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The micronutrient iron (Fe) plays a fundamental role controlling primary productivity in the upper ocean, with volcanic eruptions and deposition of airborne volcanic material (termed tephra) a potential source of Fe. Here, we investigate the geochemical and Fe isotopic (δ 56 Fe) composition of tephra layers, sediments, and mixed tephra-sediment samples from the Integrated Ocean Drilling Program (IODP) Hole 1396C, located offshore the volcanically active island of Montserrat in the Lesser Antilles, Caribbean Sea. We find that buried tephras, which have experienced diagenesis, exhibit lighter δ 56 Fe (relative to standard IRMM-524a) compositions (down to −0.26 ± 0.04‰, 2SD) than fresh tephra deposited in Montserrat (δ 56 Fe = 0.02 ± 0.02‰, 2SD). Such negative values suggest that isotopically heavier Fe has been lost from the originally deposited material. Using multivariate statistical modelling and mass balance constraints, we identify the outward Fe flux (with calculated δ 56 Fe of 0.21 ± 0.31‰, 2SD, n = 12) during non-reductive dissolution of tephra as the likely cause of the retention of these light δ 56 Fe compositions. Due to the widespread nature of tephra deposition, tephra diagenesis may provide an important source of isotopically heavy dissolved Fe (dFe) to the oceans. This process contrasts with more commonly considered reductive dissolution processes, which provide a source of dFe enriched in light isotopes to the oceans. • An iron isotope record is reported for archived tephra in the Caribbean Sea. • On average, 29% of all marine sediment in the Caribbean Sea is tephra. • Buried tephras have lighter Fe isotope composition than fresh tephra. • Diagenetic loss of iron by non-reductive dissolution is evidenced by isotope mass-balance. • Tephra diagenesis and dissolution may be important for seawater budgets. [ABSTRACT FROM AUTHOR]