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Abstract

Mercury speciation and isotopic fractionation processes have been incorporated into the HAMOCC offline ocean tracer advection code. The model is fast enough to allow a wide exploration of the sensitivity of the Hg cycle in the oceans, and of factors controlling human exposure to monomethyl-Hg through the consumption of fish. Vertical particle transport of Hg appears to play a discernable role in setting present-day Hg distributions, which we surmise by the fact that in simulations without particle transport, the high present-day Hg deposition rate leads to an Hg maximum at the sea surface, rather than a subsurface maximum as observed. Hg particle transport has a relatively small impact on anthropogenic Hg uptake, but it sequesters Hg deeper in the water column, so that excess Hg is retained in the model ocean for a longer period of time after anthropogenic Hg deposition is stopped. Among 10 rate constants in the model, steady state Hg concentrations are most sensitive to reactions that are sources or sinks of Hg(0), the evasion of which to the atmosphere is the dominant sink term in the surface ocean. Isotopic fractionations in the interconversion reactions are most strongly expressed, in the isotopic signatures of dissolved Hg, in reactions that involve the dominant dissolved species, Hg(II), including mass independent fractionation during Hg photoreduction. The Δ199Hg of MMHg in the model, subject to photoreduction fractionation, reproduces the Δ199Hg of fish in the upper 1000 m of the ocean, while the impact of anthropogenic Hg deposition on Hg isotope ratios is essentially negligible.The tar file contains data files in netcdf format, compressed with gzip. Netcdf files can be read using many standard software packages including python, matlab, ferret, and ncview. Included in the tar file is a readme file which is an edited ncdump header for the files giving the variable names, their units, and dimensions. The numbers in the file names give the simulated year A.D., where 1850 is preanthropogenic (natural), 2010 is present-day for comparison with recent measurements, and 2100 is for the end of the century. There is also an edited text output from the model with various diagnostics. Within the src directory there are fortran files and a makefile to generate the code, and supporting data files required to run the code.

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