December Geology media highlights

11/30/04

Boulder, Colo. - The December issue of GEOLOGY covers a wide variety of potentially newsworthy subjects. Topics include: impact of shifts in the North Atlantic current on European climate; new method for estimating elevations of Earth's ancient land surfaces; evidence of terrestrial causes of the Permian-Triassic mass extinction; evidence of a major Precambrian asteroid impact in northwestern Australia; the relationship of intensified hydrologic cycles and global heat transfer during greenhouse phases of Earth's history; and insights into Martian mineralogy based on experiments involving weathering of iron phases in the Martian atmosphere.

Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns at acairns@geosociety.org. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Ann Cairns for additional information or other assistance. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org

North Atlantic Current and European environments during the declining stage of the last interglacial
Ulrich C. Müller, University of Tuebingen, Institute of Geosciences, Tuebingen, Germany; and George J. Kukla, Columbia University, Lamont-Doherty Earth Observatory, Palisades, NY 10964-8000, USA. Pages 1009-1012.

The North Atlantic Current, a major element of the oceanic circulation, delivers tropical heat from the Gulf of Mexico to the northern North Atlantic. The heat is released to eastward moving air masses, thereby greatly ameliorating winter climates in northern Europe. The article presents a reconstruction of environmental shifts in Europe associated with the decline of the North Atlantic Current at the end of the last interglacial. Since the last interglacial is considered as the most recent analogy to our present interglacial, i.e., the Holocene, this reconstruction provides an idea of what may happen to Europe if such a decline of the North Atlantic Current will occur again.

Shallowly buried, enigmatic seismic stratigraphy on the New Jersey outer shelf: Evidence for latest Pleistocene catastrophic erosion?
Craig S. Fulthorpe and James A. Austin Jr., University of Texas at Austin, Institute for Geophysics, Austin, TX 78759-8500, USA. Pages 1013-1016.

A prominent buried erosional surface, 0-20 m below the seafloor of the New Jersey outer shelf, may record catastrophic erosion and redeposition of sediment following multiple breachings of glacial lake dams to the north 19,000-12,000 years ago. This event eroded linear incisions, trending northeast and east-northeast, and rapidly buried them with a chaotic fill that includes large blocks of eroded material. This implies that local processes can generate significant erosion in a shelf environment where erosional events were previously attributed to global sea-level change. Recognition of this surface adds complexity to the already complex, shallowly buried stratigraphic record of the last glacial cycle on the New Jersey continental shelf, long considered typical of periglacial shelves worldwide.

Climate-independent paleoaltimetry using stomatal density in fossil leaves as a proxy for CO2 partial pressure
Jennifer C. McElwain, Field Museum of Natural History, Department of Geology, Chicago, IL 60605, USA. Pages 1017-1020.

How high were the Sierra Nevada Mountains 10 million years ago? When did Denver-the mile high city-reach its current elevation? Simple questions it would seem, yet precise and simple methods for calculating the elevations of past land surfaces have long eluded geologists. A new method uses changes in the number of breathing pores (stomata) on fossil leaf surfaces to estimate the partial pressure of the gas carbon dioxide in the ancient atmosphere. As the partial pressure of CO2 decreases with increasing elevation according to physical principles, this new 'paleoaltimeter' enables estimation of past elevation from the estimated CO2 partial pressure. The errors associated with this new paleoaltimeter are plus or minus about 450 meters, which is significantly less than most existing methods. The great advantage of the fossil leaf method however is the fact that it appears to be independent of climate change-a factor which adversely influences the majority of other methods.

Extinction of a fast-growing oyster and changing ocean circulation in Pliocene tropical America
Michael X. Kirby, Smithsonian Tropical Research Institute, Center for Tropical Paleoecology and Archaeology, Balboa, Panama; and Jeremy B.C. Jackson, Scripps Institution of Oceanography, Geosciences Research Division, University of California, San Diego, La Jolla, CA 92093, USA. Pages 1025-1028.

Ocean circulation changed profoundly in the Caribbean Sea 6-3 million years ago as a result of constriction and final closure of the Central American seaway. In response, regional planktonic productivity, the food of many marine organisms such as oysters, is believed to have decreased in the Caribbean Sea. Our study shows that a fast-growing oyster went extinct concurrently with these changes in ocean circulation and planktonic productivity. The disappearance of this fast-growing oyster from shallow-marine environments and the continued survival of slower-growing oysters in estuaries and lagoons suggests that declines in food may have eliminated faster growing oysters from the sea, but allowed slower growing oysters to survive in estuaries and lagoons.

Iridium anomalies and shocked quartz in a Late Archean spherule layer from the Pilbara craton: New evidence for a major asteroid impact at 2.63 Ga
Birger Rasmussen, University of Western Australia, School of Earth and Geographical Sciences, Perth, Western Australia 6009, Australia; and Christian Koeberl, University of Vienna, Department of Geological Sciences, Vienna A-1090, Austria. Pages 1029-1032.

