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GEOLOGY
Yellowstone hotspot volcanism in California? A paleomagnetic test of the Lovejoy flood basalt hypothesis
Robert S. Coe, University of California, Earth Sciences, Santa Cruz, CA 95064, USA; et al. Pages 697-700.
For more than a century debate has continued over the character, age, and extent of the Lovejoy basalt, a patchwork of distinctive Tertiary basaltic outcrops in northern California separated by as much as 230 km. In particular, researchers are interested in whether or not these isolated outcrops are the remnants of flood basalt volcanism, a great sheet of rapidly erupted lava that traveled great distances. This paleomagnetic study shows that almost 90% of the Lovejoy type section erupted quickly compared to geomagnetic secular variation, probably within only a few centuries. The paleomagnetic evidence also demonstrates that the uppermost flow traveled a minimum of 75 km and that some others could have traveled 200 km or more. It appears likely that the Lovejoy basalt was part of the same 17-14 million year old outpouring of lava flows attributed to the Yellowstone hotspot, which comprise the voluminous Columbia River flood basalt and high plateau lavas of Washington, Oregon, and western Idaho, greatly expanding the extent of Yellowstone hotspot volcanism to the southwest.
Carbon isotopes from fossil packrat pellets and elevational movements of Utah Agave plants reveal Younger Dryas cold period in Grand Canyon, Arizona
Kenneth L. Cole, U.S. Geological Survey, Southwestern Biological Science Center, Colorado Plateau Research Station, Northern Arizona University, Flagstaff, AZ 86011, USA; and Samantha T. Arundel, Northern Arizona University, Department of Geography, Planning, and Recreation, Flagstaff, AZ 86011, USA. Pages 713-716.
The Grand Canyon, with its extreme vertical relief, is an ideal laboratory for studying the past elevational shifts of plant species. It also has an abundance of fossil plant records contained in trash deposits in caves left by packrats, called packrat middens (http://www.usgs.nau.edu/global_change/middens.html). This study was the first to use a detailed sequence of packrat middens to examine changes in carbon isotopes in packrat fecal pellets and the abundance of Utah agave, a succulent desert plant species, within the middens. The changes in isotopes and agave suggest that between 12,900 and 11,600 years ago, a severe cold event, the Younger Dryas period, lowered minimum winter temperatures as much as 8 oC colder than they are today. Temperatures then rapidly warmed at least 4 oC at the end of this period. Although this period has long been recognized in Europe, Greenland, and the North Atlantic Ocean, there was considerable debate about its affect on the American southwest. These results are important because the severe cooling at the beginning of the Younger Dryas occurred just as man was becoming abundant in North America, and most large mammals (horses, mammoths, camels, etc.) were becoming extinct. Also, the rapid warming at the end of the period is similar in magnitude to that expected to occur over the next 100-150 years and can be used as an analog for ecological changes during just such an event.
Event preservation in lagoonal reef systems
Richard B. Aronson, Marine Environmental Sciences Consortium, Dauphin Island Sea Lab, Dauphin Island, AL 36528, USA; et al. Pages 717-720.
Coral reefs, the world's most diverse marine ecosystems, are beset by problems ranging from local pollution and overfishing to regional outbreaks of marine diseases and global warming. Although most scientists agree that reefs are in desperate trouble, they disagree strongly over the timing and mechanisms behind the coral reef crisis. This is no mere academic exercise because different answers will dictate different actions on the part of managers and policymakers intent on saving reef ecosystems. Aronson et al. used cores extracted from reefs in Belize to reconstruct the past several millennia of ecological history. Although some scientists have claimed that reefs began their decline centuries ago, due primarily to overfishing, Aronson and colleagues found that coral populations were healthy and vibrant until the 1980s, when they were killed by disease and high-temperature bleaching. As Aronson points out, protecting fish populations is a worthy goal but it won't save the corals, which continue to succumb to forces outside local control. Saving coral reefs means addressing global environmental issues - climate change in particular - at the highest levels of government.
Cretaceous-Paleogene boundary deposits at Loma Capiro, Central Cuba: Evidence for the Chicxulub impact
Laia Alegret, University College London, Earth Sciences, London WC1E 6BT, UK; et al. Pages 721-724.
