ARCHIVES OF PREINDUSTRIAL SEASONALITY PRESERVED IN ARCHAEOLOGICAL SHELLS (MERCENARIA CAMPECHIENSIS) AND OTOLITHS (ARIUS FELIS), SOUTHWEST FLORIDA

Carbonate skeletons from Late Holocene archaeological deposits of Pine Island, Florida provide valuable archives of seasonality and associated human responses. Seasonality is reconstructed from oxygen isotopes (δ¹⁸O) in estuarine clam shells and fish otoliths. Interpreting δ¹⁸O records from estuarine skeletons is complicated because both temperature and salinity (fresh and saltwater end members have different δ¹⁸O values) vary seasonally. We present a multi-taxa approach to reconstruct preindustrial seasonality from δ¹⁸O variation in southern quahog shells (Mercenaria campechiensis) and hardhead catfish otoliths (Arius felis).

Modern and archaeological shells and otoliths were analyzed isotopically. Archaeological specimens were selected from Caloosahatchee I and II cultural periods and correspond to Roman Optimum (RO) and Vandal Minimum (VM) climate intervals. δ¹⁸O from modern shells record ambient water conditions when compared to a predictive model. Highest δ¹⁸O values from RO and VM shells are similar to modern shells indicating winter temperature and salinity were comparable to today. Lowest δ¹⁸O values from VM shells are similar to that of modern shells, whereas lowest δ¹⁸O of RO shells were 1‰ higher than modern values suggesting elevated summer salinity during the RO.

Otolith δ¹⁸O was converted to temperature using the equation from Patterson et al. (1993, AGU Monograph 78) and assuming δ¹⁸O of Gulf of Mexico water is 1‰. Temperature estimates from modern otoliths are reasonable for winter months when A. felis migrate into the Gulf. Winter temperature calculated from RO and VM otoliths indicate winter conditions during RO and VM were similar to today. Summer temperature calculated from modern and VM otoliths is overestimated, reflecting brackish salinity when catfish inhabit estuarine waters during their reproductive season. In contrast, summer temperature estimated from RO otoliths corresponds to modern temperature, suggesting higher salinity estuarine water (and perhaps lower precipitation) during that time.

Our interpretations based on geochemical data are consistent with those based on zooarchaeological data. Combining geochemical records with zooarchaeological proxies provide independent archives of climate change and seasonality.