North Atlantic Deep Water during Pleistocene interglacials and glacials

Abstract

The global ocean overturning circulation is a major means of distributing heat around the Earth, and an important trigger or amplifier of climate change. This study presents a 1.5-Myr-long neodymium (Nd) isotope record of Deep Sea Drilling Project Site 607, in the core of present-day North Atlantic Deep Water (NADW), the water mass that drives the overturning circulation and its Atlantic end-member (Broecker, 1991; Gordon, 1991), in order to document its composition through time. This time interval is marked by major changes in fundamental aspects of the Earth's climate, including the Mid-Pleistocene Transition (MPT) from ∼41-kyr to ∼100-kyr interglacial-glacial cycles and more intense glacials. The new record, mainly focusing on interglacial and glacial peaks, shows a pattern that mimics the record of benthic foraminiferal δ18O (Lisiecki and Raymo, 2005) in that the magnitude of interglacial-glacial Nd isotope shifts were smaller in the 41-kyr world than in the 100-kyr world. During the “900 ka event” (Clark et al., 2006), between ∼960 and 860 ka, marking the first 100-kyr interglacial-glacial cycle, the Nd isotope ratios shift abruptly to higher values, consistent with increased incursion of Southern Ocean water masses into the deep North Atlantic. This pattern was previously observed in the South Atlantic over the same time interval and interpreted as a weakened presence of NADW signal that reflected a period of disrupted deep ocean overturning circulation, termed the “MPT-AMOC crisis” (Pena and Goldstein, 2014). The Site 607 data support this interpretation and show the effects to be basin-wide. Following the disruption, an enhanced southern-sourced Nd isotope signature remained for ∼200 kyr during the period of “lukewarm interglacials” (Howe and Piotrowski, 2017; Jaccard et al., 2013), consistent with weaker overturning circulation. With the exception of this “MPT-AMOC crisis and recovery” interval, the Nd isotope ratios during interglacial peaks have been similar to present-day NADW, indicating similar interglacial North Atlantic ocean circulation dynamics both before and after the MPT. This contrasts with the pattern during glacial periods of the 100-kyr world, during which Nd isotopes have continued to follow the pattern of the 900-kyr event indicating a strong southern water mass signal, interpreted as intensified incursions of Southern Ocean water into the deep North Atlantic and consistent with a generally weaker overturning circulation during glacials.