SALTING THE EARTH: BIOGEOCHEMICAL CYCLING OF CHLORINATED AND BROMINATED NATURAL ORGANIC COMPOUNDS IN COASTAL ECOSYSTEMS

Abstract

This dissertation examines biogeochemical dynamics of organochlorine (org-Cl) and organobromine (org-Br) in coastal ecosystems impacted by sea level rise. Global warming results in thermal expansion of oceans and subsequent saltwater intrusion into coastal ecosystems. As previously freshwater wetlands are converted to salt-affected wetlands, bromide (Br-) is incorporated into soil organic matter, producing harmful org-Br compounds, which are both toxic to human populations and can deplete stratospheric ozone. Production of org-Br compounds is additionally maximized by high abundance of organic carbon combined with oxygenation and redox cycling.

Bromination of organic matter in salt-affected wetlands is shown to impact the stability and fate of naturally occurring org-Cl species. Org-Cl loss is observed to occur concomitantly with org-Br production within hours of initial exposure to Br-. Analysis further indicates that org-Cl species are largely lost to the gaseous phase, and thus MeBr and MeCl emissions are measured from soil incubations in oxic and anoxic conditions. Well-drained, oxic soils are shown to exhibit peak emissions of both halocarbon species within 5-6 hours of initial exposure to seawater, while strictly anaerobic soils emit solely MeBr within this time range.

Molecular scale analysis of produced org-Br compounds indicates that unique org-Br compounds form from increased Br- exposure and that these molecules are aromatic in nature and structurally similar at variable sample depth and oxygenation conditions. Mechanisms of halogenation are thus hypothesized to occur ubiquitously despite variable ecosystem conditions. With this new understanding of the critical role of coastal ecosystems in global halogen cycles, a preliminary analysis is conducted on samples from two different mangrove forests in Panama. Speciation characteristics by sample depth of Br, Cl, Mn, Fe, and S concludes that rapidly changing redox conditions play a critical role in org-Cl and org-Br fate in mangrove ecosystems. Highly diverse coastal environments throughout the globe can thus be considered in the future for their crucial contribution to organohalogen biogeochemical cycling and halocarbon emissions to the atmosphere.