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AGU journal highlights -- Jan. 13, 2012

The following highlights summarize research papers that have been recently published in Journal of Geophysical Research-Oceans (JGR-C), Journal of Geophysical Research-Planets (JGR-E), Journal of Geophysical Research- Biogeosciences (JGR-G), and Geophysical Research Letters (GRL).

In this release:

  1. Variability of North Atlantic heat transport observed from instrument data
  2. Methane exceeds nitrous oxide in rivers' contribution to warming
  3. Waste recycling primary source of energy in deep ocean
  4. Record Arctic ozone depletion could occur again
  5. Traveling supraglacial lakes observed on Antarctic ice shelf
  6. Lunar images alter understanding of impact history

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 and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2011GL049801. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions who are registered with AGU also may download papers cited in this release by clicking on the links below. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at

1. Variability of North Atlantic heat transport observed from instrument data

The Atlantic meridional overturning circulation (AMOC), which transports warm water northward and cold water back southward, redistributes energy throughout the North Atlantic Ocean. Some models predict that AMOC will slow down as Earth's temperatures rise due to anthropogenic warming, which could have serious climate consequences for the Northern Hemisphere. However, the response of AMOC to global warming is uncertain-different models predict different rates of slowdown-and there have been few continuous observations of AMOC heat transport. Two research groups have analyzed almost 10 years of ocean instrument data and compared it to the models' predictions.

Drawing on a decade of observations from the Meridional Overturning Variability Experiment (MOVE), Send et al. report for the first time the detection of strong interannual and decadal shifts in the strength of the AMOC. MOVE comprises moored instruments at three sites spread east-west over a 1,000-kilometer (621- mile) region near the Antilles Islands in the western tropical Atlantic that have been collecting temperature, conductivity, and pressure measurements since 2000. The information allowed the authors to calculate how the southbound stretch of the AMOC varied from January 2000 to June 2009. They find that the cold-water flow rate could vary substantially, ranging between 12 and 30 Sv (1 Sv = 1 million cubic meters per second, or 35 million cubic feet per second) during the study period. Further, they show that the AMOC's flow rate declined by 3 Sv over the 10-year period, corroborating recent modeling efforts. On the basis of the intricacies of their observations, and on the agreement with circulation models, the authors suggest that the decade-long drop in the strength of the AMOC is the product of long-term natural variability and need not be a consequence of recent climate change.

In a separate study, Hobbs and Willis used temperature, salinity, and displacement data measured from floats in the Argo array, combined with sea surface heights measured by satellites, to estimate a continuous time series of Atlantic meridional heat transport from 2002 to 2010 at 41 degrees north latitude. They find the mean heat transport is about 0.5 petawatts. The authors note that this estimate is consistent with previous studies in similar latitudes based on atmospheric flux data but is lower than most hydrographic estimates. Heat transport varies on an annual cycle as well as on shorter time scales, with atmospheric variability explaining most of the short-term variance. Hobbs and Willis note that the period of study was too short to infer any long-term trends, and they emphasize the need for continued monitoring of AMOC.

Send et al. Source: Geophysical Research Letters, doi:10.1029/2011GL049801, 2011

Title: Observation of decadal change in the Atlantic meridional overturning circulation using 10 years of continuous transport data

Authors: Uwe Send and Matthias Lankhorst: Scripps Institution of Oceanography, La Jolla, California, USA;

Torsten Kanzow: Leibniz Institute of Marine Sciences, Kiel, Germany.

Hobbs and Willis Source: Journal of Geophysical Research-Oceans, doi:10.1029/2011JC007039, 2012

Title: Midlatitude North Atlantic heat transport: A time series based on satellite and drifter data

Authors: Will R. Hobbs and Joshua K. Willis: NASA Jet Propulsion Laboratory, California, Institute of Technology, Pasadena, California, USA.

2. Methane exceeds nitrous oxide in rivers' contribution to warming

Nitrous oxide (N2O) emissions have been the leading area of concern for scientists investigating the role of streams and rivers in global climate change for the past decade. A potent greenhouse gas, nitrous oxide is produced in riverbed sediments through nitrification and denitrification. Efforts to understand the rate at which nitrous oxide diffuses through the water to the atmosphere have dominated the field, yet diffusion is not the only relevant mechanism nor is nitrous oxide the only relevant gas. Observations by Baulch et al. suggest that the global warming potential of methane gas, which they measured bubbling up from several riverbeds, exceeds that of nitrous oxide.

