Boulder, CO, USA - Topics include high-resolution lunar images related to the Orientale impact; possible methane release event at the icehouse-greenhouse transition 635 million years ago; evidence of oil smoke in sediment from the K-P boundary dinosaur extinction; Greenland Ice Sheet’s sensitivity to global warming; what the San Andreas fault-area landscape reveals about earthquakes; a new record of greenhouse warming from central Utah; evidence of a possible glacial land system on Mars; and a sea-level climate change fingerprint.
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Earth-based radar data reveal extended deposits of the Moon’s Orientale basin
Rebecca R. Ghent et al., Department of Geology, University of Toronto, Earth Sciences Centre, 22 Russell St., Toronto, Ontario M5S 3B1, Canada. Pages 343-346.
Analysis of new, high-resolution Earth-based radar images of the Moon by Ghent et al. reveal that the products of the Orientale impact can be found over much of the southern polar highlands. These products include a blocky, melt-rich deposit and a block-poor, fine-grained facies. These results show that production of an extensive ejecta facies depleted in rocks 10 centimeters and larger occurs for a wide range of crater sizes. This fine material can therefore contribute significantly to the physical characteristics of lunar surface materials. Furthermore, Ghent et al.’s study refines earlier views of the significance of melt in large-crater ejecta, and has implications for future exploration of the lunar south polar region, likely a key target for future landed and sample-return missions.
Carbon isotope evidence for widespread methane seeps in the ca. 635 Ma Doushantuo cap carbonate in south China
Jiasheng Wang et al., China University of Geosciences, Wuhan, Hubei 430074, China. Pages 347-350.
Earth’s most severe glaciation occurred about 635 million years ago, with the ice sheet potentially extending to tropic oceans. The end of this severe glaciation marked a major perturbation of the global carbon cycle and ocean chemistry, which was partially recorded in thin "cap carbonate" units that ubiquitously overlie glaciogenic rocks. Although the exact mechanism for cap carbonate deposition and its associated alkalinity and isotope anomaly remains debated, it has been proposed that methane release from gas hydrate destabilization may have played an important role. The finding of carbon isotope values as low as -48 parts per million (VPDB) in multiple sections of the Doushantuo cap carbonate in South China confirms the widespread distribution of methane seeps in the Doushantuo cap carbonate and supports a regional methane release event about 635 million years ago. Wang et al. propose that this methane release event may have played a critical role at the icehouse-to-greenhouse transition.
Combustion of fossil organic matter at the Cretaceous-Paleogene (K-P) boundary
Mark C. Harvey et al., 5/26 Picton Street, Ponsonby, Auckland, New Zealand. Pages 355-358.
Harvey et al. have discovered curious, tiny smoke particles in sediments deposited during the dinosaur extinction event. Close examination of the distinctive particles revealed a striking resemblance to smoke from industrial burning of coal and oil. Because this type of smoke cannot be produced by forest fires, the likely explanation is that the Cretaceous-Paleogene meteorite struck and ignited an oil field. In fact, one of Earth's largest known oil fields lies next to the meteorite crater in the Gulf of Mexico. The meteorite impact would have released enough heat to burn vast quantities of oil, injecting massive amounts of carbon dioxide into the atmosphere. This carbon dioxide may have caused an interval of intense global warming, contributing to the extinction of the dinosaurs.
Response of the southern Greenland Ice Sheet during the last two deglaciations
Anders E. Carlson et al., University of Wisconsin-Madison, Department of Geosciences, 1215 West Dayton St., 104 Wilkinson, Madison, Wisconsin 53703, USA. Pages 359-362.
The greatest uncertainty in predictions of future sea-level rise in response to global warming is the contribution from Earth’s remaining ice sheets. This uncertainty stems, at least in part, from a limited understanding of ice sheet sensitivity to a warming climate, with current ice sheet models suggesting a relatively slow response of the Greenland Ice Sheet to global warming. Carlson et al.’s new records of southern Greenland Ice Sheet summer melting for the last two deglaciations provide a continuous history of how fast this ice sheet responded to the last two "natural" climate warming events. These data indicate that the Greenland Ice Sheet responded essentially synchronously with the warming climate, suggesting that this ice sheet is very sensitive to climate change. In particular, this implies that the current negative mass loss from the Greenland Ice Sheet may actually be the beginning of an accelerating trend in response to the small temperature warming attributable to anthropogenic greenhouse gases, rather than a short-lived oscillation in mass balance.
