Imaging mantle transition zone thickness with SdS-SS finite-frequency sensitivity kernels

We invert differential SdS-SS traveltime residuals measured from stacked waveforms and finite-frequency sensitivity kernels for topography on the 410- and 660-km discontinuities. This approach yields higher resolution images of transition zone thickness than previous stacking methods, which simply average/smooth over topographic features. Apparent structure measured using simple stacking is highly dependent upon the bin size of each stack. By inverting for discontinuity topography with a variety of bin sizes, we can more accurately calculate the true structure. The inverted transition zone model is similar to simple stack models with an average thickness of 242 km, but the lateral variations in thickness are larger in amplitude and smaller in scale. Fast seismic velocities in 3-D mantle models such as SB4L18 correlate with areas of thicker transition zone. The elongated curvilinear regions of thickened transition zone that occur near subduction zones are narrow and high amplitude, which suggests relatively little lateral spreading and warming of subducted lithosphere within the transition zone. The anomalously thin transition zone regions are laterally narrow, and not broadly continuous. If these variations in transition zone thickness are interpreted as thermal in nature, then this model suggests significant temperature variations on small lateral scales.     

Compound paleovalley fills in the Lower Pennsylvanian New River Formation, West Virginia, USA

Ancient paleovalley fills are typically interpreted in the rock record using over-generalized models without carefully considering modern analogs, especially in light of recent discoveries. It is now known that many Quaternary paleovalleys are compound in origin, exhibit considerable stratigraphic complexity, contain multiple incisions, and can be orders of magnitude larger than their putative ancient counterparts. Compound paleovalley fills in the Lower Pennsylvanian New River Formation (NRF) are directly comparable to these Quaternary analogs, stimulating a paradigm shift in the interpretation of ancient paleovalleys. In the NRF, multiple laterally- and vertically-juxtaposed fill successions, separated by incision surfaces, record high-frequency fluvial responses to external controls within lower-order sequences. Lowstand incision and sediment bypass, as predicted in sequence stratigraphy, is largely discounted by the available evidence and the definition of regional sequence boundaries is not straightforward. The identification of genetic sequences may be the most effective approach to understanding the NRF and, by inference, many other ancient paleovalleys. Results from this study of the NRF promote a revised model for ancient paleovalleys that incorporates: 1) the pre-eminence of compound architecture, 2) periodic episodes of incision and subaerial exposure occurring in response to high-frequency changes in climate or relative sea level, 3) fluvial downcutting as the primary cause of paleovalley incision, although some sediments are still preserved in a net-erosional regime, and 4) composite, time-transgressive sequence boundaries that may be difficult or impossible to correlate regionally.     

Quantification of the seasonal hillslope water storage that does not drive streamflow

The relationship between seasonal catchment water storage and discharge is typically nonunique due to water storage that is not directly hydraulically connected to streams. Hydraulically disconnected water volumes are often ecologically and hydrologically important but cannot be explicitly estimated using current storage–discharge techniques. Here, we propose that discharge is explicitly sensitive to changes in only some fraction of seasonally dynamic storage that we call “direct storage,” whereas the remaining storage (“indirect storage”) varies without directly influencing discharge. We use a coupled mass balance and storage–discharge function approach to partition seasonally dynamic storage between these 2 pools in the Northern California Coast Ranges. We find that indirect storage constitutes the vast majority of dynamic catchment storage, even at the wettest times of the year. Indirect storage exhibits lower variability over the course of the wet season (and in successive winter periods) than does direct storage. Predicted indirect storage volumes and dynamics match field observations. Comparison of 2 neighbouring field sites reveals that indirect storage volumes can occur as unsaturated storage held under tension in soils and weathered bedrock and as near‐surface saturated storage that remains on hillslopes (and is eventually evapotranspired). Indirect storage volumes (including moisture in the weathered bedrock) may support plant transpiration, and our method indicates that this important water source could be quantified from precipitation and stream discharge records.     

