Justin Revenaugh

Interpolating the isotopic composition of modern meteoric precipitation

An accurate representation of the spatial distribution of stable hydrogen and oxygen isotopes in modern precipitation is required for many hydrological, paleoclimate, and ecological applications. No standardized method for achieving such a representation exists, and potential errors associated with previously employed methods are not understood. Using resampling, we test the accuracy of interpolated bD and δ 1 8 O estimates made using four methods. Prediction error for all methods is strongly related to number of data and will likely decline with the addition of new data. The best method lowers estimation error by 10-15% relative to others tested and gives an average error, using all available data, 2.5% of the global range. We present and interpret global maps of interpolated δD, δ 1 8 O, and deuterium excess in precipitation and the 95% confidence intervals for these values created using the optimal method. These depict global and regional patterns, make evident the robustness of interpolated isotopic patterns, and highlight target areas for future precipitation sampling.

Quantifying surface water–groundwater interactions using time series analysis of streambed thermal records: Method development

[1] We present a method for determining streambed seepage rates using time series thermal data. The new method is based on quantifying changes in phase and amplitude of temperature variations between pairs of subsurface sensors. For a reasonable range of streambed thermal properties and sensor spacings the time series method should allow reliable estimation of seepage rates for a range of at least ±10 m d � 1 (±1.2 � 10 � 2 ms � 1 ), with amplitude variations being most sensitive at low flow rates and phase variations retaining sensitivity out to much higher rates. Compared to forward modeling, the new method requires less observational data and less setup and data handling and is faster, particularly when interpreting many long data sets. The time series method is insensitive to streambed scour and sedimentation, which allows for application under a wide range of flow conditions and allows time series estimation of variable streambed hydraulic conductivity. This new approach should facilitate wider use of thermal methods and improve understanding of the complex spatial and temporal dynamics of surface water–groundwater interactions.

Mantle layering from ScS reverberations: 2. The transition zone

This is the second paper in a four-part sequence that examines the nature of mantle layering using the ScSn phases and internal reflections observed within the reverberative interval of SH-polarized seismograms. Mantle reflectivity profiles for 18 seismic corridors sampling portions of Indonesia, Australia, and the western and central Pacific are constructed from low-frequency (25-mHz) ScS reverberations; i.e, ScSn-type phases reflected once from internal mantle discontinuities. Modeling the reflectivity profiles using synthetic seismograms yields precise, path-specific estimates of the travel times to the 410-km and 660-km discontinuities and their reflection coefficients. The data are consistent with an olivine-dominated model of the transition zone, marked by two major phase transitions having average apparent depths of 414±2 km and 660±2 km and impedance contrasts of 0.046±0.010 and 0.072±0.010, respectively. The interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km (peak to peak) and is negatively correlated, consistent with an endothermic transition at 660 km whose Clapeyron slope magnitude is comparable to that of the 410-km (exothermic) transition. In the case of the 660-km discontinuity, we have been able to supplement these long-wavelength (1500–5000 km) observations with an estimate of intrapath topography, inferred from the frequency dependence of the 660-km reflection coefficient, which gives peak-to-peak variations less than 40 km on scale lengths of 500–1500 km. For the 660-km discontinuity, the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction. However, we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660-km discontinuity which may involve small compositional variations. Plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition, loss of reflected energy due to local curvature (or roughness) of the reflector, and/or extreme topography occurring on a small compositional component to the discontinuity (≤40% of the total impedance contrast). By stripping the 410-km and 660-km peaks from the composite path reflectivity profile, we identify three minor reflectors at mean depths of 520, 710, and 900 km. The shallowest may mark the β-phase → γ-spinel phase transition. The 710-km discontinuity can be attributed to the ilmenite (gamet) → perovskite phase transition, although changes in perovskite symmetry can potentially provide explanations for both the 710-km and 900-km features.

