Interannual variations in the local spatial autocorrelation of tropospheric temperatures

Many dimensions of the global temperature pattern have been explored intensely over the past few decades. In this investigation, we explore the underlying spatial autoregressiveness of annual tropospheric temperature anomalies measured by polar-orbiting satellites. We found that the percent of the Earth covered by areas of significantly high local autocorrelation ranged from 11.92% to 25.90% over the 1979 to 2008 study period. We gathered 13 different teleconnection indices that have been linked to regional to global temperatures and found two (Polar/Eurasia pattern and North Atlantic Oscillation) that were positively correlated to the percent area with high local spatial autocorrelation. The magnitude but not the sign of the El Ni?o Southern Oscillation is also an important factor in the variation of spatial autocorrelation. The physical mechanism driving high local spatial autocorrelation is thought to be zonal and hemispheric flow governed by the subtropical and polar jet streams.     

40Ar/39Ar ages of L4, H5, EL6, and feldspathic ureilitic clasts from the Almahata Sitta polymict ureilite (asteroid 2008 TC3)

The Almahata Sitta (AhS) meteorite consists of disaggregated clasts from the impact of the polymict asteroid 2008 TC₃, including ureilitic (70%–80%) and diverse non-ureilitic materials. We determined the ⁴⁰Ar/³⁹Ar release patterns for 16 AhS samples (3–1500 μg) taken from three chondritic clasts, AhS 100 (L4), AhS 25 (H5), and MS-D (EL6), as well as a clast of ureilitic trachyandesite MS-MU-011, also known as ALM-A, which is probably a sample of the crust of the ureilite parent body (UPB). Based on our analyses, best estimates of the ⁴⁰Ar/³⁹Ar ages (Ma) of the chondritic clasts are 4535 ± 10 (L4), 4537–4555 with a younger age preferred (H5), and 4513 ± 17 (EL6). The ages for the L4 and the H5 clasts are older than the most published ⁴⁰Ar/³⁹Ar ages for L4 and H5 meteorites, respectively. The age for the EL6 clast is typical of older EL6 chondrites. These ages indicate times of argon closure ranging up to 50 Ma after the main constituents of the host breccia, that is, the ureilitic components of AhS, reached the >800°C blocking temperatures of pyroxene and olivine thermometers. We suggest that these ages record the times at which the clasts cooled to the Ar closure temperatures on their respective parent bodies. This interpretation is consistent with the recent proposal that the majority of xenolithic materials in polymict ureilites were implanted into regolith 40–60 Ma after calcium–aluminum-rich inclusion and is consistent with the interpretation that 2008 TC₃ was a polymict ureilite. With allowance for its 10-Ma uncertainty, the 4549-Ma ⁴⁰Ar/³⁹Ar age of ALM-A is consistent with closure within a few Ma of the time recorded by its Pb/Pb age either on the UPB or as part of a rapidly cooling fragment. Plots of age versus cumulative ³⁹Ar release for 10 of 15 samples with ≥5 heating steps indicate minor losses of ⁴⁰Ar over the last 4.5 Ga. The other five such samples lost some ⁴⁰Ar at estimated times no earlier than 3800–4500 Ma bp . Clustering of ages in the low-temperature data for these five samples suggests that an impact caused localized heating of the AhS progenitor ~2.7 Ga ago. In agreement with the published work, 10 estimates of cosmic-ray exposure ages based on ³⁸Ar concentrations average 17 ± 5 Ma but may include some early irradiation.     

Climatological Drought in Arizona: An Analysis of Indicators for Guiding the Governor's Drought Task Force

Lacking a federal policy to address local water deficiencies within the United States, many states have developed their own methods for monitoring drought in an effort to mitigate its effects. This article provides an overview of efforts to standardize the use of drought indices in order to compare recent and historical drought both spatially and temporally for Arizona. Yearly averages of the Palmer Drought Severity Index and the Standardized Precipitation Index are placed into a frequency distribution to create standardization among the indices. The 1896–1904 drought ranked as the most severe, although the 1996–2004 drought was a close second.     

