Recent seismic results on the U.S. East Coast continental margin show that the zone between rifted continental and normal oceanic crust consists of thick (up to 25 km), high seismic velocity (νp of 7.2–7.3 km s−1) crust, interpreted as mafic igneous rocks emplaced during Triassic/Jurassic continental rifting. The total volume of igneous rocks in this zone, which we call the East Coast Margin Igneous Province (ECMIP), may be as much as 2.7 × 106 km3, placing the ECMIP among the world's large igneous provinces. We constrain the composition and origin of the thick, igneous crust by using a compilation of laboratory measurements to predict P wave velocities for rocks with the compositions of liquids produced by partial melting of mantle rocks. The high‐velocity crust was produced from partial melting of mantle peridotite, with smaller melt fractions (<10%) but at higher average pressures (≥2.0 GPa) than beneath normal mid‐ocean ridges. This requires higher than normal asthenospheric potential temperatures during rifting and a lid of lithosphere above upwelling asthenosphere to limit the minimum pressure of melting. Production of thick igneous crust at small melt fractions requires that the vertical flux of asthenosphere during rifting exceeded the lateral flux of lithosphere due to extension; that is, mantle “upwelling” was more rapid than lithospheric “spreading.” Thick igneous crust is strongly asymmetrical, extending up to 2000 km along the margin but only for about 80–100 km seaward. The rapid seaward transition to oceanic crust with normal thickness and seismic velocity implies that the thermal anomaly and relatively rapid upwelling lasted for only 5–8 m.y. Moreover, there is no crustal thickness anomaly in the Central Atlantic, in contrast to the North Atlantic where the influence of the Iceland plume created thick crust in a belt spanning the ocean from Greenland to the Faeroes Islands. These factors seem to preclude formation of thick igneous crust in response to a deep‐seated mantle plume. The ECMIP may have formed when high upper mantle temperatures induced asthenospheric upwelling. Magmatism and seafloor spreading dissipated the thermal anomaly in the upper mantle, after which normal oceanic crust formed along the Mid‐Atlantic Ridge.

Details of Earth's shallow subsurface-a key component of the critical zone (CZ)-are largely obscured because making direct observations with sufficient density to capture natural characteristic spatial variability in physical properties is difficult. Yet this inaccessible region of the CZ is fundamental to processes that support ecosystems, society, and the environment. Geophysical methods provide a means for remotely examining CZ form and function over length scales that span centimeters to kilometers. Here we present a review highlighting the application of geophysical methods to CZ science research questions. In particular, we consider the application of geophysical methods to map the geometry of structural features such as regolith thickness, lithological boundaries, permafrost extent, snow thickness, or shallow root zones. Combined with knowledge of structure, we discuss how geophysical observations are used to understand CZ processes. Fluxes between snow, surface water, and groundwater affect weathering, groundwater resources, and chemical and nutrient exports to rivers. The exchange of gas between soil and the atmosphere have been studied using geophysical methods in wetland areas. Indirect geophysical methods are a natural and necessary complement to direct observations obtained by drilling or field mapping. Direct measurements should be used to calibrate geophysical estimates, which can then be used to extrapolate interpretations over larger areas or to monitor changing processes over time. Advances in geophysical instrumentation and computational approaches for integrating different types of data have great potential to fill gaps in our understanding of the shallow subsurface portion of the CZ and should be integrated where possible in future CZ research.

Palaeoceanographic data have been used to suggest that methane hydrates play a significant role in global climate change. The mechanism by which methane is released during periods of global warming is, however, poorly understood. In particular, the size and role of the free-gas zone below gas-hydrate provinces remain relatively unconstrained, largely because the base of the free-gas zone is not a phase boundary and has thus defied systematic description. Here we evaluate the possibility that the maximum thickness of an interconnected free-gas zone is mechanically regulated by valving caused by fault slip in overlying sediments. Our results suggest that a critical gas column exists below most hydrate provinces in basin settings, implying that these provinces are poised for mechanical failure and are therefore highly sensitive to changes in ambient conditions. We estimate that the global free-gas reservoir may contain from one-sixth to two-thirds of the total methane trapped in hydrate. If gas accumulations are critically thick along passive continental slopes, we calculate that a 5 degrees C temperature increase at the sea floor could result in a release of approximately 2,000 Gt of methane from the free-gas zone, offering a mechanism for rapid methane release during global warming events.

[1] Results from the first joint temperature and seismic reflection study of the ocean demonstrate that water mass boundaries can be acoustically mapped. Multichannel seismic profiles collected in the Norwegian Sea show reflections between the Norwegian Atlantic Current and Norwegian Sea Deep Water. The images were corroborated with a dense array of expendable bathythermographs and expendable conductivity-temperature depth profiles delineating sharp temperature gradients over vertical distances of $5-15 m at depths over which reflections occur. Fine structure from both thermohaline intrusions and internal wave strains is imaged. Low-amplitude acoustic reflections correspond to temperature changes as small as 0.03°C implying that seismic reflection methods can image even weak fine structure.

[1] Internal waves affect many important dynamical processes in the ocean, but in situ observations of internal waves are infrequent and spatially sparse. Here we show that remote sensing of internal waves by marine seismic reflection methods can provide quantitative information on internal wave energy and its spatial variability at high lateral resolution and full ocean depth over large volumes of the ocean. Seismic images of the Norwegian Sea water column show reflections that capture snapshots of finestructure displacements due to internal waves. Horizontal wave number spectra derived from digitized reflection horizons in the open ocean compare favorably to the Garrett-Munk tow spectrum of oceanic internal wave displacements. Spectra within 10 km laterally and 200 m vertically of the continental slope show enhanced energy likely associated with internal wave-sloping boundary interactions.Citation: Holbrook, W. S., and I. Fer (2005), Ocean internal wave spectra inferred from seismic reflection transects, Geophys.