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

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

Miocene restriction of the Pacific-North Atlantic throughflow strengthened Atlantic overturning circulation

Export of warm and salty waters from the Caribbean to the North Atlantic is an essential component of the Atlantic Meridional Overturning Circulation (AMOC). However, there was also an active AMOC during the Miocene, despite evidence for an open Central American Seaway (CAS) that would have allowed low-salinity Pacific waters to enter the Caribbean. To address this apparent contradiction and to constrain the timing of CAS closure we present the first continuous Nd isotope record of intermediate waters in the Florida Strait over the past 12.5 million years. Our results indicate that there was no direct intermediate water mass export from the Caribbean to the Florida Strait between 11.5 and 9.5 Ma, at the same time as a strengthened AMOC. After 9 Ma a strong AMOC was maintained due to a major step in CAS closure and the consequent cessation of low-salinity Pacific waters entering the Caribbean

Radiogene Isotope auf Sedimenten aus der Baffin Bay : Implikationen für die Sedimenteintrag während der letzten Deglaziation

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

A 12 Million Year Record of Caribbean Outflow to the Northwest Atlantic Based on Radiogenic Nd Isotopes Obtained from ODP Site 1006

The closure of the Central American Seaway (CAS) has been an interesting research problem for several decades. The exact timing of Panama seaway closure is still unknown, though many methods have been used to try and solve this scientific problem. One such method uses the neodymium isotopic composition (Nd IC) of authigenic Fe-Mn oxihydroxides, which has been successfully applied as a paleoceanographic tool for water mass tracing due to the residence time of Nd in the oceans being shorter than the ocean overturning time and due to independence of Nd IC from biological fractionation and physical processes. The aim of our project was to reconstruct the paleocirculation in the Florida Straits for the last 12 Myrs, and to consider the results in connection with either the Panama closure or the Northern Hemisphere Glaciation. For this purpose we carried out isotopic measurements of Nd, Sr and Pb on the sediment core from ODP Site 1006 (water depth 658 m). The methods applied included the leaching of sediments to obtain the authigenic water signal from Fe-Mn oxyhydroxides and total dissolution to obtain the detrital fraction composition. Measurements were made at GEOMAR Helmholtz Centre for Ocean Research, Kiel, using an Inductive-Coupled Plasma Mass Spectrometer (ICP-MS). Pb IC results were obtained only for the detrital material since the concentrations of Pb in the leachate samples were too low for the isotopic measurements. Isotopic compositions in several core-top sediments were also measured, which could tell us if our signal was a true seawater one. The εNd curves obtained for authigenic and detrital material almost replicate each other. However we observed a strong correlation of our Nd IC record for leachates with other records for Sites close to our. This is why we suggest that we can trust our age model and consider our seawater signal as not having been contaminated by the detrital fraction. According to the Nd IC curve, most likely the Panama seaway was closed by 9.5 Ma for intermediate water exchange, but the final closure timing is hard to evaluate. It has also been shown that an intensive boundary exchange with the material from the continents plays an important role in deriving isotopic signals (since the Site is so shallow and close to the ocean margin). Core top Nd IC was different depending on whether carbonate was removed prior to leaching or not. Therefore as possible future work we would suggest to constrain measurements of Nd IC using fossil fish teeth, as they should reflect a true bottom water signal. Direct seawater measurements in this location would also be very helpful

Neodymium and Strontium isotope compositions from ODP sites 166-1006 and 165-1000

Nd isotope composition of weakly cleaned foraminifera in core tops close to ODP Site 1006. 12 Myr record of Nd isotopes measured in weakly cleaned foraminifera from ODP Site 1006. Record from 5.5 to 13.5 Myr of Nd isotopes measured in weakly cleaned foraminifera from ODP Site 1000. 12 Myr record of Nd and Sr isotopes measured in the detrital fraction from ODP Site 1006. Record from 5.5 to 13.5 Myr of Nd and Sr isotopes measured in the detrital fraction from ODP Site 1000