Boulder, Colo.--The Geological Society of America's September issue of GEOLOGY contains several potentially newsworthy items. Topics include: possible mechanism linking methane-driven oceanic eruptions, mass extinctions, and climate perturbations; time lag between the Cambrian Explosion and its manifestation in deep-marine environments; surprising origins of Jurassic sand sea sediments in the western U.S.; new estimates of atmospheric carbon dioxide extending back 60 million years; and insights from a geoscience-oriented version of the periodic table of the elements. The GSA TODAY science article challenges prevailing views of the origins of the Himalayas.
Highlights are provided below. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in stories published. Contact Ann Cairns at acairns@geosociety.org for copies of articles and for additional information or other assistance.
GEOLOGY
Holocene History of the Larsen-A Ice Shelf Constrained by Geomagnetic Paleointensity Dating
Stefanie Brachfeld, Ohio State University, Byrd Polar Research Center, 1090 Carmack Road, Columbus, Ohio 43210, U.S.A.; 614-247-6843; 614-292-4697 (fax); brachfeld.2@osu.edu; et al. Pages 749–752.
Public attention has been focused on the Antarctic Peninsula due to the rapid disintegration of the Larsen Ice Shelf. Understanding the behavior of ice shelves requires a long period of observation to watch and learn how they grow, decay, and interact with the atmosphere and the ocean. We have extended the record of ice shelf observation back in time nearly 11,000 years using marine sedimentary records. In May 2000, the United States Antarctic Program research vessel Nathaniel B. Palmer visited the former Larsen-A Ice shelf, the northerly section of the ice shelf that collapsed in 1995. Sediment cores from this area contain several diatom ooze layers. Diatoms require sunlight for photosynthesis, which they cannot get underneath an ice shelf. While questions remain about the formation of these oozes, they do suggest the presence of open water in the vicinity during the Holocene. Determining the ages of Antarctic sediments is challenging, particularly in the Larsen Ice Shelf region since the sediment lacks appropriate material for radiocarbon dating. We employed a novel method of dating these sediment cores. We used a known pattern of variations of Earth's magnetic field as an intermediary to import absolute ages to the Antarctic sediments. This study illustrates the potential of using geomagnetic field behavior as a millennial-scale dating tool.
Post-Cambrian closure of the deep-water slope-basin taphonomic window
Patrick Orr, NUI, Geology, Galway, Ireland,
353-91-524411, 353-91-750533 (fax), Patrick.Orr@nuigalway.ie; et al. Pages 769-772.
The attention of most palaeobiologists remains directed toward studying the actual fossilized remains of ancient organisms--the body fossil record of the history of life on Earth. However, an expanding research area worldwide is the study of trace fossils--the sediment fabric preserved after it has reworked by organisms. For example, the footprints you would leave wandering across wet sand on a beach are a potential trace fossil. No remains of the actual organisms are preserved and, in fact, the actual producers of many trace fossils remain unknown. The trace fossils themselves often don't look too impressive--simple trackways where arthropods have crawled over the surface, sinuous or winding scribbles where worm-like organisms have tunneled through the sediment. The important point, however, is that the behavior of the organism responsible can be interpreted from the morphology of the trace fossil that is produced--never mind that most (there are exceptions) aren't particularly photogenic. Patrick Orr (University College Dublin), Mike Benton (University of Bristol), and Derek Briggs (Yale University) have exploited this to assess how the behavior patterns of deeper-marine organisms changed between approximately 545 and 290 million years ago. They have found that the behavior of organisms in the Cambrian period (545 to 490 million years ago) is different from the following periods. What we would consider an ecosystem broadly similar to that in modern deep-marine environments doesn't really arise until near the end of, or just after, the Cambrian. What's interesting is that there is therefore a time lag between the so-called "Cambrian Explosion" near the start of the Cambrian (during which, in the view of many authorities, there is either a very rapid diversification, possibly the origination, of the major metazoan phyla) and its possible expression in deeper-marine environments. This time lag is probably why most of the deeper-marine deposits with exceptionally preserved body fossils such as the celebrated Burgess Shale fauna are found in the Cambrian period.
Combined single-grain (U-Th)/He and U/Pb dating of detrital zircons from the Navajo Sandstone, Utah
Jeffrey Rahl, Yale University, Geology and Geophysics, P.O. Box 208109, New Haven, Connecticut 06520, U.S.A.; 203-432-5686; 203-432-3134 (fax); jeffrey.rahl@yale.edu; et al. Pages 761-764.
