Publication Date:
2022-05-25
Description:
Author Posting. © The Author(s), 2006. 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 Geochimica et Cosmochimica Acta 71 (2007): 895-917, doi:10.1016/j.gca.2006.10.016.
Description:
This study examined the removal of U, Mo, and Re from seawater by
sedimentary processes at a shallow-water site with near-saturation bottom water O2 levels
(240-380 μmol O2/L), very high organic matter oxidation rates (annually averaged rate is
870 μmol C/cm2/y), and shallow oxygen penetration depths (4 mm or less throughout the
year). Under these conditions, U, Mo, and Re were removed rapidly to asymptotic pore
water concentrations of 2.2–3.3 nmol/kg (U), 7–13 nmol/kg (Mo), and 11–14 pmol/kg
(Re). The order in which the three metals were removed, determined by fitting a
diffusion-reaction model to measured profiles, was Re 〈 U 〈 Mo. Model fits also suggest
that the Mo profiles clearly showed the presence of a near-interface layer in which Mo
was added to pore waters by remineralization of a solid phase. The importance of this
solid phase source of pore water Mo increased from January to October as the organic
matter oxidation rate increased, bottom water O2 decreased, and the O2 penetration depth
decreased. Experiments with in situ benthic flux chambers generally showed fluxes of U
and Mo into the sediments. However, when the overlying water O2 concentration in the
chambers was allowed to drop to very low levels, Mn and Fe were released to the
overlying water along with the simultaneous release of Mo and U. These experiments
suggest that remineralization of Mn and/or Fe oxides may be a source of Mo and perhaps
U to pore waters, and may complicate the accumulation of U and Mo in bioturbated
sediments with high organic matter oxidation rates and shallow O2 penetration depths.
Benthic chamber experiments including the nonreactive solute tracer, Br-,
indicated that sediment irrigation was very important to solute exchange at the study site.
The enhancement of sediment-seawater exchange due to irrigation was determined for
the nonreactive tracer (Br-), TCO2, NH4
+, U and Mo. The comparisons between these
solutes showed that reactions within and around the burrows were very important for
modulating the Mo flux, but less important for U. The effect of these reactions on Mo
exchange was highly variable, enhancing Mo (and, to a lesser extent, U) uptake at times
of relatively modest irrigation, but inhibiting exchange when irrigation rates were faster.
These results reinforce the observation that Mo can be released to and removed from pore
waters via sedimentary reactions.
The removal rate of U and Mo from seawater by sedimentary reactions was found
to agree with the rate of accumulation of authigenic U and Mo in the solid phase. The
fluxes of U and Mo determined by in situ benthic flux chamber measurements were the
largest that have been measured to date. These results confirm that removal of redoxsensitive
metals from continental margin sediments underlying oxic bottom water is
important, and suggest that continental margin sediments play a key role in the marine
budgets of these metals.
Description:
We appreciate the financial support
from the National Science Foundation (OCE-0220892). Funding for this work was also
provided to JLM by the Postdoctoral Scholar Program at WHOI courtesy of the Cabot
Marine Environmental Science Fund and the J. Seward Johnson Fund. Financial support
to IMK was given by The Swedish Foundation for International Cooperation in Research
and Higher Education.
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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