Environ. Sci. Technol. 2010, 44, 5805–5811
Natural and Anthropogenic Mercury Distribution in Marine Sediments from Hudson Bay, Canada A L E X A N D E R A . H A R E , †,‡ G A R Y A . S T E R N , †,‡ Z O U Z O U A . K U Z Y K , †,‡ R O B I E W . M A C D O N A L D , †,§ SOPHIA C. JOHANNESSEN,§ AND F E I Y U E W A N G * ,†,| Centre for Earth Observation Science, Department of Environment and Geography, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2, Freshwater Institute, Fisheries and Oceans Canada, Winnipeg, Manitoba, Canada R3T 2N6, Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney, British Columbia, Canada V8L 4B2, and Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
Received March 5, 2010. Revised manuscript received June 14, 2010. Accepted June 28, 2010.
Twelve marine sediment cores from Hudson Bay, Canada, were collected to investigate the response of sub-Arctic marine sediments to atmospherically transported anthropogenic mercury (Hg). Modeling by a two-layer sediment mixing model suggests that the historical Hg deposition to most of the sediment cores reflects the known history of atmospheric Hg deposition in North America, with an onset of increasing anthropogenic Hg emissions in the late 1800s and early 1900s and a reduction of Hg deposition in the mid- to late-1900s. However, although anthropogenic Hg has contributed to a ubiquitous increase in Hg concentrations in sediments over the industrial era, the most elevated industrial-era sedimentary Hg concentrations only marginally exceed the upper preindustrial sedimentary Hg concentrations. Analysis of δ13C and relationship between Hg and organic matter capture suggests that the response of Hudson Bay sediments to changes in atmospheric Hg emissions is largely controlled by the particle flux in the system and that natural changes in organic matter composition and dynamics can cause variation in sedimentary Hg concentrations at least to the same extent as those caused by increasing anthropogenic Hg emissions.
Introduction Lake sediments often record variations in the atmospheric deposition of mercury (Hg), permitting reconstruction of the chronology of anthropogenic Hg emissions to the atmosphere from dated sediment cores (e.g. refs 1 and 2). Yet while enhanced sedimentary Hg loadings from industrial increases in atmospheric Hg are evident in many Arctic lakes (3-6), Outridge et al. (7) recently proposed an alternative hypothesis * Corresponding author phone: (204)474-6250; e-mail: wangf@ ms.umanitoba.ca. † Department of Environment and Geography, University of Manitoba. ‡ Freshwater Institute, Fisheries and Oceans Canada. § Institute of Ocean Sciences, Fisheries and Oceans Canada. | Department of Chemistry, University of Manitoba. 10.1021/es100724y
2010 American Chemical Society
Published on Web 07/09/2010
linking Hg enrichments in some Arctic lake sediments to recent increases in autochthonous primary production driven by increasing temperature and diminishing ice. This proposal highlights the importance of considering aquatic biogeochemical processes when interpreting Hg trends derived from Arctic sediment cores. Compared to lake sediments, marine sediments provide an ultimate sink for anthropogenic Hg (8-10). Extensive studies have demonstrated that Hg deposition to marine sediments occurs primarily via the association with organicrich particles (ref 10 and references cited therein). While coastal and estuarine sediments have been shown to reflect reliably the timing of transient, point source inputs of Hg (11-14), few studies have examined marine sediment cores as a record of nonpoint source atmospheric Hg emissions. The challenges in relating marine sediment core data to the chronology of atmospheric Hg deposition lie in the complex Hg pathways, dynamics of particles, and postdepositional diagenesis of Hg in marine sediments. Mercury concentrations in marine surficial sediments are often invariant with depth within a surface mixed layer (SML) due to benthic mixing (15); long-range particle transport and long residence times for Hg in the water column may also obscure the depositional history of atmospheric Hg. In the Arctic, sea ice can further intercept and redistribute particles and Hg. Several studies in the remote Arctic have demonstrated surface Hg enrichments in coastal and offshore marine sediments (16, 17), but to what extent these enrichments reflect anthropogenic contributions is not clear. Hudson Bay (HB) of Canada provides an opportunity to investigate the Hg capture in a large sub-Arctic marine system and the influence of elevated atmospheric Hg deposition to sediments observed over the last ca. 150 years (18). The coastlines of HB and their vicinities contain no major industrial Hg point sources, and a Hg mass-balance suggests that most of the particulate Hg is trapped within the Bay (19). In this paper we examine the response of the HB marine sediments to anthropogenic, atmospheric Hg flux and how such response is modified by the particle flux within the system.
Experimental Methods Study Site. Hudson Bay (50-65°N, 76-94°W) is a large, shallow, sub-Arctic marine basin almost enclosed by the North American continent (Figure 1). Although numerous small communities (population
