THE EFFECTS OF MID-HOLOCENE FLUVIO-EOLIAN INTERPLAY AND COASTAL DYNAMICS ON THE FORMATION OF DUNE-DAMMED LAKES IN NE BRAZIL

The coastal plain located in the easternmost tip of South America encompasses extensive active and stabilized dunefields along with fluvial and lacustrine systems. In addition to relative sea level and climate, fluvio-eolian interactions play a pivotal role in the evolution of this coast. The chronology obtained by optically stimulated luminescence of 36 stabilized dune samples coupled with sedimentological analysis suggest that episodes of sediment availability throughout the Holocene controlled eolian activity. Contrary to the most conventional view linking dune activity to aridity, our data show that paleodune buildup occurred between 11 ka and 6 ka. This interval falls within a period of increased humidity in NE Brazil when a rising relative sea level and higher rainfall enhanced sediment delivery benefiting the construction of transgressive dunefields. The interaction of these encroaching dunefields with existing drainage pathways is recorded in a sediment core from the dune-dammed Boqueirão Lake dated between 8.4 and 0.9 ka. During this period, data from this core, such as microfossil assemblages, organic geochemistry, and grain size conform to fluvial or lacustrine depositional conditions. Between 7.2 and 4.4 ka, during the predominantly regional humid climate, the high abundance of fluvial sponge species is consistent with a framework of competent-flow drainage systems diverting from advancing dunes. An abrupt transition from a wetter to a drier climate at 4.4–4.0 ka stimulated episodes of fluvial damming by advancing or existing dunes, as indicated by sharp changes from sandy to muddy sediments and an anomalous concentration of sponge spicules concurrent with significant mortality rates of fluvial adapted species. From 3.9 ka to the present, the disappearance of sponge spicules and peaking diatom concentration attest to a predominant lacustrine environment bounded by dunes. Thus, the establishment of Boqueirão Lake between 4.4–3.9 ka is mainly a result of the interaction between dunes and water bodies during the regionally drier climate and not a consequence of increased humidity in coastal NE Brazil.