Benthic foraminifera assemblages on the southern Portuguese margin during early Pleistocene Marine Isotope Stages 42-45

Abstract

Climate variability and glacial cycles during the Pleistocene era are well-studied phenomena, although less is known about the climate dynamics and their impact on benthic foraminiferal assemblages prior to the Mid Pleistocene Transition known as the ‘41 kyr world’.  Benthic foraminiferal assemblages are sensitive to variations in factors such as food fluxes, pore water oxygenation and bottom water conditions as well as variations in Mediterranean Outflow Water (MOW), therefore, we studied benthic foraminiferal assemblage data from IODP Site U1378 (36°48.321´N 7°43.1321´W), drilled into the Faro Drift on the southern Portuguese margin (Gulf of Cadiz) at a water depth of 559 m during IODP Expedition 339 to investigate early Pleistocene glacial/interglacial cycles of Marine Isotope Stage (MIS) 42 to late MIS 45 (1.33-1.39 Ma). In addition to the benthic assemblage data, we used epibenthic oxygen and carbon stable isotope data derived from Cibicioides pachyderma and Planulina ariminensis, insolation data of 21 June at 65°N, weight percent of organic carbon and sand (wt%). Benthic foraminiferal assemblages were picked in two fraction sizes (>250 µm and >125 µm), counting ap. 150 specimen per size fraction. A total amount of 119 species of benthic foraminifera were counted, the most abundant species being Sphaeroidina bulloides, Cassidulina laevigata, Uvigerina celtica, Cibicidoides pachyderma and Globocassidulina subglobosa (relative abundances varying between 11.04 - 5.04%). The assemblage data show variations along the analysed section, especially a significant diversity drop at 1.355 Ma (MIS 43), with high abundances (and dominance) of Bulimina mexicana striata, Uvigerina celtica and Sphaeroidina bulloides. At the same time, abundances of Cibicidoides spp., Cassidulinoides laevigata, Melonis barleeaum, and Globocassidulina subglobosa declined. These faunal changes coincided with a 0.68‰ decrease in the δ13C and a 0.22‰ increase in the δ18O values. Another change in the δ13C values, a 0.61‰ increase at 1.375 Ma (MIS 44), was associated with a high abundance peak of Globobulima spp. The abundance of Cibicidoides correlates well with the weight percentage of the sand-sized fraction (wt% sand), which likely indicates stronger current activity on the seafloor, which is also supported by the abundance of Trifarina angulosa, a species that also prefers strong currents and may be an indicator of MOW velocity at our study site.