Bioturbation depths, rates and processes in Massachusetts Bay sediments inferred from modeling of 210Pb and 239 + 240Pu profiles

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2004-07-20Author
Crusius, John
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Bothner, Michael H.
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Sommerfield, Christopher K.
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https://hdl.handle.net/1912/1606As published
https://doi.org/10.1016/j.ecss.2004.07.005Abstract
Profiles of 210Pb and 239+240Pu from sediment cores collected throughout Massachusetts
Bay (water depths of 36-192 m) are interpreted with the aid of a numerical sedimentmixing
model to infer bioturbation depths, rates and processes. The nuclide data suggest
extensive bioturbation to depths of 25-35 cm. Roughly half the cores have 210Pb and
239+240Pu profiles that decrease monotonically from the surface and are consistent with
biodiffusive mixing. Bioturbation rates are reasonably well constrained by these profiles
and vary from ~0.7 to ~40 cm2 yr-1. As a result of this extensive reworking, however,
sediment ages cannot be accurately determined from these radionuclides and only upper
limits on sedimentation rates (of ~0.3 cm yr-1) can be inferred. The other half of the
radionuclide profiles are characterized by subsurface maxima in each nuclide, which
cannot be reproduced by biodiffusive mixing models. A numerical model is used to
demonstrate that mixing caused by organisms that feed at the sediment surface and
defecate below the surface can cause the subsurface maxima, as suggested by previous
work. The deep penetration depths of excess 210Pb and 239+240Pu suggest either that the
organisms release material over a range of >15 cm depth or that biodiffusive mixing
mediated by other organisms is occurring at depth. Additional constraints from surficial
sediment 234Th data suggest that in this half of the cores, the vast majority of the presentday
flux of recent, nuclide-bearing material to these core sites is transported over a
timescale of a month or more to a depth of a few cm below the sediment surface. As a
consequence of the complex mixing processes, surface sediments include material
spanning a range of ages and will not accurately record recent changes in contaminant
deposition.
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Author Posting. © The Author(s), 2004. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Estuarine, Coastal and Shelf Science 61 (2004): 643-655, doi:10.1016/j.ecss.2004.07.005.
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Preprint: Crusius, John, Bothner, Michael H., Sommerfield, Christopher K., "Bioturbation depths, rates and processes in Massachusetts Bay sediments inferred from modeling of 210Pb and 239 + 240Pu profiles", 2004-07-20, https://doi.org/10.1016/j.ecss.2004.07.005, https://hdl.handle.net/1912/1606Related items
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