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GEOLOGY
Mega-tsunami deposits on Kohala volcano, Hawaii, from flank collapse of Mauna Loa Gary M. McMurtry, University of Hawaii, Oceanography, Honolulu, HI 96822, USA, et al. Pages 741-744.
Giant tsunamis have been invoked to explain gravels laden with marine fossils at high elevations in the Hawaiian Islands. That explanation has proved contentious, however, the islands sink and rise with the lithosphere, fueling the competing hypothesis that the deposits simply mark upraised shorelines. In this paper, the argument is resolved by considering a formation on Hawaii Island's Kohala Volcano. The formation - clearly a high-energy deposit - was emplaced about 120 thousand years ago, when local sea level, as shown by a drowned offshore reef terrace, was 430 m lower. Fossils in the formation are from a reef flat environment quite unlike the present shoreline but entirely consistent with the inferred shoreline at the time of emplacement. The 120,000-year age matches the known age of the last great landslide down the flank of nearby Mauna Loa volcano. The colossal tsunami generated by that landslide picked up the fossils as it crossed the reef and spread them over the western face of Kohala. The tsunami swept 6 km inland and deposited its dense load of entrained sediments and surface lavas up to an elevation of nearly 500 m.
A prehistorical record of cultural eutrophication from Crawford Lake, Canada Erik J. Ekdahl, University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109-1063, USA, et al. Pages 745-748.
The extent to which native societies have influenced their surroundings is a controversial topic. We show that a group of people living in the small, forested watershed of Crawford Lake, Ontario, during the 13th-15th centuries profoundly altered Crawford Lake from pristine background conditions. Lake circulation, nutrient availability, productivity, and diatom assemblages changed instantaneously with human occupation of the watershed, despite inferred low numbers of people. The Crawford Lake watershed was abandoned in the late 15th century, followed by nearly 400 years of watershed and lake recovery until resettlement of the watershed by Euro-Canadians during the 1860s. While inferred nutrient concentrations and productivity returned to predisturbance levels during this recovery period, lake circulation did not return to presettlement conditions, and diatoms remained in postdisturbance assemblages. Resettlement of the watershed during the 1860s resulted in a second period of cultural eutrophication. These results suggest that pristine lacustrine environments are highly susceptible to small variations in nutrient availability, and are the first example from the North American midcontinent of modification to the environment by native peoples.
Early Pleistocene incision of the San Juan River, Utah, dated with 26Al and 10Be Amy J. Wolkowinsky and Darryl E. Granger, Purdue University, Earth and Atmospheric Sciences, West Lafayette, IN 47907, USA. Pages 749-752.
The canyons of the Colorado River system, including Grand Canyon, Glen Canyon, and San Juan Canyon are a spectacular example of river entrenchment. The evolution of these canyons has long been the object of geologic and geomorphic studies. Although incision of the Colorado River has been dated to 5 million years ago in the western Grand Canyon, where the river exits the Colorado Plateau, it has only been loosely constrained further upstream. The timing of canyon incision is important for understanding why the canyons formed: did the canyons upstream deepen in response to the cutting of Grand Canyon, or do they exist independently of Grand Canyon? A new dating technique using cosmogenic nuclides was applied to a river terrace along the San Juan Canyon near Bluff, Utah. There, a gravel quarry exposes sediment that was deposited by the San Juan River prior to its cutting of a canyon 150 meters deep. The cosmogenic nuclides reveal that the gravels were deposited at 1.36 +/- 0.2 million years ago. This age indicates that San Juan Canyon is younger than Grand Canyon, but also that the canyon was cut more slowly than Grand Canyon. It is thus likely that San Juan Canyon has formed independently of Grand Canyon, instead being controlled by local variations in precipitation and the river's sediment load.
Magmatic vapor contraction and the transport of gold from the porphyry environment to epithermal ore deposits Christoph A. Heinrich, Eidgenössische Technische Hochschule, ETH Zentrum NO, Department of Earth Sciences, Zürich CH-8092, Switzerland, et al. Pages 761-764.
Chemical analysis of ore-forming metals in microscopic inclusions of magmatic fluids in natural crystals have been combined with experimental measurements of ore-metal solubility at the high temperatures and pressures prevailing in magma chambers beneath volcanoes. A computer model simulating the chemical reactions occurring during ascent of these fluids indicate a new mechanism by which gold, despite its inherent low solubility, can be transported in high concentrations into veins where high-grade gold ores can be formed.
