Boulder, Colo., USA - The Geological Society of America's top journal, Geology, displays its multidisciplinary best in this latest posting. Earth science disciplines covered include geoarchaeology, climatology, invertebrate paleontology, sedimentology, geomorphology, seismology, planetary geology, geochemistry, glaciology, plate tectonics, mineralogy, and environmental and medical geology. Locations include Mars; Earth's moon; India; the Tibetan Plateau; the Saskatchewan River; L'Aquila, Italy; the Antarctic; Australia; the Andes; the San Andreas fault system; and Kume Island, Japan.
Highlights are provided below. GEOLOGY articles published ahead of print can be accessed online at http://geology.
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Abrupt weakening of the summer monsoon in northwest India ~4100 yr ago
Yama Dixit et al., Godwin Laboratory for Palaeoclimate Research, Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK. First published online on 24 Feb. 2014; http://dx.
In light of contemporary concerns for the future impact of climate change on humankind, the role of climate change in the collapse of ancient civilizations has become a topic of popular interest. One such time of widespread cultural transformation occurred about 4,000 years ago (4 ka), which marked the end of several early Bronze Age civilizations (Old Kingdom in Egypt, Akkadian Empire in Mesoptamia, Indus Civilization of India). One hypothesis to explain these cultural discontinuities is a "4.2 k.y. BP aridification event" [k.y. = thousand years; BP = before present]. Although a link between climate and the decline of Indus urbanism has been suggested by some and refuted by others, the issue remains unresolved mainly because of a lack of paleoclimate data from the actual region occupied by the Indus Civilization. We report a paleoclimatic record from Haryana, India, for an abrupt climate change at ~4.1 k.y. B.P. These data provide the first paleoclimate evidence for a weakening of the monsoon and shift toward drier climate on the plains of northwest India, supporting a possible role of climate in the transformation of the Indus civilization from an urbanized to village-based rural society.
Environmental change across a terrestrial Cretaceous-Paleogene boundary section in eastern Montana, USA, constrained by carbonate clumped isotope paleothermometry
Thomas S. Tobin et al., Department of Earth and Space Sciences, University of Washington, Box 351310, 4000 15th Avenue NE, Seattle, Washington 98105, USA. First published online on 24 Feb. 2014; http://dx.
In this study, Thomas S. Tobin and colleagues uses fossil mussel shells to determine how temperatures changed immediately prior to the end-Cretaceous mass extinction 66 million years ago. The prevailing hypothesis is that this extinction, which killed the dinosaurs (except birds) and many other widespread biological groups, was caused by a large asteroid impact. However, some scientists believe that the asteroid impact was not the sole reason many animals went extinct. Instead, these scientists think the severity of the extinction event may have been compounded by prior climate changes. Tobin and colleagues built a climate record for the end of the Cretaceous using a recently developed geochemical technique that allows us to determine the temperature in ancient terrestrial environments. They discovered that there appears to have been a temperature decline (~8 degrees Celsius or 14 degrees Fahrenheit) in summer temperatures beginning about 300,000 years before the end of the Cretaceous period. The temperature drop is contemporaneous with, and may be the cause of, declining measures of animal biodiversity over the same interval. Our data is consistent with the hypothesis that ecosystems were stressed by a changing climate, which may have contributed to the mass extinction event, and made the effects of the asteroid impact more severe.
Pleistocene drainage reorganization driven by the isostatic response to deep incision into the northeastern Tibetan Plateau
Huiping Zhang et al., Current address: U.S. Geological Survey, Reston, Virginia 20192, USA. First published online on 24 Feb. 2014; http://dx.
During Pleistocene time, the Yellow River progressive excavated previously connected Tertiary-Quaternary sedimentary basins in the northeastern Tibetan Plateau. Basin top fill surfaces within basins along the Yellow River provide reliable markers to quantify the erosional unloading. The isostatic response to this incision may have led to a tilting of the landscape and drainage reorganization that includes a reversal of drainage.
Channel enlargement by avulsion-induced sediment starvation in the Saskatchewan River
Norman D. Smith et al., Department of Earth & Atmospheric Sciences, University of Nebraska, Lincoln, Nebraska 68588-0340, USA. First published online on 24 Feb. 2014; http://dx.
