Public Release:  AGU journal highlights -- July 16, 2013

American Geophysical Union

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

In this release:

  1. Why freshwater organisms survived the asteroid that killed the dinosaurs
  2. Constraining bubbling of methane from thermokarst lakes
  3. Low-cost solution optimizes water quality of reservoir effluent
  4. Characterizing the dynamics of geyser eruptions
  5. Seismic studies provide new detail on transition zone below western US
  6. New global maps of surface ocean currents made from drifter data

Anyone may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://onlinelibrary.wiley.com/ and inserting into the search engine the full doi (digital object identifier), e.g. 10.1002/jgrg.20086. The doi is found at the end of each Highlight below.

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1. Why freshwater organisms survived the asteroid that killed the dinosaurs

Roughly 65.5 million years ago, a massive asteroid smashed into present-day Chicxulub, Mexico. The impact set fire to Earth's surface. Dust and ash darkened the sky, sending the planet into an "impact winter" that lasted months to years and caused the extinction of nonavian dinosaurs and half of ocean-dwelling species. However, life in inland freshwater ecosystems largely escaped this fate. To try to understand why freshwater organisms held on while ocean life failed, Robertson et al. surveyed relevant research to understand how the mechanisms of extinction would have operated differently in the two environments.

Life in rivers and lakes, as well as the oceans, would have been protected from the initial blast of heat from the asteroid impact. Previous research suggests that the heat would have evaporated the upper half-centimeter (0.2 inches) of water, but that temperatures at depth would have been largely unaffected. In the impact winter that followed, however, a lack of sunlight would have stalled photosynthetic production. Without photosynthesis, the decomposition of existing organic matter could have caused widespread hypoxia. In addition, the veil of dust and ash would have caused temperatures to drop.

Though all of these mechanisms would have operated in both the oceans and inland waterways, the authors propose that biological adaptations and differing physical processes could have made freshwater ecosystems more resilient--an argument supported by fossil evidence previously gathered in Montana's Hell Creek formation. They hypothesize that freshwater organisms, often accustomed to annual freeze-thaw cycles and periodic hypoxia, would have held up better to the impact winter conditions. Fast-flowing river water could have reoxygenated inland waterways. Furthermore, groundwater seeps could have kept freshwater ecosystems warm and supplied with organic matter. And, even in normal conditions, many freshwater organisms have dormant stages, including eggs or adults buried in mud, that allow them to await the return of more clement conditions. Since the Chicxulub impact winter only lasted from 6 months to 2 years, the authors suggest that these differences could have helped freshwater species hold on until the sky cleared.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1002/jgrg.20086, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrg.20086/abstract

Title: K-Pg extinction patterns in marine and freshwater environments: The impact winter model

Authors: Douglas S. Robertson: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA;

William M. Lewis: Department of Ecology and Evolutionary Biology and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA;

Peter M. Sheehan: Milwaukee Public Museum, Milwaukee, Wisconsin, USA;

Owen B. Toon: Department of Atmospheric and Oceanic Sciences and Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA.


2. Constraining bubbling of methane from thermokarst lakes

In northern thermokarst lakes, which form in depressions left as permafrost thaws, methane, a greenhouse gas, can be released from lake sediments to the atmosphere through bubbling, or ebullition. Constraining the amount of methane released through bubbling would help scientists understand the role of thawing permafrost in the carbon cycle and global climate change. However, bubbling is highly variable in both space and time and thus difficult to measure accurately, so there are large uncertainties in estimates of methane emissions from northern ecosystems.

Walter Anthony and Anthony sought to better understand the spatial distribution of bubbling in lakes. They note that in many northern lakes, the bubbling sources, which they call ebullition seeps, cluster together in regular spatial patterns. They combine field data from individual seeps with models to describe the spatial patterns of ebullition in three thermokarst lakes in different regions of Alaska. The authors used these models to create simulated ebullition data sets, on which they tested various methods for estimating lake ebullition. They find that the standard method of measuring ebullition with randomly placed bubble traps is biased toward underestimating methane flux, while a method using survey transects to map ebullition bubbles trapped in lake ice only slightly underestimated methane flux. The authors suggest that transect field data from a large number of widely distributed lakes can be combined to give a good estimate of regional lake ebullition.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1002/jgrg.20087, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrg.20087/abstract

Title: Constraining spatial variability of methane ebullition seeps in thermokarst lakes using point process models

Authors: Katey M. Walter Anthony and Peter Anthony: Water and Environmental Research Center, University of Alaska Fairbanks.


