Dirk Slawinski's MSc thesis introduction
Introduction:
Human existence is dominated by cycles: solar, lunar, orbital, cosmic, climatic, and the cycle of life itself. Today, at this time of a "great human geophysical experiment" with the Earth's climate, we need to understand natural causes of climate variability. Lake sediments, tree rings, corals, and speleothems provide some of the best long-term archives of annual to century-scale variability to test the rhythms of forcing. The seasonality that is represented by an annual couplet provides direct evidence of changes in solar insolation. The amount of solar heat that a region of the Earth receives is due to the Earth's tilt and distance from the sun during the year. Areas near the poles effectively lose all solar input for a small part of the year just due to the angle of incidence (figure 1). Such variability has a predictable geometric rhythm, but what about variability in direct solar output? The output of the sun has been called a constant, but we now know that this solar "constant" of 1366.685 W/m2 is a "mean value" calculated for April 9-15 1993 (Crommelynck et al., 1994), and it has varied by 0.15% over one sunspot cycle or 11 years (Crommelynck et al., 1995) and may have varied as much as 0.30 % over the last 400 yrs (Lean, 1996). Furthermore, the Sun's output may have varied significantly in specific wavelengths, such as the Ultraviolet (UV) and the Infrared (IR), or in solar wind strength and the magnetic field (Lean and Rind, 1996). These changes still seem small relative to total solar radiation, so how much could they affect the Earth's climate? Amplifiers are needed.
The central theme of this study follows the hypothesis that we perhaps can also identify mechanisms of amplification of direct solar signals. One recent postulate suggests increased westerly winds due to the interaction of the Solar Wind and the Earth's geomagnetic dipole (Anderson et al, 1992), and another suggests cloud modification due to increased flux of cosmic rays by solar suppression of the Earth's geomagnetic field (Tinsley, 1994). Tests of these models require long, robust time-series stretching back beyond instrumental observations. Radiocarbon (14C) and 10Be now provide multiple evidence for modulations of cosmic rays in tandem with varied solar cycles (Lean and Rind, 1996, Stuiver and Braziunas, 1993). Do these translate to climatic signals in terrestrial archives?
My tenet is that the thickness variations in bundles of annual rhythms (varves) from lake sediment sequences provide a test of solar rhythms if they match known solar cycles and are shown to occur coherently across a transect of multiple sites and stratigraphic times. This thesis surveys the coherency of varve-bundle patterns from 5 lakes; Deep Lake, Elk Lake, Lake Mina, and Sandrock Site in Minnesota, and Steep Rock Lake in Ontario. These sites presented themselves as targets of opportunity, i.e. their data were readily available for my thesis research. They cover a range in time from about 15,000 ybp to today and an area roughly half the size of Minnesota.
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