Mantle layering from ScS reverberations: 3. The upper mantle

This is the third paper in a four-part sequence that examines the nature of mantle layering using the multiple-ScS phases and internal reflections observed within the reverberative interval of SH-polarized seismograms. In this paper, migration techniques are applied to ScS reverberations to image discontinuities in shear impedance beneath a tectonically diverse study area that includes the western Pacific, Japan, the Philippine Sea, and Australasia. Between the M and 410-km discontinuities (Bullens region B), the analysis reveals four reflectors, designated H, G, L, and X. The H (Hales) reflector is a positive impedance increase occurring at an average depth of about 60 km with a mean reflection coefficient of about 3.5%. It can be seen on all profiles except those where its signature is overwhelmed by the G discontinuity and is best explained as the seismic expression of the spinel → garnet facies boundary. The G (Gutenberg) reflector, marking the lid-to-low-velocity zone (LVZ) interface, is the only negative impedance contrast identified from ScS reverberations anywhere in the mantle. It is observed along most of the oceanic paths sampled in our study, where its depth generally correlates with QScS but it is not found on any of the profiles from predominantly continental corridors, consistent with the notion that the LVZ is absent or only weakly expressed beneath the cratons. G occurs at a depth of ∼60 km beneath the western Pacific, shallower than most estimates of thermal boundary layer (plate) thickness; the data are consistent with a sharp drop in shear velocity owing to the breakdown of hydrous phases such as amphibole along a steeply rising portion of the geotherm within the thermal boundary layer. The G contrasts with the largest magnitude (up to 10%) occur on profiles for back arc regions, where upper mantle melting may be accentuated by volatiles fluxing from subducted oceanic lithosphere. None of the eight profiles having a G discontinuity show an L reflector, or Lehmann discontinuity, whereas most of the ones that lack G require an L of substantial magnitude (R0(ZL) ≈ 0.02). This positive impedance contrast is not a global feature but has a continental affinity within the study area, increasing from an average depth of about 210 km beneath the continental margin north of Australia to nearly 300 km near the center of the western Australian craton. As other investigators have noted, this discontinuity is not easily explained by plausible phase transitions or chemical changes. We propose that the L reflector represents a transition from an anisotropic mechanical boundary layer to an annealed, nearly isotropic asthenosphere within the continental tectosphere. The substantial differences between the reflectivity structures of continents and oceans documented in this study thus provide new evidence for the thick plate model of the continents. The X reflector is an enigmatic feature observed at a depth near 300 km on profiles sampling regions of active subduction. Like L, this impedance increase has been observed in refraction studies but is not readily explained by standard mantle models. Based on its limited geographic occurrence, we speculatively ascribe the X discontinuity to hydration reactions occurring in parts of the mantle that have been volatile charged by subducting lithosphere. Overall, our results indicate an upper mantle where impedance layering varies substantially over a broad spectrum of scale lengths, but correlates with surface geology and plate structure down to at least 250 km (the L discontinuity), and perhaps to depths as great as 350 km (the X discontinuity). The lateral variability observed in region B reflectivity structure appears to be governed primarily by the thermal and chemical gradients related to continental deep structure and subduction. The large magnitude of this variability explains why no consistent view of upper mantle structure has developed from global data sets, which tend to average out the region B features observed by ScS reverberations.

Seismic detection of folded, subducted lithosphere at the core–mantle boundary

Seismic tomography has been used to infer that some descending slabs of oceanic lithosphere plunge deep into the Earths lower mantle. The fate of these slabs has remained unresolved, but it has been postulated that their ultimate destination is the lowermost few hundred kilometres of the mantle, known as the D″ region. Relatively cold slab material may account for high seismic velocities imaged in D″ beneath areas of long-lived plate subduction, and for reflections from a seismic velocity discontinuity just above the anomalously high wave speed regions. The D″ discontinuity itself is probably the result of a phase change in relatively low-temperature magnesium silicate perovskite. Here, we present images of the D″ region beneath the Cocos plate using Kirchhoff migration of horizontally polarized shear waves, and find a 100-km vertical step occurring over less than 100 km laterally in an otherwise flat D″ shear velocity discontinuity. Folding and piling of a cold slab that has reached the core–mantle boundary, as observed in numerical and experimental models, can account for the step by a 100-km elevation of the post-perovskite phase boundary due to a 700 °C lateral temperature reduction in the folded slab. We detect localized low velocities at the edge of the slab material, which may result from upwellings caused by the slab laterally displacing a thin hot thermal boundary layer.

A Scattered-Wave Image of Subduction Beneath the Transverse Ranges

Over 5600 short-period recordings of teleseismic events were used to create deterministic maps of P-wave scatterers in the upper mantle beneath Southern California. Between depths of 50 and 200 kilometers, the southern flank of the slab subducting beneath the Transverse Ranges is marked by strong scattering. The marked scattering indicates that the edge of the slab is a sharp thermal boundary. Such a boundary could be produced by slab shearing or small-scale convection in the surrounding mantle. The northern limb of the slab is not a strong scatterer, consistent with thicker lithosphere north of the Transverse Ranges.

