Marcus Matthew A.

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Marcus
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Matthew A.
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  • Article
    Natural organobromine in marine sediments : new evidence of biogeochemical Br cycling
    (American Geophysical Union, 2010-11-24) Leri, Alessandra C. ; Hakala, J. Alexandra ; Marcus, Matthew A. ; Lanzirotti, Antonio ; Reddy, Christopher M. ; Myneni, Satish C. B.
    Organobromine (Brorg) compounds, commonly recognized as persistent, toxic anthropogenic pollutants, are also produced naturally in terrestrial and marine systems. Several enzymatic and abiotic bromination mechanisms have been identified, as well as an array of natural Brorg molecules associated with various marine organisms. The fate of the carbon-bromine functionality in the marine environment, however, remains largely unexplored. Oceanographic studies have noted an association between bromine (Br) and organic carbon (Corg) in marine sediments. Even so, there has been no direct chemical evidence that Br in the sediments exists in a stable form apart from inorganic bromide (Brinorg), which is widely presumed conservative in marine systems. To investigate the scope of natural Brorg production and its fate in the environment, we probed Br distribution and speciation in estuarine and marine sediments using in situ X-ray spectroscopy and spectromicroscopy. We show that Brorg is ubiquitous throughout diverse sedimentary environments, occurring in correlation with Corg and metals such as Fe, Ca, and Zn. Analysis of sinking particulate carbon from the seawater column links the Brorg observed in sediments to biologically produced Brorg compounds that persist through humification of natural organic matter (NOM). Br speciation varies with sediment depth, revealing biogeochemical cycling of Br between organic and inorganic forms as part of the burial and degradation of NOM. These findings illuminate the chemistry behind the association of Br with Corg in marine sediments and cast doubt on the paradigmatic classification of Br as a conservative element in seawater systems.
  • Article
    Visualising Fe speciation diversity in ocean particulate samples by micro X-ray absorption near-edge spectroscopy
    (CSIRO Publishing, 2013-10-09) Marcus, Matthew A. ; Lam, Phoebe J.
    It is a well known truism that natural materials are inhomogeneous, so analysing them on a point-by-point basis can generate a large volume of data, from which it becomes challenging to extract understanding. In this paper, we show an example in which particles taken from the ocean in two different regions (the Western Subarctic Pacific and the Australian sector of the Southern Ocean, south of Tasmania) are studied by Fe K-edge micro X-ray absorption near-edge spectroscopy (μXANES). The resulting set of data consists of 209 spectra from the Western Subarctic Pacific and 126 from the Southern Ocean. We show the use of principal components analysis with an interactive projection visualisation tool to reduce the complexity of the data to something manageable. The Western Subarctic Pacific particles were grouped into four main populations, each of which was characterised by spectra consistent with mixtures of 1–3 minerals: (1) Fe3+ oxyhydroxides + Fe3+ clays + Fe2+ phyllosilicates, (2) Fe3+ clays, (3) mixed-valence phyllosilicates and (4) magnetite + Fe3+ clays + Fe2+ silicates, listed in order of abundance. The Southern Ocean particles break into three clusters: (1) Fe3+-bearing clays + Fe3+ oxyhydroxides, (2) Fe2+ silicates + Fe3+ oxyhydroxides and (3) Fe3+ oxides + Fe3+-bearing clays + Fe2+ silicates, in abundance order. Although there was some overlap between the two regions, this analysis shows that the particulate Fe mineral assemblage is distinct between the Western Subarctic Pacific and the Southern Ocean, with potential implications for the bioavailability of particulate Fe in these two iron-limited regions. We then discuss possible advances in the methods, including automatic methods for characterising the structure of the data.
  • Article
    Wintertime phytoplankton bloom in the subarctic Pacific supported by continental margin iron
    (American Geophysical Union, 2006-02-01) Lam, Phoebe J. ; Bishop, James K. B. ; Henning, Cara C. ; Marcus, Matthew A. ; Waychunas, Glenn A. ; Fung, Inez Y.
    Heightened biological activity was observed in February 1996 in the high-nutrient low-chlorophyll (HNLC) subarctic North Pacific Ocean, a region that is thought to be iron-limited. Here we provide evidence supporting the hypothesis that Ocean Station Papa (OSP) in the subarctic Pacific received a lateral supply of particulate iron from the continental margin off the Aleutian Islands in the winter, coincident with the observed biological bloom. Synchrotron X-ray analysis was used to describe the physical form, chemistry, and depth distributions of iron in size fractionated particulate matter samples. The analysis reveals that discrete micron-sized iron-rich hot spots are ubiquitous in the upper 200 m at OSP, more than 900 km from the closest coast. The specifics of the chemistry and depth profiles of the Fe hot spots trace them to the continental margins. We thus hypothesize that iron hot spots are a marker for the delivery of iron from the continental margin. We confirm the delivery of continental margin iron to the open ocean using an ocean general circulation model with an iron-like tracer source at the continental margin. We suggest that iron from the continental margin stimulated a wintertime phytoplankton bloom, partially relieving the HNLC condition.
  • Article
    Measuring the form of iron in hydrothermal plume particles
    (The Oceanography Society, 2012-03) Toner, Brandy M. ; Marcus, Matthew A. ; Edwards, Katrina J. ; Rouxel, Olivier J. ; German, Christopher R.
    The global mid-ocean ridge (MOR) system is a 60,000 km submarine volcanic mountain range that crosses all of the major ocean basins on Earth. Along the MOR, subseafloor seawater circulation exchanges heat and elements between the oceanic crust and seawater. One of the elements released through this venting process is iron. The amount of iron released by hydrothermal venting to the ocean per year (called a flux) is similar in magnitude to that in global riverine runoff (Elderfield and Schultz, 1996). Until recently, measurements and modeling activities to understand the contribution of hydrothermal iron to the ocean budget have been largely neglected. It was thought that hydrothermal iron was removed completely from seawater by precipitation of iron-bearing minerals within plumes and then deposited at the seafloor close to vent sites. With this assumption in place, the contribution of hydrothermal fluxes to the ocean budget was considered negligible. Recent work, however, questions the validity of that assumption, and leads to what we call the "leaky vent" hypothesis. Our goal is to measure the forms of iron, known as speciation, present in hydrothermal plume particles to better understand the bioavailability, geochemical reactivity, and transport properties of hydrothermal iron in the ocean.