Stable Carbon Isotope Evidence for Large Terrestrial Carbon Inputs to the Global Ocean

The transport of carbon from land to ocean, via rivers, groundwater, and aerosols, is an important component of the global carbon cycle that must be known to accurately assess anthropogenic CO2 storage on land and in the ocean. Current global carbon cycle budgets have adopted terrestrial carbon inputs to the ocean ranging from 0.5 to 0.9 GtC/yr, derived mainly from estimates of riverine fluxes. However, these budgets ignore the terrestrial carbon input from coastal ecosystems and through submarine groundwater discharge due to difficulties in making global assessments. Here, we use a numerical model and globally-distributed ocean observations of stable carbon isotopes to estimate terrestrial carbon inputs to the ocean as 1.4+/-0.2 GtC/yr, with 77+/-6% occurring in the Pacific and Indian Oceans. This terrestrial carbon flux is balanced by a burial of 0.2 GtC/yr in marine carbonate sediments and an efflux to the atmosphere of 1.2+/-0.2 GtC/yr, 40% of which occurs in poorly-monitored coastal regions and may have been overlooked in previous observation-based global estimates. These results suggest a more dynamic cycling of carbon in the land-ocean transition zone than previously thought, and that rivers are not the only important pathway for terrestrial carbon to the ocean.