Marine sediments are an important sink for the carbon fixed by photosynthetic algae in the sunlit ocean. Glycans represent a substantial fraction to the organic carbon buried globally in marine sediments. However, assigning the origin and contribution of specific glycans remains challenging with commonly used methods that are designed to measure the total glycan carbon content. Glycan-specific detection using monoclonal antibodies previously revealed diatom- and brown algae-derived fucoidans in marine sediments of various ages. These complex anionic glycans may contribute to long-term carbon sequestration, while their quantitative significance remains unknown. Here, we combined anion exchange chromatography with glycan-specific monoclonal antibodies to isolate and detect fucoidans from marine sediments. Applying this method to a sediment core from the Bransfield Strait in the Southern Ocean revealed that fucoidan was buried and persisted for the extent of the Holocene (11.8 ka). Our results highlight that glycan purification from sediments can help to resolve the importance of different algal glycans to the sequestration of carbon in the ocean.