Glacial Overturning Circulation Structure in the Cape Basin from Neodymium Isotope Measurements

Abstract:

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). 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 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) [1] and TN057-21/RC11-83 (4981/4718 m) [2]. 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 [3]. 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. Co-variation of εNd values at Site U1479 and sortable silt mean grain size data from the Agulhas Plateau suggests a potential role of Southern Ocean Westerly Winds in driving neodymium isotope patterns seen over the last glacial cycle. Faster sortable silt mean grain sizes correlate with less negative εNd values and smaller vertical εNd gradients in the Cape Basin. We posit that these observations are the result of enhanced wind-driven mixing during cold periods over the past 150,000 years.

[1] Hu et al. (2016) EPSL 455, 106–114. [2] Piotrowski et al. (2005) Science 307, 1933–1938. [3] Curry and Oppo (2005) Paleoceanography 20, PA1017.