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This two-stage Mn scavenging model provides a framework for integrating the exchange and dispersion processes that transport hydrothermal Mn from ridge crest to the adjacent sediments. The model links these processes presently recognized as important: Mn exchange from dissolved to fine particle (capsuled bacteria) form, fine particle exchange with large more loosely aggregated particles, particle resuspension at the seafloor, benthic flux of dissolved Mn from sediments, and physical transport by particle settling, advection, and turbulent diffusion. As a framework, the model permits concentration and fluxes in the water column and in and out of the sediments all to be quantitatively connected. Beyond its use as an interpolating tool, the model also points to regions of the water column and to component processes where additional, fruitful measurements to define the deposition of Mn are likely to be acquired. These must surely include macroaggregate distributions and rates of exchange with the fine particle population, and Mn distributions in separate phases near the seafloor.
While the model is not specific to hydrothermal regions, one advantage a hydrothermal region provides is that the ridge crest source is a localized one. This makes hydrothermal Mn useful as a tracer, albeit nonconservative, of physical and sedimentalogical processes in the deep ocean. Biogeochemical exchanges must be acknowledged, as in this model, to extract that kind of information, however. The model and data together suggest that most of the hydrothermal Mn is deposited within several hundred kilometers of the ridge, making most of the hydrothermal Mn unavailable to the distal ocean. Still there is sufficient hydrothermal Mn flux beyond that distance to elevate Mn water column concentrations several times above background levels.
Inferences based on the model and the presently available data must be considered only semiquantitative. Our present limited understanding of component processes and the difficulty of sampling, time-dependent three-dimensional hydrothermal plumes presently disallow a more elaborate model or a more comprehensive comparison of model and observations. For example, any model with just two particle classes is minimal because the diversity of particle types in the ocean is large. Going beyond this minimal scheme, however, will require exchange rates and settling velocities for a larger set of particle classes, information not likely to be quickly acquired. Even for two bulk particle classes, we are only beginning to interpret the time scales of exchange. Better definition of all model processes and parameters await future measurements.
Despite its semiquantitative nature, the model seems to contain many of the important elements of cycling of Mn in hydrothermal regions. It should serve as a useful tool to examine additional observations and to focus the need to examine the basic elements of Mn transport. The challenge is to cautiously interpret hard to acquire, limited tracer measurements; by connecting a diversity of measurements, this type of model can help do that.
Acknowledgments. We thank Kevin Roe and Geoff Lebon for determination of dissolved and particulate Mn values and Sharon Roth for many helpful comments on the manuscript. Contribution 1285 from NOAA/Pacific Marine Environmental Laboratory. Contribution 2707 from SOEST/University of Hawaii.
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