Glacial‐interglacial circulation changes inferred from ²³¹Pa/²³⁰Th sedimentary record in the North Atlantic region

Abstract

[1] Studies from the subtropical western and eastern Atlantic Ocean, using the 231 Pa/ 230 Th ratio as a kinematic proxy for deep water circulation, provided compelling evidence for a strong link between climate and the rate of meridional overturning circulation (MOC) over the last deglaciation. In this study, we present a compilation of existing and new sedimentary Th ratio measured in upper Holocene sediments indicates slow water renewal above $2500 m and rapid flushing below, consistent with our understanding of modern circulation. In contrast, during the Last Glacial Maximum (LGM), Glacial North Atlantic Intermediate Water (GNAIW) drove a rapid overturning circulation to a depth of at least $3000 m depth. Below $4000 m, water renewal was much slower than today. At the onset of Heinrich event 1, transport by the overturning circulation declined at all depths. GNAIW shoaled above 3000 m and significantly weakened but did not totally shut down. During the Bølling-Allerød (BA) that followed, water renewal rates further decreased above 2000 m but increased below. Our results suggest for the first time that ocean circulation during that period was quite distinct from the modern circulation mode, with a comparatively higher renewal rate above 3000 m and a lower renewal rate below in a pattern similar to the LGM but less accentuated. MOC during the Younger Dryas appears very similar to BA down to 2000 m and slightly slower below.

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

“…The finding of a limited reduction during HS1 suggests a greater sensitivity of climate change to variations in AMOC than previously modelled 8,9 , with important implications for regional and global climate. Both the reduction documented here for HS1 and the subsequent rapid increase in the rate of AMOC previously observed in 231 Pa/ 230 Th records at B15 ka 15,16,21 likely contributed to global deglaciation, the former by influencing the hemispheric temperature gradient and contributing to the rise in atmospheric CO 2 (refs 10,11,13) and the latter by adding enhanced meridional ocean heat transport to the warming effects of rising insolation and greenhouse gases and combining to accelerate the melting of northern ice sheets 10,12,16,19 . Supplementary Fig.…”

“…2b). The highest ratio, measured at 1.2 km in the NE Atlantic, is in an area characterized by high scavenging today 27 , while deeper cores show lower ratios 21,28,29 (Fig. 2b).…”

“…2a). The Holocene data from the main basin show a broad, overall decrease in 231 Pa/ 230 Th with increasing water depth throughout the water column 20,21 . The opal-rich areas of the Southern Ocean and North Atlantic have higher 231 Pa/ 230 Th ratios, reflecting the scavenging efficiency of opal 17 .…”

Changes in North Atlantic Deep Water strength and bottom water masses during Marine Isotope Stage 3 (45–35kaBP)

“…McManus et al (2004) suggested a near total collapse of Atlantic MOC during Heinrich event 1 based on a sedimentary Pa/Th record from the Bermuda Rise. However, since the adsorbed sedimentary Pa/Th signal is also highly sensitive to particle compositions (biogenic silica content) such an interpretation has to be treated with caution and Atlantic MOC reduction may not have been so severe (Chase et al, 2002;Hall et al, 2006;Keigwin and Boyle, 2008;Gherardi et al, 2009;Lippold et al, 2009). In addition, several of the major Heinrich IRD discharge events (H1, H2, H4) were apparently accompanied by meltwater precursor events originating from the Northwest European Ice Sheet (NWEIS) suggesting that ice sheet collapse of European origin may have pre-conditioned the MOC in the North Atlantic (Peck et al, 2007b).…”

Palaeoceanography: motivations and challenges for the future

“…There are two main ways of addressing the outstanding issues. The first is to continue to collect 231 Pa/ 230 Th down-core records covering a range of depths and locations with associated mineralogical and particle flux data [73]. Such an approach would give the mean 231 Pa/ 230 Th sedimentary ratio on a basin-wide scale, better suited to assessing the large-scale effect of particles on 231 Pa/ 230 Th and maximizing the potential for modelling studies to distinguish unique circulation patterns.…”