Gases produced in river sediments can travel to the atmosphere by diffusing through the water column, escaping as bubbles, or through plant-facilitated transport. The authors measured methane and nitrous oxide concentrations in the water and in riverbed bubbles and measured bubble accumulation in surface bubble traps for four Ontario streams to sort out whether diffusion or ebullition is dominant for each gas. They find that 10 to 80 percent of methane emissions are in the form of bubbles, while nitrous oxide emissions are almost completely through diffusion.

Additionally, the authors used streambed sediment samples to identify a relationship between gas emissions and sediment properties. They find that high levels of fine materials such as silt or clay are associated with increased emissions of both nitrous oxide and methane. They suspect the fine sediments could limit the availability of oxygen in the sediment. Depleted oxygen levels increase rates of denitrification and methanogenesis, thus increasing gas production rates. The authors also find that methane bubbles surpass diffused nitrous oxide in terms of global warming potential, which they suggest could warrant a rethinking of the importance of streams and rivers to global warming.

Source: Journal of Geophysical Research-Biogeosciences, doi:10.1029/2011JG001656, 2011

Title: Diffusive and ebullitive transport of methane and nitrous oxide from streams: Are bubble-mediated fluxes important?

Authors: Helen M. Baulch: School of Environment and Sustainability and the Global Institute for Water Security, National Hydrology Research Centre, University of Saskatchewan, Saskatoon, Saskatchewan, Canada;

Peter J. Dillon: Department of Environmental and Resource Studies, Trent University, Peterborough, Ontario, Canada;

Roxane Maranger: Department of Biological Sciences, University of Montreal, Montreal, Quebec, Canada;

Sherry L. Schiff: Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, Ontario, Canada.

3. Waste recycling primary source of energy in deep ocean

In the dark reaches of the deep ocean, far from the photosynthesizing plants and plankton that fuel life in the surface waters, ecosystems survive on chemical energy. Decades of research on the life that clusters around deep-sea hydrothermal vents has hinted at the importance of light-free food webs, but a recent analysis by Middelburg suggests that another system-waste recycling-could be the dominant pillar of life on the abyssal plains.

The realization was a result of the author's attempt to calculate the importance of chemoautotrophy to the carbon cycles of different ocean regions. Chemoautotrophs are single-celled creatures that consume carbon dioxide and other inorganic materials and convert them to forms that can then be used by other organisms-a process known as carbon fixation. The author specifically focused on chemoautotrophs that feed on biological waste, finding it to be, for the deep ocean, the largest source of new organic carbon.

However, the author's investigation was not limited to the deep ocean. He finds that for the global ocean, chemoautotrophy of inorganic waste could account for the production of 0.77 petagrams (Pg) of carbon per year. The bulk of the carbon fixation, 0.29 Pg carbon per year , occurred in the surface waters. A further 0.29 Pg carbon per year was produced in coastal and continental shelf sediments, while chemoautotrophy in the dark ocean fixed 0.11 Pg carbon per year. While the role of chemoautotrophy in supplying the ocean with organic carbon was nowhere near that of photosynthesis, it did rival the contributions of other important sources, like carbon carried in river effluent.

Source: Geophysical Research Letters, doi:10.1029/2011GL049725, 2011

Title: Chemoautotrophy in the ocean

Author: Jack J. Middelburg: Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Utrecht, Netherlands.

4. Record Arctic ozone depletion could occur again

In the winter of 2010-2011, ozone levels above the Arctic declined to record lows, creating the first Arctic ozone hole, similar to the well-known Antarctic ozone hole. Scientists believe the ozone depletion was due partly to unusually cold temperatures in the stratosphere above the Arctic, as colder stratospheric temperatures make ozone-destroying chemicals such as chlorine more active. As global climate change continues, the Arctic stratosphere is expected to get colder, but levels of ozone-destroying chemicals should decline, as emissions of these chemicals were banned by the Montreal Protocol.

