Isotopic accounts of mineral dust deposition in the northwestern subtropical Atlantic from seasons to recent millennia

Mineral dust from the Sahara is deposited over large parts of the Atlantic Ocean. In order to improve our understanding of the biogeochemical and radiative impacts of Saharan dust, we are motivated to provide measurements that document spatial and temporal variability in this dust flux. We first present point estimates of dust flux based on dissolved, particulate and aerosol thorium isotopes (Th-232/Th-230) from a transect cruise between Barbados and Bermuda in summer 2014. These measurements provide critical spatial context for interpolating dust deposition between island-based time-series studies in the western Atlantic. Additionally, we present a ~2000 year record of dust deposition from a Bahamian carbonate tidal flat at decadal-to-centennial resolution, dated by C-14 and Pb-210. We contrast two different dust flux estimates: first using thorium, which in this setting is almost entirely adsorbed from seawater onto the locally-produced carbonate sediments, and second using helium isotopes (He-4/He-3), which have a negligible dissolved component. These records surprisingly indicate relatively stable dust transport to the Bahamas despite regional climate anomalies such as the Little Ice Age or the intensification of Sahel agriculture in the 19^th and 20^th centuries. Along with our dust estimates, we find that the partial dissolution of thorium from Saharan dust and Th scavenging on carbonate banks are complicating factors in both modern and paleo- dust flux studies.