Boulder, CO -- Topics include: discovery of ancient Triassic bacteria with strong resemblance to their modern counterparts; relationship of increased atmospheric CO2 to storm frequency and severity; Southern Hemisphere recovery from the K-T impact event and mass extinction; movement of volatile compounds in Mount St. Helens leading up to its October 2004 eruption; and differences in earthquakes generated by mature vs. less well developed faults. The GSA TODAY science article examines the future of coastal development.
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GEOLOGY
Quaternary erosion-induced isostatic rebound in the western Alps
J.D. Champagnac, Geological Sciences and CIRES, University of Colorado at Boulder, Boulder, CO 80309, USA; et al. Pages 195-198.
Champagnac et al. address the relationship between the erosion of a well-known mountain range (the European Alps) and the associated vertical motion of the western Alps. The "unloading" of the mountain range is related to an upward rebound. The authors show that half of the observed vertical movement in the Alps can be explained with the Pliocene (~since 3 Ma) climate-driven enhanced erosion.
General circulation model simulation of the d18O content of continental precipitation in the middle Cretaceous: A model-proxy comparison
Christopher J. Poulsen, University of Michigan, Department of Geological Sciences, Ann Arbor, MI 48109, USA; et al. Pages 199-202.
The chemistry of ancient soils has been used to suggest that the hydrological cycle was more vigorous during the Cretaceous, a time of global warmth due to enhanced greenhouse gases. Poulsen et al. have used climate models of the Cretaceous to evaluate this interpretation and find that the soil data can be explained without requiring large increases in hydrological cycling.
Bacterial fossils and microbial dolomite in Triassic stromatolites
Edoardo Perri, Maurice Tucker
Edoardo Perri (corresponding author), Università della Calabria, Scienze della Terra, Rende, CS 87036, Italy; and Maurice Tucker, University of Durham, Department of Geological Sciences, Durham DH1 3LE, UK. Pages 207-210.
Perri and Tucker present a description and interpretation of the first convincing examples of bacteria in ancient microbial mats. Scanning electron microscopy observations have revealed spheroidal structures within a Triassic (220 million years old) stromatolite from Italy, that are directly comparable with modern bacteria. A mineralized extracellular organic substance is also described in the rock, which is interpreted to preserve original geochemical signals. Stromatolites, laminated biosedimentary deposits, provide the first evidence for the existence of life on Earth and preserve evidence of microbial mediation in carbonate precipitation through the entire geological record. In addition, the bacteria here are present within a dolomitic rock. The origin of dolomites remains one of the unsolved mysteries of earth science. However, the close association of the bacterial fossils with early formed dolomite in Perry and Tucker’s study does implicate a microbial mechanism for dolomitization.
Stable warm tropical climate through the Eocene Epoch
Paul N. Pearson, Cardiff University, Earth Sciences, Cardiff, Wales, CF10 3YE, UK; et al. Pages 211-214.
Pearson et al. reveal exceptionally warm tropical temperatures in the Eocene epoch, a past episode of greenhouse climate with high atmospheric carbon dioxide levels. The new information comes from ancient seafloor sediments that now form part of coastal Tanzania. A series of geological field expeditions have used mobile drilling rigs to bring back many new sediment cores that contain, within their layers, a detailed history of past climate change from the period roughly 55–35 million years ago. Chemical analysis of the mineral shells and organic remains of ancient planktonic organisms shows that temperatures were consistently several degrees warmer than they are in today's tropical oceans. Moreover, tropical temperatures were relatively constant, despite the fact that gradual polar cooling is known to have occurred. The findings may suggest a greater role for tropical storms in transporting heat in the ancient climate system than has hitherto been recognized.
Abrupt increase in seasonal extreme precipitation at the Paleocene-Eocene boundary
Birger Schmitz (corresponding author), Lund University, Department. of Geology, Lund, Scania SE-22362, Sweden; and Victoriano Pujalte, Department of Stratigraphy and Paleontology, University of the Basque Country, Bilbao, Basque Country 48080, Spain. Pages 215-218.
Whether increasing atmospheric carbon dioxide levels will lead to an increase in storm frequencies is a much debated issue today. At the Paleocene-Eocene boundary 55 million years ago, a "super greenhouse" world evolved during ~50,000 years because of natural emissions of carbon dioxide. Studies of Paleocene-Eocene river and flood-plain deposits exposed in the Pyrenees, Spain, show that subtropical regions experienced dramatic changes in the hydrological cycle as carbon dioxide levels rose during the greenhouse event. Paleocene semiarid "normal" coastal plains with a few river channels of 10–200 meters in width were abruptly replaced at the Paleocene-Eocene boundary by a vast fluvial "megafan" covering 2000 square kilometers or more. This change indicates that summers became longer and drier and that extreme rain and flood events became more frequent and stronger during the winter season. The observed changes in the hydrological cycle are in accordance with "worst-case" scenarios for the future of Earth.
