On the role of the southern ocean in plio-pleistocene climate evolutionorbital-scale variability of sea surface temperature, dust, iron deposition, and marine export production in the subantarctic atlantic over the last 3,650,000 years

Resumen

This PhD Thesis is focused on the reconstruction of past oceanographic variability in the Southern Ocean (SO) since the mid-Pliocene. The principal aim of this work is to asses the role of several climatic processed operating within this region in modulating glacial/interglacial (G/IG) atmospheric CO2 variations during the Pleistocene, and to asses its orbital-scale evolution through the Plio-Pleistocene transition. The study is based on the analysis of several organic and inorganic paleoclimatic tracers in a deep sea sedimentary sequence (PS2489-2/ODP1090) from Subantarctic Atlantic (SAA). In this sense, we have generated a series of millennial-scale resolution paleoclimatic records of sea surface temperatures (SST), dust, iron deposition and marine export production spanning the last 3,650,000 years. We have shown that iron fertilization has been a recurrent process operating in the SAA during the G/IG cycles of the last million of years. However, our data indicate that the increase in export production could only explain a fraction of the glacial atmospheric CO2 decrease (i.e. up to 40-50 ppmv), while the initial glacial CO2 decrease, as well as the change in the amplitude of the CO2 cycles observed around 400,000 years, was most likely driven by physical processes. The extension of the SST record back to the Pliocene reveals that the SAA temperatures where paced by Southern Hemisphere (SH) insolation forcing at the precession band during the late-Pliocene and early-Pleistocene (LP/EP). This suggest that SAA may have been sensitive to low latitude insolation forcing during the warm Pliocene period, and that Antarctic ice sheet dynamics during the LP/EP may have been more sensitive to changes in local insolation forcing than previously thought. The Plio-Pleistocene dust record has shown a 4-fold increase in the supply of dust to the SO that is closely related to the increase in the meridional temperature gradients and the subsequent intensification of the atmospheric circulation cells since the Pliocene period. These data reveal an increase in the phase lead of dust over global ice volume since 900,000 years ago, which together with the increased dust flux, suggest a more significant role for dust as an active agent of G/IG climate variability during this time period than during the LP/EP. Finally, the data presented here provide evidence of an apparent decoupling between iron supply and marine export production during the LP/EP. We hypothesise that this change in the response of marine phytoplankton to iron deposition may be related to a decrease in the supply of macronutrients (N and P) to the SAA during this interval of warm climate. Thus, the results presented in this PhD Thesis contribute to shed light on the orbital scale climate variability in the SO during the Plio-Pleistocene transition, and suggest an active role for several climatic processes operating within this region in the Plio-Pleistocene evolution of Earth's climate.