Chapter 9 - Photochemistry and the Cycling of Carbon, Sulfur, Nitrogen and Phosphorus

Abstract

This chapter presents a discussion on photochemistry and the cycling of carbon, sulfur, nitrogen, and Phosphorus. Dissolved organic matter (DOM) plays a dominant role in the absorption of ultraviolet (UV) and visible light in the open ocean. As DOM absorbance is regulated in part by photobleaching processes, light availability for photosynthesis and the penetration of UV radiation within the marine environment are influenced by photochemical transformations. In addition to its control on UV light fields, DOM photochemistry strongly impacts the biogeochemical cycling of biologically important elements in surface seawater. By the conversion of DOM into volatile species such as carbonyl sulfide, DOM photochemistry influences atmospheric chemistry and climate. The chapter illustrates an important light-driven chemical reaction in the photic zone—that is, the reduction of trace metals such as iron, manganese, and copper. The chemistry of these reduced species is quite different from their oxidized counterparts. Photochemical oxidation of DOM also produces a suite of free radicals and other short-lived species including the superoxide anion, carbonate radical, singlet oxygen, hydroxyl radical, di-bromide radical anion, and a number of poorly described organic radicals and excited state triplets. These species are much more reactive than their corresponding diamagnetic and ground state species and are expected to influence biological and chemical processes in sunlit surface waters. Furthermore, DOM photolysis is an important source or sink of a variety of atmospherically important gases that are emitted from the ocean, some of which affect the Earth's radioactive balance. Concentrations—and hence emissions—of carbon monoxide (CO), carbon dioxide (CO2), carbonyl sulfide (OCS), and di-methyl sulfide (DMS) are all partly regulated through photochemical processes involving DOM.