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

Abstract

[1] 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 0 S, 25°15.30 0 E, 2660 m water depth). This study aims to evaluate changes in the benthic paleoenvironment and its influence on benthic d 13 C 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 mm) 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 d 13 C records of E. exigua and F. wuellerstorfi suggests that organic carbon offsets d 13 C of E. exigua from ambient bottom water d 13 C DIC , while its d 13 C 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 d 13 C DIC , potentially more faithfully records the amplitude of ambient bottom water d 13 C DIC 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.Citation: Diz, P., I. R. Hall, R. Zahn, and E. G. Molyneux (2007), Paleoceanography of the southern Agulhas Plateau during the last 150 ka: Inferences from benthic foraminiferal assemblages and multispecies epifaunal carbon isotopes, Paleoceanography, 22, PA4218,

Linkages between rapid climate variability and deep‐sea benthic foraminifera in the deep Subantarctic South Atlantic during the last 95 kyr

“…The presence of E. exigua in the modern assemblage is in line with a regime of primary production characterized by seasonal phytoplankton blooms [ Machu et al , ] typically promoted by mesoscale turbulence across the Subtropical Convergence [ Llido et al ., ; Machu et al ., ]. Fluctuations in the abundance of Epistominella exigua along the core could therefore be explained as resulting from changes in the frequency and intensity of episodic phytoplankton blooms at the surface, and consequent variations in the flux of phytodetritus to the seafloor [e.g., Smart et al ., ; Thomas et al ., ; Diz et al ., ; Poli et al ., ], suggesting an overall low‐productivity environment. This interpretation is supported by the relatively high contribution of species indicative of sustained low organic carbon flux ( Oridorsalis umbonatus on average 6%, Fontbotia wuellerstorfi 7.9%, Pyrgo murrinha <5%, and Globocassidulina subglobosa <5%) and sustained intermediate carbon flux ( Pullenia osloensis 11.9%, Melonis spp.…”

“…<5%, and Siphotextularia rolshauseni , <5% on average) (see summary of species ecology in Diz et al . [] and Poli et al . []).…”

“…This interpretation is supported by the relatively high contribution of species indicative of sustained low organic carbon flux (Oridorsalis umbonatus on average 6%, Fontbotia wuellerstorfi 7.9%, Pyrgo murrinha <5%, and Globocassidulina subglobosa <5%) and sustained intermediate carbon flux (Pullenia osloensis 11.9%, Melonis spp. <5%, and Siphotextularia rolshauseni, <5% on average) (see summary of species ecology in Diz et al [2007] and Poli et al [2012]).…”

North Atlantic Deep Water and Antarctic Bottom Water variability during the last 200ka recorded in an abyssal sediment core off South Africa

“…It should be expected that a reduction in NADW export to the South Atlantic during glacials would result in lighter δ 13 C values that are typical for AABW. Molyneux et al (2007) and Diz et al (2007) suggest for the glacial periods an admixture of a well-ventilated water mass to the deep waters in the South Atlantic with a heavier δ 13 C signature similar of present day NADW. The source of that water mass could either be a continued influx of NADW, or its glacial equivalent, the Glacial North Atlantic Intermediate Water (GNAIW; Oppo and Horowitz, 2000) or a younger deep water mass of southern origin that carries a higher δ 13 C signal from recent air-sea gas exchange .…”

Sedimentary deposits on the southern South African continental margin: Slumping versus non-deposition or erosion by oceanic currents?

“…Only few details are known about the Tertiary units. Most studies have concentrated on the effect that glacial/interglacial cycles have had on the sedimentary environment and on Holocene deposits (Martin andFlemming, 1986, 1988;Rau et al, 2002Rau et al, , 2006Diz et al, 2007;Molyneux et al, 2007). Modern river input to the south coast comprises about 18% of the total terrigenous sediment material, with Pleistocene sediments on most of the Agulhas Bank consisting of relict quartz and benthic carbonate sands (Dingle et al, 1987).…”