Galen Pippa Halverson, PhD, McGill University
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.