Processes driving 231Pa/230Th in the mid-North Atlantic Basin over the last 30,000 years

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of Earth’s climate system, regulating large-scale ocean heat and nutrient transport. Paleo reconstructions indicate that the AMOC varied substantial during late Quaternary climate transitions. Sedimentary ²³¹Pa/²³⁰Th has been widely used as a tracer for reconstructing past AMOC strength. However, recent studies have questioned its applicability, as ²³¹Pa/²³⁰Th is also sensitive to particle fluxes. In particular, variations in export productivity and metalliferous particles emitted from hydrothermal vents may overprint the circulation signal.

Here, we investigate the sensitivity of sedimentary ²³¹Pa/²³⁰Th to past AMOC variability by compiling new and revised down-core ²³¹Pa/²³⁰Th records spanning the last 30,000 years from a geographically confined sector of the mid–North Atlantic, covering water depths from 2,102 to 4,110 m. This justifies the assumption of very similar particles fluxes for all core locations in this pelagic environment. However, despite their close spatial proximity, the down-core ²³¹Pa/²³⁰Th records exhibit two clearly distinguishable trends, with increasing ²³¹Pa/²³⁰Th at shallower sites and decreasing trends at deeper sites. These trends are unlikely to be the result of changes in particle scavenging alone: biogenic opal concentrations reveal similar down-core trends throughout all sites, while the absolute concentrations remain consistently below 10 wt% and bulk sediment Fe/Ti and Cu/Ti ratios at most sites provide no evidence for significant local inputs of metalliferous particles associated with enhanced hydrothermal activity despite the region's proximity to the mid-ocean ridge. The only exception is one core closest to multiple active hydrothermal vents showing intermittent intervals of elevated Fe/Ti and Cu/Ti ratios, which are associated with elevated ²³¹Pa/²³⁰Th ratios.

By incorporating the ²³¹Pa/²³⁰Th records from this geographically confined study area into a basin-wide North Atlantic compilation, we show that the inverted ²³¹Pa/²³⁰Th trends observed over the last 30,000 years are coherent North Atlantic-wide features. To investigate the underlying mechanisms, we conducted a set of conceptual Holocene and LGM AMOC simulations using the ²³¹Pa/²³⁰Th-enabled Bern3D model. The simulations show that during the LGM a weaker AMOC, relative to the Holocene, can reproduce the observed depth-depend ²³¹Pa/²³⁰Th response. This pattern is most likely caused by the spatiotemporally variable balance between particle-mediated scavenging and lateral advection of ²³¹Pa. Importantly, changes in both processes are governed on basin scale by the AMOC. These findings indicate that shallow to intermediate-depth sediment cores capture signals of past circulation strength, even when their ²³¹Pa/²³⁰Th response is inverse to the conventional deep-ocean interpretation of higher ²³¹Pa/²³⁰Th reflecting a weaker AMOC and vice versa.