Instabilities in the Labrador Sea water mass structure during the last climatic cycle
In the modern Labrador Sea, the North Atlantic deep water components are found below the ~2 km deep, intermediate Labrador Sea water (LSW) mass, which is renewed locally through winter convective mixing. This water mass structure remained relatively stable since ~9.5 14 C ka BP, as indicated by isotopic studies of foraminifer assem- blages from deep-sea cores. Almost constant differences in δ 18 O values are observed between major species. These av- erage -0.5‰ between the epipelagic species Globigerina bulloides and the mesopelagic species Neogloboquadrina pachyderma, left coiled, and -1‰ between Neogloboquadrina pachydermaand the benthic species Cibicides wuellerstorfi, after correction for Cibicides wuellerstorfi specific fractionation. These isotopic compositions represent thermohaline conditions in surface waters, in the pycnocline with the LSW, and in the deep component of the North Atlantic deep water, respectively. A drastically different structure characterized the glacial Labrador Sea. Differences in δ 18 O values of ~ -2 to -2.5‰ are then observed between Globigerina bulloides and benthic species, indicative of a strong halocline between the corresponding water masses, thus for reduced production of intermediate waters. During the same interval, Neogloboquadrina pachydermashows 13 C and 18 O fluctuations of 1 to 1.5‰ amplitude, in phase with Heinrich-Bond events and higher frequency climate oscillations. The δ 18 O values in Neogloboquadrina pachyderma vary between those of Globigerina bulloides and of benthic foraminifers, suggesting large amplitude bathymetric fluctuations of the halo-thermocline above and below the bathymetric range occupied by Neogloboquadrina pachyderma . Minimum δ 18 O values in Neogloboquadrina pachydermamatch intervals of maximum ice rafting deposition, such as the late Heinrich events, thus intervals with a deeper, more dilute buoyant surface water layer.