Constraints on oxidation-reduction chemistry in estuarine salt marsh sediments of the Hackensack Meadowlands, NJ

Abstract

The anaerobic decomposition of organic matter in anoxic estuarine sediments is governed by a series of oxidation-reduction (redox) reactions, which regulate the cycling of sulfur, carbon, iron, and phosphorus in sediments and sediment porewaters. The objective of this study is to document the co-variation of pH, dissolved SO₄^2-, ΣH2S (total dissolved sulfide), ΣCO₂ (total dissolved inorganic carbon), Fe, Mn, and PO₄^3- in a series of porewaters extracted from estuarine sediments from the Hackensack Meadowlands, NJ, in order to constrain the redox conditions in the sediments. The oxidation of organic matter via microbially mediated sulfate reduction is the dominant decay process in these salt-marsh sediments. The decomposition of organic matter results in a decrease in SO₄^2-, and a concomitant increase in ΣCO₂ and ΣH₂S with depth in the sediment porewaters. Under these conditions, reactive Fe- and Mn-oxyhydroxides are unstable, and their reductive breakdown results in the releases of dissolved Fe and Mn to the porewaters. In addition, PO₄^3- adsorbed to the Fe- and Mn- oxyhydroxides is also released. When ΣH₂S is in excess, sulfides react with the dissolved Fe to form relatively unstable Fe-sulfides (acid volatile sulfides) and pyrite. The precipitation of these Fe-sulfides depletes the concentration of Fe in the porewaters. In older sediments, in which the supply of dissolved Fe is exhausted, ΣH₂S accumulates in the porewaters up to ~160 μmol/L. In younger sediments, Fe concentrations are higher (up to ~200 μmol/l) and ΣH₂S are lower (up to ~35 μmol/l), and their cocentrations are inversely correlated. Mn-sulfides are rarely formed under these conditions so dissolved Mn accumulates in porewaters and may diffuse into overlying surface waters. The PO₄^3- concentrations are influenced by both organic matter decay and desorption from Fe-oxyhydroxides during the breakdown Fe-P adsorption complexes.