REDOX ZONATION IN THE ANOXIC OCEAN

Five principal redox zones in the open ocean: Oxic, Nitric, Sulfatic, Carbonic, and Nitronic are proposed based on the sequential thermodynamic availability of electron acceptors needed to oxidize marine planktonic organic matter. This is a chemical oceanographic refinement of the physical oceanographic model of redox conditions presented in Wilde [1987]. A sequence of such chemical zones may have been present in the ancient ocean when anoxic conditions were more common before the ventilation of the ocean with biogenically derived atmospheric oxygen and peri¬odically since then, due to a combination of extended warm climatic intervals, poorer oceanic circu¬lation and separation of basins from the global ocean. Maintenance of individual redox zones in the water column would occur in the pycnocline, where maximum stability and stratification are found. The redox boundaries in terms of equivalent oxygen for each zone is a function of the reservior of available oxidant and amount of reducing agent as planktonic organic matter and is calculated in terms of modern sea water chemistry. Generic formulae are given so that equivalent oxygen values can be calculated for varying oxidant reservoirs of the past. Equivalent oxygen values at the boun¬daries can be substituted into redox profiles with depth to obtain the thickness of each zone. The intersection of the various zones with the bottom would create different initial interstitial water chemistries for diagenetic water-sediment interactions, which may be discernible in the geologic record.

An interactive computer model is presented generating a 2-D horizontal and vertical picture of the zones with user inputs of % PAL and standard oceanic depths.