Description: Concordant detrital zircon dates from Neoproterozoic to Paleozoic continental slope and trough deposits in northern Greenland indicate Late Archean and Paleoproterozoic sediment sources. Significant numbers of dates are, however, discordant and, together with a few apparently concordant dates, give ages younger than the depositional ages of overlying fossiliferous sediments. The discordance pattern implies partial or total radiogenic-Pb loss during the Middle Devonian, possibly facilitated by postdepositional fluid movement. Such timing of radiogenic-Pb loss is supported by results from a novel modeling method, which indicate that the greatest statistical similarity between concordant and discordant detrital populations occurs when Pb loss is constrained to the interval of 380–390 Ma, i.e., long after deposition. This radiogenic-Pb loss event is interpreted to reflect fluid flow associated with Caledonian orogenic uplift to the east.

Description: The Taimyr fold-and-thrust belt records late Paleozoic compression, presumably related to Uralian orogenesis, overprinted by Mesozoic dextral strike-slip faulting. U-Pb detrital zircon analyses of 38 sandstones from southern Taimyr were conducted using laser ablation–inductively coupled plasma–mass spectrometry to investigate late Paleozoic to Mesozoic sediment provenance and the tectonic evolution of Taimyr within a regional framework. The Pennsylvanian to Permian sandstones contain detrital zircon populations of 370–260 Ma, which are consistent with derivation from the late Paleozoic Uralian orogen in northern Taimyr and/or the polar Urals. Late Neoproterozoic through Silurian ages (688–420 Ma), most consistent with derivation from Timanian and Caledonian age sources, suggest an ultimate Baltica source. Southern Taimyr represents the proforeland basin of the bivergent Uralian orogen in the late Paleozoic. Triassic sedimentary rocks contain detrital zircon populations of Carboniferous–Permian (355–260 Ma), late Neoproterozoic to Early Devonian (650–410 Ma), and minor Neoproterozoic (1000–700 Ma) ages, which suggest a similar provenance as the Carboniferous to Permian strata. The addition of a Permian–Triassic (260–220 Ma) zircon population indicates derivation of detritus from Siberian Trap–related magmatism. Jurassic samples have a dominant age peak at 255 Ma and a distinct reduction in Carboniferous–Permian and late Neoproterozoic to Early Devonian input, suggesting that erosion and contributions from Uralian sources ceased while greater input from Siberian Trap–related rocks of Taimyr dominated. Comparison of these results to the published literature demonstrates that detritus from the Uralian orogen was deposited in Taimyr, Novaya Zemlya, and the New Siberian Islands in the Permian, but not in the Lisburne Hills or Wrangel Island. In the Triassic, Taimyr, Chukotka, Wrangel Island, the Kular Dome in the northern Verkhoyansk of Siberia, Lisburne Hills, Franz Josef Land, and Svalbard shared sources from Taimyr, the Siberian Traps, and the polar Urals, indicating that there were no geographic barriers among these locations prior to opening of the Amerasia Basin. Detritus from the Uralian orogen in Taimyr was shed northward into the retroforeland basin and was then transported farther 20–30 m.y. after Uralian orogenesis. The widespread distribution of material eroded from Taimyr and the polar Urals during the Triassic is likely due to the arrival of, and sublithospheric spreading associated with, the Siberian mantle plume head at ca. 250 Ma. The subsequent motion of the lithosphere relative to the plume-swell likely caused a northwestward migration of the uplifted regions. Taimyr and the polar Urals were probably affected. In the Jurassic, detrital zircon spectra from Taimyr, Chukotka, the Kular Dome, and Svalbard show great differences, suggesting that these locations no longer shared the same provenance from Taimyr and the Urals. The restricted distribution of detritus from Taimyr and the Urals indicates that erosion of the Uralian orogen was reduced. In the Late Jurassic, the depositional setting of southern Taimyr probably changed from a foreland to an intracratonic basin.

