HOLOCENE DEPOSITIONAL HISTORY IN TWO FLORIDA GULF COAST ESTUARIES: TAMPA BAY AND CHARLOTTE HARBOR

Tampa Bay and Charlotte Harbor, the two largest estuaries on Florida’s Gulf coast, exhibit similar patterns in Holocene sediment deposition that are likely controlled more by regional environmental factors than local influences. A series of vibracores and surface sediment samples show a similar Holocene sedimentary architecture in both estuaries characterized by organic-rich muds overlain by sandy mud and muddy, shelly sand/gravel. Organic-rich muds yield radiocarbon dates ranging from ?6 ka in the lower parts of the estuaries, to ?3.5 ka in the upper parts of the estuaries, and are interpreted to represent paralic deposition during early flooding of the Holocene sea-level rise. Overlying sandy mud and muddy, shelly sand/gravel deposits represent the natural deposition of open-estuarine sediments as sea level reached its present position. Average accumulation rates of open-estuarine deposits are strikingly similar for both Tampa Bay and Charlotte Harbor ranging from ?30-65cm/ky (0.030-0.065 cm/yr). The surficial 30± cm in both estuaries show no appreciable differences in texture and/or composition from underlying sediments, but Pb-210 and Cs-137 data indicate accumulation rates have increased dramatically, ranging from 0.16-0.32 cm/yr. Once again, these surficial accumulation rates are strikingly similar for both estuaries and the dramatic increase is likely due to anthropogenic activities over the past 100 years.

The sediment distribution patterns in Tampa Bay and Charlotte Harbor have not substantially changed since being flooded by the Holocene sea-level rise, and therefore are not strongly controlled by the increase in anthropogenic activities during the last 100 yrs. Conversely, the sediment accumulation rates have been strongly influenced by anthropogenic activities with an approximate one order-of-magnitude increase over the last 100 yrs. This implies that although anthropogenic activities have had a strong impact on these estuaries by increasing the rates of sediment input, regional factors (e.g., topography, geology, climate) likely exert a greater control on how sediments are ultimately distributed.