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 fo und below the similar to 2 km deep, intermediate Labrador Sea water (LSW) m ass, 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 c ores. Almost constant differences in delta(18)O values are observed between major species. These average -0.5 parts per thousand between the epipelagi c species Globigerina bulloides and the mesopelagic species Neogloboquadrin a pachyderma, left coiled, and -1 parts per thousand between Neogloboquadri na pachyderma and the benthic species Cibicides wuellerstorfi, after correc tion for Cibicides wuellerstorfi specific fractionation. These isotopic com positions represent thermohaline conditions in surface waters, in the pycno cline with the LSW, and in the deep component of the North Atlantic deep wa ter, respectively. A drastically different structure characterized the glac ial 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 ben thic species, indicative of a strong halocline between the corresponding wa ter 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 even ts and higher frequency climate oscillations. The delta(18)O values in Neog loboquadrina pachyderma vary between those of Globigerina bulloides and of benthic foraminifers, suggesting large amplitude bathymetric fluctuations o f the halo-thermocline above and below the bathymetric range occupied by Ne ogloboquadrina pachyderma. Minimum delta(18)O values in Neogloboquadrina pa chyderma match intervals of maximum ice rafting deposition, such as the lat e Heinrich events, thus intervals with a deeper, more dilute buoyant surfac e water layer.