ALBERT

Description: The Precordillera terrane in northwestern Argentina is interpreted to be an exotic (Laurentian) continental fragment that was accreted to western Gondwana during the Ordovician. One prominent manifestation of the subduction and collision process is a Middle–Upper Ordovician clastic wedge, which overlies a passive-margin carbonate-platform succession in the Precordillera. U/Pb ages of detrital zircons from sandstones within the clastic wedge, as well as zircons from clasts within conglomerates, provide documentation for the composition of the sediment provenance. The ages of detrital zircons are consistent vertically through the succession, as well as laterally along and across strike of the Precordillera, indicating a single, persistent sediment source throughout deposition of the clastic wedge. The dominant mode (~1350–1000 Ma) of the detrital-zircon ages corresponds to the ages of basement rocks in the Western Sierras Pampeanas along the eastern side of the Precordillera. A secondary mode (1500–1350 Ma) corresponds in age to the Granite-Rhyolite province of Laurentia, an age range which is not known in ages of basement rocks of the Western Sierras Pampeanas; however, detritus from Granite-Rhyolite-age rocks in the basement of the Precordillera was available through recycling of synrift and passive-margin cover strata. Igneous clasts in the conglomerates have ages (647–614 Ma) that correspond to the ages of minor synrift igneous rocks in the nearby basement massifs; the same ages are represented in a minor mode (~750–570 Ma) of detrital-zircon ages. A quartzite clast in a conglomerate, as well as parts of the population of detrital zircons, indicates the importance of a source in the metasedimentary cover of the leading edge of the Precordillera. The Famatina continental-margin magmatic arc reflects pre-collision subduction of Precordillera lithosphere beneath the western Gondwana margin; however, no detrital zircons have ages that correspond to Famatina arc magmatism, indicating that sedimentary detritus from the arc may have been trapped in a forearc basin and did not reach the foreland. The indicators of sedimentary provenance for the foreland deposits are consistent with subduction of the Precordillera beneath western Gondwana, imbrication of basement rocks from either the Precordillera or Gondwana into an accretionary complex, and recycling of deformed Precordillera cover rocks.

Description: 〈span〉〈div〉Abstract〈/div〉Deep-sea carbonate represents Earth’s largest carbon sink and one of the least-known components of the long-term carbon cycle that is intimately linked to climate. By coupling the deep-sea carbonate sedimentation history to a global tectonic model, we quantify this component within the framework of a continuously evolving seafloor. A long-term increase in marine carbonate carbon flux since the mid-Cretaceous is dominated by a post-50 Ma doubling of carbonate accumulation to ∼310 Mt C/yr at present-day. This increase was caused largely by the immense growth in deep-sea carbonate carbon storage, post-dating the end of the Early Eocene Climate Optimum. We suggest that a combination of a retreat of epicontinental seas, underpinned by long-term deepening of the seafloor, the inception of major Himalayan river systems, and the weathering of the Deccan Traps drove enhanced delivery of Ca〈sup〉2+〈/sup〉 and HCO〈sub〉3〈/sub〉〈sup〉–〈/sup〉 into the oceans and atmospheric CO〈sub〉2〈/sub〉 drawdown in the 15 m.y. prior to the onset of glaciation at ca. 35 Ma. Relatively stagnant mid-ocean ridge, rift- and subduction-related degassing during this period support our contention that continental silicate weathering, rather than a major decrease in CO〈sub〉2〈/sub〉 degassing, may have triggered an increase in marine carbonate accumulation and long-term Eocene global cooling. Our results provide new constraints for global carbon cycle models, and may improve our understanding of carbonate subduction-related metamorphism, mineralization and isotopic signatures of degassing.〈/span〉

Description: 〈span〉Deep-sea carbonate represents Earth’s largest carbon sink and one of the least-known components of the long-term carbon cycle that is intimately linked to climate. By coupling the deep-sea carbonate sedimentation history to a global tectonic model, we quantify this component within the framework of a continuously evolving seafloor. A long-term increase in marine carbonate carbon flux since the mid-Cretaceous is dominated by a post-50 Ma doubling of carbonate accumulation to ~310 Mt C/yr at present-day. This increase was caused largely by the immense growth in deep-sea carbonate carbon storage, post-dating the end of the Early Eocene Climate Optimum. We suggest that a combination of a retreat of epicontinental seas, underpinned by long-term deepening of the seafloor, the inception of major Himalayan river systems, and the weathering of the Deccan Traps drove enhanced delivery of Ca〈sup〉2+〈/sup〉 and HCO〈sub〉3〈/sub〉〈sup〉–〈/sup〉 into the oceans and atmospheric CO〈sub〉2〈/sub〉 drawdown in the 15 m.y. prior to the onset of glaciation at ca. 35 Ma. Relatively stagnant mid-ocean ridge, rift- and subduction-related degassing during this period support our contention that continental silicate weathering, rather than a major decrease in CO〈sub〉2〈/sub〉 degassing, may have triggered an increase in marine carbonate accumulation and long-term Eocene global cooling. Our results provide new constraints for global carbon cycle models, and may improve our understanding of carbonate subduction-related metamorphism, mineralization and isotopic signatures of degassing.〈/span〉

Description: This work employs in situ measurement data and constructive simulations to examine the underlying physical mechanisms that drive spacecraft plume interactions with the space environment in low Earth orbit. The study centers on observations ofthe enhanced flux of plasma generated during a maneuver of Space Shuttle Endeavour as part of the Sensor Test for Orion Relative Navigation Risk Mitigation (STORRM) experiment in May 2011. The Canary electrostatic analyzer (ESA) instrument mounted on the port-side truss of the International Space Station (ISS) indicated an elevated ion current during the shuttle maneuver. The apparent source of enhanced ion current is a result of interaction of the spacecraft thruster plume with the rarefied ambient ionosphere, which generates regions of relatively high-density plasma through charge exchange between the neutral plume and ambient ions. To reconstruct this event, unsteady simulation data were generated using a combined direct simulation Monte Carlo/Particle in Cell methodology, which employed detailed charge-exchange cross section data and a magnetic field model. The simulation provides local plasma characteristics at the ESA sensor location, and a sensor model is subsequently used to transform the local properties into a prediction of measured ion current. The predicted and observed total current are presented as a function of time over a 30 second period of pulsed thruster firings. A strong correlation is observed in the temporal characteristics of the simulated and measured total current, and good agreement is also achieved in the total current predicted by the model. These results support conclusions that: (1) the enhanced flux of plasma observed by the ESA instrument is associated with Space Shuttle thruster firings, and (2) the simulation model captures the essential features of the plume interactions based on the observation data.