Boulder, Colo. - Topics in the July issue include: rupture models for the A.D. 900-930 Seattle fault earthquake; results from CO2 injection at Texas' Frio Formation and implications for storing greenhouse gases in sedimentary basins; a new paleothermometer for reconstructing ancient climates; the most protracted glacial surge ever recorded; desert varnish as a record of environmental change; marine predator-prey systems in the Devonian; ocean-atmosphere influences on climate variability in the Late Holocene; and a new technique for analyzing strain-distorted fossils.
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The age and origin of the Labyrinth, western Dry Valleys, Antarctica: Evidence for extensive middle Miocene subglacial floods and freshwater discharge to the Southern Ocean
Adam R. Lewis and David R. Marchant (corresponding author), Boston University, Department of Earth Sciences, 685 Commonwealth Ave, Boston, Massachusetts 02215, USA; et al. Pages 513-516.
A more than 50 kilometer-long network of bedrock channels and scoured terrain occupies the ice-free portion of a major trough that crosses the Transantarctic Mountains in southern Victoria Land. These channels, collectively termed the Labyrinth, emerge from beneath the margin of the East Antarctic Ice Sheet and represent just one of a series of spectacular channel networks within the Transantartic Mountains. These channels are reminiscent of the channeled scablands in western Washington, USA. 40Ar/39Ar analyses of volcanic tephra on the Labyrinth show that the channels are relict, that major channel incision predates 12.4 million years ago, and that the last major subglacial flood occurred sometime between 14.4 million years ago and 12.4 million years ago. The most likely source for the meltwater was catastrophic drainage of subglacial lakes from East Antarctica. As one hypothesis to test, Lewis et al. postulate that sufficient meltwater may have been released to the Ross Embayment during these outburst floods to impact regional/global climate and Southern Ocean deep-water formation.
Obsidian Hydration: A new paleothermometer
Lawrence M. Anovitz, Oak Ridge National Laboratory, Chemical Sciences Division, MS 6110, PO Box 2008, Bldg. 4500-S, Oak Ridge, Tennessee 37831-6110, USA; et al. Pages 517-520.
Obsidian, a naturally forming volcanic glass, has been used by many peoples around the world for making weapons and tools. Because freshly broken obsidian immediately begins to absorb water from its surroundings, efforts have been ongoing since the 1960's to use this hydration process to date samples from archaeological sites. Unfortunately, the sensitivity with which the rate of this hydration process responds to small changes in the environment has made this difficult. Anovitz et al. show that this liability can be turned into an advantage. Such environmental sensitivity makes obsidian hydration a unique tool for reconstructing ancient climates. Anovitz et al. present the first successful application of this approach, which combines laboratory-based experimental calibrations of the effect of temperature on the hydration rate with measurements of the hydration depths of archaeological samples and 14C results from the Chalco site in the Basin of Mexico. The results not only show that this technique is viable, but that soils in the Basin of Mexico have, overall, tended to become cooler during the last fifteen centuries.
Seasonally specific responses of the East Asian monsoon to deglacial climate changes
Takeshi Nakagawa, University of Newcastle upon Tyne, Department of Geography, Newcastle upon Tyne, NE1 7RU, UK; et al. Pages 521-524.
The film The Day After Tomorrow describes how pan-hemispheric abrupt cooling at a huge amplitude severely affected such major cities as New York, London, and Tokyo. However, the effect such an event would have on Tokyo would be slightly different than what the movie protrayed. A micropaleontological study of a Japanese lake sediment core revealed that the cooling described in the film (triggered by shut-down of the North Atlantic ocean circulation) would indeed be severe in the regions to the west of the monsoon front. However, the effect could hardly reach beyond the monsoon front. Consequently, winter in Tokyo would become colder during the climatic event, but the summer temperature in Tokyo (temperature of the Pacific air mass) would not fluctuate so much. Therefore, the Pacific and Atlantic are two geoclimatic regions that respond differently to ocean circulation changes in the North Atlantic. Nakagawa et al. reached these conclusions by analyzing signals of seasonal (summer and winter) temperature and precipitation recorded in the sediment profile.
Scleractinian corals produce calcite, and grow more slowly, in artificial Cretaceous seawater
Justin B. Ries, Johns Hopkins University, Morton K. Blaustein Department of Earth and Planetary Sciences, 301 Olin Hall, Baltimore, Maryland 21218, USA; et al. Pages 524-528.
