HALOGENATION OF ORGANIC MATTER ALONG CLIMATE AND SALINITY GRADIENTS

Abstract

Halogenated organic compounds destroy ozone when released into the atmosphere. While many such organohalogens are anthropogenic, natural biogeochemical processes can produce similar species. The formation of natural organohalogens is poorly understood, especially since climate change and sea level rise are expected to alter halogen cycling in soils and sediments. I investigated the impact of climatic conditions on organochlorines (Cl\(_{org}\)) in terrestrial systems ranging from temperate to arctic climates and the impact of sea level rise on Cl\(_{org}\) and organobromines (Br\(_{org}\)) in coastal wetlands. Temperature and precipitation affect the formation of Cl\(_{org}\) in terrestrial soils by influencing vegetation and thus the abundance and type of organic matter. At all sites, Cl\(_{org}\) increases with depth in humifying litter and decreases below the soil organic horizon. Lichens may be a major source of Cl\(_{org}\). Local increases in temperature generally decrease the concentrations of Cl\(_{org}\) retained by semiarid/temperate and boreal soils, but concentrations of total Cl and Cl\(_{org}\) do not vary significantly between climate zones. On a molecular level, thawing does not affect the composition of Cl\(_{org}\) in arctic soils. Acetone-extractable, ionizable Cl\(_{org}\) is more aromatic and oxygenated at temperate than at boreal or arctic sites, in part because aromatic Cl in boreal mosses is insoluble. Recent rises in sea level have elevated the production of Br\(_{org}\) but not Cl\(_{org}\) in salt-impacted wetland sediments, where halides interact with abundant terrestrially derived organic matter. Using the partitioning behavior of Cl and Br, I showed that Cl- and Br- sorption are similar and salt-impacted sediments retain excess Br because bromination is favored over chlorination. As climate change and sea level rise worsen, halogenation of organic matter may become a major source of ozone-depleting Br\(_{org}\).