Micrometer-scale porosity as a biosignature in carbonate crusts Tanja Bosak, California Institute of Technology, Geological and Planetary Sciences, Pasadena, CA 91125, USA, et al. Pages 781-784.
Recognizing an unambiguous microbial imprint in the rock record is often difficult. In this work, we suggest that micron-scale porosity may be a biosignature in carbonate crusts. Our laboratory experiments show that this type of porosity is only generated in the presence of bacteria, and we observe similar structures in both modern carbonates as well as ancient carbonates of putative microbial origin. This work helps to define specific criteria whereby to recognize subtle biosignatures in the rock record.
Shackleton Fracture Zone: No barrier to early circumpolar ocean circulation Roy Livermore, British Antarctic Survey, Cambridge, Cambridgeshire CB3 0ET, UK; et al. Pages 797-800.
In 1977, James Kennett, at Rhode Island, suggested a link between Drake Passage opening, global cooling, and Antarctic glaciation. He pointed out that the gateways at Drake Passage and south of Tasmania formed at about the same time that global climate took a turn for the worse (about 30-40 million years ago), and just prior to the glaciation of Antarctica. Creation of these gateways completed a deep-water pathway around Antarctica, which allowed the Antarctic Circumpolar Current (ACC)-the largest ocean current on Earth-to initiate, and effectively isolate Antarctica from warmer waters flowing south. Marine geophysical work indicated that the crust in Drake Passage formed between 29 and 6 million years ago at a spreading centre rather like the mid-Atlantic Ridge. However, it was believed that, even after the opening started, a large, circumpolar current could not form because of the existence of a shallow barrier, formed by overlapping ridges of continental crust extending from the Antarctic and South American margins. As a result, the ACC only became important 23 million years ago, when the ends of the two ridges cleared. New mapping and rock sampling of the larger of these ridges, the Shackleton Ridge, shows that it is a purely oceanic feature, just like ridges observed adjacent to large fracture zones in the Atlantic and Indian oceans, and that it could have formed much more recently than previously believed. Hence, there was probably no barrier in the embryonic Drake Passage to early ACC flow, and it may be that the opening of Drake Passage 33-34 million years ago was indeed, as Kennett suggested, the trigger for the onset of global cooling and the glaciation of Antarctica.
Rapid marine recovery after the end-Permian mass extinction event in the absence of marine anoxia R.J. Twitchett, University of Plymouth, School of Earth, Ocean and Environmental Sciences, Plymouth, Devon PL4 8AA, UK, et al. Pages 805-808.
In the past, life on Earth has suffered five major mass extinction events, the largest of which occurred during an interval of severe global warming at the end of the Permian period (a little over 250 million years ago) when 80-90% of species were wiped out. Understanding this event is crucial for understanding the subsequent evolution of the marine ecosystem. The post-Permian recovery is the longest of any extinction event: global biodiversity levels took some 100 million years to return to pre-extinction levels, although ecological recovery was slightly faster, with complex structures such as reefs appearing within 10 million years. One theory is that faster recovery was not possible because the world's oceans were stagnant, poorly oxygenated, and inhospitable for marine animals for many millions of years after the extinction event. Until now it has not been possible to test this theory. Abundant fossils of marine animals that were living in the immediate aftermath of the end-Permian extinction event have been recently discovered in Oman. These animals were living in one of the very few regions of the world's oceans that were well oxygenated and hospitable at that time. Using a new method of assessing ecological recovery, we have shown that the Oman community was surprisingly diverse and ecologically complex. The high level of recovery recorded by the Oman fossils is not seen elsewhere until many millions of years later. This result supports the theory that inhospitable conditions prevented rapid recovery: when the environment is hospitable to animal life recovery can take place 10 times faster. It also shows, for the first time, that there were dramatic regional differences in the rate of recovery after the end-Permian mass extinction event and so our current view of the post-Permian recovery is far too simplistic.
Fault segment rupture, aftershock-zone fluid flow, and mineralization Steven Micklethwaite and Stephen F. Cox, Australian National University, Research School of Earth Science, Canberra, ACT 0200, Australia. Pages 813-816.
