Neogene development and dynamic of the circum-antarctic ocean frontal system
Final Report Abstract
Abrupt cooling steps and geochemical reorganization in the circum-antarctic realm characterize the middle to late Miocene Antarctic glaciation. To identify the driving mechanism for the develoment and dynamic of subantarctic frontal system we did a combined study of documenting the late Neogene sea-surface temperatures and salinites in high-resolution records of a suitable site in the subpolar South Atlantic, and modelling the development of the Antarctic circumpolar circulation at that time using a coupled atmosphere-ocean circulation model (ECHAM5/MPI-OM) focused on the Southern Ocean by using a horizontal curvilinear grid. Applying paired Mg/Ca ratios and oxygen isotope measurements in planktonic foraminifers, sea-surface temperatures and salinites have been reconstructed. Significant upper ocean cooling and freshening started as early as 14.2 Ma, some 300 ky before the onset of Antarctic continental ice growth as indicated in benthic foraminiferal δ18O increase. We attribute this switch in surface hydrography to the northward movement of the subantarctic front across Site 1092 position. These results mirror findings from the Tasman Rise in the Pacific sector, although upper ocean hydrographic changes are much more pronounced at Site 1092 owing to its more southerly position relative to the Southern Ocean frontal system. Surprisingly, the consecutive increase in Antarctic ice volume is then associated with a warming of the surface waters by up to 2.5°C. This effect has been addressed by the modelling experiments. A focus of the coupled modelling has first been put on the potential of ocean gateway changes to cause changes in Antarctic circumpolar circulation, especially the effect of the Drake Passage and the constriction of the eastern Tethys in combination with other climatic factors. However, none of these gateway changes significantly altered the subantarctic ocean circulation. In a consecutive project, Knorr and Lohmann (2014) added a varying Antarctic ice volume and height to the sensitivity experiments. With that modification, changes in the wind field associated with the growth of the ice sheet is seen to induce changes in ocean circulation, deep-water formation and sea-ice cover that result in a sea surface warming and deep-water cooling in large parts of the Atlantic and Indian ocean sectors of the Southern Ocean. These changes are interpreted as the dominant ocean surface response to a 100 ky phase of massive ice growth in Antarctica. A rise in global annual mean temperatures is also seen in response to increased Antarctic ice surface elevation. In contrast, the longer-term surface and deep-water temperature trends are dominated by changes in atmospheric CO2 concentration. It is therefore concluded that the climatic and oceanographic impacts of the Miocene expansion of the Antarctic ice sheet are governed by a complex interplay between wind field, ocean circulation and the sea-ice system.
Publications
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(2009) Influence of Southern Ocean frontal system on surface temperature and salinity in the Atlantic sector during the middle Miocene. EGU Vienna
Kuhnert, H.; Bickert, T.
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(2009) Southern Ocean frontal system changes precede Antarctic ice sheet growth during the middle Miocene. Earth and Planetary Science Letters, 284: 630-638
Kuhnert H., Bickert T., Paulsen H.
