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Rubbing boulders, fossil mammal teeth, barrier islands, and a change in volcanic behavior

Boulder, Colo., USA In Geology: researchers experience an earthquake while studying the Atacama's rubbing boulders; information from fossil mammals, such as tooth crown height, is used to track aridity patterns; calibration of the plant transpiration of an ancient terrestrial ecosystem is presented; researchers chronicle the discovery of a new chain of barrier islands in one the highest wave-energy environments on Earth; and a change in volcanic behavior at Pisciarelli, Campi Flegrei, Italy, comes to light.

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Periglacial weathering and headwall erosion in cirque glacier bergschrunds

Johnny W. Sanders et al., Dept. of Earth and Planetary Science, University of California, Berkeley, California 94720, USA. Posted online 18 July 2012; doi: 10.1130/G33330.1.

Headwall retreat in cirques is a principal mechanism driving cirque expansion and topographic evolution near range divides. Until now, several hypotheses have been put forward to explain how cirque glaciers undermine their headwall, but the field evidence necessary to test these models has remained elusive. Johnny W. Sanders and colleagues present nearly two years of environmental measurements made in three cirque glacier bergschrunds (West Washmawapta Glacier, British Columbia, Canada and Conness Glacier, Sierra Nevada, California). Their measurements, coupled with a numerical model of ice lens growth in idealized cracks, indicate that frost weathering by ice segregation crack growth is a viable mechanism for dismantling the headwall both above and within the bergschrund. Only in the bergschrund, however, is the glacier able to entrain the loosened rock. A slight reduction in temperature -- more typical of the Pleistocene -- shifts the locus of frost weathering to within the bergschrund.

The 8.2 ka event -- Calendar-dated glacier response in the Alps

Kurt Nicolussi and Christian Schlchter, Institute of Geography, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria. Posted online 18 July 2012; doi: 10.1130/G32406.1.

The "8.2 ka event" (8.2 thousand years ago) has been identified as a major Holocene climatic cooling episode in the North Atlantic realm. This event was recognized as the most extreme climatic anomaly in the Greenland ice core records during the Holocene. However, clear evidence in terms of a major climate archive, mountain glaciers, has been missing so far for the European Alps. In the light of the tree-ring analysis of tree remains found in front of the glacier snout of the Mont Min Glacier during the past few years, the first calendar-dated evidence for a glacier advance within the 8.2 ka cooling event worldwide can now be presented. Several tree fragments date this advance from 8183 to 8168 years before A.D. 2000. The Mont Min Glacier reached the current ice extent at that time. This 8.2 thousand-year-old advance terminates a long-lasting retreat period of the Mont Min Glacier that goes back at least to about 9100 years before A.D. 2000, and during that time, the Mont Min Glacier was continuously shorter than in A.D. 2005/2010. The "8.2 ka event"-related glacier advance can be seen as one of the few interruptions of generally reduced glacier and ice extent in the Alps during the first half of the Holocene.

Neogene aridification of the Northern Hemisphere

Jussi T. Eronen et al., Dept. of Geosciences and Geography, P.O. Box 64, University of Helsinki, Helsinki FI-00014, Finland; and Biodiversity and Climate Research Centre (LOEWE BiK-F), Senckenberganlage 25, D-60325 Frankfurt am Main, Germany. Posted online 18 July 2012; doi: 10.1130/G33147.1.

This paper by Jussi T. Eronen and colleagues uses a novel approach -- information from fossil mammals --to track changing aridity patterns across the Northern Hemisphere over the past 23 million years (the Neogene). Eronen and colleagues have shown previously that the distribution of tooth crown height (hypsodonty) in communities of herbivores (ungulates, or hoofed mammals) correlates well with mean annual precipitation, and they apply those proxies here. The mammal communities show that, while both Eurasia and North America had mesic (moderately moist habitat) conditions at the start of the Neogene, and both become more arid moving toward the present day, onset of aridity in North America preceded that in Eurasia (commencing at 15 million of years ago versus 10 million years ago), and this aridity was also more severe. These predicted changes can be explained by patterns of ocean circulation and paleogeography, with key influences being the proximity of North America to the Pacific Ocean and changes in the North Atlantic resulting from the formation of the Isthmus of Panama. The difference in the history of aridity of the two continental landmasses may also explain why North American ungulate immigrants to Eurasia throughout the Neogene tended to fare well, while the reverse was not true until around 5 million years ago.

