58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Mercury has seven isotopes that are fractionated in the modern environment by both mass-dependent (MDF) and mass-independent (MIF) processes. It is only one of three elements known to produce significant MIF, and significant recent effort has gone into elucidating the driving mechanisms of this MIF. It appears that, like O and S, which also experience MIF, Hg MIF results from photochemical reactions (e.g., degradation of monomethyl mercury or photoreduction HgII) driven by ultraviolet radiation. Because the extent of ultraviolet radiation reaching the earth’s surface is dependent on the amount of ozone, and hence O₂concentrations in the atmosphere, we hypothesized that Hg MIF should have occurred throughout Earth history and the style and intensity of the MIF might be directly influenced by the evolution of atmospheric pO2. Hence we proposed to analyze black shales of various ages, but with an emphasis on intervals thought to span oxygenation events. These samples had either previously been well characterized geochemically (Fe speciation, S isotopes, TOC, d13C, trace metals) or are to be thoroughly analyzed as part of this study.

To date we have analyzed a total of 83 ancient sediment samples and 10 recent samples for bulk Hg concentrations Hg isotope ratios in Toulouse, France since the outset of this project. In addition, we have analyzed major and trace elemental concentrations and complementary isotopic ratios on some 200 samples in relation to this project, including all samples analyzed for Hg isotopes for which these data did not previously exist. The results are promising. A compilation of the Hg concentration data through time closely mirrors a recent compilation of U concentrations in organic-rich sediments. The U concentration data parallel the evolution of Earth surface oxygenation, so Hg appears to be sensitive to oxygenation as well.

We also find distinct patterns in MIF in our data. Data predating the ca. 2.4 billion year old Great Oxidation Event (GOE), define an array of D¹⁹⁹Hg vs. D²⁰¹Hg (that is, degree of MIF expressed in ¹⁹⁹Hg versus that expressed in ²⁰¹Hg) with a slope of ~1.8, whereas Paleoproterozoic samples (2.1 to 2.05 billion years old) post-dating the GEO have a slope of ~0.86. Mesoproterozoic–Neoproterozoic samples show a very distinct distribution, with a D¹⁹⁹Hg/D²⁰¹Hg slope of 1.06, which is not greatly distinct from that expressed in modern sediments (m=1.23). These samples include the highest MIF signatures (D¹⁹⁹Hg up to 0.4‰) yet documented in sediments or rocks, although these data are restricted to a single late Neoproterozoic interval of a drill core in Tasmania thought to include belong to the Sturtian glacial interval. Whereas we still do not have enough data spanning the Precambrian-Cambrian boundary to verify a major change in Hg isotope fractionation sometime in the latter Proterozoic, the data we have suggest that Hg isotope MIF record, like the Hg concentration record, at least broadly reflects the evolution of atmospheric O₂.

We are currently completing the analyses related to this project and will be writing up the mercury concentration and isotope results for publication in the following 6 months. Other data collected under the umbrella of this project will be incorporated into several manuscripts and theses currently in preparation.