Upper ocean carbon export, horizontal transport, and vertical eddy diffusivity in the southwestern Gulf of Maine

The naturally occurring radionuclides Th-234 and Be-7 were used to investig ate the magnitude of upper ocean particulate organic carbon export and the rate of vertical eddy diffusion in the southwestern Gulf of Maine. Sampling occurred during the spring (March, April) and summer (July, August) of 199 7. Both non-steady-state and horizontal transport models were assessed and found to be very important for the accurate determination of Th-234 export. Upper ocean particulate organic carbon export was estimated using modeled Th-234 export and the ratio of particulate organic carbon to Th-234 On GF/F filters. Our measurements demonstrate that the southwestern Gulf of Maine is typical of many coastal regimes, having an organic carbon export ratio ( particulate export/primary production) which ranges from 9 to 49% depending on the depth of integration and primary productivity estimate. Be-7 derive d estimates of vertical eddy diffusivity into summer surface waters ranged from 0.5 to 1.5 cm(2) sec(-1), and show that this mechanism is sufficient t o support the amount of 'new' nitrogen required for the measured particulat e carbon export. (C) 2000 Elsevier Science Ltd. Ail rights reserved.