How to quantify blue carbon sequestration rates in seagrass meadow sediment: geochemical method and troubleshooting

Abstract

Seagrasses take up carbon dioxide and transform it into organic carbon, some of which is buried in meadow sediments. Very high carbon burial rates have been claimed for seagrass meadows globally, and international protocols have been developed with a view to awarding carbon offset credits. However, recent geochemical work has shown that a misunderstanding of how marine sediment buries and processes organic carbon has led to overestimates of at least an order of magnitude. Common blue carbon methodology does not adequately account for bioturbation or remineralization in surface sediment, and there is often a conflation of standing stock with ongoing burial. To determine accurate seagrass carbon burial rates requires the following steps: (1) Determine the sediment accumulation rate below the surface mixed layer, using 210Pb and porosity; (2) Determine the burial concentration of organic carbon; (3) Multiply the sediment accumulation rate by the buried % organic carbon; (4) If excluding allochthonous carbon burial, use biomarkers to determine the proportion of seagrass-derived organic carbon; and (5) Account for the offset due to the burial of carbonate formed inside the meadow. Seagrass meadows provide valuable habitat and protect coastlines from erosion. They can also play a role in short-term carbon sequestration. However, if carbon credits are awarded based on inflated estimates and used to offset emissions elsewhere, the net effect could be an increase in carbon dioxide emissions to the atmosphere.