No abstract

In marine sediments, Fe(III) is found predominantly as a solid. Recently, however, soluble species of Fe 3ϩ complexed by natural organic ligands have been detected in coastal marine sediments with voltammetric microelectrodes. The role of soluble Fe 3ϩ complexes in diagenesis is unknown. In anoxic conditions, soluble Fe 3ϩ can effectively oxidize FeS 2 and recycle iron and sulfur for use as terminal electron acceptors during natural organic matter (NOM) degradation. Alternatively, soluble Fe 3ϩ complexes can catalyze the formation of FeS and FeS 2 through the rapid chemical reduction of Fe 3ϩ by dissolved sulfide. To better understand the role of soluble Fe 3ϩ in the transformation of iron and sulfur in marine sediments, we incubated the first few centimeters of unvegetated salt marsh sediment in plug-flow reactors. Microbial iron reduction seemed to prevail in suboxic conditions, but sulfate reduction outcompeted microbial iron reduction in the presence of reactive organic metabolites. The dominance of sulfate reduction led to the complete removal of reactive iron oxides by precipitation of FeS. Experiments mimicking the enrichment of soluble Fe 3ϩ complexes in reduced sediments show that soluble Fe 3ϩ does not reoxidize FeS and pyrite; rather, it promotes pyrite precipitation by enhancing sulfate reduction via complex bacterial interactions. The rate of pyrite formation in the presence of soluble Fe 3ϩ is much higher than previously reported in the literature, suggesting that soluble Fe 3ϩ might promote alternative pathways for microbial degradation of NOM that ultimately results in the immobilization of Fe and S as reduced iron sulfide minerals.

No abstract

Abstract. Since reclamation of the Pecos mine waste pile, area roads and campgrounds, and the Alamitos Canyon mill was begun in 1991, there has been continued monitoring of metal and trace element concentration in stream sediments and water along the Pecos River, extending from the upper Pecos River to below Brantley Dam, north of Carlsbad. In 1992, 1996, and 2000, surface water and stream sediments were sampled and analyzed for the entire reach of the Pecos River. Between 1992-1996 the upper reach of the river between the Pecos Wilderness and Villanueva was sampled on 8 occasions as part of a multidisciplinary study. These efforts were initiated so that point and non-point sources of contamination associated with the areas of concern could be identified and the effects of reclamation examined. The Pecos mine generated approximately 70,000 m 3 of waste rock, which was piled at the mine site on a slope above the river. The crushed ore from the mine was transported by aerial tramway to the Alamitos Canyon mill 18 km south of the mine. Both mine waste and mill tailings have been sources of acidic drainage contributing elevated concentrations of metals and other trace elements. This paper provides the most recent year's sample data for the stream sediments and compares the data to the earlier year's data. In this study, the <63 µ fraction was digested with aqua regia and analyzed for Cd, Cr, Cu, Fe, Mn, Pb, and Zn using flame atomic absorption spectroscopy. Geochemical trends over the period 1992-2000 confrrm a decrease in concentrations with time since reclamation began, especially in the immediate vicinity of the Pecos mine and below the confluence of Alamitos Creek which drains the mill site area. In 2000, a sharp increase was noted in Cu and Pb levels in stream sediments immediately below the fish hatchery, 19 km below the Pecos mine. Additional

No abstract