The timing and magnitude of changes to Hortonian overland flow at the watershed scale during the post-fire recovery process

Extreme hydrologic responses following wildfires can lead to floods and debris flows with costly economic and societal impacts. Process-based hydrologic and geomorphic models used to predict the downstream impacts of wildfire must account for temporal changes in hydrologic parameters related to the generation and subsequent routing of infiltration-excess overland flow across the landscape. However, we lack quantitative relationships showing how parameters change with time-since-burning, particularly at the watershed scale. To assess variations in best-fit hydrologic parameters with time, we used the KINEROS2 hydrological model to explore temporal changes in hillslope saturated hydraulic conductivity (K_sh) and channel hydraulic roughness (n_c) following a wildfire in the upper Arroyo Seco watershed (41.5 km²), which burned during the 2009 Station fire in the San Gabriel Mountains, California, USA. This study explored runoff-producing storms between 2008 and 2014 to infer watershed hydraulic properties by calibrating the model to observations at the watershed outlet. Modelling indicates K_sh is lowest in the first year following the fire and then increases at an average rate of approximately 4.2 mm/h/year during the first 5 years of recovery. The estimated values for K_sh in the first year following the fire are similar to those obtained in previous studies on smaller watersheds (<1.5 km²) following the Station fire, suggesting hydrologic changes detected here can be applied to lower-order watersheds. Hydraulic roughness, n_c, was lowest in the first year following the fire, but increased by a factor of 2 after 1 year of recovery. Post-fire observations suggest changes in n_c are due to changes in grain roughness and vegetation in channels. These results provide quantitative constraints on the magnitude of fire-induced hydrologic changes following severe wildfires in chaparral-dominated ecosystems as well as the timing of hydrologic recovery.     

The Miller Range 090340 and 090206 meteorites: Identification of new brachinite‐like achondrites with implications for the diversity and petrogenesis of the brachinite clan

Miller Range (MIL) 090340 and MIL 090206 are olivine‐rich achondrites originally classified as ureilites. We investigate their petrography, mineral compositions, olivine Cr valences, equilibration temperatures, and (for MIL 090340) oxygen isotope compositions, and compare them with ureilites and other olivine‐rich achondrites. We conclude that they are brachinite‐like achondrites that provide new insights into the petrogenesis of brachinite clan meteorites. MIL 090340,6 has a granoblastic texture and consists of ~97 modal % by area olivine (Fo = molar Mg/[Mg+Fe] = 71.3 ± 0.6). It also contains minor to trace augite, chromite, chlorapatite, orthopyroxene, metal, troilite, and terrestrial Fe‐oxides. Approximately 80% by area of MIL 090206,5 has a granoblastic texture of olivine (Fo 72.3 ± 0.1) plus minor augite and chromite, similar to MIL 090340 but also containing minor plagioclase. The rest of the section consists of a single crystal of orthopyroxene (~11 × 3 mm), poikilitically enclosing rounded grains of olivine (Fo = 76.1 ± 0.6), augite, chromite, metal, and sulfide. Equilibration temperatures for MIL 090340 and MIL 090206, calculated from olivine‐spinel, olivine‐augite, and two‐pyroxene thermometry range from ~800 to 930 °C. In both samples, symplectic intergrowths of Ca‐poor orthopyroxene + opaque phases (Fe‐oxides, sulfide, metal) occur as rims on and veins/patches within olivine. Before terrestrial weathering, the opaques were probably mostly sulfide, with minor metal. All petrologic properties of MIL 090340 and MIL 090206 are consistent with those of brachinite clan meteorites, and largely distinct from those of ureilites. Oxygen isotope compositions of olivine in MIL 090340 (δ¹⁸O = 5.08 ± 0.30‰, δ¹⁷O = 2.44 ± 0.21‰, and Δ¹⁷O = ?0.20 ± 0.12‰) are also within the range of brachinite clan meteorites, and well distinguished from ureilites. Olivine Cr valences in MIL 090340 and the granoblastic area of MIL 090206 are 2.57 ± 0.06 and 2.59 ± 0.07, respectively, similar to those of three brachinites also analyzed here (Brachina, Hughes 026, Nova 003). They are higher than those of olivine in ureilites, even those containing chromite. The valence systematics of MIL 090340, MIL 090206, and the three analyzed brachinites (lower Fo = more oxidized Cr) are consistent with previous evidence that brachinite‐like parent bodies were inherently more oxidized than the ureilite parent body. The symplectic orthopyroxene + sulfide/metal assemblages in MIL 090340, MIL 090206, and many brachinite clan meteorites have superficial similarities to characteristic “reduction rims” in ureilites. However, they differ significantly in detail. They likely formed by reaction of olivine with S‐rich fluids, with only minor reduction. MIL 090340 and the granoblastic area of MIL 090206 are similar in modal mineralogy and texture to most brachinites, but have higher Fo values typical of brachinite‐like achondrites. The poikilitic pyroxene area of MIL 090206 is more typical of brachinite‐like achondrites. The majority of their properties suggest that MIL 090340 and MIL 090206 are residues of low‐degree partial melting. The poikilitic area of MIL 090206 could be a result of limited melt migration, with trapping and recrystallization of a small volume of melt in the residual matrix. These two samples are so similar in mineral compositions, Cr valence, and cosmic ray exposure ages that they could be derived from the same lithologic unit on a common parent body.     

