The global distribution of macroalgal carbon burial in coastal sediments.

Macroalgae play a pivotal role in the global ocean carbon cycle, yet the spatial distribution of their coastal carbon burial, a long-term sink relevant to national inventories, remains poorly resolved. Here, we present a high-resolution (5-arcminute, ~9.2 km at the equator) global assessment of the Macroalgal Carbon Burial Index (MCBI), a measure of how favorable coastal environments are for burying macroalgal carbon. Synthesizing the most comprehensive dataset of sediment accumulation rates (n = 1617), we identify the depositional environment as a critical constraint on burial, acting independently of macroalgal productivity. High-favorability sinks are geographically rare (<1% of global coastlines), clustering in regions such as the Korea Bay and Okhotsk-Sakhalin region. We further identify coastline complexity as an informative predictor of MCBI (R² = 0.59), enabling low-cost identification of burial hotspots. Notably, we identify a mismatch in conservation: existing marine protected areas cover 23.9% of low-burial coastlines but only 17.1% of primary burial hotspots. Our findings map the global pattern of macroalgal carbon burial and provide a spatially explicit basis for its integration into conservation and management.