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Mafic intrusions, lava flows, and felsic plutons in southwestern Laurentia have been hypothesized to be associated with the emplacement of a late Mesoproterozoic (Stenian Period) large igneous province. Improved geochronologic data resolve distinct episodes of mafic magmatism in the region. The ca. 1,098 Ma main pulse of southwestern Laurentia large igneous province (SWLLIP) magmatism is recorded by mafic intrusions across southeastern California to central Arizona. A younger episode of volcanism resulted in eruptions that formed the ca. 1,082 Ma Cardenas Basalt, which is the uppermost unit of the Unkar Group in the Grand Canyon. With the updated geochronological constraints, we develop new paleomagnetic data from mafic sills in the SWLLIP. Overlapping poles between the Death Valley sills and rocks of similar age in the Midcontinent Rift are inconsistent with large‐scale Cenozoic vertical axis rotations in Death Valley. We also develop a new paleomagnetic pole from the ca. 1,082 Ma Cardenas Basalt (pole longitude = 183.9°E, pole latitude = 15.9°N, A95${\mathrm{A}}_{95}$= 7.4°, N= 18). The new paleomagnetic data are consistent with the pole path developed from time‐equivalent rocks of the Midcontinent Rift, supporting interpretations that changing pole positions are the result of rapid equatorward motion. These data add to the record of Laurentia's rapid motion from ca. 1,110 to 1,080 Ma that culminated in collisional Grenvillian orogenesis and the assembly of Rodinia. In Earth's geological record, voluminous magmatism over brief time frames are known as large igneous provinces. One such event took place approximately 1.1 billion years ago on the ancient North American continent (known as Laurentia) resulting in the formation of thick mafic sills. Previously, these intrusions across southwestern Laurentia were believed to have been emplaced simultaneously with the extrusive Cardenas Basalt lava flows in the Grand Canyon. We now know that the emplacement of the thick mafic intrusions represent a large igneous province, which predates the eruption of the Cardenas Basalt by 16 million years. By pairing this knowledge of the age with recorded magnetic directions in the lavas and intrusions, we can enrich our knowledge of Laurentia's motion leading up to the assembly of the supercontinent Rodinia. These data provide a new constraint on the position of the continent 1,082 million years ago and support the interpretation of rapid plate motion over the preceding 20 million years. Paleomagnetic directions from mafic sills in Death Valley are consistent with ca. 1,098 Ma LIP‐style magmatism in southwestern LaurentiaA paleomagnetic pole from the ca. 1,082 Ma Cardenas Basalt of the Grand Canyon corresponds with time‐equivalent Midcontinent Rift polesChanging southwestern Laurentia LIP pole positions support rapid equatorward motion of the continent recorded by the Keweenawan Track Paleomagnetic directions from mafic sills in Death Valley are consistent with ca. 1,098 Ma LIP‐style magmatism in southwestern Laurentia A paleomagnetic pole from the ca. 1,082 Ma Cardenas Basalt of the Grand Canyon corresponds with time‐equivalent Midcontinent Rift poles Changing southwestern Laurentia LIP pole positions support rapid equatorward motion of the continent recorded by the Keweenawan Track