Geomorphic response to uplift along the Dragon’s Back pressure ridge, Carrizo Plain, California
George E. Hilley, Dept. of Geological and Environmental Sciences, 450 Serra Mall, Braun Hall, Bldg. 320, Stanford University, Stanford, California 94305-2115, USA; and J Ramon Arrowsmith, Arizona State University, Department of Geological Sciences, Tempe, Arizona 85287, USA. Pages 367-370.
Earthquakes that have occurred along the San Andreas fault during the past 120,000 years have uplifted rocks adjacent to the fault, and this uplift allows Hilley and Arrowsmith to study how this motion may be revealed by topography. In particular, in the Carrizo Plain of Southern California, rocks of the Pacific plate are deformed as plate tectonic processes move these rocks into and through a zone of active uplift, which is produced by changes in the San Andreas fault's geometry that are linked to the North American plate. Mapping of geologic units allowed the team to reconstruct the deformation that has occurred over time, while new, high-resolution Airborne Laser Swath Mapping (ALSM) data allowed them to study how the topography and erosional processes respond to this deformation. Thus, by combining traditional geologic mapping with new tools that allow the study of landscapes in unprecedented detail, Hilley and Arrowsmith explicitly linked the response of different elements of the landscape to ongoing deformation. The measures of topographic change that they found most well-correlated with deformation may allow us to infer patterns and rates of fault-related deformation in other areas where geologic relationships alone may provide insufficient information to allow us to infer these quantities.
Mechanisms of PETM global change constrained by a new record from central Utah
Gabriel J. Bowen and Brenda Beitler Bowen, EAS Department, Purdue University, 550 Stadium Mall Dr., West Lafayette, Indiana 47907, USA. Pages 379-382.
Rapid and extreme greenhouse warming of global climate that occurred 55 million years ago at the Paleocene-Eocene boundary is widely viewed as a test case for understanding the response of Earth systems to global climate change. The number and geographic distribution of sites where this event has been studied on the continents, however, is extremely limited. Bowen and Bowen report a new record of the event from central Utah, the first North American record that is comparable in quality to the best-documented records from Wyoming's Bighorn Basin. By comparing geochemical records from the two sites, they address a number of proposed hypotheses for earth systems response to climate change at the Paleocene-Eocene boundary, finding support for aridification of the Utah site, diversion of precipitation from lower- to upper-middle latitudes, and climate-induced shifts in vegetation composition during the event.
Sub-million-year age resolution of Precambrian igneous events by thermal extraction–thermal ionization mass spectrometer Pb dating of zircon: Application to crystallization of the Sudbury impact melt sheet
Donald W. Davis, University of Toronto, Earth Sciences Centre, 22 Russell St., Toronto, ON M5S 3B1, Canada; firstname.lastname@example.org. Pages 383-386.
Uranium-lead dating of zircon is the most accurate and precise way to date igneous rocks, but age precision is rarely better than about plus or minus 1 million years for the Precambrian. Davis describes a new technique that potentially offers a considerable improvement in the accuracy of zircon ages on a wider variety of samples and at less cost than what is generally available from conventional methods. Zircon is first pre-heated in a vacuum to evaporate lead from disturbed crystal domains, leaving lead with isotopes that preserve the primary age of the sample. This refractory lead is removed from the zircon by thermal extraction at higher temperature directly into a silica-melt ionization activator within a thermal ionization mass spectrometer. This produces strong ion beams that allow the daughter lead isotope ratios to be measured to high precision with almost negligible contamination from common lead. Isotope fractionation appears to be more reproducible than with conventional solution analysis, allowing routine age determinations with precision of approximately plus or minus 0.2 million years in the Precambrian. As a test application, zircon from a mafic boundary phase of the Sudbury impact melt gives 1849.53 ± 0.21 million years ago (Ma), while a phase from several hundred meters higher in the impact melt sheet is resolvably younger at 1849.11 ± 0.19 Ma (95% confidence errors). The enhanced precision and ease of application of this method should greatly increase the scientific power and availability of zircon dating.
Reconstructing relative flooding intensities responsible for hurricane-induced deposits from Laguna Playa Grande, Vieques, Puerto Rico
Jonathan D. Woodruff et al., Massachusetts Institute of Technology-Woods Hole Oceanographic Institution Joint Program in Oceanography, Woods Hole, Massachusetts 02543, USA. Pages 391-394.
