Radiogenic isotopes on marine sediments from the Baffin Bay : implications for the sediment supply during the last deglaciation

Abstract

The Baffin Bay is a marginal sea of the Atlantic Ocean, a semi-enclosed basin, situated between Greenland, Baffin Island and the Canadian Arctic Archipelago (CAA). It is an important link in the Northern Hemisphere circulation and is connected to the Arctic Ocean and to the Labrador Sea. During the Last Glacial Maximum (LGM) the Baffin Bay was flanked by three North American Ice sheets: Laurentide, Innuitian and Greenland. Ice sheet dynamics is still poorly understood but highly valuable in the frame of recent Greenland ice sheet mass loss and recent climate change data. Understanding the mechanisms and interplay between ice sheets, ocean circulation and climate is a key to more accurate climate prediction models. Thus the deglaciation history of the Baffin Bay is a matter of interest for the research community and has been a topic for many projects in the last decades. There is a gap, however, in a thorough provenance study, which would contribute to the information on sediment supply in the Baffin Bay. Knowing the main sediment sources can further improve the research on ice sheet dynamics and paleocirculation. For this project three cores were investigated: one in the central (PC 16) and two in the southern Baffin Bay, along both eastern (SL 170) and western (SL 174) coasts. The location of these cores allows to characterize sediment provenance in the Baffin Bay as well as to reproduce transport mechanisms, in particular, water masses. The major research goal of the Ph.D project, therefore, is to perform a sediment provenance study in the Baffin Bay based on radiogenic isotope analyses (Sr, Nd, Pb) on the detrital fraction of the sediment cores and to connect these results with the information on sediment delivery mechanisms and ice sheet extent/retreat. We were able to determine major sediment sources for the core SL 170: central West Greenland (Nagssugtoqidian Mobile Belt) and southern West Greenland (Archean Block) terrains. Moreover, we observed the shift in all three radiogenic isotope records at 12 ka and attributed it to the change in the relative importance of the terrains: probably, at 12 ka the deglaciation of the central West Greenland ice sheet margin intensified or started, increasing the material supply to the location of the SL 170 core. The provenance of SL 174 and PC 16 cores was much more difficult to distinguish due to the influence of the minerals with a specific isotope composition (micas, feldspars). However, the Precambrian sources seem to prevail in their sediments. Additionally, Northern Baffin Bay Proterozoic or Paleozoic carbonate terrains could be an extra source for the PC 16 sediments. Paleocirculation study was initially the second aim of the project. Information on the water masses movement in the Baffin Bay should have been derived from the leachates fraction - 15 - of the same sediment cores. Leachates are thought to be dominated by the iron-manganese (Fe-Mn) oxyhydroxides, which are formed on the sediment grain and incorporate rare earth and trace elements reflecting the composition of the bottom seawater. However, the extraction of the true seawater signal from the Baffin Bay sediment leachates is complicated due to detrital dolomite material, which has been extracted during the leaching procedure as well. Moreover, after a brief mineralogical and chemical investigation we discovered no Fe-Mn coatings on the sediment grains or in the leachates samples. One of the potential origins of the detrital dolomite in the Baffin Bay samples was determined as the Borden Basin in Nunavut (Baffin Island, Canada). Pb isotopes appear to have a potential to trace chemical weathering and freshwater routing regime. The comparison between Baffin Bay and North Atlantic cores demonstrated, that by the end of the Baffin Bay Detrital Event 0 (around 11 ka), the modern-like circulation commenced to being established. The present-day water circulation in the Baffin Bay was possibly achieved by 9-8 ka. Overall, this PhD thesis showed that radiogenic isotopes indeed can be used for reconstructing sediment supply from the nearby land masses, and their variations can be explained in the light of changing provenance, circulation, weathering regime and ice sheet history. However, we emphasize here, that a profound chemical and mineralogical investigation is absolutely necessary along the traditional leaching procedures, in order to have a better control on the method and results and to be able to present strong arguments for the interpretation.