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 similar to 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 similar to 9.5 C-14 ka BP, as indicated by isotopic studies of foraminifer assemblages from deep-sea cores. Almost constant differences in delta(18)O values are observed between major species. These average -0.5 parts per thousand between the epipelagic species Globigerina bulloides and the mesopelagic species Neogloboquadrina pachyderma, left coiled, and -1 parts per thousand between Neogloboquadrina pachyderma and 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 delta(18)O values of similar to -2 to -2.5 parts per thousand 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 pachyderma shows C-13 and O-18 fluctuations of 1 to 1.5 parts per thousand amplitude, in phase with Heinrich-Bond events and higher frequency climate oscillations. The delta(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 delta(18)O values in Neogloboquadrina pachyderma match intervals of maximum ice rafting deposition, such as the late Heinrich events, thus intervals with a deeper, more dilute buoyant surface water layer.