Seasonal Pore Water Dynamics in Marshes of Barataria Basin, Louisiana

Abstract

AbstractSediment redox potential (Eh), interstitial pore water pH, sulfate, carbonate, and trace metals were measured monthly from August 1984 to August 1985, in a salt, brackish, and freshwater marsh in Barataria Basin, LA. The salt marsh was characterized by low Eh during the summer and fall, resulting in pyrite formation. Early spring oxidation of the sediments created extreme acid conditions, indicating that pyrite was oxidized completely to sulfate. Brackish and freshwater marshes exhibited a much greater seasonal variability, with strong vertical redox fluctuations. In the freshwater marsh, both Fe2+ and Mn2+ were controlled by temporal pH changes and were relatively independent of oxidation‐reduction phenomena, suggesting a possible organic control mechanism. However, in the brackish marsh, Fe2+ concentrations were governed by pH (r = −0.74** significant at the 0.01 probability level) and Eh (r = 0.65**) changes, indicating that Fe2+ concentrations were likely controlled by pyrite formation and oxidation. Ion activity product calculations indicated that iron and manganese phosphates and carbonates were unlikely to form in Barataria Basin marshes. However, iron and manganese sulfide supersaturation occurred in all three marshes. Brackish marshes were found to contain significantly higher pyrite concentrations (1.4% w/w), than salt (0.69% w/w) and freshwater (0.66% w/w) marshes. The dynamic pyrite cycling in Louisiana salt marshes, resulted in a low pyritic pool, characterized by single, fine grained euhedral crystals. Pyritic Fe represented 23% of the total Fe accumulation flux in the salt marsh, and 34% in the brackish marsh.