NOAA/WDS Paleoclimatology - Gardar Drift Last Interglacial Foraminiferal Stable Isotope Data

STUDY NOTES: High-resolution (centennial scale) planktonic and benthic foraminiferal stable isotope data, plus carbonate content and sediment color intensity data from the Gardar Drift, Iceland Basin, over the last interglacial. Site U1304 was sampled at a constant 5-cm spacing between 24.52 and 15.47 mcd, corresponding to the MIS 6 through MIS 5d interval. The mean sedimentation rate for MIS 5e and 5d is 40 cm/ka, which results in an average sampling frequency of 125 years. Stable oxygen (d18O) and carbon (d13C) isotope ratios were measured on three species of planktonic foraminifera (Globigerina bulloides, Neogloboquadrina incompta, and Globorotalia inflata) and the epi-benthic foraminfera Cibicidoides wuellerstorfi. Sediment lightness was measured at 2-cm intervals shortly after cores were split, using a Minolta CM1500 Spectrophotometer.
ABSTRACT SUPPLIED BY ORIGINATOR: Changes in surface and deep-water hydrography were inferred from variations in stable isotopes and sortable silt mean grain size, respectively, on the southern Gardar Drift in the subpolar North Atlantic. The bathymetric d13C gradient during the penultimate glaciation was similar to the last glaciation with high-d13C Glacial North Atlantic Intermediate water above ~2000 m, and low-d13C water derived from the Southern Ocean below. During Termination II, low-d13C water was present throughout the water column with minimum values at intermediate depths (~1500-2000 m) and below 3000 m. This pattern continued well into the early part of the Last Interglacial (LIG) period. Sortable silt mean size at 3275 m suggests that deep-water circulation on Gardar Drift was relatively weak during the earliest part of the LIG (128 to 124.5 ka) when planktonic d18O was at a minimum, reflecting warming and/or reduced salinity. We suggest that low-d13C water and slow current speed on Gardar Drift during the early part of the LIG was related to increased melt water fluxes to the Nordic Seas during peak boreal summer insolation, which decreased the flux and/or density of overflow to the North Atlantic. The resumption of the typical interglacial pattern of strong, well-ventilated Iceland Scotland Overflow Water was delayed until ~124 ka. These changes may have affected Atlantic Meridional Overturning Circulation.