Boulder, Colo. - Topics include: paleoseismology of Yucca Mountain, Nevada; impact of magnetic anomalies in Earth's oceanic crust on plate tectonics; evidence of episodic regional and possibly global glaciation during the Neoproterozoic; insights into Paleocene-Eocene global warming; impact of subduction on Earth's surface characteristics; records of environmental change in Black Coral specimens; and geology's place in the Scientific Revolution. The GSA TODAY science article addresses Hurricane Katrina sediment deposits and hydraulic conditions associated with the flooding.
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Joshua C. Galster, Lehigh University, Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA; et al. Pages 713-716.
Galster et al. examine the role that drainage area plays in determining river discharge in urban and rural watersheds. Two comparable watersheds, one rural and one urban, in eastern Pennsylvania were instrumented to measure the discharges that resulted from storms. The discharges increased at a much faster rate in the urban watershed than in the rural one, most likely as a result of the larger amounts of impervious surfaces in the urban watershed. The higher discharges in the urban stream could cause erosion, damage property, and harm aquatic habitats.
Brian Kendall, University of Alberta, Earth and Atmospheric Sciences, Edmonton, Alberta T6G2E3, Canada; et al. Pages 729-732.
The rhenium-osmium (Re-Os) radiometric dating method has been used to obtain the absolute age of deposition of black shales directly overlying ancient glacial rocks in southern and central Australia. The glacial deposits were believed to represent part of a global ice age (a "Snowball Earth" event), known to geologists as the "Sturtian" glaciation that occurred 700 million years ago. However, the absolute age constraints on the timing of this glaciation in Australia are actually very poor, and correlations of the Australian glacial deposits with other Neoproterozoic (1000–543 million years ago) glacial rocks worldwide have remained uncertain. Our new Re-Os black shale ages indicate the "Sturtian" glaciation in southern and central Australia ended by 643.0 2.4 million years ago and 657.2 5.4 million years ago, respectively. Other Neoproterozoic "Sturtian" glacial deposits previously correlated with the Australian glacial rocks have been radiometrically dated (using the uranium-lead technique on igneous rocks) to be 685 to 750 million years old. Thus, the new Re-Os ages indicate the "Sturtian" glacial deposits in Australia are younger than expected and may not have been part of a "Snowball Earth" event. Instead, the Australian glacial rocks may reflect the development of regional ice sheets in rift basins during the breakup of a large supercontinent (known to geologists as Rodinia). Consequently, other glacial deposits assigned to the "Sturtian" glaciation may also have formed in this manner. However, geological studies on other glacial rocks suggest some glaciations during the Neoproterozoic may have been unusually severe such that deposition of glacial sediment took place in marine environments at low latitudes. Thus, Kendall et al.'s results indicate a complex Neoproterozoic Earth climate characterized by episodes of regional and possibly global glaciation. Following the end of Neoproterozoic glaciation, the first large animals appear in the fossil record. In order to explore any possible link between Neoproterozoic glaciation and the evolution of animal life, it is paramount that further geochronological studies be carried out on undated Neoproterozoic glacial deposits to establish an appropriate Neoproterozoic chronology. This study represents an important step in that direction by using the relatively new Re-Os dating method for Neoproterozoic black shales.
J.C. Zachos, University of California-Santa Cruz, Earth Sciences, Santa Cruz, California 95064, USA; et al. Pages 737-740.
Imagine sea surface temperatures (SST) of 92 °F (33-34 °C) along the New Jersey coast, almost 18 °F warmer than modern peak summer temperatures. A new record of SST indicates that coastal SST reached such extreme levels during the Paleocene-Eocene Thermal Maximum roughly 55 million years ago. The estimates are based on two independent proxies of paleotemperatures, oxygen isotope ratios of plankton shells, and the concentration of cyclopentane rings in crenarchaeota bacterial lipid membranes (TEX86), both found in samples from a sediment core drilled near Wilson Lake, New Jersey. Both records reveal an increase in mean annual SST of 7-8 °C during this global warming event, which lasted tens of thousands of years and was driven by a rapid increase in greenhouse gas levels.
Laurent Husson, Massachusetts Institute of Technology, EAPS, Cambridge, Massachusetts 02139, USA. Pages 741-744.
