Chromophoric DOM as a Tracer of North Atlantic Subtropical Mode Water Formation

Global surveys of chromophoric dissolved organic matter (CDOM) profiles have revealed that subtropical mode water layers represent a local minimum in the CDOM profile, reflecting their source in surface waters that have been bleached by solar radiation. Biogeochemical changes in CDOM below the surface layer have also been shown to be relatively slow, suggesting the use of CDOM as a semiconservative tracer of mode water formation. Estimates of CDOM formation rate in the mode water from concurrent measurements of CDOM and CFCs and salinity could be used to separate the biogeochemical signal from that of mixing. In this study, we examine mode water formation and transport using a 7-year record of CDOM concentrations in the subtropical mode water and sections from the CO2/CLIVAR Repeat Hydrography program. We used this in conjunction with hydrographic and biogeochemical data collected by the BATS and Repeat Hydrography programs, as well as nutrient data from the same sites. We found that profiles of CDOM concentrations at Bats tended to be inversely proportional to those of excess nitrogen (DINxs), reflecting remineralization throughout the water column. Furthermore, typical CDOM profiles were altered when the mode water was compressed due to factors such as eddies, indicating a need to consider such factors in characterizing CDOM distribution in areas where mode water exists.