Abstract

The potential importance of sedimentary Mn as a secondary oxidant and redox intermediate between O2 and Corg is often discounted in nearshore sediments. Study of the Mn cycle at 19 stations in Long Island Sound (LIS) demonstrates that sedimentary Mn can be a significant redox intermediate, accounting in many cases for 30–50% of the benthic O2 flux (annual mean ≈40 ± 35%). At some sites and times, the import of solid oxidant as Mn in suspended matter is also apparently greater than the dissolved O2 flux to the bottom. The relative importance of the Mn cycle as redox intermediate varies substantially both seasonally (summer > fall > spring > winter) and spatially as a function of biogenic reworking, Corg flux, and O2 concentration in the overlying water. During warmer seasons, the net flux of Mn++ from the bottom decreases (≈5–10x) generally from west to east, correlating directly with the benthic flux of planktonic debris and storage of residual Corg. Average fluxes are −0.003, 0.43–0.94, 2.2, and 0.43 mmol Mn/m2/d during winter, spring, summer, and fall respectively. Mn++ fluxes are relatively elevated during the spring bloom despite low temperatures. At most sites and times, surface sediments are enriched in excess Mn (4–17 μmol/g) above lithogenic background, with exponentially decreasing concentrations to 2–3 cm depth. A regular seasonal and spatial cycle of excess Mn occurs. Excess Mn inventories are often ≈5–10 μmol/cm2 but range from ≈0–25 μmol/cm2. The highest inventories are found in mid LIS, in the transition area between high Corg flux and seasonally low O2 regions of the western Sound and the lower Corg flux, better oxygenated regions of central LIS. Excess Mn decreases at most sites following the spring plankton bloom and is lost entirely from westernmost sediments of highest Corg flux, several months before noticeable O2 depletion in overlying water. The bloom is a major factor in mobilization of metals into suspended matter and promotes lateral redistribution of Mn. Destratification and oxygenation of the water column in the fall results in the capture and reestablishment of excess sedimentary Mn in all regions of the Sound. Bioturbation transports Mn and Corg into anoxic sediment zones. When overlying water is well-oxygenated, the resulting Mn++ is efficiently irreversibly adsorbed or reoxidized (≈80–90%, during summer–fall), closing the sedimentary Mn cycle and inhibiting net Mn++ fluxes. The internal Mn cycle is therefore most important as an intermediate oxidant during warm periods of high bioturbation, well-oxygenated overlying water, and moderate Corg flux. S species apparently dominate the direct reduction of Mn.

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