| Summary: | The study reconstructed seasonal climate change in mid-latitude northwest Scotland during the climate episodes Neoglacial (~3300-2500 BP) and Roman Warm Period (RWP; ~2500-1600 BP) and in subtropical southwest Florida during the latter part of RWP (1-550 AD) based on archaeological shell accumulations in two study areas. In northwest Scotland, seasonal sea surface temperature (SST) during the Neoglacial and RWP was estimated from high-resolution oxygen isotope ratios of radiocarbon-dated limpet (Patella vulgata) shells accumulated in a cave dwelling on the Isle of Mull. The SST results revealed a cooling transition from the Neoglacial to RWP, which is supported by earlier studies of pine pollen in Scotland and European glacial events and also coincident with the abrupt climate deterioration at 2800-2700 BP. The cooling transition might have been driven by decreased solar radiation and weakened North Atlantic Oscillation (NAO) conditions. In southwest Florida, seasonal-scale climate conditions for the latter part of RWP were reconstructed by using high-resolution oxygen isotope ratios of archaeological shells (Mercenaria campechiensis) and otoliths (Ariopsis felis). The reconstructions agree with archaeological observations and are partially coherent with the history of sea-level change. Moreover, the results suggest a marked drying and cooling trend across the RWP to Vandal Minimum (VM; ~500-800 AD) transition, which is consistent with falling sea level and reduced solar radiation. Reduced solar radiation might have triggered a change in atmospheric circulation patterns that precipitated the observed climate transition. Although NAO and other atmospheric circulation patterns have been proposed as the internal climate mechanism responsible for the climate fluctuations in the late Holocene, other studies suggest that stochastic processes are possibly associated with the internal climate mechanism. Therefore, I also examined the extent to which the internal climate mechanism in the late Holocene is stochastic. Our results indicate that the stochastic extent of climate change over the past millennium generally decreased during the intervals of climate transition.
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