To try to learn more about Arctic ozone dynamics and determine whether the Arctic ozone hole is likely to recur, Sinnhuber et al. looked at satellite observations of temperature, ozone, water vapor, and chemicals that affect ozone in the Arctic atmosphere. They also used a model to determine how sensitive ozone levels are to stratospheric temperatures and chemistry. They find that their model accurately reproduced measured conditions. Their model suggests that stratospheric temperatures 1 degree Celsius (1.8 degrees Fahrenheit) lower than in the 2010-11 winter would result in locally nearly complete ozone depletion in the Arctic lower stratosphere with current levels of chemicals. A 10 percent reduction in ozone- depleting chemicals would be offset by a 1 degree Celsius decrease in stratospheric temperatures. The researchers conclude that although ozone-depleting substances should decline in coming decades, temperature changes could offset those effects, potentially leading to future severe Arctic ozone depletions similar to that during the winter of 2010-11.

Source: Geophysical Research Letters, doi:10.1029/2011GL049784, 2011

Title: Arctic winter 2010/2011 at the brink of an ozone hole

Authors: B.-M. Sinnhuber, G. Stiller, R. Ruhnke, T. von Clarmann, and S. Kellmann: Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany;

J. Aschmann: Institute of Environmental Physics, University of Bremen, Bremen, Germany.

5. Traveling supraglacial lakes observed on Antarctic ice shelf

A sequence of lakes on top of the George VI ice shelf in Antarctica has been observed to move along the boundary of the ice shelf with Alexander Island. LaBarbera and MacAyeal analyzed satellite images of the George VI ice shelf in Antarctica taken from 2001 through 2010. They find that the supraglacial lakes move in a direction and with a speed that differs from ice shelf flow: parallel to the grounding line of the ice shelf, in the manner of a traveling wave. The authors present a simple model to show how the lakes form in the depressions associated with compressions of the ice sheet. The study could help researchers better understand the dynamics of the ice shelf.

Source: Geophysical Research Letters, doi:10.1029/2011GL049970, 2011

Title: Traveling supraglacial lakes on George VI Ice Shelf, Antarctica

Authors: C. H. LaBarbera: Department of Geology, Cornell College, Mount Vernon, Iowa, USA.

D. R. MacAyeal: Department of Geophysical Sciences, University of Chicago, Chicago, Illinois, USA.

6. Lunar images alter understanding of impact history

New images from the Lunar Reconnaissance Orbiter could change our view of the history of impacts on the Moon. In 1972 the Apollo 17 mission took samples from the region of the Serenitatis impact basin. Scientists believed that the impact that created the Serenitatis basin was responsible for the formation of the Apollo 17 impact melt samples, which were dated to 3.8 billion years ago. A geological feature known as the Sculptured Hills, which surround the Apollo 17 landing site, was also believed to have been created from ejecta from the Serenitatis impact.

The new images show that the Sculptured Hills lie on top of the rims of several craters known to be created after the Serenitatis basin. In addition, the Sculptured Hills have similar morphology to features created by ejecta from the impact that formed the nearby Imbrium crater. Thus, Spudis et al. argue that the Sculptured Hills, and possibly also the Apollo 17 samples, may have been formed from the Imbrium impact, not the Serenitatis. The Serenitatis basin could be older than scientists had thought, the authors suggest.

If the Apollo 17 samples are indeed formed from the Imbrium impact, not the Serenitatis, scientists will have to revise their understanding of the impact melting process of large body impacts because it was based in part on the assumption that those samples were formed from the Serenitatis impact. Alternatively, if the Apollo 17 samples are indeed from the Serenitatis basin-forming impact and the Serenitatis basin is 3.8 billion years old, then many of the impact basins and large craters on the Moon all must have been created within a short 30-million-year time window around 3.8 billion years ago-definitely a global impact "cataclysm"-the authors suggest. Either of these interpretations changes scientists' view of the impact process and the history of the Moon.

Source: Journal of Geophysical Research-Planets, doi:10.1029/2011JE003903, 2011

Title: The Sculptured Hills of the Taurus Highlands: Implications for the relative age of Serenitatis, basin chronologies and the cratering history of the Moon

Authors: Paul D. Spudis: Lunar and Planetary Institute, Houston, Texas, USA;

Don E. Wilhelms: U. S. Geological Survey (retired), San Francisco, California, USA;

Mark S. Robinson: ASU School of Earth and Space Exploration, Tempe, Arizona, USA.


Contact: Mary Catherine Adams
American Geophysical Union

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