April GEOLOGY and GSA TODAY media highlights

Following are highlights from the April issue of GEOLOGY and a summary of the science article from the April issue of GSA TODAY, published by the Geological Society of America. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in stories published. Contact Ann Cairns at GSA to request advance copies of articles and for additional information or assistance.

GEOLOGY

Evidence for a deep asthenosphere beneath North America from western United States SKS splits. Derek L. Schutt and Eugene D. Humphreys, pages 291-294.

A traditional view of Earth has a weak layer - the asthenosphere � lying immediately beneath plates, with plate motion accommodated by deformation within this layer. Such deformation would create fabric in asthenospheric rocks that could be detected with S waves; S waves split into 2 waves with first-arriving S waves vibrating in the direction of asthenosphere shear. S waves recorded in the western United States are inconsistent with this expectation, implying that the weak layer accommodating plate motion occurs at greater depths, where the minerals do not create rock fabric when deformed. This suggests that North America plate motion occurs largely by deformation below 200 km, which is below the traditional depth of the asthenosphere.

Warm deepwater ocean conveyor during Cretaceous time. Bernd J. Haupt and Dan Seidov, pages 295-298.

This study discusses oceanic high-latitude oceanic circulation during the Cretaceous (135-65 Ma), a time during much of which it is thought that there was no polar ice. As it currently stands, there is no feasible mechanism that could maintain warm subpolar surface oceans in both hemispheres. The goal of our study is to explore a hypothesis that a warm deep ocean can coexist with relatively cool subpolar (high-latitude) sea surface in one hemisphere and a warmer subpolar sea surface in the other. A series of numerical ocean circulation experiments confirms that the ocean meridional circulation can keep the abyssal ocean warm despite the northern subpolar surface water staying relatively cool.

Are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle? Simon M. Peacock, pages 299-302.

Deep in the Earth where temperatures and pressures are high, we expect rocks to deform ductilely. Yet earthquakes, which represent brittle deformation, occur up to 660 km (410 miles) deep in the Earth in subduction zones where tectonic plates dive into Earth's mantle. Recent work has shown that many deep earthquakes may be triggered by mineral reactions that occur during subduction. In this paper the author proposes that the breakdown of serpentine minerals may trigger a particular set of earthquakes that occur at 50�200 km depth within the subducting oceanic mantle. The breakdown of serpentine releases water that can lubricate pre-existing faults and trigger earthquakes. If this hypothesis is correct, then current models may significantly underestimate the amount of water entering subduction zones.

Evidence for a small (~ 0.000 030) but resolvable increase in seawater 87Sr/86Sr ratios across the Cretaceous-Tertiary boundary. Kenneth G. MacLeod et al., pages 303-306.

The nature of the Cretaceous-Tertiary boundary has been one of the more actively disputed topics in the geosciences over the last 20 years, but most workers now agree that the boundary coincides with globally distributed evidence for an impact that hit near Chicxulub on the Yucatan Peninsula. How this impact affected the biosphere and punctuated the history of life, though, is less well understood. New strontium isotopic measurements on submillimeter-sized fossils of marine plankton (planktic foraminifera) that lived in the subtropical western Atlantic suggest that one effect of the impact was an increase in the amount of strontium in continental runoff large enough to shift the ratio of strontium isotopes in the world's oceans. Analysis of the geochemical patterns in different foraminifera shows that extinction at the boundary among the species analyzed was virtually complete; that is, there is no evidence for survivors of the event at this site among the planktic foraminif era. The size of the isotopic excursion suggests that there must have been a large increase in the weathering of continental rocks immediately after the impact. Such weathering supports the hypothesis that one of the deleterious paleoenvironmental effects of the impact was acid rain on a dramatic scale.

Global dinoflagellate event associated with the late Paleocene thermal maximum. Erica M. Crouch et al., pages 315-318.

Because rapid global warming is of considerable concern today, the Earth's previous global warming events can be studied to give insights as to how ecosystems respond to such events. Here, the authors discuss such the effect of a short-term (220,000 year-long) global warming event that occurred in the Late Paleocene (ca. 55 million years ago). Benthic microplankton underwent a drastic and catastrophic extinction. Surface-dwelling microplankton were also severely affected. The event has been linked to massive gas-hydrate release.

Icosahedral fracture tessellation of early Mesoproterozoic Laurentia. James W. Sears, pages 327-330.