A newly discovered Upper Cretaceous to lower Paleogene section at Loma Capiro (central Cuba) has provided new evidence for a Cretaceous-Paleogene (K/Pg) boundary age for the Chicxulub impact. The studied sediments at Loma Capiro consist of a bathyal, foraminiferal-rich, marl and sandstone sequence, and a 9.6-m-thick intercalated clastic complex. The clastic complex contains impact material, and its microfacies and foraminiferal content suggest deposition from gravity flows that eroded sediments from upper-slope and shelf settings and redeposited them in deeper, bathyal environments in coincidence with the K/Pg boundary. Alegret et al. suggest that the origin of the clastic complex may be linked to the collapse of the Cuban platform, triggered by the Cretaceous-Paleogene impact at Chicxulub.
U-Pb zircon age constraints on the Hamersley spherule beds: Evidence for a single 2.63 Ga Jeerinah-Carawine impact ejecta layer
Birger Rasmussen, University of Western Australia, School of Earth and Geographical Sciences, Crawley, Perth, Western Australia 6009, Australia; et al. Pages 725-728.
Up to seven spherule layers have previously been found in rocks from northwestern Australia and southern Africa, which have been interpreted as impact ejecta layers that formed during major asteroid impacts between 2.65 and 2.5 billion years ago. However, the lack of age constraints has led to confusion as to the number of impacts they represent and their correlation. Rasmussen et al. report the first robust U-Pb zircon date for one of these layers (the Carawine spherule bed; 2630 ± 6 million years old) in the Hamersley region of Western Australia, showing that it is equivalent to the ~2.63 billion-year old Jeerinah impact layer (Hamersley region). The new date negates previous correlations between the Carawine and Wittenoom spherule layers. In addition, an isotopic date for the Mount McRae Shale (Hamersley Group) provides a new minimum age for the Wittenoom spherule layer and a new maximum age for the Dales Gorge layer, aiding correlation between the three Hamersley spherule beds with three layers in similar-aged rocks in the Griqualand West Basin in South Africa. The new dates show that the Hamersley spherule layers represent three impacts that occurred at ca. 2.63 billion years old, 2.56-2.50 billion years old, and 2.50-2.48 billion years old. The three Hamersley spherule beds broadly correlate with three beds in South Africa, highlighting the possible global extent of the impact ejecta fallout. The layers potentially provide three geologically instantaneous markers spanning 150 million years across the Archean-Proterozoic boundary, possibly underpinning a chronostratigraphic framework across two continents and aiding paleogeographic reconstructions.
Taphonomic tradeoffs in tropical marine death assemblages: Differential time averaging, shell loss, and probable bias in siliciclastic vs. carbonate facies
Susan M. Kidwell, University of Chicago, Department of Geophysical Sciences, Chicago, IL 60637, USA; et al. Pages 729-732.
Today, tropical areas contain the highest total diversity of multicellular life on Earth. Predicting and managing the fates of this biodiversity in the face of environmental change requires knowing something about how these organisms have responded to changes over the last several millennia to millions of years: Which groups have waxed versus waned, or shifted their geographic distribution in response to changes of similar magnitude in the past? The sedimentary record contains this information. Using Caribbean reef and other shallow water environments in Panama as a field area, Kidwell et al. used a combination of dating methods to improve their understanding of how marine organisms actually become fossilized, so that they could interpret the geohistorical record with greater confidence, and in particular aimed to quantify the degree to which multiple generations of individuals might be mixed within individual sedimentary layers (time-averaging). These data are also relevant to questions of bias in the quality of preservation in different habitats, and relevant to rates of shell carbonate burial (versus recycling of that calcium and carbon-dioxide back into seawater and the atmosphere).
Hafnium systematics of the Mariana arc: Evidence for sediment melt and residual phases
Darren L. Tollstrup and James B. Gill, University of California, Earth Sciences, Santa Cruz, CA 95064, USA. Pages 737-740.
New evidence from submarine volcanoes in the Northern Mariana Trust Territories confirms that sediment from the Pacific Ocean sea floor is reborn in circum-Pacific volcanoes. By using isotopes of the elements hafnium (Hf) and neodymium (Nd), scientists found that lavas collected from the volcanoes using the submersible Alvin lie on a mixing line with the isotopically distinctive sediments. They also found a deficit in the concentration of Hf relative to Nd that is best explained by melting the sediment during its subduction to depths about 100 km beneath the volcanoes. This discovery provides another argument against disposing of nuclear waste by burying it in seafloor sediment that will be subducted back into the interior of Earth.
Karoo large igneous province: Brevity, origin, and relation with mass extinction questioned by new 40Ar/39Ar age data
Fred Jourdan, University of California Berkeley, Berkeley Geochronology Center, Berkeley, CA 94709, USA; et al. Pages 745-748.
