Fig 1.
Map of the Georgia coast showing the location of Sapelo Island and shell rings.
The map was created by CJG and VDT using ArcGIS Pro and wetland shapefile data from Georgia GIS Clearinghouse (https://data.georgiaspatial.org/index.asp).
Fig 2.
AMS models: (A) Probability distributions; (B) Posterior probability of the chronological relationships for the start and end date of the Sapelo shell rings.
Table 1.
Modeled dates from Sapelo Shell Rings I, II, and III.
Fig 3.
Box plots comparing (A) estimated salinity and (B) mean LVH between the three shell rings, showing significantly lower estimated salinity and smaller shells at Ring III.
The shell rings are in chronological order based on the radiocarbon model, and red diamonds show mean values for each ring.
Table 2.
Descriptive statistics for oyster measurements and oxygen isotope analysis.
Fig 4.
Examples of individual shell δ18Ocarbonate profiles showing seasonal fluctuations in oxygen values and estimated season of harvest.
The data sequence follows ontogeny from right to left, with the first value representing time of capture. The dashed lines in each graph represent the values that divide the sample range for each profile into equal thirds. If the first sample value falls below the bottom line the shell was collected in the summer, and if it falls above the top line then the shell was harvested in the summer. If the first value falls between the lines, then the shell was harvested in either the fall or spring.
Table 3.
Estimated summer δ18O water (‰ VSMOW) values modeled after Andrus and Thompson’s (2001) oxygen isotope-temperature equations (Eqs 1 and 2), assuming shell growth cessation at 28°C for oysters and 31°C for clams.
Salinity (psu) calculated based on Eq 3.
Fig 5.
Temporally relevant portion of the multimillennial tree-ring chronology derived from a deposit of ancient buried bald cypress trees at the mouth of the Altamaha River.
The chronology is in indices (standardized units representing average ring width, largely indicative in this locale of winter-spring precipitation), with “1000” indicating an annual ring of average width. Enhanced interannual rainfall variability and numerous very dry years are evident beginning around the earliest occupation of Ring I.