Dynamic topography is the deflection of Earth's surface as a response to mantle flow. This flow is driven by differences in mantle density. Subducting plates are formed of dense material sinking in Earth's mantle, subsequently sucking down the topography. Husson's work shows that the dynamic component of topography can be evaluated from a joint analysis of seismic tomography, theoretical modeling, gravity, and bathymetry above subduction zones like the Aegean, Mariana, and East Scotia seas.
Bridget Y. Lynne, University of Auckland, Geology, Auckland, New Zealand; et al. Pages 749-752.
Hot springs are important because it is believed that these extreme environments may have hosted early microbial life on Earth. Recognizing ancient biosignals in siliceous hot spring deposits (sinter) requires an understanding of how they formed, and the types of post-depositional overprints that can affect the archived biotic signatures. Siliceous sinter forms in terrestrial environments wherever alkali chloride hot springs discharge at the surface. As these fluids cool to <100 ºC, silica precipitates on all biological and non-biological surfaces. Microbial communities thrive in the thermal waters. Shifting channel flows drown surrounding plants, and pollens and insects are often blown into the spring water. All may become entombed and fossilized in the silica, potentially recording biotic indicators of past environments. However, over thousands of years, sinters undergo a series of silica phase transformations, from opal (noncrystalline or partially crystalline) to microcrystalline quartz, affecting the preservation of their fossil components. In addition, the physical and chemical milieu of geothermal systems also can change, shifting from discharging alkaline hot springs to acidic steam vents (fumaroles) as the water table drops. To record the effects of a changing geothermal system on the silica phases of microbe-bearing hot spring deposits, a newly deposited opaline sinter was placed inside an active fumarole at Orakei Korako, New Zealand, for two years and systematically monitored. This field and laboratory experiment showed that an acidic fumarole environment (common in many geothermal systems worldwide), overprints and alters original microbial textures and accelerates the rate of silica phase transitions. Distinct micro-environments developed within the sinter sample, and patches of quartz formed after only 21 weeks.
Brooks B. Ellwood (corresponding author), Louisiana State University, Geology and Geophysics, Baton Rouge, Louisiana 70803, USA; and Wulf A. Gose, University of Texas–Austin, Austin, Texas 78712, USA. Pages 753-756.
Sediments from Hall's Cave, central Texas, have been sampled for regional paleoclimate evaluation. Magnetic susceptibility measurements (how susceptible a sample is to becoming magnetized) were performed on a continuous vertical sequence of samples from an excavation in the cave, to develop a magnetic stratigraphy susceptibility profile for the site. Such measurements in cave sediments are sensitive to climate due to changes in soil formation rates. Results showed a number of important major climatic events were recorded in the cave, including the H1 Heinrich Event at ~17,200 B.P., the end of the last major glaciation at ~14,300 B.P., and the 8,200 B.P. climatic event. These results are consistent with other independent indicators of climate for the region, but provide better precision on the timing of events and indicate that caves can provide a unique laboratory where climatic variations are preserved.
Liviu Giosan, Woods Hole Oceanographic Institution, Geology and Geophysics, Woods Hole, Massachusetts 02543; et al. Pages 757-760.
Sea level in the Black Sea has long been controversial, at a variety of time scales. Sea level curves from the region show large magnitude (up to 15 meters) and high frequency level variations since the early Holocene despite the fact that the Black Sea was already connected to the World Ocean. A new chronology for beach ridges in the Danube delta -based on mass spectrometry radiocarbon measurements and supported by optically stimulated luminescence dates - shows that the Black Sea water level remained within -2 m and +1.5 m of the current level for the last five millennia. Hydroisostatic effects related to a catastrophic reconnection of the Black Sea to the World Ocean in the early Holocene may have played a role in reaching the Holocene highstand earlier than estimated by models. The new dates also demonstrate that the Holocene Danube delta started to build out of a Black Sea embayment only ~5200 years ago, rather than ~9000 years ago as previously proposed. The new chronology allows for a better understanding of the Danube delta paleogeography, including the demise of Histria, the main ancient Greek-Roman city in the region, whose harbor has been shoaled with sands transported alongshore from the updrift Danube coast. Further, submergence at several ancient Chalcolithic/Bronze Age to Greek-Roman settlements along the Black Sea coast may be better explained by local factors such as subsidence, rather than by basin-wide sea level fluctuations.
