FRESHWATER PEAT-OVER-MARL SEQUENCES: STRONG DIFFERENCES WITH EVERGLADES MARLS

Many Holocene-age peat bogs in eastern North America have freshwater calcitic sediments below the organics. The calcitic, sometimes shelly, fine granular carbonate is unconsolidated and commonly called marl, though silicious matter often is minor. Most date only slightly older and are thought to be conformable with the overlying organic sediment, which is usually fibrous peat in upper layers but sometimes fine-grained organic muds at depth. For northern areas the interpretation is generally the "textbook" shallow-lake infilling, but with bog encroachment from the edges and the special condition of a marl lake at the onset. Familiar lake marl forms in open-water, continuously inundated, environments. The infilling and conversion to bog involves shallowing but not necessarily any change in average water elevation; thus there may be no strong hydrologic or climatic signal in the carbonate-to-organic shift. In southern Florida, much of the Everglades peatland as well as certain other important bogs such Corkscrew Swamp similarly are underlain by marl, but of an apparently much different origin and the peat-over-marl sequence there invokes a different hydrologic history. Freshwater calcitic mud forms now in the southern Everglades (and in similar terrain at Andros, Bahama Islands) in seasonal marshes that are dry for more of the year than nearby deeper peat-forming marshes. The calcite is precipitated in mats of blue-green algae that require the abundant sunlight of these sparse marshes and the recurring long drying helps destroy the accompanying organic matter. Peat-over-marl sequences in southern Florida thus required a significant change in surface-water flooding at the shift, including in average (though fluctuating) water elevation, and thus in the hydrologic controls on flooding. Rising sea level as hydrologic drainage baselevel and hydrologic feedback by sediment sealing of drainage by marl accretions, do not seem sufficient as main causes because peat also initiated at more elevated sites in the SE US region above siliceous sediments at about the same time (mid-Holocene). A climatic shift to wetter conditions is strongly suggested as the cause for the increase in "wetness" indicated by the marl-to-peat shift in southern Florida.