Insights on geochemical cycling of U, Re and Mo from seasonal sampling in Boston Harbor, Massachusetts, USA

Abstract

3Benthic 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 -), TCO 2 , NH 4 + , 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.4

Phosphate-mineral interactions and potential consequences for nutrient cycling

“…Ingall explains high C:P values by a combination of contributions of terrigenous C org into the coastal waters and preferential remineralization of PO 4 relative to C. Although no major rivers discharge into Buzzards Bay, terrestrial inputs are likely to be significant and both Mass Bay and Hingham Bay have historically had significant anthropogenic input of C org via sewage discharge and organic pollutants in the overlying seawater. (Morford, Martin et al 2007) These results show that the oceanic Redfield ratio of 106:1 is an overestimation of the amount of organic-PO 4 contained in C org in the sediment column, in agreement with Ingall's findings. (Morford, Martin et al 2007) A bioturbation mixing coefficient was calculated for each site and combined with our SEDEX data to determine the apparent flux of solid-bound PO 4 as a result of this biological activity (Table 7) solid-phases, is highest at the core tops and diminishes deeper into the sediments.…”

“…(Morford, Martin et al 2007) These results show that the oceanic Redfield ratio of 106:1 is an overestimation of the amount of organic-PO 4 contained in C org in the sediment column, in agreement with Ingall's findings. (Morford, Martin et al 2007) A bioturbation mixing coefficient was calculated for each site and combined with our SEDEX data to determine the apparent flux of solid-bound PO 4 as a result of this biological activity (Table 7) solid-phases, is highest at the core tops and diminishes deeper into the sediments. The largest burial of PO 4 seems to be associated with the Ca-bound phase and P org .…”

“…Martin, W.) (Morford, Martin et al 2007). Visual identification and a trial and error approach located the regions of highest slope for the flux determinations.…”

A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia

“…To be consistent with previous studies on trace element redox modeling, we define the suboxic setting as areas where the O 2 penetration depth below the sediment-water interface is <1 cm (Partin et al, 2013;Reinhard et al, 2013a). This condition accounts for areas of the seafloor where dissolved O 2 is present in bottom waters, but typically at levels low enough (<10-100 lM; higher values are also possible in shallow coastal locations; Morford and Emerson, 1999;Morford et al, 2005Morford et al, , 2007Morford et al, , 2012 so that Fe-Mn oxyhydroxides are not permanently buried, and that reduction of Re occurs in the anoxic sediment pore-waters. The result is appreciable authigenic accumulation of Re in sediments.…”

Surface-cycling of rhenium and its isotopes

“…Likewise, C org -weathering (of which the burning of fossil fuels can be considered an anthropogenic "short-circuit") is a source of atmospheric CO 2 . The association between C org and metals such as V, Cr, Zn, Mo, Cd, Re, and U has been widely documented for both modern and ancient sediments, and these metals are used to infer the redox characteristics of the depositional and diagenetic environments Klinkhammer and Palmer, 1991;Calvert and Pederson, 1993;Colodner et al, 1993a;Quinby-Hunt and Wilde, 1996;Tribovillard et al, 2006;Morford et al, 2007).…”