2. Long-Term Solar Activity Variations in the Pleistocene and Their Connection With Abrupt Climate Change

[3]  It is well known that the North Atlantic region is one of the key regions where conditions for abrupt climate change of the global scale can arise [Meeker and Mayewsky, 2002; Rogers and Van Loon, 1979; Van Loon and Rogers, 1978]. One of stimulators of abrupt climate change can be dynamics of the North Atlantic overturning circulation (NAOC), i.e., the dynamics of the position of the Gulf Stream edge when the surface current converts into deep-water current of an opposite direction. The position of the northern edge of NAOC is governed by the temperature factor, when waters increase their density on cooling and sink into the ocean depths, and desalination of the ocean surface layer under the action of increasing precipitation in the Arctic or considerable ice rafting from Greenland and North America glaciers. Note that these are the processes with positive feedback because southward displacement of the Gulf Stream tongue contributes to even a greater temperature decrease in the Arctic region.

Figure 1
[4]  Analysis of variations in the surface temperature of the Atlantic Ocean and also temperature variations in Greenland for the last ice age revealed sharp and large temperature variations, the so-called Dangaard-Oeschger (D-O) and Henrich events with a periodicity of around 1570 and 4800 years, respectively [Dansgaard et al., 1993; Grootes and Stuiver, 1997] (Figure 1). Later analysis of bottom sediments from the North Atlantic showed that these temperature oscillations are associated with ice rafting from glaciers of Greenland and the North American continent [Bond and Lotti, 1995; Bond et al., 1999]. For the time intervals indicated above, large amounts of gravel and sand carried from the continent were found in bottom sediments. It is natural to suppose that warming periods corresponded to the time intervals preceding ice-rafting events. In this respect it is reasonable to compare the development of D-O events with the solar activity periodicity. As a criterion of solar activity oscillations with durations of hundreds and thousands of years, variations in the concentrations of cosmogenic isotopes 14C and 10Be in dated terrestrial archives can be used.

[5]  Generation of cosmogenic isotopes occurs in the atmosphere under the action of high-energy cosmic rays. In its turn, the intensity of cosmic rays in the Earth's atmosphere depends on the degree of turbulization of the solar wind and, hence, solar activity. Thus, the periodicity in solar activity oscillations governs the periodicity of variations in the cosmic ray flux intensity in the atmosphere. Galactic cosmic rays and high-energy solar cosmic rays entering the Earth's atmosphere form a number of cosmogenic nuclides, such as carbon isotope 14C (radiocarbon) and beryllium isotope 10Be. Radioisotopes 14C and 10Be are suitable for studying natural processes, such as time variations of the geomagnetic field and solar activity. As a result of exchange processes in the environment, these isotopes are recorded in dated natural archives: 14C is found in tree rings and 10Be is contained in glaciers and bottom sediments. Studies of dated natural archives is a unique tool of the investigation of dynamic processes at the Earth and in the near-Earth space on the time scales from tens to several thousands of years on the basis of 14C data (the half-life of 14C is around 5730 years) and to hundreds of thousands of years on the basis of 10Be data (the half-life of 10Be is around 1.5 million years). For the time interval of tens of thousands of years, it is reasonable to use the data on the 10Be concentration to characterize the solar activity level. Note that variations in concentrations of both 14C and 10Be depend on not only solar activity, but also on the exchange processes. Along with this, Bard et al. [1997], by comparing variations in 14C and 10Be for the last millennium, have shown that variations in 14C and 10Be trace well solar activity variations. Beer [2000] has shown that variations in 14C and 10Be give information on solar activity variations for the Holocene as well. In addition, Finkel and Nashiizumi [1997] have demonstrated that variations in the 14C and 10Be concentrations in time intervals from 8000 to 30,000 years are similar. For this reason, it can be believed that the data on 10Be trace solar activity variations, at least qualitatively, in the Pleistocene.

Figure 2
[6]  The presently available experimental data allow one to compare solar activity variations (variations in the 10Be concentration) and variations in surface temperature δ18O) [Finkel and Nishiizumi, 1997; Grootes et al., 1993] for the last 40,000 years by using cores from one and the same borehole in Greenland (GISP2). These data are presented in Figure 2. As can be seen, the graphs are similar, the intervals of a sharp temperature decrease are preceded by the intervals of an enhanced solar activity. Figure 2 also shows temperature variations (  δ18O ) for another Greenland borehole (GRIP) [Dansgaard et al., 1993] and the graph of development of ice-rafting events (IRE) in the North Atlantic derived from the data of borehole VM23-081 in the ocean bottom [Bond and Lotti, 1995; Bond et al., 1999]. It is apparent from Figure 2 that the variations in surface temperature inferred from the data of both boreholes are similar. Each development of the D-O event corresponds to the formation of layers with an increased content of gravel as evidenced by the data for the borehole in the ocean bottom, which indicates that ice-rafting events occurred in relevant time interval. Thus, the data presented in Figure 2 show that the IRE are preceded by the time intervals of a high solar activity and, hence, a stronger heating of the ocean surface. This suggests that in the Pleistocene, when the ice age developed, a high solar activity level and, most probably, a high solar irradiance stimulated development of such surface processes as ice-rafting events in the Northern part of the Atlantic ocean, and thereby caused abrupt climate changes. Note that the experimental data given above are confirmed by simulation of the IRE development [Shaffer et al., 2004]. The results of simulation point to a periodicity in the occurrence of ice-rafting events, consistent with experimental data, and also to the fact that the IRE onset is preceded by an increase in the ocean temperature.


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