Northern-sourced water dominated the Atlantic Ocean during the Last Glacial Maximum

Abstract

Increased carbon sequestration in the ocean subsurface is commonly assumed to have been one of the main causes responsible for lower glacial atmospheric CO2 concentrations. Remineralized carbon must have been stored away from the atmosphere for thousands of years, yet the water mass structure accommodating such increased carbon storage continues to be debated. Here, we present new sediment-derived bottom-water neodymium isotope records that allow fingerprinting of water masses and provide a more complete picture of the Atlantic Meridional Overturning Circulation geometry during the Last Glacial Maximum. These results suggest that the vertical and meridional structure of the Atlantic water mass distribution only experienced minor changes since the last ice age. In particular, we find no compelling evidence supporting glacial southern-sourced water substantially expanding to shallower depths and farther into the Northern Hemisphere than today, which had been previously inferred from stable carbon isotope (δ13C) reconstructions. We argue that depleted δ13C values observed in the deep Northwest Atlantic do not necessarily indicate the presence of southern-sourced water. Instead, these values may represent a northern-sourced water mass with lower than modern preformed δ13C values that were further modified downstream by increased sequestration of remineralized carbon, facilitated by a more sluggish glacial deep circulation, corroborating previous evidence.

A Ternary Mixing Model Approach Using Benthic Foraminifer δ13C‐δ18O Data to Reconstruct Late Pliocene Deep Atlantic Water Mass Mixing

“…This contradicts the traditional δ 13 C-based hypothesis that SCW dominated the LGM deep Atlantic (e.g., Boyle & Keigwin, 1982;Ferrari et al, 2014;Negre 10.1029/2019PA003804 et al, 2010. Instead, it supports the more nuanced view presented in recent papers (Du et al, 2020;Gebbie, 2014;Howe et al, 2016;Pöppelmeier et al, 2020), where low δ 13 C-values in the North Atlantic are explained by reduced ventilation and sluggish circulation. However, even if the additional δ 18 O tracer may improve water mass reconstructions, the TMM estimates are still partially determined by δ 13 C variability, and sluggish circulation might have interfered with the conservative properties of δ 13 C during the LGM.…”

Evidence for influx of Atlantic water masses to the Labrador Sea during the Last Glacial Maximum

“…Evidence for active formation of NADW during the LGM has recently also been provided by Klockmann et al 60 , while Ezat et al 61 suggest the possibility of enhanced Arctic Ocean convection during the LGM. Also Pöppelmeier et al 62 support a northern source of both intermediate and deep water during the LGM, suggesting that the distribution of water masses in the Atlantic was not much different than today.…”

Neodymium Isotopes in Glauconite for Palaeoceanographic Reconstructions at Continental Margins: A Preliminary Investigation From Demerara Rise

“…∼ 1200 and 4000 m depth, and finally, the Antarctic Bottom Water (AABW; ε Nd ∼ −10.5) below 4000 m depth (Huang et al, 2014). In previous investigations, the application of Nd isotope measurements to various paleoceanographic archives retrieved from Demerara Rise (Huang et al, 2014) and other locations in the Atlantic have proven particularly useful to reconstruct past ocean circulation patterns, indicating prominent changes in the Atlantic Meridional Overturning Circulation (AMOC) over glacial-interglacial timescales, with impact on heat transport and carbon storage in the surface and deep ocean, respectively (e.g., Rutberg et al, 2000;Piotrowski et al, 2004Piotrowski et al, , 2005Pahnke et al, 2008;Böhm et al, 2015;Howe et al, 2016;Lippold et al, 2016;Pöppelmeier et al, 2020). A survey of the spatial distribution of Nd isotopes in the Atlantic during both the recent Holocene and the last glacial periods indicated relatively unradiogenic NADWlike ε Nd values (∼ −13) below 1500 m water depth at Demerara Rise (Howe et al, 2016).…”