Glacial/interglacial instabilities of the Western Boundary Under Current during the last 365 kyr from Sm/Nd ratios of the sedimentary clay-size fractions at ODP site 646 (Labrador Sea)

Abstract

We present 40 Sm-Nd isotope measurements of the clay-size (<2 µm) fractions of sediments from the Southern Greenland rise (ODP-646) that span the last 365 kyr. These data track changes in the relative supply of fine particles carried into the deep Labrador Sea by the Western Boundary Under Current (WBUC) back to the fourth glacial-interglacial cycles. Earlier studies revealed three general sources of particles to the core site: (i) Precambrian crustal material from Canada, Greenland, and/or Scandinavia (North American Shield -NAS), (ii) Palaeozoic or younger crustal material from East Greenland, NW Europe, and/or western Scandinavia (Young Crust -YC) and (iii) volcanic material from Iceland and the Mid-Atlantic Ridge (MAR). Clay-size fractions from glacial sediments have the lowest Nd isotopic ratios. Supplies of young crustal particles were similar during glacial oxygen isotope stages (OIS) 2, 6, and 10. In contrast the mean volcanic contributions decreased relative to old craton material from OIS 10 to OIS 6 and then from OIS 6 to OIS 2. The glacial OIS 8 interval displays a mean Sm/Nd ratio similar to those of interglacials OIS 1,5, and 9. Compared with other interglacials, OIS 7 was marked by a higher YC contribution but a similar ~30% MAR supply. The overall NAS contribution dropped by a factor of 2 during each glacial/interglacial transition, with the MAR contribution broadly replacing it during interglacials. To decipher between higher supplies and/or dilution, particle fluxes from each end member were estimated. Glacial NAS fluxes were systematically higher than interglacial fluxes. During the time interval examined, fine particle supplies to the Labrador Sea were strongly controlled by proximal ice-margin erosion and thus echoed the glacial stage intensity. In contrast, the WBUC-carried MAR supplies from the eastern basins did not change significantly throughout the last 365 kyr, except for a marked increase in surface-sediments that suggests unique modern conditions. Distal WBUC-controlled inputs from the Northern and NE North Atlantic seem to have been less variable than proximal supplies linked with glacial erosion rate.

Foraminifer isotope study of the Pleistocene Labrador Sea, northwest North Atlantic (IODP Sites 1302/03 and 1305), with emphasis on paleoceanographical differences between its “inner” and “outer” basins

“…The difference in the isotopic composition of Npl at the two sites during the MIS 5e is relatively small (2.3‰ vs. 2.5‰ on the average at Site 1302/1303 and Site 1305, respectively). This suggests a relatively well-homogenized mesopelagic water mass throughout the inner and outer Labrador Sea, assuming that both species of foraminifers developed during MIS 5e on top of the pycnocline between surface and intermediate waters, as they did during the Holocene (e.g., de Vernal and Hillaire-Marcel, 2006). On the contrary, the differences in the δ 18 O values of Cw are very large (~3.4 ‰ vs~2.7‰ at Site 1302/ 1303 and Site 1305, respectively) demonstrating that distinct deepwater mass bathed the two sites.…”

“…A survey of the literature, prior 1994, about the late Quaternary paleoceanography of the Labrador Sea can be found in a special issue of the Canadian Journal of Earth Sciences (cf. Hillaire-Marcel et al, 1994b) whereas more recent studies are briefly reported in Hillaire-Marcel et al (2001a-b), de Vernal and Hillaire-Marcel (2006) and Fagel and Hillaire-Marcel (2006), for example. Already drilled in 1985 (ODP Leg 105; Site 646), the Eirik Ridge area was revisited in 2004 during IODP Expedition 303 in order to retrieve fresh material for higher resolution investigations from the closely located Site 1305 (57°28.5′ N, 48°31.8′ W; water depth: 3460 m; cf.…”

“…Thirdly, we intended to further document deep-current pathways and deepwater mass origins from clay mineral assemblages and their Nd and Pb-isotopes and sea-surface conditions from dinoflagellate cyst assemblages (e.g., de Vernal and Hillaire-Marcel, 2006). These data are already published (e.g., de Vernal and Hillaire-Marcel, 2006;Fagel and Hillaire-Marcel, 2006) or will be published elsewhere. In this paper we focus on the first two objectives listed above.…”

“…HillaireMarcel et al, 1994aHillaireMarcel et al, , 2001a. The high interglacial sedimentation rates over Eirik Ridge can be due to a combination of factors: i) enhanced sediment focusing under a higher velocity WBUC resulting from high production of DSOW (e.g., Hunter et al 2007), ii) enhanced biogenic carbonate fluxes, and iii) important detrital supplies accompanying ice-retreat phases on southern Greenland, especially during late glacial/early interglacial intervals (see also Fagel and Hillaire-Marcel, 2006;Stoner et al, 2000).…”

South Greenland ice-sheet collapse during Marine Isotope Stage 11

“…1). These sediments are transported to the core site in the Western Boundary Undercurrent [15][16][17] (Fig. 1 and Methods), and magnetic and geochemical properties of Holocene Eirik Drift sediments document the close coupling between GIS subglacial erosion, sediment transport and terrigenous silt deposition at MD99-2227 [16][17][18] .…”