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Terrestrial cooling and changes in hydroclimate in the continental interior of the United States across the Eocene-Oligocene boundary
Published on December 27, 2017
One of the most pronounced climate transitions in Earth s history occurred at the Eocene-Oligocene transition, 34.0 - 33.6 m.y. ago. Marine sedimentary records indicate a dramatic decline in pCO2 coeval with global cooling during the transition. However, terrestrial records are relatively sparse, with conflicting interpretations of hydroclimate in continental interiors. Here, we provide quantitative constraints on the response of the continental hydroclimate in the western United States across the Eocene-Oligocene boundary by studying clumped isotope temperatures in eastern Wyoming. Our results show that T(Delta 47) dropped from ~28 C to ~21 C, indicating ~7 C cooling in air temperature, which occurred parallel to the decrease in atmospheric pCO2 during the latest Eocene early Oligocene. We find that aridity and the biome were stable, and ice-volume corrected precipitation decreased ~1.6 percent across the Eocene-Oligocene boundary, attributable to reduced vapor condensation temperatures. These new quantitative data add to the growing body of evidence suggesting a marked terrestrial response in temperature and hydroclimate across the Eocene-Oligocene transition. Our findings indicate a pattern of greenhouse-gas-induced global temperature change in the continental interior of the United States that was roughly 1.5-2 X the magnitude of cooling in the global ocean.
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Paleozoic sediment dispersal before and during the collision between Laurentia and Gondwana in the Fort Worth Basin
Published on December 20, 2017
We report detrital zircon U-Pb ages in the Fort Worth Basin (southern USA) aimed at understanding sediment dispersal patterns on the southern margin of Laurentia before and during the Laurentia-Gondwana collision. The ages from two Cambrian fluvial-marginal marine sandstone and six Pennsylvanian deltaic-fluvial sandstone samples span from Archean to early Paleozoic time. In the Cambrian sandstones, 80% of zircons are of Mesoproterozoic age (1.451 - 1.325 Ga) and 18% are of Grenvillian age. The high abundance of the Mesoproterozoic population suggests that the grains were dispersed by a local river draining the midcontinent granite-rhyolite province located in the Texas Arch to the northwest of the Fort Worth Basin. In the Pennsylvanian sandstones, 26% of zircons are of Archean - early Mesoproterozoic age, 47% are of Grenvillian age, 15% are of Neoproterozoic earliest Paleozoic age (800 - 500 Ma), and 10% are of early Paleozoic age (500 - 318 Ma), indicating a different dispersal pattern during the Pennsylvanian relative to the Cambrian. Compared to other early Paleozoic detrital zircon records on the southern margin of Laurentia, our Pennsylvanian sandstones have a distinct age peak at ca. 650 - 550 Ma, which we interpreted to be a result of transport by local rivers draining a peri-Gondwana terrane, most likely the Sabine terrane in the Ouachita orogen. The high abundance of Grenvillian zircons reflects either direct transport from the Appalahians by an axial river or recycling from Mississippian - Pennsylvanian sedimentary rocks incorporated in the Ouachita orogenic front. The similarity of detrital zircon age distributions in the Fort Worth Basin, the Arkoma Basin, and the southern Appalachian forelands seems to favor sediment dispersal by a major river with headwaters in the southern Appalachians.
