Modelling the impact of biogenic particle flux intensity and composition on sedimentary Pa/Th

There is compelling evidence that millennial climate variability of the last glacial period was associated with significant changes in the Atlantic Meridional Overturning Circulation (AMOC). Several North Atlantic sedimentary Pa/Th records indicate a consistent and large Pa/Th increase across millennial-scale events, which has been interpreted as considerable reduction in North Atlantic Deep Water (NADW) formation. However, the use of sedimentary Pa/Th as a pure kinematic circulation proxy is challenging because Pa and Th are also highly sensitive to changes in particulate flux intensity and composition that might have occurred across these millennial scale events. In this study, we use the Pa/Th enabled iLOVECLIM Earth System Model of intermediate complexity to evaluate the impact of changes in biogenic particle flux intensity and composition on the Atlantic Pa/Th. We find that in our model, changes in Particulate Organic Carbon (POC), and to a lesser extent biogenic opal production, can significantly affect the sedimentary Pa/Th, possibly explaining up to 30% of the observed North Atlantic Pa/Th increase across Heinrich stadial 1. The sedimentary Pa/Th response is also likely sensitive to shifts in the geographical distribution of the particles, especially in high scavenging regions. Our study suggests that a decrease in opal production in the northwest Atlantic can induce a far field Pa/Th increase in a large part of the North Atlantic basin. Therefore, local monitoring of particle fluxes may not be sufficient to rule out any influence of changing particle fluxes on sedimentary Pa/Th records.