ALBERT

Description: Climate variability of the late Quaternary, especially the Last Glacial (LG) to the Holocene, has become the most heated topic for the recent decades, which helps to better understand the shape of current and future climate on our planet. The long term glacial/interglacial changes have been associated to insolation changes controlled by earth’s orbit, whereas the millennial scale variations are mostly accepted to be modulated by the “bipolar seesaw” mechanism which redistributes heat between the northern and southern hemispheres through the Atlantic Meridional Overturning Circulation (AMOC). The validation of such hypothesis is hampered by the very limited high resolution records from the high latitudes Southern Ocean. Situated at the southern end of the AMOC, Southern Ocean Atlantic sector represents one of the key regions for understanding the global climate change. The warm and cold water routes (WWR, south of Africa and CWR, Drake Passage/Scotia Sea) connect the South Atlantic to South Indian and Pacific Oceans, respectively; and the Weddell Gyre connects the open ocean South Atlantic to the Western Antarctic Shelf Ice (WASI), where nowadays the cold surface water and Antarctic Bottom Water (AABW) are generated beneath. These water masses represent the most important constituents of the AMOC in the Southern Hemisphere. This PhD project generated a series of new diatom based high resolution marine records covering wide area of the high latitudes South Atlantic, including from the Bouvet Island area and the Scotia Sea, aimed to provide new insights of the response and drive in Southern Ocean in the context of late Quaternary global climate change. With focusing on the LG to Holocene time period, by integration of our new generated and other existing records from the Southern Ocean Atlantic and Western Indian sectors, a detailed regional age model for the past 30 kyrs is established by AMS 14C dating and regional core correlation, which can be a template for further paleoenvironment reconstructions in this area. Our reconstructions suggest 2-3_C cooling in the LG south of the modern Polar Front compared to modern conditions. Winter sea ice in the Bouvet Island area expanded by 5_ latitude, the more expanded sea ice field resulted in the stronger tropical Atlantic cooling than other tropical oceans by the reduction of warm water to the South Atlantic via the WWR, and intensive export of carbon to the deep Southern Ocean. The two steps of deglacial warming are mainly concurrent with the Heinrich stadial 1 and Younger Dryas cooling in the northern hemisphere which support the bipolar seesaw from the marine archives. The North Atlantic Deep Water (NADW) provided the second source of warming from below to the high latitudes South Atlantic which extended the first step warming till 14 cal. ka (kyr BP). The temporal cooling at ca. 14-12 cal. ka is possibly caused by the melt water input from the Antarctica. Our records support the Southern Ocean control of the deglacial atmospheric CO2 rise. The early Holocene optimum marks the warmest period and the strongest cold water reduction and confined at ca. 11-10 cal. ka in the southern cores in the study area. Sea ice probably retreated south of modern conditions and maximum opal deposition southward expanded to at least 55_S. The mid-late Holocene cooling in the study area may be related to the cold water expansion from the Weddell Gyre with the developing cavity under the WASI. The Holocene climate development may have also interplayed with the NADW which represents a rapid resumption at the early Holocene and slight decline during the mid-late Holocene. The high resolution Holocene record from the central Scotia Sea suggests a stable climate at the core site. The early Holocene high productivity is maintained by the enhanced upwelling at that time. The Holocene reservoir change at the core site is evidenced, which is in agreement with the Southern II Abstract Ocean ventilation history. The pre-Holocene release of CO2 from the Southern Ocean strongly lowered the d14Catm which caused the low surface ocean reservoir at the early Holocene at the core site. The centennial scale climate variability may be mainly linked to solar activity and also influenced by the sea ice induced freshwater variability. Stratigraphis of long term records from the Scotia Sea covering the past 300 kyrs were established by a combination of radiocarbon chronology, correlation of magnetic susceptibility (MS) to Antarctic ice core dust/climate records, diatom biofluctuation stratigraphy, and geomagnetic chronology. Good consistency of the age models by these proxies improves the reliability of our stratigraphies and indicates the applicability of these approaches. However, detailed investigation show that, the radiocarbon chronology can be affected by the changes in carbon reservoir and fossil carbon contamination in the study area; the abundance pattern of diatom species Eucampia antarctica can be used to identify the past 6 marine isotope stages (MIS) while the fluctuation weakens during MIS 7 and 8; the reliability of geomagnetic chronology weakens at low sedimentation rates conditions. The MS-Antarctic ice core correlations represent high efficiency and the best reliability due to the high sensitivity to the changes of surrounding source regions and current systems, which is closely related to the climate changes in the Southern Ocean. In addition, a possible correlation between the ash layers found in our Scotia Sea cores and the Antarctic ice cores is established, which can be used as additional age markers for further studies in this area.