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Utility of small-angle neutron scattering to quantifying accessible and wettable pores in shale
Published on November 13, 2017
Since 2000, the technological advances of horizontal drilling and hydraulic fracturing in the United States have led to a dramatic increase in hydrocarbon (gas and oil) production from shale formations, changing the energy landscape in the US and worldwide. However, total gas recovery is only 12 to 30%, while the tight-oil recovery rate from shale formations is even lower at 5 to 10%. Dr. Hu s research group has been studying the nano-petrophysical aspects, mainly pore connectivity and wettability, which are unique to shale to lead to steep production decline and low overall production. The group uses an integrated approaches of pycnometry ((liquid and gas), pore and bulk volume measurement after vacuum saturation,, porosimetry (mercury injection capillary pressure, low-pressure gas physisorption isotherm, water vapor adsorption/desorption isotherm, nuclear magnetic resonance cyroporometry), imaging (X-ray computed tomography, Wood s metal impregnation, field emission-scanning electron microscopy), scattering (ultra- and small-angle neutron, small angle X-ray), as well and the utility of both hydrophilic and hydrophobic fluids as well as fluid invasion tests (imbibition, diffusion, vacuum saturation) followed by laser ablation-inductively coupled plasma-mass spectrometry imaging of different nm-sized tracers. Among a total of 18 SCI papers published in 2017 from Dr. Hu s group, a recent paper of using SANS (small angle neutron scattering) to quantify accessible pore spaces of various shale is published in Scientific Report, is an online, open access journal from the publishers of Nature, with an impact factor of 4.26. The group has been continuing the work to tease out the relationship between topological connectivity, wettability and effective pore accessibility, using a contrast-matching technique with SANS.
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Late Paleozoic subsidence and burial history of the Fort Worth basin
Published on November 01, 2017
The Fort Worth basin in northcentral Texas is a major shale-gas producer, yet its subsidence history and relationship to the Ouachita fold-thrust belt have not been well understood. We studied the depositional patterns of the basin during the late Paleozoic by correlating well logs and constructing structure and isopach maps. We then modeled the one-dimensional (1-D) and two-dimensional subsidence history of the basin and constrained its relationship to the Ouachita orogen. Because the super-Middle Pennsylvanian strata were largely eroded in the region, adding uncertainty to the subsidence reconstruction, we used PetroMod 1-D to conduct thermal-maturation modeling to constrain the postMiddle Pennsylvanian burial and exhumation history by matching the modeled vitrinite reflectance with measured vitrinite reflectance along five depth profiles. Our results of depositional patterns show that the tectonic uplift of the Muenster uplift to the northeast of the basin influenced subsidence as early as the Middle Mississippian, and the Ouachita orogen became the primary tectonic load by the late Middle Pennsylvanian when the depocenter shifted to the east. Our results show that the basin experienced 3.7 -- 5.2 km (12,100 to 17,100 ft) of burial during the Pennsylvanian, and the burial depth deepens toward the east. We attributed the causes of deep Pennsylvanian burial and its spatial variation to flexural subsidence that continued into the Late Pennsylvanian in response to the growth of the Ouachita orogen and southeastward suturing of Laurentia and Gondwana. The modeling results also suggest that the Mississippian Barnett Shale reached the gas maturation window during the Middle Late Pennsylvanian.
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In situ peridotitic diamond in Indus ophiolite sourced from hydrocarbon fluids in the mantle transition zone
Published on June 09, 2017
In recent years ophiolitic diamonds have been reported mostly from podiform chromitites. However, the mechanism of such diamond formation remains unknown. We report in situ diamond, graphite pseudomorphs after diamond crystals, and hydrocarbon (C-H) and hydrogen (H2) fluid inclusions in ultrahigh-pressure (UHP) peridotitic minerals of the Nidar ophiolite, Indus suture zone. Diamond occurs as octahedral inclusion along with nitrogen (N2) in orthoenstatite. Methane (CH4) also occurs with UHP clinoenstatite (>8 GPa) in orthoenstatite. The graphite pseudomorphs after diamond crystals and primary hydrocarbon (C-H), and hydrogen (H2) fluids are included in olivine. Oriented hematite (?-Fe2O3) exsolutions are also present in the olivines, indicating a precursory ?-Mg2SiO4 phase of the host olivines. This assemblage of diamond, graphite, C-H and H2 has not previously been reported from any ophiolitic peridotite. The hydrocarbon fluids in UHP clinoenstatites and retrogressed ?-Mg2SiO4 strongly suggest their source from the mantle transition zone or base of the upper mantle. We conclude that the peridotitic diamonds precipitated from C-H fluids during mantle upwelling beneath the Neo-Tethys Ocean spreading center.
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