Seismicity and the strange rubbing boulders of the Atacama Desert, northern Chile

Jay Quade et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Posted online 18 July 2012; doi: 10.1130/G33162.1.

Jay Quade and colleagues discovered clusters of more than one ton-sized boulders in the Atacama Desert of northern Chile worn to smoothness around their mid-sections. They suggest that the boulder smoothing is the cumulative result of at least a million years of rubbing between boulders during earthquakes. Dating of the boulder surfaces shows that they have been exposed for greater than a million years. During the research team's second field visit to one major boulder site, they experienced an earthquake that rocked but did not tip the boulders, causing them to rub against each other for about a minute. This 5.6 magnitude earthquake was centered about 100 kilometers northeast of the site. In the seismically active Atacama, earthquakes of this energy or greater occur about once every four months, suggesting that the average boulder has experienced ~40-70 kilohours of abrasion over the past 1.3 million years. This unusual evidence underscores the largely unrecognized role that seismicity probably plays in hillslope sediment transport in the nearly rainless Atacama Desert and perhaps on other seismically active but now dry worlds like Mars.

Glacier expansion in southern Patagonia throughout the Antarctic cold reversal

Juan L. Garca et al., Instituto de Geografa, Facultad de Historia, Geografa y Ciencia Poltica, Pontificia Universidad Catlica de Chile, Campus San Joaqun, Avenida Vicua Mackenna 4860, comuna Macul, Santiago 782-0436, Chile. Posted online 23 July 2012; doi: 10.1130/G33164.1.

Juan L. Garca and colleagues built a glacial chronology based on Beryllium-10 cosmogenic exposure dating of boulders resting on moraines in Torres del Paine National Park, Chile (51 degrees S, 72 degrees W). Recent improvements of geochronologic techniques plus detailed geomorphic and geologic mapping allowed them to produce strong evidence for an atmospheric cooling in south Patagonia throughout the Antarctic cold reversal chronozone (ACR; about 14.6-12.9 thousand years ago). This work shows that ice in the southern mid-latitudes expanded and actively built prominent moraine systems not only at the end of the ACR, but throughout this stadial, interrupting the last deglacial warming trend. Concurrent intensive westerly circulation in the region brought cold and wetter conditions to Torres del Paine, triggering glacial expansion. For the first time, work dating glacial fluctuations strongly demonstrates that the late-glacial southern criosphere responded closely to changes in the atmosphere and ocean throughout the ACR, providing another piece of evidence for paleoclimate teleconnections at the southern part of the planet during the last glacial termination.

How steady are steady-state landscapes? Using visible-near-infrared soil spectroscopy to quantify erosional variability

Kristin E. Sweeney et al., Dept. of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA. Posted online 23 July 2012; doi: 10.1130/G33167.1.

Erosion rates enable scientists to infer how landscapes respond to climate and tectonics through time. However, most methods for measuring erosion rates are quite costly, limiting the number of samples per study. Kristin E. Sweeney and colleagues present a new, inexpensive method for measuring relative erosion rates, which uses visual/near infrared spectroscopy to infer the residence time of hilltop soils. Soil residence time is directly linked to erosion rate: longer residence times indicate slower erosion rates. Results from two study watersheds in the Oregon Coast Range suggest that this method can capture both the natural unsteadiness of hillslope erosion and larger landscape dynamics such as divide migration.

Deep-time evidence of a link between elevated CO2 concentrations and perturbations in the hydrological cycle via drop in plant transpiration

Margret Steinthorsdottir et al., Dept. of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden. Posted online 23 July 2012; doi: 10.1130/G33334.1.

In this manuscript, Margret Steinthorsdottir and colleagues propose a novel mechanistic link between marine and terrestrial mass extinction via atmospheric CO2-driven changes in plant transpiration across the Triassic-Jurassic mass extinction boundary (200 million years ago). This study is the first to calibrate the plant transpiration of an ancient terrestrial ecosystem during a period of high CO2-induced global warming and to link plant physiological forcing to broader environmental degradation and loss of biodiversity through effects on the hydrological cycle. The effect of the current rise in CO2 concentration on the hydrological cycle is receiving increasing attention, and it has been repeatedly suggested (but not tested) that the physiological forcing of atmospheric CO2 (i.e., the effects on plant transpiration) may equal or exceed the effects of the radiative forcing of CO2. The results reported in this paper may not only provide a mechanism to link the effects of rapid rise in atmospheric CO2 concentration and environmental degradation/loss of biodiversity at the Triassic-Jurassic mass extinction boundary, but also may have a significant relevance for the discussion of future impacts of global warming. Plant transpiration responses may thus be of key importance in both past and future climate change, through physiological forcing of the hydrological cycle.

