Description: We present an SiF4 separation line, coupled to a laser fluorination system, which allows for an efficient combined silica d18O and d30Si analysis (50 min per sample). The required sample weight of 1.5-2.0 mg allows for high-resolution isotope studies on biogenic opal. Besides analytical tests, the new instrumentation set-up was used to analyse two marine diatom fractions (〉63 µm, 10-20 µm) with different diatom species compositions extracted from a Bølling/Allerød-Holocene core section [MD01-2416, North-West (NW) Pacific] to evaluate the palaeoceanographic significance of the diatom isotopic signals and to address isotopic effects related to contamination and species-related isotope effects (vital and environmental effects). While d30Si offsets between the two fractions were not discernible, supporting the absence of species-related silicon isotope effects, systematic offsets occur between the d18O records. Although small, these offsets point to species-related isotope effects, as bias by contamination can be discarded. The new records strengthen the palaeoceanographic history during the last deglaciation in the NW Pacific characterized by a sequence of events with varying surface water structure and biological productivity. With such palaeoceanographic evolution it becomes unlikely that the observed systematic d18O offsets signal seasonal temperature variability. This calls for reconsideration of vital effects, generally excluded to affect d18O measurements.

Description: Sea surface temperatures and sea-ice extent are the most critical variables to evaluate the Southern Ocean paleoceanographic evolution in relation to the development of the global carbon cycle, atmospheric CO2 variability and ocean-atmosphere circulation. In contrast to the Atlantic and the Indian sectors, the Pacific sector of the Southern Ocean has been insufficiently investigated so far. To cover this gap of information we present diatom-based estimates of summer sea surface temperature (SSST) and winter sea-ice concentration (WSI) from 17 sites in the polar South Pacific to study the Last Glacial Maximum (LGM) at the EPILOG time slice (19,000-23,000 cal. years BP). Applied statistical methods are the Imbrie and Kipp Method (IKM) and the Modern Analog Technique (MAT) to estimate temperature and sea-ice concentration, respectively. Our data display a distinct LGM east-west differentiation in SSST and WSI with steeper latitudinal temperature gradients and a winter sea-ice edge located consistently north of the Pacific-Antarctic Ridge in the Ross sea sector. In the eastern sector of our study area, which is governed by the Amundsen Abyssal Plain, the estimates yield weaker latitudinal SSST gradients together with a variable extended winter sea-ice field. In this sector, sea-ice extent may have reached sporadically the area of the present Subantarctic Front at its maximum LGM expansion. This pattern points to topographic forcing as major controller of the frontal system location and sea-ice extent in the western Pacific sector whereas atmospheric conditions like the Southern Annular Mode and the ENSO affected the oceanographic conditions in the eastern Pacific sector. Although it is difficult to depict the location and the physical nature of frontal systems separating the glacial Southern Ocean water masses into different zones, we found a distinct temperature gradient in latitudes straddled by the modern Southern Subtropical Front. Considering that the glacial temperatures north of this zone are similar to the modern, we suggest that this represents the Glacial Southern Subtropical Front (GSSTF), which delimits the zone of strongest glacial SSST cooling (〉4K) to its North. The southern boundary of the zone of maximum cooling is close to the glacial 4°C isotherm. This isotherm, which is in the range of SSST at the modern Antarctic Polar Front (APF), represents a circum-Antarctic feature and marks the northern edge of the glacial Antarctic Circumpolar Current (ACC). We also assume that a glacial front was established at the northern average winter sea ice edge, comparable with the modern Southern Antarctic Circumpolar Current Front (SACCF). During the glacial, this front would be located in the area of the modern APF. The northward deflection of colder than modern surface waters along the South American continent leads to a significant cooling of the glacial Humboldt Current surface waters (4-8K), which affects the temperature regimes as far north as into tropical latitudes. The glacial reduction of ACC temperatures may also result in the significant cooling in the Atlantic and Indian Southern Ocean, thus may enhance thermal differentiation of the Southern Ocean and Antarctic continental cooling. Comparison with temperature and sea ice simulations for the last glacial based on numerical simulations show that the majority of modern models overestimate summer and winter sea ice cover and that there exists few models that reproduce our temperature data rather well.