Dynamic domain kinematic modelling for predicting interflow over leaky impeding layers

Traditional Boussinesq or kinematic simulations of interflow (i.e., lateral subsurface flow) assume no leakage through the impeding layer and require a no-flow boundary condition at the ridge top. However, recent analyses of many interflow-producing landscapes indicate that leaky impeding layers are common, that most interflow percolates well before reaching the toe slope, and therefore, the downslope contributing length is shorter than the hillslope length. In watersheds characterised by perched interflow over a low conductivity layer through permeable topsoil, interflow with percolation may be modelled with a kinematic wave model using a mobile upslope boundary condition defining the hillslope portion contributing interflow to valleys. Here, we developed and applied a dynamic interflow model to simulate interflow using a downslope travel distance concept such that only the active contributing length is modelled at any time. The model defines a variable active area based on the depth of the perched layer, the topographic slope and the ratio of the hydraulic conductivity of topsoil to that of the impeding layer. It incorporates a two-layer soil moisture accounting water balance analysis, a pedo-transfer function, and percolation and evaporation routines to predict interflow rates in continuous and event-based scenarios. We tested the modelling concept on two sets of data (2-year dataset of rainfall observations for the continuous simulation and a multi-day irrigation experiment for the event simulation) from a 121-m-long open interflow collection trench on an experimental hillslope at the Savannah River Site, South Carolina. The continuous model simulation partially represented the observed interflow hydrograph and perched water depth in the experimental hillslope with correlation coefficients of 0.85 and 0.35, respectively. Model performance improved significantly at event-scale analysis. The modelling approach realistically represents interflow dynamics in hillslopes with leaky impeding layers and can be integrated into catchment-scale hydrology models for more detailed hillslope process modelling.     

