(Scripps Institution of Oceanography, University of California, San Diego, USA, California 92093-02 15)
In the past 200ka, the important role of deep water in the North Atlantic (NADW) in driving climate change at high latitudes has been well understood, but little is known about the early history of NADW. Here, the author gives the sedimentological and paleontological data obtained from the North Atlantic Ocean and the Norwegian Sea-Greenland Sea, and points out that NADW started at 1 1.5Ma, that is, 2 ~ 3 Ma after the expansion of the Antarctic ice sheet, so the former cannot be used as the reason for the latter as previously speculated. Recent data show that the appearance of NADW may be caused by the cooling climate, rather than the sinking of Greenland-Scotland Ridge. In the near future, climate warming may reverse this process, stop NADW, and thus trigger a fundamental change in the climate.
Key words: paleoceanography and paleoclimate Miocene North Atlantic deepwater area, Norwegian Sea-Greenland Sea.
1 Introduction
The relatively warm and salty North Atlantic surface water flows to the Norwegian Sea-Greenland Sea, releasing a lot of heat to the cold area. Finally, the high-density water sinks and flows out from three sea sills on the shallow sea ridge (Greenland-Scotland Ridge) connecting Greenland and Scotland. During the decline period, the outflow of aquatic products produced the stranded North Atlantic water, which combined to form the North Atlantic Deep Water (NADW), which flowed southward and finally to the South Atlantic, Indian Ocean and Pacific Ocean. NADW plays an important role in the global circulation between deep seas, the distribution of nutrients in deep waters and the redistribution of global total heat. Atmospheric CO: and the large-scale climate fluctuation in the late quaternary is related to the change of NADW. Recently, many data about the origin of NADW are based on carbon isotope data from mid-latitude to low-latitude. However, due to the different positions and lithofacies of samples taken by different authors, there are still differences in the age determination of data obtained from a large number of boreholes.
5 carbon isotope data
About 1 1.5Ma, the initial formation of NADW is also reflected in the carbon isotope data, but little is known about the subsidence history of this ridge. Here, the newly determined age of NADW is 1 1.5Ma, which urges us to reevaluate the previous general hypothesis. During the period from 14Ma to 1 1Ma, according to Haq et al. (1987)[36], the sea level dropped by 220m, while according to Sclater et al. (1985)[37] or Miller. If the sinking of seamounts triggered the start of NADW, it should have formed before 14Ma, and should have stopped when the sea level decline rate exceeded the sinking rate of seamounts. In addition, the reconstruction of the subsidence history of Greenland-Scotland Ridge shows that the eastern part of the Danish Strait region fell below sea level in the Middle Eocene and early Middle Miocene [20]. The Wyville-Thompson Ridge in the east of Greenland-Scotland Ridge is considered to be on a stretched continental crust, which may have appeared in the early Eocene, and then the ridge deepened [37]. In addition, a feature of No.552 hole is that the overflow water from the Wavell-Thompson Ridge is formed at the same time as the overflow water from the Danish Strait, although the bed depth of the Danish Strait is quite different from that of the Wavell-Thompson Ridge. Finally, the threshold critical depth as the initiation depth of NADW has not been successfully simulated.
On the other hand, the data of dinoflagellate, which is similar to the location of hole 642, shows that the cold climate may trigger the start of NADW, and the start of NADW is in the period of significant cooling in the Norwegian Sea (Figure 2). The beginning of NADW coincides with the oxygen isotope enrichment period (MI5), which is a sign of global cooling and/or ice increase. In fact, since 15Ma, the global climate has cooled, when the Antarctic ice sheet began to expand rapidly. The oxygen isotope data of planktonic foraminifera in hole DSDP408 in the Danish Strait show that its value is about 1+0 ~ 1 2ma [38]. This shows that the surface seawater temperature is around 7℃, and it is estimated that the ice volume at that time was 1/2 now. This means that in hole 408, the surface water has cooled to the present temperature 1 1 ~ 12ma. In addition, the sporopollen data of Iceland [39] show that the annual average temperature is 3 ~ 5℃ around 100 Ma. During the period of 10 ~ 12ma, the relatively low temperature and the steep temperature gradient from the equator to the pole in the northern high latitudes greatly strengthened the atmospheric circulation. The North Atlantic Ocean current flows to the Norwegian Sea, and its surface seawater is cooled below the critical temperature, and a large-scale hot salt cycle begins.
In a word, climate cooling is an important factor in the formation of NADW, and large-scale hot-salt cycle can be carried out without the relatively cold Norwegian Sea-Greenland Sea. It is of far-reaching significance to understand the important role of climate cooling in the early stage of NADW. As predicted by climate modelers, the large-scale temperature rise at high latitudes in the next few centuries may reverse the process that led to the start-up and final shutdown of NADW. If so, the North Atlantic and Western Europe may get cold, and the global climate may be fundamentally different. The potential serious consequences of this possibility require correct research on NADW at different scales.
As a result of this research, part of the funds were sponsored by the National Science Foundation and the Petroleum Research Fund. The discussion with E.Jansen on paleooceanography in the Norwegian-Greenland Sea was also helpful. Thanks to W.Berger and W.Hay for reading the first draft of the paper. Professor berggren and Professor Wang put forward valuable suggestions for further improvement. The samples used in the study were provided by ODP.
(Translated by Zhou Lijun, Xu Dong Language School)
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