||December 2, 2003
GSA Release No. 03-40
Boulder, Colo. - The December issue of GEOLOGY covers a wide variety of subjects and includes several newsworthy items. Topics include: new evidence regarding thermal power of the K-T boundary impact event; causes of selective extinction in the early Jurassic; controversy over mammalian dispersal when India and Eurasia first collided; and evidence of wildfire impacts on dating of rocks. Two possible greenhouse events in the late Cretaceous are the subject of the GSA TODAY science article.
Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in articles published. Contact Ann Cairns for additional information or other assistance.
Non-media requests for articles may be directed to GSA Sales and Service.
- Influence of wildfires on apatite and zircon (U-Th)/He ages
Sara Mitchell, University of Washington, Department of Earth and Space Sciences, Seattle, WA, U.S.A.; and Peter Reiners, Yale University, Geology and Geophysics, New Haven, CT, U.S.A.. Pages 1025-1028.
- Measurements of the timing and rates of erosional exhumation in Earth's crust are typically made by low-temperature thermochronometry, a form of geochronology that is sensitive to temperature changes below about 100-200 °C, as in the shallow crust. One technique in particular, apatite (U-Th)/He dating, relies on the decay of U and Th to He, and the fact that He does not significantly accumulate in apatite until the crystal has cooled below about 70 °C. Typical thermochronometric studies assume that temperatures decrease linearly as bedrock approaches the surface in an eroding terrain, and thus the He "clock" records the amount of time since that rock was some distance below the surface. However, once exposed at the surface, many rock surfaces are subjected to intense heating by wildfires. In this study, we demonstrate that wildfires can strongly affect measured (U-Th)/He ages of rocks, because the short (10-40 minutes) and hot (450-600 °C) heating by wildfires can cause very large He losses and apparently young ages in the outermost 3 cm of rock and throughout small rocks and sand on hillslopes. Besides the implications for care in the use of the apatite (U-Th)/He method as typically applied, our results also demonstrate that this technique can be used to constrain the locations, durations, and intensities of past wildfires in modern landscapes.
- Zn-Pb-Cu massive sulfide deposits: brine pool types occur in collisional orogens, black smoker types in back arc/arc basins
Mike Solomon, University of Tasmania, Hobart, Tasmania 7001, Australia; and Cecilio Quesada, Instituto Geologico y Minero de Espana, Direccion de Geologia y Geofisica, Madrid, 28003, Spain. Pages 1029-1032
- Massive sulfide ores have been mined for zinc, lead, copper, silver, and gold for thousands of years. Following research on massive sulfides currently forming on the sea floor, and on Tertiary ores in Japan, it was thought that ancient examples developed in extensional basins like the Sea of Japan. Solomon and Quesada have pointed out that ancient examples in southern Iberia and western Tasmania, among the world's richest and largest, formed during continent-continent or arc-continent collision zones. This finding will lead to reinterpretation of the tectonic setting of other ancient provinces, and refocus mineral exploration.
- North Atlantic Oscillation dynamics recorded in shells of a long-lived bivalve mollusk
Bernd Schöne, Johann Wolfgang Goethe University, Institute for Geology and Paleontology, INCREMENTS research group, Frankfurt, Germany; et al. Pages 1037-1040.
- Climate over the Northern Hemisphere is largely controlled by the so-called North Atlantic Oscillation (NAO). This climate index measures the sea-level pressure difference between the Azores and Iceland. On average, every 7.7 years the pressure difference reaches a maximum resulting in strong winds over Europe, extreme winters in North America, and severe droughts in lower latitudes. The NAO influences the global economy in many ways; e.g., by dramatic fluctuations in fish catch and crop yields or sharp fluctuations in the energy consumption. A primary focus is thus on forecasting changes of the NAO. This in turn requires modeling the NAO's past based upon observational data and various NAO records preserved in other climate archives such as trees and ice cores. Unfortunately, we do not have any NAO records from the area most affected by the NAO: the Atlantic Ocean. Our study reconstructed the state of the NAO for the first time from shells of extremely long-lived bivalve mollusks. The new record covers a period of 245 years and can be used to improve models of the NAO.
- Bleaching of Jurassic Navajo Sandstone on Colorado Plateau Laramide highs: Evidence of exhumed hydrocarbon supergiants?
Brenda Beitler, University of Utah, Department of Geology and Geophysics, Salt Lake City, UT 84112-0111, U.S.A.; et al. Pages 1041-1044.