Climate, water management, and policy in the San Pedro Basin: results of a survey of Mexican stakeholders near the U.S.–Mexico border

This paper reviews regional climate knowledge and vulnerability in the northern Mexico San Pedro River Basin, with a focus on water quality, quantity, and management issues on the Mexican side of the border. A discussion based on the available literature is supplemented by a survey assessing concerns about water and the quality and usability of climate and hydrologic information available to water managers and communities. The surveys indicate that the central concern for urban residents is the lack of reliable potable water due to frequent service breakdowns–with climate change and variability, specifically drought and high temperatures, as contributing factors. Water managers desire appropriate meteorological and hydrologic information to improve planning strategies, but access to this information remains limited. Considerable disagreement exists about who should pay for previously free or low-cost water and wastewater treatment. Urban users have little incentive to conserve because of the present flat, low rate and frustration with service. In rural areas, while a majority of ranchers recognize that variable climate and water loss could increasingly jeopardize their lifestyle, they seldom use meteorological information in planning or modify their water consumption. Climate vulnerability also includes potential for serious environmental health issues due to the presence of heavy metals and organic contaminants in the San Pedro.     

Ureilites: A critical review

Abstract— Ureilites are coarse-grained ultramafic rocks whose petrography, mineral chemistry, lithophile element bulk chemistry, and Sm-Nd isotopic systematics suggest that they are highly fractionated igneous rocks and thus are products of common planetary differentiation processes. However, they also have primitive characteristics that are difficult to reconcile with extensive igneous processing. These include high abundances of siderophile elements, planetary-type noble gases, and the oxygen isotopic signature of unequilibrated solar system materials. The incongruity between igneous and primitive features constitutes the most important problem in understanding ureilite petrogenesis. In this review the petrographic, chemical, and isotopic characteristics of ureilites are summarized, and the petrogenetic implications of these characteristics are discussed. The most important constraints on ureilite petrogenesis are: 1) Ureilites have lost a basaltic complement; 2) Ureilites had a two-stage cooling history; 3) Ureilites are probably residues but partly crystallized from melts; 4) Ureilites are derived from a minimum of six reservoirs which were distinct in oxygen isotopic composition and did not equilibrate with one another; 5) A correlation between oxygen isotopic composition and mg ratio was established in ureilite parent material in the solar nebula; 6) If carbon-metal-silicate-CO/CO₂ equilibrium was maintained then the mg ratios of ureilites were pressure/depth-dependent; however, if the pressure was sufficiently high (> 100–200 bars) that a CO/CO₂ gas phase was not present then carbon and metal could have been at equilibrium with all ureilite mg ratios at the same pressure; 7) Ureilites either lost a low-melting temperature metal fraction or gained a refractory-rich metal component; 8) Primordial noble gases were retained in carbon in ureilites; 9) The ultramafic ureilite assemblage formed at ～4.55 Ga, but Sm-Nd and Rb-Sr isotopic systematics have been subsequently disturbed. Recently proposed models for ureilite petrogenesis are evaluated in terms of how well they satisfy these constraints; no models unequivocally satisfy all of them. Reconciling constraints 5 and 6 requires a large ureilite parent body.     

Comment on “Parent body depth‐pressure‐temperature relationships and the style of the ureilite anatexis” by P. H. Warren (MAPS 47:209–227)

Ureilites are carbon‐rich ultramafic (olivine + dominantly low‐Ca pyroxene) achondrites with poorly understood petrogenesis. One major problem concerns the origin of extensive variation in FeO content (olivine core Fo values ranging from approximately 75 to 95) among the individual ureilites. The two main competing hypotheses to explain this variation are: (1) equilibrium smelting, in which ureilite Fo values were established by pressure‐dependent (depth‐linked) carbon redox reactions on the ureilite parent body during partial melting; or (2) nebular inheritance, in which the variation in FeO contents was derived from ureilite precursors and was preserved during partial melting. The paper “Parent body depth‐pressure‐temperature relationships and the style of the ureilite anatexis” by Warren (2012) discusses a series of topics related to ureilite petrogenesis. In each case, an argument is presented within the context of smelting versus nonsmelting models. Collectively, these arguments create the impression that there are many valid arguments against smelting. The purpose of this comment is to point out flaws in some of these arguments, and/or to show that the issues they address are independent of smelting versus nonsmelting models. Both equilibrium smelting and nebular inheritance (simple anatexis) models face challenges in explaining all the properties of ureilites, but both remain viable.