Improved Isotope‐Based Source Apportionment of Iron Reveals Strong Anthropogenic Input in the North Pacific Ocean

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 influence of natural dust. The iron isotope technique has emerged as a powerful tool for quantifying anthropogenic iron in the marine environment. However, anthropogenic contributions derived from such an apportionment approach are highly sensitive to the selection of isotopic end‐members, which are mainly determined by source tests and inconsistent across literature. Analysis of aerosol samples collected during a spring 2022 cruise in the Chinese marginal seas showed that 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 literature. To address 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 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 in ambient aerosol. Applying these updated end‐members to iron sources based on previously published iron isotope data sets from the North Pacific indicates 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.