Continental flood basalts consist of huge amounts of magma emplaced over several million km² (e.g., Deccan traps, Central Atlantic magmatic province, Parrana-Etendeka) and are thought to be related to continental breakup. The peak durations of continental flood basalt emplacement are currently considered to be relatively brief (~1 million year) magmatic events. This short time span is commonly believed to have strong implications for global geodynamics as well as major biotic crises through global climate forcing. New 40Ar/39Ar dates on the Karoo flood basalts (3 millions km²; southern Africa) show different characteristics compared to other continental flood basalts including (1) a significantly longer duration of magmatism (~8 million years, with 6 million years for the main volume), (2) apparent south-to-north migration of the eruption centers, and (3) briefer distinctive pulses inside the province. This suggests that the Karoo province does not fit the general mantle plume model invoked for most continental flood basalts (including the Karoo itself) and may also explain the absence of a major contemporaneous mass extinction.
Mid-Cretaceous pCO2 based on stomata of the extinct conifer Pseudofrenelopsis (Cheirolepidiaceae)
Matthew Haworth, University of Oxford, Earth Science, Oxford, Oxfordshire OX1 3PR, UK; et al. Pages 749-752.
Atmospheric carbon dioxide (CO2) is a major factor that determines global climate, and in order to predict future climate change it is important to understand the role CO2 levels have played in climate systems in the past. One method for reconstructing CO2 concentration involves the relationship between the number of stomata on a leaf and the atmospheric CO2 concentration in which the plant grew. Stomata are the tiny pores through which plants take up CO2, but also lose water at the same time. This means that plants have to balance CO2 uptake against water loss, and can respond to variations in CO2 levels by altering the number of stomata. During periods of high atmospheric CO2, plants can have lower numbers of stomata, and so lose less water while maintaining the same rate of CO2 uptake. This inverse relationship between the number of stomata and atmospheric CO2 allows estimation of the ancient atmospheric CO2 concentration from the density of stomata on fossil leaves. Using the fossil conifer Pseudofrenelopsis, Haworth et al. have reconstructed CO2 levels for the mid-Cretaceous period. The Cretaceous is known to have been a time of relative global warmth, with high levels of atmospheric CO2 responsible for this global greenhouse climate. Their results suggest, however, that atmospheric CO2 levels were lower than previously thought and relatively stable throughout the mid-Cretaceous, rather than dramatically fluctuating, as has been suggested in other reconstructions. Mid-Cretaceous CO2 levels were still significantly higher (2 to 3 times) than current concentration. Predictions of CO2 emissions suggest we will reach levels of atmospheric CO2 equivalent to those of the mid-Cretaceous between 2050 and 2100.
Climate simulation of the latest Permian: Implications for mass extinction
Jeffrey T. Kiehl and Christine A. Shields, National Center for Atmospheric Research, Climate Change Research Section, Boulder, CO 80305, USA. Pages 757-760.
Life at the Permian-Triassic boundary (~251 million years ago) experienced the largest disruption in Earth's history. Paleoclimatic data indicate that Earth was significantly warmer than at present and that much of the ocean had very low levels of oxygen. This study presents results from the first fully coupled comprehensive climate model using paleogeography for this time period. This climate model includes an atmospheric model, a full three-dimensional ocean model, a sophisticated land surface model, and a sea-ice model. The climate system model simulates very warm high latitude surface air temperatures related to high carbon dioxide levels and a stagnate global ocean circulation, which supports paleodata indicating low oxygen levels in the deep ocean. This is the first comprehensive coupled climate simulation using a realistic continental configuration, which has integrated to a point where the deep ocean state is in equilibrium, and that captures these observed features of this time period.
GSA TODAY Science Article
The non-equilibrium landscape of the Sierra Nevada, California
Marin K. Clark et al., California Institute of Technology, MS 100-23, Pasadena, California 91125, USA. Pages 4-10.
High mountains owe their origins to deep Earth processes: It has long been known that mountain belts form where tectonic plates collide, but just how river systems sculpt the topography of mountain ranges and at what rates is still unknown. Using a wide range of approaches that merge geochronology, classic geomorphology, and geodynamics, Marin Clark and colleagues have tackled the long-standing problem of the origin and evolution of the topography of the Sierra Nevada mountain range in California. Older low-relief topography is preserved in the Sierras and is relict from a time dominated by slow erosion rates between 83 and 32 Ma. Younger topography, characterized by greater bedrock incision, records rejuvenation of uplift and erosion of the Sierras, developed over the past 3 m.y. The mechanism for this younger uplift is uncertain but may relate to processes deep in the North American plate or subducting Pacific plate. Clark received GSA's prestigious Subaru Outstanding Women in Geoscience award in 2003.
To view the complete table of contents for the September issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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