Did Plinian eruptions in California lead to debris flows in Nevada? An intriguing stratigraphic connection
John W. Bell (corresponding author) and P. Kyle House, University of Nevada, Nevada Bureau of Mines and Geology, MS 178, Reno, NV 89557, USA. Pages 219-222.
Plinian eruptions are explosive volcanic eruptions that eject large quantities of volcanic ash high into the atmosphere. During the last several thousand years, multiple plinian eruptions have occurred at the Mono-Inyo Craters in eastern California, with the last event occurring about 500 years ago. During the course of many field investigations in western Nevada, Bell and House have noted that thin (1–2 cm) volcanic ash (tephra) beds derived from these eruptions are commonly buried beneath large (kilometer-scale) debris-flow flood deposits produced by intense precipitation. Such relationships have been found throughout a 250-km-long corridor extending from the Mono-Inyo Craters into western Nevada. Considering that the thin tephra beds are preserved at the surface for no more than few years, and that the average return periods for these intense storms are on the order of hundreds of years, it is highly improbable that this association is a random coincidence. Bell and House propose that the most plausible explanation for this association is that some volcanic eruptions from Mono-Inyo Craters produced intense thunderstorms throughout the western Nevada region, resulting in the large floods that buried the tephra beds.
Hotspots and edge-driven convection
Scott D. King, Purdue University, Department of Earth and Atmospheric Sciences, West Lafayette, IN 47907-1397, USA. Pages 223-226.
Regions of intra-plate, or excessive plate margin, volcanism are called hotspots. While conventional wisdom is that hotspots are related to deep mantle plumes, a growing number of observations question the plume mechanism for some hotspots. King makes the argument that hotspots driven by small-scale convection should be located 600–1000 kilometers from the edges of the most stable and oldest parts of the continents, called cratons, which have deep fast-seismic-velocity anomalies interpreted as cold roots. The group of hotspots that is outside the 600–1000 kilometer craton zone correspond remarkably well with seismic imaging of slow-velocity anomalies that may indicate deep plumes and another study that uses five different criterion to identify possible deep mantle plumes. Thus, there is little overlap between the hotspots located favorably for small-scale convection and those proposed to be of deep mantle origin
Faunal evidence for reduced productivity and uncoordinated recovery in Southern Hemisphere Cretaceous-Paleogene boundary sections
Martin Aberhan, Humboldt-Universität, Museum für Naturkunde, Berlin 10115, Germany; et al. Pages 227-230.
A catastrophic bolide impact is widely recognized as the trigger of the mass extinction that occurred 65 million years ago at the Cretaceous-Tertiary boundary, when animals such as dinosaurs (on land) and ammonites (in the sea) became extinct. One possible killing mechanisms involves a global collapse of food chains due to the shutdown of photosynthesis by sun-blocking dust clouds and soot from fires. It is still unclear whether the intensity of these perturbations is related to the distance from the impact site in today’s Mexico and whether it is related to paleolatitude. Therefore, Aberhan et al. analyze poorly known faunal data from high southern latitudes. They found that the ecological composition of organisms that lived on the sea floor at that time, such as clams, snails, corals, and sea urchins, changed significantly across the boundary and in a way that was consistent with the scenario of reduced availability of food after the impact event: Animals were less abundant for some time; starvation-resistant groups and animals not relying directly on photosynthetic organisms as a food source became more dominant; individuals with larval stages that do not need to feed on phytoplankton became proportionally more common; the average body size of individuals within communities became smaller; and individuals with low metabolic rates or inactive lifestyles became better represented. A return to pre-extinction conditions of the various ecological attributes occurred over unequal time spans, indicating that recovery from the mass extinction was uncoordinated with respect to ecological traits.
Vapor transfer prior to the October 2004 eruption of Mount St. Helens, Washington
Adam J.R. Kent, Oregon State University, Department of Geosciences, Corvallis, OR 97333, USA; et al. Pages 231-234.
It is known that many volcanic eruptions are driven by the behavior of volatile compounds—primarily water, sulfur dioxide, carbon dioxide, and chlorine—that have dissolved into liquid magma. Just like releasing the top of a cola bottle, the upward movement of magma can reduce pressure, allowing these volatiles to separate from magma and form a separate vapor (i.e., bubbles). Kent et al. use the trace element lithium (Li), which will dissolve into a vapor that separates from magma, to delineate the path of volatile movement prior to the October 2004 eruption of Mount St. Helens. Their study reveals an accumulation of vapor in the upper portions of the St. Helens’ magma chamber immediately prior to the 2004 eruption. The accumulation of this vapor may have been an important factor in initiating the eruption, perhaps by increasing pressure enough to rupture the overlying rock.
