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

Abstract

The naturally occurring radionuclide 234Th (t1/2 = 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 234Th flux combined with measurements of the POC/234Th 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. Copyright 2001 by the American Geophysical Union.