1. Radiometric dating shows the earth to be 4.5 billion years old.
Loess deposits (deposits of wind-blown silt) in China are 300 m thick.
They give a continuous climate record for 7.2 million years. The
record is consistent with magnetostratigraphy and habitat type inferred
from fossils. [Donghuai et al, 1997; Ding et al, n.d.; Russeau & Wu,
1997]
2. Varves are annual sediment layers that occur in large lakes.
They are straightforward to measure, cover millions of years, and
correlate well with other dating mechanisms.
a. In seasonal areas, sedimentation rates vary across the year, so
sediments often show annual layers (varves) distinguished by texture
and/or composition. We can be confident that the layers are
seasonal because we see the same sorts of layers occurring today.
Even if they were not seasonal, the fineness of the sediments is
often such that each layer would require several days, at least, to
form. Some formations have millions of layers, such as the varve
record from Lake Baikal with 5 million annual layers [Williams et
al, 1997], and the 20,000,000 layers in the Green River formation.
They must have taken hundreds of thousands of years to form at the
very least.
b. Dates obtained by counting annual layers of varves match dates obtained
from radiometric dating. One varve formation, covering 45,000
years, was used to calibrate C-14 dating using terrestrially
produced leaves, twigs, and insect parts that also appeared in the
sediments. The varves were easy to count because they included an
annual diatom bloom. [Kitigawa & van der Plicht, 1998].
c. Varves record climate changes, too, since climate affects the amount of
sediments. Climate is affected by orbital cycles known to occur at
about 400,000, 600,000, and million year periods (the so-called
Milankovitch cycles). Climate cycles of these durations occur in
the varve records.
* Lake Baikal contains annual layers from 12
million years ago to the present. These sediments contain periodic
changes matching the orbital cycles. [Kashiwaya et al, 2001]
References:
- Ding, Z.L. et al. Rearrangement of atmospheric circulation at about
2.6 Ma over Northern China: Records of evidence from grain size
loess-red clay sequences.
http://fadr.msu.ru/inqua/nl-15/llz-abs.html#11
- Donghuai Sun et al., 1997?
Magnetostratigraphy and paleoclimatic interpretation of a continuous
7.2Ma Late Cenozoic eolian sediments from the Chinese Loess Plateau.
http://www.agu.org/pubs/gap/DonghuaiS/DonghuaiS.html
- Kashiwaya, Kenji, S. Ochiai, H. Sakai & T. Kawai, 2001. Orbit-related
long-term climate cycles revealed in a 12-Myr continental record from
Lake Baikal. Nature 410, 71-74.
- Kitagawa, H. and van der Plicht, J., 1998. Atmospheric Radiocarbon
Calibration to 45,000 yr B.P.: Late Glacial Fluctuations and Cosmogenic
Isotope Production. Science 279: 1187-1190. See also Kitagawa,
H. and van der Plicht, J., 2000. PE-04. A 45.000 year varve chronology
from Japan. http://www.cio.phys.rug.nl/HTML-docs/Verslag/97/PE-04.htm
- Russeau, D.-.D., and Wu, N., 1997. A new molluscan record
of the monsoon variability over the past 130,000 yr in the
Louchuan loess sequence, China. Geology 25(3): 275-278.
- Williams, D. F., J. Peck, E. B. Karabanov, A. A. Prokopenko,
V. Kravchinsky, J. King, and M. I. Kuzmin. Lake Baikal Record of
Continental Climate Response to Orbital Insolation During the Past 5
Million Years. Science 278: 1114-1117.