The iron cycle is a fundamental component of the Earth system. Growing evidence indicates that anthropogenic emissions can significantly perturb the iron cycle, beyond the contribution of natural dust. The iron isotope technique has emerged as a powerful tool for quantifying anthropogenic iron inputs in marine environments. However, anthropogenic contributions derived from such apportionment approaches are highly sensitive to the selection of isotopic end-members, which are mainly determined by source sample analyses but remain inconsistent across the literature.
By investigating aerosol samples collected during a spring 2022 cruise in the Chinese marginal seas, anthropogenic iron contributions traced by isotope analysis ranged from 0.12 ± 0.12 to 0.43 ± 0.24, depending on the end-members derived from previous studies. To address this uncertainty, an observation-based approach was developed to constrain isotopic end-members directly from ambient measurements. Shipborne high-time-resolution measurements of multiple elements were conducted, and a Positive Matrix Factorization (PMF) receptor model was used to quantitatively distinguish dust and anthropogenic contributions to atmospheric iron. By combining the constraints from PMF results with filter-based iron isotope measurements, isotopic end-members of +0.44‰ for dust iron and −1.43‰ for anthropogenic iron were newly determined for ambient aerosols. Applying these updated end-members to iron source apportionment based on previously published iron isotope datasets from the North Pacific revealed that anthropogenic contributions were ubiquitously underestimated, with maximum underestimation likely exceeding 30%.
The application of observation-based end-members, as opposed to the conventional source-test-based end-members, allows for more accurate quantification of anthropogenic iron inputs by better capturing isotopic fractionation during atmospheric transport, thereby advancing isotope biogeochemistry.