Relative abundance and biometry of Bolivina ordinaria on the northern continental slope of the Pelotas basin during the late Quaternary

This study examines the variations in relative abundance, test size, and biomass of the species Bolivina ordinaria (Foraminifera) over the last 112 ka BP on the northern slope of the Pelotas Basin, Southern Brazil. The analyses were based on 651 individuals from 42 samples distributed along the cores SIS-249 (Marine Isotope Stage 5 to 3) and SIS-188 (Marine Isotope Stage 3 to 1). Bolivina ordinaria was the most abundant species of the Bolivinitidae family in these cores, particularly during warmer periods (MIS 1 and 5). Nine other species of this family were identified: B. compacta, B. doniezi, B. lowmani, Bolivina sp., B.striatula, Brizalina sp., Brizalina spinescens, Brizalina subaenariensis, and Brizalina subspinescens. Larger test lengths were observed during MIS 3 in the both cores. The summed biomass was higher during MIS 5 in the SIS-249 core and lower during MIS 3 in the SIS-188 core. The fluctuations observed in the relative abundance and summed biomass of B. ordinaria are consistent with the oscillations in total organic carbon flow, and the variations in primary productivity between the interglacial and glacial periods. The results suggest that this infaunal species is favored by the regular supply of refractory organic matter during warmer periods but faces increased competition from shallow infaunal and epifaunal species during glacial periods, when the flow of labile organic matter increases.