June 2008 Geology and GSA Today media highlights

Boulder, CO, USA - GEOLOGY topics include Samoa on the hotspot trail, South Carolina's offshore iceberg scours; Yellowstone's climate-induced geyser periodicity; coralline red algae as a high-resolution climate recorder; the effects of extreme storm events on landscape and carbon dioxide; the iron isotope record and the first emergence of atmospheric and oceanic oxygen; and eastern California's shear zone earthquakes. GSA Today's science article discusses the Canadian Shield, Earth's oldest continental crust, where rocks may have originated under primordial seas.

Highlights are provided below. Representatives of the media may obtain complementary copies of articles by contacting Christa Stratton at cstratton@geosociety.org. 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 Christa Stratton for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Anthony A.P. Koppers et al., College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Administration Building, Corvallis, Oregon 97331-5503, USA. Pages 435-438.

Hotspots are focused regions of anomalous intra-plate volcanism, yet the mechanisms driving this type of volcanism are subject to vigorous debate. One model assumes that deep-seated mantle plumes drive hotspot volcanism and generate linear age progressions in the hotspot trails they leave behind. These are the so-called "primary" hotspot trails, of which only three have been identified in the Pacific basin. However, the absence of clear age progressions along other hotspot trails has been used as an argument against the role of mantle plumes in seamount trails, favoring models of lithospheric extension and the release of melt from shallow mantle sources or plumelets. In the case of Samoa, the omnipresence of subaerial volcanic rocks younger than 0.39 million years old on Savai'i Island appears to argue against the mantle plume model, because a 7.1 cm/yr Pacific plate motion would predict a hotspot age of 5.1 million years for this volcano. Koppers et al. present new 40Argon/39Argon data on volcanic samples from the deep flanks of Savai'i Island that confirm the predicted 7.1 cm/yr age progression for the Samoan hotspot. Three different volcanic samples from the deepest portion of the southwest flank of Savai'i Island give ages ranging from 4.99 to 5.21 million years. These results demonstrate that the onset of shield-building on Savai’i occurred much earlier than the oldest 0.39 million-year-old volcanics sampled subaerially on the island. Strontium-neodymium-lead isotopes and trace-element data show that the dredge samples have a typical Samoan shield pedigree and are distinct from the subaerial rejuvenated volcanics on Savai'i and Upolu. Together, this reinstates Samoa as a hotspot trail with similar characteristics as the "primary" Hawaiian and Louisville hotspot trails. It eliminates the major argument against a plume origin for Samoa.

Robert Moucha et al., CP 8888, Succursale Centre-Ville, Montréal, Québec H3C 3P8, Canada. Pages 439-442.

There is a longstanding debate among geologists about the origin of the Colorado Plateau's unusually high elevation. Using new high-resolution seismic images of Earth's interior structure, Moucha et al. demonstrate through numerical modeling that the Colorado Plateau is overlying a hot mantle plume that drives some of this unusually high elevation. This new view of the deep dynamics and structure below the southwestern United States also has important implications for some of the geologically recent volcanic activity in New Mexico, as well as for the rifting dynamics of the nearby Rio Grande Rift. Understanding the rifting dynamics in this area is important for evaluating the seismic hazard that the Rio Grande Rift may pose.

Jenna C. Hill et al., Center for Marine and Wetland Studies, Coastal Carolina University, P.O. Box 261954, Conway, South Carolina 29528-6054, USA. Pages 447-450.

Hill et al. present high-resolution swath bathymetry data collected offshore of South Carolina as part of the National Oceanic and Atmospheric Administration's Ocean Explorations program in 2006 and 2007 to show evidence of extensive iceberg scouring along the upper slope of the continental margin (170–220 m water depth). The seafloor morphology in this region is characterized by numerous west-southwest–oriented grooves. These features are flanked by piles of sediment and boulders along each side, suggestive of iceberg keels plowing into the seafloor. The grooves often terminate in circular pits ringed by ridges several meters high, where icebergs came to rest on the seafloor. The location and orientation of the keel marks suggest icebergs were entrained in a southwestward-flowing coastal current. Currently, the warm waters of the rapid, northeastward-flowing Gulf Stream bathe the upper slope across the region. An offshore shift in the Gulf Stream axis during a period of lowered sea level may have allowed glacially fed coastal currents to penetrate farther south, transporting icebergs to this portion of the margin. This is the first evidence of iceberg transport this far south along the U.S. Atlantic margin.

Shaul Hurwitz et al., U.S. Geological Survey, MS 439, 345 Middlefield Road, Menlo Park, California 94025, USA. Pages 451-454.

By statistically analyzing the variations in intervals between geyser eruptions, Hurwitz et al. demonstrate that several geysers in Yellowstone's Upper Geyser Basin respond to long-term precipitation trends and to the seasonal hydrologic cycle.

