Identifying Blue Carbon sources – Organic signatures of Zostera Marina meadows near Poel Island (SW Baltic Sea)

Sandy proximal sediments underlying Zostera marina meadows near Poel Island (SW Baltic Sea) were cored to a depth of ~ 60 cm and studied for the nature and amount of particulate organic carbon. Special emphasis was placed on the questions (i) whether seagrass coverage would lead to an enhanced C sequestration in this high-energy depositional setting, and (ii) whether the organic matter stored in these sediments has a seagrass origin or derives from other, marine or terrestrial sources. We observed that the C sequestration in the rhizosphere of seagrass vegetated areas is currently very low and similar to unvegetated areas. However, our analyses of selected lipid biomarkers (sterols, ω-hydroxyacids, and α,β-dihydroxyacids), scanning electron microscopy, ¹⁴C age dating, and a ‘heavy’ δ¹³C-value clearly identified a layer enriched in fossil seagrass biomass that has been buried about 2.500 years ago. This material now resides at a depth of 40 – 60 cm as a several cm thick organic-rich layer and contains most of the organic carbon stored in the uppermost ~ 0.5 m of the sediments near Poel Island. Thus, event-driven burial of seagrass biomass may provide a critical pathway for enabling Blue Carbon storage over relevant (10³ yrs-) timespans in coarse-grained proximal sediments.