Rate and apparent quantum yield of photodissolution of sedimentary organic matter

Abstract

We quantified rates of photochemical dissolution (photodissolution) of organic carbon in coastal Louisiana suspended sediments, conducting experiments under well-defined conditions of irradiance and temperature. Optical properties of the suspended sediments were characterized and used in a radiative transfer model to compute irradiances within turbid suspensions. Photodissolution rate increased with temperature (T), with activation energy of 32 ± 7 kJ mol−1, which implicates indirect (non-photochemical) steps in the net reaction. In most samples, dissolved organic carbon (DOC) concentration increased approximately linearly with time over the first 4 h of irradiation under broadband simulated sunlight, after higher rates in the initial hour of irradiation. Four-hour rates ranged from 2.3 µmol DOC m−3 s−1 to 3.2 µmol DOC m−3 s−1, but showed no relation to sample origin within the study area, organic carbon or reducible iron content, or mass-specific absorption coefficient. First-hour rates were higher—from 3.5 µmol DOC m−3 s−1 to 7.8 µmol DOC m−3 s−1—and correlated well with sediment reducible iron (itself often associated with organic matter). The spectral apparent quantum yield (AQY) for photodissolution was computed by fitting DOC photoproduction rates under different spectral irradiance distributions to corresponding rates of light absorption by particles. The photodissolution AQY magnitude is similar to most published dissolved-phase AQY spectra for dissolved inorganic carbon photoproduction, which suggests that in turbid coastal waters where particles dominate light absorption, DOC photoproduction from particles exceeds photooxidation of DOC.