Evidence for influx of Atlantic water masses to the Labrador Sea during the Last Glacial Maximum

Abstract

The Last Glacial Maximum (LGM, 23-19,000 year BP) designates a period of extensive glacial extent and very cold conditions on the Northern Hemisphere. The strength of ocean circulation during this period has been highly debated. Based on investigations of two marine sediment cores from the Davis Strait (1033 m water depth) and the northern Labrador Sea (2381 m), we demonstrate a significant influx of Atlantic-sourced water at both subsurface and intermediate depths during the LGM. Although surface-water conditions were cold and sea-ice loaded, the lower strata of the (proto) West Greenland Current carried a significant Atlantic (Irminger Sea-derived) Water signal, while at the deeper site the sea floor was swept by a water mass comparable with present Northeast Atlantic Deep Water. The persistent influx of these Atlantic-sourced waters entrained by boundary currents off SW Greenland demonstrates an active Atlantic Meridional Overturning Circulation during the LGM. Immediately after the LGM, deglaciation was characterized by a prominent deep-water ventilation event and potentially Labrador Sea Water formation, presumably related to brine formation and/or hyperpycnal meltwater flows. This was followed by a major re-arrangement of deep-water masses most likely linked to increased overflow at the Greenland-Scotland Ridge after ca 15 kyr BP.

Figure 1

Regional setting for sediment gravity cores TTR13-AT-479G (labelled 479G) located at 64°24.37′ N, 55°45.08′ W (1033 m present water depth), and TTR13-AT-455G (labelled 455G) located at 62°52,17′ N; 55°11,22′ W (2381 m water depth). (A) The modern surface ocean circulation pattern (modified after^, ) (red: warm surface currents; blue: cold surface currents; purple: cool current formed as a mixture of warmer and cold currents; black hatched: NADW = North Atlantic Deep Water, NEADW = Northeast Atlantic Deep Water and DSOW = Denmark Strait Overflow Water). LC = Labrador Current. WGC = West Greenland current. (B) Detailed map of the study region combined from GEBCO and BedMachine data. It illustrates that core 479G is located on a plateau, while core 455G is located in the basin not far away from the mouth of a major canyon (Fylla Canyon). The orange hatched line designates the track of the hydrographic transect shown in (C). (C) Simplified vertical distribution of the water masses in the study region modified from World Ocean Atlas. LC = Labrador Current; WGC = West Greenland Current; DWBU = Deep Western Boundary Undercurrent; PW = Polar Water; WGIW = West Greenland Irminger Water (Atlantic-soured water); TrW = Transitional Water, where the Atlantic-source WGIW is partly mixed with PW; LSW = Labrador Sea Water; NEADW = Northeast Atlantic Deep Water. Note that LSW and NEADW is only found in the northern Labrador Sea, not in the Davis Strait. For further details see supplementary Fig. S1.

Figure 2

Age models of core TTR13-AT-455G and the top of core TTR13-AT-479G. The age model of core 455G is based on eight calibrated AMS ¹⁴C datings on the planktic foraminifera Neogloboquadrina pachyderma (sinistral) (Table S1), while the age model of core 479G is based on a combination of AMS ¹⁴C datings on planktic foraminifera (the date on benthic foraminifera was not included in the age model) and correlation to the NGRIP ice core and Heinrich events.

Figure 3

Selected data from core 445G: Stable oxygen and carbon isotope values on the planktic foraminifera Neogloboquadrina pachyderma (sinistral) (in ‰ vs. V-PDB), sediment grains > 1 mm and 0.1–1.0 mm and > 1 mm (in % of total sample) obtained through wet sieving, % of Fe²⁺ as well as the % of the high-productivity species Alabaminella weddellensis, the oxygen-demanding species Cibicidoides wuellerstorfi and the high-energy indicator Cibidides lobatulus (all calculated based on the total calcareous benthic foraminiferal assemblage). The cumulative relative frequencies of the polar planktic foraminifera Neogloboquadrina pachyderma (sinistral), the frontal indicator Turborotalita quinqueloba and other planktic species are calculated from the total planktic foraminiferal fauna (note that the scales starts at 50%). Marine Isotope Stages (MIS) are according to Lisiecki and Raymo and the Last Glacial Maximum (LGM; interval shown in blue-green colour) is placed according to Mix et al.. “x” designated the location of ¹⁴C dates. The MIS 3/2 and MIS 2/1 boundaries are marked as gray horizontal lines, while the expected location of Heinrich (H) events are marked in light brown.

Figure 4

Stable isotope and foraminiferal distribution from cores TTR13-AT-455G (solid lines, dark colour) and TTR13-AT-479G (hatched lines, lighter colour). Benthic foraminifera are grouped according to environmental preference; calculated as percentage of the total benthic calcareous foraminiferal community. Sea-ice species: Islandiella helenae, Stainforthia feylingi; Polar water species: Cassidulina reniforme, it may also tolerate highly chilled Atlantic water. Chilled Atlantic water: Islandiella norcrossi; Atlantic water species: Cassidulina neoteretis, Miliolida. Deep-water species (today found in NEADW): Astrononion echolsi, Epistominella exigua, Ioanella tumidula, Melonis pompilioides, Nuttallides umbonifera, Oridorsalis tenerus, Pullenia bulloides, Pullenia subcarinata, Pullenia simplex, Tosaia hanzawai. High-energy species: Cibicides lobatulus, Discorbinella spp. (mainly Discorbinella araucana), Trifarina angulosa, Astrononion gallowayi, Astrononion stelligerum. Species that tolerates unstable conditions: Elphidium clavatum. Note that Cibicidoides wuellerstorfi is not included here but shown separately in Fig. 3. Planktic stable isotope values (in ‰ vs V-PDB) were measured on left-coiled specimens of Neogloboquadrina pachyderma, while benthic and planktic foraminiferal concentrations are reported as number of specimens per gram wet sediment. Marine Isotope Stages (MIS) are according to Lisiecki and Raymo and the Last Glacial Maximum (LGM) is placed according to Mix et al.. “x” designated the location of ¹⁴C dates for 455G (black) and 479G (purple). The MIS 3/2 and MIS 2/1 boundaries are marked as gray horizontal lines, while the expected location of Heinrich events are marked in light brown.

Figure 5

Schematics of interpretation of water mass distribution near our core sites off the shelf of West Greenland: (A) Last Glacial Maximum, (B) ca. 18.5–15 kyr BP, and (C) Modern. The modern conditions are interpreted from Hall et al. and the World Ocean Atlas. PW, Polar Water; WGIW, West Greenland Irminger Water; TrW, Transitional Water; NEADW, North East Atlantic Deep Water; LSW, Labrador Sea Water; WGC, West Greenland Current; DWBU, Deep Western Boundary Undercurrent.