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AGU journal highlights for April 16, 2012

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Journal of Geophysical Research-Solid Earth (JGR-B), and Journal of Geophysical Research-Oceans (JGR-C).

1. Active faults newly identified in Pacific Northwest

The Bellingham Basin, which lies north of Seattle and south of Vancouver, British Columbia, around the border between the United States and Canada in the northern part of the Cascadia subduction zone, is important for understanding the regional tectonic setting and current high rates of crustal deformation in the Pacific Northwest. Using a variety of new data, Kelsey et al. identify several active faults in the Bellingham Basin that have not been previously known. These faults lie more than 60 kilometers (37 miles) farther north of the previously recognized northern limit of active faulting in the area. The authors note that the newly recognized faults could produce earthquakes with magnitudes between 6 and 6.5 and thus should be considered in hazard assessments for the region.

Source: Journal of Geophysical Research-Solid Earth, doi:10.1029/2011JB008816, 2012

Title: Holocene faulting in the Bellingham forearc basin: Upper-plate deformation at the northern end of the Cascadia subduction zone

Authors: Harvey M. Kelsey: Department of Geology, Humboldt State University, Arcata, California, USA;

Brian L. Sherrod: U.S. Geological Survey, University of Washington, Seattle, Washington, USA;

Richard J. Blakely: U.S. Geological Survey, Menlo Park, California, USA;

Ralph A. Haugerud: U.S. Geological Survey, University of Washington, Seattle, Washington, USA.

2. In the Greek isles, a volcano has awakened

In 1650 B.C.E., a series of massive volcanic eruptions decimated the ancient seafaring Minoan civilization. Over the next 4 millennia, the largely subaquatic Santorini caldera had a series of smaller eruptions, with five such events within the past 600 years, and ending most recently in 1950. From the air, the Santorini caldera appears as a small cluster within the larger collection of Greek islands in the southern Aegean Sea. Following a 60-year lull, Santorini woke up on 9 January 2011 with a swarm of low-magnitude earthquakes.

A GPS monitoring system installed in the area in 2006 gave Newman et al. a stable background against which to compare the effects of the reawakened volcano. By June 2011 the regional GPS stations showed that they had been pushed 5-32 millimeters (0.2-1.3 inches) farther from the caldera than they had been just six months earlier. Following these initial results, the authors bolstered the GPS network and conducted a more extensive survey in September 2011, which confirmed that the land near the volcano was swelling. Continued monitoring from September through January 2012 showed the expansion was accelerating, reaching a rate of 180 mm (7 in) per year.

Using a model that interpreted the source of the deformation as an expanding sphere, the authors suggest that the expansion is due to an influx of 14.1 million cubic meters (498 million cubic feet) of magma into a chamber 4-5 kilometers (2.5-3.1 miles) below the surface. The authors suggest that the ongoing expansion is not necessarily the signal of an impending eruption, adding that the recent swelling represents only a fraction of that which led to the Minoan eruption. However, they warn that even a small eruption could trigger ash dispersion, tsunamis, landslides, or other potentially dangerous activity.

Source: Geophysical Research Letters, doi:10.1029/2012GL051286, 2012

Title: Recent Geodetic Unrest at Santorini Caldera, Greece

Authors: Andrew V. Newman: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, USA;

Stathis Stiros, Fanis Moschas, and Vasso Saltogianni: Department of Civil Engineering, University of Patras, Greece;

Lujia Feng: Nanyang Technological University, Earth Observatory of Singapore, Singapore;

Panos Psimoulis: Institute of Geodesy and Photogrammetry, Switzerland;

Yan Jiang: University of Miami, Rosenstiel School of Marine and Atmospheric Sciences, USA;

Costas Papazachos, Dimitris Panagiotopoulos, Eleni Karagianni, and Domenikos Vamvakaris: Geophysical Laboratory, Aristotle University of Thessaloniki, Greece.