River channels downstream of large dams commonly become larger after the dam is constructed. This happens because entrapment of river-borne sediment in the upstream reservoir causes outflowing water to become sediment-deficient and ultimately more erosive, resulting in widening, deepening, and sometimes coarsening of the channel bed. In a study of the Saskatchewan River in south-central Canada, Norman D. Smith and colleagues discovered that the process of avulsion (i.e., the natural diversion of flow from an established river channel to a new location in the floodplain) creates effects similar to dams by depositing the diverted river sediment on the floodplain surface, thereby creating erosive, sediment-deficient flows in downstream channels. Results of three surveys (1911, 1954-1956, 2011-2012) conducted on a 130-km channel segment downstream of avulsion deposits initiated in the 1870s show that enlargement has occurred in the upper 95 km, including a greater than 50% increase in average channel size in the upper 35 km. Because avulsions are basic features of aggrading river systems, this outcome bears on assessments of channel size, flood frequency, bank stability, and sediment budgets in both modern and ancient floodplains. Erosion by sediment-deficient flows exiting areas of avulsive deposition may be an important but unrecognized process in floodplain evolution.
Earthquakes and fault zone structure
Luisa Valoroso et al., Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy. First published online on 24 Feb. 2014; http://dx.
Earthquakes are one of the most damaging manifestations of the tectonic plate motions that deform the Earth's crust. In order to mitigate the risk to human populations and cultures, it is important to characterize the fault zone structure and related properties (such as permeability, anisotropy, fluid content, and lithology) which are essential for understanding earthquake mechanics and earthquake rupture nucleation, propagation and arrest. Most of our knowledge about fault structure derives from field studies of ancient faults. Here, by using precise aftershocks locations of the 2009 Mw 6.1 L'Aquila (Italy) earthquake, Luisa Valoroso and colleagues reconstruct the complex structure of a normal fault at a resolution directly comparable with field geological investigations and investigate the mechanisms that generate damage around faults. They observe minor antithetic and synthetic fault segments in both the hanging wall and footwall blocks; fault bendings; dilational jogs; and parallel slipping planes at the base of the seismogenic volume. The fault zone thickness varies along strike from 0.3 to 1.5 km at the fault terminations with an increase in geometrical complexity. The strong similarities between seismological and geological images of fault structure indicate that earthquakes have a key role in the evolution of fault architecture.
Rapid soil accumulation in a frozen landscape
Martin Schiller et al., Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen, Denmark. First published online on 24 Feb. 2014; http://dx.
Much of what we know about Mars comes from understanding processes in the upper reaches of the Antarctic Dry Valleys. Until now, large polygon structures covering the surface of central Beacon Valley, which are also present on Mars, have been thought to be frozen in time for millions of years. However, new data presented in this article by Martin Schiller and colleagues, provide evidence for soil accumulation within thousands of years within a single polygon structure potentially indicating that these surfaces are much more active than previously thought. This is a tantalizing find because it suggests that the apparent stable surfaces of cold desert environments such as that of Beacon Valley and Mars might be much more active than it appears at first glance.
Oxic facies and the Late Devonian mass extinction, Canning Basin, Australia
Annette D. George et al., School of Earth & Environment, University of Western Australia, Crawley, WA 6009, Australia. First published online on 24 Feb. 2014; http://dx.
The Late Devonian mass extinction is widely regarded as one of the "Big Five" biotic crises in the last 550 million years of Earth's history, yet the causal mechanisms remain arguably the most difficult to decipher. This paper presents results from the Canning Basin of northwestern Australia, where it has been known for many years that the well-preserved Late Devonian reef complexes record global changes in reef-building organisms through two major extinction events. Annette D. George and colleagues examined the sedimentary record in a drillcore and obtained geochemical data on organic content, carbon isotopic ratios, and inorganic elemental ratios. Their data do not support anoxic conditions as a major factor in causing extinctions in this region in contrast to many other Late Devonian localities worldwide. Instead, they interpret the effects of relative sea-level changes and sediment influx from the adjacent Kimberley landmass as significant regional controls causing stress-inducing environmental changes such as fluctuations in light, salinity, and nutrient composition and abundance.
Late Holocene fluctuations of Qori Kalis outlet glacier, Quelccaya Ice Cap, Peruvian Andes
Justin S. Stroup et al., Department of Earth Sciences, HB 6105 Fairchild Hall, Dartmouth College, Hanover, New Hampshire 03755, USA. First published online on 25 Feb. 2014; http://dx.
Understanding the causes of tropical glacial fluctuations is important for predicting the response of these glaciers to future climate change. Records of glacial fluctuations also provide insight about past climate. This study by Justin S. Stroup and colleagues provides the first comparison of the past extents of Quelccaya Ice Cap, the largest tropical ice mass, with the annually dated Quelccaya ice core records of accumulation and temperature (Thompson et al., 2013) and shows that temperature was the driving force of glacial expansion and retreat during the last millennium. Furthermore, the past fluctuations of other glaciers in tropical South America are similar to Quelccaya, indicating a regionally consistent pattern of past climate conditions. When compared with glacial fluctuations at higher latitudes, the results suggest that glaciers were larger than present and depositing moraines in both northern and southern hemispheres at about the same time, indicating that climate mechanisms which caused late Holocene cooling likely influenced a globally synchronous pattern of cooling.
