so, claims have been made that a correlation may exist between the timing of major LIP growth and major mass extinctions during the Phanerozoic (Haggerty, 1996; Morgan et al., 2004).

GLACIAL-INTERGLACIAL TIMESCALES

The magnitude and distribution of solar radiation, or insolation, received by the Earth’s surface varies due to change in Earth’s location and orientation relative to the Sun. Over periods of 100,000 and 400,000 years, the Earth’s orbit around the Sun varies from nearly circular (eccentricity = 0.00) to slightly elliptical (eccentricity = 0.06). Received total radiative energy changes by about 0.1 percent as a result of the altered distance of the Earth from the Sun. The tilt, or obliquity, of Earth’s orbit, which is currently approximately 23.5° from an axis perpendicular to the plane of orbit, is primarily responsible for the existence of seasons. Variations in the obliquity from 22.1° to 24.5° alter the seasonal distribution of radiation on the Earth’s surface with a period near 41,000 years. Polar regions receive greater insolation when the tilt is largest. Precession of the Earth’s orbit, which occurs with a roughly 22,000-year periodicity, further modulates seasonality, influencing the relative timing of Earth’s closest approach to the Sun (perihelion) relative to the timing of seasons. Currently, perihelion coincides approximately with the Northern Hemisphere winter solstice (favoring decreased seasonal changes in response to seasonal changes in insolation), but the reverse was true 12,000 years ago, at the beginning of the Holocene period.

Periods of approximately 22,000, 41,000, and 100,000 years are prevalent in paleoclimate records and are generally considered to relate, at least in part, to orbital forcing. However, interpreting the dominant 100,000-year cycle in this way is problematic because insolation changes of the order 0.1 percent are too small to have produced the extensive glaciation and cooling of the glacial cycles (Raymo, 1998) and are insufficient to generate the observed variations in model simulations (Kukla and Gavin, 2004). The weak eccentricity forcing must be amplified by the climate system, but the amplifying mechanisms are not well understood. Broecker (1994) noted that glacial terminations are abrupt, in marked contrast to the gradual (sinusoidal) changes in orbital variations. Wunsch (2004) has argued from statistical analyses of climate records that orbital forcing of the 100,000-year glacial cycles accounts for only 20 percent of the variance and is likely indistinguishable from chance. He concluded that broadband stochastic processes are probably the dominant control on glacial cycles.



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