Beach Washed and Modern Corals (Siderastrea siderea) Reveal Shifts in Sea Surface Temperature and Trade Winds in the Central Caribbean Sea for the Common Era

Understanding climate variability during times of flourishing civilizations in the past, such as the Maya civilization in the Yucatan peninsula, may provide valuable insights into how those cultures thrived before their downfall. In this study, we use trace elemental ratios in coral skeletons as climate proxies — Sr/Ca for sea surface temperature (SST) and Ba/Ca for upwelling, river runoff, and productivity — to create monthly-resolved reconstructions of past climate variability. The study site is Little Cayman Island (19º41'44″N, 80º03'42″W) located ~750 km west of the Yucatan in the central Caribbean Sea. This island is far from large land masses, has no rivers, and low human impacts, thus coral Ba/Ca there is driven by upwelling via trade winds and sea surface productivity. We examine two corals (Siderastrea siderea), a beach-washed (18LIC10) and a modern (16LIC2) coral, both from the north side of the island. 16LIC2 was cored in the backreef using a pneumatic drill (26.5 cm, 27 years long). 18LIC10 was collected from coral rubble washed onshore (15.2 cm, 32 years long) and is 1318 ±11 years old (702 CE) as determined by ²³⁰thorium dating. Coral Sr/Ca-SST reveals a mean shift of ~1°C from colder conditions at 702 CE (18LIC10) to warmer modern SST (16LIC2), likely due to 18LIC10 living prior to the Medieval Warm Period (950-1250 CE). The coral Sr/Ca seasonal cycle is reduced at 702 CE compared to the modern coral and our results generally agree with coral Sr/Ca seasonality in Bonaire in the southern Caribbean though those corals are younger (865-956 CE). Similarly, coral Ba/Ca seasonal cycles are reduced in 18LIC10 and a mean shift occurs (5.71 ±1.47 (1σ) μmol/mol 18LIC10; 9.37 ±1.80 (1σ) μmol/mol 16LIC2). The mean shift and seasonality change in coral Ba/Ca is the result of increased upwelling induced by easterly trade winds, which can increase productivity in the water column, suggesting the trade winds were weaker at 702 CE. These findings provide evidence for ~1ºC drop in SST, reduced SST seasonal cycles, and weaker trade winds 50 years before the Terminal Classic Period (750-1050 CE) of the Maya civilization in Mesoamerica.