Surface and deep ocean circulation in the subpolar North Atlantic during the mid‐Pleistocene revolution

Abstract

[1] We investigated surface and deep ocean variability in the subpolar North Atlantic from 1000 to 500 thousand years ago (ka) based on two Ocean Drilling Program (ODP) sites, Feni drift site 980 (55°29 0 N, 14°42 0 W) and Bjorn drift site 984 (61°25 0 N, 24°04 0 W). Benthic foraminiferal stable isotope data, planktic foraminiferal faunas, ice-rafted debris data, and faunally based sea-surface temperature estimates help test the hypothesis that oceanographic changes in the North Atlantic region were associated with the onset of the 100-kyr world during the mid-Pleistocene revolution. Based on percentage of Neogloboquadrina pachyderma (s) records from both sites, surface waters during interglacials and glacials were cooler in the mid-Pleistocene than during marine isotope stages (MIS) 5 and 6. In particular, interglaciations at Bjorn drift site 984 were significantly cooler. Faunal evidence suggests that the interglacial Arctic front shifted from a position between the two sites to a position northwest of Bjorn drift site 984 after ca. 610 ka. As during the late Pleistocene, we find faunal evidence for lagging surface warmth at most of the glacial initiations during the midPleistocene. Each initiation is associated with high benthic d13 C values that are maintained into the succeeding glaciation, which we term ''lagging NADW production.'' These findings indicate that lagging warmth and lagging NADW production are robust features of the regional climate system that persist in the middle to late Pleistocene.

Expansion of subarctic water masses in the North Atlantic and Pacific oceans and implications for mid‐Pleistocene ice sheet growth

“…One exception to this trend is the high C 37:4 concentrations during the later stages of MIS 21, when d 18 O indicates relatively warm and/or low ice volume conditions. However, high abundances of the polar water foraminifer Neogloboquadrina pachyderma (s.) (Figure 4) [Wright and Flower, 2002], support the inference made from the %C 37:4 data that cooler and fresher waters prevailed at Site 983 at this time.…”

“…Thus the %C 37:4 trends in Figure 3 reflect an equatorward expansion of arctic/polar water masses in the Atlantic and Pacific Oceans from 1.15 Ma that spanned both glacials and interglacials. For the North Atlantic, this interpretation is supported (Figure 4) by the correlation between the peaks in %C 37:4 at Site 983 to the intervals dominated by the polar planktonic foraminifer N. pachyderma (s.) at Site 984, for the time interval over which these two records overlap (1.0 -0.6 Ma) [Wright and Flower, 2002]. The difference in age models, the high degree of variability in %C 37:4 and SST within both glacial and interglacial stages, and the potential for advection-induced temporal offsets at Site 983, limit our ability to investigate leads and lags between sites.…”

“…As our time series do not continue from 0.5 Ma to the present day, we cannot conclusively demonstrate that the retreat of the subarctic/ subpolar water masses after 0.9 Ma was a permanent one at either site. We have noted above that our %C 37:4 record at Site 983 is closely aligned with the N. pachyderma (s.) record from Site 984 [Wright and Flower, 2002]. The latter study demonstrated that a relatively southern position of the Arctic Front in the North Atlantic was maintained between 1.0 and 0.6 Ma, after which the interglacial position of the Arctic Front achieved the southwest to northeast alignment that is found at the present day.…”

“…Site 983 is ideally positioned to register changes in the position of the Arctic Front and thus the extent of the Arctic water mass in the North Atlantic. This has been demonstrated in previous examinations of Site 983 sediments and of its closest neighbor, Site 984, using diatoms and foraminifera respectively [Koç et al, 1999;Wright and Flower, 2002]. The sensitive location of Site 983 may account for the high degree of variability in SSTs and %C 37:4 within glacial/ interglacial stages.…”

“…We present results from two Ocean Drilling Program (ODP) sites covering the time interval 1.5-0.5 Ma (Figure 1): North Atlantic Site 983 and northwest Pacific Site 882. Although the Arctic Front that divides Atlantic and Arctic waters presently lies to the north of Site 983, it is known to have migrated southward during glacials of the Pleistocene bringing much cooler waters and potentially also sea ice to the site [e.g., Bard et al, 1987;Rosell-Melé et al, 1998;Wright and Flower, 2002]. In contrast, sea ice does not appear to have reached Site 882 over the Pliocene and Pleistocene; rather, glacials are associated with increasing polar stratification [Haug et al, 2005[Haug et al, , 1999Jaccard et al, 2005].…”