The ancient fracture zones that outline the Precambrian core of North America are of the same scale and configuration as part of an icosahedral tessellation of Earth's surface. The icosahedral tessellation is familiar as the pattern of the buckyball, soccer ball, and wart and herpes viruses. The fracture pattern originated about 1.5 billion years ago, after consolidation of a supercontinent of which North America was part. The fracture pattern may have resulted from accumulation of heat beneath the supercontinent, which acted as an insulating blanket on the mantle. The regular icosahedral pattern was deformed by collisions that created the Grenville and Appalachian mountain systems. It may prove to be useful in restoration of North America's fragmented and dispersed parental supercontinent.

Age and composition of dikes in Southern Tibet: New constraints on the timing of east-west extension and its relationship to postcollisional volcanism. Helen Williams et al., pages 339-342.

The Tibetan Plateau is a huge influence on our climate and has been linked with global cooling and the development of the Asian monsoon. However, there is little agreement on when the plateau formed or how such a large area could have been raised to form a plateau with a mean elevation of over 5000 m. New ages for dikes in southern Tibet indicates that the plateau has been spreading under its own weight from 18 million years before present, and must have reached a high elevation by this time. The formation of a high plateau 18 million years ago, considerably earlier than has been previously thought, is of fundamental importance to models for the onset of the monsoon, which is thought to have started only 8 million years ago. The uplift of the Tibetan Plateau is linked to surface volcanism, indicating that the processes that raised the plateau are related to deep-mantle processes, over 80 km below the surface.

Low-gradient outlet glaciers (ice streams?) drained the Laurentide ice sheet. M.R. Kaplan et al., pages 343-346.

Researchers studying the glacial geology in a remote area of the eastern Canadian Arctic, have found that the former Laurentide Ice Sheet, which once covered most of Canada and much of the northeastern United States, was more dynamic than previously documented. The authors show that the Laurentide Ice Sheet had some characteristics similar to those of the present Antarctic Ice Sheet. In particular, they have documented the presence of fast-moving, low-sloped ice streams or outlet glaciers that may have drained much of the ice sheet. Scientists had previously noted that sediments from the bottom of the North Atlantic Ocean, far from North America, contain iceberg debris that originally came from the Laurentide Ice Sheet. The presence of such material on the ocean bottom, far from its source area, suggests that the Laurentide Ice Sheet must have occasionally undergone catastrophic collapse, releasing iceberg armadas into the adjacent ocean. These iceberg calving events would have affected ocean salinity and ocean circulation, with global consequences for Earth's climate. The authors� findings add a critical component to understanding the mechanism by which the Laurentide Ice Sheet could have experienced such collapses: the presence of ice streams and outlet glaciers allowed rapid transfer of ice from the interior sections of the Laurentide Ice Sheet to the ocean in a brief period of time, facilitating interaction between the ice sheet, the adjacent ocean, and the global climate system.

Tsunami deposits from major explosive eruptions: An example from the 1883 eruption of Krakatau. Steven Carey et al., pages 347-350.

The 1883 eruption of Krakatau volcano in Indonesia produced devastating tsunamis that killed over 30,000 people. On several islands to the north of Krakatau evidence of inundation by the tsunamis is preserved in the form of unusual pumice-rich deposits near the coasts. Analysis of pumice shapes using fractal techniques indicate that the deposits were formed from floating pumice rafts that were carried onshore as the tsunamis struck the coasts. The rafts were created earlier in the eruption from pumice fallout and pyroclastic flows that traveled over water. These pumice-rich deposits are characteristic of tsunamis associated with explosive eruptions and may provide a valuable indicator of tsunami hazards in areas of active volcanism.

Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis. Richard J. Twitchett et al., pages 351-354.

The end-Permian mass extinction was the largest such event of the Phanerozoic, yet the cause is still hotly debated. This article reports on a complete Permian-Triassic boundary section from East Greenland that sheds new light on the duration and extent of this event. The section contains well-preserved shallow marine organisms, as well as spores and pollen from terrestrial plants. For the first time it has been possible to compare in detail, using the same samples, the biotic changes that took place on land and in the sea. In both cases, the high-diversity end-Permian ecosystems completely collapsed over a short interval of time. In addition, the devastation occurred at the same time on land as in the sea. By correlating with accurately dated sections in China, it appears that these events took just a few tens of thousands of years: a geological instant. Despite rapid and widespread ecological upheaval, some organisms still managed to hang on for a few hundred thousand years before finally disappearing. The ultimate cause of this devastation is still unknown, but the changes that the authors observe are consistent with suggestions of rapid global warming in a runaway greenhouse world.

Geomorphic control of persistent mine impacts of a Yellowstone Park stream and implications for the recovery of fluvial systems. W. Andrew Marcus et al., pages 355-358.

