Morphosedimentary evolution and the formation of a Holocene peatland on a tropical coastal plain (SE Brazil)

Abstract The Brazilian coastal zone underwent significant changes through climatic variations and relative sea level (RSL) fluctuations during the Late Pleistocene and Holocene, resulting in notable geomorphological alterations. These transformations led to the formation of estuarine‐lagoonal environments in the lowlands of the Brazilian coastal plains. With the RSL falling and coastline regression, depressed areas experienced siliciclastic and organic sedimentation, promoting the development of peatlands, which are wetland ecosystems dominated by Histosols. This study examines the depositional dynamics of a broad forested peatland, surrounded by hydromorphic mineral soils and adjacent to a large river, located 25 km from the coastline, which initially suggests a fluvial origin with no connection to estuarine environments in SE Brazil. Profiles of Histosols from the mire and Cambisols from the surrounding lower terraces were analysed for granulometry, and radiocarbon dating ( 14 C) and optically stimulated luminescence (OSL) dating were performed. The geomorphological model indicates that, approximately 120,000 years ago, a rise in the RSL submerged the coastal plain, depositing sediments of the Marine Isotope Stage (MIS 5). Around 18 000 years ago (MIS 2), a fall in the RSL of over 100 m caused intense landscape dissection and the formation of incised valleys. During MIS 1, 7000 years ago, estuarine‐lagoonal sedimentation occurred in the incised valleys. Subsequent regression and sediment filling transformed paleolagoons into lakes, fostering terrestrialisation and paludisation. At the southwestern edge of the peatland, high gravel content indicates colluvial deposition. The lower terrace near the peatland was dated to 5500 years ago and provided the surface for the deposition of the shell mound (sambaqui), which consists mainly of oyster shells, evidencing human occupation (shell mound builders) and indicating an ancient estuarine ecosystem surrounding the Brazilian coastal plain at that time.