HIGHLY VARIABLE SEDIMENT DISTRIBUTION IN SHALLOW WIND-STRESSED LAKES - A CASE FOR SEDIMENT-MAPPING SURVEYS IN PALEOLIMNOLOGICAL STUDIES

We examine sediment distribution patterns in seven Florida lakes and d iscuss implications for paleolimnological studies of shallow, subtropi cal lakes. The study lakes are highly productive and should exhibit th ick organic sediment deposits, but organic sediments are often grossly lacking because basins are shallow, and frequent mixing, lack of stra tification, and warm temperatures lead to breakdown of organic materia l. Organic sediment distribution patterns are highly variable. We obse rve three types of distribution patterns. When organic sediments are a bundant, there may be (1) uniform sediment distribution. In lakes lack ing organic sediments, there are (2) distribution to deeper areas if p resent, or (3) distribution to peripheral areas and embayments when de ep waters are absent. We advocate the use of systematic mapping survey s to locate optimal coring sites for paleolimnological studies of shal low, wind-stressed lakes. Because numerous factors affect diagenesis a nd sediment redistribution, sediment abundance and location are not pr edictable. Sediment chronologies may be discontinuous and disturbed ev en in accumulation zones. The extent to which sedimentary records are discontinuous or disturbed is not quantifiable in any practical manner . Pb-210 and Cs-137 radioisotopic profiles provide qualitative evidenc e of the degree of stratigraphic disturbance. Total excess Pb-210 inve ntories show that sediments are focused into depositional zones when s ediment distribution is uneven. Excess Pb-210 inventories are not info rmative about the completeness of sedimentary profiles unless small in ventories suggest discontinuous sedimentation or erosional events. We present examples of disturbed and undisturbed profiles, and discuss ho w we use radioisotopic and geochemical evidence, and multiple cores to assess stratigraphic continuity.