Background: Early diagenetic processes involved in natural organic matter (NOM) oxidation in marine sediments have been for the most part characterized after collecting sediment cores and extracting porewaters. These techniques have proven useful for deep-sea sediments where biogeochemical processes are limited to aerobic respiration, denitrification, and manganese reduction and span over several centimeters. In coastal marine sediments, however, the concentration of NOM is so high that the spatial resolution needed to characterize these processes cannot be achieved with conventional sampling techniques. In addition, coastal sediments are influenced by tidal forcing that likely affects the processes involved in carbon oxidation.

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Gold/mercury amalgam (Au/Hg) microelectrodes with a diameter of 25 microm were developed for the detection of environmentally relevant analytes such as manganese and iron by scanning electrochemical microscopy (SECM), and applied to investigate the controlled dissolution of manganese carbonate (MnCO(3); rhodochrosite) in acidic conditions. Characterization of the amalgam electrode geometry via approach curves recorded during SECM experiments revealed Au/Hg microelectrodes with sphere cap geometry. Quantitative determination of Mn(2+) has been achieved by calibration of the Au/Hg microelectrode in bulk solution experiments. Subsequent SECM imaging experiments confirm the applicability of amalgam microelectrodes for imaging of Mn(2+) production during dissolution of MnCO(3) at pH 3.9. This study confirms feasibility and provides the fundamental basis of SECM imaging with amalgam microelectrodes to address biogeochemically relevant questions.

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