CARBON-CYCLE IMBALANCES IN THE SARGASSO SEA

THE net exchange of carbon dioxide between the atmosphere and the ocea n, and thus the nature of the oceanic carbon sink, is dominated by the seasonal dynamics of carbon cycling in the upper ocean. This cycle re presents a balance between abiotic and biotic carbon transport into, a nd export out of, the ocean's upper layer. Here we report measurements of these processes made over five years in the Sargasso Sea off Bermu da, as part of the US Joint Global Ocean Flux Study (JGOFS). We find t hat the decrease in carbon stocks from the spring to the autumn in the upper 150 m of the ocean is three times larger than the measured sum of biotic and abiotic fluxes out of this layer, This discrepancy can b e explained either by failure to account for horizontal advection of c arbon or by inaccuracies in the fluxes of sinking particles as measure d using sediment traps. Either the traps miss 80% of the sinking parti cles, or 70% of the carbon cycling is due to advection (or a combinati on of both processes is responsible). Sediment-trap measurements of th e Th-234 flux during this period suggest that most of the discrepancy may be due to inaccuracies in the trap methods, which would require a very general reassessment of existing ideas about particle export and remineralization of carbon in the oceans. If, on the other hand, advec tion is the main source of the discrepancy, the traditional one-dimens ional (vertical) modelling of the oceanic carbon cycle cannot give a f ull account of carbon dynamics.