Influence of Atlantic Meridional Overturning Circulation changes on deglacial deep-water ecosystems off NW-Africa

The Atlantic Ocean is an important component of the global climate system. In this ocean basin, the Atlantic Meridional Overturning Circulation (AMOC), partially driven by the formation of North Atlantic Deep Water (NADW) in the northern hemisphere, uptakes an important percentage of the anthropogenically induced excess heat of our planet and distributes it around the globe through a complex ocean circulation system that influences global warming rate (e.g. Cummins et al., 2016; Levitus et al., 2000). As described in Chapter 2 and Chapter 4, the instrumental record of AMOC strength provides evidence of a potential decrease or weakening also known as “AMOC slowdown” in the 19th and 20th centuries, synchronous with the increasing trend of CO2 in the atmosphere, as well as greenhouse gases concentrations (Caesar et al., 2021; Dima et al., 2021; IPCC, 2022; Rahmstorf et al., 2015). Furthermore, AMOC will very likely decrease in the 21st century (IPCC, 2023). However, the instrumental AMOC record is still too short to fully understand the effects of such slowdown in the climate system, therefore our knowledge must rely on paleoceanographic proxies. Although controversial, AMOC slowdown has been recorded by different proxies in the last 30,000 years, offering the opportunity to observe the effects of this slowdown in the deep ocean through the marine fossil record. This is discussed in detail in the Introduction of Chapter 4.

The last deglaciation period in the Eastern North Atlantic (NE Atlantic) starts after the end of the Last Glacial Maximum (LGM, ⁓ 19 kyrs BP). Temporary returns to cold periods occurred during the Heinrich Stadial 1 (HS1, ⁓18.3 – 15.4 Kyr BP) and Younger Dryas (YD, ⁓12.9 – 11.7 Kyr BP), and between these two cold periods, the Bølling–Allerød warming (B-A) was a transient warming time in the northern hemisphere. The record of AMOC, includes kinematic proxies like 231Pa/230Th (Böhm et al., 2015; Gherardi et al., 2005; McManus et al., 2004), paleo-circulation proxies like the radiogenic neodymium (Nd) isotope composition (Böhm et al., 2015; Du et al., 2020; Roberts et al., 2010) and bottom water ventilation proxies like the benthic foraminifera δ13C (e.g., Oppo et al., 2015), that show a consistent AMOC decline during the HS1 and YD, alternated by a short B-A resumption period.

In this research, deglacial sedimentary records of gravity cores GeoB9512-5 (15.34 °N, 17.37 °W, 793 m water depth), GeoB9508-5 (15.50 °N, 17.95 °W, 2,384 m WD) and GeoB9506-1 (15.61 °N, 18.35 °W, 2,956 m WD) were analyzed to extract and investigate the benthic foraminifera content, and reconstruct the bottom water changes in the NE Atlantic during the last 27,000 years. Three components of bottom waters were investigated; (1) bottom water oxygenation reconstructed from the detailed taxonomic and quantitative analyses of benthic foraminifera (GeoB9512-5 and GeoB9506-1); (2) bottom water temperatures (BWTs) reconstructed from benthic foraminifera Magnesium-Calcium (Mg/Ca) ratios (GeoB9512-5, GeoB9508-5 and GeoB9506-1); and (3) changes in ventilation and salinity inferred from benthic foraminifera δ13C and δ18O respectively (GeoB9512-5, GeoB9508-5 and GeoB9506-1). The key outcomes of this research are: (1) deep-ocean heat uptake stagnation in times of reduced AMOC in the NE Atlantic, suggesting AMOC strength sets the oceanic heat storage depth; (2) a better ventilated and oxygenated deglacial Eastern Tropical North Atlantic Oxygen Minimum Zone, related to an intensified subtropical cell due to AMOC slowdown; than in turn (3) affected calcification processes of some benthic foraminifera at intermediate depths introducing uncertainties on Mg/Ca-based paleotemperature reconstructions at intermediate depths in the eastern North Atlantic. Finally, the integrated paleotemperature records show that during the LGM and most of the last deglacial period, heat storage took place in the intermediate waters in the tropical eastern Atlantic, and shift to a deep mode in the Holocene as AMOC strength is restored.