Abstract

Giant Antarctic iceberg calving is projected to increase with climate change, affecting ocean circulation, nutrient supply, and carbon cycling. These icebergs can stimulate primary production and influence Southern Ocean carbon fluxes through physical and biogeochemical modification of the upper ocean, yet the underlying biogeochemical mechanisms remain poorly constrained. Here we investigate the coupled effects of meltwater input and nutrient dynamics around two of the largest known icebergs—A-76A and A-23A—using an approach that incorporates silicon isotopes to examine nutrient cycling. Around A-76A, enhanced glacial meltwater input coincides with dynamic nutrient variability and strong silicon isotope fractionation, consistent with a diatom bloom. In contrast, waters around A-23A show minimal enhancement of glacial meltwater input and are nutrient-rich but show no silicon isotope fractionation, indicating minimal biological uptake. These contrasting regimes reveal that iceberg influence on ocean biogeochemistry is highly heterogeneous, likely reflecting differences in meltwater delivery and local circulation. Our findings demonstrate that silicon isotopes serve as sensitive tracers of biological response in iceberg-affected environments and highlight the need to consider iceberg diversity when assessing their cumulative role in future Southern Ocean productivity and carbon cycling.