Modelling the modern oceanic cycle of beryllium-10 and beryllium-9

Beryllium isotopes, i.e. cosmogenic meteoric ¹⁰Be and stable ⁹Be, enter the oceans through distinct pathways. Beryllium-10 is produced in the atmosphere and enters the oceans mainly via precipitation, while ⁹Be is sourced from continents. Beryllium isotopes with a short oceanic residence time (10²-10³ yrs) display non-conservative behaviour in seawater. The ¹⁰Be/⁹Be proxy has been utilized as a powerful tool for quantifying diverse processes, including geomagnetism, sedimentation, continental input, and ocean circulation. Substantial effort has been invested in understanding external sources and internal cycling of Be isotopes in the recent decade, such as constraints on the global distribution of ¹⁰Be depositional fluxes and on riverine and benthic ⁹Be inputs. Hence, it offers an excellent opportunity to revisit their modern oceanic cycle. Here, we investigate the controls on the modern oceanic cycling of Be isotopes using a three-dimensional ocean biogeochemical model constrained by water-column distributions of ⁹Be and ¹⁰Be compiled from the literature. In addition to modelling the previously identified controls, we highlight the critical role of marine benthic fluxes and scavenging on particulate organic matter and opal in governing the mass balance and spatial distribution of Be isotopes. The transport of Be isotopes between basins by circulation is of lesser importance compared to external inputs at continent/atmosphere–ocean boundaries, except in the South Pacific. Consequently, the basin-wide ¹⁰Be/⁹Be ratio predominantly reflects the pattern of external inputs across most basins in the modern ocean. Based on our data-constrained oceanic model, we can further assess the sensitivity of basin-wide ¹⁰Be/⁹Be ratios to changes in external sources, such as continental denudation, and internal cycling, such as particle scavenging. The mechanistic understanding developed from this Be cycling model provides important insights into the various applications of marine Be isotopes, and offers additional tools to assess the individual effects of geomagnetism and environment on cosmogenic ¹⁰Be/⁹Be records in marine sediments.