Colleen A. Dalton

Geophysical and Geochemical Evidence for Deep Temperature Variations Beneath Mid-Ocean Ridges

Deep, Driving Temperatures Convection in Earths mantle is largely controlled by the physical properties of the mantle such as density and viscosity. Because these factors are influenced by both temperature and composition, it has been difficult to ascribe one as the primary control over mantle convection or explain the long-wavelength features associated with mid-ocean ridges. Examining correlations between a global seismic velocity model with constraints on the depth and geochemical signature of mid-ocean ridges, Dalton et al. (p. 80; see the Perspective by Kelley) suggest that large temperature variations extending into the upper mantle explain most of the geophysical and geochemical observations. Moreover, the analysis provides support for deeply rooted mantle plumes as the source of hot spot volcanism. Temperature variations in the upper mantle drive mantle convection. [Also see Perspective by Kelley] The temperature and composition of Earth’s mantle control fundamental planetary properties, including the vigor of mantle convection and the depths of the ocean basins. Seismic wave velocities, ocean ridge depths, and the composition of mid-ocean ridge basalts can all be used to determine variations in mantle temperature and composition, yet are typically considered in isolation. We show that correlations among these three data sets are consistent with 250°C variation extending to depths >400 kilometers and are inconsistent with variations in mantle composition at constant temperature. Anomalously hot ridge segments are located near hot spots, confirming a deep mantle-plume origin for hot spot volcanism. Chemical heterogeneity may contribute to scatter about the global trend. The coherent temperature signal provides a thermal calibration scale for interpreting seismic velocities located distant from ridges.

Financial Implications of Hypothetical San Francisco Bay Oil Spill Scenarios: Response, Socioeconomic, and Natural Resource Damage Costs

ABSTRACT This study provides a comprehensive examination of the use of trajectory modeling to estimate financial impacts of oil spills, including natural resource damages, response costs, and socioeconomic costs, as well as an opportunity to examine how spill size, oil type, response strategy, and probabilistic trajectory factors impact costs. The inclusion of NRDA, response, and socioeconomic costs in the modeling allows for an assessment of the relative proportion of NRDA costs to response and socioeconomic costs to further support the findings of past studies that refute the myth that NRDA costs are the overriding factors in most spill cases. The study demonstrates the overall financial and NRDA benefits of dispersant use. Estimated total bio-economic costs for oil spill scenarios involving four oil types and three spill sizes for two locations in San Francisco Bay, were modeled. Assuming present-day mechanical-only response, total costs range from

Rayleigh wave phase velocities in the Atlantic upper mantle

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MODELING FATES AND IMPACTS OF HYPOTHETICAL OIL SPILLS IN DELAWARE, FLORIDA, TEXAS, CALIFORNIA, AND ALASKA WATERS, VARYING RESPONSE OPTIONS INCLUDING USE OF DISPERSANTS

520 million. Estimated total bioeconomic costs...

Constraints on shear velocity in the cratonic upper mantle from Rayleigh wave phase velocity

Phase velocity in the period range 30–130 s is measured for approximately 10,000 fundamental-mode Rayleigh waves traversing the Atlantic basin. In order to isolate the signal of the oceanic upper mantle, paths with >30% of their length through continental upper mantle are excluded. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two-dimensional parameterization is utilized in order to capture perturbations to age-dependent structure. In both scenarios, phase velocity shows a strong dependence on seafloor age at all periods, with higher velocity associated with older seafloor. Removing age-dependent velocity from the 2-D phase-velocity maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. The age-dependent phase velocities for the Atlantic are not consistent with a half-space cooling model and are best explained by a plate cooling model with thickness of 75 km and mantle temperature of 1400°C. In contrast, age-dependent phase velocities for the Pacific basin determined by Nishimura and Forsyth (1989) can be fit reasonably well by a half-space cooling model with mantle temperature approximately 50°C warmer than the Atlantic. Comparison of Rayleigh wave phase velocity and fractionation-corrected Na concentrations in mid-ocean ridge basalts erupted at 87 axial ridge segments reveals a positive correlation coefficient that increases with period, as expected if along-ridge variations in mantle potential temperature are controlling both quantities.

