||29 June 2007
GSA Release No. 07-23
Boulder, CO, USA - Topics include: new insights into dynamics of seafloor spreading; erosion of Alaska's Arctic coast; a challenge to hypothesized glaciation in the mid-Cretaceous; evidence of vegetation causing erosion rather than preventing it; a case for a Hadeon ocean; and dating of Earth's earliest and largest global carbon cycle imbalance. The GSA TODAY science article addresses deep-time mountain building in Tibet.
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- Rapid dike emplacement leads to eruptions and hydrothermal plume release during seafloor spreading events
R.P. Dziak, Oregon State University/NOAA. CIMRS. Hatfield Marine Science Center, Newport, Oregon 9736, USA; et al. Pages 579-582.
- One of the most important discoveries in the study of the global mid-ocean ridge system was the first real-time detection of a deep-ocean volcanic eruption and the associated release of huge volumes of anomalously warm water (called hydrothermal plumes) into the ocean. Real-time detection of this event allowed for the rapid assembly of a multidisciplinary oceanographic research team to investigate, in situ, the many biological, chemical, and hydrothermal after-effects on the attendant seafloor and water-column ecosystems. The key to rapid on-site investigations is the accurate evaluation of the remotely recorded earthquake data indicating that a large-scale seafloor eruption is in progress. Nevertheless, after nearly two decades and the real-time detection of seven mid-ocean ridge seafloor spreading events, evaluation of the likelihood of a seafloor eruption based on remote seismicity has had a success rate of only ~60%. Moreover, verification has required the application of substantial resources to muster oceanographic vessels and personnel to investigate the eruption sites. Through analysis of earthquake records, Dziak et al. have found that rapid, high-velocity injection of magma during seafloor spreading events typically leads to eruption of lavas onto the ocean floor and the release of massive hydrothermal plumes. Thus, these findings offer a new method for predicting remotely the likelihood of a seafloor eruption and hydrothermal plume release during future deep-sea earthquake swarms.
- Quantitative remote sensing study indicates doubling of coastal erosion rate in past 50 yr along a segment of the Arctic coast of Alaska
J.C. Mars and D.W. Houseknecht, U.S. Geological Survey, EMRT, Reston, VA 20151, USA. Pages 583-586.
- Rates of coastal erosion along the Beaufort Sea coast north of Teshekpuk Lake, in the national Petroleum Reserve in Alaska, have been documented by image analysis of topographic maps and Landsat Thematic Mapper Data that span 1955-2005. Some areas have undergone up to 0.9 km of coastal erosion in the last 50 years. Land loss attributed to coastal erosion more than doubled ,from 0.48 km2 per year from 1955-1985 to 1.08 km2 per year from 1985-2005. Coastal erosion also has breached thermokarst lakes, causing initial draining of the lakes followed by marine flooding. Image analysis also documents thermokarst lake expansion and drainage into adjacent lakes and watersheds.
- Depleted mantle wedge and sediment fingerprint in unusual basalts from the Manihiki Plateau, central Pacific Ocean
Stephanie Ingle, University of Hawaii, School of Ocean and Earth Science and Technology, 1680 East-West Road POST 606, Honolulu, HI 96822, USA; et al. Pages 595-598.
- The Early Cretaceous, ~145-100 million years ago, is notable as a time when Earth's mantle dynamics may have operated differently than at other times in Earth's history. The evidence for this includes numerous large igneous provinces (LIPs), thought to form when hot jets of material ascend from deep within Earth to produce abundant, but geologically short-lived, volcanism. Several of these LIPs in the Pacific Ocean, the Hikurangi, Manihiki, and Ontong Java plateaus, may have formed as a single, immense feature, implying an event that would have paved over as much as 1% of Earth's surface with basaltic lava around 120 million years ago. New Argon-dating of basalts from the Manihiki Plateau support the proposed relationship among the three plateaus, despite their current separation by thousands of kilometers. The chemistry of these basalts is, however, unusual and has not previously been observed in other oceanic rocks, including those from the Ontong Java and Hikurangi plateaus. Ingel et al. present a model to explain the unusual basalts and the relationship between the three plateaus. The implication is that these plateaus may indeed have formed as one, but the mantle that melted to produce them was either chemically heterogeneous on a very large scale, or the previously sampled and studied rocks from each plateau do not adequately reflect the variety of rocks present.
