Basin scale distribution of chromophoric dissolved organic matter in the Pacific Ocean

Yamashita, Youhei, Eiichiro Tanoue

Limnol. Oceanogr., 54(2), 2009, 598-609 | DOI: 10.4319/lo.2009.54.2.0598

ABSTRACT: Chromophoric dissolved organic matter (CDOM) was surveyed along the 160°W transect from the equatorial to the subarctic Pacific. CDOM characteristics were evaluated through the measurement of fluorescence intensity at 320 nm excitation and 420 nm emission and of absorption coefficient (a) at 320 nm, both indicative of marine humic-like CDOM. In the surface layer (~200 m), different levels but similar optical characteristics of CDOM were found among the oceanic regions studied. In the mesopelagic (200-1000 m) and abyssal layers (1000 m-bottom), levels of fluorescence intensities were linearly correlated to those of absorption coefficients, with a similar slope between these layers. However, the intercept of the linear relationship between the two optical parameters was significantly lower for the mesopelagic layer than for the abyssal layer. Differences between intercepts were consistent with the transport of optically distinct CDOM to the mesopelagic layer through the formation of the North Pacific intermediate water (NPIW). At wavelengths shorter than 300 nm, the absorption coefficients of CDOM in the surface layer systematically deviated from the natural logarithmic relationships between absorption coefficients and wavelengths in the 320-350 nm range. This class of CDOM, estimated using absorption coefficient at 275 nm, was defined as surface-specific CDOM and may be derived from biocomponents. Surface-specific CDOM was higher in the relatively young mode water than in old NPIW, suggesting that this class of CDOM is semi-labile.

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