Following are highlights from the August issue of Geology and a summary of the science article from the August issue of GSA Today, published by the Geological Society of America. Stories written regarding these articles are embargoed until August 1. We ask that you discuss articles of interest with the authors before publishing stories on their work, and that reference be made to Geology in stories published. Contact Anika Burkard for additional information and assistance.
Geology
Decoupling of taxonomic and ecologic severity of Phanerozoic marine mass extinctions, by Mary L. Droser et al., p. 675
Mass extinctions have occurred throughout the history of life on Earth. We typically recognize mass extinctions by the number of species, genera, families, etc. that go extinct in a very short time interval. However, taxonomic data record only one aspect of a mass extinction. The ecological degradation associated with a mass extinction may not be the same as the taxonomic shifts. In this paper, the authors examine the ecological shifts associated with the Late Ordovician and Late Devonian mass extinctions, two of the 5 largest mass extinctions in the history of life, utilizing a system of paleoecological levels that allow the ranking of ecological change. While these extinctions show similar levels of extinction, about 21% of all marine families go extinct. The Late Ordovician mass extinction does not have the magnitude of ecological shifts associated with the Late Devonian mass extinction. This demonstrates that taxonomic numbers are not adequate to predict ecological change and that some species are more important than others in maintaining ecosystem structure. This has important implications for management of our modern biodiversity crises.
Global cooling accelerated by early late Eocene impacts?, by Hubert B. Vonhof et al., p. 687
Oxygen isotope analyses of fossil marine organisms from a seafloor sediment core near Antarctica suggest that distinct climate cooling occurred 35.5 Ma ago as a result of two large extraterrestrial impacts on Earth. A comparison with diversity patterns of contemporaneous dinoflagellate algae from Italy suggests that this cooling occurred on a global scale. Unlike the Cretaceous-Tertiary boundary impact, the 35.5 Ma impacts did not cause significant extinctions. Nevertheless, distinct marine biodiversity changes, probably related to climate cooling are observed.
Carbon isotopic composition of individual Precambrian microfossils, by Christopher H. House et al., p. 707
Using an ion microprobe, the authors studied biochemical attributes of thirty 1 - 2 billion year old bacterial fossils. Because different enzymatic reactions alter carbon isotope ratios to different degrees, the carbon isotopic composition of cell mass is related to the carbon fixation pathway operating in an organism. The 850 million-year-old and 2 billion-year-old microfossils studied were petrified in samples of chert, a granular microcrystalline quartz. These cellular fossils were composed of organic carbon from the original living cells, preserving the isotopic signature produced by the carbon fixation enzymes operating when the organisms were alive. The results of the study were consistent with the assignment of these particular fossil specimens to the cyanobacteria. This research is part of an ongoing effort to understand the biosphere billions of years ago and understand the origin of life on Earth.
Oily old ores: Evidence for hydrothermal petroleum generation in an Archean volcanogenic massive sulfide deposit, by Birger Rasmussen and Roger Buick, p. 731
The world's oldest oil and bitumen are preserved in a 3.25 billion-year-old sulfide deposit of volcanic origin in the Pilbara region of northwestern Australia. The oil is trapped in microscopic fluid pockets in iron, copper, and zinc sulfides that precipitated from the hydrothermal fluids associated with volcanism. Because the ore deposit is so well preserved, it is possible that it may still contain organic molecules derived from the ancient source microbes. If so, these may provide information on the type of aquatic and subsurface life that once thrived very early in Earth's history.
Active displacement partitioning and arc-parallel extension of the Aleutian volcanic arc based on Global Positioning System geodesy and kinematic analysis, by Hans G. Avé Lallemant and John S. Oldow, p. 739 An integrated study of faults and fractures combined with geodetic results employing the Global Positioning System indicates a complex history of crustal deformation in the Aleutian Islands of Alaska. This volcanic island chain marks the boundary between the northwestern part of the North American plate and the subducted section of the Pacific plate, and is the locus of some of the greatest earthquakes in the world. In addition to crustal shortening oriented at a high angle to the tectonic boundary, the convergence between these two plates is causing a substantial westerly migration of the Aleutian island chain. The migration is not constant, and the islands to the east are moving at about 3 mm/year, whereas to the west they are moving at 31 mm/year. The difference in rate indicates that the island chain is being stretched along its axis.
Thermal monitoring of North Pacific volcanoes from space, Jonathan Dehn et al., p. 755
Thermal infrared satellite imagery is used to monitor over 100 remote yet dangerous volcanoes in the North Pacific, extending from Alaska to the Kamchatkan Peninsula. The data are collected multiple times per day from weather satellites and show distinct signatures for different types of volcanic activity. By establishing long-term thermal records for each of these volcanoes, small yet significant changes in temperature can be detected. This tool can be used to search for precursors to explosive eruptions, in turn refining our ability to predict potentially catastrophic events. Automated techniques are being developed to monitor these changes and to provide alerts to analysts.
Fine-resolution pollen record of late-glacial climate reversal from New Zealand, by Rewi M. Newnham and David J. Lowe, p. 759 Do rapid shifts in climate that occur in the Northern Hemisphere register in the same way in the Southern Hemisphere? The answer to this question is crucial to understanding how the global climate system operates during periods of major change. This study presents a detailed pollen record from New Zealand that shows clear evidence of abrupt changes in climate at the end of the last ice age that are similar in structure to those in the Northern Hemisphere. The record also suggests that global climatic events at this time may have registered earlier in the Southern Hemisphere.
Impact Events and Their Effect on the Origin, Evolution, and Distribution of Life, by David A. Kring, p. 1-7.
Impact cratering has affected the geologic and biologic evolution of Earth, from the earliest stages of accretion to the present. The environmental consequences of impact cratering and their biologic repercussions are illustrated by the Chicxulub impact event and its link to the Cretaceous-Tertiary mass extinction event. While smaller impact events are more common, there were probably four to five additional impact events of this size during the Phanerozoic. These types of large impact events, and even larger ones, occurred more frequently earlier in Earth history. A particularly intense period of bombardment appears to have occurred ~3.8-3.9 Ga, corresponding to the earliest isotopic traces of life on Earth. These impact events may have made it difficult for preexisting life to survive or may have provided the necessary environmental crucibles for prebiotic chemistry and its evolution into life. Some effects of impact bombardment were shock wave, air blast, heat, tsunamis, and earthquakes (regional); ejecta in the whole atmosphere of Earth making it too dark to see for one to six months and too dark for photosynthesis for two months to one year (global); increased concentrations of poisonous gases (SO2, SO3, CO2, and ozone-depleting Cl and Br) - with enhanced stratospheric S 10^5 - 10^6 times relative modern abundances (global).
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Full text Geology articles and the science article from GSA Today are available on the first of each month on the GSA Web site, http://www.geosociety.org/pubs/journals.htm.