||25 September 2007
GSA Release No. 07-44
Boulder, CO, USA - Topics include: discovery of exceptionally preserved soft-bodied biotas in Ontario and Manitoba, Canada; discovery of an arctic lake containing sediments 200,000 years old; effects of ancient Mayan deforestation and agriculture on soil erosion in northern Guatemala; a new catalog of episodic tremor and slip for the Cascadia subduction zone; and a new model of Sierra Nevada volcanism and uplift. The GSA TODAY science article takes a broad look at agriculture and soil erosion.
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- Eramosa Lagerstätte—Exceptionally preserved soft-bodied biotas with shallow-marine shelly and bioturbating organisms (Silurian, Ontario, Canada)
Peter H. von Bitter, Royal Ontario Museum, Department of Natural History, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada; et al. Pages 879-892.
- Because soft-bodied organisms quickly decay after death, preservation of soft body parts is the exception rather than the rule, and paleontologists rarely truly know what has been lost from ancient ecosystems. von Bitter et al. describe such exceptions preserved in the 425-million-year-old Silurian Eramosa Lagerstätte of Ontario, Canada, including abundant articulated conodont skeletons with eye traces and heterostracan jawless fish with the first recorded traces of preserved soft tissue (vertebrates), annelids and arthropods with soft body parts (invertebrates), and diverse marine plants. The exceptionally preserved biota of the Eramosa Lagerstätte lived in marginally marine to fully marine environments; those in the latter were associated with carbonate-secreting invertebrates such as brachiopods and corals, as well as burrowing organisms. It is the association of exceptionally preserved biota with the more commonly preserved shelly fauna and trace fossils that distinguishes the Eramosa Lagerstätte from other shallow-marine Silurian Lagerstätten, such as the Waukesha Lagerstätte of Wisconsin. The Eramosa Lagerstätte is not the product of exceptional preservation in an atypical environment, a bias claimed for many post-Cambrian Lagerstätten, and may provide a more reliable, balanced measure of Silurian marine life than previously available.
- Exceptionally preserved Late Ordovician biotas from Manitoba, Canada
Graham A. Young, The Manitoba Museum, Geology and Paleontology, 190 Rupert Avenue, Winnipeg, Manitoba R3B 0N2, Canada; et al. Pages 883-886.
- Much of our knowledge of life in the distant past has come from studies of those rare sites at which soft tissues such as skin and muscle are preserved. Very few such sites are known from the Ordovician Period of geological time (about 488 to 444 million years ago). Young et al. describe the fossils from two recently discovered Ordovician sites in Manitoba, Canada, where the rocks are formed from sediment deposited along and near ancient shorelines about 445 million years ago. Fossils in these rocks add significantly to our knowledge of life in the seas shortly before a major mass extinction. These fossils provide important clues about the anatomy and genealogy of several groups of organisms, including jellyfish, horseshoe crabs, eurypterids ("sea scorpions"), and seaweeds. Because they occur between the well-known Burgess Shale-type deposits of the Cambrian Period (which are more than 500 million years old) and younger Paleozoic soft-bodied fossils such as the Mazon Creek biota of Illinois (slightly more than 300 million years old), their position in the geological record helps to fill an important gap in our knowledge of soft-bodied fossils.
- Multiple generations of interglacial lake sediment preserved beneath the Laurentide Ice Sheet
J.P. Briner, University at Buffalo, Department of Geology, 876 NSC, Buffalo, New York 14260, USA; et al. pages 887-890.
- Global warming is a reaction of Earth's climate system to anthropogenic greenhouse gasses emitted throughout the industrial era. The amount and pattern of future climate change, and the complex set of interactions between Earth's natural systems (for example, oceans, biosphere, and ice sheets) are not well understood. Paleoclimate studies are important sources of information about our planet prior to the industrial era. Paleoclimate studies tell us what the relationship is between greenhouse gas concentrations in the atmosphere and global temperature, provide information on how different earth systems are connected, and determine how rapid climate and biological changes can occur under natural conditions. Because the Arctic plays a critical role in the global climate system,as it has the capability to amplify global warming, paleoclimate studies are of particular importance. One key source of paleoclimate information is the variety of climate change indicators preserved in lake sediments, which are deposited through time much like the layers in ice cores. However, long lake sediment records are rare in the Arctic because ice sheets during the last Ice Age (10,000 years ago) scooped out most lake basins or otherwise altered previously existing lakes. Briner et al. have made a unique discovery of an arctic lake containing sediments that span the last 200,000 years. The finding demonstrates that in settings where ice sheets preserved the underlying terrain—a situation in the Arctic that is not as rare as it once was thought—valuable records of paleoclimate exist.
