Foraminifera and ostracods from the semiarid Carrizal Bajo coastal wetland, southern Atacama Desert

Abstract

This study analyzes both the texture and foraminiferal and ostracod assemblages of recent (Quaternary) sediments from the Carrizal Bajo wetland, in the southern Atacama Desert of northern Chile. A transect carried out from the infralittoral zone to the landward margin of the active wetland allowed four coastal sectors to be differentiated based on the distribution and abundance of these faunal groups. The infralittoral, shallow marine areas (sector 1) are composed of bioclastic sandy sediments extremely rich in foraminifera and ostracods, which contrast with their marked scarcity or absence in the very fine sands and very coarse muds of the adjacent intertidal-supratidal zones and washover fans (sector 2). Scarce resedimented shells of ostracods and the gastropod Heleobia copiapoensis characterize the peaty sandy muds of the lagoon bottom (sector 3), a hostile environment for the groups studied due to intermittent anoxic conditions and frequent changes in salinity. The innermost (landward) margin of the wetland (sector 4) is characterized by sandy silts with a rich freshwater fauna in the vicinity of springs, as well as an unexpected occurrence of resedimented marine foraminifera ~650 m inland. These results can be applied in future research related to investigating: 1) the environmental degradation of the system, should the density and diversity of these faunal groups decrease or even disappear over time; 2) the reconstruction of paleoenvironments in recent Quaternary deposits, in case the taxa determined are present in the fossil record; or 3) the identification of recent Quaternary tsunamigenic layers along this coastline, characterized by the presence of infralittoral species within aeolian or wetland sedimentary sequences.