Organic signatures from modern and ancient seagrass in coarse-grained sediments near Poel Island, Baltic Sea

• Sandy nearshore sediments studied for organic matter below seagrass meadows. • Background TOC below seagrass meadows very low and similar to unvegetated areas. • Fossil seagrass recognition by combined microscopy, biomarkers, δ 13 C, 14 C-age dating. • Seagrass-derived organic matter preserved in ancient event layer at depth. • Event-driven in situ burial as a major storage pathway for seagrass-derived Blue Carbon. Seagrass meadows are regarded to play an important role in mitigating climate change by acting as a sink of organic carbon (C) in their underlying sediments. This study investigates the C sequestration potential of seagrass in coarse-grained nearshore deposits near Poel Island in the southwestern Baltic Sea. Four ∼60 cm long sediment cores from Zostera marina meadows were analyzed for the nature and quantity of particulate organic C. Emphasis was placed on the questions (i) whether seagrass coverage would lead to an enhanced C storage in this high-energy proximal setting, and (ii) whether the stored C has a seagrass origin or derives from other, marine or terrestrial sources. We observed that total organic C (TOC) contents in the rhizosphere of seagrass vegetated areas are currently very low (0.05–0.22%) and similar to unvegetated areas. However, a several-cm-thick, dark brown layer, residing at a depth of 40–60 cm, was strongly enriched in TOC (up to 17%) and contained most of the organic matter stored in the sediment profiles studied. A combination of lipid biomarkers, scanning electron microscopy, 14 C age dating, and 13 C measurements in TOC and individual lipids revealed that the organic matter in this layer originated from seagrass. This material was abruptly buried ∼2000 years ago, possibly because of a storm event. Our findings highlight that event-driven burial of seagrass biomass can provide a critical pathway for Blue Carbon storage over relevant (10 2 –10 3 yrs) timespans in coarse-grained proximal sediments.