The North American craton interior preserves a >1 Ga history of near surface processes that inform ongoing debates regarding timing and drivers of continental‐scale deformation and erosion associated with far‐field orogenesis. We tested various models of structural inversion on a major segment of the Midcontinent Rift along the Douglas Fault (DF) in northern Wisconsin, which accommodated ≳10 km of total vertical displacement. U‐Pb detrital zircon and vein calcite Δ47/U‐Pb thermochronometry from the hanging wall constrain the majority of uplift (≳8.5 km) and deformation to 1052–1036 Ma during the Ottawan phase of the Grenvillian orogeny. Combined U‐Pb zircon dates, Δ47/U‐Pb calcite thermochronometry, and field data that document syn‐ to early post‐depositional deformation in the footwall constrain a second stage of uplift (1–1.5 km) ca. 995–980 Ma during the Rigolet phase of the Grenvillian orogeny. A minor phase of Appalachian far‐field orogenesis is associated with minimal thrust reactivation. Our combined analyses identified the 995–980 Ma Bayfield Group as a Grenvillian foreland basin with an original thickness 0.5–2 km greater than currently preserved. By quantifying flexural loading and other subsidence mechanisms along the Douglas Fault, we identify dynamic subsidence as a mechanism that could be consistent with the development of late‐Grenvillian transcontinental fluvial systems. Minimal post‐Grenvillian erosion (0.5–2 km) in this part of the craton interior has preserved the Bayfield Group and equivalent successions, limiting the magnitude of regional erosion that can be attributed to Neoproterozoic glaciation. Around 1.1 billion years ago, volcanic rocks and sediments began accumulating in a large rift basin around the present‐day Lake Superior region of North America. Continent‐continent collision on the edge of North America caused rifting to cease and major uplift to occur on thrust faults in the rift. The timing of the tectonic shift and the amount of crustal uplift and the number of uplift episodes are all poorly constrained. Our new geochronology precisely identifies the age of two major uplift episodes, ∼1.05 and ∼0.98 billion years ago. Mineral veins that formed during uplift record the age and temperature of formation, which enables reconstruction of an uplift history. We conclude that the collisional event ∼1.05 billion years ago resulted in ≳8.5 km of uplift followed by 1.0–1.5 km of uplift ∼0.98 billion years ago. A sedimentary basin associated with collision ∼1.0 billion years ago may have been part of a vast transcontinental river system. The surface underlying the Great Unconformity, a surface representing missing time, regionally did not undergo much erosion following uplift, suggesting long‐term stability of the North American craton interior over the last ∼0.98 billion years. U‐Pb zircon and calcite dates constrain two‐stage Grenvillian inversion of the Midcontinent Rift on the Douglas Fault (DF) in northern WisconsinΔ47/U‐Pb thermochronometry constrains vertical displacement of ≳8.5 and ∼1.5 km in the Ottawan and Rigolet Grenvillian phases, respectivelyBasin analysis and clay mineralogy constrain original thickness, subsidence drivers, and erosion of a ca. 1010–980 Ma Grenvillian foreland U‐Pb zircon and calcite dates constrain two‐stage Grenvillian inversion of the Midcontinent Rift on the Douglas Fault (DF) in northern Wisconsin Δ47/U‐Pb thermochronometry constrains vertical displacement of ≳8.5 and ∼1.5 km in the Ottawan and Rigolet Grenvillian phases, respectively Basin analysis and clay mineralogy constrain original thickness, subsidence drivers, and erosion of a ca. 1010–980 Ma Grenvillian foreland

AbstractA goal of undergraduate geoscience programs is to prepare students for the geoscience workforce. In order to understand the extent to which programs succeed at this goal, we identified desired workforce skills, compiled undergraduate geoscience program requirements, and mapped workforce skills onto faculty responses to the 2016 National Geoscience Faculty Survey categorized by respondent-provided course names for majors-level courses (n = 1037). We used a Bayesian statistical model to determine the posterior probability of students practicing desired workforce skills in undergraduate geoscience programs, using program profiles as the model prior, conditioned by the likelihood of a student practicing workforce skills in each course category based upon survey responses. Probabilistic Monte Carlo simulations of students in each example program were used to calculate the likely number of courses in which a student practiced each skill. Activities to help students develop skills related to geologic reasoning, working as part of a team, quantitative skills (algebra), applying skills in new scenarios, evaluation of scientific literature, temporal thinking, spatial thinking, written communication, and managing uncertainty are frequently encountered by students; on average, students encounter these activities at least seven times during an undergraduate program. In contrast, students encounter activities to help them develop an understanding of societal relevance and systems thinking, on average, fewer than three times during an undergraduate program. Our results provide a snapshot, via instructors’ reporting, of the state of workforce skill development across undergraduate degree programs, and both the approach and data set can be built upon for future studies.

