Paleoenvironmental interpretation of sinkhole sediments on a flat low-lying carbonate platform, a case study from Chassahowitzka River, west-central Florida

After rapid increases in ocean volumes during the early Holocene, sea levels rose more slowly from the mid- to late-Holocene. The flat shelf of the eastern Gulf of Mexico meant even small rises in sea level shifted the coastline inland many kilometers. The impact of this transgression on the karst environments of western Florida is not well known. Here we investigate how one Floridian location responded to changing climate from the mid- and late-Holocene. Chassahowitzha River, representing one of many west-central Florida rivers, is spring fed and discharges into the Gulf of Mexico. Sinkholes along this river have the potential to preserve complete and un-disturbed sedimentary records. Three sediment cores were collected from a sinkhole complex just north of the river. All three cores contain their sedimentary record down to the limestone bedrock. Ten radiocarbon dates were used to create the core chronologies. Interpretation of paleoenvironmental changes is based on sediment grain size analysis, microphotography, loss-on-ignition organic carbon contents and identification of microfossils in the sediments. Prior to 7 ka BP, sediments are dominated by quartz sand followed by an organic-rich layer ending at 6 ka BP. We interpret the organic sediments as the onset of a freshwater wetland/marsh environment along the river. Most of the marsh sediments are deposited between 7 and 6 ka BP when the coastline was 30 kms seaward. Next, there is a 3.8 kyr gap in deposition as demonstrated by the radiocarbon chronology. This hiatus could have been caused by changes in sea level, periods of aridity, draining of the sinkhole lakes or the development of the sinkhole complex. After this interval the sedimentary record is characterized by inorganic deposits which contain an abundance of microfossils. The topmost sediments of each core were interspersed with discrete shell layers which may be indicative of extreme storm events. • Record spans 7 ka from coastal sinkholes. • Paleo-sea level lead to a transition from an arid terrestrial setting to a freshwater wetland and the present salt marsh. • Changinging sea levels between 5 ka and 2 ka possibly caused a cessation of deposition or internal geomorphic processes. • Several discrete shell layers found within the upper 2 ka of record correspond with tropical storm overwash.