3. Low-cost solution optimizes water quality of reservoir effluent

A large reservoir can provide reliable access to water, control flooding, and be used to generate hydroelectricity. On the other hand, large dams can upset local ecosystems by changing river flow patterns or by affecting nutrient and oxygen concentrations in downstream flows. Damming a river and constructing a reservoir requires balancing these benefits and risks.

In tropical reservoirs, water often becomes highly stratified, with nutrient-depleted but well-oxygenated upper layers and nutrient-rich but oxygen-depleted waters at depth. When water is let out of the reservoir to power turbines or stave off drought, its quality has consequences for life downstream.

Engineers plan to use the dam at the Itezhi-Tezhi Reservoir that blocks the Kafue River in Zambia for power generation. Compared with more-industrialized regions, the Kafue River area is poor in nutrients, as farmers do not use fertilizers. Therefore, the presence of the Itezhi-Tezhi Reservoir is currently removing nutrients needed for productivity in the downstream regions. Using the Itezhi-Tezhi Reservoir as a case study, Kunz et al. devise an approach that should allow for optimizing the quality of the water being loosed from a reservoir without affecting the dam's capacity for power generation.

The authors' approach revolves around drawing from the two layers of the stratified reservoir water, balancing the mix of oxygenated shallower water and nutrient-rich deeper water. The authors note that though technology exists to re-oxygenate depleted reservoir water, the necessary equipment is often expensive to operate and maintain. Using a biogeochemical model of nutrient cycling, the authors calculate that by using a mix of water from around 13 meters (43 feet) depth and from more than 30 meters (about 100 feet) depth from the Itezhi-Tezhi Reservoir, they could maximize the nutrient load while avoiding releasing hypoxic water.

Source: Water Resources Research, doi:10.1002/wrcr.20358, 2013 http://onlinelibrary.wiley.com/doi/10.1002/wrcr.20358/abstract

Title: Optimizing turbine withdrawal from a tropical reservoir for improved water quality in downstream wetlands

Authors: Manuel J. Kunz, David B. Senn, Bernhard Wehrli and Alfred Wüest: Eawag: Swiss Federal Institute of Aquatic Science and Technology, Surface Waters - Research and Management, Kastanienbaum, Switzerland, and Institute of Biogeochemistry and Pollutant Dynamics, Zurich, Switzerland;

Elenestina M. Mwelwa: Hydrology Department, ZESCO Limited, Lusaka, Zambia.


4. Characterizing the dynamics of geyser eruptions

Some geysers have predictable eruptions that make them ideal for study. Understanding geyser eruption dynamics can provide insight into other intermittent natural processes, such as volcanic eruptions, but most studies of geysers have focused on the processes that trigger geyser eruption. The eruption jet dynamics are less well characterized.

In one of the most comprehensive studies of geyser eruption dynamics to date, over the course of a 4-day experiment at Lone Star Geyser in Yellowstone National Park, Karlstrom et al. measured water discharge, acoustic emissions, and infrared intensity, and recorded visible and infrared video of the geyser. They selected Lone Star Geyser because it has relatively frequent and vigorous eruptions, about every 3 hours.

They find that the eruption cycle has four distinct phases: first, a main eruptive phase with liquid and steam fountaining and maximum jet velocities of 16 to 28 meters per second (36 to 63 miles per hour), concluded by a period in which the ratio of erupting steam to liquid increases; second, a posteruption phase with no discharge but periodic acoustic and infrared emissions; third, a recharge period during which the geyser cone refills; and fourth, a "preplay" period characterized by a series of 5- to 10-minute pulses of steam, periodic acoustic emissions, and small volumes of liquid water discharge. The authors also estimate the total heat output from the geyser to be about 1.4 megawatts, which is less than 0.1 percent of the total heat output from Yellowstone caldera.

