Cooler and More Variable Medieval Climate in the Northern Caribbean indicated by Pseudodiploria strigosa and Orbicella faveolata Coral Sr/Ca-based Temperature Records from the 11th Century

Coral-based paleoclimate records are relatively sparse before 1600AD because colonies don’t usually live more than a few centuries and it is difficult to know the age of dead corals until after expensive collection and dating. We present two rare Medieval coral-Sr/Ca based paleotemperature records from specimens that grew off shore of Anegada, British Virgin Islands, in the northeastern Caribbean.Uranium series dating indicates that the corals grew at the same time during the first half of the 11^th Century (C).One coral is a Pseudodiploria strigosa, 13AN4, and the other is an Orbicella faveolata, 13AN8. Their mean Sr/Ca values and standard deviations (1 sigma) are 9.34±0.06 mmol/mol and 9.27±0.08 mmol/mol respectively. This is significantly higher (p=0.026 in a 2 sample ANOVA) than the three previously published 20^th Ccoral Sr/Ca records from this site, which together average 9.03±0.08. The differences between individual corals of a particular era is much smaller than the difference between time periods, indicating that the approximate 0.3 mmol/mol difference between the 20^th Century and Medieval corals represents an environmental difference between the two time periods, rather than just inter-colony variations. If interpreted as a temperature difference alone, it would indicate 3.75-6˚C cooler conditions in the first half of the 11^th C relative to the 20^th C, based on Sr/Ca-SST relationships of -0.08 mmol/molC^-1 to -0.05 mmol/molC^-1. Such large magnitude changes are unlikely, and the signal more likely represents a combination of changing baseline seawater Sr/Ca and cooler temperatures. The initial data indicate larger interannual variability in both corals relative to that observed in the modern corals. We explore these signals in relation to current hypotheses and paleoclimate evidence for the state of large-scale climate patterns such as the North Atlantic Oscillation, Atlantic Multidecadal Variability, and centennial-scale variability over the last millennium.