Computer Science
Scientific paper
Aug 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.1807t&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 50, Issue 8, pp.1807-1818
Computer Science
11
Scientific paper
Manganese porewater and solid phase data are considered from two different Atlantic carbonate sediments, both with good geochemical characterisation. The first case is a continuously accumulated alternating pelagic marl/carbonate ooze, where there is a correlation in post-oxic conditions between Mn(II) porewater levels and an acetate leachable Mn fraction of the sediment. This leachable Mn appears to have an association with calcite and, in this example, the leached residue Mn content approximates to that of shale. In the second case, a very recent, compositionally homogeneous calcareous turbidite is underlain by Mn oxyhydroxide at the former sediment surface which is now acting as a source of Mn(II) by reduction. Sorption of Mn by the turbidite sediment and shale-like leached residue Mn levels in anoxic conditions are again observed as Mn(II) diffuses towards present oxic conditions, although no unique equilibrium is achieved. Similar older buried turbidites have concentration decreases in solid-phase Mn content preserved from bottom to top as relict evidence that this process has occurred. In these turbidites 54-64% of the total Mn is leachable by the acetate method and therefore presumed to be sorbed by calcite. The oldest example has persisted for 125 kyr. It is suggested that the slow breakdown of metastable Mn oxyhydroxide and Sorption of Mn(II) to calcite surfaces are important controls on the behaviour of Mn in pelagic carbonates in mildly reducing conditions. Where accumulation fluctuations occur, these processes act together with accumulation to limit overall Mn migration and to prevent the large Mn enrichments seen near the sediment surface in more reducing environments.
Colley Stephen
Higgs N. C.
Hydes David J.
Jarvis Ian
Thomson J. J.
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