The ratio of magnesium to calcium in seawater has varied significantly over the geologic past, mainly due to chemical reactions along the mid-ocean ridge. Such reactions are ultimately driven by fluctuating rates of ocean crust production. The high magnesium-to-calcium ratio of modern seawater (5.2) favors the formation of the aragonite mineral, while low magnesium-to-calcium ratios (less than 2) favor the formation of the calcite mineral. Likewise, modern scleractinian corals, which are the primary reef builders in today's oceans, produce their skeletons exclusively from this aragonite mineral. However, when Ries et al. grew modern scleractinian corals in artificial seawaters formulated at the low magnesium-to-calcium ratios that existed during Late Cretaceous time (100–65 million years ago), the corals actually produced a portion of their skeleton as the calcite mineral, instead of the normal aragonite. These are the first experiments to show that the Late Cretaceous ancestors of modern aragonitic scleratinian corals probably produced at least part of their skeleton as calcite. Furthermore, corals grown in the artificial Late Cretaceous seawater calcified at significantly slower rates than those grown in modern seawater. This suggests that the scleractinian corals' diminished reef-building activity during the Late Cretaceous was caused by the unfavorable magnesium-to-calcium ratios that existed at that time.
Clast size controls and longevity of Pleistocene desert pavements at Lathrop Wells and Red Cone volcanoes, southern Nevada
Greg A. Valentine (corresponding author) and Charles D. Harrington, Los Alamos National Laboratory, Earth and Environmental Sciences Division, MS D462, Los Alamos, New Mexico 87545, USA. Pages 533-537.
Desert pavements, composed of thin surface layers of pebbles, are an important component of arid landscapes. They promote the accumulation and stabilization of eolian sediment to form soils, and the degree of development of pavements is commonly used as a relative age indicator for geomorphic surfaces. Valentine and Harrington's studies focus on the development of pavements and soils on surfaces that originate as fragmental deposits of volcanic material ejected from scoria cone volcanoes. The volcanic fragments form an excellent parent material for pavement formation and allow scientists to test the effects of clast size on the development of pavements. Valentine and Harrington show that there is a range of clast sizes that promote pavement formation, and above or below this size range, pavements and related soils do not form. This information can be useful in restoration of desert landscapes after human disruption. Desert pavements can be disrupted by vegetation and animal activity. Previous work, at the altitude of the two studied volcanoes in southern Nevada, has suggested that denser plant and animal communities during glacial times would have completely disrupted any previous pavements such that no modern pavements could be older than about 15,000 years. The work reported in this paper shows that this is not correct, and that the pavements at Red Cone and Lathrop Wells volcanoes were not "reset," but instead show different levels of maturity, even though both are older than 15,000 years.
Baking black opal in the desert sun: The importance of silica in desert varnish
Randall S. Perry, Imperial College, University of London, Earth Science and Engineering, South Kensington Campus, London, OX1 3 PR, UK; et al. Pages 537-541.
In many of the world's arid environments, rocks take on a shiny coating known as desert varnish. For many years early scientists, including Darwin, were intrigued by its black color and wondered at the mystery of its origin. Yet just how this lustrous veneer is created has remained a subject of controversy. A new study by Perry et al. has discovered the source of desert varnish and reveals how it can be used as a record of environmental change on Earth, and possibly on other planets such as Mars. Previously, it was thought that the activity of microbes helped to produce the rock coating. It was believed that the dark appearance is produced by the mineral manganese oxide and that bacteria exist that oxidize this manganese. This theory was strengthened when scientists discovered organic compounds and DNA fragments bound within the shiny coating. Using a battery of techniques, including high resolution electron microscopy, Perry et al. reveal that silica is the most important mineral present, indicating that biology is not important for desert varnish formation and that the source of the organic components is outside rather than within the varnish. On desert rock surfaces, silica is slowly dissolved from other minerals, then gels together to form a glaze which binds within it other substances on the rock face. Detrital grains, organic compounds, and pollutants from local environments become entombed in the coating as it forms. The desert varnish record extends through time because it occurs in fine layers that develop one over the other, over tens of thousands of years, and the deepest, oldest layers in the varnish may have formed in very different conditions than the shallowest, youngest layer. The layers represent a record of environmental change. The study also suggests that if silica exists in varnish-like coatings in deserts, caves, or hydrothermal deposits on Mars, then it may entrap ancient microbes or chemical signatures of previous life on that planet.
Submarine pyroclastic deposits formed at the Soufrière Hills volcano, Montserrat (1995–2003): What happens when pyroclastic flows enter the ocean?
