Agulhas leakage as a key process in the modes of Quaternary climate changes

Abstract

Heat and salt transfer from the Indian Ocean to the Atlantic Ocean (Agulhas leakage) has an important effect on the global thermohaline circulation and climate. The lack of long transfer record prevents elucidation of its role on climate changes throughout the Quaternary. Here, we present a 1,350-ka accumulation rate record of the planktic foraminiferal species Globorotalia menardii. We demonstrate that, according to previous assumptions, the presence and reseeding of this fauna in the subtropical southeast Atlantic was driven by interocean exchange south of Africa. The Agulhas transfer strengthened at glacial ice-volume maxima for every glacial-interglacial transition, with maximum reinforcements organized according to a 400-ka periodicity. The long-term dynamics of Agulhas leakage may have played a crucial role in regulating meridional overturning circulation and global climate changes during the Mid-Brunhes event and the Mid-Pleistocene transition, and could also play an important role in the near future.

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“…This pattern is consistent with the hypothesis that peak Agulhas leakage occurred during glacial terminations and plausibly aided the AMOC to shift to its full-strength interglacial mode Lohmann, 2003, 2007;Peeters et al, 2004). A high-resolution faunal record of the accumulation rate of tropical species Globorotalia menardii at ODP Site 1087 in the southern Benguela region was used to infer that Agulhas leakage strengthened during every glacial termination of the past 1.3 my (Caley et al, 2012). The Caley et al (2012) interpretation of increased leakage during terminations in the Late Pleistocene is also supported by combined alkenone (Uk 37 and δD) and dinoflagellate cyst assemblages, SST, and salinity estimates from Site 1087 (Petrick et al, 2015).…”

“…A full synthesis of the paleoceanographic evidence is yet to emerge, but there appears to be several aspects of the Agulhas paleotime series about which different workers are in broad agreement. Based on published records, Agulhas leakage has been a persistent feature of the past 1.3 my, with less leakage during full glacial times than interglacial times and greatest leakage at the glacial terminations (Peeters et al, 2004;Caley et al, 2012;Petrick et al, 2015). There is also general agreement about evidence for a more northerly position of the STF during glacial times (Rau et al, 2002(Rau et al, , 2006Bard and Rickaby, 2009;Martínez-Méndez et al, 2010), and some suggestion that this may play a role in reducing Agulhas leakage.…”

“…A high-resolution faunal record of the accumulation rate of tropical species Globorotalia menardii at ODP Site 1087 in the southern Benguela region was used to infer that Agulhas leakage strengthened during every glacial termination of the past 1.3 my (Caley et al, 2012). The Caley et al (2012) interpretation of increased leakage during terminations in the Late Pleistocene is also supported by combined alkenone (Uk 37 and δD) and dinoflagellate cyst assemblages, SST, and salinity estimates from Site 1087 (Petrick et al, 2015). SST and salinity records at an upstream location along the southeast African margin (Core MD96-2048; 26°S) display prominent 41 ky cyclicity that was interpreted to suggest that long-term Agulhas Current variations were associated with high-latitude (as opposed to regional tropical) climate forcing.…”

Pathways of the Agulhas waters poleward of 29°S

“…This transport of warm and saline Indian water by the Agulhas leakage is referred to as the ''warm water route'' in the global conveyer belt schematic [Gordon, 1986]. It compensates the southward export of the North Atlantic Deep Water (NADW) formed in the sub-Arctic North Atlantic and plays an important role in the global ocean circulation and climate [Beal et al, 2011;Caley et al, 2012]. Recent studies have recognized that an increase in Agulhas leakage could strengthen the Atlantic overturning circulation in response to the poleward shift of the westerly jet in the Southern Hemisphere [Biastoch et al, 2009a;Beal et al, 2011;Biastoch and B€ oning, 2013].…”

Moving beyond the age-depth model paradigm in deep sea palaeoclimate archives: dual radiocarbon and stable isotope analysis on single foraminifera

“…Many such studies (Barker et al, 2015;Caley et al, 2012;Simon et al, 2016) use tuning to the LR04 (Lisiecki and Raymo, 2005) benthic stack to produce an age-depth chronology. Were the LR04 stack to display similar PDSM uncertainty as T86-10P, one could expect an intra-sample heterogeneity (1σ age value for a 1 cm slice) of between 1500-2000 years (Fig.…”