Miller Christian A.

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Christian A.

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  • Thesis
    Surface-cycling of rhenium and its isotopes
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2009-06) Miller, Christian A.
    The application of elemental and isotopic metal palaeoredox tracers to the geologic past rests on an understanding of modern metal cycles. This study reevaluates the surface-cycling of Mo and Re in near-surface reservoirs. Revised river averages of Mo and Re are 1.8- and 7.9-fold larger than previous estimates. The river concentrations of 8.0 nmol Mo kg−1 and 11.2 pmol Re kg−1 (pre-anthropogenic), result in shorter seawater response times of 4.4×105 yr (τMo) and 1.3×105 yr (τRe pre-anthropogenic). These metals, especially Re, are more sensitive to changing source and sink fluxes than previously thought. Evaluation of Mo and Re concentrations in high temperature fluids from the Manus Basin indicate that Re is essentially absent from the hydrothermal end member and Mo is present at concentrations considerably lower than ambient seawater. The sink fluxes represented by hydrothermal circulation are negligible in comparison to the revised river source fluxes. Anthropogenic contributions to the Re flux to seawater are seen in the high concentrations of certain impacted water samples such as those associated with mining sites. It may also be seen in a significant, variable, Re enrichment feature in the Hudson River estuary. This Re enrichment feature is not the result of estuarine mixing or the remobilization of sediment-hosted Re. On the basis of a Re - SO2− 4 correlation we are able to quantify and correct for the anthropogenic Re, which corresponds to ~33% of the modern river average. This study documents the development of an analytical method for stable Re isotopes. Though complicated by analyte requirements and 187Re – 187Os decay, Re isotope measurements have a reproducibility of ±0.05h for analyte concentrations of 20 ng Re mL−1. Total Re isotopic variability to date is 0.9h. This includes 0.3h across five commercially available Re products, and 0.5h across a black shale weathering profile. δ187Re variability across the weathering profile was systematic with the most weathered samples showing the most significant 187Re depletions. The Re isotopic weathering profile is well described by both two-component mixing and Rayleigh fractionation. There are currently insufficient data to discriminate between the two models.
  • Preprint
    Strontium isotopes in Chilean rivers : the flux of unradiogenic continental Sr to seawater
    ( 2009-08-26) Fiege, Katherina ; Miller, Christian A. ; Robinson, Laura F. ; Figueroa, Ricardo ; Peucker-Ehrenbrink, Bernhard
    Analyses of Chilean river waters indicate that the average yield of unradiogenic Sr (~ 517 mol Sr km− 2 yr− 1, 87Sr/86Sr ~ 0.7057) from western South America (1,220,853 km2) into the southeastern Pacific Ocean is ~ 2–4 times higher than that from Iceland (~ 110 mol Sr km − 2 yr− 1, 87Sr/86Sr ~ 0.7025) and the Deccan traps, but lower than fluxes of unradiogenic Sr from ocean islands in the Lesser Antilles and Réunion. The Sr flux from western South America accounts for about 1.8% of the annual dissolved Sr delivered to the ocean via rivers. If Chilean rivers analyzed in this study accurately characterize runoff from western South America, active convergent continental margins release about as much unradiogenic Sr to seawater as a 0–1 Myr old mid-ocean ridge segment of equivalent length. Modulations of the flux of unradiogenic Sr from active margins over geologic time scales have to be considered as an additional driving force of change in the marine Sr isotope record, supplementing temporal variations in the submarine hydrothermal flux as a source of unradiogenic Sr to seawater. Such modulations can be driven by changes in the surface exposure of volcanic arc terrains, changes in climate, ocean currents and geographic latitude due to plate tectonics, as well as topographic changes that can affect local rainfall, runoff and erosion.