Suboxic sediments

Recent suboxic sediments Suboxic pore fluids (range) ... 

It seems much more likely that Mo isotopes are fractionated within the ocean system during removal from seawater, as hrst proposed hy Barling et al. (2001). This possibility is considered helow with respect to Mo removal to ferromanganese oxides, euxinic sediments and suboxic sediments. 

Evidence for this hypothesis can be formd in the rough correlation between 5 Mo and [Mo] in suboxic sediments (Siebert et al. 2003) Higher [Mo] is associated with 5 Mo approaching the seawater value, as expected from mass balance in a closed reservoir (the reservoir is the diffusive zone beneath the sediment-water interface in suboxic settings see following section). 

Based on these considerations, the importance of fractionation in suboxic sedimentary systems for the global ocean Mo isotope budget is probably modest. This conclusion follows from the magnitude of relative to ag. Mno, and the relatively modest importance of suboxic sediments as compared to Mn oxides for the global ocean Mo elemental budget (Morford and Emerson 1999 Emerson and Breckel personal communication). However, this prediction must be assessed by further studies in open ocean reducing sediments. 

Figure 9. A schematic depiction of the apparent Mo isotope budget in the modem oceans, neglecting suboxic sediments. Mean isotopic values for inputs and outputs are indicated, along with the fractional importance of each sediment type for Mo removal. See text for details.

Nevertheless, it is important to stress that uncertainties persist in our rmderstanding of the Mo ocean budget, particularly as regards Mo removal to suboxic sediments. The Mo isotope system may ultimately prove useful in reducing such rmcertainties. 

Mo isotope fractionation in the oceans results from an equilibrium isotope effect between dissolved and Mn oxide-associated Mo (Barling et al. 2001 Siebert et al. 2003 Arnold et al. 2004 Barling and Anbar 2004), then 5 MOg,v - 8 MOox 1000 x ln(asw M Ox)- Additionally, it seems reasonable that 5 MOem approximates 8 MOs , at least in settings similar to the Black Sea and Cariaco Basin. Using these relationships and assuming / x + /e = 1 (i.e., neglecting suboxic sediments), we obtain ... 

As seen in Figure 6, mean 8 Mo in these ancient sediments is significantly offset from that of modem seawater and from mean 8 Mo of recent euxinic sediments. The offset is in the direction of less fractionation of Mo isotopes in the oceans (closer to the likely 5 MOjj ), as would be expected if removal of Mo to euxinic sediments expanded at the expense of removal to oxic (and suboxic) sediments. The result is consistent with the hypothesis of expanded ocean anoxia, and consequent increase in the areal extent of euxinic depositional settings, during this time. A similar, albeit smaller, offset is seen in repeated measurements of a Devonian black shale sample (USGS SDO-1 Fig. 6). This shift could he interpreted as consistent with somewhat expanded euxinic deposition during this time—an interpretation consistent with other evidence of expanded Paleozoic ocean anoxia in inland hasins. While further work is needed, these initial results are promising. 

Local water column paleoredox. The offset between recent euxinic and suboxic sediments (Fig. 6) suggests that such data could provide an additional tool to differentiate between euxinic and suboxic conditions in ancient sediments. Such application requires independent constraint on 5 Mo, as might be obtained from analyses of coeval Mn oxide sediments. 

Figure 14.14 Porewater profiles of dissolved Fe and Mn in suboxic sediments from the Gulf of Papua. (Modified from Alongi et al., 1996.)...

Galloway, F., and Bender, M. (1982). Diagenetic models of interstitial nitrate profiles in deep sea suboxic sediments. Limnol. Oceanogr. 27(4), 624—638. 

Rosenthal Y., Lam P., Boyle E. A., and Thompson J. (1995a) Authigenic cadmium enrichments in suboxic sediments precipitation and postdepositional mobility. Earth Planet. Set Lett. 132, 99-111. 

Diagenesis and elemental cycling in oxic or suboxic sediments in Saguenay Fjord is to some extent governed by different processes than in fully anoxic systems, where organic matter oxidation and sulfate reduction have been taking place without interruption for a considerable length of time. This has led to a situation where sulfide is present in the water column. 

De Lange, G.J., 1992a. Shipboard routine and pressure-filtration system for pore-water extraction from suboxic sediments. Marine Geology, 109 77-81. 

Buried nodules Most workers on the subject agree that the preferential concentration of nodules at the sediment surface is due to the activity of benthic organisms which can slightly move the nodules. Buried nodules have, however, been found in all the oceans of the world. Their abundance is highly variable, but it is possible that it may not be entirely random. Buried nodules recovered in large diameter cores are sometimes concentrated in distinct layers. These layers may represent ancient erosion surfaces or surfaces of nondeposition on which manganese nodules were concentrated in the past. By contrast, in the Peru Basin large asymmetrical nodules get buried when their bottoms get stuck in tenacious suboxic sediment just below the surface layer. 

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