Investigating the Atlantic meridional oceanic circulation variability in the Iceland Basin over the Holocene : a multi-proxy approach

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) is a fundamental component of the global climate system, responsible for regulating the exchange of heat, carbon, and nutrients between the surface and deep ocean across latitudes. It is composed of three components: surface circulation, deep circulation, and deep water formation. While these components are linked, each operates through distinct mechanisms, adding complexity to the AMOC system. In the past, variations in AMOC strength have been associated with reduced deep water formation, leading to significant impacts on global climate by altering heat and nutrient distribution. Understanding these mechanisms and their variability is essential for predicting future climate behaviour. In this thesis, we reconstruct the past variations of the AMOC’s three components within the Iceland Basin, a critical region of the AMOC pathway. Using a multi-proxy approach that includes benthic and planktonic foraminiferal assemblages, stable isotopic measurements (δ¹³C and δ¹⁸O), and analogue method reconstructions, we examine surface circulation, deep water formation, and deep circulation during the Holocene. Our findings highlight the dynamic role of the Subpolar Gyre (SPG) in influencing surface circulation in the Iceland Basin from the Mid to the Late Holocene. A strong SPG is marked by increased cold-water planktonic species, lower sea surface temperatures (SSTs), and lighter δ¹⁸O values of N. incompta, reflecting enhanced inflow of cold Labrador Sea water and potential vertical mixing in the upper layers. Interestingly, during notable Late Holocene climatic events, such as the Little Ice Age, the Iceland Basin exhibited contrasting trends, with SSTs warming instead of cooling. To reconstruct deep circulation, we analysed benthic foraminiferal assemblages and the stable isotopic compositions of H. elegans and L. wuellerstorfi. Variations in the Iceland-Scotland Overflow Water (ISOW), a major component of the North Atlantic Deep Water (NADW), were identified. ISOW strength intensified during the early to mid-Holocene, supporting a more vigorous AMOC. This was followed by a gradual weakening after 6,500 years BP as Denmark Strait Overflow Water (DSOW) strengthened. Despite evidence of vertical mixing in the Iceland Basin during the early to mid-Holocene, our records did not confirm the formation of dense deep water masses, likely because these water masses probably remained confined to the upper limb of the NADW due to insufficient density. Lastly, we tested a novel method of paleo-density reconstruction using planktonic foraminifer abundances, δ¹⁸O, and analogue method calculations. Results confirmed vertical mixing impacting the subsurface in the Iceland Basin during the early to mid-Holocene, but methodological limitations, such as potential overestimations of surface parameters, introduce some uncertainties. In conclusion, this thesis provides new insights into the past variability of the AMOC’s surface circulation, deep circulation, and deep water formation in the Iceland Basin. By highlighting the distinct mechanisms driving each component, this study offers a deeper understanding of the AMOC’s complexity and its role in shaping past, present and future climate systems.