Ballast Effect on the Export of Sea‐Ice Biomarker IPSO ₂₅ in Prydz Bay, Antarctica

Abstract Understanding past sea‐ice variability in Antarctica is crucial for studying global climate change mechanisms. While a di‐unsaturated C 25 highly branched isoprenoid (IPSO 25 ) and its combination with phytoplankton biomarkers (PIPSO 25 ) are promising sea‐ice proxies, their applicability in Antarctica is limited by an incomplete mechanistic understanding of the processes transferring these biomarkers from the surface ocean to the sediment. Here, to explore these critical export processes across different sea‐ice seasons, we investigate the first time‐series sediment‐trap record from Prydz Bay, Antarctica. The record includes IPSO 25 , the tri‐unsaturated C 25 highly branched isoprenoid (HBI III), brassicasterol, and PIPSO 25 . Our findings reveal that the export of the sea‐ice biomarker IPSO 25 is regulated by both sea‐ice dynamics and the ballast effect. During the marginal ice zone or polynya stage, ballast materials (e.g., lithogenic materials) enhance the export of particles, synchronizing the export of various biomarkers and organic matter. At other times, biomarker fluxes are very low. Given this transport process affecting single biomarkers, we then explored whether a ratio‐based index could provide a more robust signal. Our results show that P III IPSO 25 (the HBI III‐based PIPSO 25 ) reflects general seasonal co‐variation with satellite‐derived sea‐ice conditions, indicating P III IPSO 25 is less affected by sinking processes compared to single‐biomarker‐based sea‐ice reconstructions. This study emphasizes the need to account for both ballast effects and the ice retreat processes in paleoclimate interpretations. We show that ratio‐based proxies like P III IPSO 25 are promising tools for Antarctic sea‐ice reconstructions, and we recommend further mechanistic and spatial validation studies across other Antarctic regions.