Similar mid-depth Atlantic water mass provenance during the Last Glacial Maximum and Heinrich Stadial 1
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 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 values (Curry and Lohmann, 1983), lower values in the Atlantic are most commonly interpreted as a greater fraction of low- 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 may be greater than previously appreciated and that it may have contributed significantly to the observed HS1 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 decrease has also been attributed to a decrease in the northern-sourced end-member 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 decrease in the mid-depth North and South Atlantic, their relative importance could not be determined on the basis of benthic and 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 εNd) act as quasi-conservative water mass tracer that is independent of the remineralisation of organic matter (Frank, 2002). In the modern Atlantic, εNd (143Nd/144Nd normalised to 143Nd/144NdCHUR = 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- 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 εNd records from the mid-depth South Atlantic. These εNd 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 εNd 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 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).
Section snippets
Core sites
KNR159-5-33GGC (27.6°S, 46.2°W, 2082 m; 33GGC hereafter) and GL1090 (24.9°S, 42.5°W, 2225 m) were cored on the southern Brazil margin in the South Atlantic (Fig. 1). The age model of 33GGC is based upon planktic foraminiferal radiocarbon dates (Tessin and Lund, 2013) and yields a sedimentation rate of 27 cm/kyr in the deglacial section and 2 cm/kyr in the Holocene (Tessin and Lund, 2013). The age model for GL1090 was constructed using planktic foraminiferal radiocarbon dates (Santos et al., 2017
Veracity of neodymium isotopes
The core top foraminiferal εNd value of 33GGC (−10.4 ± 0.4) agrees well with the interpolated values from nearby seawater (Fig. 2, Jeandel, 1993) consistent with an authigenic foraminiferal signal derived from seawater. The core top foraminiferal εNd value of GL1090 (−13.0 ± 0.1) is less radiogenic than that of 33GGC and of the least radiogenic εNd value observed from nearby seawater (−12.3 ± 0.4; Jeandel, 1993). There are a number of possible explanations for the 2.6 ε unit difference in the
Conclusions
We present two new foraminiferal εNd records from the mid-depth South Atlantic. The deeper, and slightly more northerly, site shows consistently less radiogenic values and the sustained influence of northern-sourced waters throughout the deglaciation. Both sites display similar εNd values during the LGM and HS1, suggesting that there were similar water mass mixing proportions at these times, despite significantly lower values during HS1 than the LGM at the same sites. Consistent with other
Acknowledgments
Ana Luiza S. Albuquerque is thanked for providing core material from GL1090. Vicky Rennie, KR Pietro, Jo Clegg, Jason Day, Mervyn Greaves and Caroline Daunt are thanked for technical support and Thiago Pereira dos Santos is thanked for providing stable isotope data from GL1090. Nd isotope analyses were supported by NERC grants NE/K005235/1 and NE/F006047/1 to AMP and NSF Grant OCE-1335191 to DWO. DWO acknowledges funding from WHOI's investment in Science Program. KFH acknowledges financial
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Neodymium isotopes as a paleo-water mass tracer: A model-data reassessment
2022, Quaternary Science ReviewsCitation Excerpt :Yet, here we find that this depth range is relatively insensitive to changes in water mass mixing (albeit very short excursions appear after the Younger Dryas that are however unlikely to be resolvable in sediment cores) as εNd signatures vary between −9 and −11 throughout the past 20 kyr and independently of the LGM circulation state. Pöppelmeier et al. (2020a) expanded the dataset by Howe et al. (2018) to greater water depths (2.8 km), but also found no distinct millennial scale variability in Nd isotopes at the southern Brazil Margin. Combining these datasets with our simulations suggests first, that the LGM exhibited a relatively weak circulation so that the transition to HS1 was not particularly pronounced (or instead both, the LGM and HS1, exhibited a moderately weak circulation), and second, that during the Younger Dryas the AMOC experienced only a minor weakening, which did not lead to a strong response in the South Atlantic (as also attested by Pa/Th-based reconstructions, McManus et al., 2004; see also Figure S13 for simulations with a smaller Younger Dryas freshwater forcing).
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2020, Earth and Planetary Science LettersCitation Excerpt :Further, the transition from the LGM to Heinrich Stadial 1 (HS1) was characterized by only small shifts in the absolute Nd isotope values, while all five records remain invariant. This finding suggests that the large AMOC reorganizations observed in the North Atlantic during HS1 (Bradtmiller et al., 2014; Gherardi et al., 2009; Lippold et al., 2016; McManus et al., 2004) did not notably affect the Southwest Atlantic Nd isotope values at mid-depths (Howe et al., 2018). Nd isotope signatures suggest that at the onset of the Bølling-Allerød (B/A) the relative influence of Northern Sourced Water (NSW) increased as it partly contributed to the water mass mixture of the Southwest Atlantic.