Sedimentation in the coastal reefs of Abrolhos over the last decades

Abstract

Coral reefs of the coastal area of Abrolhos are located in an environment with a high influx of terrigenous sediments that are carried out to the sea, either as a result of natural processes (river output, coastal erosion, and torrents) or due to anthropogenic influences (deforestation, coastal development, and dredging). Excessive terrigenous sediment in coastal areas has been identified as one of the major threats to coral reefs, leading to their worldwide decline. The present study assessed the evolution of sedimentation in the Abrolhos coastal reefs during the past decades by analyzing samples from sediment cores collected near the reefs of Coroa Vermelha (located 15 km from the coast), Pedra de Leste (located 12 km from the coast) and Popa Verde (located 35 km from the coast). The purpose of this assessment was to observe whether the previously described pattern of surficial bottom sediment distribution in Abrolhos, which consisted of terrigenous mud in the nearshore reefs, to carbonate-dominant sediments towards the offshore reefs, is still a prevalent feature. Sediment color, texture, CaCO3 percentage, biogenic compounds and clay minerals, as well as the sedimentation rate and the geochronology of the sediment cores were analyzed. The results showed indications of an increase in the deposition of terrigenous mud, over the last decade, in the vicinity of the reefs nearest to the coast, though this does not yet constitute a definitive evidence of such a change. However, this observation therefore suggests that local processes resulting from anthropogenic actions are most likely causing an increase of the sedimentation rate of continent-derived sediment runoff in the Abrolhos coastal areas. To minimize this situation, there is an urgent need for the development of new management strategies to protect the already disturbed Abrolhos coastal reefs, especially during times of global changes.