Ocean circulation changes off southern Greenland during the abrupt climate events of mid-to-late MIS3
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Marine sediment cores from the North Atlantic and ice cores from the Greenland Ice Sheet serve as natural archives of past climate variability. Ice cores have revealed that during the last glacial period the climate comprises both relatively stable intervals such as the Last Glacial Maximum (LGM, ca. 26-19 ka) and unstable climate intervals such as Marine Isotope Stage 3 (MIS3, ca. 59-29 ka). The climate of MIS3 is characterized by abrupt changes from colder stadial to warmer interstadial conditions, well-known as Dansgaard-Oeschger events (DO). Although DO events had repercussions over the climate system outside of the high latitude Northern Hemisphere, they are especially pronounced in records from the North Atlantic region. North Atlantic marine sediments record changes in sea ice cover, surface productivity and iceberg discharge, which are all associated with DO events. Iceberg discharge from the Northern Hemisphere ice sheets, and hence release of fresh water into the North Atlantic was especially pronounced during so-called Heinrich (H) Stadials. H-Stadials occurred approximately every 7 to 10 kyrs and are particularly cold and long stadials within the DO event oscillations. The freshwater discharge further contributed to sea ice formation in the North Atlantic and Nordic Seas. This near-perennial sea ice cover during stadials prevented ocean-atmosphere interaction while during interstadials the Nordic Seas were seasonally ice-free. These interstadial periods allowed moisture to evaporate from the ocean´s surface where after it was transported towards the Northern Hemisphere Ice Sheets, and thereby sustain their growth during MIS3 and the LGM. This thesis aims to give insights into the complex interactions between ice sheets, sea ice and ocean circulation during MIS3 and the LGM. Herein, the main focus lies on changes along the South-East Greenland margin, complemented by thoroughly studied sediment core sites from the Nordic Seas and Greenland ice core records. Paper I investigates environmental conditions between 65 and 25 ka recorded by a deep-sea sediment core from the northeastern Labrador Sea. Stable isotope data, ice-rafted debris (IRD) and planktic foraminifer fluxes and assemblage counts are compared to sediment cores from the Irminger Basin and the Greenland Sea. The results are used to draw conclusions about the sensitivity of the northwest North Atlantic to changes in sea ice extent, surface hydrography and iceberg discharge during the DO events of MIS3. We find, that an expanded sea ice cover seems to be characteristic for most stadials in the Labrador Sea while climate conditions during most interstadials allowed for open waters with a productive sea surface. Major iceberg calving events associated with H-Stadials 4 and 3 are documented. The influence of Atlantic Water is enhanced during Greenland Interstadials 14, 12 and 8. Paper II takes a detailed look at the source areas that may have contributed to each IRD event observed in Paper I. X-ray spectroscopy and single grain Lead-isotope measurements are performed. Additionally, several petrologic tracers and X-ray fluorescence Calcium-to-Strontium ratios are analyzed. The main results indicate that there are no specific trends of IRD sources throughout MIS3 or during interstadials, stadials and H-Stadials. Greenland is the main contributor of the IRD deposits observed at the Eirik Drift, although icebergs from the Laurentide Ice Sheet melted at the core site during the ice sheet collapse associated with H-Stadial 4. In Paper III, the knowledge gained within the scope of Papers I and II is combined with previously published and new datasets from three regions in the Nordic Seas and the subtropical North Atlantic. The datasets comprise biomarker, IRD and planktic foraminifer concentration as well as sea surface temperature records to evaluate changes in the North Atlantic sea ice extent during late MIS3 DO events and the LGM. The results from these proxy-data are then compared to stable oxygen isotope (δ18O) and deuterium excess (d-excess) records from the GRIP ice core. We find a strong link between the variability of d-excess data and the location of the sea ice edge in the North Atlantic. During interstadials and the LGM, sea surface conditions were similar and allowed for a northern moisture source of precipitation as sea ice retreated to the northwestern Nordic Seas. Additionally, the investigated regions were characterized by high plankton productivity during the LGM. The surface of the subtropical North Atlantic was as warm or warmer during the LGM than during MIS3 interstadials. The results of this thesis reveal that the northeastern Labrador was a highly dynamic region during the DO events of MIS3. The high-resolution MIS3 data close the gap between the well-studied LGM and MIS5e and connect this part of the North Atlantic to the well-constrained sea surface conditions of the Nordic Seas and the mid-latitude North Atlantic. Furthermore, by mapping the North Atlantic sea surface conditions during MIS3 DO events and the LGM, a link between ice core d-excess and the Nordic Seas sea ice cover is demonstrated.
Består avPaper I: Griem, L.; Voelker, A. H. L.; Berben, S. M. P.; Dokken, T. M. and Jansen, E.: Insolation and glacial meltwater influence on sea-ice and circulation variability in the northeastern Labrador Sea during the last glacial period. Paleoceanography and Paleoclimatology. 2019, 34(11):1689-1709. The article is available at: https://hdl.handle.net/11250/2726974
Paper II: Griem, L.; Storey, C.; Berben, S. M. P.; Dokken, T. M. and Jansen, E.: Climate and ice sheet responses in northern latitudes during Marine Isotope Stage 3: A provenance study of ice-rafted debris from the Eirik Drift. The article is not available in BORA.
Paper III: Griem, L.; Dokken, T. M.; Risebrobakken, B.; Faber, A.-K.; Berben, S. M. P.; Vinther, B. M.; Sadatzki, H.; Gkinis, V. and Jansen, E.: Similar conditions in the North Atlantic region during the Last Glacial Maximum and interstadials: Evidence from ice core and sediment core records. The article is not available in BORA.