Anticorrelated Seismic Velocity Anomalies from Post-Perovskite in the Lowermost Mantle

Earths lowermost mantle has thermal, chemical, and mineralogical complexities that require precise seismological characterization. Stacking, migration, and modeling of over 10,000 P and S waves that traverse the deep mantle under the Cocos plate resolve structures above the core-mantle boundary. A small –0.07 ± 0.15% decrease of P wave velocity (Vp) is accompanied by a 1.5 ± 0.5% increase in S wave velocity (Vs) near a depth of 2570 km. Bulk-sound velocity \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \([V_{\mathrm{b}}=(V_{\mathrm{p}}^{2}-\frac{4}{3}V_{\mathrm{s}}^{2})^{\frac{1}{2}}]\) \end{document} decreases by –1.0 ± 0.5% at this depth. Transition of the primary lower-mantle mineral, (Mg1-x-y FexAly)(Si,Al)O3 perovskite, to denser post-perovskite is expected to have a negligible effect on the bulk modulus while increasing the shear modulus by ∼6%, resulting in local anticorrelation of Vb and Vs anomalies; this behavior explains the data well.

Mantle layering from ScS reverberations: 1. Waveform inversion of zeroth-order reverberations

This is the first paper in a four-part sequence that examines the nature of mantle layering required by the multiple-ScS phases and internal reflections observed within the reverberative interval of SH-polarized seismograms. Here we present path-averaged estimates of the two-way travel times through the crust+mantle (τScS) and crust alone (τM), the whole mantle quality factor (QScS) and the reflection coefficient at the M discontinuity (R0(zM)) obtained by iterative waveform inversion of ScS reverberations. We show that QScS estimates derived from direct waveform inversion can be biased significantly if reverberations originating within the crust are ignored, especially in continental regions where the crust is thick. Applying our technique, which explicitly models crustal reverberations, we substantially expand the geographical coverage of τScS and QScS measurements available from previous studies. The τScS data correlate well with whole mantle travel times predicted by the Harvard tomographic models. Both the τScS data and the QScS data support the hypothesis that variations in velocity and attenuation structure between old continents and oceans persist to depths of several hundred kilometers. In particular, the value of QScS derived for paths crossing stable continent is very high (280±30 at 30 mHz) compared with that for mature ocean basin (189±16), consistent with the thick-plate model of cratons requiring that differences in thermal structure extend to several hundred kilometers. The crustal thicknesses implied by the observations of path-averaged τM are consistent with those inferred from short-range refraction data, gravity measurements, and surface topography. The path-averaged reflectivity of the M discontinuity is also tectonically correlated, becoming brighter as the percentage of continental crust encountered along the path increases. This result implies that, on average, the impedance at the base of the continental crust is significantly lower than at the base of the oceanic crust.

Ancient subduction, mantle eclogite, and the 300 km seismic discontinuity

A seismic discontinuity is frequently observed near 300 km depth beneath continents and island arcs. Here we show that this discontinuity is generated by SiO2-stishovite formation in eclogitic assemblages. Such free silica is petrologically anticipated within materials of mid-oceanic-ridge basalt chemistry at these depths, and the 300 km discontinuity is likely associated with either the coesite to stishovite transition or exsolution of stishovite from clinopyroxenes containing excess silica. The presence and amplitude of this seismic feature provide a means for determining how much subducted, or delaminated, formerly basaltic material is present at deep upper-mantle depths. Thus, the distribution of the 300 km discontinuity yields a measure of mantle geochemical heterogeneity and provides a means for locating the residue of ancient subduction within the upper mantle.

Microseismic measurement of wave-energy delivery to a rocky coast

Rocky coasts are attacked by waves that drive sea-cliff retreat and etch promontories and embayments into the coastline. Understanding the evolution of such coastlines requires knowledge of the energy supplied by waves, which should depend upon both the deep-water waves and the coastal bathymetry they cross. We employ microseismic measurements of the wave-induced shaking of sea cliffs near Santa Cruz, California, as a proxy for the temporal pattern of wave-energy delivery to the coast during much of the winter 2001 storm season. Visual inspection of the time series suggests that both deep-water wave heights and tide levels exert considerable control on the energy delivered. We test this concept quantitatively with two models in which synthetic time series of wave power at the coast are compared with the shaking data. In the first model, deep-water wave power is linearly scaled by a fitting parameter; because this model fails to account for the strong tidal signal, it fits poorly. In the second model, the wave transformation associated with shoaling and refraction diminishes the nearshore wave power, and dissipation associated with bottom drag and wave breaking is parameterized by exponential dependencies on two length scales; this model reduces the variance by 32%-45% and captures the essence of the full signal. Shoaling and refraction greatly modulate the wave power delivered to the coast. Energy dissipated by bottom drag across the shelf is relatively small; the dissipation length scale is many times the path length across the shelf. In contrast, much energy is dissipated in the surf zone; the tidal-dissipation depth scale is of the same order as the tidal range (1-2 m), which accounts for the strong dependence of the cliff shaking on the tide.