Several spherule layers in South Africa and Australia, with ages of ca. 3.4 to 2.6 Ga, have been interpreted as the result of large asteroid or comet impacts onto the early Earth. Some of these spherule layers show extreme enrichments in the PGEs, unlike modern ejecta deposits, which caused some questions regarding the initial impact interpretation. On the other hand, until now, no shocked minerals-the hallmark for all confirmed impact structures and ejecta-have been found in any of these spherule layers. Even rocks from the 2 Ga Vredefort impact structure contain abundant shocked minerals, so it is unlikely that Archean impacts would, for some reason, not produce shocked minerals. In the present work, Rasmussen and Koeberl document, for the first time, the presence of shocked quartz in a sample of the ~2.63 Ga spherule layer from the Jeerinah Formation (Pilbara craton, northwestern Australia). The survival of shocked quartz in ~2.63 Ga rocks, which have undergone multiple metamorphic events, suggests that their absence in other impact ejecta layers may not only be a question of preservation. The presence of shocked quartz in a layer containing melt spherules provides compelling evidence for an extraterrestrial impact with a target area that was at least partly silicic, favoring a continental impact site. In addition, enrichments in Ir and other siderrophile elements in the spherule layer indicate that they contain as much as 2-3 wt% of a chondritic meteorite component. If proposed correlations between the Australian spherule layer and similar South African layers are correct, then the combined ejecta blanket represents fallout from a single major impact with an areal distribution of >32,000 km2, which is among the largest yet documented in the Precambrian rock record. Thus so far the impact record on Earth is quite limited: nothing for the first billion years, then some spherule layers until about 2.5 Ga, and then some impact craters. Nevertheless, the discovery of these spherule layers aids in the discussion of the importance of impact events in the early parts of Earth's history. Clearly the "early" impact record on Earth, which spans more than half of the age of our planet, is still a wide-open field of research.

Weathering of iron rich phases in simulated Martian atmospheres
Vincent Chevrier, Univ. Aix-Marseille 3, Aix en Provence - 13545, France; et al. Pages 1033-1036.

We report about experimental weathering of iron phases (metal, sulfide, oxide) in carbon dioxide and water vapor, i.e., present day Martian atmosphere. Such experiments have never been conducted. We evidence that the neoformed mineral phases are similar to the ones invoked in the Martian regolith according to spatial probes data (including the MER rovers). Therefore the occurrence of such phases (carbonate, sulfate, oxyhydroxides) can no longer be taken as an evidence for formation of these minerals in presence of liquid water and oxidants not present today (oxygen, acids, etc.). Moreover these results emphasize the key contribution of "extramartian" materials (metal and sulfide due to meteorite bombardment) in the mineralogy of the Martian regolith. This contribution should be of wide interest for the present debate on the processes having affected the Martian surface, and thus on the interpretation of present rover data (Spirit and Opportunity) and on the question of life on Mars.

Evidence for sulfidic deepwater during the Late Permian in the East Greenland Basin
Jesper K. Nielsen, University of Tromsų, Department of Geology, Tromsų, Norway; and Yanan Shen, Harvard University, Botanical Museum, Cambridge, MA 02138, USA. Pages 1037-1040.

The most severe biological disaster over Earth's history occurred around the Permian-Triassic (P-Tr.) boundary of 251 million years ago. The causes of this biological crisis have been greatly debated. In order to find the killers of the P-Tr life, we went to the East Greenland Basin where the sedimentary rocks across the P-Tr. boundary are perfectly preserved. In the laboratory, we measured diameters of framboidal (strawberry-shaped) pyrite crystals from the Late Permian black shales. We discovered a remarkable size difference of framboidal pyrite between bioturbated shales and laminated organic-rich black shales. In bioturbated shales, the larger (>5 µm) and wider distribution of framboidal pyrites clearly indicates oxic bottom water conditions. By contrast in laminated shales, the smaller (<5 µm) and less variable distribution of framboidal pyrites provides compelling evidence for sulfidic deepwater during the Late Permian in the East Greenland Basin. In combination with S-isotope data, our geological and geochemical study indicates that the Late Permian oceans were quite similar to the modern Black Sea whose deepwater is enriched in malodorous H2S gas. We believe that high levels of noxious H2S gas in deepwater of Late Permian oceans severely restricted the ecological space available for animal survival. This oceanic H2S gas could have caused the P-Tr mass extinction if H2S-enriched waters had reached shallow water regions triggered by oceanographic processes and if H2S gas outgassed in to the atmosphere, to affect the marine and terrestrial environments.

Evidence for increased latent heat transport during the Cretaceous (Albian) greenhouse warming
D.F. Ufnar, University of Southern Mississippi, Geology, Hattiesburg, Mississippi 39406, USA; et al. Pages. 1049-1052.

The mid-Cretaceous was a time of greenhouse warmth with reduced equator-to-pole temperature gradients. Ancient soil carbonates formed in coastal plain deposits throughout the North American Western Interior Basin and North Slope, Alaska, have been used to reconstruct aspects of the mid-Cretaceous hydrologic cycle. Stable isotope mass-balance modeling suggests that the hydrologic cycle was greatly intensified with enhanced precipitation and evaporation rates. The intensified hydrologic cycle has implications for global heat transfer during greenhouse phases of Earth history. Quantitative estimates suggest that 2-4 times as much latent heat was transferred toward the poles with an intensified hydrologic cycle. The increased heat transfer through the atmosphere coupled with ocean heat transport and increased atmospheric carbon dioxide levels may help explain the polar warmth during the mid-Cretaceous.

