Late Quaternary paleoceanography of the Southern Ocean.

摘要

The focus of this thesis is the reconstruction of the sedimentary record of changes in Southern Ocean circulation and biological production preserved in the composition and chemistry of fossil biota and biogenic sediments. The aim of this work has been to relate ideas about the sensitivity of climate change, and carbon cycling in particular, to biogeochemical and physical processes operating in the Southern Ocean.; Three key results emerge: (1) Southern Ocean polar and subpolar planktonic biota and ice-rafted debris, shifted equatorward during glacial stages. These changes indicate an equatorward shift of the corresponding water masses and of the zero point of the wind stress curl, which controls gyre boundary positions in the modern ocean. (2) Southern Ocean deep-water carbonate preservation and surface-water carbonate production decreased during glacial stages. Enhanced glacial carbonate dissolution is consistent with carbonate-mediated increases in deep-ocean alkalinity, by which some models explain lowered atmospheric pCO{dollar}sb2{dollar} during glacial stages. The implied lower carbonate ion concentration during glacial stages places constraints on the amplitude and rapidity of Antarctic deep-water nutrient changes. Reduced carbonate production rates similarly suggest higher alkalinity and lower pCO{dollar}sb2{dollar}. (3) Vertical carbon isotope gradients between subantarctic surface waters and circumpolar deep waters were enhanced during glacial stages. This vertical difference indicates the extent of biologically-mediated removal of dissolved carbon, relative to that being delivered to the surface by upwelling deep waters. The glacial increases in {dollar}Deltadeltasp{lcub}13{rcub}{dollar}C are consistent with increases in vertical dissolved inorganic carbon gradients. The implications of the above changes in planktonic and benthic {dollar}deltasp{lcub}13{rcub}{dollar}C, SST, and carbonate accumulation and preservation for past changes in pCO{dollar}sb2{dollar} in Southern Ocean surface waters are explored. Implied changes in pCO{dollar}sb2{dollar} in subantarctic waters have the timing and amplitude necessary to explain most glacial-interglacial variance in atmospheric CO{dollar}sb2{dollar} indicated by ice cores over the past 150,000 years, are highly coherent with and precede changes in ice volume in the primary Milankovitch frequency bands over the past 500,000 years, thus strengthening the role of CO{dollar}sb2{dollar} as a critical paleoclimatic feedback mechanism in the climate response to orbital variations.