Evidence for Stable Holocene Basin‐Scale Overturning Circulation Despite Variable Currents Along the Deep Western Boundary of the North Atlantic Ocean

Abstract

While substantial changes in thermohaline circulation related to deglacial climate variability are well established, the role of this circulation in Holocene climate variability remains uncertain. Here we use two dynamical proxies, 231Pa/230Th ratios and mean sortable silt size ( trueSS¯), to reconstruct Holocene bottom water circulation at the intermediate‐depth Carolina Slope. We find no substantial change in deep current speed or 231Pa export at this site during the Holocene, suggesting consistent 231Pa export via the Deep Western Boundary Current. trueSS¯ shows increasing millennial‐scale variability in the middle‐late Holocene, which may reflect Labrador Sea Water contribution to current speed. We conclude that deepwater export from the North Atlantic has remained remarkably stable during the Holocene, decoupled from changing rates of specific water masses, while production of these water masses varied at millennial to centennial time scales. The persistence of the large‐scale overturning may reflect the ocean's stabilizing influence on Holocene climate.

230Th Normalization: New Insights on an Essential Tool for Quantifying Sedimentary Fluxes in the Modern and Quaternary Ocean

“…Over 50 years' (1966–2019) worth of data have been compiled to create the global thorium database ( n = 1,167) presented here (Adkins et al, ; Anderson et al, , Anderson et al, , Anderson et al, , Anderson et al, ; Bausch, ; Böhm et al, ; Bohrmann, ; Borole, ; Bradtmiller et al, , , ; Broecker, ; Broecker et al, ; Brunelle et al, , ; Causse & Hillaire‐Marcel, ; Chase et al, , ; Chong et al, ; Costa, McManus, & Anderson, ; Costa & McManus, ; Crusius et al, ; Dekov, ; Denis et al, ; Dezileau et al, , ; Durand et al, ; Fagel et al, ; Francois et al, , Francois et al, ; Frank, Eisenhauer, Bonn, et al, , Frank, Eisenhauer, Kubik, et al, , ; Fukuda et al, ; Galbraith et al, ; Geibert et al, ; Gherardi et al, , ; Gottschalk et al, ; Hickey, ; Hillaire‐Marcel et al, ; Hoffmann et al, , ; Jaccard et al, , ; Jacobel et al, ; Jonkers et al, ; Kienast et al, ; Ku & Broecker, ; Kumar et al, ; Lam et al, ; Lamy et al, ; Lao et al, ; Lippold et al, , Lippold et al, , Lippold et al, , Lippold et al, ; Loubere et al, ; Loveley et al, ; Lund et al, ; Mangini & Dominik, ; Marcantonio et al, , Marcantonio et al, ...…”

“…Over 50 years ' (1966-2019) worth of data have been compiled to create the global thorium database (n = 1,167) presented here (Adkins et al, 2006;Anderson et al, 2006Bausch, 2018;Böhm et al, 2015;Bohrmann, 2013;Borole, 1993;Bradtmiller et al, 2006Bradtmiller et al, , 2007Bradtmiller et al, , 2009Broecker, 2008;Broecker et al, 1993;Brunelle et al, 2007Brunelle et al, , 2010Causse & Hillaire-Marcel, 1989;Chase et al, 2003Chase et al, , 2014Chong et al, 2016;Costa, McManus, & Anderson, 2017;Crusius et al, 2004;Dekov, 1994;Denis et al, 2009;Dezileau et al, 2000Dezileau et al, , 2004Durand et al, 2017;Fagel et al, 2002;Francois et al, 1990, Francois et al, 1993Frank, Eisenhauer, Bonn, et al, 1995Fukuda et al, 2013;Galbraith et al, 2007;Geibert et al, 2005;Gherardi et al, 2005Gherardi et al, , 2009Gottschalk et al, 2016;Hickey, 2010;Hillaire-Marcel et al, 2017;Hoffmann et al, 2013Hoffmann et al, , 2018Jaccard et al, 2009Jaccard et al, , 2013…”

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

“…Here we aim at reconstructing large-scale AMOC variability for the Holocene, combining high-resolution 231 Pa/ 230 Th, εNd, and δ 13 C records with existing data from Bermuda Rise sediments, which sensitively recorded past changes in AMOC dynamics (Böhm et al, 2015;Deaney et al, 2017;Gutjahr & Lippold, 2011;Henry et al, 2016;Keigwin & Boyle, 2000;McManus et al, 2004;Rempfer et al, 2017;Roberts et al, 2010). Existing Holocene data sets from this area are not available in sufficient temporal resolution or have been obtained from potentially less sensitive locations, hindering identification of short-scale AMOC oscillations (Hoffmann et al, 2018;Lippold et al, 2016;McManus et al, 2004;Ng et al, 2018;Rempfer et al, 2017).…”

Constraints on the Northwestern Atlantic Deep Water Circulation From 231Pa/230Th During the Last 30,000 Years

“…Previous studies using the 231 Pa/ 230 Th proxy have concluded that a strong circulation of the North Atlantic prevailed during the Holocene (Hoffmann et al, 2018) and pronounced weakenings of AMOC strengths occurred during past cold phases (Bradtmiller et al, 2014;Gherardi et al, 2009;Lippold et al, 2016; Figure 1. Overview map of the northwest Atlantic.…”

“…Previous studies using the 231 Pa/ 230 Th proxy have concluded that a strong circulation of the North Atlantic prevailed during the Holocene (Hoffmann et al, ) and pronounced weakenings of AMOC strengths occurred during past cold phases (Bradtmiller et al, ; Gherardi et al, ; Lippold et al, ; McManus et al, ; Ng et al, ). Nevertheless, a clear picture of the vertical structure of the deglacial circulation and the evolution of AMOC is still lacking, in particular due to insufficient spatial and temporal proxy coverage.…”