Strong glacial-interglacial variability in upper ocean hydrodynamics, biogeochemistry, and productivity in the southern Indian Ocean

Abstract

In the southern Indian Ocean, the position of the subtropical front – the boundary between colder, fresher waters to the south and warmer, saltier waters to the north – has a strong influence on the upper ocean hydrodynamics and biogeochemistry. Here we analyse a sedimentary record from the Agulhas Plateau, located close to the modern position of the subtropical front and use alkenones and coccolith assemblages to reconstruct oceanographic conditions over the past 300,000 years. We identify a strong glacial-interglacial variability in sea surface temperature and productivity associated with subtropical front migration over the Agulhas Plateau, as well as shorter-term high frequency variability aligned with variations in high latitude insolation. Alkenone and coccolith abundances, in combination with diatom and organic carbon records indicate high glacial export productivity. We conclude that the biological pump was more efficient and strengthened during glacial periods, which could partly account for the reported reduction in atmospheric carbon dioxide concentrations.

Paleoproductivity Modes in Central Mediterranean During MIS 20—MIS 18: Calcareous Plankton and Alkenone Variability

“…Despite the subject it is out of the main aim of the present study and conscious of the possible factors such as degradation, ecological conditions and alkenone contribution from non‐calcifying species possibly involved in the relation between alkenone concentration and abundance of taxa (Beltran et al., 2011; Malinverno et al., 2008; Plancq, 2015; Prahl et al., 1989), we point out that the pattern of absolute abundance of G. oceanica and of G. caribbeanica displays similarities with that of total alkenone concentration, supported by positive correlations ( r = +0.80 and +0.82, respectively; Figures 5c and 5d). This may imply, although not necessarily demonstrate, that these taxa are both the main alkenone producers during the studied interval, in agreement with recent data from the southern Indian Ocean during MIS 8 (Tangunan et al., 2021).…”

Rapid diversification underlying the global dominance of a cosmopolitan phytoplankton

“…g: generation time. c Absolute abundance of G. huxleyi in sites U1475 [87] and SO139-74KL [88]. d Global Δ Sea Surface Temperature [89] (ΔSST; blue line) and LR04 [90] (red line) over 350 ka; (MIS: Marine Isotopic Stage).…”

Glacial Southern Ocean Expansion Recorded in Foraminifera‐Bound Nitrogen Isotopes From the Agulhas Plateau During the Mid‐Pleistocene Transition

“…Given sufficient iron supply to Site U1475, the increase in nutrients would fuel local productivity further enhancing glacial carbon drawdown (Anderson et al., 2014; Martin, 1990; Martínez‐García et al., 2011, 2014). Indeed, northward STFZ migrations during glacial periods of the past 1.4 million years are met with increased productivity at Site U1475 with the most significant northward frontal migrations yielding the greatest increases in biogenic sediment deposition (Cartagena‐Sierra et al., 2021; Tangunan et al., 2021).…”

“…The evidence for the northward expansion of the Southern Ocean during the MPT comes from the Atlantic Sector of the Southern Ocean (Diekmann & Kuhn, 2002; Kemp et al., 2010) and more recently the Indian‐Atlantic Ocean Gateway, south of Africa (Cartagena‐Sierra et al., 2021; Starr et al., 2021; Tangunan et al., 2021). Expansion of the Southern Ocean via northward migrations of the Subtropical Frontal Zone (STFZ) and South Westerly Winds slows global overturning circulation and increases deep ocean carbon storage (Ferrari et al., 2014; J. Marshall & Speer, 2012; Russell et al., 2006; Sigman et al., 2021; Toggweiler et al., 2006).…”