Glacial-interglacial oceanic changes in the central Pacific sector of the Southern Ocean during the past 500 ka

Ralf.Tiedemann [ at ]


Earth’s climate has undergone repeated, dramatic changes between glacial and interglacial states troughout the past 500 ka years with dominant 100 ka cycles. Global mean temperatures during glacials were significantly lower than during interglacials, rising and falling in close correspondence with atmospheric CO2 concentrations. To understand underlying climate mechanisms that operated throughout the past is an important topic of climate research, particularly as this provides information to better constrain predictions of future climate change. In this regard, changes in the Southern Ocean, both in physical ocean circulation and biological productivity, are commonly thought to have played a critical role in regulating the ocean-atmosphere CO2 balance. However, paleoceanographic data from this oceanic region is sparse, thus rendering much of the understanding of the underlying processes incomplete, particularly for the central parts of the Pacific sector of the Southern Ocean. This thesis reconstructs interglacial-glacial scale oceanographic changes in the high latitude (~55°S) central Pacific sector of the Southern Ocean with the aim to further improve the current understanding of climate dynamics. For this purpose, a number of proxy data were generated from marine sediment cores that were collected during R/V Polarstern cruise ANT-XXVI/2. The main outcome of this study is compiled in three manuscripts (Capture 4, 5 and 6) as follows: 1) Based on benthic foraminiferal δ13C data, past Circumpolar Deep Water δ13C compositional changes were reconstructed in the Southern Ocean Pacific sector, which displayed high glacialinterglacial amplitude fluctuations over the last 500 ka. When compared to reconstructions from the Atlantic sector, the results imply that a common Circumpolar Deep Water δ13C composition was maintained throughout the last four glacial-interglacial cycles, thus evolving parallel in both basins. This finding modifies the current LGM picture on the deep water exchange between Atlantic and Pacific sector. In turn, new implications arise regarding the interpretation of past Antarctic Bottom Water lateral distribution and formation in the Atlantic Sector, and on past CO2 storage in the deep Atlantic interior. 2) Based on planktonic foraminiferal Mg/Ca and δ18O, the surface water temperature and δ18O evolution was reconstructed for the central Pacific sector over the past 500 ka. Sea surface temperatures in this open marine, high latitude (~55°S) setting closely co-evolved with Antarctic air temperatures, whereas the glacial-interglacial changes were characterized by moderate amplitudes. By contrast, the ice volume-corrected water δ18O displayed no clear glacial-interglacial modulation. Overall, the results fit well into the current picture for the South Pacific, which supposes that surface oceanic changes in the eastern and western marginal areas were amplified by changes in South Pacific gyre circulation over glacial-interglacial cycles, thus underpinning a link of the gyre’s boundary current circulation with Earth’ s climate change.3) Using downcore carbonate contents in combination with planktonic foraminiferal δ13C, basic characteristics of carbonate sedimentation and surface carbonate productivity are outlined for the central Pacific sector with focus on the Marine Isotope Stage 11 interglacial. The sedimentation between the Subantarctic and Polar Front stands out during Marine Isotope Stage 11, when compared to other interglacials, by massive coccolith deposition as already documented for the Atlantic sector of the Southern Ocean. Contemporary changes in surface water δ13C of approximately 1 ‰ occurred, pointing toward a turnover to a coccolithophorid dominated phytoplankton productivity, assumedly on cost of silica consuming diatoms. The beginning and end of this Marine Isotope Stage 11 changes can be within a few thousand years precisely determined in the central Pacific sector. Under presumption that the dominance of coccolithophorides has resulted in an excess of silica in central Pacific Sector surface waters, inferences were drawn on the validly of the Silica Leakage Hypotheses from an interglacial perspective – a hypotheses which links glacial diatom morphology and/or community structure changes in the Southern Ocean, which conceivably have caused a higher equatorward export of silica via Subantarctic Mode Water, to a potential strengthening of the ‘biological pump’ in the equator upwelling systems and which may constitute an important mechanism contributing to the recurring atmospheric CO2 drawdown over glacial-interglacial cycles.

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Ullermann, J. (2015): Glacial-interglacial oceanic changes in the central Pacific sector of the Southern Ocean during the past 500 ka , PhD thesis, Universität Bremen, Fachbereich 5, Geowissenschaften.

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