North Atlantic Ice Sheet Fluctuations 10,000–70,000 Yr Ago as Inferred from Deposits on the Reykjanes Ridge, Southeast of Greenland

Abstract

Marine records from the Reykjanes Ridge indicate ice sheet variations and abrupt climate changes. One of these records, ice-rafted detritus (IRD), serves as a proxy for iceberg discharges that probably indicates ice sheet fluctuations. The IRD records suggest that iceberg discharge 68,000–10,000 yr B.P. happened more frequently than the 7000- to 10,000-yr spacing of the Heinrich events. An IRD peak 67,000 to 63,000 yr B.P. further suggests that the Middle Weichselian glaciation started about 12,000 yr earlier in the North Atlantic than in the Norwegian Sea. Several later IRD events, in contrast, correlate with Norwegian Sea IRD-rich layers and imply coeval ice sheet advances in the North Atlantic and the Norwegian Sea. Coccoliths in a core from the Reykjanes Ridge show distinct peaks in species that record occasional inflow of warm surface water during the last glaciation, as previously reported from the eastern Labrador Sea. High abundances of coccoliths, together with a decrease of Neogloboquadrina pachyderma sin. and relatively low δ18O values, imply enhanced advection of the North Atlantic Current 69,000–67,000 yr B.P., 56,000–54,000 yr B.P., 35,000–33,000 yr B.P., and 26,000–23,000 yr B.P. This advection provided a regional moisture source for extension of ice sheets onto the shelf. In contrast, most of the IRD events are characterized by cold polar surface water masses indicating rapid variations in ocean surface conditions.

Meltwater and ice rafting in the southern Norwegian Sea between 20 and 40 calendar kyr B.P.: Implications for Fennoscandian Heinrich events

“…The increase in carbonate associated with H2 was associated with a strong increase in the number of foraminifera in core MD99‐2283, up to 900 foraminifera per gram of sediment. Similar findings from the Faroe‐Shetland Channel [ Rasmussen et al , 1996, 1997] and the northern North Atlantic [ Lackschewitz et al , 1998] suggest increased productivity during this period.…”

“…The carbonate content is a bulk parameter influenced by marine productivity, dilution and dissolution and reflects a strong relationship with the number of foraminifera during the deglaciation and the Holocene outside the coast of northern Norway [ Knies et al , 2003]. The carbonate content in surface and cored sediments from the Nordic Seas is interpreted to reflect the influx of temperate Atlantic waters through time [ Kellogg , 1976; Ramm , 1988; Baumann et al , 1995; Hebbeln et al , 1998; Lackschewitz et al , 1998]. Throughout the northern North Atlantic and the Nordic Seas periods with high carbonate levels have been observed in records between 28 and 40 cal kyr B.P.…”

“…During the period between 25 and 20 cal kyr B.P., increases in IRD have been observed in the Faroe‐Shetland Channel [ Rasmussen et al , 1996], the central Norwegian Sea [ Bauch et al , 2001], the Vøring Plateau [ Dokken and Jansen , 1999; Berstad , 2003] and the Denmark Strait [ Hagen and Hald , 2002] indicating near simultaneous increased calving on the glaciated margins of the Nordic Seas. During the same time period, improving oceanic conditions are indicated by increases in the amount of planktonic foraminifera Turborotalia quinqueloba and of the benthic foraminifera Cibicidoides wuellerstorfi in the central Norwegian Sea [ Bauch et al , 2001], SST reconstruction in the Denmark Strait [ Hagen and Hald , 2002], coccoliths and planktonic foraminifera in the northern North Atlantic [ Lackschewitz et al , 1998] and an increased reconstructed heat flux to the Nordic Seas between 21 and 24 cal kyr B.P. [ Weinelt et al , 2003].…”

Centennial‐scale evolution of Dansgaard‐Oeschger events in the northeast Atlantic Ocean between 39.5 and 56.5 ka B.P.

“…Planktonic foraminifera oxygen isotopes ( δ 18 O) were measured on approximately 10 individuals of Neogloboquadrina pachyderma ( s ) >150 μ m per sample throughout the study period. This species is common in polar waters <7°C [ Be and Tolderlund , 1971] and has commonly been used for isotopic measurements of surface waters at high and midlatitudes during glacial periods [e.g., Bond et al , 1993; Fronval et al , 1995; Lackschewitz et al , 1998; Elliot et al , 1998; van Kreveld et al , 2000]. However, N. pachyderma ( s ) calcifies below the surface at depths up to 200 m [ Bauch et al , 1997], and so may not reveal additional information on water hydrography at shallower depths.…”

“…The importance of the Atlantic Meridional Overturning Circulation (MOC) in the amplification and transmission of abrupt climate signals has been highlighted because of its ability to affect the global transfer of heat, and because of a close relationship between the activity of the MOC and abrupt climate signals recorded at high northern latitudes [e.g., Boyle and Keigwin , 1987; Broecker , 1994; Clark et al , 2002; McManus et al , 2004]. Many studies point to a reduction in deepwater formation in the Nordic Seas and northern North Atlantic Ocean and the northward spread of southern‐sourced deep waters during D‐O stadials [ Keigwin and Boyle , 1999; Hagen and Hald , 2002], linked to the release of freshwater and icebergs to sites of overturning from the circum‐North Atlantic ice masses [ Bond and Lotti , 1995; Elliot et al , 1998; Lackschewitz et al , 1998; Knutz et al , 2001]. Dokken and Jansen [1999] and van Kreveld et al [2000] suggested that the formation of sea ice brines in the Nordic Seas during D‐O stadials could have destabilized stadial boundary conditions by generating an embryonic form of overturning circulation that eventually forced the MOC into a “warm” interstadial mode.…”