Geoscience Reference
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Precipitation of d 18 O at this cave site is believed to be largely a measure of the
fraction of water vapor removed from air masses moving between the tropical
Indo-Pacific and southeastern China. Rainfall integrated between tropical sources
and southeast China was found to be significantly lower during glacial times than
interglacial times, perhaps related to lower relative humidity. This reduced precipi-
tation pattern correlated to some degree with reduced temperatures during the last
ice age, as measured at Greenland.
Because the cave data were dated without use of any tuning process, they
provide (like Devil's Hole) an independent chronology for events around the
termination of the previous ice age. They found a large sudden increase in
precipitation at 129 kybp , a fairly constant level of high precipitation from 129 to
120 kybp , and a large sudden decrease in precipitation at 120 kybp .
While the authors claimed that this behavior mirrored the variability of solar
input (as Figure 9.9 shows), the timing of these cave variations does not match
solar timing. Nor does it match the ice core data in Figure 6.3 . The Devil's Hole
data (expanded version of Figure 6.1 ) show a sharp increase in temperature from
about 140 to 133 kybp , high temperatures persisting from 133 to about 121 kybp ,
and a sharp decrease in temperature from about 121 to 112 kybp . This seems to
parallel the Chinese cave data except that the Devil's Hole dates are about 10,000
years earlier.
6.3 MAGNETISM IN ROCKS AND LOESS
6.3.1 Magnetism in loess
The magnetic properties of thick (100-300m) deposits of wind-borne dust, called
loess, from China may provide a continuous record of climate variations over the
last 2.6 million years. Although the loess paleoclimate records are not as detailed
as those from ice cores, they are a potential source of continental paleoclimate
information for non-Arctic land regions.
When sediment is deposited, magnetic particles within the sediment tend to
align with the Earth's magnetic field. As they are compacted and consolidated
they become immobile, preserving a record of the direction in which the magnetic
field was oriented at the time of deposition. Because the Earth's field reverses
polarity from time to time and the reversal chronology has been determined with
good accuracy for the last 100 million years or so, sediments can be dated by
comparing the alignment of magnetic particles with the established reversal
history.
According to Banerjee and Jackson (1996), the thick loess deposits of central
China have accumulated at an average rate of about 10 cm every thousand years,
and the rate increased by a factor of 3-4 during ice ages due to stronger winds
and a greater preponderance of dust in the air. During interglacial periods, soils
developed on the loess surface. As the cycle of glaciation and deglaciation
continued, alternating layers of loess and soil built up. The soils and loesses can
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