Photo-lability of deep ocean dissolved black carbon

[摘要] Dissolved black carbon (DBC), defined here as condensed aromatics isolatedfrom seawater via PPL solid phase extraction and quantified asbenzenepolycarboxylic acid (BPCA) oxidation products, is a significantcomponent of the oceanic dissolved organic carbon (DOC) pool. Thesecondensed aromatics are widely distributed in the open ocean and appear tobe tens of thousands of years old. As such DBC is regarded as highlyrefractory. In the current study, the photo-lability of DBC, DOC andcoloured dissolved organic matter (CDOM; ultraviolet-visible absorbance)were determined over the course of a 28 day irradiation of North AtlanticDeep Water under a solar simulator. During the irradiation DBC fell from1044 ± 164 nM-C to 55 ± 15 nM-C, a 20-fold decrease in concentration.Dissolved black carbon photo-degradation was more rapid and more extensivethan for bulk CDOM and DOC. The concentration of DBC correlated with CDOMabsorbance and the quality of DBC indicated by the ratios of different BPCAscorrelated with CDOM absorbance spectral slope, suggesting the opticalproperties of CDOM may provide a proxy for both DBC concentrations andquality in natural waters. Further, the photo-lability of components of theDBC pool increased with their degree of aromatic condensation. These trendsindicate that a continuum of compounds of varying photo-lability existswithin the marine DOC pool. In this continuum, photo-lability scales witharomatic character, specifically the degree of condensation. Scaling therapid photo-degradation of DBC to rates of DOC photo-mineralisation for theglobal ocean leads to an estimated photo-chemical half-life for oceanic DBCof less than 800 years. This is more than an order of magnitude shorter thanthe apparent age of DBC in the ocean. Consequently, photo-degradation is posited as theprimary sink for oceanic DBC and the apparent survival of DBC molecules inthe oceans for millennia appears to be facilitated not by their inherentinertness but by the rate at which they are cycled through the surfaceocean's photic zone.

[学科分类] 地球化学与岩石