J. Halfar et al., Dept. of Chemical and Physical Sciences, University of Toronto at Mississauga, 3359 Mississauga Rd. N, South Building, Room 3006, Mississauga, Ontario L5L 1C6, Canada. Pages 463-466.

Assessing human impact on climate and ecosystems and predicting future climate evolution require knowledge of climates from past centuries. While we have achieved a good understanding of century-scale climate change in mid- and high-latitude terrestrial areas (mainly through the analysis of tree-ring records), such data are incomplete in many extratropical marine regions, due to a lack of appropriate climate archives. Halfar et al. have investigated the potential of long-lived marine algae as recorders of past climates. Coralline red algae, which live in many coastal regions worldwide, produce a skeleton made up of calcium carbonate. Individual calcified plants exhibit annual growth bands, very similar to tree rings, and can live continuously for several centuries in temperate and subarctic oceans. By monitoring coralline red algae of the genus Clathromorphum for a one-year period in the gulf of Maine, United States, Halfar et al. were subsequently able to relate geochemical information contained in the calcified algal growth bands to locally measured ocean temperatures, thus highlighting the suitability of coralline red algae as extratropical climate archives. By applying the results of the monitoring experiment to a 30-year coralline-algal oxygen isotope record, they demonstrated that the algae archived past water temperatures. In addition, the specimen contained a record of past variations of the Labrador ocean current and related climate oscillations in the northwestern Atlantic—a key region in Earth's oceans where poorly understood climate and oceanographic changes have recently had a dramatic impact on ecosystems and fishery yields.

Alison M. Anders et al., Dept. of Geology, University of Illinois at Urbana-Champaign, NHB 245, 1301 W. Green St., Urbana, Illinois 61801, USA. Pages 479-482.

Mountainous topography influences the spatial distribution of rain and snowfall. Precipitation, in turn, shapes mountain ranges through erosion. Using a coupled numerical model, Anders et al. explore the interrelationship between precipitation and mountain evolution. This model suggests profound differences between rain- and snow-dominated mountain ranges. If most precipitation falls as rain, modeled mountain ranges tend to be steep and narrow with deeply set river valleys in the center of the range and low-relief range shoulders. In contrast, if most precipitation falls as snow, modeled mountain ranges are broader, less high and steep, and the relief between river valleys and ridges is uniform from foothills through the center of the range. These results suggest that complex relationships between climate and topography may be present in the natural system, and indicate an increased efficiency of erosion in snow-dominated climates.

Steven T. Goldsmith et al., The Ohio State University, Dept. of Earth Sciences, 267 Mendenhall Laboratory, Columbus, Ohio 43210, USA. Pages 483-486.

Goldsmith et al. completed one of the first known semicontinuous monitoring studies of large-scale particulate organic carbon (POC) delivery coupled with hyperpycnal suspended sediment concentrations during a typhoon event. This linkage provides evidence for the removal and potential rapid burial of significant quantities of organic carbon from terrestrial systems. These POC fluxes, when combined with storm-derived carbon dioxide consumption from silicate weathering, elucidate the important role these short-term aperiodic storm events on small mountainous rivers have on modifying Earth's climate over geologic time. As recent studies suggest, cyclone intensity and monsoonal frequency may be increasing owing to global warming; even larger POC fluxes, greater silicate weathering-driven carbon dioxide consumption, and more rapid organic carbon burial may occur in Taiwan and similar locales in the future. Goldsmith et al.'s study contributes to the understanding of delivery fluxes from small mountainous rivers and how these fluxes may change with future climate change. This work was completed under funding from the U.S. National Science Foundations' East Asia and Pacific Summer Institute and GEO-EAR Hydrologic Sciences programs.

Silke Severmann et al., Dept. of Earth Sciences, 2258 Geology Building, University of California, Riverside, California 92521, USA. Pages 487-490.

Iron shuttling from shallow shelf sediments to deep basin sediments is a process that is characteristic of marine environments where oxygen is absent in the water column, such as the modern Black Sea. Severmann et al. show that this iron shuttle has a distinct iron isotope signature, which is preserved in the sediments, and which can be used to identify similar environments in the rock record. Although such environments are rather rare in the modern oxic ocean, they were much more widespread in the earlier parts of Earth's history. Severmann et al.'s isotope data from the Black Sea can therefore guide our interpretation of the geological record and help us to reconstruct the chemical composition of ancient oceans. Because the cycling of iron is very sensitive to the presence of oxygen, the iron isotope record may tell us when oxygen first appeared in the surface ocean and in the atmosphere.

Takeshi Nakagawa et al., Dept. of Geography, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK; takeshi.nakagawa@newcastle.ac.uk. Pages 491-494.

