AMOC Variability During the Last 400 ka from Nd Isotopes in a Southern Ocean Core

Abstract:

Paleoceanographic studies have used Nd isotope ratios to characterize paleo-ocean deep-water mass distributions. The Southern Ocean has nearly homogeneous εNd that reflects the relative inputs from the north Atlantic and north Pacific, offering the possibility to quantify the AMOC strength through time. However, obtaining an εNd record from the Southern Ocean is a challenge because carbonates are poorly preserved there, hindering stratigraphic control and generally precluding use of this important substrate. Alternatively, studies of deep-sea corals and ferromanganese crusts provide snapshots or poor time resolution.

Deep-sea core E33-22 (55°S, 120°W, 2743m) lies in the polar frontal zone of the Pacific sector, above the carbonate compensation depth, where foraminifera are preserved. It spans ~400,000 years (from the Holocene to MIS11) with a sedimentation rate of ~2.5 cm/kyr. We measured its εNd record with the purpose of determining the best estimate of the export of NADW out of the Atlantic system, which we suggest is our best measure of the vigor of the global thermohaline circulation system. The record shows expected εNd variability during orbital and sub-orbital time-scales, that is lower εNd during warm and higher εNd during cold climate intervals. Moreover, the record passes a critical test in that εNd-values are almost always between South Atlantic and Pacific values. The only exceptions are glacial periods when they coincide with South Atlantic values, indicating extension of Southern Ocean system into the South Atlantic. We conclude that these data represent a conservative record of AMOC variability through the late Pleistocene.