Axial high topography and partial melt in the crust and mantle beneath the western Galapagos Spreading Center

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q12005, doi:10.1029/2008GC002100.The hot spot-influenced western Galápagos Spreading Center (GSC) has an axial topographic high that reaches heights of ∼700 m relative to seafloor depth ∼25 km from the axis. We investigate the cause of the unusual size of the axial high using a model that determines the flexural response to loads resulting from the thermal and magmatic structure of the lithosphere. The thermal structure simulated is appropriate for large amounts of cooling by hydrothermal circulation, which tends to minimize the amount of partial melt needed to explain the axial topography. Nonetheless, results reveal that the large axial high near 92°W requires that either the crust below the magma lens contains >35% partial melt or that 20% melt is present in the lower crust and at least 3% in the mantle within a narrow column (35% in the crust are considered unreasonable, it is likely that much of the axial high region of the GSC is underlain by a narrow region of partially molten mantle of widths approaching those imaged seismically beneath the East Pacific Rise. A narrow zone of mantle upwelling and melting, driven largely by melt buoyancy, is a plausible explanation.Ito was supported by grants NSF-OCE- 0327051 and NSF-OCE-0351234

Submesoscale Coherent Vortices in the Gulf Stream

International audienceSeismic images and glider sections of the Gulf Stream front along the U.S. eastern seaboard capture deep, lens-shaped submesoscale features. These features have radii of 5-20 km, thicknesses of 150-300 m, and are located at depths greater than 500 m. These are typical signatures of anticyclonic submesoscale coherent vortices. A submesoscale-resolving realistic simulation, which reproduces submesoscale coherent vortices with the same characteristics, is used to analyze their generation mechanism. Submesoscale coherent vortices are primarily generated where the Gulf Stream meets the Charleston Bump, a deep topographic feature, due to the frictional effects and intense mixing in the wake of the topography. These submesoscale coherent vortices can transport waters from the Charleston Bump's thick bottom mixed layer over long distances and spread them within the subtropical gyre. Their net effect on heat and salt distribution remains to be quantified. Plain Language Summary The interior of the ocean is populated by small-scale coherent vortices, which redistribute water properties on the scale of basins. These structures are very difficult to observe. They have no surface signature and small dimensions, on the order of 1-50 km, such that they are missed by satellites and sampled only by chance. Furthermore, climate-scale ocean models do not resolve these type of motions and do not take into account their impacts for the large-scale transport and distribution of heat, nutrients, and other materials. Understanding and parameterizing these phenomena within models is critical for a better prediction of climate. Here we present new observations of submesoscale coherent vortices from seismic images and glider sections in the region of the Gulf Stream. We use a numerical model at very high resolution to reproduce vortices with the same characteristics and to analyze their generation mechanism. These vortices are generated where the Gulf Stream interacts with a deep topographic feature called the Charleston Bump due to frictional effects and intense mixing in the wake of the topography. These vortices transport waters from the Charleston Bump's thick bottom mixed layer and act to spread them all around the subtropical gyre