OXYGEN ISOTOPE CHEMOSTRATIGRAPHY

McHugh and Olson (2002) have constructed an oxygen isotope record for the Pleistocene at Site 1073 and by comparison with the SPECMAP oxygen isotope timescale have significantly improved the age calibration of this important section. Four sequence boundaries are recognized in the section on the basis of seismic stratigraphic correlation from the shelf: pp4 at 520 mbsf, pp3 at 325 mbsf (cf. McCarthy et al., in press), pp2 at 145 mbsf, and pp1 at 79.5 mbsf. Oxygen isotope values range from 3.3 to 1.5. These are lower than those observed at North Atlantic deep sea sites, owing to proximity to the shelf and a continental ice sheet. The data are noisy, owing to disequilibrium effects in the planktonic foraminifers used to construct the record as well as the high-resolution sampling. Nannofossil datums are the base of the Emiliania huxleyi acme zone around 70 mbsf (~85 ka), the first appearance of Emiliania huxleyi at 120 mbsf (260 ka), and the last occurrence of Pseudoemiliania lacunosa at 330 mbsf (460 ka). (See Wei, Chap. 5, this volume, for slightly modified datums.) Surface pp1 is correlated by McHugh and Olson (2002) with an inferred hiatus between oxygen isotope Stages (OISs) 8 and 5; surface pp2 is correlated with the transition between OIS 9 and 8; pp3 is correlated with a hiatus between OIS 12 and 11. Although not the interpretation of McHugh and Olson (2002), these data and uncertainty in the distribution and duration of hiatuses permit the hypothesis that the surfaces correlate at least approximately with prominent glacial intervals and corresponding oxygen isotopic maxima: pp1 with OIS 6 (~150 ka), pp2 with OIS 8 (~300 ka), and pp3 with OIS 12 (~450 ka). Glacially modulated changes in sediment supply strongly influenced sedimentation at the continental slope, with rates of accumulation greatest during interglacial intervals (cf. Savrda et al., 2001b; McCarthy et al., in press). During glacial times, sediment was effectively trapped behind terminal moraines or captured at canyon heads and delivered to the continental rise.