A MODERN ANOXIC ENVIRONMENT IN COASTAL WETLANDS

A complex of three closely related wetlands is being studied to determine whether organic-rich mucks are the precursor to black shale formation. The wetland complex includes a brackish tidal marsh associated with the San Francisco Bay delta and the San Joaquin River. Saline bay water mixes twice daily with freshwater from the San Joaquin River to form a tule dominated backwater marsh. Dissolved oxygen levels at the boundary layer routinely measure 0 mg/L at high tide regardless of temperature during 3 seasons of the year. Hypoxic conditions (2-3 mg/L DO) exist at the boundary layer during winter, which results in a benthic environment rich with macro-invertebrates.

The complex also includes a lentic 1-meter deep freshwater marsh fed by groundwater and dominated by cattail reeds. Decades of cattail growth have created an oligotrophic system marked by anaerobic decomposition. Dissolved oxygen levels at the boundary layer measure 0 mg/L regardless of water temperature. Hydrogen sulfide gas bubbles regularly from the black sediment. Iron sulfide regularly forms on the water’s surface. The benthic environment is dominated by amphipods that regularly succumb to rapid die-offs.

The third wetland in the complex is a brackish lentic pond dominated by tule fringe. The pond is 3 meters deep with boundary layer DO measured at 0 mg/L during 3 seasons. During winter hypoxic conditions prevail with DO measured at 2-3 mg/L. Gastropods are plentiful during winter but quickly die-off as sediment from runoff settles, bringing DO levels to 0 mg/L.

These repeated anoxic occurrences and depositions of plant and animal debris contribute to the organic-rich sediment that may be a precursor to black shale. Future studies will focus on studying the seasonal layers of organic deposition as well as gradient effects on the degree of anoxia along the boundary layer.