Variability in mid‐depth ventilation of the western Atlantic Ocean during the last deglaciation

Abstract

Negative stable carbon isotopic excursions have been observed throughout most of the mid‐depth (~1000–3000 m) Atlantic Ocean during Heinrich Stadial 1 (HS1) and the Younger Dryas (YD). Although there is an agreement that these mid‐depth excursions were in some way associated with a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), there is still no consensus on the precise mechanism(s). Here we present benthic stable carbon and oxygen isotopic (δ13C and δ18O) records from five cores from the western equatorial Atlantic (WEA). Together with published benthic isotopic records from nearby cores, we produced a WEA depth transect (~800–2500 m). We compare HS1 and YD data from this transect with data from previously published North and South Atlantic cores and demonstrate that the largest negative δ13C excursions occurred in the WEA during these times. Moreover, our benthic δ18O records require the presence of two water masses flowing from the Southern Ocean, bisected by a Northern Component Water (NCW). Given that δ18O is a conservative water mass tracer, we suggest that δ13C was decoupled from water mass composition and does not correspond to simple alternations between northern and southern sourced waters. Instead, δ13C behaved non‐conservatively during HS1 and the YD. Consistently with our new 231Pa/230Th record from the WEA transect, that allowed the reconstruction of AMOC strength, we hypothesize that the negative δ13C excursions reflect an increase in the residence time of NCW in response to a weakened AMOC, allowing for a marked accumulation of 13C‐depleted respired carbon at the mid‐depth WEA.

Data Constraints on Glacial Atlantic Water Mass Geometry and Properties

“…The western Atlantic glacial δ 13 C section includes published benthic data compiled previously (Curry & Oppo, 2005), data published since then (Burckel et al, 2015;Freeman et al, 2015;Hodell et al, 2008Hodell et al, , 2010Hoffman & Lund, 2012;Keigwin & Swift, 2017;Lund et al, 2015;Lynch-Stieglitz et al, 2014;Oppo et al, 2015;Poggemann et al, 2017;Praetorius et al, 2008;Rickaby & Elderfield, 2005;Tessin & Lund, 2013;Thornalley et al, 2010;Voigt et al, 2017), and our new data from the eight cores on the Demerara Rise, ranging in water depth from 376 to 2,642 m ( The inversion is also informed by Cd/Ca and δ 13 C data from the eastern basin compiled by Marchitto and Broecker (2006; Figure S14 in the supporting information). An earlier inversion of glacial data gave unrealistically low δ 13 CAS values in the surface South Atlantic (not shown).…”

“…The western Atlantic glacial δ 13 C section includes published benthic data compiled previously (Curry & Oppo, ), data published since then (Burckel et al, ; Freeman et al, ; Hodell et al, , ; Hoffman & Lund, ; Keigwin & Swift, ; Lund et al, ; Lynch‐Stieglitz et al, ; Oppo et al, ; Poggemann et al, ; Praetorius et al, ; Rickaby & Elderfield, ; Tessin & Lund, ; Thornalley et al, ; Voigt et al, ), and our new data from the eight cores on the Demerara Rise, ranging in water depth from 376 to 2,642 m (Table and supporting information tables). The Western Atlantic CdW section includes data compiled previously (Marchitto & Broecker, ), data published since then (Makou et al, ; Poggemann et al, ), and new data from the Demerara Rise and Blake Outer Ridge.…”

230Th normalization: New insights on an essential tool for quantifying sedimentary fluxes in the modern and Quaternary ocean

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Constraining the Variability of the Atlantic Meridional Overturning Circulation During the Holocene

“…NADW is characterized by distinctively less radiogenic (i.e., lower) εNd signatures (mostly due to the contribution of very unradiogenic LSW (εNd~−15) compared to more radiogenic signatures of Southern Component Water (εNd~−9, Figure 1; e.g., (Lambelet et al, 2016;Stichel et al, 2015). Similarly, the stable carbon isotope composition (δ 13 C) of seawater recorded by epibenthic foraminifera, indicative of the accumulation of remineralized carbon in the ocean interior, represents a debated (Gebbie et al, 2015;Howe, Piotrowski, Noble, et al, 2016;Schmittner et al, 2013;Voigt et al, 2017), yet long-standing proxy for reconstructions of water mass distribution (Curry & Oppo, 2005;Keigwin, 2004;Oppo et al, 2003). Both tracers thus provide largely independent and complementary evidence reflecting past changes in the relative contributions of water masses originating from the North Atlantic and the Southern Ocean.…”