Investigating the carbon cycle in the Gulf of Maine using the natural tracer thorium 234

Abstract

The naturally occurring radionuclide 234 Th ( t 1/2 = 24.1 days) was used to examine the organic carbon cycle in the Gulf of Maine. A seasonal study (March, June, and September 1995) was conducted in the central Gulf of Maine in Wilkinson and Jordan Basins, and a regional survey, which included the Scotian Shelf, was conducted during August‐September 1997. Particulate organic carbon (POC) export (particulate export production) was estimated from a three‐dimensional steady state model of 234 Th flux combined with measurements of the POC/ 234 Th ratio on >53‐μm particles. The POC export for this region was seasonally variable; average values ranged from 15 to 34 mmol C m −2 d −1 , between 11% and 25% of the regionally integrated primary production. The Gulf of Maine was a net source (to the Mid‐Atlantic Bight) of dissolved organic carbon (2.4 mmol C m −2 d −1 ) amounting to ∼2% of carbon uptake rates. Organic carbon burial in the sediments was a minor fraction of the primary production, averaging 1.6 mmol C m −2 d −1 . Though only a fraction of total export production was buried in the sediments, these estimates close the budget for organic carbon in the Gulf of Maine. An implication is that off‐shelf export may not be as important as previously estimated in this shelf region.