Eastern Atlantic deep-water circulation and carbon storage inferred from neodymium and carbon isotopic compositions over the past 1.1 million years

Abstract

The Mid-Pleistocene transition (MPT; 1200 to 800 thousand years, kyr) is marked by the shift from 41-kyr to 100-kyr interglacial-glacial cyclicity without substantial change in the astronomical forcing. This change in climate response relied on internal feedback processes including interaction between ice sheet/sea ice, ocean circulation and the carbon cycle. It was suggested that a major perturbation of global oceanic carbon chemistry occurred at around 900 ka (Marine Isotope Stage, MIS, 24–22) although the mechanism responsible for the change is still to be elucidated. To investigate the link between the Atlantic Meridional Overturning Circulation (AMOC) and oceanic carbon storage for the past 1100 kyr, we combined neodymium isotopic composition (143Nd/144Nd or εNd) recorded in foraminiferal authigenic fractions with epibenthic foraminiferal δ13C and δ18O from two cores in the North- and South-east Atlantic Ocean. Glacial/interglacial εNd amplitude is smaller before the 900-ka event than after the event. The 900-ka event is marked by increase in seawater εNd at both sites. These observations are consistent with previous studies, suggesting basin-wide εNd changes. Combined with existing data, these new results reveal a persistent meridional gradient of seawater εNd in the Atlantic Ocean over the past 1100 kyr. By comparing the reconstructions with numerical modelling results, we propose that weaker AMOC and changes in Nd sources to the North Atlantic were the main reasons for the observed εNd shift at the 900-ka event in relation to the evolution of the Northern hemisphere cryosphere. The influence of enhanced Southern Ocean overturning circulation on εNd values was estimated to be minor. Seawater εNd and benthic δ13C relationship for the whole study period indicates the presence of carbon-rich glacial deep water (>3000 m) in the North and the South Atlantic, in particular at MIS 22 and 24. This suggests that, in addition to weaker AMOC, reduction of deep-water ventilation and/or air-sea exchange in the Southern Ocean could have been responsible for the observed low benthic δ13C values. Together with increased biological productivity due to iron fertilization in the Southern Ocean, the physical process significantly contributed to the deep Atlantic carbon storage during the 900-ka event and the subsequent glacial periods.