Intermediate water links to Deep Western Boundary Current variability in the subtropical NW Atlantic during marine isotope stages 5 and 4

Abstract

Records from Ocean Drilling Program Sites 1057 and 1059 (2584 m and 2985 m water depth, respectively) have been used to reconstruct the behavior of the Deep Western Boundary Current (DWBC) on the Blake Outer Ridge (BOR) from 130 to 60 kyr B.P. (marine isotope stage (MIS) 5 and the 5/4 transition). Site 1057 lies within Labrador Sea Water (LSW) but close to the present-day boundary with Lower North Atlantic Deep Water (LNADW), while Site 1059 lies within LNADW. High-resolution sortable silt mean (inline equation) grain size and benthic δ 13C records were obtained, and changes in the DWBC intensity and spatial variability were inferred. Comparisons are made with similar proxy records generated for the Holocene from equivalent depth cores on the BOR. During MIS 5e, inline equation evidence at Site 1057 suggests slower relative flow speeds consistent with a weakening and a possible shoaling of the LSW-sourced shallower limb of the DWBC that occupies these depths today. In contrast, the paleocurrent record from the deeper site suggests that the fast flowing deep core of the DWBC was located close to its modern depth below 3500 m. During this interval the benthic δ 13C suggests little chemical stratification of the water column and the presence of a near-uniform LNADW-dominated water mass. After ∼111 kyr B.P. the inline equation record at Site 1057 increases to reach values similar to Site 1059 for the rest of MIS 5. The strengthening of flow speeds at the shallow site may correspond to the initiation of Glacial North Atlantic Intermediate Water formation also suggested by a divergence in the benthic δ 13C records with Site 1057 values increasing to ∼1.2‰. Coupled suborbital oscillations in DWBC flow variability and paleohydrography persisted throughout MIS 5. Comparison of these data with planktonic δ 18O records from the sites and alkenone-derived sea surface temperature (SST) estimates from the nearby Bermuda Rise suggest a hitherto unrecognized degree of linkage between oscillations in subtropical North Atlantic SST and DWBC flow.