Bidirectional sulfate diffusion in saline-lake sediments: Evidence from Devils Lake, northeast North Dakota

Chemical and isotopic gradients in pore water in Devils Lake indicate that maximum rates of sulfate reduction occur between 1 and 3 cm depth in the bottom sediments. Dissolved sulfate diffuses into the sulfate-reduction zone upward from deeply buried saline pore water at an average rate of 1.4 x 10^-5 μmol ⋅ cm^-2 ⋅ s^-1, and downward from the overlying water column at an average rate of 2.4 x 10^-5 μmol ⋅ cm^-2 ⋅ s^-1. The result is a bidirectional flux of sulfate into the sulfate-reduction zone. Upward-diffusing sulfate provides a ready supply of electron acceptors for sulfate-reducing bacteria even at fairly great depths in the sediments. The abundance of electron acceptors enables sulfate-reducing bacteria to outcompete methanogenic bacteria for organic material and thereby suppress methane production. Suppression of methanogenesis may be widespread in sulfate-rich lakes and wetlands and may limit methane fluxes from these water bodies to the atmosphere.