Iron isotope variability in the suspended load across the Changjiang (Yangtze) River basin: The tally of source and weathering contributions

Iron isotopes are increasingly used to trace the sources and biogeochemical cycling of Fe in Earth's surface environments. Despite the recognized significance of riverine input of Fe to the primary productivity throughout the oceans, uncertainties remain over the riverine Fe sources and fractionation of Fe isotopes. This study presents the first dataset on Fe isotope compositions in Suspended Particulate Matter (SPM) of the Changjiang (Yangtze) mainstream and its major tributaries, from its headwater on the Tibetan Plateau to its estuary, aiming to better determine the sources of Fe-bearing particles and to explore the relationship of particulate Fe isotopic variation with weathering regimes across watershed spatial scales.

Our results demonstrate that the suspended load (>0.45 μm) transports approximately 99 % of the total riverine Fe budget along the course of the Changjiang River, with δ⁵⁶Fe values ranging from −0.15 to 0.16 ‰, which are either lower than or comparable to those of the Upper Continental Crust (UCC; ∼0.1 ‰). The strong positive correlation between Fe and Al, along with the negative correlation between Fe and Ca concentrations in SPM, highlights the presence of Fe in close association with clay-rich aluminosilicate minerals. The data suggest that provenance rock types are not the dominant factor causing spatial SPM δ⁵⁶Fe variation. Furthermore, evidence from Fe and Zn enrichment factors (relative to Al) and their relationships with δ⁵⁶Fe in SPM indicates minimal anthropogenic Fe inputs. Rather, the spatial variability of δ⁵⁶Fe in SPM more likely reflects changes in weathering regimes and intensities under varying climatic conditions, as supported by the relationship between δ⁵⁶Fe and the Chemical Index of Alteration (CIA) weathering proxy. Specifically, in the mountainous headwaters and upper reaches, characterized by a cold, dry alpine climate, we observe slightly negative δ⁵⁶Fe values at close to −0.1 ‰, with minimal variation relative to CIA values, reflecting the dominance of a weathering-limited regime. In contrast, in the mid-to-lower reaches, under a warm, humid climate, a positive correlation between δ⁵⁶Fe and CIA is observed, consistent with the progressive loss of isotopically light Fe during chemical weathering. A global assessment of SPM δ⁵⁶Fe across different rivers suggests significant variability in the isotopic signatures of riverine particulate Fe delivered to the oceans, driven primarily by climate and its influence on weathering regimes. Altogether, our new data demonstrate the robustness of Fe isotopes as tracers of Fe sources and fractionation processes in large river systems, providing crucial insights into the links between continental weathering, climate change, and the riverine Fe cycle.