During the Jurassic era (ca. 190 million years ago), a huge sand sea similar to today's Sahara desert covered much of the western United States. In southern Utah, these desert sands are exposed at Zion National Park in the Navajo Sandstone, a rock unit famous for its large fossil sand dunes visible in steep cliff exposures. The tremendous amount of sediment preserved in the sand sea leads to the question: where did all of the sedimentary material come from? In the September issue of Geology, researchers from Yale and the Australian National Universities present evidence that the Navajo sediment was originally derived from the Appalachian Mountains in eastern North America. To show this, they developed a new technique for providing a distinctive "fingerprint" of the original source of most of the sediment found within sedimentary rocks. In single crystals of zircon, they measured both crystallization ages, by U/Pb dating, and cooling ages, by (U-Th)/He dating. The crystallization age corresponds to the time a rock formed deep in the earth, while the cooling age roughly corresponds to when a rock was eroded at Earth's surface. Together, these two ages provide a relatively precise constraint on the source of sediment. The bulk of the zircons analyzed show a combination of crystallization and cooling ages that is consistent with an origin from only one place in North America: the Appalachian Mountains. This surprising finding suggests that an ancient westward-flowing river system transported sediment across the continent of North America during the Jurassic, perhaps in a fashion similar to the Amazon in modern day South America, which carries material across the continent from the Andes.
Fragment of an ancient outlet-glacier system near the top of the Transantarctic Mountains
Stephen Hicock, The University of Western Ontario, Department of Earth Sciences, Department of Earth Sciences, The University of Western Ontario, London, Ontario N6A 5B7, Canada, 519-661-3189, shicock@uwo.ca; et al. Pages 821-824.
At 2500 meters above sea level in Antarctica there is an uplifted remnant of a glacial paleo-valley >20 million years old. It was part of the outlet glacier system of the time and is not the product of an ice sheet that overrode the Transantarctic Mountains < 3 million years ago--a hypothesis that is popular among some scientists. Our new field observations of a Sirius Group glacial deposit at Mount Feather, Antarctica, establish the wet-based style of glaciation (in contrast to today's cold, dry-based regime at the same elevation) recorded by this ancient and controversial deposit, as well as the ancient ice flow direction. We build on these data to provide the first plausible explanation of the deposit, taking into account previously established tectonic uplift rates and recent cosmogenic and ash ages that demonstrate the antiquity of the landscape and the sediments resting on it.
Methane-driven oceanic eruptions and mass extinctions
Gregory Ryskin, Northwestern University, Chemical Engineering, Evanston, Illinois 60208, U.S.A. Pages 741-744.
In his book "Extinction: bad genes or bad luck?" David M. Raup of the University of Chicago posed the question: "Did we choose a safe planet?" This paper attempts to provide the answer. If the hypothesis presented in the paper is correct, the answer is negative; however, it may be possible to prevent the impending catastrophe. Focusing on the most severe mass extinction in Earth's history, the Permian-Triassic boundary 250 million years ago, the paper proposes a mechanism that may have caused this and other extinctions (which collectively define the geological time scale), as well as climate perturbations including the ice ages. The mechanism is purely terrestrial, and likely still in operation today.
Generation of metal-rich felsic magmas during crustal anatexis
Andrew G. Tomkins, University of Calgary, Geology and Geophysics, Calgary, Alberta T2N 1N4, Canada, 403-220-6801, 403-284-0074 (fax), atomkins@ucalgary.ca; and John A. Mavrogenes, Australian National University, Geology/rses, Canberra, ACT 0200, Australia, 612-6125-3678, 612-6125-5544 (fax), john.mavrogenes@anu.edu.au. Pages 765-768.
Some of the world's largest and most valuable ore deposits are associated with granitic intrusions. These granite bodies essentially represent portions of Earth's lower crust that have melted and subsequently migrated upward before cooling and crystallizing closer to Earth's surface where we see them today. In association with this process, we have found that when metal-rich zones are subjected to high temperatures deep in Earth's crust, gold and other ore minerals can melt to form metallic magma. We show that if the surrounding rock also melts, to form granitic magma, both of these very different magmas can migrate together. As the granitic melt migrates upward, it can act as a carrier for the metallic melt, which is gradually dissolved. New ore deposits are likely to form when this metal-rich granitic magma starts to crystallize. Understanding this important process helps in exploration for new ore reserves.
Atmospheric pCO2 since 60 Ma from Records of Seawater pH, Calcium, and Primary Carbonate Mineralogy
Robert Demicco, State University of New York, Binghamton, Department of Geological Sciences and Environmental Studies, P.O. Box 6000, Binghamton, New York 13902-6000, U.S.A., 1-607-777-2264; 1-607-777-2288 (fax), demicco@binghamton.edu; et al. Pages 793–796.
