Deep circulation changes in the South Atlantic since the Last Glacial Maximum from Nd isotope and multi-proxy records

• Published in: Earth and planetary science letters 2016-01, Vol.434, p.18-29
• Main Authors: Wei, R., Abouchami, W., Zahn, R., Masque, P.
• Format: Article
• Language: English
• Subjects: Amplitudes; Basins; Circulation; deep circulation; Deep water; Holocene; Isotopes; Last Glacial Maximum; Leachates; Nd isotopes; Neodymium; South Atlantic; Southern Ocean
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2015.11.001

We report down-core sedimentary Nd isotope (εNd) records from two South Atlantic sediment cores, MD02-2594 and GeoB3603-2, located on the western South African continental margin. The core sites are positioned downstream of the present-day flow path of North Atlantic Deep Water (NADW) and close to the Southern Ocean, which makes them suitable for reconstructing past variability in NADW circulation over the last glacial cycle. The Fe–Mn leachates εNd records show a coherent decreasing trend from glacial radiogenic values towards less radiogenic values during the Holocene. This trend is confirmed by εNd in fish debris and mixed planktonic foraminifera, albeit with an offset during the Holocene to lower values relative to the leachates, matching the present-day composition of NADW in the Cape Basin. We interpret the εNd changes as reflecting the glacial shoaling of Southern Ocean waters to shallower depths combined with the admixing of southward flowing Northern Component Water (NCW). A compilation of Atlantic εNd records reveals increasing radiogenic isotope signatures towards the south and with increasing depth. This signal is most prominent during the Last Glacial Maximum (LGM) and of similar amplitude across the Atlantic basin, suggesting continuous deep water production in the North Atlantic and export to the South Atlantic and the Southern Ocean. The amplitude of the εNd change from the LGM to Holocene is largest in the southernmost cores, implying a greater sensitivity to the deglacial strengthening of NADW at these sites. This signal impacted most prominently the South Atlantic deep and bottom water layers that were particularly deprived of NCW during the LGM. The εNd variations correlate with changes in 231Pa/230Th ratios and benthic δ13C across the deglacial transition. Together with the contrasting 231Pa/230Th: εNd pattern of the North and South Atlantic, this indicates a progressive reorganization of the AMOC to full strength during the Holocene. •Depth gradient in εNd consistent with glacial shallowing of southern waters.•Continuous southward export of Northern Component Water during the past 24 kyrs.•Northward glacial penetration of southern waters supported by multi-proxy records.•The AMOC progressively reinvigorates to full strength during the Holocene.