J. Trofimovs, University of Bristol, Earth Sciences, Bristol, Avon BS81RJ, UK; et al. Pages 549-553.
A major question in volcanology is what happens when pyroclastic flows enter the ocean. To date, the topic remains controversial due to the difficulty in sampling in situ submarine deposits and equivocal data from ancient successions. The Soufrière Hills volcano, Montserrat, West Indies, has undergone a series of andesite lava dome growth and collapse events since the eruption began in 1995. Over 90% of the pyroclastic material produced has been deposited into the surrounding ocean. The submarine flanks of the Soufrière Hills volcano have been investigated by a combination of swath bathymetry and sediment coring during cruise JR123 of the RRS James Clark Ross (9–18 May 2005). Sampling of these submarine deposits reveals that the pyroclastic flows mix rapidly and violently with the water as they enter the sea. Flows entering the ocean have been observed to generate small-scale explosions and a large explosion at the culmination of the July 2003 collapse. Mixing between the pyroclastic flow and ocean water takes place between the shoreline and 500 meter depth, where the deposition of basal coarse-grained parts of the pyroclastic flow first starts on slopes of 15 degrees or less. The coarse components (pebbles to boulders) are deposited proximally from dense basal slurries to form steep-sided, near-linear ridges that intercalate to form a submarine fan. The finer ash-grade components are mixed into the overlying water column to form turbidity currents that flow distances in excess of 30 kilometers from the source.
Low-velocity glacial surges - Processes unlocked by modern surge on Svalbard
Ida Lønne, NOKUT, International Recognition Unit, Oslo, N-0121, Norway. Pages 553-556.
Fridtjovbreen, a small glacier on Svalbard, 78°N, has recently gone through a 12-year-long surge event. This is the most protracted glacial surge ever recorded, with an advance rate reaching a maximum of 4.2 meters per day, considerably slower than surge advances at lower latitudes. Push moraines and ice-cored moraines formed along the land-based part of the ice margin. However, the most distinct landforms were the numerous sediment ridges that developed below sea level as the ice front retreated. These observations show that the surge terminated gradually over several years, which is in strong contrast to the abrupt termination of surges mapped outside Svalbard.
Predatory drill holes and partial mortality in Devonian colonial metazoans
Mark A. Wilson (corresponding author), College of Wooster, Geology, Wooster, Ohio 44691, USA; and Paul Taylor, Natural History Museum, Department of Palaeontology, London, SW7 5BD, UK. Pages 565-568.
One of the puzzles in the history of life is the changing relationship between predators and prey. How have predators affected the evolution of their prey and vice versa? Predator-prey systems have been difficult to distinguish in Paleozoic marine invertebrate communities because little evidence remains in the fossil record. In this study, Wilson and Taylor report the discovery of beveled holes drilled in the calcareous tubes of a Devonian colonial animal (about 375 million years old) called a hederellid. The form and distribution of the holes show that they were made by a predator that selected particular hederellid species to drill and consumed the soft parts under the shell. The hederellids in response produced patches to repair the holes, and they shut down parts of their colonies to reduce the damage. This is the earliest example of drilling predation on a colonial organism. It is thus important new evidence for the reconstruction of predator-prey relationships over time.
Subdecadal to multidecadal cycles of Late Holocene North Atlantic climate variability preserved by estuarine fossil pigments
J. Bradford Hubeny, University of Rhode Island, Oceanography, Narragansett, Rhode Island 02882, USA; et al. Pages 569-572.
In order to understand how climate may change in the future, it is necessary to understand how it has changed in the past. University of Rhode Island scientists Brad Hubeny, John King, and Antelmo Santos recently improved our understanding of natural climate cycles around the North Atlantic Ocean and how different climate influences act in tandem. Hubeny et al. reconstructed productivity in a Rhode Island estuary over the past 1000 years by measuring the concentrations of chlorophyll used for photosynthesis. The calculated productivity was driven by temperature, and therefore gives a good approximation of fluctuating temperatures over time. The results demonstrate that in New England, both atmospheric and oceanic circulation patterns influence air and coastal water temperatures. The relative roles of atmospheric and oceanic influences has varied over the past 1000 years, and this new evidence of their interaction advances scientific understanding of how climate varies from decade to decade and century to century.
Gas-water-rock interactions in Frio Formation following CO2 injection: Implications for the storage of greenhouse gases in sedimentary basins
Y.K. Kharaka, U.S. Geological Survey, Water Resources Discipline, Menlo Park, California 94025, USA; et al. Pages 577-581.