- Development of en echelon magmatic segments along oblique spreading ridges
J.W. van Wijk, Los Alamos National Laboratory, Earth and Environmental Sciences Division, Los Alamos, NM 87545, USA; and D.K. Blackman, Scripps Institution of Oceanography, La Jolla, CA 92093, USA. Pages 599-602.
- The general process of seafloor spreading at mid-ocean ridges has become widely known since the 1960s, but details of the process are still not fully understood. At mid-oceanic ridges, two tectonic plates move apart, while magmatic material wells up beneath the ridge. van Wijk and Blackman report the results of a study on oblique spreading mid-ocean ridges. Mid-ocean spreading is sometimes oblique (in contrast to orthogonal spreading); examples of oblique spreading mid-ocean ridges include the Reykjanes Ridge south of Iceland and the Mohns Ridge north of Iceland in the North Atlantic. At oblique spreading ridges, en echelon magmatic segments are observed in the mid-ocean ridge valley, and it is thought that plate spreading originates at these magmatic segments. van Wijk and Blackman used numerical models of lithosphere deformation and mantle upwelling to study the formation of these magmatic segments. The models show that segment formation (location, orientation, spacing, length) is controlled by plate kinematic processes, and results from the change in spreading direction of the tectonic plates by far field forces. The models furthermore suggest that the longevity of the magmatic segments can be explained by the shallow nature of the oblique spreading process. These results are important for our understanding of seafloor spreading. One of the questions debated today is whether magmatic upwelling beneath mid-ocean ridges causes spreading of the plates, or whether magmatic upwelling is the result of spreading of the plates. The results presented by van Wijk and Blackman support the latter; i.e., magmatic upwelling is the result of spreading.
- Cyclothem ["digital"] correlation and biostratigraphy across global Moscovian-Kasimovian-Gzhelian stage boundary interval (Middle-Upper Pennsylvanian) in North America and eastern Europe
Philip H. Heckel, University of Iowa, Department of Geoscience, Iowa City, IA 52242-1379, USA; et al. Pages 607-610.
- Marine rocks of Pennsylvanian age (~300-320 million years ago) have been difficult to correlate on a global scale because most fossils evolved slightly differently in the major basins where these rocks are well studied (central U.S., western Russia, and eastern Ukraine). However, repeated buildup and melting of large ice sheets on the southern continent (Gondwana) during that time caused worldwide sea-level drops that exposed large areas of the northern (at that time tropical) continents to erosion, followed by large-scale sea-level rises and highstand flooding, which produced deposits called cyclothems. These cyclothems were deposited at the same time everywhere in the world. During some highstands, a few species of microfossils called conodonts spread across two, and sometimes three, of these basins, which allows these particular cyclothems to be correlated with one another among the widely separated basins, providing a loose framework of age-correlation. The remaining cyclothems with different fossils can be 'digitally' fit by position in the succession into the loose framework, to correlate with other cyclothems that have different fossils but are in the same position, and therefore provide a tight global framework of age-correlation of strata over several million years of Earth history.
- Microstructures developed by coseismic and aseismic faulting in near-surface sediments, San Andreas fault, California
Susan M. Cashman, Humboldt State University, Department of Geology, Arcata, CA 95521, USA; et al. Pages 611-615.
- Movement on faults occurs both by sudden, earthquake-producing slip events (coseismic rupture) and by gradual displacement that does not generate damaging earthquakes (aseismic creep). Both coseismic and aseismic slip occur on the San Andreas fault and onother major faults in the densely populated San Francisco Bay area. Because seismic hazard analyses are based on evaluations of paleoearthquake history, differentiating between features produced by creep and those produced by earthquakes is crucial for evaluating seismic hazards in regions containing creeping faults. Cashman et al. addressed this problem by examining microstructures in near-surface sediments (materials that record deformation over relatively short time spans) at sites on two sections of the San Andreas fault: the creeping section, and the site of the 1906 earthquake rupture. Their results show that the packing, crushing, and orientation of grains in fault zone sediment at the two study sites differ, suggesting that sediment in active fault zones may preserve a record of coseismic slip.
- Testing for ice sheets during the mid-Cretaceous greenhouse using glassy foraminiferal calcite from the mid-Cenomanian tropics on Demerara Rise
Kazuyoshi Moriya, National Oceanography Centre, Southampton, School of Ocean & Earth Science, Southampton, N/A SO14 3ZH, UK; et al. Pages 615-618.
