ANAEROBIC BOTTOM WATERS NEED NOT BE DEEP

Modern and ancient shallow-marine sedimentary sequences contain numerous examples of black anaerobic sediment sequences caused by anaerobic conditions from excessive organic and nutrient discharge from adjacent low-lying coastal wetlands and/or from coastal sediment recycling during transgression.

Jupiter Estuary, southeast Florida, contains over 4 m of layered organic mud and quartz sand which formed over the past 6,000 years in water depths never more than 3 meters. The sequence central estuary is layered without bioturbation except for a brief interval early in the estuary's history and near the surface (because of human-termination of drainage from the adjacent freshwater wetland). Eutrophication and low light maintained a near anaerobic shallow environment and bottom nepheloid bottom surface through persistent input of tannin-rich waters, fine organics, and nutrients from the adjacent Loxahatchee Swamp.

With accelerating sea level rise, coastal wetland erosion is producing similar anoxic bottom water conditions in shallow portions of Florida Bay and adjacent bays.

In the ancient, zones of non-burrowed and non-skeletal layered carbonates are thought to reflect shallow anoxic phases in the marine transgressive portion of meter-scale cycles in the upper Cambrian (Conocheague Ls.) and Lower Ordovician (St. Paul’s Group) of Appalachian shallow marine to tidal flat sequences – a time without major ice-driven sea level fluctuations.

The late Lower Cambrian Forteau Formation of northwest Newfoundland and Labrador has repeated alternations of layered black shales with fossiliferous limestone and clean archaeocyath reefal limestone. In a time of waning glacial influence, fluctuations in nutrients, sediment recycling and water clarity during small transgressions are considered more likely controls than large sea level fluctuations.