Geochemistry of the end-Permian extinction event in Austria and Italy: No evidence for an extraterrestrial component
Christian Koeberl, University of Vienna, Department of Geological Sciences, Vienna A-1090, Austria; et al. Pages 1053-1056.

The Permian-Triassic (P-Tr) boundary is associated with the largest mass extinction known in Earth history. Following the association of the K-T boundary mass extinction with a large impact event, speculations bloomed that other major mass extinctions might also be related to impact events. However, so far the evidence in favor of such a proposal is controversial. Siderophile element anomalies (e.g., enhanced Ir contents) were found at some P-Tr boundary locations; their presence was also confirmed in the present work from elemental and isotopic analysis (using platinum group element abundances and osmium isotopes), but Koeberl et al. clearly showed that purely terrestrial processes were at work in concentrating these rare metals, and that there is no evidence for an extraterrestrial component. The present work also indicates that there are no traces the extraterrestrial helium-3 isotope, the alleged presence of which in the so-called fullerenes (large carbon molecules) has been the subject of a lot of debate. The sparse evidence for impact has recently resulted in suggestions of a possible buried underwater impact structure near Australia (Becker et al., Science May 2004), but it became rapidly clear that the evidence for the existence of such an impact structure (and any age information) is tenuous at best. On the other hand, recent research (e.g., Mundil et al., Science 305, 1760, 2004) succeeded in demonstrating that the P-Tr boundary event is exactly synchronous with the Siberian flood volcanism, indicating a causal link. This was also supported by sulfur isotope data for P-Tr samples, indicating a volcanic source (Maruoka et al., EPSL 206, 101, 2003). The work by Koeberl and coworkers provides, for the first time, clear evidence that most of the alleged indicators for an extraterrestrial signature are, in fact, of purely terrestrial origin.

Can marine anoxic events draw down the trace element inventory of seawater?
Thomas J. Algeo, University of Cincinnati, Department of Geology, Cincinnati, OH 45221-0013, USA. Pages 1057-1060.

Trace elements such as Mo, U, and V are sensitive to the redox conditions of marine environments. Modern seawater is for the most part well oxygenated, and, consequently, these elements are removed slowly from seawater. However, under strongly reducing conditions in which seawater is depleted of oxygen, these trace elements are removed rapidly to organic-rich sediments. Such conditions have prevailed at various times in the geologic past, for example, during the Late Devonian Period. In this study, I show that removal of redox-sensitive trace elements to sediments of Late Devonian age may have depleted their concentration in contemporaneous seawater by up to 70%. Furthermore, trace-element concentration patterns in the Upper Devonian Ohio Shale of the Central Appalachian Basin are consistent with such an inference.

Neogene tilting of crustal panels near Wrangell, Alaska
Chris Butzer, University of Arizona, Department of Geosciences, Tucson, AZ 85721-0077, USA; Robert F. Butler, University of Portland, Chemistry and Physics, Portland, OR 97203-5798, USA; et al. Pages 1061-1064.

Paleomagnetic observations indicate that a ~20-million-year-old igneous complex south and west of Wrangell, Alaska, has tilted east-side-up by almost 20°. This tilting of crustal blocks along the margin of the continent is similar to a stack of dominoes slowly falling over. Individual tilt blocks (the dominoes) may correspond to individual islands in this region of southeast Alaska with faults separating the islands (the shearing planes between the dominoes) occupying the maze of ocean canals in southeast Alaska. It is suggested that tilting of crustal blocks may account for a majority of the difference between expected and observed paleomagnetic directions that has preciously been interpreted to indicate that portions of southeast Alaska moved north by 1000s of kilometers since 100 million years ago.

Post–Little Ice Age record of coarse and fine clastic sedimentation in an Alaskan proglacial lake
Michael G. Loso, Univ. of California, Santa Cruz, Earth Sciences Dept., Santa Cruz, CA 95064, USA; et al. Pages 1065-1068.

In the remote mountains of Alaska's Wrangell-St. Elias National Park, the surprise drainage of a glacier-dammed lake revealed a centuries-long record of glacier erosion. Until just a few years ago, picturesque Iceberg Lake was best known from its photo on the cover of a popular hiking guidebook. Late in the summer of 1999, however, the shrinking glacier dam failed and a massive flood emptied the previously-stable lake. The next summer, scientists armed with shovels and rubber boots found an unusually complete record of glacier erosion in the layers of sediment that coat this muddy lake bottom. According to Michael Loso, a geologist from the University of California Santa Cruz, the detailed sedimentary record exposed by Iceberg Lake's surprising demise tells us much about how quickly glaciers erode their beds (at a rate of about a millimeter and half per year) and also how they do it: primarily through the plucking of large blocks.

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