AGU Journal highlights -- Jan. 10, 2008

The structure, shape, and properties of mid-ocean ridges depend on the rate at which plates separate. As spreading rates decrease, magma cools and the lithosphere thickens along the ridge axis. At ultraslow spreading rates, the ridge axis becomes sufficiently cold that magma crystallizes and volcanism is limited to localized centers widely spaced along the ridge. Noting that some slow spreading ridges adopt ultraslow characteristics when their axes are oblique to the spreading direction, Montési and Behn analyze oblique ridges worldwide. Through comparing observations with theoretical calculations, the authors verify that the thermal structure and crustal thickness beneath an oblique ridge are controlled by the ridge’s effective spreading rate, defined as the contribution to spreading that is perpendicular to the oblique axis. Through these calculations, the authors define the threshold effective spreading rate at which spreading shifts from slow to ultraslow, and conclude that this transition is not related to a change in melt productivity, but rather to the efficiency with which magma is able to breach the surfaces.

Title:

Mantle flow and melting underneath oblique and ultraslow mid-ocean ridges

Authors:

Laurent G. J. Montési: Department of Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.; Now at Department of Geology, University of Maryland at College Park, Maryland, U.S.A;
Mark D. Behn: Department of Marine Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, U.S.A.

Source:

Geophysical Research Letters (GRL) paper 10.1029/2007GL031067, 2007; http://dx.doi.org/10.1029/2007GL031067

5. A younger, thinner Arctic ice cover: Increased potential for rapid, extensive sea ice loss

Over the past two decades, reductions in the amount of Arctic sea ice that survives the summer melt have resulted in more newly formed ice (first-year ice) and less of the thick, old ice that forms perennial ice cover. Though past studies have described the extent of multiyear ice, defined as ice that survives at least one melt season, little is known about changes within the multiyear ice cover itself. Using satellite-derived estimates of sea ice age and thickness, Maslanik et al. constructed a thickness record for 1982 to the present. The authors find that 58 percent of multiyear ice now consists of relatively young and thin 2- to 3-year-old ice, compared with 35 percent in the mid-1980s, with nearly complete loss of the oldest, thickest ice. The authors expect that this decline helps to expose more open water, which in turn increases the absorption rate of solar energy. Not only do such feedbacks help explain recent large ice loss trends, but they also increase the potential of the current younger, thinner Arctic ice cover to rapidly melt.

Title:

A younger, thinner Arctic ice cover: Increased potential for rapid extensive sea-ice loss