- Spectacular color variations in the Jurassic Navajo Sandstone in southern Utah have long attracted the attention of geologists and laypersons alike. In this paper, we explain the cause of the color variations and discuss the implications. The Navajo Sandstone is perhaps the largest eolian (sand dune) complex on Earth, past or present. Abrupt red-white color transitions are believed to be the result of reducing fluids, likely gas hydrocarbons, flowing through the sandstone pores and removing the red pigment. Field mapping and analysis of satellite imagery indicate both stratigraphic and structural control on where fluids have left the sandstone "bleached." The most extensive regional bleaching occurs on eroded crests of broad asymmetrical uplifts produced during Laramide deformation (Cretaceous-Tertiary age). Alteration patterns suggest that the faults that core these uplifts were carriers for hydrocarbons and brought the buoyant bleaching fluids to the crests of the anticlines where they bleached the sandstone in both structural and stratigraphic traps. The extent of bleaching indicates that the Navajo Sandstone may have been one of the largest hydrocarbon reservoirs known. These ancient hydrocarbon traps have been extensively eroded, potentially releasing the bleaching gas into the atmosphere. The magnitude of the reservoir suggests that hydrocarbon escape could be significant in global carbon fluxes and possibly influence climate.
- Fireball passes and nothing burns — The role of thermal radiation in the K/T event: Evidence from the charcoal record of North America
Claire Belcher, Royal Holloway University of London, Geology, Egham, Surrey TW20 0EX, U.K.; et al. Pages 1061-1064.
- The amount of thermal power delivered by the impact of an asteroid on the Yucatan Peninsula, Mexico, 65 million years ago has been predicted to have been sufficient to have ignited extensive wildfires (Melosh et al., 1990; Kring and Durda, 2002). The presence of soot in the Cretaceous-Tertiary (K-T) boundary rocks from across the globe has been considered evidence that globally extensive wildfires occurred at this time (Wolbach et al., 1990). The K-T boundary and the lowermost Tertiary rocks from six non-marine sequences stretching from Colorado to Sakatchewan contain no charcoal or below background amounts of charcoal and a significant quantity of non-charred material, suggesting that there was no distinctive wildfire across North America related to the K-T event. This indicates that the thermal power delivered from the impact across North America did not have the destructive potential previously predicted. Therefore, high amounts of thermal radiation were not responsible for the extinctions associated with the K-T event.
- Extremely high sea-surface temperatures at low latitudes during the mid-Cretaceous as revealed by archaeal membrane lipids
Stefan Schouten, NIOZ, Marine Biogeochemisty and Toxicology, Den Burg, Texel 1790 AB, Netherlands. Pages 1069-1072.
- The middle Cretaceous (125-88 Ma) "greenhouse" world was characterized by high atmospheric CO2 levels, the general absence of polar ice caps, and much higher global temperatures than at present. Both foraminifera-based and model-based temperature reconstructions indicate extremely high sea-surface temperatures (SSTs) at high latitudes. However, there are a number of uncertainties with SST reconstructions due to diagenetic effects and tenuous assumptions with respect to Cretaceous seawaters. In this paper we applied a novel SST proxy that is based on the membrane lipids of ancient marine microbes called archaea. This new proxy called TEX86 indicates that tropical SSTs in the proto-North Atlantic were extremely high at 32-36 °C between 112-88 Ma, which is ~5-8 °C higher than today. This finding agrees with SST estimates based on global circulation model calculations. The TEX86 proxy indicates cooler SSTs (27-32 °C) for the equatorial Pacific between 120-115 Ma.
- Synchronous compression and extension in East Gondwana: Tectonic controls on world-class gold deposits at 440 Ma
Richard Squire and John Miller, The University of Melbourne, School of Earth Science, Melbourne, Victoria 3010, Australia. Pages 1073-1076.
- The occurrence of several extremely large gold deposits in eastern Australia, that all formed ca. 440 million years ago, may represent the products of a major geological event that occurred near the eastern margin of the ancient supercontinent Gondwana. Upwelling of extremely hot melt from deep below Earth's crust provided not only the source for some of the gold, but also the heat-engine to drive hot fluids several hundreds of kilometers inboard of East Gondwana's margin. As a result, several different styles of gold deposits were formed over an area at least 700 km in extent.
- Selective extinction among early Jurassic bivalves: A consequence of anoxia
Martin Aberhan, Institut fuer Palaeontologie, Museum fuer Naturkunde, Berlin D-10115, Germany; and Tomasz Baumiller, University of Michigan, Museum of Paleontology, Ann Arbor, MI 48109-1079, U.S.A. Pages 1077-1080.
- Extinctions often do not occur at random, but affect some groups of organisms more severly than others. Such selective extinction patterns can tell us more about the underlying causes of the extinctions. In our study on marine Jurassic bivalves we demonstrate that at certain times extinctions were significantly enhanced as compared to other time intervals, and that organisms living within the sediment suffered distinctly more than those living on the sea floor. This pattern can be explained by oxygen concentrations getting low within the sediment while oxygen supply in bottom waters was restricted less severely. The explanation is supported by sedimentological and geochemical data suggesting that widespread anoxia was a principal cause of the diversity crisis. This kind of approach should be applicable to other extinction events of the geological past as well.
