Glacial overturning circulation structure and Agulhas Leakage from Nd isotope and K/Ar measurements in the Cape Basin

The Agulhas Current is a key part of the warm water surface return route in the ocean’s overturning circulation. At the Agulhas Retroflection, a portion of this current “leaks” into the Atlantic, transporting salt and heat and contributing to the formation of North Atlantic Deep Water (NADW). The Agulhas Current also carries suspended clay sediment, and the K/Ar Age of these clays traces the path of the current and records it in the underlying sediments [1,2]. NADW has a neodymium isotopic composition that is distinct from water originating in the Pacific, and mixing between these water masses in the subsurface is quasi-conservative. We present authigenic neodymium isotope and K/Ar measurements from IODP Site U1479, located in the Cape Basin (35°03.53′S; 17°24.06′E, 2615 m water depth). In the modern ocean, this Site is bathed in remnant NADW, characterized by its salinity and temperature, as well as its εNd minimum. Neodymium isotope variations at Site U1479 are consistent with other Cape Basin εNd records, including ODP Sites 1087 and 1088 (1372 and 2082 m, respectively) [3] and TN057- 21/RC11-83 (4981/4718 m) [4]. We do not see evidence for an increase in the influence of NADW at Sites 1087/1088 when it decreases at Site U1479. This would be expected if NADW shoaled in the water column, as is indicated by benthic δ¹³C data from the western Atlantic [5]. Instead, Site U1479 has the most negative εNd values of these sites across the last glacial cycle, implying a modified glacial circulation structure in the Cape Basin. Our record of Agulhas Leakage from K/Ar measurements shows a more complex picture than the canonical record of Peeters et al. (2004) [6]. Rather than simply recording increases in Agulhas Leakage at the glacial terminations, we see increased Agulhas Leakage during the middle of the last glacial period (~100–40 ka) as well. Ongoing work examining potential changes in sediment redistribution by bottom currents will hopefully help to reconcile these apparent differences.

[1] A. M. Franzese et al., EPSL, 250, 72 (2006). [2] A. M. Franzese, PhD Thesis, Columbia University (2008) [3] Hu et al. (2016) EPSL 455, 106–114. [4] Piotrowski et al. (2005) Science 307, 1933–1938. [5] Curry and Oppo (2005) Paleoceanography 20, PA1017. [6] F. J. C. Peeters et al., Nature, a, 661 (2004).