Global patterns of organic carbon transferand accumulation across the land–oceancontinuum constrained by radiocarbon data

As an effective tracer for identifying the origin and cycling of carbon in aquatic ecosystems, the distribution pattern of the radiocarbon (Δ¹⁴C) of organic carbon (OC) in riverine particles and coastal sediments are essential for understanding the contemporary carbon cycle, but are poorly constrained due to under-sampling. This hinders our understanding of OC transfer and accumulation across the land–ocean continuum worldwide. Machine learning approaches and >3,800 observations have been used to construct a high-spatial resolution global atlas of Δ¹⁴C values in river–ocean continuums and show that Δ¹⁴C values of river particles and corresponding coastal sediments can be similar or different. Four characteristic OC transfer and accumulation modes are recognized: the old–young mode for systems with low river and high coastal sediment Δ¹⁴C values; the young–old and old–old modes for coastal systems with old OC accumulation receiving riverine particles with high and low Δ¹⁴C values, respectively; and the young– young mode with young OC for both riverine and coastal deposited particles. Distinguishing these modes and their spatial patterns is critical to furthering our understanding of the global carbon system. Specifically, among coastal areas with high OC contents worldwide, old–old systems are largely neutral to slightly negative to contemporary atmospheric carbon dioxide (CO₂) removal, whereas young–old and old–young systems represent CO₂ sources and sinks, respectively. These spatial patterns of OC content and isotope composition constrain the local potential for blue carbon solutions.