- Evidence for a rapid sea-level rise 7600 yr ago
Shi-Yong Yu, Large Lakes Observatory, Univiversity of Minnesota, Duluth, Minnesota 55812, USA; et al. Pages 891-894.
- Dating the transgression and subsequent regression in marginal basins of the southeastern Swedish Baltic Sea provides a new perspective of global ice-volume changes and the isostatic adjustment of the mantle after the retreat of the Scandinavian Ice Sheet. Yu et al. attribute a rapid local sea-level rise of roughly 4.5 meters ca. 7600 calibrated (cal) years before present to a sudden increase in ocean mass, most likely caused by the final decay of the Labrador sector of the Laurentide Ice Sheet. This finding suggests that land-based ice sheets can melt relatively quickly, and thus more attention should be given to the Greenland Ice Sheet when predicting future sea level, particularly within the context of global warming.
- Changes in coral-reef structure through the Miocene in the Mediterranean province: Adaptive versus environmental influence
Luis Pomar, Universitat de les Illes Balears, Ciencies de la Terra, Ctra. Valldemossa, km 7.5, Palma de Mallorca, Baleares/España E-07122, Spain; and Pamela Hallock, University of South Florida, College of Marine Science, 140 7th Avenue South, St. Petersburg, Florida 33701, USA. Pages 899-902.
- Both global climate change and the worldwide decline of reef-building coral populations now make the news on a regular basis. Well-constrained studies of fossil reef communities during times of environmental change can provide invaluable insights and perspective on the current situation. Pomar and Hallock examine changes in reef-building corals in the Mediterranean approximately 6.5-7 million years ago. These changes took place during a time of global cooling, and also correspond with recently documented diversification of the symbiotic zooxanthellae (algae) that shallow-water coral depend upon for rapid growth and calcification.
- Damage and permeability around faults: Implications for mineralization
Heather A. Sheldon, CSIRO, Exploration & Mining, PO Box 1130, Bentley, WA 6102, Australia; +1-61 8 64368915, +1-61 8 64368555 (fax); and Steven Micklethwaite, Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. Pages 903-906.
- Gold deposits and other valuable minerals are commonly located close to bends or gaps in major faults. Recent work has drawn parallels between the location of aftershocks around modern faults, such as the San Andreas fault, and the location of mineral deposits around ancient faults. Sheldon and Micklethwaite explain these relationships in terms of "damage," or cracking, that occurs after a fault rupture event or earthquake. Damage creates pathways that allow fluid to pass through the rock, transporting and depositing gold and other minerals. The ability to simulate this process using computers may lead to the prediction and discovery of previously unknown mineral deposits.
- Segmentation in episodic tremor and slip all along Cascadia
Michael R. Brudzinski, Miami University, Geology Department, Oxford, Ohio 45056, USA; +1-513-280-0660, +1-513-529-1542 (fax); and Richard M. Allen, Seismological Laboratory, Department of Earth & Planetary Science, University of California-Berkeley, Berkeley, California 94720, USA. Pages 907-910.
- The recent discovery of episodic tremor and slip (ETS) in subduction zones is based on slow slip episodes visible in global positioning system (GPS) observations correlated with nonvolcanic tremor signals on seismometers. ETS occurs just inboard from a region capable of great megathrust earthquakes; however, whether there is any communication between these two processes remains unknown. In this study, Brudzinski and Allen use new single-station methods to compile an ETS catalog for the entire Cascadia subduction zone, offshore western North America, and compare the patterns with a variety of along-strike trends for the subducting and overriding plates. Correlated ground vibrations and strain observations are found all along the subduction zone, demonstrating that ETS is an inherent part of the subduction process. There are three broad (300-500 km), coherent zones with different recurrence intervals (14 ± 2, 19 ± 4, 10 ± 2 months), where the interval duration is inversely proportional to upper plate topography and the spatial extent correlates with geologic terranes. These zones are further divided into segments of ETS that occur at times typically offset from each other. The seven largest (100-200 km) segments appear to be located immediately landward from forearc basins interpreted as manifestations of megathrust asperities, implying that there is a spatial link between ETS and earthquake behavior. It is not yet clear if any temporal link exists, but the regional time between ETS episodes could be controlled by strength variations due to composition of geologic terranes.