Description: Apatite geochronology is a versatile method for providing medium temperature history constraints of magmatic and metamorphic rocks. The LA-ICP-MS technique is widely applied to U/Pb geochronology using various minerals. Apatite U/Pb geochronology, in contrast to e.g., zircon, is compromised by variable amounts of common Pb incorporated into the crystal during growth. Magmatic apatite often shows a sufficient spread in data to obtain a precise and accurate lower intercept age. If this is not the case, the initial Pb isotopic composition needs to be estimated to obtain accurate and precise age information from apatite. Two approaches are common, one being the estimation of common Pb from a Pb evolution model and the other being the measurement of a coexisting mineral phase that tends to incorporate Pb but not U, e.g., feldspar. Most recent studies applying LA-ICP-MS to the analysis of Pb isotopes in feldspar utilize either multicollector or magnetic sector mass spectrometers. In this study we first evaluate the application of quadrupole mass spectrometry for apatite U/Pb geochronology combined with Pb isotopic measurements in feldspar and compare the results with modeled initial Pb isotopic compositions. The resulting age information is accurate and precise despite using plagioclase rather than K-feldspar, as is normally used, to define initial Pb isotope compositions. We apply this method to apatite-bearing gabbroic rocks from layered intrusions (Bushveld, Bjerkreim-Sokndal, Hasvik, and Skaergaard) ranging in age from ca. 2 Ga to ca. 55 Ma and generate metamorphic/cooling ages generally consistent with the known geologic history of these intrusions.

Description: Switchgrass ( Panicum virgatum L.) has been the principal perennial herbaceous crop investigated for bioenergy production in North America given its high production potential, relatively low input requirements, and potential suitability for use on marginal lands. Few large trials have determined switchgrass yields at field scale on marginal lands, including analysis of production costs. Thus, a field-scale study was conducted to develop realistic yield and cost estimates for diverse regions of the USA. Objectives included measuring switchgrass response to fertility treatments (0, 56, and 112 kg N ha −1 ) and generating corresponding estimates of production costs for sites with diverse soil and climatic conditions. Trials occurred in Iowa, New York, Oklahoma, South Dakota, and Virginia, USA. Cultivars and management practices were site specific and field-scale equipment was used for all management practices. Input costs were estimated using final harvest-year (2015) prices, and equipment operation costs were estimated with the MachData model ($2015). Switchgrass yields generally were below those reported elsewhere, averaging 6.3 Mg ha −1 across sites and treatments. Establishment stand percent ranged from 28 to 76%, and was linked to initial year production. No response to N was observed at any site in the first production year. In subsequent seasons, N generally increased yields on well-drained soils; however, responses to N were nil or negative on less well-drained soils. Greatest percent increases in response to 112 kg N ha −1 were 57% and 76% on well-drained South Dakota and Virginia sites, where breakeven prices to justify N applications were over $70 and $63 Mg -1 , respectively. For some sites, typically-promoted N application rates may be economically unjustified; it remains unknown whether a bioenergy industry can support the breakeven prices estimated for sites where N inputs had positive effects on switchgrass yield. This article is protected by copyright. All rights reserved.

Description: Apatite geochronology is a versatile method for providing medium temperature history constraints of magmatic and metamorphic rocks. The LA-ICP-MS technique is widely applied to U/Pb geochronology using a variety of minerals. Apatite U/Pb geochronology, in contrast to e.g. zircon, is compromised by variable amounts of common Pb incorporated into the crystal during growth. Magmatic apatite often shows a sufficient spread in data to obtain a precise and accurate lower intercept age. If this is not the case, the initial Pb isotopic composition needs to be estimated to obtain accurate and precise age information from apatite. Two approaches are common, one being the estimation of common Pb from a Pb evolution model and the other being the measurement of a coexisting mineral phase that tends to incorporate Pb but not U, e.g. feldspar. Most recent studies applying LA-ICP-MS to the analysis of Pb isotopes in feldspar utilize either multicollector or magnetic sector mass spectrometers. In this study we firstly evaluate the application of quadrupole mass spectrometry for apatite U/Pb geochronology combined with Pb isotopic measurements in feldspar and compare the results with modelled initial Pb isotopic compositions. The resulting age information is accurate and precise despite using plagioclase rather than K-feldspar, as is normally used, to define initial Pb isotope compositions. We apply this method to apatite-bearing gabbroic rocks from layered intrusions (Bushveld, Bjerkreim-Sokndal, Hasvik, and Skaergaard) ranging in age from ca. 2 Ga to ca. 55 Ma and generate metamorphic/cooling ages generally consistent with the known geologic history of these intrusions.