3. Voyager 1 might have farther to go to exit to the heliosheath

The Voyager 1 spacecraft is exploring the outer heliosheath past about 111 astronomical units (AU) from the Sun. The heliosheath is the region where the outgoing solar wind is slowed by the interstellar medium. The Voyager 1 and Voyager 2 spacecraft have been sending back interesting new information about the structure of this previously uncharted boundary region at the edge of the solar system. Webber et al. now report that Voyager 1 recently observed two sudden increases in the intensity of low-energy cosmic ray electrons as the spacecraft traveled farther from the Sun. At the outer boundary of the heliosheath the electron intensity is usually assumed to be equal to that in interstellar space, outside the heliosphere. The authors suggest that the sudden changes in electron intensity are evidence of significantly different regions in the structure of the outer heliosheath. They also suggest that as of early 2012, Voyager 1 has not quite reached the undisturbed interstellar medium outside of the heliosheath.

Source: Geophysical Research Letters, doi:10.1029/2012GL051171, 2012

Title: Sudden intensity increases and radial gradient changes of cosmic ray MeV electrons and protons observed at Voyager 1 beyond 111 AU in the heliosheath

Authors: W. R. Webber: Department of Astronomy, New Mexico State University, Las Cruces, New Mexico, USA;

F. B. McDonald: Institute of Physical Science and Technology, University of Maryland, College Park, Maryland, USA;

A. C. Cummings and E. C. Stone: Space Radiation Laboratory, California Institute of Technology, Pasadena, California, USA;

B. Heikkila and N. Lal: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA.

4. El Nino related to changes in sardine spawning

The Pacific sardine, an important species for commercial fishing, spawns off the coast of California. The area and quality of its spawning habitat have been observed to vary from year to year, sometimes changing with El Nino-Southern Oscillation conditions, but smaller-scale mechanisms also play a role. To sort out the physical forcing mechanisms affecting sardine spawning, Song et al. used data assimilation, combining observational data on sea surface height, sea surface temperature, and salinity data with a physical ocean model. They then looked at how the physical mechanics could explain observed interannual variations in sardine spawning range and egg density.

The researchers find that increased wind-driven offshore transport during the April 2002 and 2007 La Nina conditions led to extension of the sardine spawning habitat farther offshore. In contrast, in April 2003, under El Nino conditions, spawning habitat was smaller but of higher quality, leading to a higher density of eggs and greater survival of larvae. Although there are still unknown factors that control sardine egg distribution, the work advances understanding of interannual variability in sardine spawning.

Source: Journal of Geophysical Research-Oceans, doi:10.1029/2011JC007302, 2012

Title: Application of a data-assimilation model to variability of Pacific sardine spawning and survivor habitats with ENSO in the California Current System

Authors: Hajoon Song: Department of Ocean Sciences, University of California, Santa Cruz, California, USA;

Arthur J. Miller: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA;

Sam McClatchie, Edward D. Weber, and Karen M. Nieto: Southwest Fisheries Science Center, NOAA, La Jolla, California, USA;

David M. Checkley Jr.: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA.

5. Lake Erie's thermal structure and circulation are backward

A series of high-resolution measurements has shown that Lake Erie, one of the North American Great Lakes, is, in some respects, backward. In the majority of thermally stratified lakes, the thermocline, a thin subsurface layer of rapid temperature change, is deeper near the coast than near the center of the lake. Lake Erie, however, has an inverted thermocline, which is deeper offshore than at the coast. Beletsky et al. first mapped this bowl-shaped thermocline during the summer of 2005 with a large network of temperature sensors.

In 2005, and again in 2007, moored instruments that collected temperature readings at 1 meter (3.3 feet) depth intervals were spread 30 to 50 kilometers (19 to 31 miles) apart around the central basin of the lake. Supporting these point measurements, the authors collected higher-resolution temperature profiles with a boat-towed sensor. The authors find that the 2 to 3 meter (6.5 to 10 feet) thick thermocline, which was most pronounced in late summer, sat up to 8 m (26 ft) deeper offshore than at the coast. In addition to the anomalous thermocline behavior, the authors find that the circulation of central Lake Erie flows in a direction opposite of most Northern Hemisphere lakes. Using circulation sensors placed on the lake floor, the authors observed a pronounced clockwise (anticyclonic) circulation.

