Much of our understanding of Late Holocene climate change is derived from historical and geological records from mid- to high-latitudes; however, there is a paucity of data documenting Late Holocene seasonality and climate change in low latitudes. To address this issue, we examined high-resolution subtropical climate records contained in clam (Mercenaria campechiensis) shells from archaeological middens on Pine Island, SW Florida that accumulated over the last 2000 yr. Four climate intervals are represented based on ¹⁴C and pottery dates: Roman Optimum (RO), Vandal Minimum, Medieval Warm Period, Little Ice Age (LIA).

Modern climate records provide baseline conditions for distinguishing climate change from natural variability. Most of the year-to-year variability in air temperature occurs during winter months. Based on a best-fit sinusoid, annual low temperature (coldest month of the year) over the last century indicates a multidecadal oscillation of cold and warm periods at ~60-yr cycles (1.3 ºC amplitude, 17 ºC mean). Annual high temperature (warmest month of the year) has a similar periodicity but 0.6 ºC amplitude, 28 ºC mean. Given the high variability in cold months and summer growth cessation inherent in M. campechiensis, winter temperatures recorded in modern and midden shells from warm and cold periods are compared to evaluate climate change.

d¹³C from a live-collected clam ranges from -4.00 to -0.72‰ reflecting fluctuating salinity (25-35 ppt), where most positive values coincide with winter dry season. Midden shell d¹³C varies by ~1-1.5‰ (LIA +0.28 to +1.80‰; RO +0.10 to +1.11‰) near subtropical seawater values indicating dominantly marine conditions.  d¹⁸O in the modern clam ranges from -1.97 to +1.10‰ and records anomalously cold Winters 1977 and 1978. In comparison, d¹⁸O of LIA shell ranges from -1.87 to +1.19‰ and records cold temperature similar to Winters 1977 and 1978. d¹⁸O of RO shell is positively offset by ~0.5‰ relative to LIA shell, ranging from -1.28 to +1.53‰. This positive offset likely indicates evaporative conditions and not abnormally cold temperatures. Seasonal temperature range during RO and LIA is similar to modern conditions (spanning ~12 ºC, where 1‰ shift in d¹⁸O = ~4 ºC shift in temperature), though the warmest months are probably not recorded in the shell due to summer growth cessation.