Surface complexation of Cu on birnessite (δ-MnO2): Controls on Cu in the deep ocean

Hexagonal birnessite (δ-MnO₂) is a close analogue to the dominant phase in hydrogenetic marine ferromanganese crusts and nodules. These deposits contain ∼0.25 wt.% Cu which is believed to be scavenged from the overlying water column where Cu concentrations are near 0.1 μg/L. Here, we measured the sorption of Cu on δ-MnO₂ as a function of pH and surface loading. We characterized the nature of the Cu sorption complex at pH 4 and 8 using EXAFS spectroscopy and find that, at pH 4, Cu sorbs to birnessite by inner-sphere complexation on the {0 0 1} surface at sites above Mn vacancies to give a three to fourfold coordinated complex with 6 Mn neighbors at ∼3.4 Å. At pH 8, however, we find that some Cu has become structurally incorporated into the MnO₂ layer by occupying the vacancy sites to give 6 Mn neighbors at ∼2.91 Å. Density functional calculations on and clusters predict a threefold coordinated surface complex and show that the change from surface complexation to structural incorporation is a response to protonation of oxygens surrounding the vacancy site. Consequently, we propose that the transformation between sorption via surface complex and vacancy site occupancy should be reversible. By fitting the Cu sorption as a function of surface loading and pH to the formation of the observed and predicted surface complex, we developed a surface complexation model (in the basic Stern approximation) for the sorption of Cu onto birnessite. Using this model, we demonstrate that the concentration of inorganic Cu in the deep ocean should be several orders of magnitude lower than the observed total dissolved Cu. We propose that the observed total dissolved Cu concentration in the oceans reflects solubilization of Cu by microbially generated ligands.     

Possible evidence for on-going volcanism on Mars as suggested by thin, elliptical sheets of low-albedo particulate material around pits and fissures close to Cerberus Fossae

High quality CCD images obtained at two different observatories in North Italy allowed the identification of four morphological structures near the nuclear region of the comet Ikeya-Zhang (I-Z): haloes, jets, shells and spirals. The interpretation of the nature of these structures has been attempted by means of a comparison of different up-to-date image processing techniques, which led to a single common estimate of the rotation period (p=1.48±0.20 days).     

Satellites that see the Earth move

Sheila Peacock summarizes the British Geophysical Association's annual Advances in Geophysics meeting, which took place on 10 and 11 February 2005, on the theme of "The Impact of Satellite Measurements on the Observation and Modelling of Continental Deformation".