Todd A. Ehlers, University of Michigan, Department of Geological Sciences, Ann Arbor, Michigan 48109-1063; et al. Pages 765-768.
Glaciers are efficient agents of erosion and responsible for sculpting many of the spectacular views found in mountain ranges around the world. Ehlers et al. document the degree to which glaciers modify mountains over millions of years and successfully quantify a rapid increase in erosion rates across the Coast Mountains of British Columbia, which was coincident with the onset of glaciation of this range. Furthermore, they found that the present topographic high point of the range (Mount Waddington) used to be located 16 km to the southwest of its current position. These results confirm that glaciation can significantly modify the erosion and topographic history of mountain ranges, particularly ranges located at high latitudes.
B. Williams, UQAM, GEOTOP-UQAM-McGill, Montreal, Quebec H2V 2N1, Canada; et al. Pages 773-777.
Williams et al. examine four Black Coral specimens collected by the Johnson Sealink submersible from the southeastern continental shelf of the United States and the Gulf of Mexico. Cross sections of the base of the coral skeleton show that corals lay down concentric coeval skeletal bands, which allow the stratigraphy of the record to be constrained. Lead-210 dating indicates the specimens are several centuries old, and annual radial growth estimates are 0.0145 mm, far slower than estimates for shallower water Black Corals. Carbon and nitrogen stable isotope analyses of the skeleton are reproducible within and among specimens from the same location; therefore, these organisms may provide records of past environmental change.
************** Kinking of the subducting slab by escalator normal faulting beneath the North Island of New Zealand
Stuart Henrys, Geological and Nuclear Sciences, Resources, Wellington 1, New Zealand; et al. Pages 777-780.
In many subduction zones, the repeat time between megathrust earthquake events is longer than the historical period of observation. In these cases, it is important to identify the updip limit of the seismogenic zone. Along the east coast of the North Island of New Zealand a key indictor of the updip limit is a change in dip of the down-going Pacific plate.,Henrys et al. suggest that the simple shear along the steep normal faults provides the mechanism by which the change in dip occurs, using an analogy for the subduction zone interface to the block motions that occur on an escalator.
Elizabeth Veenstra, Geophysical Institute, University of Alaska-Fairbanks, Fairbanks, Alaska 99775-7320, USA; et al. Pages 781-784.
Earthquake data recorded in south central Alaska was used to determine crustal thickness in the region because such data yield clues about how mountains are supported. The study collected high-quality data from continuously recorded broadband sensors over a period of 27 months. Earthquakes occurring at specific distances from the recording station provided near-vertical incident waves, ideal for studying boundaries such as the crust mantle boundary. During the field deployment, 751 earthquakes occurred in an appropriate distance range, and by recording these events at up to 36 seismic stations across south central Alaska, 14,754 three-component seismograms were isolated for studying crustal thickness. Results revealed typical crust beneath the interior lowlands to be 26 km thick. Additionally, the transition from thick to thin crust coincides with a major crustal break. By comparing observed crustal thickness with crustal thickness predicated by topography, Veenstra et al. find that variations in both crustal thickness and density are required to explain the seismic and gravity data.
Tom Parsons, U.S. Geological Survey, Menlo Park, California 94025, USA; et al. Pages 785-788.
Paleoseismologists discovered a remarkable series of events that happened about 80,000 years ago at Yucca Mountain, Nevada, site of the proposed U.S. high-level nuclear waste repository. Trenches across three faults showed volcanic ash from the Lathrop Wells cone at the bottom of earthquake fissures, indicating that volcanic and earthquake events were tightly clustered in space and time. Parsons et al. use numerical Earth models of Yucca Mountain faults and volcanic intrusions to determine the influence that these processes have on one another. They found that each was mutually reinforcing. That is, an earthquake could encourage volcanic events, and a volcanic event could encourage earthquakes. In their model, slip on the faults in the central Yucca Mountain block tended to favor volcanic intrusions about 15 km south of the proposed repository.
Maurice A. Tivey, Woods Hole Oceanographic Institution, Department of Geology and Geophysics, Woods Hole, Massachusetts 02543-1541, USA; et al. Pages 789-792.