Although streams actively flush contaminated sediments from past metal mining activities downstream, impacts may persist for centuries or more in watersheds. Researchers reached this finding after investigating plants, aquatic insects, and metal concentrations in sediments along Soda Butte Creek, which drains the New World Mining District of Montana before entering Yellowstone National Park. Although mining last occurred in 1953, aquatic insect populations have not recovered significantly since they were first monitored in 1967. In the flood-plain, fewer grass species are found and grasses are sparser or absent in areas of metal-rich sediments deposited when a mine tailings dam failed in 1950 that deposited mine tailings at high levels, out of the reach of most natural floods. Fifty and 100 year floods in 1996 and 1997 did very little to remove or cover up the contaminated sediments, indicating that the floodplain tailings will continue to suppress biodiversity for decades to centuries. The impacts of mine wastes are severe near the mine waste sources, but becoming increasingly subtle with increasing distance from the mines. Similar situations occur throughout the world and, without remediation, metal mining impacts that persist for centuries may be typical for mountainous streams where mining has occurred historically or is occurring now.

Middle Cretaceous greenhouse hydrologic cycle of North America. Tim White et al., pages 363-366.

This study involves a geochemical approach to reconstructing Cretaceous (~100 million years old) precipitation rates in central North America. The authors accomplished this by sampling small spherules of siderite (a mineral composed of iron and carbonate) that were precipitated around roots in ancient soils exhibiting characteristics of nonmarine wetland soils recharged by precipitation. Combined with the observation that siderite records the chemistry of the water from which it precipitated and is most readily formed under relatively freshwater conditions, the authors suggest that the chemistry of the siderite records the chemistry of Cretaceous precipitation. The authors used this rationale to reconstruct a paleolatitudinal gradient in Cretaceous precipitation chemistry, which was then compared to modern precipitation chemistry profiles. The authors concluded that a consistent difference between the Cretaceous and modern profiles is best described by Cretaceous precipitation rates up to 4-5 times modern values. This conclusion is consistent with the presence of the Cretaceous Western Interior Seaway in central North America as a source for higher precipitation, and an intensification of the hydrologic cycle associated with the ice-free Cretaceous greenhouse Earth.

Discovery of high-pressure ZrSiO4 polymorph in naturally occurring shock-metamorphosed zircons. B. P. Glass and S. Liu, pages 371-373.

This paper describes the discovery of a new high-pressure mineral produced from zircon (ZrSiO4) during the impact of an extraterrestrial body. The impact took place approximately 35 million years ago and produced a large impact structure, about 90 kilometers in diameter, that today is buried beneath the mouth of the Chesapeake Bay. This high-pressure phase has been produced in laboratory experiments since 1969, but this is the first time that it has been found in natural samples. Laboratory experiments indicate that this high-pressure mineral forms at pressures in excess of about 30 billion pascals, which is equivalent to a depth of greater than 800 kilometers in Earth�s mantle. This high-pressure mineral will provide geologists with another tool for determining peak shock pressures generated during impact events. Because zircon is such a stable mineral, this new high-pressure mineral may allow identification of ancient impact events (greater than 2 billion years old) for which most other evidence of their impact origin may have been destroyed.

GSA TODAY

Are lithospheres forever? Tracking changes in subcontinental lithospheric mantle through time. Suzanne Y. O'Reilly et al.

Increasingly, geologic and geophysical data suggest that the mantle part of continental plates plays an integral role in helping control the buoyancy, and hence the long-lived nature, of continental lithosphere. This paper provides a synthesis of information gleaned from a large database of mantle xenoliths and xenocrysts. The xenoliths are brought up by volcanic eruptions and provide some of the best direct information on the nature of the mantle beneath continents. Petrologic analysis helps decipher what depth the xenoliths came from, then the authors map the distribution of different rock types with depth, similar to drill-hole logs through continents. The resulting chemical tomography reveals that the mantle lithosphere columns generally were stabilized at the same time as their superjacent crust, and that the composition of lithospheric mantle has changed through earth history. Archean lithospheric mantle is lower density; Phanerozoic mantle is higher density and more fertile. A major change in the processes that produce mantle lithosphere coincided with the archean-Proterozoic boundary (2.5 billion years ago). A consequence of these different mantle columns is that the mantle beneath Archean cratons is buoyant and relatively difficult to delaminate or transform, whereas Phanerozoic lithosphere can delaminate and be destroyed. The answer to the question of whether lithospheres are forever seems to be that some Archean lithospheres are indeed a permanent, buoyant record of early earth processes, but that mantle lithospheres became less permanent through earth history.

*To view abstracts and the complete table of contents of GEOLOGY, as well as that of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, see http://www.gsajournals.org. To obtain full text of these articles and articles from back issues, contact Ann Cairns, [email protected].