- The mid-Cretaceous (100-90 million years before present) is widely considered to have been the archetypal greenhouse interval in Earth's history, with much warmer climates than today. However, contemporaneous glaciations have been hypothesized based on some geological evidence. Moriya et al. present new palaeoclimate records for the mid-Cenomanian (96 million years before present) Demerara Rise (off Suriname) that reveal no evidence of glaciation, and call into question past interpretation of the data upon which glaciations were hypothesized.
- Synchronous millenial-scale climatic changes in the Great Basin and the North Atlantic during the last interglacial
Rhawn F. Denniston, Cornell College, Geology, Mount Vernon, Iowa 52314, USA; et al. pages 619-622.
- Analysis of glacial ice cores from Greenland demonstrate that the climate of the north Atlantic experienced several dramatic swings during the last glacial (~100,000-20,000 years ago). These climate oscillations have been observed in numerous areas around the globe, but their existence and impact in the western United States has remained unclear. Stalagmites from Nevada that grew between ~100,000 and 80,000 years ago record these shifts and demonstrate that the climate of the Great Basin responded synchronously with the north Atlantic, likely by changes in atmospheric circulation patterns and/or mean annual temperature.
- Oceanic core complexes and crustal accretion at slow-spreading ridges
B. Ildefonse, CNRS, Laboratoire de Tectonophysique, Université Montpellier 2, Montpellier, 34095 Cedex 05, France; et al. Pages 623-626.
- Ildefonse et al. propose a new model of oceanic crust development to explain periods of plate rifting that produce a shallow, domal seafloor massif on one side of a rift. The new model is based on ocean drilling results and seafloor mapping. New findings from Integrated Ocean Drilling Program expeditions 304 and 305 (www.iodp.org) to the Mid-Atlantic ridge were instrumental in the development of a revised model of ocean core complex formation, by which deep crustal rocks are exposed at the seafloor through detachment faulting. Ildefonse et al. summarize the findings of their drilling project and compare them to prior results from other oceanic core complexes. At every drilling site, only gabbroic rocks were cored, despite more variable lithology, including serpentinites, being sampled on the nearby seafloor. The observations suggest that processes associated with relatively enhanced periods of mafic intrusion, within typically magma-poor sections of slow and ultra-slow spreading ridges, favor development of oceanic core complexes.
- Resolving the timing of orogenesis in the Western Blue Ridge, southern Appalachians via in situ ID-TIMS monazite geochronology
Stacey L. Corrie and Matthew J. Kohn, University of South Carolina, Department of Geological Sciences, Columbia, SC 29208, USA. Pages 627-630.
- Despite extensive study, the age of metamorphic rocks in Great Smoky Mountains National Park (North Carolina and Tennessee), has remained controversial, with data supporting rock ages of both 400 and 450 million years old. Corrie et al.'s U-Pb data from monazite, a common mineral in metamorphic rocks, however, unequivocally indicate that the Great Smoky Mountains were formed during a mountain-building event called the Taconian orogeny, 450 million years ago.
- Vegetation causes channel erosion in a tidal landscape
S. Temmerman, University of Antwerp, Department of Biology, Antwerp 2610, Belgium; et al. Pages 631-634.
- Many landscapes on Earth are dissected by channels, such as rills and rivers, that are formed by the erosive forces of flowing water. Earth scientists traditionally believe that vegetation tempers the erosion of such channels. They believe that dense plant structures, such as stems and leaves, attenuate the flow velocities of water, and hence reduce the erosive forces exerted by flowing water. Temmerman et al.'s study revealed that this erosion-reducing effect of vegetation indeed occurs within static vegetation patches, but that dynamic vegetation patches, which may expand or shrink, have a contrasting effect, promoting the erosion of channels in between expanding vegetation patches. This was especially shown for a coastal landscape, where water flow is caused by tidal action, and where the establishment of vegetation on a bare mud flat led to the erosion of channels in between expanding vegetation patches. This study, therefore, demonstrates that interactions between plant growth, water flow, and landforms significantly impact the evolution of landscapes.
- Temperature spectra of zircon crystallization in plutonic rock
T. Mark Harrison, University of California-Los Angeles, Department of Earth & Space Sciences, Los Angeles, CA 90095-1567, USA; et al. pages 635-638.
- The first 500 million years of Earth's history (known as the Hadean Eon) has been difficult to study because there is no known rock of that age. Nonetheless, it has been widely assumed that there were no oceans or continents during that period. A new method of examining tiny grains of zircon from that period suggests that the Hadean Earth contained abundant water that fluxed continental crust to produce low-melting-temperature granites. Alternate explanations have been advanced suggesting these zircons were derived from igneous rocks initially at much higher temperatures. Harrison et al. use calculations, confirmed by observation, to show that such magmas yield zircon crystallization temperatures well above that observed in the Hadean grains. Their results therefore support the interpretation of a water-rich, pre-4-billion-year-old crust.
