July media highlights: GEOLOGY and GSA TODAY

June 18, 2001

BOULDER, Co. - Following are highlights from the July issue of GEOLOGY and a summary of the science article from the July issue of GSA TODAY, published by the Geological Society of America. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in stories published. Contact Ann Cairns at GSA for copies of articles and for additional information or assistance.


Upper Cenozoic chronostratigraphy of the southwestern Amazon Basin. Kenneth E. Campbell, Jr., Natural History Museum of Los Angeles County, Los Angeles, CA, USA; et al. Pages 595-598 The biological diversity of the Amazon Basin of South America has been poorly understood due to a lack of information about the geologic history of the Amazon Basin, and in particular, the lack of accurate dates or ages for the rocks of lowland Amazonia. This paper provides the first two accurate dates, based on samples of volcanic ashes taken from lowland strata, of the most recent sedimentary deposits of Amazonia. These ash ages allowed the authors to date fossil deposits within southwestern Amazonia and confirm the presence of a mammal derived from North American ancestors in South America more than 9 Ma. This mammal--an extinct, elephant-like gomphothere--thus becomes the first known participant in the late Cenozoic faunal exchange between the two continents. From the younger ash age the authors were able to postulate that deposition within southwestern Amazonia continued until ~2.5 Ma, when the modern aspects of the region began to develop.

Sedimentary evidence of intense hurricane strikes from New Jersey. Jeffrey P. Donnelly, Brown University, Providence, RI, USA (Current Address: Woods Hole Oceanographic Institution, Woods Hole, MA, USA); et al. Pages 615-618 Intense, landfalling hurricanes, such as Hurricane Andrew in 1992, often result in substantial loss of life and resources. Several hurricanes (1938, 1944, Belle-1976, and Gloria-1985) brushed the New Jersey coast in the twentieth century, but an intense hurricane has not made landfall there since September 3, 1821. Since that time the New Jersey coast has experienced unprecedented human development. A concentration of resources and population in areas at risk of intense hurricane strikes, and potential links between human-induced climate change and the frequency and intensity of tropical cyclones, necessitate examination of long-term records of hurricane activity. Analysis of storm deposits recovered from Strathmere, New Jersey, provides a 700-year record of historic and prehistoric intense storms striking the New Jersey coast. Intense hurricanes typically result in storm surge of more than 10 feet above normal tides. Given the extremely low elevation of many of the heavily developed barrier islands on the New Jersey coast, the next intense hurricane strike will likely result in extreme damage to property. Due to a lack of public awareness, insufficient time to warn the population, and inadequate evacuation routes, significant loss of life may occur as well.

Variation in Holocene El Niño frequencies: Climate records and cultural consequences in ancient Peru. Daniel H. Sandweiss, University of Maine, Orono, ME, USA; et al. Pages 603-606 This article identifies prehistoric changes in the frequency of El Niño from the distribution of mollusk shells in archaeological sites. Between ca. 5.8 and 3.0 ka, El Niño apparently occurred less frequently than it does today. At that time, people along the coast of Peru began living in settled communities and building increasingly large temple mounds with elaborate public art. About 3 ka, El Niño started to recur much more frequently. This change in climate took place at the same time as the construction of temples ceased after nearly three millennia, suggesting that the increase in El Niño frequency may have played some role in the collapse of the religious system expressed in the temples.

Accelerated Pleistocene coral extinctions in the Caribbean Basin shown by uranium-lead (U-Pb) dating. Stephen R. Getty, University of New Mexico, Albuquerque, NM, USA; et al. Pages 639-642 These authors applied new and sophisticated dating mechanisms for corals in the Caribbean. What they learned was that a coral extinction occurred at 1.0 - 0.9 million years ago, much more recently than had been previously thought. The date of the extinction, however, now nicely coincides with global changes associated with glacial-interglacial cycles and sea level. These factors may have provided strong environmental mechanisms for rapid habitat modification and coral extinction. They suggest what might be expected in reef environments, as sea level and temperature fluctuate on short timescales, such as may be occurring as a result of human activity.