Woodruff et al. estimate the relative flooding intensities for hurricanes striking eastern Puerto Rico over the past 5000 years. Flooding conditions are inversely modeled based on the grain-size distribution of sediment deposited in a coastal lagoon during each hurricane event. A deposit associated with the infamous A.D. 1928 San Felipe hurricane is used as a modern analogue to test the technique, which produces reasonable estimates for wave heights that exceeded the site’s barrier during this Category 5 event. The lagoon’s sedimentary record exhibits large-scale changes in the frequency of hurricane flooding on centennial-to-millennial time scales; however, Woodruff et al. show that the magnitude of flooding at the site during these events appears to have stayed relatively constant through time. Over the past 5000 years, no evidence exists for an anomalously large hurricane or tsunami event with a competence for sediment transport any greater than historical hurricane events.
Groundwater and climate dynamics derived from noble gas, 14C, and stable isotope data
Stephan Klump et al., EBA Engineering Consultants Ltd., Calcite Business Centre, Unit 6, 151 Industrial Road, Whitehorse, Yukon Y1A 2V3, Canada. Pages 395-398.
Groundwater is not only the most important source of drinking water worldwide, but it can also be used as an archive of past environmental conditions. Using a suite of environmental tracers, Klump et al. investigated a deep aquifer system in southeastern Wisconsin. These tracers allow the residence time of the groundwater and the environmental conditions prevailing during groundwater recharge to be assessed. Results show that the aquifer contains water recharged before, during, and after the last glacial advance across the area. The findings of this study shed light on the glacially driven climatic and hydraulic changes that occurred during the late Pleistocene and the early Holocene. The work also contributes to the thorough understanding of the present and past hydrogeologic conditions of the aquifer, which is a prerequisite for the sustainable use of this important regional water resource.
Permeability control on magma fragmentation
Sebastian Mueller et al., Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS81RJ, UK. Pages 399-402.
Mueller et al. address an aspect of the geosciences that has a direct impact on lives, infrastructure, and capital-explosive volcanic eruptions. The balance and competition between gas loss via permeable flow (avoiding explosivity) and gas retention and subsequent fragmentation (generating explosivity) is a decisive factor in controlling such eruptions. Using an experimental approach, this study relates fragmentation behavior to permeability, thus taming the nagging issue of short-circuiting of explosions via gas flow. The results will enable refined modeling of various volcanological scenarios and, subsequently, lead to a more reliable hazard assessment.
Rock varnish evidence for latest Pleistocene millennial-scale wet events in the drylands of western United States
Tanzhuo Liu and Wallace S. Broecker, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964-8000, USA. Pages 403-406.
Obtaining continuous millennial-scale climate records for the world’s deserts has proven to be a difficult, but often critical, step in understanding the mechanism and spatial variations of Earth’s climate. Rock varnish is a slowly accreting (about few micrometers per thousand years), dark coating on subaerially exposed rock surfaces in arid to semiarid deserts. Because of its sedimentary origin, rock varnish often displays a layered microstratigraphy that records past climatic variations. Liu and Broecker present rock varnish evidence for latest Pleistocene (from 18,000 to 11,500 calendar years before present) millennial-scale wet events in the drylands of the western United States. Preliminary radiometric age calibration of the varnish record indicates that these wet events are largely associated with millennial-scale cooling events documented in the Greenland ice core records. This finding suggests that such wet oscillations in the western United States may be parts of regionally widespread manifestation of pervasive millennial-scale cycles of the North Atlantic climate.
Rapid ecosystem response to abrupt climate changes during the last glacial period in western Europe, 40–16 ka
Barbara Wohlfarth et al., Department of Geology and Geochemistry, Stockholm University, 106 91 Stockholm, Sweden. Pages 407-410.
Current and future climate change may lead to dramatic consequences for our remaining natural ecosystems, but how fast may these occur and how severe might these be? Looking into the future is quite difficult, but a glimpse into the past offers an excellent opportunity to further our understanding of the impact and rapidity of climatic changes. Wohlfarth et al. recovered and analyzed sediments from the bottom of the former lake at Les Échets in central France, which offer such a glimpse into the past. Their careful study revealed that the lake’s ecosystem and the lake’s surroundings responded distinctly and within 40-230 years to climatic changes between 40,000 and 20,000 years before present. The observation that the lake’s response, and response time, were unique for each of these climatic shifts clearly shows how sensitive ecosystems are once critical thresholds are crossed.
