Seasonal climate change across the Roman Warm Period/Vandal Minimum transition using isotope sclerochronology in archaeological shells and otoliths, southwest Florida, USA

Archaeological evidence suggests that southwest Florida experienced variably warmer and wetter climate during the Roman Warm Period (RWP; 300 BC–550 AD) relative to the Vandal Minimum (VM; 550–800 AD). This hypothesis was tested by reconstructing seasonal-scale climate conditions for the latter part of the RWP (1–550 AD) by using high-resolution oxygen isotope ratios (δ18O) of archaeological shells (Mercenaria campechiensis) and otoliths (Ariopsis felis). Eight shells radiocarbon-dated to 150–550 AD recorded that the RWP summers at 150–250 AD were insignificantly different from today and the RWP summers at 250–350 AD and 450–550 AD were drier relative to today. Eight otoliths indicate that the winters were variable during the RWP, colder than today at 150–200 AD and 250–300 AD, similar to today at 200–250 AD, 300–350 AD and 450–500 AD, and warmer than today at 500–550 AD. The climate reconstructions agree with archaeological observations and are partially coherent with the history of sea-level change, with a drying and cooling trend at the 95% confidence level across the RWP/VM transition. The climate transition is not only consistent with falling sea level, but also coherent with reduced solar radiation. Reduced solar radiation may have triggered a change in atmospheric circulation patterns that precipitated the observed climate transition.