The temperate conifer forests flanking the Cascade Mountains of the Pacific Northwest are a product of the regional climate. Frequent rain and fog encourage the growth of lush vegetation in a part of the world where cool Pacific storms march relentlessly shoreward from the north and west.
A million years from now, a geoscientist examining the fossils and sediments of the rocks that are currently being formed in this region will be able to say confidently that the climate had been moist and cool. Unless, that is, that scientist happened to be looking at rocks formed near Mount St. Helens or another of the active volcanoes in the range. Looking at these rocks, the scientist might think that the Pacific Northwest was a savannah or even a desert. According to Judith Harris of the University of Colorado Museum and John Van Couvering of the American Museum of Natural History, volcanically influenced ecosystems may look, in retrospect, as if they developed in a much drier regional climate than actually existed.
They named the phenomenon "mock aridity. " Their idea may explain why computer models tend to predict wetter climates than those suggested by the fossils. And it may mean that some paleoecologists will have to reevaluate their evidence. Explains Harris, "Volcanic activity makes for a barren environment. " After an isolated volcanic episode, a pioneering biological community will develop, followed by successional communities and, eventually, a climax community. When an ecosystem has reached climatic climax, it is a steady—state community that reflects the regional climate. But if volcanism is persistent, the biological community will never have the chance to reach climatic climax. It will bounce around between several pioneer and successional stages.
The possible implications are that some well—established paleoclimate stories might have to be rewritten. The two most significant examples are equatorial East Africa from the Miocene epoch (about 23 million years ago) to the present and the North American Great Plains from the Oligocene epoch (about 38 million years ago) to the present. Both have been interpreted as having woodland or savannah mosaic ecologies throughout each period. A savannah mosaic may consist of woodlands, treed grasslands and grasslands. Both areas, however, experienced persistent volcanism throughout each period. Had there been no volcanic activity, both regions might have appeared wetter. There might have been deciduous forests in North America and deciduous rain forests in East Africa.
A test of this idea, asserts paleoanthropologist Richard Potts, is to look at the global climate record in places where volcanism is not a factor. In the oceans, global temperature fluctuations are recorded in the oxygen—isotope ratios of marine sediments. These records are relatively immune to the local effects of terrestrial volcanism. "There you see the climatic change occurring later than what people have been seeing on land, " says Potts. This is consistent with Harris's suggestions about the regional climates of the North American Great Plains and equatorial East Africa. However, Potts says, the change is not steady and, in fact, fluctuations in the global climate have increased over the past 50 million years.
The ecosystems in these two examples developed during the transition from globally warm and wet climates of the Cretaceous period, which ended about 65 million years ago, to the climatically fluctuating ice ages of the past few million years. A big question among paleoecologists, particularly those who study human evolution, has been, when did the climate become cool and dry enough for forests to give way to savannah environments?
In using ocean records to test the idea that the Miocene savannah environments in the fossil record are an effect of volcanism, which of the following assumptions does Potts make?[A] II only
[B] III only
[C] II and III
[D] I, II and III
With which of the following statements would Harris and Van Couvering most likely agree?[A] Ecosystems near volcanoes were more successful than fossils indicate.
[B] Computer models are more accurate gauges of ancient climate than fossils.
[C] Persistent volcanism destroys the fossils and sedimentary rocks in the surrounding region.
[D] There have been in fact no true deserts in the past few million years.
If a geoscientist were to discover fossils that indicated that the Pacific Northwest was a savannah environment a million years ago, what effect would this development have on the argument of Harris and Van Couvering?[A] It would strongly support the argument.
[B] It would support the argument somewhat, but not conclusively.
[C] It would neither support nor weaken the argument.
[D] It would substantially weaken the argument.
Which of the following would most challenge the idea that equatorial East Africa may have had a wetter climate than was previously suspected?[A] Previous climate estimates were based on fossils from climax communities.
[B] The fossil record contains no evidence of deciduous rain forests.
[C] The level of volcanic activity in East Africa has never declined since the Miocene.
[D] Computer models suggest that the climate in East Africa was warm.
In the passage, Harris is cited as making a claim that volcanic activity makes for a barren environment. This claim is:[A] supported by the fact that no evidence of biological activity has been found in volcanic regions.
[B] supported by an analogy of volcanic regions to another type of barren environment.
[C] perhaps true, but not explicitly supported in the passage.
[D] challenged by the author's claim that biological communities do develop in volcanic regions.
Suppose that the marine sediment record was found to show that global temperatures did not drop until well after the Miocene epoch. How would this finding be relevant to the passage?[A] It would support the claim that volcanism had an extensive impact on global climate.
[B] It would weaken the claim that the transition to a cooler climate occurred later than scientists thought.
[C] It would weaken the claim that marine sediment records are immune to the effects of volcanism.
[D] It would support the claim that some Miocene savannah environments are an effect of volcanism.
Suppose that the fossil record in a Pacific Rim country suggests that the region had a savannah ecology 38 million years ago. Harris and Van Couvering would most likely respond to this information by asserting that:[A] the record inaccurately reflects the regional climate due to persistent volcanism.
[B] the record may not be accurate if there was volcanic activity in the region.
[C] the fossil record should no longer be used a source of information regarding global climate.
[D] this supports their theory that volcanism can cause mock aridity.