Sediment biogeochemistry of iron and sulfur in an acidic lake

Abstract

The biogeochemical cycling of Fe and S in the sediments of acidic Big Moose Lake, Adirondack Park, NY, U.S.A., was investigated. Sediment cores and porewater samples were collected along a depth transect from the hypolimnion (24 m water depth), metalimnion (17 m), and epilimion (8, 12 m). Four vertically distinguishable zones in the sediment environment were observed at each site: 1.1) NO3− disappearance near the sediment/water interface;2.2) accumulation of solid-phase Fe in the top 5 cm;3.3) coincident accumulation of chromium reducible sulfur (CRS), disappearance of SO4− and minima in C:S and δ34S at or slightly below maxima in oxide-bound Fe; and4.4) homogenous background concentrations in S and Fe below 30 and 10 cm, respectively. Iron and sulfur accumulations in the upper 10 cm occurred at the same depths in cores of different ages, indicating that diagenetic rather than depositional processes played a dominant role in determining near-surface Fe and S profiles. Although sediment accumulation rates varied along the transect, the four zones were located at similar depths in the sediment column at all sites. Diagenetic processes play a major role in the development of these features in Big Moose Lake.The extent of Fe enrichment was considerably greater in the metalimnetic and epilimnetic sediments. In contrast, concentrations of Fe were lower in hypolimnetic sediments, which appeared to be losing Fe to the water column. At all sites, increases in total S from a background concentration of 60–80 μmol g−1 of dry mass occurred in sediments dated approximately 1850. More recent increases to 560 μmol Sg−1 occurred asynchronously in the cores, indicating an important role of sulfate reduction in adding S to sediments. However, organic sulfur accumulation accounted for 22–56% of the recent increase in sediment S. Concentrations of iron and organic carbon were high in lake sediments and probably do not limit S storage in this lake.