GEOCHEMISTRY OF A MODERN ANOXIC ENVIRONMENT AND IMPLICATIONS FOR ANCIENT ANOXIC ENVIRONMENTS

A small pool of approximately 8 m in diameter in Buck Run, Livingston County, New York, becomes anoxic during periods of low flow. The water in this pool is stratified, where the boundary between oxygenated and anoxic water is approximately 0.5 m beneath the surface. Water samples were collected using a Masterflex peristaltic portable pump. Measurements of pH, Eh, temperature, TDS, and dissolved oxygen were taken in October 2000. The temperature of both waters was 18 ^oC. The anoxic water had a pH of 6.2 and Eh of -199 mV. The oxic water had a pH of 7.6, and Eh of 100 mV. The dominant ions in the anoxic water were Na⁺, Ca²⁺, Cl^-, and SO₄^2-. The dominant ions in the oxic water were the same, but at approximately half the concentration. The chemistry of the anoxic water closely resembles the chemistry of groundwater in shale, which underlies the stream.

Development of an anoxic environment in a clear flowing stream conducive to mixing is unusual. The anoxia seems to be due to discharge of groundwater that has percolated through pyritic black and gray shales. This stimulates us to think of larger scale settings where shale dewatering or degassing could result in basin-wide stratification and anoxia.