Coriolis effect entombed in Pleistocene Bahamian dune fields

Abstract:

The windward islands of the Bahamas-Caicos archipelago form a nearly continuous Atlantic Ocean-facing transect that spans >950 km in length and nearly 7° of latitude. Heavy vegetation has relegated most studies of Bahamian aeolianites to exposures along roadcuts, quarries, and sea cliffs offering windows into internal architecture but without the benefit of holistic landform visualization. New digital elevation data from satellite radar interferometry (TanDEM-X) enables a step change in the ability to map and quantify aspects of Bahamian aeolian landforms across the archipelago at a scale that would be impractical through reconnaissance field study alone. A semi-automated mapping approach that leverages object-based image analysis and manual refinement yields a total aeolian dune area of ~1674 km² across the islands of Great Abaco, Eleuthera, Cat, San Salvador, Long, Crooked, Acklins, Mayaguana, and the Turks and Caicos Islands, representing ~38% of exposed land. Longitudinal axis measurements from 747 Pleistocene coastal parabolic dunes record a systematic shift towards increasing consistency of E-W orientation with decreasing latitude. Three National Data Buoy Center (NDBC) data buoys provided modern wind direction and velocity measurements (n=730,933 ea.) along this transect. Analysis of wind vectors (>P90, n=70,095) demonstrate increasing organization of easterlies at southern latitudes and an offset in directionality compared to formational winds of Pleistocene MIS5e deposits as interpreted from landform orientation analysis. Southward increase in wind strength and consistency from modern buoys and ancient dune axes alike is best explained by the transition from weak, erratic winds near the northern end of the study area fringing the Horse latitudes to strong easterly trade winds in the south. Ultimately, this trend is produced by geostrophic flow driven by relative influence of atmospheric circulation within the Hadley Cell and right-hand deflection of the Coriolis effect in the Northern Hemisphere. We speculate that the offset in directionality between Pleistocene dune axes and modern wind vectors is related to differences in breadth of the Hadley Cell between the MIS5e interglacial and today.