Were the past glacial ages wetter or dryer" Though it is often asked, this question does not actually make much sense because temperature and precipitation do not oscillate with the same rhythm. Glacial ages (periods of globally low temperature and a larger volume of polar ice) typically recur at 100,000 year intervals. By contrast, precipitation at several places on Earth is oscillating at 23,000 year cycles. However, the reason two different climatic parameters (temperature and rain fall) show different rhythms is not well understood. Nakagawa et al. reconstructed climate changes of the past 450,000 years using fossil pollen grains from a Japanese lake sediment. The results clearly showed that the temperatures of both continental and oceanic air masses fluctuate at 100,000 year cycles, whereas the land-ocean temperature gradient and summer rainfall oscillates at 23,000 year cycles. The land-ocean temperature gradient is the direct cause of the East Asian monsoon, which in turn is responsible for the Japanese summer rain fall. The 23,000 year climatic cycle coincides with the strongest periodicity of changes in the brightness of the sun. Based on these findings, Nakagawa et al. propose that the direct solar beam is regulating the land-ocean temperature gradient (and hence monsoon intensity and rainfall) through different heat capacities between land and oceans, and this process is essentially independent from glacial-interglacial cycles.

S. Dyksterhuis, University of Sydney, Institute of Marine Science, School of Geosciences, Edgeworth David Building F05, NSW 2006, Australia; and R.D. Müller, School of Geosciences, Madsen Building H9, University of Sydney, NSW 2006, Australia. Pages 495-498.

Dyksterhuis and Müller demonstrate how the complex interplay between juxtaposed weak and strong geological plate elements and changes in far-field plate boundary forces can cause intraplate orogeny. The time dependence of the intraplate stress regime of the Australian continent provides a cause for an enigmatic intra-plate orogeny in southeastern Australia dubbed the "Sprigg's Orogeny." In agreement with geological observations, modeled stress directions, which rotate around geologically weaker regions and towards stronger plate elements, show ideal orientations to cause uplift along preexisting faults during two distinct stages during the past 55 million years. Dyksterhuis et al.'s work demonstrates how mountain building can occur far away from plate boundaries and has implications for the evaluation of the suitability and longevity of hazardous waste storage locations and resource exploration.

Elevated shear zone loading rate during an earthquake cluster in eastern California

M. Oskin et al., Dept. of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27516, USA. Pages 507-510.

Oskin et al. studied six active faults in the eastern California shear zone and found that the sum of fault slip rates is at most 6.2 ± 1.9 mm per year, which is only half the present-day displacement rate of 12 ± 2 mm per year as measured from global positioning system (GPS) and triangulation surveys. Oskin et al.'s study indicates that displacement rate, and thus seismic hazard, has varied over time in the eastern California shear zone. The dates of past earthquakes on these faults, determined from trenching studies, are consistent with the slower long-term displacement rate. These previously published ages of prehistoric earthquakes also show that earthquake activity occurs in clusters, with increased activity over periods that last approximately 1,000 years. The latest cluster of activity, which includes the 1992 Landers and 1999 Hector Mine earthquakes, occurred while the rate of far-field displacement was double the average rate of fault slip. Far-field displacement rates, usually assumed to be constant, are used to determine the rate of stress increase (loading) on faults and estimate the chance of an earthquake occurring. Because a higher rate of loading has been shown to directly increase the likelihood of an earthquake, the elevated present-day displacement rate determined by this study implies that seismic hazard is currently high. Likewise, an area where the present-day displacement rate is diminished could have a decreased seismic hazard.

Canada's Craton: A bottom's-up view

Dante Canil, School Earth & Ocean Sciences, University of Victoria, PO Box 3055 STN CSC, Victoria, British Columbia V8W 3P6, Canada. Pages 4-11.

How did the continents first form, and how have they evolved" For answers to these fundamental earth science questions, we have to look to the ancient cratons that lie at the heart of many of today’s continents. The stable heart of the North American continent is the Canadian Shield, an ancient craton that includes the oldest continental crust on Earth. The Canadian Shield is rich with mineral deposits, including diamonds, and is underpinned by a 200-km-thick keel of cold, strong mantle. In the June GSA Today, Dante Canil of the University of Victoria-British Columbia presents an extensive compilation of geological and geophysical data that shows that this mantle keel consists of rocks that originally lay at the base of the primordial seas that covered the early Earth. Even more surprising is his assertion that this durable keel, which is largely responsible for the longevity and the stability of the continent, didn’t develop until 500 to 1000 million years after the formation of the craton. Much further research is required to determine how rocks that lay at the surface of Earth ended up being inserted beneath and adhering to an already ancient craton.

To review the abstracts for these articles, go to www.gsajournals.org. View the complete table of contents for the current issue of GEOLOGY at www.gsajournals.org/perlserv/?request=index-html&issn=0091-7613.

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