Late Amazonian glaciation at the dichotomy boundary on Mars: Evidence for glacial thickness maxima and multiple glacial phases
James W. Head et al., Department of Geological Sciences, Brown University, Box 1846, Providence, Rhode Island 02912, USA. Pages 411-414.
A terrain type characterized by lineated terrain oriented parallel to the strike of long valleys fills many valleys along the distinctive boundary between the southern uplands and the northern lowlands of Mars. This lineated valley fill has been interpreted as glacial in origin, representing the former presence of extensive glaciation at these mid-latitudes. A glacial flow lobe trending uphill for over 900 meters elevation provides impressive evidence that the glaciation was even more extensive than previously thought. Taken together with the elevation of the lineated valley fill, Head et al.'s new data suggest that a glacial land system at least 2 km thick extended over these regions of Mars in recent geological history.
Optical dating of abrupt shifts in the late Pleistocene East Asian monsoon
Thomas Stevens et al., School of Earth Science and Geography, Kingston University, Penrhyn Road, Kingston upon Thames, Surrey, KT1 2EE, UK. Pages 415-418.
Chinese loess (wind-blown silt) contains one of the most important records of terrestrial Quaternary climate change. The deposits contain a detailed and long-term record of the alternating influence of part of the monsoon system responsible for sustaining two-thirds of the world's population. However, previous findings from loess have been limited by the lack of independently dated sequences. Using optically stimulated luminescence dating at high sampling resolution, Stevens et al. provide time-continuous chronologies for two sites on the Chinese Loess Plateau. The age models shed new light on loess sedimentation rates, monsoon forcing, response, and dynamics. This new information is only obtainable using radiometric dating and allows the first independent comparison to other regional records of monsoon change.
The sea-level fingerprint of the 8.2 ka climate event
Roblyn Kendall et al.; Jerry X. Mitrovica, University of Toronto, Physics, 60 St. George St., Toronto, ON M5S 1A7, Canada. Pages 423-426.
8200 years ago, Earth experienced a cooling event that was the most abrupt, widespread climate instability of the last 10,000 years. There is general agreement that the event was triggered by the release of fresh water into the North Atlantic associated with the catastrophic drainage of glacial Lakes Agassiz and Ojibway, with a resulting disruption in ocean circulation. The event has received significant attention because it occurred during a period in which the climate was otherwise similar to the present-day; therefore, it may provide clues as to how sensitive the modern ocean/climate system is to the melting of ice reservoirs associated with global warming. Unfortunately, the volume of the ancient Lakes Agassiz and Ojibway is uncertain. Could sea-level records provide a robust measure of this volume? Kendall et al. show that the drainage of these lakes produced a highly distinct fingerprint in sea level. For example, their analysis demonstrates that sites relatively close to the lakes, including the west and Gulf coasts of the United States, had small signals due to the lake release, and thus may not be optimal field sites for constraining the outflow volume. Other sites, such as the east coast of South America and western Africa, have significantly larger signals associated with the lake release, and are thus better choices in this regard.
Stress-forecasting (not predicting) earthquakes: A paradigm shift?
Stuart Crampin et al., Professor of Seismic Anisotropy, Shear-Wave Analysis Group, School of GeoSciences, The University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW, Scotland, UK. Pages 427-430.
A paradigm shift has been suggested before earthquakes can be predicted. Seismic shear-wave splitting monitors fluid/rock deformation of the most compliant elements of in situ rock: fluid-saturated stress-aligned microcracks in almost all crustal rocks. Because rocks are vulnerable to shear stress, stress before large earthquakes has to accumulate over enormous volumes of rock, in order for shear-wave splitting to monitor the effects on the microcracks at substantial distances from impending epicenters. The paradigm shift is to ignore the source zone that is locally heterogeneous and complicated by initial conditions and instead monitor stress accumulation away from the source. Using small earthquakes as the source of shear waves, characteristic effects have been observed with hindsight before 14 earthquakes worldwide (M 1.7 to M 7.7), and on one occasion, when changes were recognized early enough, the time, magnitude, and fault-break of an M 5 earthquake in southwest Iceland were successfully stress-forecast in a narrow time-magnitude window. The key effect is that the various effects of the observed deformation are logarithmically proportional (self-similar) to earthquake magnitudes, just like the Gutenberg-Richter relationship. Using controlled-source cross-hole seismics, Crampin et al. suggest techniques or routine forecasting.
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