Effects of midlatitude westerlies on the paleoproductivity at the Agulhas Bank slope during the penultimate glacial cycle: Evidence from coccolith Sr/Ca ratios

Abstract

Modern primary productivity on the Agulhas Bank, off South Africa, has been proposed to be linked to the midlatitude westerlies. A paleoproductivity record from this area may therefore resolve temporal changes in the westerly dynamics. Accordingly, we produced a coccolith Sr/Ca-based paleoproductivity record from core MD96-2080 (Agulhas Bank slope) during the penultimate glacial-interglacial cycle. Deriving the productivity signal from Sr/Ca requires a correction for a temperature effect, here constrained using Mg/Ca sea surface temperatures from the foraminifer Globigerina bulloides from core MD96-2080. Phases of depressed productivity coincided with periods of stratification in the same core, indicated by high relative abundances of the coccolithophore Florisphaera profunda and with low relative abundances of the upwelling indicator G. bulloides in the nearby Cape Basin. These observations collectively suggest that productivity was regulated by upwelling throughout this region. We infer that, as in the present, periods of low productivity result from a more northerly position of the westerlies, potentially accompanied by subtropical front displacements, and blockage of upwelling promoting easterlies. Productivity minima also coincide with periods of increased ice-rafted detritus (IRD) deposition on the Agulhas Plateau, which also indicates extreme northward positions of the westerlies. The influence of the westerlies appears to be obliquity conditioned, as productivity minima (and IRD maxima) occur during low-obliquity intervals. The dynamic connection between productivity and the westerlies is supported by coeval salinity changes in the South Indian Gyre that likewise respond sensitively to a poleward contraction of the westerlies.

Insolation forcing of coccolithophore productivity in the western tropical Indian Ocean over the last two glacial‐interglacial cycles

“…However, EPP values during these terminations are in similar scale, which are still comparably low (Figure c). This low productivity trend was also observed during T II in the Agulhas Corridor, south of the study area, where an abrupt warm and saline episode during T II did not coincide with a productivity peak [ Mejia et al , ]. A study by Rickaby et al [] shows similar trend in CF Sr/Ca values and suggested the influence of day length and light intensity, in addition to insolation in coccolithophore productivity (Figure b).…”

“…For this study, we used the raw Sr/Ca ratios to measure coccolithophore productivity because a study by Rippert et al [] suggested that the temperature difference in the tropical IO from the Last Glacial Maximum to the Holocene is ~2 to 4°C in maximum. Moreover, temperature‐corrected and raw Sr/Ca data attributed to productivity from the Agulhas Bank showed similar variations, which indicated only a small contribution of temperature to the Sr/Ca record [ Mejia et al , ]. We also calculated the relative carbonate contribution of the individual species to the total assemblage by multiplying the coccolith concentration (species CC/g of sed.)…”

“…The influence of temperature on Sr/Ca was corrected in previous studies using the equation based on Stoll et al [2002b] and from independent temperature proxies such as Mg/Ca of planktonic foraminifera [e.g., Mejia et al, 2014;Saavedra-Pellitero et al, 2017]. For this study, we used the raw Sr/Ca ratios to measure coccolithophore productivity because a study by Rippert et al [2015] suggested that the temperature difference in the tropical IO from the Last Glacial Maximum to the Holocene is~2 to 4°C in maximum.…”

“…The Sr/Ca is a well-developed proxy, reflecting coccolith growth rates and thus interpreted to record variations in coccolith carbonate production [Stoll et al, 2002a]. This new method resulted in a number of paleoenvironmental reconstructions in the recent decades, exploring geochemical compositions in the coccolith fraction [e.g., Bolton et al, 2016;Bolton and Stoll, 2013;Mejia et al, 2014;Rickaby et al, 2002;Saavedra-Pellitero et al, 2017;Stoll and Schrag, 2000;Stoll et al, 2002bStoll et al, , 2007aStoll et al, , 2007b.…”

The drivers of late Quaternary climate variability in eastern South Africa

“…Consequently, in a comparable way to the LGM, the increased sea ice during the late Holocene (Fig. 5g), may have displaced the westerlies equatorward, increasing winter wind strength and the length of the dry season at Mfabeni, leading to a decreased influence of the moisture bearing tropical easterlies (Mejía et al, 2014;Toggweiler et al, 2006;Williams and Bryan, 2006). Furthermore, although the westerlies may have had a more northerly position during this time, simultaneous high local summer insolation and warm SSTs (causing strong convective rainfall during summer; Fig.…”