- Terrestrial impact of abrupt changes in the North Atlantic thermohaline circulation: Early Holocene, UK
Jim D. Marshall, University of Liverpool, Earth and Ocean Sciences, Liverpool, L69 3GP, UK; et al. Pages 639-642.
- Abrupt cooling events marked the period after the last glaciation on Earth. They were most likely triggered by general climate warming, and melting of ice sheets in North America. Marshall et al. use lake records from Hawes Water, in England, to quantify the impact of two prominent early Holocene climatic events during which the North Atlantic circulation ('Gulf stream') is thought to have slowed down. Very-high-resolution oxygen isotope records from sedimentary carbonate mud from the lake provide evidence for two abrupt cold events, respectively lasting about 50 and about 150 years, at around 9,300 and 8,200 years ago. Using fossil midges, Marshall et al. demonstrate that summer air temperatures dropped by about 1.6 °C from normal levels, during each event. The isotope results demonstrate that the cooling and freshening of northwest Atlantic waters (near Canada) was detectable in the composition of rainwater in northwest Europe where they caused significant, if relatively short-term, climate deterioration.
- Temporal constraints on the Palaeoproterozoic Lomagundi-Jatuli carbon isotopic event
Victor A. Melezhik, Geological Survey of Norway, Regional Mapping, Trondheim, N-7491, Norway; et al. Pages 655-658.
- At 2500-2000 Ma (million years ago), Earth went through a critical interval when the biosphere and geosphere were experiencing global-scale changes. The sequence of events includes the first global icehouse event, oxidation of the atmosphere and the emergence of an aerobic world, and global perturbations in the carbon cycle — all representing the greatest challenge to life on Earth since its beginning. Overall, the understanding of the timing, cause(s), and, specifically, the biological consequences with respect to the most profound change in surface environments during the early history of Earth is incomplete. The earliest, and the greatest, imbalance of the carbon cycle, named the Lomagundi-Jatuli Event, is one of those environmental upheavals that remains poorly understood, particularly in terms of its causes and timing. Melezhik et al.'s study of volcanic rocks from northern Scandinavia has yielded a more precise date of the event and provided a time constraint for the end of Earth's earliest and unprecedented perturbation of the global carbon cycle at 2058 million years ago, thus linking it to the break-up of the old, Late Archaean supercontinent, known as Kenorland.
- Ages for the Big Stone Moraine and the oldest beaches of glacial Lake Agassiz: Implications for deglaciation chronology
Kenneth Lepper, North Dakota State University, Geosciences, Fargo, ND 58105, USA; et al. Pages 667-670.
- Knowing the chronology of glacier retreat, and when glacial lakes formed, is the necessary first step in linking physical events on the paleolandscape with paleoclimate records. At the close of the last ice age, lobes of glacial ice in the upper Midwest of the United States retreated very quickly, likely in response to a warming climate. The retreating ice paused near the junction of Minnesota, North Dakota, and South Dakota long enough to form the Big Stone Moraine, which today is part of the subcontinental drainage divide between the Gulf of Mexico and Hudson Bay. After the ice sheet retreated from this moraine, the world's largest freshwater lake, glacial Lake Agassiz, began forming. Although this lake disappeared about 8200 years ago, its presence has been known for well over a century — its sediments having formed the rich farmland in the Red River Valley. Although the lake's routing of meltwater has been implicated in abrupt climate change events, Lepper et al. can now say with some degree of certainty that the lake started forming 13,950 years ago.
- The Gangdese retroarc thrust belt revealed
P. Kapp, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA; et al.
- The Himalayas and the high Tibetan Plateau are two of the most spectacular topographic features of Earth. Understanding their origins and evolution remains a major goal of plate tectonic investigations. While the collision of India with Asia, beginning about 50 million years ago, explains much of the geological evolution of the region, many questions remain unresolved, including the nature of the region prior to the arrival of India. Based on geological mapping in Tibet, Paul Kapp and his colleagues at the University of Arizona, together with L. Ding of the Chinese Academy of Science, have recently demonstrated that deformation and the development of high topography may have commenced as early as 105 million years ago. In doing so, Kapp and his colleagues have more than doubled the deep time history of mountain building in this region. Their results indicate that the evolution of great mountain belts may be far more complicated than previously thought.
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