Description: Enhanced glacier and polar ice sheet melting during the last decades is one of the major focuses of geosciences. The understanding of the effects of future global warming is important due to raising sea level. Antarctic ice masses play a key role in global climate. Melting of Antarctic ice would result in a sea level rise of about ~3-6 m due to the West Antarctic Ice Sheet and 60 m due to the East Antarctic Ice Sheet. Because of its marine-based nature, especially the West Antarctic Ice Sheet is sensitive to rising temperatures. Additionally, the Ross Sea, as a part of the west Antarctic system, and its shelf are a key region stabilizing the west Antarctic ice masses. It is thus essential to understand the processes and changes in this area in order to interpret the past and predict the future climate developments. Sedimentary archives are a unique opportunity to get insights into past climate variability and the ice response due to increased temperature (~3°C), as the earth had experienced in the last 14 Ma since the mid Miocene cooling. The multi-national drilling program ANDRILL (ANtarctic geological DRILLing, McMurdo Ice Shelf project, MIS) focuses on the changes of climatic influences on the West Antarctic Ice Sheet during the past ~14 Ma. During austral summer 2006/07, an approximately 1300 m long sedimentary succession beneath the northwestern Ross Ice Shelf was cored. In this study geochemical investigations were carried out and interpreted using a multiproxy approach. The results of major element measurements from X-ray fluorescence on discrete samples and high-resolution non-destructive XRF analyses on split-cores, using an XRF core scanner, total organic carbon, total inorganic carbon, opal, and mineral data, as well as optical microscope and visual colour reflectance investigations were used to reveal different processes controlling the depositional environment in the southern Ross Sea. Provenance analyses reveal three main sources for fine-grained terrigenous sediments at the MIS site. First, close-by McMurdo Group volcanoes (MVG) are a source for sedimentary deposits of AND-1B (mainly diamictites) during times of extended glaciations. Second, in interglacial periods, sediment composition (mainly mudstones) is controlled by southern Transantarctic Mountains (S TAM) and finally a geochemical mixture of both sources is visible in the record, which also can be indicative for a western Transantarctic Mountain source (W TAM). According to sediment architecture (McKay, 2008) and source, different transport mechanisms are existing. MVG sediments are mainly transported by subglacial erosion, whereas S TAM and W TAM sediments represent ice proximal to distal conditions with transport processes such as fluvial meltwater and gravity flows. The entire AND-1B core can be divided into 5 major sections of provenance. These geochemical facies (GCF) represent colder phases during Late Miocene and Late Pleistocene (GCF1, dominated by MVG); warm intervals during early Pliocene and late Pliocene are dominated by input from southern provenances (GCF 2), and oscillations between material from MVG and W TAM dominate the mid Pliocene (GCF 3). In sedimentary successions indicating glacial terminations and extended sea ice conditions dolomite, Fe-dolomite siderite and other traces of carbonate minerals were found. Micritic crystal size and distribution within the sediment matrix reveal an authigenic, early diagenetic precipitation. These precipitates form in concert with freezing processes that result in a hyper-saline brine formation, saturating pore waters with respect to dolomite phases. Within the layers of hyper-saline brines, dolomite or unstable carbonate phases occur that are later on recrystallized to dolomite. An additional factor for dolomite formation is sulphate reduction, indicated by pyrite and biomarkers of sulphate reducing bacteria. The features of the AND-1B dolomite are likely to be transferable to global cap-carbonates of Precambrian times and can be likely an explanation for dolomite formation due to rapid deglaciations at glacial terminations. During open water conditions in the southern Ross Sea, when no West Antarctic Ice Sheet had existed (Naish et al., 2009), mass accumulation and paleoproductivity estimations from the AND-1B record reveal the Ross Sea as a high production area during Pliocene and Pleistocene. Opal accumulation is in the same ranges as for modern analogues in front of the present Ross Ice Shelf edge and even higher during specific times in the Pliocene. In contrast, paleoproduction and organic carbon accumulation rates are low and reveal a decoupling between organic carbon and opal preservation, where opal evidences good preservation efficiency while organic carbon shows low preservation rates. These results suggest that the Ross Shelf was an oxic environment with strong organic carbon degradation during the Pliocene. Nevertheless, accumulation of organic carbon was high enough to make the Ross Shelf a major sink for organic carbon. In a global comparison, the Ross Sea area removed more carbon from the atmospheric CO2 cycle as some highly productive up-welling systems do.