Similar mid-depth Atlantic water mass provenance during the Last Glacial Maximum and Heinrich Stadial 1

Highlights

• We present two authigenic neodymium records from the mid-depth South Atlantic.

• These records show no change between the Last Glacial Maximum and Heinrich Stadial 1.

• Water mass provenance was similar at both sites during these time periods.

Abstract

The delivery of freshwater to the North Atlantic during Heinrich Stadial 1 (HS1) is thought to have fundamentally altered the operation of Atlantic meridional overturning circulation (AMOC). Although benthic foraminiferal carbon isotope records from the mid-depth Atlantic show a pronounced excursion to lower values during HS1, whether these shifts correspond to changes in water mass proportions, advection, or shifts in the carbon cycle remains unclear. Here we present new deglacial records of authigenic neodymium isotopes – a water mass tracer that is independent of the carbon cycle – from two cores in the mid-depth South Atlantic. We find no change in neodymium isotopic composition, and thus water mass proportions, between the Last Glacial Maximum (LGM) and HS1, despite large decreases in carbon isotope values at the onset of HS1 in the same cores. We suggest that the excursions of carbon isotopes to lower values were likely caused by the accumulation of respired organic matter due to slow overturning circulation, rather than to increased southern-sourced water, as typically assumed. The finding that there was little change in water mass provenance in the mid-depth South Atlantic between the LGM and HS1, despite decreased overturning, suggests that the rate of production of mid-depth southern-sourced water mass decreased in concert with decreased production of northern-sourced intermediate water at the onset of HS1. Consequently, we propose that even drastic changes in the strength of AMOC need not cause a significant change in South Atlantic mid-depth water mass proportions.

Introduction

The most recent glacial termination was apparently accompanied by changes in Atlantic meridional overturning circulation (e.g. Böhm et al., 2015, McManus et al., 2004) that are thought to be important in communicating changes in the climate between the Northern and Southern Hemispheres (Broecker, 1998). One key event in this transition was Heinrich Stadial 1 (HS1), a Northern Hemisphere cold period during which massive iceberg rafting into the North Atlantic occurred (Hemming, 2004). The freshwater from melting icebergs is believed to have reached the regions of deep water formation where it may have caused a weakening (Bond et al., 1992, Bradtmiller et al., 2014, Broecker, 1994;Gherardi et al., 2005, Oppo et al., 2015) or even a near-complete shut-down (McManus et al., 2004) of Atlantic overturning.

Benthic foraminiferal carbon isotope records from the intermediate (here, 1000–1500 m) and mid-depth (here, 1500–2500 m) Atlantic show pronounced excursions to low ${\delta }^{13}\mathrm{C}$ values during HS1 (Lund et al., 2015, Oppo et al., 2015, Rickaby and Elderfield, 2005, Tessin and Lund, 2013, Thornalley et al., 2010, Zahn and Stüber, 2002). While it has long been recognized that changes in remineralization can impact ${\delta }^{13}\mathrm{C}$ values (Curry and Lohmann, 1983), lower ${\delta }^{13}\mathrm{C}$ values in the Atlantic are most commonly interpreted as a greater fraction of low-${\delta }^{13}\mathrm{C}$ southern-sourced water (SSW) (Boyle and Keigwin, 1987, Duplessy et al., 1988, Keigwin and Lehman, 1994, Sarnthein et al., 1994). However, several recent studies suggest the influence of remineralization on deglacial ${\delta }^{13}\mathrm{C}$ may be greater than previously appreciated and that it may have contributed significantly to the observed HS1 ${\delta }^{13}\mathrm{C}$ decrease (Lacerra et al., 2017, Oppo et al., 2015;Schmittner and Lund, 2015, Voigt et al., 2017). In addition to more SSW and greater remineralization, the LGM to HS1 ${\delta }^{13}\mathrm{C}$ decrease has also been attributed to a decrease in the northern-sourced end-member ${\delta }^{13}\mathrm{C}$ value (Lund et al., 2015, Waelbroeck et al., 2011). These explanations are not mutually exclusive and a recent study suggested that while a combination of these mechanisms could explain the LGM to HS1 ${\delta }^{13}\mathrm{C}$ decrease in the mid-depth North and South Atlantic, their relative importance could not be determined on the basis of benthic ${\delta }^{13}\mathrm{C}$ and ${\delta }^{18}\mathrm{O}$ data alone (Oppo et al., 2015). This ambiguity limits our knowledge of how the water mass provenance within the Atlantic varied between the LGM and HS1. As a result, our fundamental understanding of how ocean circulation responds to perturbations such as the freshwater forcing thought to have occurred during HS1 still contains a significant element of uncertainty.

The isotopes of the radiogenic element neodymium (expressed as ε_Nd) act as quasi-conservative water mass tracer that is independent of the remineralisation of organic matter (Frank, 2002). In the modern Atlantic, ε_Nd (¹⁴³Nd/¹⁴⁴Nd normalised to ¹⁴³Nd/¹⁴⁴Nd_CHUR = 0.512638, Hamilton et al., 1983, Jacobsen and Wasserburg, 1980) in parts per ten thousand) distinguishes upper North Atlantic Deep Water (NADW) (−13.2) (Lambelet et al., 2016, Piepgras and Wasserburg, 1987) in the subtropical North Atlantic from both Antarctic Intermediate Water (AAIW) (−8.3) and Antarctic Bottom Water (AABW) (−8.5) (Stichel et al., 2012). In addition, the conservative behaviour of seawater Nd isotopes has been suggested for intermediate/deep depths of the Atlantic Ocean (i.e. around 2500 m and below) (Goldstein and Hemming, 2003, Lambelet et al., 2016). As a result, neodymium isotopes are the ideal candidate to investigate whether the low-${\delta }^{13}\mathrm{C}$ values observed in the mid-depth Atlantic during HS1 were the result of changes in water mass provenance. Although numerous deglacial records of authigenic neodymium isotopes from throughout the Atlantic do exist (Gutjahr et al., 2008, Huang et al., 2014, Lippold et al., 2016, Piotrowski et al., 2004, Roberts et al., 2010, Skinner et al., 2013, Wei et al., 2016), the mid-depth Atlantic – a key region for understanding how Atlantic overturning varied between the LGM and HS1 – remains underinvestigated. In this paper we present two deglacial foraminiferal ε_Nd records from the mid-depth South Atlantic. These ε_Nd records display little change between the LGM and HS1. Although uncertainty remains in the neodymium composition of water mass end-members during these time periods, we propose that the simplest explanation for this lack of change in ε_Nd values is that the provenance of the water masses in the mid-depth Atlantic was similar during the LGM and HS1. This interpretation suggests that the low ${\delta }^{13}\mathrm{C}$ values observed in the mid-depth South Atlantic during HS1 were caused by other mechanisms, most likely the greater accumulation of organic matter in slower circulating water (Lacerra et al., 2017, Voigt et al., 2017).