Archeological implications of the geology and chronology of the Soa basin, Flores, Indonesia. Paul B. O'Sullivan, Syracuse University, Syracuse, NY, USA; et al. Pages 607-610 This paper addresses the importance of understanding the geological history of archaeological sites prior to sampling the site for dating purposes. The authors contend that the geologic history of archeological sites, in this case those associated with early hominids, must be thoroughly understood before deposits containing artifacts can be properly dated, and artifacts recovered from these deposits cannot be placed into their proper chronostratigraphic or archaeological context until proper dating has been completed. Following a detailed geologic investigation, the authors completed new radiometric dates from early hominid artifact localities within the Soa Basin on the island of Flores, in eastern Indonesia. These dates indicate that Homo erectus had successfully navigated across the major biogeographical boundary known as Wallace's Line and colonized Flores by ca. 840 ka.

Hot, shallow mantle melting under the Cascades volcanic arc. Linda T. Elkins Tanton, Massachusetts Institute of Technology, Cambridge, MA, USA; et al. Pages 631-634 Melting occurs at progressively greater depths and higher temperatures from west to east across the Cascades volcanic arc in northern California. Young lavas from seven vents defining a 75-km-long, east-west transect across the Cascades volcanic arc in Northern California were analyzed to determine their temperatures and depths of melting. The increase in melting depth across the arc parallels modeled surfaces of constant temperature in the mantle under the arc, but does not parallel the inferred dip of the subducting oceanic slab beneath the arc. The depth of mantle melting at which the lavas were created is ~36 km at the western end of the transect and 66 km at the eastern end at temperatures from 1300 to 1450 °C. The very high temperatures of dry melting so close to the crust indicate a transitory condition of the mantle.

Hypersulfidic deep biosphere indicates extreme sulfur isotope fractionation during single-step microbial sulfate reduction. Ulrich G. Wortmann, GEOMAR Research Center for Marine Geosciences, Kiel, Germany; et al. Pages 647-650 Bacteria living in oxygen-free environments use organic matter and sulfate as food sources. Depending on their metabolism, this process can alter the sulfur-isotopic composition of the sulfate reservoir the bacteria live on, and the reduced sulfur they produce as a by-product. It has been the belief that bacteria are not able to cause more than 46 per mil difference between source and product, although a 70 per mil difference is usually observed. The authors describe for the first time direct evidence of bacteria that fractionate ~65 per mil. This finding may have important consequences for the understanding of certain ore-deposits and the early evolution of life.

Mountain erosion over 10-year, 10,000-year, and 10,000,000-year times scales. James W. Kirchner, University of California, Berkeley, CA, USA; et al. Pages 591-594 Silt from forest roads and logging operations can clog mountain streams, making it difficult for fish to spawn. Because mountain erosion controls sediment supply to streams, understanding erosion processes is crucial for assessing the erosional consequences of land use. The authors compared erosion rates in the mountains of central Idaho over three different time scales: the past few decades, the past 5-25 k.y., and the past 20-50 m.y. Their results show that erosion rates measured over the past few decades are, on average, roughly 20 times lower than the average erosion rates over thousands or millions of years. This is because erosion is extremely episodic, with over 90 percent of the long-term sediment yield occurring during extreme events (such as rainstorms following catastrophic wildfires) that are very large but very rare. The authors' results suggest that activities like a logging and road-building may affect mountain sediment yields in two different ways: by inc! re! asing the rate of incremental, day-to-day erosion and by altering the risk and magnitude of catastrophic erosion events, which are extremely rare, but make up most of the long-term average.


Experimental Stratigraphy. Chris Paola, University of Minnesota, Twin Cities, Minneapolis, MN, USA; et al. This paper reports on efforts to experimentally produce stratigraphic sequences in a large tank in which base-level change, sediment supply, and (basin) subsidence can be precisely controlled. Heretofore, stratigraphers have had to work backwards from sedimentary sequences preserved in the rock record in order to decipher the effects of diverse controlling factors--a difficult task, the results of which are not testable. With the new experimental apparatus, the effects of changes in single factors (e.g., subsidence rate, rate of sediment supply) on the resulting sedimentary sequence can isolated and documented. This allows testing of hypotheses regarding the response of a sedimentary sequence to different conditions; the test of one such hypothesis is described in the paper. Given the huge effort that geologists put into interpreting sedimentary sequences, there will be great interest in this experimental approach and in this GSA Today science paper.
*To view abstracts and the complete table of contents of GEOLOGY, as well as that of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, see http://www.gsajournals.org. To obtain full text of these articles and articles from back issues, contact Ann Cairns, acairns@geosociety.org.

Geological Society of America

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