Global warming and resulting climate change are among the most important environmental challenges facing the world. There is broad consensus among scientists that global warming results from increases in the concentrations of atmospheric carbon dioxide (CO2) emitted largely from the burning of coal and petroleum. An attractive mechanism to decrease CO2 emissions to acceptable levels is to separate, compress, and store the gas in sedimentary basins. Kharaka et al. discuss the geochemical results from a multidisciplinary field experiment funded by the US Department of Energy. Approximately 1,600 tons of CO2 were injected at 1,500-meter depth into a sandstone section of the Frio Formation - a major brine and oil reservoir in the Texas Gulf Coast. Brine and gas samples were collected and analyzed from both the injection and observation wells before, during, and after CO2 injection to track the flow of injected CO2 and the resulting changes in brine and gas compositions. This first field test of CO2 storage in sandstone beds having no petroleum proved highly successful, demonstrating the relatively straightforward method of CO2 injection and its flow and storage in the Frio Formation. The chemical data, including pH, alkalinity, iron, and gas compositions, proved highly effective for tracking the injected CO2. Geochemical modeling, however, indicated rapid dissolution of minerals, especially calcite and iron oxyhydroxides, likely caused by low pH brine in contact with injected CO2. This is a potential problem because mineral dissolution could create pathways in the rock seals and well cements that may facilitate leakage of CO2 and brine. Maintaining reservoir integrity against the ultimate escape of CO2 back to the atmosphere is essential to the success of injection operations. Preventing brine leakage into overlying drinking water supplies is also paramount because of its high salinity and the toxicity of the mobilized metals and organic compounds.
Rupture models for the A.D. 900–930 Seattle fault Earthquake from uplifted shorelines
Uri S. ten Brink, U.S. Geological Survey, Woods Hole Science Center, Woods Hole, Massachussetts 02543-1598, USA; et al. Pages 585-588.
The active Seattle fault, which runs under the city of Seattle and across the Puget Sound area, last ruptured around A.D. 900–930 and generated a tsunami. The details of this earthquake are largely unknown, because it happened in prehistoric time, and no other large earthquakes have occurred along this fault since then. The earthquake raised the shoreline at Restoration Point on Bainbridge Island by up to 7 meters (23 feet), and the shoreline of parts of Seattle by almost as much. Other shorelines in the area experienced subsidence or lesser amounts of uplift. ten Brink et al. use the pattern and magnitude of shoreline uplift and subsidence to model the size of the earthquake and its rupture depth, geometry, and magnitude. This will allow scientists and policymakers to understand what could happen should the fault rupture again and prepare for the expected damage.
Deepak C. Srivastava (corresponding author) and Jyoti Shah, IIT Roorkee, Earth Sciences, Department of Earth Sciences, Roorkee, Uttaranchal 247667, India. Pages 593-596.
The shapes of natural objects, e.g. fossils, are commonly distorted by the strain in the in Earth's crust. The distorted fossils, despite being useful to structural geologists for estimation of strain, are a curse to paleontologists because the distortion makes identification of a fossil very difficult. Scientists, therefore, need methods that not only decipher strain from distorted fossils, but also "return" the fossils to their original shapes by removing the effects of deformation. Srivastava and Shah present a very easy and rapid digital technique that analyzes distorted fossils to decipher the amount of strain and restores their pre-distortion shape in a single operation. The new technique, using any graphic software, takes only a few minutes and produces precise results.
Laacher See revisited: High-spatial-resolution zircon dating indicates rapid formation of a zoned magma chamber
Axel K. Schmitt, UCLA, Department of Earth and Space Sciences, Los Angeles, California 90095-1567, USA. Pages 597-600.
About 12,900 years ago, the violent eruption of Laacher See volcano in the Eifel region (Germany) wreaked havoc on its immediate surroundings and impacted the environment and climate in central Europe. How long it takes to amass significant volumes of magma at depth will constrain the chances for an early detection of magma bodies that are at risk of erupting. Schmitt determined the compositions and ages of individual zircon crystals from Laacher See rocks by high-spatial-resolution ion microprobe analysis. The results indicate that zircons crystallized rapidly a few thousand years prior to the eruption in a precursor magma that was different from the 12,900 B.P. Laacher See magma body proper. This suggests that the erupted Laacher See magma might have accumulated much faster than over the 10,000–20,000 year period previously thought.
To view the complete table of contents for the July issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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