- Quantification of soil erosion rates related to ancient Maya deforestation
Flavio S. Anselmetti, Geological Inst. ETHZ, Earth Sciences, Universitätsstr. 16, CHN H70, Zurich, ZH 8092, Switzerland; et al. Pages 915-918.
- Anselmetti et al. address an important issue of how the ancient Maya civilization of Mesoamerica impacted the environment through its agricultural activities. For the first time, Anselmetti et al. have quantified soil erosion rates over 6000 years during the period of Maya occupation in a lake basin in northern Guatemala. They found that soil erosion was greatest (almost 1000 t/ km2yr–1) in the Pre-Classic period when estimated population densities were still low. Erosion rate unexpectedly declined in the Early and Late Classic periods as Maya population densities rose exponentially. Anselmetti et al. infer that a pulse of intense erosion accompanied initial land clearance in the Pre-Classic period when highly erodable soils were first exposed. These results suggest that the Maya had to contend with soil erosion long before their accelerated population growth and ultimate demise in the Classic period. This example of ancient human impact on the environment is relevant to contemporary issues of tropical deforestation and soil erosion.
- The tectonic significance of hgh-K2O volcanism in the Sierra Nevada, California
Keith Putirka, California State University, Fresno, Earth and Environmental Sciences, 2576 E. San Ramon Ave., MS/ST24, Fresno, California 93710, USA; and Cathy J. Busby, Department of Earth Science, University of California-Santa Barbara, Santa Barbara, California 93106, USA. Pages 923-926.
- Putirka and Busby present new field and geochemical data from the central Sierra Nevada, California, that suggest a new model for Sierra Nevada volcanism and uplift. The rise of the Sierra Nevada has been attributed to the removal of dense rock that lies beneath the granitic crust, a process called "lithosphere delamination." This process works as would a block of wood, floating in water, with a metal plate underneath; remove the metal plate and the wood block rises. Seismic studies show that "lithosphere delamination" beneath the Sierra is nearly certain—but its timing and linkages to volcanism are not. This new data show that special volcanic rocks, high in the element potassium (K), erupt at the onset of transtensional "normal" faulting, not delamination. These normal faults, which control range uplift, began their activity 10 million years ago. This interpretation stands in contrast to earlier models that link the onset of delamination to volcanic activity 3.5 million years ago. Putirka and Busby's new age dates, and published dates, demonstrate that volcanism was initiated throughout the Sierra Nevada 10 million years ago. Putirka and Busby surmise that range front faulting began throughout the Sierra Nevada 10 million years ago as well, and that "delamination" was initiated then, or earlier. A later episode of volcanic eruption and range uplift, beginning 3.5 million years ago, may have been triggered by a later accelerated phase of delamination, range-front faulting, or both.
- Dual role of seawater and hydrothermal fluids in early Archean chert formation: Evidence from silicon isotopes
Sander H.J.M. van den Boorn, Utrecht University, Earth Sciences, Budpastlaan 4, Utrecht 3584 CD, Netherlands; et al. pages 939-942.
- Our knowledge of Earth's earliest surface heavily relies on information from scarce sediments that have survived since their formation billions of years ago. Rock sequences in the Archean (the period between 4.0 and 2.5 billion years ago) are unusually rich in chert. This rock type, with very high silica content, is a key element in reconstructing the conditions of the early Earth and of the environments where the earliest forms of life developed. Not only are the oldest fossil remains intimately associated with cherts, the properties of cherts have also been used as evidence that Archean ocean water had higher temperatures than later in the geological history.
- However, how these abundant cherts formed has remained controversial. Van der Boorn et al. tackle the problem from a new angle by using silicon isotopes to unravel their origin. Their data from 3.5- to 3.0-billion-year-old chert deposits from the Pilbara Craton (Western Australia) demonstrate that silicon isotopes (together with other geological and geochemical data) are diagnostic for the origin of chert. Van der Boorn et al. argue that Archean seawater is the dominant source of silica for one group of cherts and that hydrothermal fluids venting at the seafloor produced a different set of chert deposits. These findings reconcile competing hypotheses, and have important implications for the use of Archean cherts as traces of the earliest environmental conditions of our planet.
