increase in the export of organic carbon to the ocean floor at a global scale. This is interpreted to reflect a strengthening of the biological carbon pump, which may have maintained or even driven the cooling trend by drawing down atmospheric CO2 concentrations.