Photodissolution of particulate organic matter from sediments

Irradiation of particulate organic matter (POM) at light intensities found at the earth's surface should induce reduction in molecular weight, as found for dissolved organic matter, and hence result in transfer to the dissolved phase. We studied Mississippi River suspended sediments to test if photodissolution can induce losses of POM similar to those observed between delivery and burial in coastal sediments. Irradiation experiments in a solar simulator demonstrated dissolution of tens of percent of the POM after several days of exposure to strong sunlight. Neither water type nor iron oxyhydroxide removal had large effect on the reaction extent, but temperature may be a strong controlling parameter. Ultraviolet and visible wavelengths drive this reaction. A hyperbolic response of reaction extent to photon flux allows significant reaction to occur in highly turbid suspensions, despite significant light penetration into the suspensions of only millimeters to centimeters. Our data do not yet allow quantitation of this reaction's contribution to POM loss between the Mississippi River and its depocenter, but they do demonstrate its potential significance under nearshore resuspension regimes. More importantly, these results point to a heretofore ignored role for photodissolution of particulate organic matter at the earth's surface.