Photochemical and Oceanographic Controls on Mercury Cycling in the Arctic Ocean

Abstract

Mercury （Hg） is a global pollutant that reaches remote regions through long-range atmospheric transport. The Arctic Ocean plays a distinctive role in this global cycle, acting as a net sink that accumulates Hg delivered from the atmosphere and, increasingly, from rivers. Gaseous elemental Hg（0） （GEM） carried from lower latitudes is oxidized to Hg（II） and deposited to the surface ocean. Within the water column, part of the retained Hg（II） is methylated to methylmercury （MeHg）, the form that biomagnifies through Arctic food webs. This pathway explains why Hg burdens in Arctic biota often exceed regulatory guidelines, a pattern that has sustained concern for ecosystem and human health. The elevated Hg concentrations have been attributed to spatial contrasts in seawater Hg and to the invasion of atmospheric Hg（0） at the sea surface. Yet despite extensive work on the biogeochemical cycling of Hg and its broad distribution, two aspects of this cycle remain poorly constrained. The first is the photochemistry that sets the Hg（0） pool, and the second is the oceanographic structure that governs Hg speciation across the region. This dissertation addresses both gaps. It investigates Arctic Hg cycling from the surface ocean to the deep water column by combining laboratory experiments, field observations, and box modeling. The work is organized into four chapters. Chapter 2 examines how the Hg（II） photoreduction rate constant （kr） in seawater varies with dissolved organic matter and inorganic sea salts. We measured kr in controlled experiments and modeled the reaction as a two-step reversible process with Hg（I） as the key intermediate. The results identify thiols and bicarbonate as the main controls on Hg（II） photoreduction. Chapter 3 characterizes the distribution of Hg（0） in the surface water and overlying air of the Arctic Ocean and its relationship to hydrographic and meteorological conditions. We measured Hg（0） during a summer expedition and developed a two-box model of the air and surface ocean. The model quantifies the fluxes that control dissolved gaseous mercury （DGM）, identifies its main sources and sinks along the latitudinal gradient, and uses satellite-derived data to resolve the seasonal variation of DGM from May to September. Chapter 4 compares the vertical profiles of total Hg （THg） and MeHg between Arctic regions, using Hg data compiled from previous summertime expeditions. We find that the regional variability of Hg species depends on the Pacific and Atlantic inflows, which differ in Hg content and hydrographic properties. Chapter 5 extends Chapter 4 with a six-box model that represents the polar mixed layer, halocline, and Atlantic water in the Pacific and Atlantic sectors in September. The model reproduces the hydrologic residence times and the observed Hg concentrations during the GEOTRACES Arctic expeditions. A sensitivity analysis identifies the main environmental drivers and projects future changes in Arctic Hg cycling. These results advance our understanding of Hg biogeochemical cycling and bioaccumulation in the Arctic Ocean.