Paleoceanography of the southern Agulhas Plateau during the last 150 ka: Inferences from benthic foraminiferal assemblages and multispecies epifaunal carbon isotopes

Abstract

Benthic foraminiferal assemblages and the carbon isotope composition of the epifaunal benthic foraminifera Epistominella exigua and Fontbotia wuellerstorfi have been investigated along core MD02‐2589 located at the southern Agulhas Plateau (41°26.03′S, 25°15.30′E, 2660 m water depth). This study aims to evaluate changes in the benthic paleoenvironment and its influence on benthic δ13C with a notable focus on E. exigua, a species associated with phytodetritus deposits and poorly studied in isotope paleoceanographic reconstructions. The benthic foraminiferal assemblages (>63 μm) show large fluctuations in species composition suggesting significant changes in the pattern of ocean surface productivity conceivably related to migrations of the Subtropical Convergence (STC) and Subantarctic Front (SAF). Low to moderate seasonality and relatively higher food supply to the seafloor are indicated during glacial marine isotope stages (MIS) 6, 4, and 2 and during MIS 3, probably associated with the northward migration of the SAF and confluence with the more stationary STC above the southern flank of the Agulhas Plateau. The lowest organic carbon supply to the seafloor is indicated from late MIS 5b to MIS 4 as a consequence of increased influence of the Agulhas Front (AF) and/or weakening of the influence of the STC over the region. Episodic delivery of fresh organic matter, similar to modern conditions at the core location, is indicated during MIS 5c–MIS 5e and at Termination I. Comparison of this paleoenvironmental information with the paired δ13C records of E. exigua and F. wuellerstorfi suggests that organic carbon offsets δ13C of E. exigua from ambient bottom water δ13CDIC, while its δ13C amplitude, on glacial‐interglacial timescales, does not seem affected by changes of organic carbon supply to the seafloor. This suggests that this species calcifies preferentially during the short time span of the year when productivity peaks and phytodetritus is delivered to the seafloor. Therefore E. exigua, while offset from δ13CDIC, potentially more faithfully records the amplitude of ambient bottom water δ13CDIC changes than F. wuellerstorfi, notably in settings such as the Southern Ocean that experienced substantial changes through time in the organic carbon supply to the seafloor.