Temperature in C

Chronozones

Chronozones

FIGURE 6.52 Reconstruction of SSTs in August (open circles) and February (dots) in the southeastern Norwegian Sea (-63° N) over the last 13,400 (MC) yr, based on diatom assemblages calibrated with modern SSTs. Oscillations of temperature within the Bolling-Allerad chronozones (Older Dryas I and II, Boiling Cold Periods I and II) and meltwater peaks IA and IB are indicated (Ko? Karpuz and Jansen, 1992).

FIGURE 6.52 Reconstruction of SSTs in August (open circles) and February (dots) in the southeastern Norwegian Sea (-63° N) over the last 13,400 (MC) yr, based on diatom assemblages calibrated with modern SSTs. Oscillations of temperature within the Bolling-Allerad chronozones (Older Dryas I and II, Boiling Cold Periods I and II) and meltwater peaks IA and IB are indicated (Ko? Karpuz and Jansen, 1992).

(the start of the Younger Dryas period) with summer SSTs dropping to ~1 °C in <50 yr, and remaining low for the next 1000 yr. During this interval, glaciers advanced in many areas and quite dramatic changes in the distribution of flora and fauna took place throughout western Europe. An equally rapid warming ensued at -10,200 B.P., followed by a short cool spell centered on 9800 B.P. at the beginning of the Pre-Boreal period. SSTs then rose to Holocene maxima (13-14.5 °C in summer) from 9500-5000 yr B.P. These changes are remarkably synchronous with the 8lsO record from Dye-3 in southern Greenland, which reflects the changes in oceanic heat flux and associated atmospheric temperature signal (Fig. 6.53). The 8lsO records from the same area indicate that meltwater from the Fennoscandian and Barents Sea ice sheets played a critical role in bringing about these oscillations. The strongest meltwater signal (resulting in low sea surface salinities, as seen in unusually low 8lsO values) began -14,700 yr B.P., reaching a maximum at -13,500 yr B.P. (Sarnthein et al., 1992), at which point SSTs at 63° N rose to the level where diatoms could survive (Duplessy et al., 1992). Thus, initial melting was probably related to the orbitally induced insolation increase, amplified by calving into a rising sea level, as the ice continued to melt back. Once warmer waters penetrated northward, the subsequent melting of the ice sheet was very much controlled by associated warm air advection. However, meltwater played a key role in controlling SSTs and led to the series of warm-cold oscillations in late Glacial and early Holocene time. Ko$ Karpuz and Jansen (1992) suggest that as meltwater from the Fennoscandian ice sheet flooded the Norwegian Sea, the low salinity surface waters would have frozen over in winter, minimizing mixing in the upper layers of the ocean and restricting the warmer Atlantic water to below the halocline. The ensuing cooler conditions would have reduced melting, eventually leading to erosion of the shallow low-salinity mixed layer and rapid warming as the warmer waters reappeared at the surface. This in turn led to renewed melting and the entire sequence began again, as recorded in the Boiling-Allered Interstadial Complex of oscillating warm and cold episodes (see Fig. 6.52). The Younger Dryas cold event may have lasted longer because of major drainage of meltwater from the Laurentide ice sheet (mwp-IA), which in some way set up the conditions necessary to prolong disruption of circulation in the North Atlantic, as noted earlier. Similar conditions may have occurred during the final cool episode around 9800 yr B.P., which coincided with mwp-IB (Fig. 6.53). The 813C in benthic forams from the Bermuda rise record reductions in NADW flux from the Norwegian Sea during the Younger Dryas episode, though a less dense intermediate water ("upper NADW") from the Labrador Sea may have continued to form, as registered even as far away as the Southern Ocean at that time (Lehman and Keigwin, 1992a, b; Charles and Fairbanks, 1992).

An alternative scenario for the Younger Dryas involves Arctic Ocean sea ice. During the LGM, when sea level was 120 m lower and warm waters did not penetrate to the northern reaches of the North Atlantic Basin, the Arctic Ocean was extremely isolated. The exposed Bering Land bridge prevented North Pacific water from entering the Arctic Basin, and the only source of ice export from the Arctic Ocean was through a very restricted channel (a narrower Fram Strait, between northeast Greenland and Svalbard) as the Barents Sea region was occupied by a grounded ice

Apparent discharge rate (km3 yr-1) 5.000 10.000

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