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

The naturally occurring radionuclides 234Th and 7Be were used to investigate 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 1997. Both non-steady-state and horizontal transport models were assessed and found to be very important for the accurate determination of 234Th export. Upper ocean particulate organic carbon export was estimated using modeled 234Th export and the ratio of particulate organic carbon to 234Th 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. 7Be derived estimates of vertical eddy diffusivity into summer surface waters ranged from 0.5 to 1.5 cm2 sec−1, and show that this mechanism is sufficient to support the amount of `new’ nitrogen required for the measured particulate carbon export.