- Repeated fracture and healing of silicic magma generates flow banding and earthquakes?
Hugh Tuffen, University of Munich, Earth and Environmental Science, München, Bavaria 80333, Germany. Pages 1089-1092.
- Shallow earthquakes occur in silica-rich volcanoes just prior to explosions, and are a vital monitoring tool used to save lives worldwide. Although they are thought to originate in the rising magma, the earthquake source has been highly controversial for many years because we cannot directly observe what happens within the volcano. We have discovered the youngest and best-preserved volcanic conduit yet documented, and have found numerous fractures, or fossil earthquakes, which formed during flow of the now-solidified magma. The fractures formed because the magma had cooled sufficiently to become solid-like and brittle, and provide the first direct evidence of a source mechanism for volcanic earthquakes. Since fracturing introduces bands into the rising magma, it may also explain the origin of the flow bands that are ubiquitous in obsidian.
- Stratigraphic response and mammalian dispersal during initial India-Asia collision: Evidence from the Ghazij Formation, Baluchistan, Pakistan
William Clyde, University of New Hampshire, Department of Earth Sciences, Durham, NH 03824-3589, U.S.A; et al. Pages 1097-1100.
- Thick sedimentary deposits along the border between Pakistan and Afghanistan have magnetic signatures that indicate they were deposited ca. 55 Ma in response to the initial tectonic collision between India and Eurasia. Fossil mammals from the same sediments show an increasing biological connection between the two continents through time. Many modern groups of mammals abruptly first appear during a short global warming event at about this same time (Paleocene-Eocene Thermal Maximum) and some researchers have hypothesized that they originated on India. Our new results suggest the opposite. Modern groups of mammals seem to have dispersed into India during initial collision rather than out of it.
- Paleomagnetic determination of vertical-axis rotations within the Charleston-Nebo salient, Utah
Bob Butler, University of Arizona, Department of Geosciences, Tucson, AZ 85721, U.S.A.; et al. Pages 1113-1116.
- In many mountain chains, there are layers of rock that have been folded and stacked on top of each other like a thin layer of snow on a sidewalk pushed in front of a snow shovel. These parts of mountain systems are called fold-thrust belts. Many fold-thrust belts have curved segments called salients where rocks have been pushed farther than in adjacent parts of the mountains. A classic example is the Charleston-Nebo salient that is a horseshoe bend in the mountains southeast of Salt Lake City, Utah. Geologists have argued for decades about how this curve was formed. A point of argument has been whether rocks that form the Charleston-Nebo salient were originally straight and were bent into a horseshoe outline or instead the horseshoe form is the original shape of a zone of weakness. Paleomagnetism is the field of geological research that studies ancient directions of Earth's magnetic field recorded in rocks. Just like a magnetic compass points north today, the magnetic directions frozen in rocks point toward north at the time the rocks were formed millions of years ago. So rocks of the same age over a large area will retain the same magnetic direction unless deformed by bending forces during mountain building. By studying the paleomagnetism of 225 million year old rocks that are found around the Charleston-Nebo salient, we have determined that rocks on the southern limb of the salient were rotated clockwise by ~65° while rocks on the northern limb were rotated counterclockwise by ~35°. This study has shown that the rocks of the Charleston-Nebo salient were originally straight and were bent into their present horseshoe shape. These results are significant to petroleum exploration because fold-thrust belts are common sites of oil and gas accumulations.
- Terrestrial evidence for two greenhouse events in the latest Cretaceous
Lee Nordt, et al., Department of Geology, Baylor University, Waco, Texas 76798, USA.
- Geologic records of past fluctuations in climate and in greenhouse gas content can provide powerful insight for modeling future global change. In this paper, Baylor University researchers Lee Nordt, Stacy Atchley, and Steve Dworkin report evidence that fluctuating atmospheric temperature and carbon dioxide content generally rose and fell together over the interval 71 to 64 million years ago. The Baylor group analyzed carbonate nodules in a succession of ancient soils of latest Cretaceous to earliest Tertiary age that are exposed in Big Bend National Park, Texas. Nordt and his colleagues calculated temperature and atmospheric CO2 content using models that relate atmospheric conditions to measured isotopic compositions of carbon and oxygen in soil. The researchers found that by adjusting their initial age estimates of the succession they could align it to a previously established pattern of fluctuating ocean temperatures. Dramatically elevated CO2 levels and atmospheric and ocean temperatures occur twice in the age-adjusted succession, and imply that Earth experienced greenhouse conditions estimated as 4 million years before and again half a million years before the Cretaceous ended with a bang.
To review the abstracts for these articles, go to www.gsajournals.org.
To review the complete table of contents for the current issue of GEOLOGY, go to www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
Representatives of the media may obtain a complimentary copy of any GEOLOGY article by contacting Ann Cairns.
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