Lead


Lay summary
Abrupt climate shifts occurring during the last ice age were first recognized in the temperature record deduced from Greenland ice cores and have subsequently been observed in various paleoclimate records from all over the world. Nevertheless, their underlying cause(s) remain a matter of debate. This project investigated sedimentary sequences from the Pacific, focusing on the time interval when abrupt climate changes were the most prominent. The equatorial Pacific plays a key role in the present-day global climate and has been suggested as the possible starting place of past climate shifts. The eastern equatorial Pacific was chosen because it is a very dynamic region. It is influenced by physical/chemical exchanges between the ocean and atmosphere and by important vertical and horizontal movements of cold waters, coming both from the deep and from the Polar regions. The principal objective was to reconstruct the dynamics of the tropical Pacific during these abrupt climate changes, and to determine whether past changes at the sea surface are caused primarily by variations in the atmospheric or oceanic circulation. I investigated 6 cores along a N-S transect. In each core, I used a variety of sedimentary proxies, i.e. measurable descriptors of an environmental parameter, to reconstruct past temperatures, salinities, chemical composition and productivity of surface waters. Alkenone-based sea surface temperature reconstructions reveal a strenghtening of the eastern Pacific Cold Tongue during the Last Glacial Maximum. Changes in glacial surface hydrography computed from planktonic foraminiferal oxygen isotopes and temperature estimates suggest a southward shift of the Intertropical Convergence Zone. On a shorter timescale, millennial-scale cooling events seem to reflect fast readjustments of surface waters to Northern Hemisphere climatic conditions.In addition to these surface forcings, the EEP is also significantly affected by subsurface oceanic circulation. This project confirms the strong influence exerted by the Southern Ocean on the biogeochemistry of the EEP. Records of bulk sediment nitrogen isotopes, opal and organic carbon content suggest that millennial-scale variations in Southern Ocean overturning rates modulated the delivery of nutrients and oxygen to the EEP throughout the last glacial stage. Finally, independent opal export and diatom productivity proxies suggest that the sedimentary opal record reflects both production and preservation events, and that opal burial in the eastern Pacific is controlled both by the physiological response of diatoms to low-latitude Fe inputs and by the supply of silicic acid from the high southern latitude.