Iron (Fe) availability impacts marine primary productivity, influencing the efficiency of the biological pump. Stable Fe isotope analysis has emerged as a tool to understand how Fe is sourced and cycled in the water column. However, distinguishing the major Fe sources in sediment records based on their Fe isotope compositions alone is challenging as sources can exhibit overlapping values. To address this issue, we studied three pelagic clay sequences spanning the past 90 million years. First, we used osmium isotope chronostratigraphy to date the sequences. Second, we analyzed leachates of the sediments for their multi-element concentration and Fe isotope compositions. Third, we used statistical modeling to reveal five principal Fe sources to the sites—dust, distal background, two hydrothermal fluids, and a magnesium-rich volcanic ash—which share similar depositional histories. Initially, hydrothermal inputs dominated Fe deposition, shifting to increased contributions from other sources as the sites migrated away from their respective mid-ocean ridges. Notably, between 66–40 Ma, distal background sources became significant, before the sites shifted to increasing dust dominance around 30 million years ago. Our analysis suggests that the modern South Pacific is the most dust-dominated it has been in ≈90 million years, which is significant considering the low rates of dust deposition to this region. Our approach establishes a framework combining isotope measurements, geochemical analysis, and statistical modeling to explore the history of Fe delivery to the ocean. This approach holds potential for understanding the interplay between Fe delivery, the biological pump, and Earth’s climate across geological timescales.

Mount Early and Sheridan Bluff (87 degrees south) are the above-ice expression of Earth’s southernmost volcanic field and are isolated by more than 1000 km from any other exposed Cenozoic volcano in Antarctica. These monogenetic, Early Miocene volcanoes are constructed mostly of olivine-phyric basaltic pillow lavas and breccias (Mount Early) and pāhoehoe lavas (Sheridan Bluff) whose differentiation is controlled by the fractional crystallization of olivine with lesser quantities of clinopyroxene, plagioclase and magnetite. Fractional crystallization or contamination by crust cannot account for the coexistence of olivine tholeiite and alkaline compositions but their relationship can be explained by change from higher (5-6%) to lower (1.5-2%) degrees of mantle partial melting, respectively, of a source that is heterogeneous on a small-scale. Both alkaline and subalkaline magma types have geochemical and isotopic signatures that differentiate them from volcanism of the West Antarctic rift system. Data trends in Sr-Nd-Pb isotope space along with major and trace element characteristics indicate mixing of at least two-distinct mantle sources; 1) a relatively depleted component similar to sources for mid-ocean ridge basalt from the extinct Antarctic-Phoenix spreading center, and 2) an enriched component similar to sources for mafic magmas of the Jurassic Karoo‒Ferrar large igneous provinces. The availability of these two mantle source types was facilitated by the detachment, sinking and heating of metasomatized continental lithosphere (enriched source) that released volatiles into the surrounding asthenosphere (depleted source) to promote flux melting. Volcanism triggered by lithospheric detachment is therefore uniquely applied to Mount Early and Sheridan Bluff to explain their isolation and enigmatic tectonic setting but can also account for source heterogeneity and the ephemeral change in degree of mantle partial melting recorded in their mafic compositions.