Success in discovery of viable gold deposits is a bottom line in the mineral industry and has driven our need to understand how faults and fluids interact. It just so happens that many gold deposits are formed in structures such as faults and quartz veins, at a point in time when these geological beasts were hosting earthquakes. Even more telling is the observation that the majority of gold deposits cluster together on small structures around much larger ones, just like aftershocks around large earthquakes. In applying our understanding of how earthquakes operate, perhaps we can begin to make predictions about mineral deposits. Stress transfer modeling (STM) is one technique that has enjoyed success in predicting aftershock distribution. It is now enjoying equal success in predicting the location of gold deposits around ancient fault systems; a result with important implications for our understanding of faulting and fluid flow as well as the exploration for future blind deposits. For the mineral industry it seems there is good news when the future looks shocking.
Rapid transport pathways for geothermal fluids in an active Great Basin fault zone Jerry P. Fairley and Jennifer J. Hinds, University of Idaho, Geological Sciences, Moscow, Idaho 83844-3022, USA. Pages 825-828.
The way in which faults control groundwater movement is important in many areas of the environmental and earth sciences. For example, during the 1990s, investigators working on the U.S. Department of Energy's Yucca Mountain Project found evidence that groundwater may have moved along faults from the land surface to the level of the proposed nuclear waste repository in less than about 50 years. In addition to its implications for safe disposal of nuclear waste, understanding these so called fast flow paths has applications in petroleum recovery, earthquake prediction, and the study of life in the deep, hot biosphere. To examine the distribution of fast flow paths, researchers from the University of Idaho took more than 700 temperature measurements in a fault controlled geothermal system in southeast Oregon. Analysis of the data showed that less than 0.01% of groundwater moved through the fastest pathways in the fault, and only about 2% of the total flow in the fault could be classified as fast. Furthermore, the investigation revealed that fast flow takes place through a limited number of discrete, isolated pathways that are widely separated by areas of lower permeability. Although the existence of fast flow paths has been hypothesized for more than 10 years, the University of Idaho study is the first to determine their prevalence and contribution to the total flow of groundwater in an active fault zone. In addition, the statistical models developed by the researchers may provide an important tool for environmental and earth science investigations.
d18O and Marion Plateau backstripping: Combining two approaches to constrain late middle Miocene eustatic amplitude Cédric M. John, University of California, Department of Earth Sciences, Santa Cruz, CA 95064, USA, et al. Pages 829-832.
One of the big challenges in geosciences is to reconstruct past sea level variations. The amplitude of truly global sea level changes, called eustatic changes, is important as it determines which portion of the continental shelf is being flooded, and also because one of the main mechanism behind these fluctuations is the waxing and waning of ice caps. Both of these parameters have a profound influence on climate. The present paper used two different techniques in parallel to pin down the exact amplitude of the sea level lowering that took place between 13.6 and 11.5 millions of years ago, a time when the Antarctic continent became heavily glaciated. One of the approaches used is based on the geometry of deposition on an eastern Australian shallow-marine Plateau before and after the sea-level fall, whereas the other one relies on the effect that high-latitude ice volume formation has on the ratio of oxygen atoms of different mass. The results obtained (55±5.0 m of sea level change) imply that the formation of ice on Antarctica during that period was larger than previously thought (at least 73% of the East Antarctic ice sheet).
GSA TODAY Science Article
Wrightwood and the earthquake cycle: What a long recurrence record tells us about how faults work Ray Weldon, University of Oregon, Department of Geological Sciences, Eugene, OR 97405-1272, USA, et al.
A new study reports that the interval between great earthquakes on the San Andreas fault depends surprisingly little on the size of the last earthquake. Ideas like "the Big One is overdue" or "the longer it waits, the bigger the earthquake will be" appear to be much too simple. Ray Weldon and his colleagues analyze an exceptionally long record of past earthquakes on the fault at a site near Los Angeles, and find that earthquakes over the past 6000 years have clustered irregularly in time and in size. By studying the walls of many trenches dug across broken sediments at the site, the scientists were able to piece together the amount of ground offset and timing of at least 30 ground-rupture events. The team's analysis calls into question a popular model of the earthquake cycle, in which the interval of time for strain to accumulate between earthquakes portends the size of the next earthquake, and the size of an earthquake predicts the time to the next one. Instead, the reported patterns suggest that cycles of strain accumulation along the fault may span several earthquakes.
To view the complete table of contents for the August issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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