Ancient coastal forests reborn as modern marine blue carbon ecosystems

Sea-level rise and coastal erosion can submerge terrestrial carbon-rich landscapes, yet the ecological fate and climate significance of such blue-carbon deposits remain largely unknown. We report the discovery of submerged peat reefs in the shallow coastal waters of the Bazaruto Archipelago, Mozambique, relict of ancient coastal forests now transformed into benthic marine habitats. Radiocarbon (¹⁴C) dating reveals ages spanning ~300 to ~5000 years BP, indicating mid- to late-Holocene origins as coastal peat swamps subsequently submerged by rising seas. The peat contains up to 40% organic carbon by weight, representing a substantial blue-carbon stock, and δ¹³C values around -27‰ confirm a predominantly terrestrial plant origin for this carbon. Peat reefs are shallower and exhibit greater variability in temperature, salinity, oxygen, pHₜₒₜₐₗ, light, and tidal exposure than adjacent hard coral reefs, influencing biological structure and habitat function. Biodiversity surveys across multiple scales (benthic quadrats, invertebrate, small fish, and megafauna transects) show that these peat reefs support a rich and distinctive assemblage of marine life, significantly different from nearby hard coral reefs. The peat substrate is colonized by diverse sessile and mobile invertebrates providing structural complexity for fish communities and as nursery habitat, with richness comparable to nearby hard coral reefs. Importantly, overlapping communities highlight ecological connectivity across reef types (41 and 44 small fish/megafauna species shared), even as habitat composition differs markedly: unlike adjacent hard coral reefs dominated by reef-building corals, peat reefs host no living corals and are instead characterized by seagrass patches, macroalgae, sponges, and detritus-based food webs. Our findings illustrate how ancient terrestrial ecosystems can persist and support contemporary ecological functions, and the ecological reinvention of Earth’s landscapes under shifting sea levels. This underscores how substrate-driven ecosystem baselines shape biodiversity, calling for the inclusion of such relict ecosystems in emerging conservation frameworks and blue-carbon strategies.