The authors attribute the unusual circulation and thermocline patterns to anticyclonic winds that tend to blow over Lake Erie. Such anticyclonic winds would cause the warm surface waters to converge in the center of the lake, driving down the depth of the thermocline. They suggest that the depressed thermocline squashes the cool region near the lake bed, where many species hide from the summer heat. The depressed thermocline could also be responsible for amplifying deep-water summer hypoxia, reducing the oxygen available to lake-bottom plants and animals.

Source: Geophysical Research Letters, doi:10.1029/2012GL051002, 2012

Title: Summer thermal structure and anticyclonic circulation of Lake Erie

Authors: Dmitry Beletsky and Raisa Beletsky: CILER, School of Natural Resources and Environment, University of Michigan, Ann Arbor, Michigan, USA;

Nathan Hawley, Henry A. Vanderploeg, David J. Schwab, and Steven A. Ruberg: Great Lakes Environmental Research Laboratory, NOAA, Ann Arbor, Michigan, USA;

Yerubandi R. Rao: National Water Research Institute, Environment Canada, Burlington, Ontario, Canada.

6. Fossilized plant matter points to desertification near Tibetan Plateau

Roughly 22 million years ago, at the onset of the Miocene, the Tibetan Plateau started to lift upward. The rising land curbed the flow of moist air from the south, sparking the onset of central Asian desertification. Or, perhaps, the supposedly arid region to the northeast of the Tibetan Plateau harbored shallow lakes or wetlands until as recently as 8 million years ago, at which point the historical desertification was initiated by some other mechanism. The current debate between these two proposals, of either a 22- or 8-million-year-old onset of desertification, hinges, to a sizeable degree, on the history of the fine sediments of the Tianshui Basin in central China.

One line of research, which looked at grain sizes, rock magnetic properties, and bulk geochemistry, among other factors, suggested that the early Miocene sediments were transported to the Tianshui Basin by the wind. The existence of wind-borne sediment, known as loess, would support the 22-million-year desertification hypothesis. Other researchers, however, suggest that though many of the sediment properties are similar to loess, they also show a good match for lake bed or wetland material.

To discriminate between the two hypotheses, Peng et al. measured the chain length distributions of n-alkanes-a type of hydrocarbon found in waxy plant material-from preserved organic material found in Tianshui Basin sediment samples. Long n-alkane chains, those with 27-31 carbon atoms, mainly stem from terrestrial plants, while midsized chains, with 23-25 carbon atoms, derive from aquatic plants or wetland flora. The authors find an abundance of the shorter carbon chains in the Tianshui sediments, an important difference from nearby loess samples. Supported by the observation of preserved pollen and algae, the authors suggest that the Miocene Tianshui Basin was a lake, mudflat, or floodplain region and not the arid loess hypothesized by some researchers.

Source: Geophysical Research Letters, doi:10.1029/2012GL050934, 2012

Title: Biomarkers challenge early Miocene loess and inferred Asian desertification

Authors: Tingjiang Peng, Jun Zhang, and Zhengchuang Hui: Key Laboratory of Western China's Environmental Systems (Ministry of Education) and College of Earth and Environmental Sciences, Lanzhou University, Lanzhou, China;

Jijun Li: Key Laboratory of Western China's Environmental Systems (Ministry of Education) and College of Earth and Environmental Sciences, Lanzhou University, Lanzhou, China and College of Geography Science, Nanjing Normal University, Nanjing, China;

Chunhui Song: School of Earth Sciences and Key Laboratory of Western China's Environmental Systems (Ministry of Education), Lanzhou University, Lanzhou, China;

Zhijun Zhao: College of Geography Science, Nanjing Normal University, Nanjing, China;

John W. King: Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, USA.


Contact: Mary Catherine Adams
American Geophysical Union

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