Description: Reduced surface-deep ocean exchange and enhanced nutrient consumption by phytoplankton in the Southern Ocean have been linked to lower glacial atmospheric CO2. However, identification of the biological and physical conditions involved and the related processes remains incomplete. Here we specify Southern Ocean surface-subsurface contrasts using a new tool, the combined oxygen and silicon isotope measurement of diatom and radiolarian opal, in combination with numerical simulations. Our data do not indicate a permanent glacial halocline related to melt water from icebergs. Corroborated by numerical simulations, we find that glacial surface stratification was variable and linked to seasonal sea-ice changes. During glacial spring-summer, the mixed layer was relatively shallow, while deeper mixing occurred during fall-winter, allowing for surface-ocean refueling with nutrients from the deep reservoir, which was potentially richer in nutrients than today. This generated specific carbon and opal export regimes turning the glacial seasonal sea-ice zone into a carbon sink.

Description: Dust deposition in the Southern Ocean constitutes a critical modulator of past global climate variability, but how it has varied temporally and geographically is underdetermined. Here, we present data sets of glacial-interglacial dust-supply cycles from the largest Southern Ocean sector, the polar South Pacific, indicating three times higher dust deposition during glacial periods than during interglacials for the past million years. Although the most likely dust source for the South Pacific is Australia and New Zealand, the glacial-interglacial pattern and timing of lithogenic sediment deposition is similar to dust records from Antarctica and the South Atlantic dominated by Patagonian sources. These similarities imply large-scale common climate forcings such as latitudinal shifts of the southern westerlies and regionally enhanced glaciogenic dust mobilization in New Zealand and Patagonia.

Description: 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.

Description: During the last glacial termination, the upper North Pacific Ocean underwent dramatic and rapid changes in oxygenation that lead to the transient intensification of oxygen minimum zones (OMZs), recorded by the widespread occurrence of laminated sediments on circum-Pacific continental margins. We present a new laminated sediment record from the mid-depth (1100 m) northern Bering Sea margin that provides insight into these deglacial OMZ maxima with exceptional, decadal-scale detail. Combined ultrahigh-resolution micro-X-ray-fluorescence (micro-XRF) data and sediment facies analysis of laminae reveal an alternation between predominantly terrigenous and diatom-dominated opal sedimentation. The diatomaceous laminae are interpreted to represent spring/summer productivity events related to the retreating sea ice margin.We identified five laminated sections in the deglacial part of our site. Lamina counts were carried out on these sections and correlated with the Bølling–Allerød and Preboreal phases in the North Greenland Ice Core (NGRIP) oxygen isotope record, indicating an annual deposition of individual lamina couplets (varves). The observed rapid decadal intensifications of anoxia, in particular within the Bølling–Allerød, are tightly coupled to short-term warm events through increases in regional export production. This dependence of laminae formation on warmer temperatures is underlined by a correlation with published Bering Sea sea surface temperature records and d18O data of planktic foraminifera from the Gulf of Alaska. The rapidity of the observed changes strongly implies a close atmospheric teleconnection between North Pacific and North Atlantic regions.We suggest that concomitant increases in export production and subsequent remineralization of organic matter in the Bering Sea, in combination with oxygen-poor waters entering the Being Sea, drove down oxygen concentrations to values below 0.1ml/l and caused laminae preservation. Calculated benthic–planktic ventilation ages show no significant variations throughout the last deglaciation, indicating that changes in formation rates or differing sources of North Pacific mid-depth waters are not prime candidates for strengthening the OMZ at our site. The age models established by our correlation procedure allow for the determination of calendar age control points for the Bølling–Allerød and the Preboreal that are independent of the initial radiocarbon-based chronology. Resulting surface reservoir ages range within 730–990 yr during the Bølling–Allerød, 800–1100 yr in the Younger Dryas, and 765–775 yr for the Preboreal.