Mathematics – Logic
Scientific paper
Sep 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.2019p&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 50, Issue 9, pp.2019-2031
Mathematics
Logic
4
Scientific paper
Relatively rapidly-accumulating (>7 cm kyr - ) and organic-rich (~1.5% C org ) EPR sediments near 21 °N are characterized by pronounced nitrate reduction in the upper decimetre and by an enrichment of Mn and increased Fe / Al ratio in surficial sediments. Joint consideration of solid-phase and pore water data indicates that the distributions of both Mn and Fe in the sediments are in disequilibrium with their respective pore water profiles: precipitation of upward-diffusing Mn and Fe is occurring at present below the enriched horizons, producing "perched" enrichments indicative of non-steady-state diagenesis. Consideration of a high resolution 14 C age vs. depth curve and a decrease toward the interface (from 1.51 to 1.37 wt%) over the top 7 cm of the C org profile suggests that the diagenetic disequilibrium evident for Mn and Fe may be due to a geologically recent decrease in productivity. Such a decrease would be manifest in the sediments by a downwardly-migrating oxidation front which would progressively erode the Mn and Fe pore water profiles, producing the existing decoupled surface enrichments. In contrast to Mn, the distribution of Cu has apparently not been perturbed by non-steady-state phenomena. A first-order decrease with depth of the dissolved copper concentration is accompanied by a first-order increase in solid-phase Cu. Application of a diffusion-reaction model to these data predicts a sedimentation rate of ~8 cm kyr -1 which agrees well with the 14 C-determined rate of ~7.3 cm kyr -1 , implying that the pore water and solid-phase distributions are in diagenetic equilibrium. Available evidence suggests that the major host for non-detrital Cu in hemipelagic sediments is authigenic nontronite. Because this phase appears to form slowly, there may be a kinetic constraint on the amount of copper which can be incorporated authigenically in rapidly-accumulating hemipelagic deposits. Support for this hypothesis is given by the observed inverse proportionality between the sedimentation rate and the concentration of non-detrital Cu in surface sediments from at least seven widespread locations in the eastern Pacific. It is suggested that the variation observed in the salt- and CaCO 3 -free Cu concentration in hemipelagic sediments is a consequence largely of the residence time of sediments at the surface, where high dissolved Cu concentrations prevail.
Pedersen Thomas F.
Southon John R.
Vogel John S.
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