Localized shear in the deep lithosphere beneath the San Andreas fault system
Heather A. Ford et al., Current address: Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520-8109, USA. First published online on 25 Feb. 2014; http://dx.
The San Andreas fault system accommodates a significant portion (~75%) of relative motion between the Pacific and North American plates. At the surface, deformation is accommodated by transform motion along the strike-slip faults within the fault system. In contrast, the distribution of deformation at lithospheric mantle depths is debated. In this study, Heather A. Ford and colleagues utilize scattered seismic waves to image contrasts in lithospheric mantle properties across the central segment of the San Andreas fault and at the Calaveras-Green Valley-Bartlett Springs faults (part of the San Andreas fault system) to the north. The correlation between the surface expressions of the faults, and the change in lithospheric mantle properties at depth, points to deformation on a narrow shear zone less than 50 km in width. This result supports the view that the mantle lithosphere behaves in a "plate-like" manner.
Apatites in lunar KREEP basalts: The missing link to understanding the H isotope systematics of the Moon
Romain Tartèse et al., Planetary and Space Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK. First published online on 25 Feb. 2014; http://dx.
Recent re-investigations of samples from the Moon using modern analytical techniques have been able to eventually measure indigenous lunar water, challenging the long-standing paradigm of a dry Moon. Mare basalts erupted more than three billion years ago indirectly sample the lunar mantle, and provide us with the opportunity to re-assess its volatile inventory. However, most basaltic magmas were affected by petrogenetic processes such as crystallization and degassing that modified the primary characteristics of indigenous lunar water. In this study, we have analyzed the water content and its isotopic composition in the volatile-bearing mineral apatite in some phosphorus-rich KREEP basalts. Because this type of basalt contains elevated contents of incompatible elements such as P, apatite likely crystallized earlier than in other types of lunar basalts. As a result, these apatites likely preserved information regarding the volatile inventory of lunar magmas before substantial late-stage modifications occurred. Overall, the data obtained on apatites in these KREEP basalts suggest that the H isotopic composition of the water they contain was similar to that of the Earth's interior. These findings may imply that significant amounts of water present in the Moon were inherited from the proto-Earth, surviving the purported Moon-forming impact event.
Evidence for a Noachian-aged ephemeral lake in Gusev crater, Mars
Steven W. Ruff et al., School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA. First published online on 25 Feb. 2014; http://dx.
The Mars Exploration Rover Spirit was remarkably successful in its exploration of Gusev crater, but seemed to have failed in its efforts to locate evidence of the ancient lake that drew it there. Notable discoveries were made related to the role of water, including identification of carbonate-rich outcrops dubbed Comanche. Originally, the carbonate was thought to be a product of volcanic hydrothermal activity. But scientists now recognize abundant evidence that the carbonate is more likely due to evaporative precipitation from an ephemeral lake early in the history of Gusev crater. Floodwaters entering the crater through the huge Ma'adim Vallis inlet channel, perhaps repeatedly, appear to have ponded with sufficient duration to alter the existing rocks and leave behind a residue of carbonates. This scenario adds significance to Gusev crater as a candidate landing site for future missions oriented toward the search for past life on Mars.
Lead concentration and isotopic composition in the Pacific sclerosponge (Acanthochaetetes wellsi) reflects environmental lead pollution
Kazuto Ohmori et al., Department of Natural History Sciences, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan. First published online on 25 Feb. 2014; http://dx.
Sclerosponges (coralline sponge) are possible powerful recorders of lead (Pb) change in ocean surface, because their skeletons contain 10 times higher Pb concentration than that of another carbonate proxy. In addition, sclerosponge skeletons grow slowly (0.01-1.34 mm/year), long living, up to several hundreds of years, can be found throughout geological time scale. In this study, Kazuto Ohmori and colleagues measured Pb concentration and isotopes in high-Mg calcite skeleton of the sclerosponge (Acanthochaetetes wellsi) collected from Kume Island, Japan, East China Sea, to assess the usefulness as a proxy for lead pollution history. Results show that the Pb/Ca time series recorded from 1967 through 2007 CE correspond to historical changes in atmospheric lead flux in anthropogenic aerosols. Pb isotopes in the sclerosponge skeleton document that the main source of lead emissions shifted from Japan (1970-1980 CE) to China (1995-2005 CE), as expected from the timing of legislation against the use of leaded gasoline in Japan and China. These results indicate that the skeleton of the Pacific sclerosponge is a powerful proxy to monitor environmental lead pollution. Applying this methodology to long-living and/or fossil specimens could be useful in determining the interannual variability of atmospheric transport and dynamics over geologic time scales.