Pre-eruption recharge of the Bishop magma system
D.A. Wark, Rensselaer Polytechnic, Earth & Environmental Sciences, Department of Earth Sciences, Troy, NY 12180, USA; et al. Pages 235-238.
Wark et al. describe the distribution of titanium in quartz phenocrysts from the Bishop Tuff (eastern California, USA), which is a sheet of rhyolite ejecta deposited during the super-volcanic eruption that accompanied collapse of the Long Valley caldera. Based on a new technique that translates titanium concentrations into temperatures of quartz crystallization, Wark et al. show that the sub-volcanic magma chamber had been strongly heated only a short time before the 0.76 Ma eruption. The heating, in turn, is attributed to injection of hot basaltic melts into deeper levels of the sub-volcanic system. This raises the interesting possibility that the Bishop super-eruption was triggered by the introduction of new "recharge" melts.
Short-term soil mixing quantified with fallout radionuclides
James M. Kaste, Dartmouth College, Earth Sciences, HB 6105, Hanover, NH 03755, USA; et al. Pages 243-246.
Soils are an extremely valuable natural resource that we rely on for the growth of food plants, and they play a major role in regulating greenhouse gases in the atmosphere. Despite the critical role that soils have in our everyday lives and in our climate system, our understanding of certain important soil processes is limited. Kaste et al. study physical soil mixing, which is driven by bioturbation, freeze-thaw cycles, and other natural processes that are traditionally difficult to measure. They use natural and artificial radioactive fallout from the atmosphere to determine physical soil mixing rates across a variety of landscapes. These radioactive atoms are delivered to the soil surface primarily with precipitation, and physical mixing processes disperse them deeper into the soil profile. By using migration models, and treating physical mixing as a diffusion-like (gradient-driven) process, Kaste et al. calculate soil-mixing timescales. They find that in regions of the northeastern US - where little evidence of burrowing organisms is observed, but freeze-thaw cycles are possible - soils are "turned over," or mixed every 8000 years. At other sites where burrowing mammals and insects are more obvious, soils are mixed ten times faster. This work helps to further scientific understanding of soil mixing rates, and can be used to improve contaminant transport, carbon sequestration, and landscape evolution models.
Numerical evaluation of mechanisms driving Early Jurassic changes in global carbon cycling
David J. Beerling (corresponding author) and Stuart J. Brentnall, University of Sheffield, Department of Animal and Plant Sciences, Sheffield, S. Yorks S10 2TN, UK. Pages 247-251.
The Early Jurassic (early Toarcian, ca. 183 million years ago) carbon cycle perturbation is characterized by an ~?5‰ d13C excursion in the exogenic carbon reservoirs, a 1000 parts per million rise in atmospheric carbon dioxide (CO2), and a 6–7 ºC warming. Two proposed explanations for this presumed global carbon cycle perturbation are the liberation of massive amounts of isotopically light methane (CH4) from (1) Gondwanan coals by heating during the intrusive eruption of the Karoo-Ferrar large igneous province (LIP) or (2) the thermal dissociation of gas hydrates. Carbon cycle modeling indicates that the release of CH4 from Gondwanan coals synchronous with the eruption of the Karoo-Ferrar LIP fails to reproduce the magnitude or timing of the CO2 and d13 C excursions. However, sensitivity analyses constrained by a marine cyclostratigraphically dated d13C record indicate that both features of the geologic record can be explained with the huge input of ~15,340–24,750 billion tons of carbon over ~220 thousand years, a result possibly pointing to the involvement of hydrothermal vent complexes in the Karoo Basin. The simulated release of >6000 billion tons of carbon from gas hydrates also reproduces aspects of the early Toarcian rock record, but the large mass involved raises fundamental questions about its formation, storage, and release.
Extensive storage of basal meltwater in the onset region of a major West Antarctic ice stream
Leo E. Peters, The Pennsylvania State University, Geosciences, University Park, PA 16802, USA; et al. Pages 251-254.
New observations by Peters et al. in West Antarctica reveal the storage of water at the base of the ice sheet in a region where the ice is in transition to fast flow. This basal water is a vital component in lubricating the bed of the ice, leading to enhanced flow and drainage of the West Antarctic ice sheet. The amount of water present at the bed varies in the study area, with a water pocket present in the downstream direction where such water storage is favored. When combined with earlier observations of rapid vertical motions of the nearby ice surface, this variation suggests that the water moves under the ice in discrete pulses or "drips," building up in a favorable place and then moving quickly to another similar location. Current models of ice-flow lubrication and sediment transport do not include this process. The amplitude variation with offset seismic technique that Peters et al. applied to identify this subglacial water pocket is a powerful tool for imaging the bed of the ice sheet and for locating features that may be related to past and present ice flow patterns.
