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152 PART III • Orbital-Scale Climate Change

in lake depth rises and falls roughly every five or six Orbital-scale Tectonic Combined uplift
cycles separated by 20–25 m of sediment, or a little less insolation uplift and insolation
than 100,000 years. An even larger-scale change in
amplitude of the monsoon-cycle peaks occurs between
approximately 530 and 620 meters depth in the core
(see Figure 8-16), or over a time interval of about
400,000 years.
These two longer-term patterns match the expected
monsoon signature shown in Figure 8-13 remarkably
well. They reflect a modulation of the strength of the
20,000-year precession cycles by eccentricity changes
at intervals of about 100,000 and 400,000 years. The
full imprint of ancient monsoons is amazingly clear
in the sediments of this basin despite the passage of
200 Myr.
Time

8-8 Joint Tectonic and Orbital Control
of Monsoons

We saw in Chapter 5 that tectonic changes affect the
intensity of monsoon circulations. Large landmasses
such as Pangaea intensify monsoons by offering a larger
area for the Sun to heat. Positioning of landmasses at
lower latitudes is important because solar radiation is Monsoon
more direct and albedos are much lower than at higher, threshold
snow-covered latitudes. Topography is a key control
over monsoon strength at tectonic time scales because
high-elevation regions focus strong monsoonal rains on
their margins.
The processes that control monsoon intensity over
tectonic time scales interact with those at orbital scales. FIGURE 8-17 Combined tectonic and orbital forcing of
Tectonic-scale processes alter the average strength of monsoons Monsoons are driven by orbital-scale variations in
the monsoon over millions of years, while the orbital- insolation (left) and by slower-acting tectonic factors such as
scale insolation changes drive shorter-term monsoon plateau uplift (center). The combined tectonic and orbital
forcing causes the amplitude of orbital-scale monsoon
strength at a cycle of about 20,000 years. One way the responses to increase and gradually exceed critical thresholds
tectonic and orbital factors might interact is suggested (right). (Adapted from W. F. Ruddiman et al., “Late Miocene to
in Figure 8-17. On the left is a schematic version of a Pleistocene Evolution of Climate in Africa and the Low-Latitude
low-latitude summer insolation curve, with individual Atlantic: Overview of Leg 108 Results,” Ocean Drilling Program
maxima and minima at the 20,000-year precessional Initial Reports 108B [1989]: 463–84.)
cycle and modulation of this cycle every 100,000 and
400,000 years. The smooth curve in the center repre-
sents gradually changing tectonic-scale processes, such monsoon cycles never exceeded this threshold. Later, as
as the slow uplift that gradually intensifies the average tectonic processes created conditions more favorable to
strength of the monsoon over millions of years. This monsoons, peaks in summer insolation would have dri-
slow tectonic-scale increase in monsoon strength com- ven monsoons that began to exceed the threshold by
bines with the orbital-scale monsoon cycles to produce small amounts and then later by steadily increasing
the response shown on the right—a slow increase in the amounts.
amplitude of the orbital-scale cycles caused by tectonic Something like this kind of evolving climatic
amplification. response is thought to have occurred in Southeast Asia
We can hypothesize the existence of a threshold over the last 30 or 40 Myr. A long-term tectonic increase
value above which key climatic indices record monsoon in monsoon intensity due to uplift progressively intensi-
responses but below which no response is registered (as fied the amplitude of orbitally driven monsoon cycles in
in Figure 8-13). In the changes shown on the right in this region. Simulations run on general circulation mod-
Figure 8-17, the tectonic influence may have been weak els indicate that the combined effects of orbital-scale
enough during the earlier intervals that the orbital-scale insolation and uplift are not additive in a simple linear