Page 175 - Earth's Climate Past and Future

CHAPTER 8 • Insolation Control of Monsoons 151

drying up of the lakes. Individual layers in these sequences
are continuous over large areas, indicating that the wet-
dry variations in climate affected the entire basin.
Extensive investigations show that these fluctua-
tions in lake depth over millions of years were cyclic
(Figure 8-16). The shortest cycles occur over rock
thicknesses averaging 4–5 m, equivalent to about
20,000 years in time based on the average thickness of
each annual varve (0.2–0.3 mm). These cycles were dri-
ven by precession. Monsoons filled and emptied these
Pangaean lakes (Chapter 5) in response to orbital pre-
cession in the same way that North African lakes have
filled and emptied during much more recent times.
Because we are looking much farther back in time, the
periods of the orbital cycles were slightly shorter than
they are today (Chapter 7).
Two larger-scale groupings of cycle peaks are also
evident. The amplitude of individual 20,000-year peaks

Lake depth
Shallow Deep
FIGURE 8-15 Evidence of changing lake levels Dinosaur Length of orbital cycles
footprints in lake muds that have since hardened into rock
show that the Pangaean lakes occasionally dried out
completely. These footprints are from a basin in Connecticut 500 20,000 years
formed at the same time as the Newark Basin in New Jersey.
(Dinosaur State Park, Rocky Hill, CT.)

and dark layers, with one light/dark pair deposited each
year. Darker organic-rich layers were deposited in sum-
mer, lighter mineral-rich layers in winter. Dissolved
oxygen concentrations must have been low or zero Core depth (m)
in the deeper levels of the lake when the organic-rich
layers accumulated to prevent destruction of the delicate 100,000 400,000
varves by animals moving across and within the sedi- years years
ments. Use of these varves as an internal chronometer to
count elapsed time (Chapter 2) confirms that the total 600
time of lake-sediment deposition was about 20 Myr.
The types of sediment deposited in the Newark
Basin varied widely in response to changes in lake
depth. When the lake was deep (100 m or more), the
sediments tended to be gray or black muds with large
amounts of organic carbon. These sediments contain 20,000 years
finely laminated varves and are rich in well-preserved
remains of fish. Sediments deposited when the lake was
FIGURE 8-16 Fluctuations of Pangaean lakes Newark
shallower or entirely dried out tend to be red or purple
Basin lake sediments varied in depth from very shallow to over
because they were oxidized (rusted) by contact with air, 100 m deep at three tempos. Individual cycles in lake depth
and they often contain mud cracks due to exposure to every 4–5 m occur at a period of 20,000 years, clusters of
the dry air. Dinosaur footprints and the remains of plant larger deep-lake maxima every 20–25 m occur at intervals near
roots are also common in sediments from the dried-out, 100,000 years, and unusually large deep-lake clusters at
vegetated parts of the lakebeds (Figure 8-15). intervals of 90–100 m occur every 400,000 years. (Adapted
The thick sequences preserved in the Newark Basin from P. E. Olsen and D. V. Kent, “Milankovitch Climate Forcing in
repeatedly fluctuate between sediments typical of deep the Tropics of Pangaea During the Late Triassic,” Palaeogeography,
lakes and those that indicate a shallowing or complete Palaeoclimatology, Palaeoecology 122 [1996]: 1–26.)