Radiocarbon evidence for a smaller oceanic carbon dioxide sink than previously believed

Abstract

RADIOCARBON produced naturally in the upper atmosphere or arti-ficially during nuclear weapons testing is the main tracer used to validate models of oceanic carbon cycling, in particular the exchange of carbon dioxide with the atmosphere^1–3 and the mixing parameters within the ocean itself^4–7. Here we test the overall consistency of exchange fluxes between all relevant compartments in a simple model of the global carbon cycle, using measurements of the long-term tropospheric CO² concentration⁸ and radiocarbon composition^9–12, the bomb ¹⁴C inventory in the stratosphere^13,14 and a compilation of bomb detonation dates and strengths¹⁵. We find that to balance the budget, we must invoke an extra source to account for 25% of the generally accepted uptake of bomb ¹⁴C by the oceans³. The strength of this source decreases from 1970 onwards, with a characteristic timescale similar to that of the ocean uptake. Significant radiocarbon transport from the remote high stratosphere and significantly reduced uptake of bomb ¹⁴C by the biosphere can both be ruled out by observational constraints. We therefore conclude that the global oceanic bomb ¹⁴C inventory should be revised downwards. A smaller oceanic bomb 14C inventory also implies a smaller oceanic radiocarbon penetration depth¹⁶, which in turn implies that the oceans take up 25% less anthropogenic CO₂ than had previously been believed.