Historical Loading Record of a Post-Depositionally Reactive Substance: A Model for Sulfur in an Adirondack Lake

Abstract

A model for the distribution of sulfur in the sediments of a lake in the Adirondack Mountains, New York State, is developed to gain insight into the timing and magnitude of anthropogenic increases in sulfur loading. Surficial sulfur concentrations are about 3500 µg/g, increase to 6000-7000 µg/g at a depth of about 6 cm, and then decrease downcore to background levels of about 2500 µg/g. Sulfate concentrations are about 60-80 µM in the overlying water and decrease rapidly below the sediment water interface. This reflects assimilation and/or dissimilatory reduction of sulfate into the solid phase fractions of the sediment. A mathematical model is constructed assuming sulfur is incorporated into sediments by burial of detrital organic matter and by diffusion with subsequent fixation of sulfate from the overlying water. Several historical scenarios of atmospheric sulfate loading rates were used as model boundary conditions. Model results are compared with observed sedimentary sulfur profiles. The observed sediment sulfur profiles are best described using model boundary conditions showing increases in lake water sulfate concentrations, and thus presumably in atmospheric deposition, within the last 50-60 years.