Barrier islands on bedrock: A new landform type demonstrating the role of antecedent topography on barrier form and evolution

J.A.G. Cooper et al., University of Ulster, School of Environmental Sciences, Coleraine BT52 1SA, Northern Ireland, UK. Posted online 23 July 2012; doi: 10.1130/G33296.1.

A new chain of barrier islands has been discovered in one the highest wave energy environments on Earth. The 90-km-long chain of sandy barrier islands flanks the western shore of Scotland's Western Isles (Outer Hebrides). Uniquely among barrier islands, these are migrating over a solid bedrock surface and the shape of the underlying surface is fundamental in controlling their response to rising sea level. In some parts, the islands resemble typical barrier islands with tidal inlets and deltas, but where the bedrock surface is high, the islands enclose perched lagoons and lack tidal deltas. The planar bedrock surface is crucial in permitting barrier islands to form in such high energy settings -- it reduces incoming wave energy to the point that islands can stabilize.

Early signals of new volcanic unrest at Campi Flegrei caldera? Insights from geochemical data and physical simulations

Giovanni Chiodini et al., Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, via Diocleziano 328, 80124 Naples, Italy. Posted online 23 July 2012; doi: 10.1130/G33251.1.

Many hundreds of thousands people live at Campi Flegrei, a caldera located near Naples, Italy, which had a large ground inflation in 1982-1984 followed by twenty years of subsidence. Recently, the behavior of the volcano has changed: the fumaroles have increased in activity; swarms of low-magnitude earthquakes have become more frequent; and the ground started a new general uplifting trend, indicating that the system is undergoing repeated injections of magmatic fluid. Physical simulations of the process show that total injected fluid masses are the same order of magnitude as those emitted during small to medium size volcanic eruptions, and their cumulative curve highlights a current period of increasing activity.

Upper plate tectonic stress state may influence interseismic coupling on subduction megathrusts

Laura M. Wallace et al., GNS Science, Lower Hutt 5040, New Zealand. Posted online 23 July 2012; doi: 10.1130/G33373.1.

Faults that accommodate subduction of one tectonic plate beneath another produce the largest and most destructive earthquakes and tsunamis on Earth -- such as the March 2011 M9.0 earthquake and tsunami in northern Japan. The increasing use of GPS techniques to measure deformation of tectonic plates at subduction zones has revealed that in some cases the subducting and overriding plates are locked or "coupled" together between large earthquakes due to friction along the fault, while other subduction plate boundaries tend to creep aseismically. Using GPS data from subduction zones in southwest Japan, New Zealand, and Vanuatu to document the degree of fault locking versus fault creep, Laura M. Wallace and colleagues show that a lateral change from aseismic creep to deep locking also correlates with a change in the tectonic behavior of the overriding plate from rifting to contraction. To explain this observation, they suggest that interactions between fluid pressure and the tectonic stress regime within the overriding plate may influence the degree of locking versus creep on the subduction interface fault. This new hypothesis may help to explain why some subduction zones lock up and produce great megathrust earthquakes while others do not.

Thickness of the chemical weathering zone and implications for erosional and climatic drivers of weathering and for carbon-cycle feedbacks

A. Joshua West, Dept. of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, California 90089, USA. Posted online 23 July 2012; doi: 10.1130/G33041.1.

The thin veneer at Earth's surface, known as the Critical Zone, plays host to a range of chemical reactions that are critical for generating and sustaining the resources that support life and shape the natural environment. The chemical breakdown of minerals -- known as chemical weathering -- is one of the most important of these chemical reactions. Until now, little has been known about where chemical weathering takes place, with soils typically thought to define the most important interface across which rocks break down chemically and physically. The analysis presented here challenges this assumption, indicating that chemical weathering takes place across a distributed zone that is often at least partly hosted in soils, but may also include bedrock, for example along fractures at depth. As a result, even landscapes with minimal soil may generate significant weathering fluxes. Moreover, because the total flux of material associated with erosion is high in these settings, they may play disproportionately important roles in global geochemical cycles. The ultimate message is that just because landscapes don't have soils doesn't mean that they are not significant players in the global exchange between the solid rock that makes up the bulk of the Earth and the life-sustaining surface.

Contact: Kea Giles
Geological Society of America

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