Rapid Coastal Dune Migration into Temperate and Equatorial Forests: Optical Chronology of Imaged Upper Slipface Strata

This study presents the first sets of optical dates that constrain dunefield evolution along a temperate, forested, microtidal barrier (Curonian Spit, southeast Baltic Sea, Lithuania) and a macrotidal equatorial mangrove coast (Path State, northern Brazil). In both areas, high-resolution geophysical (georadar) images were used to map the subsurface architecture, relative chronology, and orientation of dune migration surfaces (slipfaces) and vertical accretion strata. Besides relative chronology of dune accretion, subsurface images reveal bounding (superimposition) surfaces indicative of distinct activity phases. A cross-barrier Preila transect on the Curonian Spit, served as the basis for establishing dune chronology in this historically active part of the Great Dune Ridge. Due to barrier width (> 1 km) and dune height (>30 m), aeolian sand transfer, rather than overwash and breaching, has been the dominant process of landward migration for this barrier spit since mid-Holocene. OSL dating of the upper sections of buried slipfaces in shallow trenches revealed dune migration of more than 600 m between mid-1500s and late 1700s at the Preila site. These ages are consistent with historical records and the time of burial of a coastal village near the edge of a lagoon by 1797. In equatorial Brazil (Atalaia Beach), large reactivated parabolic dunes have been migrating over coastal roads and mangrove forests over at least the past 150 years. In geophysical images of a parabolic dune, numerous high-amplitude hyperbolic anomalies produced by buried trees contrast with steeply landward-dipping slipface reflections. Due to water table elevation, the lower older part of the dune sequence has been stabilized in the blowout area. Similarly, interdunal lake has partially arrested the migration of a transverse dune ridge, while the adjacent segment continues to advance onto a sparsely vegetated plain. Our preliminary data indicate that whereas the two regions differ markedly in their precipitation patterns, vegetation types, oceanographic setting, and land-use history, both experienced episodes of rapid (average: 1-2 m/year) migration of massive dunes (10-50 m high), which continues locally at the present time. Aside from their implications to coastal evolution, accurate reconstructions of dune dynamics on decadal to centennial time scales should be integrated into coastal development and management strategies.