Reversed flow of Atlantic deep water during the Last Glacial Maximum

Abstract

The meridional overturning circulation (MOC) of the Atlantic Ocean is considered to be one of the most important components of the climate system. This is because its warm surface currents, such as the Gulf Stream, redistribute huge amounts of energy from tropical to high latitudes and influence regional weather and climate patterns, whereas its lower limb ventilates the deep ocean and affects the storage of carbon in the abyss, away from the atmosphere. Despite its significance for future climate, the operation of the MOC under contrasting climates of the past remains controversial. Nutrient-based proxies and recent model simulations indicate that during the Last Glacial Maximum the convective activity in the North Atlantic Ocean was much weaker than at present. In contrast, rate-sensitive radiogenic (231)Pa/(230)Th isotope ratios from the North Atlantic have been interpreted to indicate only minor changes in MOC strength. Here we show that the basin-scale abyssal circulation of the Atlantic Ocean was probably reversed during the Last Glacial Maximum and was dominated by northward water flow from the Southern Ocean. These conclusions are based on new high-resolution data from the South Atlantic Ocean that establish the basin-scale north to south gradient in (231)Pa/(230)Th, and thus the direction of the deep ocean circulation. Our findings are consistent with nutrient-based proxies and argue that further analysis of (231)Pa/(230)Th outside the North Atlantic basin will enhance our understanding of past ocean circulation, provided that spatial gradients are carefully considered. This broader perspective suggests that the modern pattern of the Atlantic MOC-with a prominent southerly flow of deep waters originating in the North Atlantic-arose only during the Holocene epoch.

231Pa/230Th evidence for a weakened but persistent Atlantic meridional overturning circulation during Heinrich Stadial 1

“…Data north of 45°S were used for the LGM and HS1 to account for the northward shift of the Southern Ocean opal belt during the LGM, in keeping with Yu et al 1 The average 231 Pa/ 230 Th ratio in LGM sediments (0.065±0.005; n ¼ 79) is similar to the value obtained by Yu et al 1 (0.059±0.007; n ¼ 26; Fig. 2); this study 15,16,[20][21][22][23]25,27,28,30,39,40,[54][55][56] . The average ratio for HS1 (0.074 ± 0.009; n ¼ 26) is somewhat greater than the LGM value, but not significantly different at the 95% confidence level; this study 15,16,21,22,25,27,30,39,40,54 .…”

“…2); this study 15,16,[20][21][22][23]25,27,28,30,39,40,[54][55][56] . The average ratio for HS1 (0.074 ± 0.009; n ¼ 26) is somewhat greater than the LGM value, but not significantly different at the 95% confidence level; this study 15,16,21,22,25,27,30,39,40,54 . The average 231 Pa/ 230 Th ratio in Holocene sediment north of 50°S (0.070 ± 0.004; n ¼ 165) is greater than Yu et al, a result that is significant at the 95% confidence level (Yu et al Holocene 231 Pa/ 230 Th: 0.060 ± 0.004; n ¼ 68); this study 1,15,16,[20][21][22]25,27,28,30,39,40,[54][55][56][57][58][59][60][61][62][63][64]…”

“…In fact, most cores selected for inclusion have multiple control points within and surrounding HS1. Six of the cores (OCE326-GGC5, SU90-44, SU81-18, MD02-2594, MD95-2037 and DAPC2) come from high-resolution studies designed specifically to identify and analyse HS1, and have excellent age control; see age models in original references for details 15,16,21,25,27,30,39,40,54 . Three of the cores (RC24-01, RC24-07, RC13-189) contain at least one age-control point within HS1 (ref.…”

The impacts of deglacial meltwater forcing on the South Atlantic Ocean deep circulation since the Last Glacial Maximum

“…The weaker dense water production in the NH during YD is discussed by several studies based on proxy data (Boyle, 1987;Hughen et al, 1998;Piotrowski et al, 2005;Praetorius et al, 2008;Roberts et al, 2010). Furthermore, through 231 Pa / 230 Th ratio analysis, Negre et al (2010) as well as the modeling study of Wainer et al (2012) -argue that the modern Atlantic circulation was only fully established during the Holocene, which is also consistent with the evolution of the salinity-based water mass structure presented in Fig. 6.…”

Meridional circulation across the Antarctic Circumpolar Current serves as a double 231Pa and 230Th trap

“…Glacial-Interglacial changes in 67 to record the history of AABW flow (Lippold et al, 2016). Interpretations in terms of northward flow of 73 SCW or AABW depend on assumptions on the scavenging history and resulting preformed 231 Pa/ 230 Th 74 ratio in these southern source waters (Jonkers et al, 2015;Lippold et al, 2016;Negre et al, 2010). 75…”