Veränderung des Tiefenwassers in der Labradorsee in den letzten 30000 Jahren
Zusammenfassung der Projektergebnisse
Deepwater convection in the Labrador Sea is assumed to be one of the driving mechanisms that stabilizes the modern Atlantic Meridional Overturning circulation (AMOC) mode. Classically, it has been assumed that no Labrador Sea Water (Elsworth et al.) formation occurred during the last glacial maximum (LGM). Yet, newer results indicate that convection in the Labrador Sea may still have been active during the LGM and that changes in the Labrador Sea convection may have played a role in the deglacial change from a shallower to deeper NADW circulation. The aim of the DEEPTELLS project was to gain a better understanding of the role of the Labrador Sea in deepwater circulation during the Last Glacial Maximum and the Deglaciation. The main question was whether deepwater convection in the Labrador Sea occurred during both periods and whether during the Deglaciation LSW formation contributed to deep water export to into the abyssal ocean. Furthermore, it was planned to investigate deepwater formation mechanisms using combined Mg/Ca of ostracods and stable isotope data from benthic foraminifera and to estimate current speed changes using sortable silt records. Due to an overprint from changing deglacial sediment sources sortable silt records could not be used for current speed reconstruction. However, they were used in combination with sediment facies analyses to reconstruct the position of the ice margin. We investigated the benthic δ18O and δ13C records from 4 sediment cores from the Inner Labrador Sea and compared the results to respective records from cores situated in positions that monitor the North Atlantic deepwater masses at different depth levels. Combined δ18O and δ13C data indicate an active deepwater convection in the Labrador Sea throughout the last 30 ka BP. Yet, deepwater export most probably was weak, mainly restricted to the mid depth (1200-2800 m water depth) and not connected to the abyssal ocean until the Bølling- Allerød (14.6 ka BP). During Bølling- Allerød a collapse of deepwater stratification is observed in the western subpolar North Atlantic and the Labrador Sea. This collapse most probably set the conditions for the onset of modern deepwater circulation. Though the original plan to measure Mg/Ca on ostracods could not be followed due to a lack in ostracods, Mg/Ca and Na/Ca was measured on benthic foraminifera, instead. Mg/Ca records on epibenthic and infaunal foraminifera indicate a deglacial warming of the deepwater in the Labrador Sea corroborating an increased inflow of warm saline Atlantic waters as a cause for increasing deglacial deepwater formation in the Labrador Sea over the deglaciation. First results indicate that Na/Ca is a promising tool for deepwater salinity reconstructions, that still need further investigation and calibration.
