Frame Caitlin H.

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Frame
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Caitlin H.
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  • Article
    Basin-scale inputs of cobalt, iron, and manganese from the Benguela-Angola front to the South Atlantic Ocean
    (Association for the Sciences of Limnology and Oceanography, 2012-07) Noble, Abigail E. ; Lamborg, Carl H. ; Ohnemus, Daniel C. ; Lam, Phoebe J. ; Goepfert, Tyler J. ; Measures, Christopher I. ; Frame, Caitlin H. ; Casciotti, Karen L. ; DiTullio, Giacomo R. ; Jennings, Joe C. ; Saito, Mak A.
    We present full-depth zonal sections of total dissolved cobalt, iron, manganese, and labile cobalt from the South Atlantic Ocean. A basin-scale plume from the African coast appeared to be a major source of dissolved metals to this region, with high cobalt concentrations in the oxygen minimum zone of the Angola Dome and extending 2500 km into the subtropical gyre. Metal concentrations were elevated along the coastal shelf, likely due to reductive dissolution and resuspension of particulate matter. Linear relationships between cobalt, N2O, and O2, as well as low surface aluminum supported a coastal rather than atmospheric cobalt source. Lateral advection coupled with upwelling, biological uptake, and remineralization delivered these metals to the basin, as evident in two zonal transects with distinct physical processes that exhibited different metal distributions. Scavenging rates within the coastal plume differed for the three metals; iron was removed fastest, manganese removal was 2.5 times slower, and cobalt scavenging could not be discerned from water mass mixing. Because scavenging, biological utilization, and export constantly deplete the oceanic inventories of these three hybrid-type metals, point sources of the scale observed here likely serve as vital drivers of their oceanic cycles. Manganese concentrations were elevated in surface waters across the basin, likely due to coupled redox processes acting to concentrate the dissolved species there. These observations of basin-scale hybrid metal plumes combined with the recent projections of expanding oxygen minimum zones suggest a potential mechanism for effects on ocean primary production and nitrogen fixation via increases in trace metal source inputs.
  • Thesis
    The biogeochemistry of marine nitrous oxide
    (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2011-06) Frame, Caitlin H.
    Atmospheric nitrous oxide N2O concentrations have been rising steadily for the past century as a result of human activities. In particular, human perturbation of the nitrogen cycle has increased the N2O production rates of the two major sources of this greenhouse gas, soil and the ocean. Nitrification, and particularly ammonia oxidation, is one of the major processes that produces N2O in the ocean. In this thesis, a series of stable isotopic methods have been used to characterize the biogeochemical controls on N2O production by marine nitrification as well as the natural abundance stable isotopic signatures of N2O produced by marine nitrifiers. This thesis shows that in addition to chemical controls on N2O production rates such as oxygen (O2) and nitrite (NO-2) concentrations, there are also biological controls such as nitrifier cell abundances and coastal phytoplankton blooms that may influence N2O production by ammonia oxidizers as well. Ammonia oxidizers can produce N2O through two separate biochemical mechanisms that have unique isotopic signatures. Using culture- based measurements of these signatures, we conclude that one of these pathways, nitrifier- denitrification, may be a significant source of N2O produced in the South Atlantic Ocean and possibly the global ocean.