Variations in the carbon dioxide content of the atmosphere are implicated in Earth's climatic history. This study calculates the carbon dioxide content of Earth's atmosphere over the past 60 million years based on two sets of measurements: (1) pH estimates of the surface ocean of Earth made from measurements on microscopic plankton skeletons preserved in a core taken from the Pacific Ocean; and (2) Ca ion content of the surface ocean from measurements of evaporated seawater preserved in marine halite deposits. Together these measurements allow thermodynamic calculation of the carbon dioxide content of the atmosphere through the carbonate buffer system of the world ocean. We consider our results to be the most reliable estimates of carbon dioxide in Earth's atmosphere over the past 60 million years, showing up to an ~100 times rise in atmospheric carbon dioxide during times when the fossil record indicates globally warmer temperatures.
Evidence for orbitally controlled size variations of the East Antarctic Ice Sheet during the late Miocene
Jens Gruetzner, Bremen University, Geosciences Department, P.O. Box 330440, Bremen, Bremen 28334, Germany, 49-421-218-2482, 49-421-218-3116 (fax), gruetzn@allgeo.uni-bremen.de; et al. Pages 777–780.
The history of the East Antarctic Ice Sheet has been largely inferred from low-latitude climate proxy data (mainly the oxygen isotope ratio in shells of microfossils) but the amplitude and period of ice-sheet variations remain undefined in these data sets because the isotope changes reflect a combined effect of ice volume and temperature. As yet, proximal Antarctic high-resolution records that could document size fluctuations of the East Antarctic Ice Sheet with a high resolution are rare because of low core recovery, discontinuities, and dating problems. We studied sediment cores recovered from East Antarctic continental rise during Ocean Drilling Program Leg 188. Our results indicate that prominent cyclic color variations observed in these climate archives were largely controlled by 41 and 20 thousand year variations in solar insolation. On the basis of these cycles we derived a high-resolution chronology for a8 500,000 year long time interval ca. 7 million years before present. This time scale together with high-resolution iron measurements allowed us to track fluctuations in the amount of continental material transported to the drilling location. The large magnitude of these fluctuations (up to 100% change during each 41 thousand year cycle) and the episodic recurring input of iceberg–transported material suggest that the ice sheet underwent large size variation ca. 7 million years ago. Such evidence indicates a more dynamic behavior than previously inferred and implies that a significant proportion of the variability seen in low-latitude climate proxy data reflects Antarctic ice volume changes.
An Earth Scientist's Periodic Table of the Elements and Their Ions
L. Railsback, Department of Geology, University of Georgia, Athens, Georgia 30602-2501, U.S.A., 706-542-3453, 706-542-3453 (fax), rlsbk@gly.uga.edu. Pages 737–740.
This paper presents a new periodic table of the elements more useful to earth scientists than the conventional periodic table used by chemists. The new periodic table presented here acknowledges that most natural matter occurs in charged form as ions, rather than in elemental form. The immediate result is a completely rearranged table in which many elements appear multiple times, because many elements assume different charge under different natural conditions. The practical result is that many trends in mineralogy, seawater chemistry, soil chemistry, the chemistry of Earth's crust and mantle, the chemistry of sediments, and nutrient chemistry become apparent in ways that are not recognizable on conventional, elementally constructed, periodic tables.
GSA TODAY Science Article
Initiation of the Himalayan orogen as an early Paleozoic thin-skinned thrust belt
G.E. Gehrels, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, ggehrels@geo.arizona.edu; et al.
The Himalayas: Older than they look: The Himalayan mountain range, recently in the news with the 50th anniversary of the ascent of Mount Everest by Sir Edmund Hillary, has been a focus of geological research for more than a century as one of the great mountain belts on Earth. Using different dating techniques in combination with geological mapping, the traditional story has been that the Himalayas owe their origin to the collision of the Indian continent with Eurasia beginning about 55 million years ago. However, scientists at University of Arizona are challenging that view. George Gehrels and colleagues present new evidence that demonstrates that some of the features commonly attributed to the formation of the Himalayas over the past 55 million years may in fact owe their origin to an older period of mountain building more than 450 million years ago. The vestiges of this older mountain-building event within the Himalayas appear to be widespread. They may rewrite some of the history of this mountain belt, which is cited widely as having influenced the chemistry of sea water and global climate over the past 50 million years.
To review the abstracts for these articles, go to http://www.gsajournals.org. To review 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. Representatives of the media may obtain a complimentary copy of any GEOLOGY article by contacting Ann Cairns at acairns@geosociety.org.
Geological Society of America
Release No. 03-24
Contact: Ann Cairns
303-357-1056
acairns@geosociety.org