Computer Science
Scientific paper
Dec 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52.2925b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 52, Issue 12, pp.2925-2939
Computer Science
8
Scientific paper
Sediment cores up to 35 m long have been recovered from turbidite sequences in the Madiera Abyssal Plain and the Southern Nares Abyssal Plain. Detailed geochemical analyses of these sediments (Si, Al, Fe, Mn, Ca, organic and inorganic carbon) and associate pore waters (nitrate, ammonia, silicate, Fe 2+ , Mn 2+ and alkalinity) provide representative profiles of both steady-state and nonsteady-state diagenetic reactions initiated by oxidation and reduction processes. Models demonstrate that chemical transport in the pore water system is diffusion dominated. Over periods of a few thousands of years to several hundreds of thousands of years Mn and Fe mobilized in the subsurface mildly reducing sediments are precipitated as easily reducible oxyhydroxides in the near-surface oxidized sediments. Nonsteady-state profiles show the influence of episodic and rapid turbidite deposition, and the imprint of oxidation zones that developed as the result of oxygen diffusion into the upper portions of turbidites during intervals of slow pelagic deposition. The time required for development of oxidation zones between turbidites ranges from 2.0 to 15 ka in the Madeira Abyssal Plain, and from 0.2 to 2.4 ka in the Southern Nares Abyssal Plain. Paleo-oxidation zones can be recognized in old turbidites by distinct changes in color between lower and upper portions of the deposit. In the basal unoxidized portion of the turbidite, the sediments are characteristically dark green, gray or brown. In the paleo-oxidized zone, the sediments are pale green, blue or yellowish brown. These zones in both the Madeira and Southern Nares Abyssal Plains contain distinctly less organic C and carbonate in the paleo-oxidized zone as compared with the unoxidized sediments. Depletion of carbonate content in the oxidized zones in the Southern Nares Abyssal Plain is particularly significant in that as much as 80% of the initial carbonates have been lost. Also the relative amounts of weak acid soluble Fe and Mn are generally less in the paleo-oxidized zones, due to the initial oxidation of metal-carbonate and metal hydroxide forms to easily reducible oxide forms, which are then subsequently solubilized under the mildly reducing conditions at depth in the sediment column.
Buckley Dale E.
Cranston R. E.
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