Hydrography of chromophoric dissolved organic matter in the North Atlantic

The distribution and optical absorption characteristics of chromophoric dissolved organic matter (CDOM) were systematically investigated along three meridional transects in the North Atlantic Ocean and Caribbean Sea conducted as part of the 2003 US CLIVAR/CO2 Repeat Hydrography survey. Hydrographic transects covered in aggregate a latitudinal range of 51 to 621 north along longitudes 201W (line A16N, Leg 1), 521W (A20), and 661W (A22). Absorption spectra of filtered seawater samples were collected and analyzed for depths ranging from the surface to 6000m, sampling all the ocean water masses in the western basin of the subtropical North Atlantic and several stations on the North and South American continental slopes. The lowest surface abundances of CDOM (o 0.1m 1 absorption coefficient at 325 nm) were found in the central subtropical gyres while the highest surface abundances ( 0.7m ) were found along the continental shelves and within the subpolar gyre, confirming recent satellite-based assessments of surface CDOM distribution. Within the ocean interior, CDOM abundances were relatively high (0.1–0.2m 1 absorption coefficient at 325 nm) except in the subtropical mode water, where a local minimum exists due to the subduction of low CDOM surface waters during mode water formation. In the subthermocline water masses of the western basin, changes in CDOM abundance are not correlated with increasing ventilation age as assessed using chlorofluorocarbon (CFC) concentrations and the atmospheric CFC history. But dissolved organic carbon (DOC) mass-specific absorption coefficients of CDOM increase with increasing ventilation age in the deep sea, indicating that CDOM is a refractory component of the DOC pool. The overall CDOM distribution in the North Atlantic reflects the rapid advection and mixing processes of the basin and demonstrates that remineralization in the ocean interior is not a significant sink for CDOM. This supports the potential of CDOM as a tracer of ocean circulation processes for subducted water masses. r 2007 Elsevier Ltd. All rights reserved.