Basic Earth Science and Society - Basic Research Opportunities in Earth Science - page 18

of different paleoenvironments (earthquakes), or characteristic mineralogy or
chemistry (bolide impacts). On the other hand, extreme events of longer duration
can be easier to recognize than smaller, more transient changes. Climatic
extremes such as the “hothouse” conditions of the Cretaceous resulted in
widespread bauxite deposits (lateritic weathering), petroleum generation (high
marine stands and biologic productivity), and iron- and phosphorous-rich
sedimentary rocks (marine upwelling). Precambrian mantle plumes have been
postulated as the cause of the major magmatic eruptions that led to the formation
of large igneous provinces and their associated mineral deposits, which are
themselves chemical extremes.
Similarly, relatively recent extreme events are recognizable in sediment and
ice cores. Analysis of such cores has led to a greater appreciation of the transient
nature of climate change and the potential for abrupt and amplified responses
following small perturbations in atmospheric and oceanic processes. For
example, marine records from the Paleocene-Eocene boundary reveal an intense
warming period, lasting no more than 10,000 years, associated with a large
benthic extinction, changes in ocean circulation patterns, and increased ocean
temperatures, especially in higher latitudes. Isotopic analysis suggests that a
catastrophic release of methane, possibly from marine clathrates, resulted in a
sharp increase in greenhouse warming. On land, soil and vertebrate fossil
evidence links this event to the sudden onset of warm terrestrial climates and the
first appearance of modern mammal lineages.
The combination of precise geochronology and systematic field
investigations can reconstruct surprisingly complete and detailed accounts of
what happened during major events in Earth history. One of the most fascinating
detective stories in all of geoscience is the discovery of the Chicxulub crater off
the Yucatan peninsula, the “smoking gun” that confirmed the Alvarez hypothesis
that a bolide impact killed the dinosaurs at the end of the Cretaceous. 14 Research
of this type is yielding an increasingly rich picture of Earth processes, which in
turn is helping to assess the significance of future changes and extremes, the
mechanisms that might trigger them, and the hazards to human life that could
result.
Observing the Active Earth
Until recently, only rudimentary instrumentation was available for collecting
synoptic data on global processes in real time, and the monitoring
14 W. Alvarez, T. Rex and the Crater of Doom. Princeton University Press, Princeton,
N.J., 236 pp., 1997.