U.S. Department of the Interior
U.S. Geological Survey
OFR 2007-1203

Discussion and Future Directions

The temporal resolution necessary in interpreting questions of human impact, sea level rise and climate change in near-shore Holocene sediments can be limited by the constraints of radiometric dating. As mentioned above, statistical uncertainty increases in samples less than 1000 years old due to the decrease in ¹⁴C activity and counting error (Stuiver, 1986). For estuarine carbonates, the question of which correction factor to apply is a significant issue. Nine estuarine mollusks from nearshore sites had ¹³C values ranging from -4.4 to -7.1 ‰ (Table 3), which is within the range for freshwater shells (Gupta and Polach, 1985); however, because they are known only from saline waters, we have applied the marine correction factor. In addition, problems exist with the dating of freshwater carbonates. Beta Analytical states in their explanation of “Calendar Calibration” that “reservoir corrections for fresh water carbonates are usually unknown” (http://www.radiocarbon.com/calendar.htm, last accessed 3/12/2007). Other researchers have reported on the problems and constraints of using freshwater shells for ¹⁴C dating (Culleton, 2006; Meadows, 2005; Mienis, 2005; Pilcher, 1991; and Zhou and others, 1999).

Contamination of a sample by older or younger carbon can be a factor in any environment (Lowe and Walker, 1997; Stuiver, 1986), but in south Florida contamination from the underlying Plio-Pleistocene limestone is of particular concern (Scholl and Stuiver, 1967). Groundwater moving through the porous limestones and surficial waters are dissolving the “old” carbonates and introducing “old” carbon into the modern environment for uptake by the living organisms. Concurrently, the same groundwater can be carrying “younger” radioisotopes, thus resetting the decay clock at the base of some of the cores (this process is seen in the ²¹⁰Pb diagrams for some of the cores in nearshore settings.) The introduction of old carbon from the bedrock introduces error that can artificially age the sample (Lowe and Walker, 1997; Stuiver, 1986). In addition, atmospheric input may not be a constant in all reservoirs as assumed (Reimer and others, 2002b; Stuiver, 1986). Stuiver and others (1998) discuss variations in marine reservoir age over time due to changes in ¹⁴C production rates as a result of geomagnetic and solar-related fluctuations.

Reimer and others (2002a and 2002b) discuss the importance of establishing regional reservoir correction factors for calibration of marine ages. The potential sources of error discussed above for south Florida indicate the importance of establishing a reservoir correction specifically for the relatively enclosed basins of Florida Bay and Biscayne Bay. As Beta Analytical Laboratories cautions, “In the case of carbonates, reservoir correction is theoretical and the local variations are real, highly variable and dependent on provenance.” (Beta Analytical, http://www.radiocarbon.com/calendar.htm, last accessed 3/19/2007).

The age models presented here represent the most accurate models we can produce with the data and methods we currently have available and they provide a framework within which we can interpret ecosystem history data from these cores. To improve these models would require a multi-tiered approach to address the questions of reservoir correction factors, biological fractionation, uptake of old carbon by living organisms, and the effects of groundwater migration in distribution of isotopes. Such an undertaking is outside the scope of our current study, but warrants further investigation by researchers in order to address societal needs in questions of Holocene environmental changes.