Page 116 - Earth's Climate Past and Future

92 PART II • Tectonic-Scale Climate Change

long-term succession of glacial (icehouse) versus 50 Background
nonglacial (greenhouse) climates. In this view, high sea Iridium concentrations
levels caused warm climates by moderating the harsh
winters, and low sea levels caused cold climates by per-
mitting the very cold winters typical of continental con- 40
ditions. Although the timing of the high sea level and
largely ice-free climate 100 Myr ago compared with the Typical
low sea level and glacial climate today fits this explana- sediment
mixing
tion, this view is no longer viable. 30 profile
The major criticism of this idea centers on the fact
that summer-season ablation is a powerful factor in Sediment (cm)
determining the extent of snow and ice (companion 20
Web site, pp. 9-11). The problem is that low sea levels
and withdrawal of the ocean from continental interiors
lead to more extreme continental climates, including
very hot summers. No matter how cold winters become 10
in a continental climate, hot summers should easily melt
any snow that accumulated and thereby oppose glacia-
tion. Conversely, high sea levels should cause cooler,
more maritime summers that favor the persistence of 0 2 4 6 8 10
snow and ice through the summer ablation season at Iridium (parts per billion)
very high latitudes. A Iridium spike
The record of the last 100 Myr supports this criti-
cism. The high sea levels of 100 Myr ago were not
accompanied by glaciation, and the low sea levels of
today are. As a result, the hypothesis that sea level is the
major control of long-term glaciation finds little or no
support today. Glaciation is now seen as a cause of low
sea level (because of storage of ocean water in ice
sheets) rather than a result.

Asteroid Impact (65 Myr Ago)

The greatest catastrophes known to have affected Earth
are the rare but massive impacts of large extraterrestrial
asteroids and comets. An inverse relationship exists
between the sizes of these objects and the frequency B Quartz grain subject to shock waves
with which they hit Earth. The largest bodies (more
than 10 km in diameter) arrive only every 50 to 100 Myr FIGURE 5-13 Evidence of an asteroid impact (A) Ocean
but result in much greater environmental effects than sediments containing a layer enriched in the element iridium are
the smaller, more frequent impacts. evidence of a large asteroid impact 65 Myr ago. (B) Sediments
The impact event 65 Myr ago coincided with a deposited in Montana 65 Myr ago contain grains of quartz
global-scale extinction of some 70% of the species and crisscrossed by multiple lineations produced by high-pressure
40% of the genera living at the time, including all the shock waves from an asteroid impact. (A: adapted from
dinosaurs and all but one of twenty-five species of W. Alvarez et al., “Extraterrestrial Cause for the Cretaceous-
planktic foraminifera. The geologic evidence for this Tertiary Extinction,” Science 280 [1095–1108]. B: Glenn Izett,
impact includes the worldwide distribution of a thin Williamsburg, VA.)
layer of sediment enriched in iridium (Ir), an element
that is rare on Earth but 10,000 times more abundant in
some kinds of meteorites (Figure 5-13A). This element of sudden pressures much larger than those found on
was deposited in a thin layer that was later mixed by Earth, even in highly explosive volcanoes (Figure 5-13B).
burrowing animals. The best candidate for the site of the impact 65 Myr ago
Other supporting evidence for an impact event is a crater in eastern Mexico on the Yucatán Peninsula,
includes small grains of quartz with distinctive textures between the Caribbean Sea and the Gulf of Mexico
called “shock lamellae” that are formed by the shock wave (Figure 5-14).