The ocean crust provides an almost complete record of Earth's magnetic field history, from the present day back through the Jurassic period, approximately 175 million years ago. Over this period, Earth's magnetic field has reversed polarity many times and has varied in intensity significantly. During this time, there have also been two periods when the magnetic field remained fixed in one polarity and thus relatively quiet. Although the Cretaceous Quiet Zone (84-118 million years ago) is well-documented, the Jurassic Quiet Zone (JQZ) (157-175 million years ago) is more controversial. Terrestrial records suggest that Earth's magnetic field was reversing rapidly during the Jurassic, but corroboration from the marine record has been poor because of the scarcity of Jurassic-aged ocean crust, poorly resolved magnetic signals due to deep seafloors, and noise due to the equatorial location of Jurassic crust. Tivey et al. report the results of a deep-towed magnetic survey of the oldest portion of the Pacific that overcomes these noise and resolution problems and shows that magnetic anomaly reversals continued into the JQZ, but became weaker in amplitude. Tivey et al. find very rapid reversals, with over 10 reversals per million years at 160 and 167 million years ago. Anomaly amplitudes decreased from 155 to 162 million years ago, where low amplitude anomalies become difficult to correlate. Prior to 167 million years ago, anomalies regained amplitude and remained strong until the end of Tivey et al.'s record at 170 million years ago. The JQZ thus appears to be a combination of low-amplitude magnetic anomalies combined with rapid field fluctuations, which could be due to either intensity or polarity changes.
Gary D. Rosenberg, IUPUI, Geology, Indianapolis, Indiana 46202, USA. Pages 793-796.
Nicholas Steno, who founded geology in the 17th century, compiled a notebook as a student in 1659 that he titled "Chaos," which is rediscovered for English-speaking audiences in Rosenberg's paper. "Chaos" provides uncanny witness into Steno's thought process and unprecedented insight into geology's place in the Scientific Revolution. One can see numerous seeds of modern geology and evolutionary thought sprouting in "Chaos." It shows Steno using geometry to understand the structure of the human body and Earth, and illustrates how close Steno was to understanding the modern concept of fractionation of the planet (separation of Earth materials due to heat within the Earth). At the same time, it explains why Steno would later reject the theological pronouncements of one of the most important figures of the Scientific Revolution, René Descartes, despite the fact that Steno would become a Catholic bishop. Descartes had proposed that the heart was an oven warmed by the passions of the soul, but Steno would later use geometry the way he had in "Chaos" to prove it was a muscle, not an oven like Earth.
Boyan K. Vakarelo, University of Texas - Dallas, Department of Geosciences, Richardson, Texas 75083, USA; et al. Pages 797-800.
Valarelov et al. demonstrate that in a 2.2 million year interval in the Upper Cretaceous, four tectonically driven erosional surfaces can be mapped. Biostratigraphic correlation to a sea-level curve for the Cenomanian - where coeval high-frequency low-amplitude eustatic cyclicity has been demonstrated - allows the first direct comparison of the effects of eustasy and tectonics at temporal scales of hundreds of thousands of years during a global greenhouse time. It is suggested that minor tectonic pulses locally overshadow the effects of eustasy and exert the dominant control over preserved stratigraphy.
Katrina's unique splay deposits in a New Orleans neighborhood
Stephen A. Nelson and Suzanne F. Leclair, Department of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana 70118, USA.
We are all familiar with the devastation that Hurricane Katrina brought to the Gulf Coast when it came ashore a year ago. New Orleans' Tulane University geology professors Steve Nelson and Suzanne LeClair acted quickly to map the distribution and composition of flood deposits left after the southern breech in the levee of the London Avenue Canal in order to provide insight into the hydraulic conditions associated with the flood event. The deposition of sediment from flood events provides a record of flood conditions not generally available simply by observing surface water conditions. In the case of Katrina, flood deposits were removed quickly as emergency crews attempted to bring residential neighborhoods back to normal. The results of Nelson and LeClair's work are presented in the September issue of GSA Today. Their article describes how the breech of the levee and resultant flood waters mobilized unconsolidated marsh and beach sediments and left a 1.8-m-thick deposit that included fine sand, cars, household items, and close to the breech, a house. Geologic mapping of flood deposits in an urban neighborhood provides insights into flow directions and flood power and provides additional observational data of deposits that by necessity were removed by recovery crews.
To view the complete table of contents for the September issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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