New Micropalaeontological Evidence for an Early Pleistocene Existence of the Agulhas Leakage

The Agulhas Leakage is considered to influence the Atlantic Meridional Overturning Circulation and the climate system via transport of saltier and warmer water masses from the Indian Ocean into the Atlantic Ocean (Laxenaire et al. 2018). Therefore, reconstructing the time of the possible onset of the Agulhas Leakage will allow an improvement of global palaeo-climate models. Since the Agulhas Leakage is known to disperse the Indian Ocean biota into the Atlantic Ocean, Caley et al. (2012) proposed that the Agulhas Leakage exists since about 1.3Ma.

Here, we provide new evidence for an early Pleistocene existence of the Agulhas Leakage by comparing the test size evolution of G. menardii between the tropical eastern Atlantic Ocean ODP Site 667 and the Indian Ocean IODP Site 1476, which is located in the Mozambique Channel.

At Site 667 and parallel to a climate cooling trend, we observe a test size decrease from a maximal axial length (max δY) of 875µm during the Mid-Pliocene Warmth Period (3.2Ma) to a maximal axial length of 520µm by the end of Pliocene (ca. 2.6Ma). This trend is followed by a relatively rapid test size increase until ca. 2.1Ma, during which the size more than doubles (max δY = ca. 1200µm). This pattern in the test size evolution of G. menardii was also observed in the western tropical Atlantic Ocean (Knappertsbusch 2016).

In the Mozambique Channel, we do not observe a decrease of the test size at the end of the Pliocene. The values stay almost stable throughout the Pliocene (max δY = ca. 900µm) until 2.3Ma. Between 2.3 and 2Ma, the maximal test size increases to a value very similar to that observed in the eastern tropical Atlantic (max δY = ca. 1250µm).

It has been observed that relatively large G. menardii specimens occurred in the Mozambique Channel, while the Atlantic only harboured relatively small specimens during the late Pliocene and earliest Pleistocene, and that both localities show a similar test size at ca. 2Ma. This suggests the possibility of a dispersal of the Indian Ocean giant G. menardii into the Atlantic between 2.3 and 2Ma, probably via a strengthening Agulhas Leakage.

 

Caley, T., Jiraudeau, J., Malaizé, B., Rossignol, L. & Pierre, C. (2012), ‘Agulhas leakage as a key process in the modes of Quaternary climate changes’, PNAS 109(18), 6835–6839.

Knappertsbusch, M. W. (2016), ‘Evolutionary prospection in the Neogene planktic foraminifer Globorotalia menardii and related forms from ODP Hole 925B (Ceara Rise, western tropical Atlantic): evidence for gradual evolution superimposed by long distance dispersal?’, Swiss Journal of Palaeontology.

Laxenaire, R., Speich, S., Blanke, B., Chaigneau, A., Pegliasco, C. & Stegner, A. (2018), ‘Anticyclonic Eddies Connecting the Western Boundaries of Indian and Atlantic Oceans’, Journal of Geophysical Research: Oceans 123, 7651–7677.