From river mouth to reef: benthic foraminifera assemblages reveal ecological dynamics in a mixed carbonate–siliciclastic Brazilian continental shelf

Abstract. The recent benthic foraminiferal assemblages have been considered to be a very useful tool as analogues for understanding environmental changes in the past oceans. In order to identify distribution patterns, the benthic foraminiferal assemblages (total fauna, >63 µm), along with sedimentological data, have been investigated on the continental shelf of the Espírito Santo Basin (ESB, between 18°20′ and 21°20′ S). The ESB is distinguished from the other Brazilian basins due its geomorphologically diverse continental shelf and slope; this is apart from it being economically important for oil exploration. Seafloor samples (0–2 cm) from 18 to 150 m were distributed in seven transects arranged perpendicularly to the coast. The density, taxonomic diversity, and assemblage of the total biota composition have changed significantly along the Espírito Santo continental shelf (ESCS). This study identified five main benthic foraminiferal assemblages along the ESCS, each associated with distinct sedimentary and environmental characteristics. The cluster analysis reveals five groups which were named after the abbreviation of the main species or genera. Group M–C is characterized by the marked presence of Miliamina subrotunda and Cibicides spp., as well as by high values of diversity and richness, indicating a stable environment. Group H–Q is dominated by Hanzawaia boueana and Quinqueloculina spp. in areas with terrigenous sediments and biogenic carbonate deposits. Group G–T is associated with high organic matter content and a marked presence of Globocassidulina rossensis and Trifarina angulosa. Group Q–B is linked to high-energy environments with bioclastic sediments and is dominated by Quinqueloculina cuvieriana and Bigenerina textularioidea. Group A–P is found on the Abrolhos Shelf, characterized by high abundances of porcelaneous symbiotic foraminifera, such as Articulina sulcata and Peneroplis planatus, in carbonate sediments. The distribution of these assemblages is primarily controlled by sediment composition, grain size, organic matter flux, and hydrodynamic conditions. Sediments rich in carbonate seem to favor symbiotic-bearing foraminifera species adapted to oligotrophic environments, while regions with higher organic matter content support opportunistic and infaunal species. The results highlight the interplay between sedimentary and oceanographic processes and ecological factors in structuring benthic foraminiferal assemblages along the ESCS.