Late Quaternary paleoceanographic history of the Pacific sector of the Southern Ocean based on diatom evidence

Ralf.Tiedemann [ at ]


The last transition from a full glacial to a full interglacial state is of special importance to investigate processes that control the Earth’s climate evolution. Out of phase interhemispheric climate variability over the last deglaciation has been associated with orbital induced insolation changes as well as with the “bipolar seesaw”, hence related to changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC). The Southern Ocean (SO) as only water territory connecting the Pacific, Atlantic and Indian Ocean, plays a crucial role as southern limb of the AMOC in propagating signals within its basins and into the different world oceans. The Antarctic Circumpolar Current (ACC), steered by the strong Southern Westerly Winds (SWW), redistributes heat, salt and nutrients via wind-driven upwelling and thus has the high potential of regulating atmospheric CO2 concentration via the biological pump as well as surface and deep-water ventilation. Sea surface temperature and sea-ice extent are important surface water parameters related to the oceanic frontal and current systems as well as to water mass formation via brine release and bioproductivity changes. Despite numerous marine studies from the Pacific sector of the SO, the (sub)antarctic realm is still underrepresented in paleoceanographic research. This thesis examines the environmental changes of the last 30,000 years (30 kyr) in the Pacific sector of the SO using diatom-based transfer function estimates of summer sea surface temperature (SSST) and winter sea-ice (WSI) concentrations reconstructed from 17 selected sediment cores. Including available sea surface temperatures and sea-ice records from the Pacific sector the thesis objectives are primarily a basin and circum-Antarctic wide comprehension of last glacial, deglacial and Holocene climate variability with respect to forcing mechanisms, lead-lag conditions and ice-ocean-atmosphere-ocean feedbacks. The first manuscript deals with the reconstruction of temperature and sea-ice signals during the Last Glacial Maximum (LGM; 19-23 kyr before present, BP) in the Pacific sector of the SO using new diatom data from a total of 17 cores. Consistent with estimates from previous studies, the Pacific sector shows a distinct basin-wide cooling with a temperature decrease of ≥4 K in the present Subantarctic Zone. Most prominent is an E-W gradient concerning the cold-water expansion and the maximum extent of winter sea ice (WSI) that results from strong topographic forcing also steering the frontal system. Hence, the frontal system was characterized by colder SSSTs Atmospheric forcing mechanisms such as the SWW and the El Niño Southern Oscillation (ENSO) are proposed to amplify the E-W gradient and to have a high regional impact in the Pacific sector. Regarding the average latitudinal expansion of the cold-water realm and the WSI extent in the different SO sectors, a coherent and uniform circum-Antarctic picture of the LGM time slice arises. The second manuscript carefully examines the deglacial history of the Pacific sector of the SO, based in the same set of sediment cores. A major outcome is a decoupling of the eastern Pacific sector to its western counterpart and the other SO sectors, which has not been shown before. An early deglacial warming around 22 kyr BP observed in the eastern sector is in close agreement with a warming in the adjacent West Antarctic Ice Sheet (WAIS), and is most likely related to the rising Southern Hemisphere insolation. Hence, the synchronous CO2 rise recorded in East and West Antarctica might have been triggered by the shutdown of the AMOC, rapid sea-ice retreat due to intense warming and strong upwelling due to strengthened SWW. Over the course of the deglaciation, the Pacific records show the common “Antarctic timing” consisting of increasing temperatures until the Antarctic Holocene Optimum (AHO; ~12-9 kyr BP) only interrupted by the Antarctic Cold Reversal (ACR; ~14.5-12.5 kyr BP). A sole contribution of the WAIS to meltwater pulse 1 A (14,400 yr BP) can not be ascertained as less cooling occurred during the ACR than expected by model simulations. The Holocene climate in the SO is of special importance in deciphering small-scale changes induced by atmospheric forcings, which allos to infer possible present climate changes. The sediment cores, presented in the third manuscript, are relatively high resolved for SO sediments (8-34 cm/kyr) and were retrieved in the western Pacific’s Antarctic Zone. The SSST and WSI estimates show a Mid-Holocene cooling which corroborates results from model simulations of freshwater shedding from the rapid WAIS retreat. This sea surface cooling, most likely originating in the Pacific sector is propagated via the “cold water route” into the other SO sectors. The variability of warm and cold periods during the Mid- and Late Holocene reveals a strong dependence to regional influencing factors such as the close vicinity to the sea ice edge as well as to the atmospheric shift of a SWW-to a ENSO governed climate state. In summary, this thesis provides for the first time SSST and winter sea-ice estimates in the Pacific sector of the SO on a wide spatial range and for time slices whose pale oceanographic history is crucial for the understanding of global climate change. The investigated environmental parameters point to the sensitivity of this SO sector, concerning the drainage of the WAIS and the impact of atmospheric changes, that has the high potential of triggering climate change.

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Benz, V. (2016): Late Quaternary paleoceanographic history of the Pacific sector of the Southern Ocean based on diatom evidence , PhD thesis, Universität Bremen, Fachbereich 5, Geowissenschaften.

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