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CHAPTER 8 • Insolation Control of Monsoons 149

that the tropical ocean may have delayed full develop- Monsoon Forcing Earlier in Earth’s History
ment of the summer monsoon by controlling the supply
of latent heat that fuels the monsoons. The concept of insolation forcing of summer monsoons
An alternative view is that the peak monsoon response can be used to investigate environments that existed in
should have the phase of insolation forcing during the the distant geologic past. Orbital-scale changes in sum-
midsummer month of July (or February). This view has a mer insolation at the precession cycle drive changes in
reasonable physical basis because modern summer mon- monsoonal precipitation, and precipitation is a key con-
soons reach their peak development during these hot trol of processes that leave evidence in ancient climate
midsummer months. With the continents heated to their records, such as sediment-laden runoff and lake depth. As
maximum summer temperatures, the temperature con- a result, many ancient sedimentary rocks contain valu-
trast with the cool oceans also reaches a maximum, and able information about varying monsoonal precipitation.
this temperature difference drives the strongest monsoon If high-quality time control is available for ancient
circulations of the year. In this view, precession-cycle deposits, we can look for evidence of the kind of mon-
insolation changes aligned with this intense midsummer soon signature shown in Figure 8-5. Because many of
heating should have the greatest impact in boosting conti- these deposits extend over millions of years, we can
nental temperatures and in driving the strongest mon- expect to see records that look like those in Figure 8-13.
soons. From this perspective, the midsummer monsoon A wide range of sediment indicators linked to precipita-
responses are forced by midsummer insolation with no tion, erosion, runoff, transport, and deposition may
significant lags in the climate system. have this appearance. In some cases, this relationship
At this point, climate scientists cannot prove which can even be used to refine (“tune”) time scales initially
of these views is correct. In addition, the phasing of determined by radiometric dating (Box 8–1).
peak summer monsoons at orbital time scales is not yet As in the case of North African lakes, we expect the
fully known for all regions. In any case, the evidence monsoon signature to show clusters of two or three
supports Kutzbach’s original hypothesis that summer strong maxima separated by clusters of two or three
monsoon variations every 23,000 years should have a weak maxima, with these clusters repeating in the record
July phase in the north (and February in the south). at intervals of about 100,000 years because of control of

BOX 8-1 LOOKING DEEPER INTO CLIMATE SCIENCE
CONTINUED

Assume that a sediment core contains two magnetic rever- East Africa, volcanic eruptions deposit beds of basalt
sal boundaries that have been dated by correlation to the (lava) or volcanic ash that can be radiometrically dated
global magnetic stratigraphy established by dating basalt by K/Ar methods (see Chapter 2). The volcanic deposits
layers on land (Chapter 4). The ages of these reversals con- provide the initial time framework for the tuning
strain the intervening sequence of sediments to a particular process, analogous to the use of magnetic reversal
interval of time. Also assume that this sediment sequence boundaries in marine sediments. The monsoon-driven
contains a record that is directly tied to the strength of the variations in lake size during the sequence lying
tropical monsoons, such as a sequence of sapropel layers or between the basalt layers can then be tuned to the
of changes in the composition of marine plankton. astronomically dated record of summer insolation.
In many cases, the monsoon-related response mea- Because records on land are generally much more diffi-
sured in the sediments will show an obvious correlation to cult to date than those in ocean sediments, tuning of
the summer insolation forcing. Both the insolation changes these lake sequences provides an enormous improve-
and the monsoon responses recorded will show cycles near ment over other dating methods.
23,000 years and obvious modulation of these cycles at For sedimentary records that contain climatic responses
eccentricity periods near 100,000 and 400,000 years. The at the cycles of both precession and obliquity, the tun-
tuning process (matching maxima and minima in the mon- ing method can be tested even more rigorously. In this
soon response to correlative features in the insolation sig- case, the “tuned” time scale must match not just the
nal) allows the ages within the sedimentary sequence to be amplitude-modulated precession cycle at 23,000 years
assigned to specific precession cycles in the past at a finer but also the tilt cycle at 41,000 years. This added require-
resolution than the 23,000-year length of each cycle. ment makes the tuning process an even more demanding
This method has also been applied to sediments exercise but one that is even more likely to yield a unique
deposited in long-lived lakes on continents. In regions like time scale.