Seafloor age dependence of Rayleigh wave phase velocities in the Indian Ocean

ABSTRACT Oil spill response may include use of chemical dispersants and in situ burning equipment, in addition to traditional mechanical response equipment. To evaluate the potential impacts of var...

How seismic waves lose energy

Seismic models provide constraints on the thermal and chemical properties of the cratonic upper mantle. Depth profiles of shear velocity from global and regional studies contain positive velocity gradients in the uppermost mantle and often lack a low-velocity zone, features that are difficult to reconcile with the temperature structures inferred from surface heat-flow data and mantle-xenolith thermobarometry. Furthermore, the magnitude and shape of the velocity profiles vary between different studies, impacting the inferences drawn about mantle temperature and composition. In this study, forward modeling is used to identify the suite of one-dimensional shear-velocity profiles that are consistent with phase-velocity observations made for Rayleigh waves traversing Precambrian cratons. Two approaches to the generation of 1-D models are considered. First, depth profiles of shear velocity are predicted from thermal models of the cratonic upper mantle that correspond to a range of assumed values of mantle potential temperature, surface heat flow, and radiogenic heat production in the lithosphere. Second, shear velocity depth profiles are randomly generated. In both cases, Rayleigh wave phase velocity is calculated from the Earth models, and acceptable models are identified on the basis of comparison to observed phase velocity. The results show that it is difficult but not impossible to find acceptable Earth models that contain a low-velocity zone in the upper mantle, and that temperature structures that are consistent with constraints from mantle xenoliths yield phase-velocity predictions lower than observed. For most acceptable randomly generated Earth models, shear velocity merges with the global average at approximately 300 km. This article is protected by copyright. All rights reserved.

The mean composition of ocean ridge basalts

Variations in the phase velocity of fundamental-mode Rayleigh waves across the Indian Ocean are determined using two inversion approaches. First, variations in phase velocity as a function of seafloor age are estimated using a pure-path age-dependent inversion method. Second, a two-dimensional parameterization is used to solve for phase velocity within 1.25° × 1.25° grid cells. Rayleigh wave travel time delays have been measured between periods of 38 and 200 s. The number of measurements in the study area ranges between 4139 paths at a period of 200 s and 22,272 paths at a period of 40 s. At periods < 100 s, the phase velocity variations are strongly controlled by seafloor age and shown to be consistent with temperature variations predicted by the half-space-cooling model for a mantle potential temperature of 1400°C. The inferred thermal structure beneath the Indian Ocean is most similar to the structure of the Pacific upper mantle, where phase velocities can also be explained by a half-space-cooling model. The thermal structure is not consistent with that of the Atlantic upper mantle, which is best fit by a plate-cooling model and requires a thin plate. Removing age-dependent phase velocity from the 2-D maps of the Indian Ocean highlights anomalously high velocities at the Rodriguez Triple Junction and the Australian-Antarctic Discordance and anomalously low velocities immediately to the west of the Central Indian Ridge.

The global attenuation structure of the upper mantle

Energy loss varies with frequency in the lithosphere but not in the asthenosphere Ocean basins record the life history of a tectonic plate—its creation at a mid-ocean ridge, its thickening over time, and its consumption at a subduction zone. The movement of tectonic plates is possible because the lithosphere, Earths stiff outermost shell, slides on top of a weak asthenosphere. Despite its fundamental role in facilitating plate tectonics, the nature of the lithosphere-asthenosphere boundary is poorly understood. The asthenosphere is on average warmer than the lithosphere, but the temperature contrast alone may not provide the necessary viscosity reduction. Previous work has also proposed a dehydrated lithosphere and damp asthenosphere (1), and a solid lithosphere and partially molten asthenosphere (2). On page 1593 of this issue, Takeuchi et al. (3) present an analysis of aftershocks of the 2011 Tohoku earthquake and show how the attenuation of seismic waves has a different frequency response in the lithosphere versus the asthenosphere.