Precise determinations of the rates and durations of Palaeozoic biogeochemical events are largely unavailable. Here, we present two new high-precision U-Pb (zircon) dates from volcanic ash deposits from the Ludlow Series (Silurian System) of Podolia, Ukraine, that yielded weighted mean 206Pb/238U dates of 424.08 ± 0.20 (0.29) [0.53] Ma and 422.91 ± 0.07 (0.21) [0.49] Ma (analytical, tracer and total uncertainties). These new dates bracket the largest post-Cambrian global carbon cycle perturbation (Lau Excursion) and constrain the 'Ludlow Rise' in 87Sr/86Sr. These chronostratigraphically well-controlled dates improve the calibration of the Silurian time scale and provide the first determinations of the rates of biogeochemical change during the Ludlow Epoch. [ABSTRACT FROM AUTHOR]

In developing the national biosolids recycling rule (Title 40 of the Code of Federal Regulation Part 503 or Part 503), the USEPA conducted deterministic risk assessments whose results indicated that the probability of groundwater impairment associated with biosolids recycling was insignificant. Unfortunately, the computational capabilities available for performing risk assessments of pollutant fate and transport at that time were limited. Using recent advances in USEPA risk assessment methodology, the present study evaluates whether the current national biosolids pollutant limits remain protective of groundwater quality. To take advantage of new risk assessment approaches, a computer‐based groundwater risk characterization screening tool (RCST) was developed using USEPA's Multimedia, Multi‐pathway, Multi‐receptor Exposure and Risk Assessment program. The RCST, which generates a noncarcinogenic human health risk estimate (i.e., hazard quotient [HQ] value), has the ability to conduct screening‐level risk characterizations. The regulated heavy metals modeled in this study were As, Cd, Ni, Se, and Zn. Results from RCST application to biosolids recycling sites located in Yakima County, Washington, indicated that biosolids could be recycled at rates as high as 90 Mg ha−1, with no negative human health effects associated with groundwater consumption. Only under unrealistically high biosolids land application rates were public health risks characterized as significant (HQ ≥ 1.0). For example, by increasing the biosolids application rate and pollutant concentrations to 900 Mg ha−1and 10 times the regulatory limit, respectively, the HQ values varied from 1.4 (Zn) to 324.0 (Se). Since promulgation of Part 503, no verifiable cases of groundwater contamination by regulated biosolids pollutants have been reported.

A large data set of single and multi-grain zircon and titanite analyses from a sample of the Oligocene Fish Canyon Tuff (FCT), a voluminous ash flow from the San Juan Mountains of Colorado and widely used 40Ar/39Ar geochronological standard, has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuff by the isotope dilution U-Pb zircon method. Twenty-three single zircon grains and seven small multi-grain fractions of the FCT yield an inverse-variance weighted mean 206Pb/238U date of 28.402 ± 0.023 Ma (2σ; MSWD 0.93) and a slightly older weighted mean 207Pb/235U date of 28.529 ± 0.030 Ma (MSWD 0.74), which are insensitive to common Pb corrections. Initial 230Th disequilibria calculated from a newly measured Th/U = 2.2 for FCT pumice shards indicate Th-deficiency for these zircons; its correction brings the weighted mean 206Pb/238U date to 28.478 ± 0.024 (MSWD 0.97), minimizing the discordance of the FCT zircon analyses. These calculations culminate in Concordia ages (Ludwig, 1998)for the crystallization of the FCT zircons of 28. 476 ± 0.029 Ma (MSWD 1.50) without propagating decay constant errors, or 28.498 ± 0.035 Ma (MSWD 1.03) with propagated decay constant errors.

Using data from 1979 itemized tax returns, this paper investigates the quantitative effect that state-level social-welfare transfers have upon private-charitable giving. The empirical estimates suggest that the need of recipients and the level of governmental aid play significant and substantial roles in determining private charitable contributions.