Arctic vs. North Atlantic water mass exchanges in Fram Strait from Pb isotopes in sedimentsThis article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.

Surface sediment samples (n = 10), collected between Spitzbergen and Greenland, and two cores raised east (C04) and west (C16) from Fram Strait were analyzed for their chemical and isotopic (Pb) compositions to trace the source of sediments and water masses exchanging between the Arctic and the North Atlantic oceans. In surface sediments, variable major and trace element concentrations suggest variations in both the mineralogy (carbonate and quartz dilution of other silicate minerals) and source regions of detrital supplies, based on Th/Zr and, to a lesser extent, on Th/U ratios. Each core site shows specific but nearly constant Th/Zr ratios, indicating homogeneous source supplies. At both core sites, Pb concentrations and isotopic compositions display similar patterns: homogeneous low Pb and radiogenic crustal signals below 5–10 cm, contrasting with high Pb and less radiogenic anthropogenic inputs at core-tops. However, the differing pre-anthropogenic Pb isotopic ratios in C04 and C16 confirm the involvement of distinct source supplies east and west of Fram Strait. We suggest that this isotopic specificity is mainly owing to inputs of material carried from northwestern Europe by the North Atlantic water mass and from the Laptev Sea by the Transpolar Drift, respectively. Some material from the Greenland margin and possibly from the North Atlantic Ocean may reach this zone as well. Sediments from the western Arctic are not significantly transported into the Fram Strait area, suggesting that the Canadian and the Eurasian basins remained decoupled, at least during the time span of the cored sediments (∼2000 years).

Role of cabbeling in water densification in the Greenland Basin

The contribution of cabbeling mixing to water mass modification in the Greenland Sea was explored from hydrographic observation across the Greenland Basin in summer 2006. Neutral surface was chosen as a reference frame, and the strength of cabbeling mixing was determined by the dianeutral velocity magnitude. Water types in the area were classified into North Atlantic Water (NAW), modified North Atlantic Water (mNAW), water from Barents Sea near Bear Island (BIW), Arctic Intermediate Water (AIW) and Deep Water (DW), and significant cabbeling-induced velocity (>1 m/day) appeared at the interfaces of these water types below the seasonal pycnocline. The mixing between BIW and NAW in the eastern periphery was the most vigorous, where mixing-induced velocity reached 7.5 m/day which accompanied NAW production of 123 m3/day through transformation of BIW. Cabbeling in the Arctic Frontal Zone was found of two types; mixing within NAW in the upper layer and mixing within mNAW in the lower layer with a maximum velocity of 3 m/day. Source waters in the central Greenland Basin were AIW and mNAW and produced a vertical velocity of 4 m/day. In the western part of the Greenland Basin, the areas of active cabbeling were widely separated and each mixing point appeared rather weak, with a maximum velocity of 2.5 m/day. The average density gain in the eastern periphery was 0.003 kg/m3 while it was 0.001 kg/m3 in the other areas, though the impact of cabbeling on the bulk buoyancy change was highest in the western Greenland Sea. The frontal areas occupied approximately 50% of the whole analysis area and the total density gain due to cabbeling mixing in the Greenland Basin as a whole was estimated as 6.7×10−4 kg/m3.

Paleoenvironments along the Eastern Laurentide Ice Sheet Margin and Timing of the Last Ice Maximum and Retreat

ABSTRACT Palynological and isotopic analysis in a few deep-sea cores from the Labrador Sea reveals strong environmental changes related to the Late Pleistocene glacial fluctuations over eastern Canada. On the whole, the Labrador Sea was characterized by strong exchanges between North Atlantic water masses, Arctic outflows, and meltwater discharges from Laurentide, Greenland and lnuitian ice sheets. The penetration of temperate Atlantic waters persisted throughout most of the Late Pleistocene, with a brief interruption during the Late Wisconsinan. During this glacial substage, a slight but continuous meltwater runoff from the Laurentide ice margins grounded on the northern Labrador Shelf is indicated by relatively low 18O values and low-salinity (< 30‰) dinocyst assemblages. The calving of the ice margin, the melwater outflow and the subsequent dilution of surface waters offshore Labrador probably contributed to the dispersal of floating ice and, consequently, to a southward displacement of the polar front restraining the penetration of North Atlantic waters into the Labrador Sea. The advection of southern air masses along the Laurentide ice margins, shown by pollen assemblages, was favourable to abundant precipitation and therefore, high ice accumulation rates, especially over northern Labrador during the Late Wisconsinan. The déglaciation is marked by a brief, but significant, melting event of northern Laurentide ice shortly after 17 ka. The main glacial retreat occurred after ca. 11 ka. It allowed restoration of WSW-ENE atmospheric trajectories, increased phytoplanktonic productivity, and penetration of North Atlantic water masses into the Labrador Sea.