Stable Carbon Isotopes Suggest Large Terrestrial Carbon Inputs to the Global Ocean. Issue 4 (28th April 2021) - Kwon, Eun Young
Abstract: 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 GtC/yr to 0.9 GtC/yr, derived mainly from estimates of riverine fluxes. However, these budgets ignore the terrestrial carbon inputs from coastal ecosystems and through submarine groundwater discharge (collectively referred to as coastal margin inputs in this study) due to difficulties in making global assessments. Using a numerical model and globally distributed ocean observations of stable carbon isotopes, we estimate terrestrial carbon inputs to the ocean at 1.4 ± 0.5 GtC/yr, with 95% of the coastal margin inputs (0.8 ± 0.5 GtC/yr) occurring in the Pacific and Indian Oceans. This terrestrial carbon flux is largely balanced by an efflux to the atmosphere of 1.2 ± 0.5 GtC/yr, 40% of which occurs in poorly monitored coastal regions and may have been overlooked in previous observation‐based global estimates. Our results suggest more dynamic cycling of carbon in the land‐ocean transition zone than previously thought, and that rivers may not be the only important pathway for terrestrial carbon to the ocean. Plain Language Summary: The transfer of carbon from the land to the ocean is an important part of the global carbon cycle, but its magnitude is not well known. By combining global oceanic compilations of observed stable carbon isotopes with an ocean inverse model, we provide new constraints on terrestrial carbon inputs to the ocean within the overall cycling of carbon in the atmosphere‐land‐ocean system. Our results show that the terrestrial carbon flux to the ocean could be up to two times larger than previously believed. The results highlight the importance of vegetated coastal margins that transport carbon into the ocean, which are often overlooked in biogeochemical cycles and have important implications for the global carbon cycle and the fate of anthropogenic CO2 . Key Points: Total terrestrial carbon input to the global ocean is about two times larger than riverine inputs alone Most of the non‐riverine coastal margin carbon inputs occur in the Indian and Pacific Oceans The terrestrial carbon input is largely balanced by an efflux to the atmosphere, 40% of which occurs in poorly monitored coastal regions