- Geodetic constraints on areal changes in the Pacific-North America plate boundary zone: What controls Basin and Range extension?
Corné Kreemer and William C. Hammond, University of Nevada, Reno, Bureau of Mines and Geology, 1664 N. Virginia Street, Reno, Nevada 89557-0178, USA. Pages 943-946.
- The crustal deformation in the wide region between the Pacific and North America plates has been quantified using over 1500 published velocities inferred from global positioning system (GPS) measurements. Kreemer and Hammond have converted the deformation into estimates of present-day changes in surface area. Their study shows that between the Gulf of California and British Columbia, the entire plate boundary does not undergo any net areal change. The greater Basin and Range area, on the other hand, increases its size by over 5000 square meters per year. Kreemer and Hammond show that this areal growth is offset by an equal amount of areal reduction in the northern California Coast Ranges and Klamath Mountains. The study enforces the idea that the southern Cascadia subduction zone provides a "window of escape" for the highly elevated Basin and Range to extend toward. Such an idea is consistent with independent information about stress orientations in the Basin and Range, and anomalously low-stress magnitudes along the southern Cascadia subduction zone.
- A Paleocene lowland macroflora from Patagonia reveals significantly greater richness than North American analogs
Ari Iglesias, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, División Paleobotánica, Paseo del Bosque s/n°, La Plata, La Plata / Buenos Aires 1900, Argentina; et al. Pages 947-950.
- Very little is known about the vegetation of the South American continent during the Paleocene epoch, the 10 million years following the mass extinction that ended the age of dinosaurs 65.5 million years ago. Iglesias et al. report the discovery of diverse and well-preserved fossil plants from Paleocene rocks in the Patagonian deserts of Argentina. The flora show a much warmer and wetter climate than today and more than 40 species of plants—many more than the better-known North American floras of the same age that tended to have 10-15 species. The discovery raises new questions about whether extinction effects were much less severe in southern South America, whether recovery was faster, or if pre-extinction floras were more diverse.
- Evidence for a sedimentary origin of clay minerals in the Mawrth Vallis region, Mars
Joseph R. Michalski, Jet Propulsion Laboratory, Mail Stop 183-501, Pasadena, California 91109, USA; and Eldar Z. Noe Dobrea, Malin Space Science Systems, P.O. Box 910148, San Diego, California 92121, USA. Pages 951-954.
- Clay minerals detected spectroscopically in the Mawrth Vallis region of Mars correspond to a complex, layered, thick (>600 m) stratigraphic section of ancient bedrock. Because the light-toned, clay-bearing rocks are lithologically diverse over a broad area (>80,000 km2), have significant internal layering and complexity, and contain buried impact craters within the section (demonstrating that they were deposited over a geologically significant duration of time), Michalski and Dobrea interpret the host rocks as sedimentary or possibly pyroclastic. Crater counts date the clay-bearing rocks to early-middle Noachian time (estimated as 3.8-4.1 Ga). Geomorphic observations suggest that the rocks were lithified early, and deeply eroded during the late Noachian-early Hesperian. The combination of a probable ancient sedimentary context of the clays and a moderate pH formation environment implied by the occurrence of smectites clearly places these rocks among the most important targets for future astrobiological exploration.
- Is agriculture eroding civilization's foundation?
David R. Montgomery, Quaternary Research Center and Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA
- Making mole hills out of mountains: Ever since humans disturbed Earth's surface in the name of agriculture, we have witnessed increased erosion and soil loss. In the October 2007 GSA Today science article, geomorphologist David Montgomery quantifies the rate at which soil is lost due to erosion of agricultural land and compares that to geologically established rates of soil formation and rates of erosion on undisturbed land. The results paint a clear picture that the human race is "mining" the soil for agriculture because the rate of soil loss and, hence, fertility, is several times to several orders of magnitude greater than the rate of soil production. While this is clearly unsustainable and should be concern for a humanity seeking to feed an expanding population, its slow and inexorable pace make it less sensational than climate change and associated dramatic meteorological events. Montgomery shows that erosion rates from conservation tillage techniques, if widely adopted, may be enough to stem the loss of soil from farmland and bring it closer to a steady-state with respect to rates of soil formation.
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