Quantifying organic carbon burial rates and stocks in seagrass meadow sediments influenced by sargassum-brown tides

Seagrass meadow sediments are efficient organic carbon (Corg) sinks and can store Corg for hundreds of years. The temporal variation of Corg burial rates and stocks over recent decades at nearshore seagrass meadows in the Puerto Morelos Reef Lagoon, Mexico, was evaluated in 210Pb-dated sediment cores from nearshore meadows dominated by Thalassia testudinum. The sediments were predominantly sandy (>52% sand) rich in carbonate grains (11.8–12.5% Cinorg) with minor Corg (0.24–1.12%) and Norg (0.02–0.13%) concentrations. The C:N ratio (9.4–13.0) indicated that marine-derived Corg was prevalent. Corg stocks in the upper 30 cm sediment were 15.9 ± 3.0–24.8 ± 4.6 Mg ha−1. Sedimentary mass accumulation rates (MAR) (0.7–1.5 g cm−2 yr−1) were higher than those previously recorded in seagrass sediments from the reef lagoon and other parts of the world. The highest MAR values, recorded in 2015 (±0.13) and 2018 (±0.03), coincided with the peak sargassum influx years. MAR and Corg burial rates (11.4–133 g m−2 yr−1) were correlated (r2 = 0.76), indicating that the massive influxes of sargassum have accelerated Corg burial rates in the region since 2015. This study marks the initial evaluation of the interaction between the massive influx of sargassum, MAR, and Corg burial rates in seagrass sediments, potentially laying the groundwork for future extended monitoring initiatives. •Organic carbon (Corg) stock and burial rates were measured in seagrass sediment cores.•The 210Pb-dated cores were from a natural protected area in the Mexican Caribbean.•Sargassum influx since 2015 accelerated Corg burial rates in the region.•Sargassum tides seemed to influence Corg dynamics, guiding future monitoring.