Marine Isotope Stages 11 and 12: Rapid Variability in Subtropical North Atlantic Circulation

The period of time represented by Marine Isotope Stages (MIS) 11 and 12 contains what may be the most extreme climate conditions of the last 500,000 years. In order to study the nature of rapid variability in North Atlantic ocean circulation during this time, we have selected two high deposition rate cores from sediment drifts in the western North Atlantic Basin (ODP Site 1059, 31.6° N, 75.4° W, 2985 m water depth; and ODP Site 1055, 32.6° N, 76.2° W, 1799 m water depth). Site 1059 is located within the depth range of modern Lower North Atlantic Deep Water (LNADW). Site 1055 is located near the boundary between modern Upper North Atlantic Deep Water (UNADW) and LNADW. The sedimentation rates at both sites exceed 10 cm/k.y. throughout MIS 11 and 12, making it a prime location to study rapid changes in the strength of NADW production and its relationship to variability in sea-surface conditions. We have produced detailed records of δ ¹⁸O and δ ¹³C in the benthic foraminifera species Cibicidoides wuellerstorfi and the planktonic species Globigerinoides ruber (white), as well as a detailed record of carbonate content. These records span the interval of time ranging from 500 - 350 k.y.b.p. (Site 1059) and 430-350 k.y.b.p. (Site 1055), which includes glacial stages 10 and 12, and interglacial stage 11 (2 periods of ice growth, and the 12/11 glacial termination). The benthic δ ¹³C at Site 1059 varies rapidly throughout this time period with the highest amplitudes, ~ 1.0-1.5 per mil, occurring during periods of ice growth. Planktonic δ ¹⁸O records exhibit 0.5-1.0 per mil changes occurring on millennial time scales throughout the entire interval, indicating rapid changes in surface water hydrography. Most of the increases in δ ¹³C on the 13/12 ice growth are associated with decreases in planktonic δ ¹⁸O. This suggests that increases in benthic δ ¹³C may represent times of strong LNADW production, which pulled warm salty water northward and increased SST's in this region. The oscillations observed suggest that this mode of ocean circulation could be rapidly reorganized especially during periods of ice growth. Major decreases in δ ¹³C in the Site 1055 record seem to correlate with similar decreases in the 1059 record, suggesting that changes in the strength of UNADW and LNADW production occurred synchronously. The change in benthic δ ¹⁸O over the 12/11 termination at both sites is approximately 2.3 per mil. It has been suggested that the ice-volume contribution to the δ ¹⁸O change at the termination was ~1.5 per mil (Chappell 1998). The large residual in our record indicates that bottom water temperatures may have changed 3° C-4° C, comparable to the change in surface temperature at this time in this region.