HIGH RESOLUTION ISOTOPE SCLEROCHRONOLOGY REFLECTS CHANGES IN THE SEASONAL CYCLE AT ROCKY POINT, BELIZE BETWEEN THE LAST INTERGLACIAL AND PRESENT

In this study, we used sclerochronology and stable oxygen isotopic records (δ¹⁸O) of both modern and Pleistocene corals from Rocky Point, Belize to compare seasonality at the last interglacial with that of today. The last interglacial interval is believed to have experienced stronger Northern Hemisphere solar insolation than today, which is thought to have resulted in a more northward and latitudinally extensive Intertropical Convergence Zone (ITCZ) over the Caribbean, presumably leading to higher amplitude seasonal temperature and/or precipitation cycles. We compared geochemical data from fossil and modern Orbicella sp. corals in northern Belize to explore changes in seasonality between the Pleistocene and present. The Pleistocene coral was dated at 124-128 ka using ²³⁰Th/²³⁴U dating methods and the modern coral was collected in 2014. Linear extension rates (LER) from the modern coral averaged 10.3±1.6 mm/year (n= 31) and showed higher normalized variability (stdev. = 1.4, n = 29) compared with the Pleistocene coral (mean LER of 2.5±0.9 mm/yr, stdev. = 0.9, n = 19). Both corals were sampled at a range of resolutions to examine the optimal method for reproducing seasonality. At the lowest sampling resolution (4 samples per year), the average δ¹⁸O values for the Pleistocene and modern coral were -4.19‰ and -3.96‰ respectively, but the Pleistocene coral had a seasonal δ¹⁸O range of 0.66‰ compared to the modern coral range of 0.40‰. At the highest sampling resolution (10 samples per year), the Pleistocene and modern δ¹⁸O indicated seasonal ranges of 2.47‰ and 0.99‰. Our data show that at the highest sampling resolutions, the Pleistocene seasonal δ¹⁸O ranges were greater than those of the modern coral by 1.48‰, representing a maximum potential temperature range of ~10°C. This exceeds the current seasonal temperature range of 7°C recorded from near-by in situ instrumental data. The larger seasonal range of the Pleistocene coral supports the hypothesis that the ITCZ was located at higher latitudes in the Northern Hemisphere and fluctuated within a greater latitudinal extent during the last interglacial, if δ¹⁸O is driven primarily by temperature in this coral.