Arrival of extremely volatile-rich high-Mg magmas changes explosivity of Mount Etna
Vadim S. Kamenetsky, University of Tasmania, School of Earth Sciences, Hobart, TAS 7001, Australia; et al. Pages 255-258.
It is well known that magmatic volatiles play a key role in explosive volcanism. They control vesiculation and fragmentation processes of the magma and consequently the style of eruptions. Notably, Mount Etna is one of the world's most actively degassing volcanoes, yielding roughly 10% of the global carbon dioxide and sulfur dioxide fluxes. However, because volatiles are almost completely degassed during subaerial eruption, it is difficult to measure pre-eruption volatile concentrations directly. One way around this problem is to analyze tiny samples of non-degassed melt trapped inside crystals. Kamenetsky et al.’s study of unique picritic magmas, the eruption of which marked the beginning of the vigorous volcanic activity of Mount Etna and olivine-hosted melt inclusions, provides explanations for the “behind the scenes” processes and controls in the volcano’s conduit.
Kinematic evidence for downdip movement on the Mormon Peak detachment
Christopher D. Walker, Lamont-Doherty Earth Observatory, Department of Earth and Environmental Science, Columbia University, Palisades, NY 10964, USA; et al. Pages 259-262.
It is well known that the Basin and Range province of the western United States is being stretched, as the Sierra Nevada is pulled obliquely away from the Colorado Plateau. The magnitude of stretching and how the deformation is distributed across specific structures, however, is less certain. According to one view, large amounts of extension may be accommodated by gently inclined faults. Walker et al. examined directional indicators along one such fault, the Mormon Peak detachment in the Mormon Mountains of southeastern Nevada. Their data support an alternative interpretation: that the detachment marks the base of a series of gravitationally driven slide blocks and accounts for no extension. These findings raise issues about the significance of other similar structures in the Basin and Range province.
Long-term tectonic control on Holocene shelf sedimentation offshore La Jolla, California
Leah J. Hogarth, Scripps Institution of Oceanography, Geosciences Research Division, University of California–San Diego, La Jolla, CA 92093-0208, USA; et al. Pages 275-278.
Although many researchers have emphasized the importance of tectonics in controlling stratigraphic architecture, there are few compelling examples that illustrate how tectonics actually controls sediment accumulation. Hogarth et al. present new data from offshore Southern California that shows how tectonic deformation parallel to the shoreline controls sediment accumulation and preservation in shallow-water regions. Jogs in the right lateral strike-slip Rose Canyon fault create uplift and subsidence on the continental shelf. Similar jogs on other strike-slip faults in the region control much of the uplift and subsidence in the Continental Borderlands of Southern California. In the lows, sediments accumulate and are potentially preserved, while on the highs, there is enhanced erosion and transport of sediments. In addition, this research shows where thick sand repositories may occur offshore that could be used for potential beach re-nourishment programs.
Evolution of fault-surface roughness with slip
Amir Sagy, University of California–Santa Cruz, Earth & Planetary Sciences, Santa Cruz, California 95064, USA; et al. Pages 283-286.
The surfaces of faults record the cumulative effects of many earthquakes. They also set the conditions for future earthquakes. Despite the critical role of fault surfaces, measurements of their shape have been scarce. Sagy et al. use cutting-edge laser technology (light detection and ranging [LiDAR]) to fill the observational gap. The new measurements, with sub-centimeter resolution, reveal that faults that have slipped large distances are measurably smoother than new faults. Furthermore, the authors observe that large-slip faults have distinct 10-meter-long bumps that have never previously been observed. These differences between old and new faults suggests that mature structures like the San Andreas fault will result in a different kind of earthquake with smoother rupture than less well-developed faults.
GSA TODAY Science Article
Fetch-limited barrier islands: overlooked coastal landforms
J. Andrew G. Cooper, Centre for Coastal and Marine Research, School of Environmental Sciences, University of Ulster, Coleraine BT52 1SA, UK; et al.
The world over, open ocean barrier islands have proved to be highly valued places to live. Due to intense development activity, these ocean-facing islands are now reaching development capacity, despite the fact that increasing sea levels pose significant problems for the future of ocean-side living. What then is the future of coastal development? This is the question posed by Andrew Cooper of the University of Ulster and his colleagues in a recent GSA Today article. They show that much of the world’s bay, lagoon and estuarine coasts also consist of lower-energy, sheltered islands—what they refer to as fetch-limited barrier islands. As these sheltered islands, which number more than 15,000, become increasingly attractive for development, questions are arising concerning the origin and evolution of these hitherto unstudied landforms. How these islands will respond to development and, more importantly, to the ongoing rise in sea level are first order questions of geological and societal significance.
To review the abstracts for these articles, go to www.gsajournals.org. To view the complete table of contents for the current issue of GEOLOGY, go to www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.