PALEOCEANOGRAPHY

With the higher resolution of paleoceanographic studies at sites such as those collected during Leg 172, the more important becomes the stratigraphic correlations among sites and their chronology. Grutzner et al. (in press) have provided an important baseline for study of these sites through their development of age models based on tuning estimated carbonate content (from sediment lightness) to the orbital parameters precession and obliquity. They found that sites tend to group into those from shallow locations where precession dominates carbonate content (2200-3000 m) and deeper sites where obliquity dominates (3000-4800 m). Time series analyses of the resulting age series of carbonate reveal several significant periods in the range 12 to 1.5 k.y. Although these results are preliminary and will be improved upon as more oxygen isotope data become available, recognition of periodicities as short as 1500 yr by Grutzner et al. (in press) shows the promise of Leg 172 sites for high-resolution studies of the ocean and climate.

Using stable isotopes, Bianchi et al. (in press) established the bathymetric gradients between the deep Blake Outer Ridge (Site 1060) and the Bahama Outer Ridge (Site 1062) from MIS 5e-5d. They found no ¹³C contrast, and high ¹³C during MIS 5e, which indicates strong Lower North Atlantic Deep Water (LNADW) flow over both sites. A ¹³C gradient developed late in MIS 5e and early in MIS 5d, but by ~113 ka both locations were overlain by a nutrient-rich water mass. In contrast to ¹³C, the sortable silt parameter indicates significant and coherent variability at both sites throughout the MIS 5e-5d interval.

Oppo et al. (2001) also concentrated on MIS 5e, but at Site 1059. They compared stable isotope results there with those from two locations in the subpolar North Atlantic (including ODP Site 980). At each location they found millennial-scale variability in sea-surface temperature (SST) proxies, despite the minimum in ice volume. However, as reported by Bianchi et al. (in press) for deeper sites, they found no strong geochemical evidence for NADW change during the last interglacial.

Site 1059 is also featured in a paper by Hagen and Keigwin (in press). All stadial-interstadial changes in climate recognized for MIS 3 in Greenland ice are evident at this site. SST oscillations were as large as 4°C, based on ¹⁸O of Globigerinoides ruber, which also typically varies between 20% and 40% of the planktonic fauna. Neogloboquadrina pachyderma (left coiling) occasionally exceeds 2% of the fauna during the coldest episodes of MIS 2 and 3. In general, there is a relationship between interstadial climate and high production of NADW (high ¹³C). However, notable exceptions to this pattern occur during the long interstadials 12 and 14 (~45 and 52 ka). At those times, there seems to be a lag of >1 k.y. between interstadial warming and increased NADW flow.

Keigwin (Chap. 9, this volume) assessed the significance of red sediments (red lutites) through an oxygen isotope study of select intervals within MIS 1 to 6 at Sites 1054, 1055, and 1063. For the deeper Sites 1055 and 1063, maxima in planktonic values coincided with intervals with redder sediments, indicating a connection between colder SST and deep circulation that transports the red lutites from their source in eastern Canada. Site 1054, which is only 24 km away from Site 1055 but is 500 m shallower, has a notable absence of reddish sediment. From measurements of planktonic ¹⁸O through the last glacial maximum (LGM) interval at these two sites, Keigwin noted lower values at Site 1054 than at Site 1055. He suggested this could result from higher SST at Site 1054, possibly related to the proximity of this site to the core of the Gulf Stream. He surmised that the lack of red sediment at Site 1054 may indicate that the shallowest components of the DWBC were deeper than Site 1054 during the LGM.

The group of Thunell, Poli, and Rio investigated the interval MIS 9-12. Thunell et al. (in press) used carbonate stratigraphy to identify particular time slices for paleohydrographic reconstruction within the interval MIS 10-12. They found nutrient depletion in MIS 12 above 2000 m, whereas MIS 11 was similar to today (with high ¹³C between 3500 and 2000 m and lower ¹³C below 3500 m). In general, MIS 10 and 12 were similar. This group also conducted a time series study of the same time interval at Bermuda Rise Site 1063 (Poli et al., 2000). Millennial-scale variability is typical in both MIS 12 and 11. MIS 12 had interstadials every several thousand years and traces of ice rafting during stadial events. Interstadial events in MIS 11 (high carbonate and high NADW flow) were found to correlate with warm events in the Vostok ice core.

Franz and Tiedemann (in press) sampled MIS 8-10 at better than 1-k.y. resolution for stable isotope and sedimentology studies. From MIS 9 to 8.5, waters as deep as 3 km were well ventilated, but during the glacial epochs before and after AABW shoaled to 2200 m. Leg 172 sites recorded the subtle interplay between source of sediment, redistribution by DWBC, varying corrosiveness of bottom waters, and carbonate dissolution driven by organic carbon fluxes.