Origin of δ13C minimum events in thermocline and intermediate waters of the western South Atlantic

R.A Nascimento,T.P Santos,I.M Venancio,C.M Chiessi,J.M Ballalai,H Kuhnert,A Govin,R.C Portilho-Ramos,D Lessa,B.B Dias,T.M.L Pinho,S Crivellari,S Mulitza,A.L.S Albuquerque

doi:10.1016/j.quascirev.2021.107224

Abstract

Stable carbon isotopic (δ13C) minimum events have been widely described in marine archives recording the properties of thermocline and intermediate waters during glacial terminations. However, the mechanisms associated with these events remain ambiguous. Here we present three high temporal resolution deep-dwelling planktonic foraminifera δ13C records from the main thermocline and one benthic δ13C record from the modern core of Antarctic Intermediate Water (AAIW). Our sediment cores are distributed along the western South Atlantic from the equator to the subtropics, with the longest record spanning the last ∼300 kyr. The results show that δ13C minimum events were pervasive features of the last three glacial terminations and Marine Isotope Stage 4/3 transition in the western South Atlantic. Two distinct mechanisms were responsible for the δ13C minima at the thermocline and intermediate depths of the Atlantic, respectively. We suggest that the δ13C minimum events at the thermocline were mostly driven by the thermodynamic ocean-atmosphere isotopic equilibration, which is supported by calculated δ13C of dissolved inorganic carbon in the subtropical western South Atlantic as well as by previously published model simulations. On the other hand, intermediate depths δ13C minimum events in the tropics were likely caused by the slowdown of the Atlantic meridional overturning circulation and the associated accumulation of isotopically light carbon at mid and intermediate depths of the Atlantic Ocean.

Highlights

Upwelling in the Southern Ocean (SO) connects the deep ocean with the atmosphere, by drawing deep waters to the SO surface (Marshall and Speer, 2012)
Planktonic δ13C records from all studied cores present a similar pattern with remarkable negative excursions during glacial terminations that only recover after the onset of the following interglacial period (Fig. 2)
We present three high temporal resolution thermocline and intermediate depth (1037 m) foraminiferal δ13C records from the western South Atlantic spanning the last 300 kyr

Summary

Introduction

Upwelling in the Southern Ocean (SO) connects the deep ocean with the atmosphere, by drawing deep waters to the SO surface (Marshall and Speer, 2012). The breakup of deep-ocean stratification (e.g., Du et al., 2018; Basak et al, 2018) and the intensification of upwelling in the SO have been linked to the advection of nutrient-rich 13C-depleted waters from the deep ocean to the SO surface (Anderson et al, 2009; Toggweiler et al, 2006) This was followed by atmospheric CO2 increase and stable carbon isotopic (δ13C) minimum events at thermocline and intermediate depths of the oceans (between 100 and 1200 m water depth) (e.g., Martínez-Botí et al, 2015; Ziegler et al, 2013; Schmitt et al, 2012; Spero and Lea, 2002). Δ13C minimum events have been observed in regions far from the influence of SAMW and AAIW as, for example, in the North Atlantic (e.g., Lynch-Stieglitz et al, 2019; Rickaby and Elderfield, 2005)

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Results

Conclusion