Tracking a Governance Transition: Identifying and Measuring Indicators of Social Forestry on the Willamette National Forest

Abstract

National Forests in the United States have undergone a spatially and temporally uneven governance transition in response to social and economic pressures and contemporary policy changes, with many national forest units moving from a wholly government-led “dominant federal” model to a more collaborative “social forestry” model in which nonfederal actors have greater influence and authority. Here we report on an effort to develop a suite of indicators designed to capture some of the most tangible elements of a transition from dominant federal to social forestry modes of governance. We pilot test these data on the Willamette National Forest using data from a variety of sources internal and external to the USDA Forest Service. We assess the suitability of these indicators for tracking governance transitions and discuss their applicability to other national forest units nationwide.     

The effect of El Niño Southern Oscillation on U.S. corn production and downside risk

ENSO teleconnections imply anomalous weather conditions, causing yield shortages, price fluctuations, and civil unrest. We estimate ENSO’s effect on U.S. county-level corn yield distributions and find that temperature and precipitation alone are not sufficient to summarize the effect of global climate on agriculture. We find that acreage-weighted aggregate impacts mask considerable spatial heterogeneity at the county-level for the mean, variance, and downside risk of corn yields. Impacts for mean yields range from ??24 to 33 % for El Niño and ??25 to 36 % for La Niña, with the geographical center of losses shifting from the Eastern to Western corn belt. ENSO’s effect on the variance of crop yields is highly localized and is not representative of a variance-preserving shift. We also find that downside risk impacts are large and spatially correlated across counties.     

On amplitude information carried by the ambient seismic field

The use of the ambient seismic field (ASF) to extract Earth's response has received significant attention in the last several years. Multiple studies demonstrate the utility of the ASF for estimating high-resolution velocity models in various locations. In this paper, we discuss the amplitude information carried by the ASF. Amplitude information includes both amplification effects due to elastic structure, such as low velocity sedimentary basins, and attenuation effects in the crust and upper mantle or even in buildings. As has been suggested by other authors, amplitude measurements may be biased due to non-uniformities in ambient field excitation; however, we find very similar and stable results for different time intervals for both amplification and attenuation, suggesting that this bias may not be as large as feared. We conclude that valuable amplitude information can be recovered from the ASF through careful processing. Amplitude measurements may be particularly valuable due to the enhanced sensitivity of attenuation to fluids and/or temperature fluctuations.     

U-Th-Pb and Trace Element Evaluation of Existing Titanite and Apatite LA-ICP-MS Reference Materials and Determination of 208Pb/232Th-206Pb/238U Date Discordance in Archaean Accessory Phases

The last decade has seen a marked increase in U-Th-Pb petrochronological studies focused on titanite and apatite. This has motivated the need for detailed, phase-specific method development and well-characterised reference materials with a wide variety of ages and chemical compositions. This study presents new U-Th-Pb isotope and major-, minor- and trace-element mass fraction data for ten titanite and five apatite reference materials based on integrated EPMA, LA-ICP-MS and isotope-dilution (ID) multi-collector (MC)-ICP-MS characterisation. Cross-comparison of EPMA and LA-ICP-MS data for the same titanite reference material suite demonstrates that careful selection of EPMA primary reference materials is necessary to minimise inaccuracies. We further identify a significant X-ray interference when measuring low Ce abundances in Ti-bearing phases and outline a method for its correction. New ID-MC-ICP-MS and LA-ICP-MS data suggest tri-concordance (defined as concordant ages for the ²³⁸U-²⁰⁶Pb, ²³⁵U-²⁰⁷Pb and ²³²Th-²⁰⁸Pb decay systems) of the MKED-1 titanite reference material and demonstrate the reproducibility of ²⁰⁸Pb/²³²Th measurements in some secondary titanite reference materials. Integration of ²⁰⁸Pb/²³²Th results with U-Pb geochronology provides a meaningful tool to help interpret complex U-Th-Pb isotopic age spectra. We provide example applications of ²⁰⁸Pb/²³²Th LA-ICP-MS measurements toward interpreting Archaean titanite ages from the Acasta Gneiss Complex, Northwest Territories, Canada.