Copper and manganese in hemipelagic sediments at 21 °N, east pacific rise: Diagenetic contrasts

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 14C-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 CaCO3-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.