High sulfur content of coal is generally attributed to marine inundation during or shortly following peat formation. When a large peat deposit on the Caribbean coast of Panama subsided during a 1991 earthquake, sulfate-rich seawater inundated the margin of the deposit, providing the opportunity to evaluate the role of short-term marine flooding on sulfur abundance and chemistry. Salinity and pH measurements across the new marine margin indicate that penetration of saline waters into the peat is restricted to <2 m both vertically and horizontally. A comparison of forms of sulfur in the newly flooded peat vs. adjacent, subaerial peat reveals that total sulfur content is not increased by flooding, but that the distribution of sulfur forms differs markedly after marine inundation: inorganic forms, particularly mineral sulfate, make up a higher proportion of the total; the organic sulfate fraction becomes highly variable; and the carbon-bonded sulfur content remains seemingly unaffected. Heightened bacterial activity is seen as a likely mediator in this redistribution of sulfur forms. Our results indicate that a high sulfur content in coals and peats cannot form by short-term periodic flooding events such as storms but must reflect long-term infiltration of marine or brackish waters measured on time scales of hundreds to thousands of years.