Millennial-scale variability in the local radiocarbon reservoir age of south Florida during the Holocene

A growing body of research suggests that the marine environments of south Florida provide a critical link between the tropical and high-latitude Atlantic. Changes in the characteristics of water masses off south Florida may therefore have important implications for our understanding of climatic and oceanographic variability over a broad spatial scale; however, the sources of variability within this oceanic corridor remain poorly understood. Measurements of Delta R, the local offset of the radiocarbon reservoir age, from shallow-water marine environments can serve as a powerful tracer of water-mass sources that can be used to reconstruct variability in local-to regional-scale oceanography and hydrology. We combined radiocarbon and U-series measurements of Holocene-aged corals from the shallow-water environments of the Florida Keys reef tract (FKRT) with robust statistical modeling to quantify the millennial-scale variability in Delta R at locations with (nearshore) and without (open ocean) substantial terrestrial influence. Our reconstructions demonstrate that there was significant spatial and temporal variability in Delta R on the FKRT during the Holocene. Whereas Delta R was similar throughout the region after similar to 4000 years ago, nearshore Delta R was significantly higher than in the open ocean during the middle Holocene. We suggest that the elevated nearshore Delta R from similar to 8000 to 5000 years ago was most likely the result of greater groundwater influence associated with lower sea level at this time. In the open ocean, which would have been isolated from the influence of groundwater, Delta R was lowest similar to 7000 years ago, and was highest similar to 3000 years ago. We evaluated our open-ocean model of Delta R variability against records of local-to regional-scale oceanography and conclude that local upwelling was not a significant driver of open-ocean radiocarbon variability in this region. Instead, the millennial-scale trends in open-ocean Delta R were more likely a result of broader-scale changes in western Atlantic circulation associated with an increase in the supply of equatorial South Atlantic water to the Caribbean and shifts in the character of South Atlantic waters resulting from variation in the intensity of upwelling off the southwest coast of Africa. Because accurate estimates of Delta R are critical to precise calibrations of radiocarbon dates from marine samples, we also developed models of nearshore and open-ocean Delta R versus conventional C-14 ages that can be used for regional radiocarbon calibrations for the Holocene. Our study provides new insights into the patterns and drivers of oceanographic and hydrologic variability in the Straits of Florida and highlights the value of the paleoceanographic records from south Florida to our understanding of Holocene changes in climate and ocean circulation throughout the Atlantic. Published by Elsevier B.V.