INTEGRATED MORPHOLOGICAL AND SUBSURFACE ANALYSIS OF RECENT COASTAL DUNE MIGRATION, EQUATORIAL BRAZIL

In the high-precipitation region of Pará State, northern Brazil, 240 km east of the Amazon River, abundant sediment supply and strong easterly winds lead to formation of isolated coastal dunes. Two westward-migrating dunes at Atalaia Beach were analyzed using aerial and satellite images, high-resolution 400 MHz ground-penetrating radar (GPR) records, and optically-stimulated luminescence (OSL) ages of slipface strata. Dune 1 (L=700 m, W=270 m) has a well-developed parabolic shape, exhibits a series of trailing vegetation marks, and is advancing into a mangrove forest. Based on imaged and OSL-dated paleo-dune surfaces, the time-averaged dune migration rate decreased from 1.6 to 1.1 m/yr, with a stabilization of the dune limb approximately 90 years ago. Based on the most recent changes in the position of the dune crest (using geo-referenced 2008 ground surveys, 2001 vertical aerial photographs, and 2009 satellite images), Dune 1 continues migrating at a rate of 1.0 m/yr. A rapidly evolving Dune 2 (L=500 m, W=150 m) has superimposed three-dimensional megaripples (L=20 m, W=13 m), and is migrating into a low-relief residential area. This landform is located 2,000 m upwind of Dune 1 and is proximal to a large active sediment source – a wide sandy beach backed by an extensive dune ridge. Subsurface images indicate that Dune 2 consists of four sedimentary packages (average thickness 4.0 m), which likely represent distinct phases of dune migration. Based on partially buried man-made structures, this dune has migrated at an average rate of at least 1.6 m/yr, while accreting vertically at a rate of 0.3 m/yr. Both recent and future migration patterns and rates of the two dunes are affected by downwind topography (tidal channels, vegetation density and height, and intensity of development). Our study demonstrates that for a region with limited historical records, integrated morphological and subsurface datasets provide valuable information about the rapidly evolving coastal dunefields of equatorial Brazil.