Calusa archaeological deposits in SW Florida preserve cultural information and consequences of paleoenvironmental change. Geochemical records from associated clam shells provide subtropical climate archives in an archaeological framework. Here, we test climatic interpretations based on chronostratigraphic and zooarchaeological evidence and reconstruct seasonality and climate change using d¹⁸O of shells (Mercenaria campechiensis) that accumulated during the Caloosahatchee IIA (CIIA) cultural period (AD 500-800). CIIA coincides with the Vandal Minimum (VM) climate interval.

Calusa mound complexes are vertical structures that rise 8-9 m above mean sea level (MSL). Their bases (AD 500-800) are expansive middens that accumulated along a former shoreline, suggesting MSL was ~50 cm lower than today. Transition from midden to mound building may have resulted from rising sea level. Faunal analyses suggest low salinity and sea level during Early CIIA. Abundant remains of migratory ducks, not found in any other period, suggest Early CIIA was probably the coldest period between AD 50 and AD 1500.

O isotope compositions of shells from Early CIIA (d¹⁸O-VM) were compared to shells from the Little Ice Age (d¹⁸O-LIA) and to a shell (d¹⁸O-REC) that grew during a recent cold period (1976-1979). d¹⁸O-VM ranges from +1.78 to -1.67‰, d¹⁸O-LIA from +1.19 to -1.87‰, and d¹⁸O-REC from +1.25 to -1.87‰. The most negative d¹⁸O values in VM and LIA shells occur at dense bands corresponding to summer hibernation, typical of shells growing at low latitudes. In contrast, the most positive values in d¹⁸O-REC are associated with dense bands similar to patterns found in shells at mid to high latitudes suggesting winter hibernation. Air temperature records from Ft. Myers indicate 1976-1979 were among the coldest years over the last century. Shells from 1976-1979 likely grew through Summer and shut down during Winter. Thus, d¹⁸O-REC serves as a lower temperature boundary for comparison to shells from prior cold periods. Neither d¹⁸O-VM nor d¹⁸O-LIA grew during periods that were as cold as 1976-1979. The most positive value of d¹⁸O-VM may reflect winter temperature 2-3ēC colder than that of d¹⁸O-LIA or may reflect evaporative conditions. To better constrain temperature, we are calibrating Sr/Ca in M. campechiensis as an independent proxy of temperature.