Source: Journal of Geophysical Research-Solid Earth, doi:10.1002/jgrb.50251, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrb.50251/abstract

Title: Eruptions at Lone Star Geyser, Yellowstone National Park: Energetics and eruption dynamics

Authors: Leif Karlstrom: Department of Geophysics, Stanford University, Stanford, California;

Shaul Hurwitz, Fred Murphy and Malcolm J. S. Johnston: U.S. Geological Survey, Menlo Park, California;

Robert Sohn: Woods Hole Oceanographic Institute, Woods Hole, Massachusetts;

Jean Vandemeulebrouck: ISTerre, Universite de Savoie, CNRS, Le Bourget-du-Lac, France;

Maxwell L. Rudolph: Department of Physics, University of Colorado, Boulder, Colorado;

Michael Manga: Department of Earth and Planetary Science, McCone Hall University of California at Berkeley, Berkeley, California;

R. Blaine McCleskey: U.S. Geological Survey, Boulder, Colorado.


5. Seismic studies provide new detail on transition zone below western US

At certain depths in Earth's mantle, the increasing pressure causes minerals to undergo phase changes, transforming to different crystal structures. Seismic waves change speed at these discontinuities, so analyzing seismic waves gives scientists information about the structure of the mantle.

To gain more detail about the fine structure of the transition zone beneath the western United States--the zone between the upper and lower mantles, bounded by discontinuities at 410 kilometers (255 miles) depth and 660 kilometers (410 miles) depth--Tauzin et al. analyzed seismic waves recorded at seismic stations of the U.S. transportable array. For instance, they imaged the area where the Gorda plate, which subducted under northern California, flattens and causes uplift of the 410 kilometer (255 mile) discontinuity under northern Nevada.

They also find that the transition zone is thicker below Washington, Oregon, and Idaho. In addition, they identify minor negative discontinuities (where seismic wave velocity decreases with depth rather than increases) at around 350, 590, and 630 kilometer (217, 367 and 391 mile) depths, and show that the 350 kilometer (217 mile) and 590 kilometer (367 mile) discontinuities extend over a wide region under those states. The authors suggest that this might be related to increased water content in the transition zone there, or to a significant amount of oceanic material accumulated in the area.

Source: Journal of Geophysical Research-Solid Earth, doi:10.1002/jgrb.50182, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrb.50182/abstract

Title: Multiple transition zone seismic discontinuities and low velocity layers below western United States

Authors: B. Tauzin and Y. Ricard: Laboratoire de Geologie de Lyon, Terre, Planetes, Environnement, Universite Lyon, Villeurbanne Cedex, France;

R.D. van der Hilst: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA;

G. Wittlinger: Institut de Physique du Globe de Strasbourg, Ecole et Observatoire des Sciences de la Terre, CNRS and Universite de Strasbourg, Strasbourg Cedex, France.


6. New global maps of surface ocean currents made from drifter data

The Global Drifter Program, which began in 1979 in the tropical Pacific Ocean, now spans the globe with a total of 1,250 sensors. Tracked from orbit by satellites the array provides a means to observe global near-surface ocean circulation patterns.

To create an improved representation of the global ocean near-surface currents Lumpkin and Johnson devised a new technique to analyze drifter records. Using 6-hourly drifter observations from 1979 to 2012 the authors isolated the drifter movement caused by near-surface currents. They then calculated the current properties on a 0.5 by 0.5 degree grid. Their approach was novel in that the data were analyzed in elliptical bins, with the shape of each ellipse representing the range of variability of the surface currents observed within that bin. The authors model both seasonal variations and interannual changes caused by the El Niño--Southern Oscillation. In this way the authors were able to compile monthly climatologies for each bin, visually representing not just the mean current speed but also its variability. Knowing the variability for each model bin gives an indication of the importance of both mean and eddy-driven currents in each region.

In their model the authors represent many of the world's major ocean currents. They capture the seasonal cycle in current behavior, along with patterns and seasonal shifts in near-surface eddies.

Source: Journal of Geophysical Research-Oceans, doi:10.1002/jgrc.20210, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrc.20210/abstract

Title: Global ocean surface velocities from drifters: Mean, variance, El Niño-Southern Oscillation response, and seasonal cycle

Authors: Rick Lumpkin: NOAA/Atlantic Oceanographic and Meteorological Laboratory, Miami, Florida, USA;

Gregory C. Johnson: NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington, USA.

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Contact: Mary Catherine Adams
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E-mail: mcadams@agu.org

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