GSA release 02-55: Dec. Geology & GSA Today media highlights

November 25, 2002

Boulder, Colo.--The Geological Society of America's December issue of GEOLOGY contains several potentially newsworthy items. Topics of particular interest include: new results from study of Lake Tanganyika sediment cores and impacts of deforestation on the lake's biodiversity; changes in Earth's biogeography and its relationship to changes in plate tectonics; new analysis of Martian gullies based on images supplied by the Mars Global Surveyor spacecraft; a Norian-Rhaetian extinction event that may have preceded the end-Triassic mass extinction; and new evidence regarding the Paleocene-Eocene thermal maximum (PETM) and the dramatic global warming that followed it 55 million years ago.

Highlights are provided below. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in stories published. Contact Ann Cairns at acairns@geosociety.org for copies of articles and for additional information or other assistance.

Please note that GSA will close for the Thanksgiving holiday at 12:00 p.m. MST on Wednesday, November 27. The office will reopen Monday, December 2.

GEOLOGY

Effects of land-use change on aquatic biodiversity: A view from the paleorecord at Lake Tanganyika, East Africa.
Simone R. Alin, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, et al. Pages 1143-1146.

Researchers at the University of Arizona and their colleagues used sediment cores from Lake Tanganyika in East Africa to study the effects of deforestation on biodiversity in the lake. Lake Tanganyika is home to over 2,000 species, making it the most biologically diverse tropical lake on the planet. The research, coordinated by Dr. Simone Alin, compared sediment cores collected offshore from Gombe Stream National Park in Tanzania, the site of Jane Goodall's chimpanzee research, with cores collected offshore from a deforested watershed just outside the national park. Changes in sedimentation rates, nutrient input, and biodiversity were the major manifestations of deforestation's effects on the lake habitat at the deforested site since the late 1800s. Severe flooding that occurred throughout East Africa in the early 1960s apparently amplified these environmental changes at the deforested site. In contrast, the sediment record offshore from Gombe Stream National Park indicated that no comparable changes occurred offshore from the park. The study by Dr. Alin and her colleagues reflects the importance of conserving landscapes in order to protect aquatic biodiversity from the combined impacts of rapid population growth and severe climate events.

Euconodont paleobiogeography and the closure of the Iapetus Ocean.
Howard A. Armstrong, Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, UK; and Alan W. Owen, Division of Earth Sciences, University of Glasgow, Lilybank Gardens, Glasgow G12 8QQ, UK. Pages 1091-1094.

Today, distinctive suites of animals and plants characterize different parts of the world. On the basis of these distribution patterns Earth is divided into biogeographical regions, or provinces. Provinciality is also known in the geological past, particularly when temperature gradients were steep between the poles and the equator. The number and distribution of biogeographical provinces affects global biodiversity. But how do biogeographical provinces breakdown? This paper documents the changes in provinciality of conodonts, an extinct group of early marine vertebrates, during the Late Ordovician and Early Silurian periods (about 460 to 430 million years ago). During this time, the microcontinent of Avalonian drifted northward from cool, high temperate latitudes into the tropics toward the Laurentian plate, closing the Iapetus Ocean. As the gap between Avalonia and Laurentia decreased and the climate of Avalonia warmed, the indigenous fauna became progressively extinct. Laurentian species were then able to migrate and insinuate themselves into Avalonian niches. This pattern has strong similarities to the Great Faunal Interchange of land animals between North and South America during the Pliocene epoch, suggesting there may be underlying fundamental processes operating when barriers to migration are closed through plate tectonics.

Role of aquicludes in formation of Martian gullies.
Martha S. Gilmore and Eleyne L. Phillips, Department of Earth and Environmental Sciences, Wesleyan University, 265 Church Street, Middletown, Connecticut 06459, USA. Pages 1107-1110.

Images provided by the Mars Orbiter Camera (MOC) aboard the Mars Global Surveyor (MGS) spacecraft reveal erosional landforms previously interpreted to be geologically young gullies formed by groundwater seepage. As liquid water is not currently stable on the surface of Mars, it has been suggested that the gullies are relicts of a time when climatic conditions were warmer, perhaps as a consequence of changes in the planet's obliquity. But gullies do not occur everywhere on Mars, suggesting to us that, as on Earth, the location of these gullies is controlled in part by the presence of an impermeable rock layer (aquiclude) that collected and concentrated local groundwater. Indeed, each of the gullies we observed emanates from a specific, cliff-forming layer, even if the layer is faulted suggesting a causal relationship. We also measured the depths of the gully heads below the local surface and found that the variation in gully depths tend to correlate to rock units interpreted to contain more impermeable layers (e.g., volcanic ash). Gully formation is therefore dependent upon both favorable climatic conditions to produce and sustain liquid water and the presence of impermeable subsurface layers to collect the groundwater. Gullies may mark the distribution of subsurface impermeable layers globally, and are prime targets for the search for present water and life on Mars.

Carbon and nitrogen isotope disturbances and an end-Norian (Late Triassic) extinction event.
Mark A. Sephton, Planetary and Space Sciences Research Institute, Open University, Milton Keynes MK7 6AA, UK, et al. Pages 1119-1122.

Five great mass extinctions disrupt the geological record, but they have not been studied in equal depth. The end-Triassic extinction, around 200 million years ago, has been relatively neglected by scientists. Now it has been discovered that this "Cinderella" of mass extinctions is preceded by an earlier extinction event. The first extinction occurs at the Norian-Rhaetian boundary, where chemical changes in the rock record indicate that the giant Panthalassa Ocean became stagnant. Oceanic torpor led to the demise of deep-water marine life. There may have been a trigger for the Norian-Rhaetian extinction. One possibility is climate change caused by the Manicouagan impact. The age of the enormous Manicouagan Crater excludes it from involvement in the end-Triassic extinction, but it is within estimates for the Norian-Rhaetian boundary. Other options include the onset of massive volcanism associated with the Central Atlantic magmatic province created when the supercontinent Pangea began to tear itself apart, or simply a sick, sluggish world predisposed to extinction.Whatever the cause, the magnitude and complexity of the chemical changes at the Norian-Rhaetian boundary suggest that the associated environmental disturbances may rival those at the close of the Triassic.

Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Paleocene-Eocene thermal maximum.
Deborah J. Thomas, Department of Geological Sciences, University of North Carolina-Chapel Hill, Chapel Hill, North Carolina 27599-3315, USA, et al. Pages 1067-1070.

The Paleocene-Eocene thermal maximum (PETM) was a dramatic global warming event that occurred approximately 55 million years ago. The warming is believed to have been the result of a massive input of methane (a potent greenhouse gas) released from frozen hydrate reservoirs in deep-sea sediments, based on a pronounced change in the isotopic composition of carbon throughout marine and terrestrial reservoirs. However, geologists have been unable to test this hypothesis using traditional records. We present new data based on analyses of the oxygen and carbon composition of individual marine microfossils from a location off the coast of Antarctica (Ocean Drilling Program Site 690, Maud Rise). These data demonstrate that the initial change in carbon isotopes occurred in a geologic instant and was preceded by a brief period of gradual surface water warming. Both of these findings support the thermal dissociation of methane hydrate as the cause of the PETM carbon isotope change. Furthermore, the data reveal that the methane-derived carbon was mixed from the waters of the surface ocean down to the bottom of the oceans. Such a progression implies a rapid and major release of methane that permitted a significant fraction of the initial dissociated hydrate methane to reach the atmosphere prior to oxidation.

Mineral biosignatures in evaporites: Presence of rosickyite in an endoevaporitic microbial community from Death Valley, California.
Susanne Douglas and Heixong Yang, California Institute of Technology, Jet Propulsion Laboratory, Center for Life Detection, 4800 Oak Grove Drive, MS 183-301, Pasadena, California 91109-8099, USA. Pages 1075-1078.

In most natural environments, microorganisms live as interdependent microbial communities in the form of biofilms or mats. When the environment becomes inhospitable, microbial communities are predominantly found inside rocks where they find shelter from temperature extremes and dessication. In Death Valley, a very hot and dry environment, an extensive microbial community exists within the evaporite deposits found on the central playa floor. These endoevaporitic organisms interact with the underlying groundwater from which they gain necessary nutrients. As a consequence, their metabolic activity and the presence of the cells themselves, affect the mineralogy of the evaporite deposit. The organisms have a profound effect on the texture of the mineral deposit, promoting smaller grain size, and also on the types of minerals found. Among the dominant minerals--bassanite, gypsum, and halite--are found ones which are uniquely associated with the microbial community--rosickyite and strontianite, as well as calcite. We suggest that the microbial communities in Death Valley are uniquely tied to evaporite mineralogy, affecting its composition.

Nearly frictionless faulting by unclamping in long-term interaction models.
Tom Parsons, U.S. Geological Survey, MS-999, 345 Middlefield Road, Menlo Park, California 94025, USA. Pages 1063-1066.

A baffling characteristic of the San Andreas fault is its ability to slip with almost no frictional resistance despite high measured laboratory friction of fault-zone rocks. Various explanations have been postulated over the years for this, ranging from fluid lubrication to ball-bearing like granular materials. This new study suggests that faults arrange themselves into sets, so that when slip on all of them is considered, the major fault ends up unclamped, enabling it to slip more freely. In this way, a fault like the San Andreas could experience much less friction across it, at the expense of the crust surrounding the fault, which would be more deformed. Models indicate that this mode is a lower energy state than a single, simple fault.

GSA TODAY

Slow crawl across the salinity divide: Delayed colonization of freshwater ecosystems by invertebrates. Molly Fritz Miller, Geology Department, Box 117 Station B, Vanderbilt University, Nashville, Tennessee 37235, USA, molly.f.miller@vanderbilt.edu, and Conrad C. Labandeira, Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560-0121, USA, labandeira.conrad@nmnh.si.edu.

How does the colonization history of freshwater habitats, particularly that of sand or mud substrates, compare to that of soft bottom habitats in the marine realm? In less than 100 million years from the late Neoproterozoic (600 million years ago) into the Cambrian, marine animal life appeared and burgeoned from a few animals with a limited range of body plans and lifestyles to ecologically diverse faunas with representatives of all major phyla. Lakes and streams are aquatic habitats that are connected to oceans. If all major phyla appeared and marine habitats were colonized within less than 100 million years after the first appearance of multicellular animals ~600 million years ago, one would predict that lakes and streams would have been invaded by invertebrates soon thereafter. Instead, this paper demonstrates that substrate ecospace in streams and lakes was barely used for about 200 million years after the rapid colonization of marine habitats. The factors that inhibited colonization in freshwater may have included an inability of marine animals to acquire the osmoregulatory capabilities and reproductive and dispersal mechanisms required for success in fresh water, and an inability of aquatic insects to adapt to living within the sediment. The present may be the key to the past, and this paper also raises questions about the extent to which modern freshwater substrate ecospace is used. Human-aided transport has facilitated rapid invasion of marine animals into fresh water where they have adapted quickly. Study of the evolutionary processes occurring in historical time may elucidate the macroevolutionary processes that controlled the colonization of fresh water through the Phanerozoic.

To review the abstracts for these articles, go to www.gsajournals.org. To obtain a complimentary copy of any GEOLOGY article, contact Ann Cairns at acairns@geosociety.org. To review the complete table of contents for the November issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-toc&issn=0091-7613&volume=030&issue=12
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Geological Society of America

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