Causes of Global Cooling

It is still not clear what triggered the collapse of the Frasnian hot climates and the rapid fall of global temperature into the cold Famenn-ian world. The temporal structure of the Late Devonian extinction (Figure 1) consists of a series of extinction pulses that occurred on multiples of 100,000-year scales over a period of1 to 2 million years, and it is thus similar to the temporal pattern seen in the glacial cycles of the Pliocene and Pleistocene. However, all attempts to find geologic evidence of glaciation in the Late Frasnian to Early Famennian interval of time have failed.

At the present, two alternative theoretical scenarios have been proposed to explain the rapid global cooling that apparently triggered the Late Devonian extinction. Both of these hypotheses are catastrophic: one proposes that an interval of major flood basalt volcanism occurred during the Late Devonian, while the other proposes that an interval of impact bombardment occurred during the Late Devonian, during which a series of asteroids or comets struck the earth.

In the catastrophic impacts scenario, the earth is envisioned to have been impacted by a series of asteroids or comets, impacts that occurred sequentially over a span of 1 to 2 million years. One of the climatic effects of the impact of a large asteroid with the earth is rapid global cooling, caused by the vast amount of dust and debris injected into the earth's atmosphere by the explosive vaporization of both the asteroid and the earth's crust at the impact site. An impact-produced global dust cloud would block light from the sun from reaching the earth's surface, triggering a plan-etwide "impact winter" and lethally cold temperatures, even at the equator. A string of such impacts is hypothesized to have produced a series of cooling pulses during the Late Devonian, and it is thought that these triggered the observed extinction pulses (Figure 1).

The catastrophic volcanism scenario is based upon the recognition that there have been flood basalt episodes in earth's history, during which enormous amounts of lava, gasses, and volcanic dust have been produced in volcanic eruptions of almost unimaginable magnitude. These flood basalt fissure eruptions are produced by gigantic plumes of molten rock that originate deep in the earth's mantle and that slowly rise to produce paroxysms of volcanic eruptions over huge geographic areas when they intersect the earth's surface. Several flood basalt episodes are known to have occurred in other times of biotic crisis, such as the eruption of the Siberian flood basalts during the Permo-Triassic extinction and the Deccan flood basalts during the Cretaceous-Tertiary extinction. It has been hypothesized that the enormous volume of volcanic gasses and dust injected into the earth's atmosphere during such events would produce catastrophic climatic effects very similar to those produced by the impact of an asteroid with the earth—a "volcanic winter" scenario rather than an "impact winter" one.

Vast regions of the bottoms of the earth's

Table 1

Late Devonian Impact Craters

Table 1

Diameter

Crater

(kilometers)

Known Late Devonian Impact Events

Woodleigh (Western Australia)

120

Alamo (New Mexico)

70

Siljan (Sweeden)

52

Charlevoix (Quebec, Canada)

46

Flynn Creek (Tennessee)

4

Probable Late Devonian Impact Events

Taihu Lake (China)

100

Aorounga (Chad)

13

Panther Mountain (New York)

10

oceans were depleted of oxygen during the Late Devonian. It has been argued that the great geographic extent of these anoxic water masses was produced in part by extensive submarine volcanism, and hence might be evidence for catastrophic volcanic episodes during the Late Devonian. In addition, a major continental rift system is now known to have been active in the Ukraine region of Europe during the Late Devonian (the Pripyat-Dnieper-Donet rift), and many of the earth's flood basalt fissure eruptions are associated with rifting and spreading of the earth's tectonic plates. On the other hand, the volume of volcanic material erupted in the Pripyat-Dnieper-Donet rift appears to be relatively small (less than 10,000 cubic km); thus the intensity of the volcanism associated with the rift was not near the magnitude associated with the Siberian or Deccan flood basalt fissure eruptions (both of which produced well over a million cubic km of volcanic material).

In contrast to the catastrophic volcanic scenario, much more evidence exists for the hypothesis that the earth was impacted by a series of asteroids or comets during the Late Devonian. This evidence consists of craters blasted into the earth's surface by an impact event: eight impacts are known to have occurred during the Late Devonian (Table 1), and another three impact events are probable.

These impacts all occurred on the earth's continents; thus an unknown number of additional impacts may have occurred in the earth's oceans, for which we have no geologic record.

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