High Resolution Profiles and Fluxes of Redox Species in Continental Margin Sediments: Implications for the Cycles of Nitrogen, Iodide, Manganese, and Iron

Abstract

The preferential use of the electron acceptor that yields the highest amount of free energy in the bacterially mediated oxidation of organic matter constitutes a long-standing paradigm in biogeochemistry. In marine sediments, this leads to a well-established vertical sequence of redox reactions in which oxygen is reduced near the sediment-water interface, followed in turn by the reduction of nitrate and manganese oxide, iron oxide, sulphate, and finally carbon dioxide (e.g. Froelich et al., 1979). The early expressions of the paradigm were limited to these major redox elements, but thermodynamic calculations and experimental observations have shown that other redox elements (for example I, Se, Cr) can also be involved in organic carbon oxidation, either directly or by responding to changes in redox potential. Progress in understanding the zonation of redox reactions in sediments is closely tied to the ability to make high resolution measurements. Here we report new measurements of redox species in undisturbed sediment cores from 100m, 200 m and 325 m depth along a cross-channel section in the Laurentian Trough in the Gulf of St. Lawrence. The vertical distributions of 02, Mn(II), Fe(II), I(-I), and HS(-I) were measured with millimeter resolution in the top several cm of the cores using a voltammetric Hg/Au amalgam microelectrode (Brendel and Luther, 1995). The distributions of NO3, NH2 and HPO]were measured colorimetrically, using porewater samples obtained by conventional slicing and centrifugation of replicate cores. Solid-phase Mn was extracted with HC1 and measured by AA-spectrometry. Benthic fluxes of 02, NO3 and NH~ were measured by incubating cores at the in s i tu temperature. The vertical distributions we measured were qualitatively similar at the three sites. In all cores, 02 disappeared at about 4 mm depth, followed by the successive appearance of I(I-), Mn(II) and Fe(II). The appearance of Fe(II) was very abrupt and was accompanied by a broad peak in the voltammogram consistent with the presence of colloidal Fe(III). As in our previous studies with this microelectrode, there was a depth interval in which neither 02 nor Mn(II) could be detected. The concentration of iodide increased sharply below the sediment surface and displayed a sharp subsurface maximum of about 5 gM at the very depth where Mn(II) was first detected. NO3 decreased rapidly across the sediment-water interface to levels below detection limits at 10 mm depth. Below this depth NO3 increased again and displayed a broad concentration maximum with concentrations as high as 15-40 gM. These data are consistent with the successive use of electron acceptors during organic carbon degradation according to free energy yield. However, a close examination of the data reveals new details about diagenetic processes. Thus, a nitrate minimum located near the base of the oxic sediment layer supports a flux of nitrate from the water column into the sediment which agrees with the measured nitrate flux. This is consistent with classical anaerobic denitrification. However, a sharp ammonium gradient across the oxic surface layer is not reflected in a corresponding release of ammonia from the sediment, indicating there is an important ammonium sink within the oxic layer. A possible sink could be the coupled nitrification-denitrification within this layer; this would require the presence of