DECADAL DISTRIBUTION PALEOECOLOGY OF UNILOCULAR CALCAREOUS FORAMINIFERA FROM THE SE BRAZILIAN CONTINENTAL MARGIN

Unilocular calcareous foraminifera (UCF) have increasing importance in biostratigraphy of the Neogene, however the ecology of the group still needs additional case studies. Samples from the southeastern Brazilian continental margin have high diversity species of UCF, in a pattern determined by sediment type and total organic carbon (TOC).

A coastline-perpendicular transect with three box corer stations was carried out from the coast (BCCF10-15) until the edge of the continental shelf (BCCF10-01). Fifty-five foraminiferal samples of 10cm³ were collected, with a resolution of 1cm slices, and washed through a 0.063 mm mesh sieve. The geochronological model of the box-cores was based on ²¹⁰Pb. Although BCCF10-15 presents the highest flow of benthic foraminifera and UCF comprises 20% of the total fauna, BCCF10-01 presents the greatest Shannon-Wiener (H’) and a greater variation of foraminifera assemblages over time, with UCF totaling 17% of the total fauna. However, the central station (BCCF10-09) presented abundant phytodetritic species and the greatest diversity of UCF species (23% of the total fauna), which caught my attention since the occurrence of UCF in general is negligible. In order to contribute to the ecology conditioning this anomalous increase of UCF in the central station, this work aims to detail the occurrence of UCF for the last 180 years, the time span sampled by the box cores.

This middle shelf core evidenced a stable environment, with excellent preservation of the fauna, which revealed some delicate morphological features of rare species of UCF. δ¹³C values of Uvigerina peregrina indicate an environment with more degraded organic matter in the center of the shelf. The average δ¹³C composition of Cibicides spp. is 0.25‰ impoverished in relation to δ¹³C_DIC values. Such values and the high frequency of phytodetritic species indicate the formation of a phytodetritic layer at the bottom of the continental shelf, produced by western boundary current eddies in a region of high primary productivity induced by the South Atlantic Central Water upwelling. The influence of different primary productivity regimes was imprinted on the distribution and composition of UCF.