Galen Pippa Halverson, PhD, McGill University
Jeroen Sonke, PhD, Midi-Pyrenees Observatory
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 much 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 O2 concentrations 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. HgII is concentrated in black shales due to its affinity for sulfide. Hence we proposed to survey black shales of various ages, but with an emphasis on intervals thought to span oxygenation events, the test whether or not Hg MIF responded to the purported steps in atmospheric O2 levels. 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 63 ancient sediment samples and 10 recent samples over two 1-month analytical sessions in Toulouse, France. The results are promising, if not as predicted based on prior work. Specifically, we find three distinct patterns in MIF in our data. First, 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 to Mesoproterozoic samples (2.1 to 1.45 billion years old) post-dating the GEO have a slope of ~0.87. The late Neoproterozoic samples analyzed to date (n=4) show a very distinct distribution, with a D199Hg/D201Hg slope similar to that expressed in modern sediments (m=1.23), but with the highest MIF signatures (D199Hg up to 0.4ä) yet documented in sediments or rocks. Hence, while 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 does at least broadly record the evolution of atmospheric O2.
We are currently striving to double our dataset, with two specific goals. First, we will produce a relatively low-resolution survey spanning all of Earth history, and second, we will analyze a more detailed sample suite spanning the Neoproterozoic. A new post-doc (Pierre Sansjofre) is due to join the project in February, who will expedite the analyses of the Neoproterozoic samples. We expect to present these results at the 2013 Goldschmidt Conference in Florence, Italy.