Collaborative Research: Towards a Better Understanding of Tl Isotope Cycling under Different Redox Conditions

The element thallium (Tl) is gaining momentum as a tool for reconstructing the history of molecular oxygen (O2) in Earth’s ancient oceans. Reconstructing this history is important because the availability of O2 in Earth’s ancient oceans played a key governing role in the origin and evolution of life on our planet. Furthermore, ongoing deoxygenation of modern oceans will affect every human on Earth, and we can better predict and prepare for this deoxygenation if we understand comparable events in Earth’s past.

The utility of Tl to track changes in marine O2 stems from the fact that its isotopes are strongly fractionated by interactions with manganese (Mn) oxide minerals, which are formed and buried in marine sediments today only where O2 is present. Indeed, many studies show that Tl isotope ratios preserved in ancient marine rocks can provide important information about past O2 availability in the ocean.

Yet, our understanding of the modern Tl isotope cycle is far from complete, due largely to the extremely low abundances of Tl found in environments today, which make it difficult to collect enough material for accurate Tl isotopic analysis. The PIs’ preliminary work in a brackish pond on Cape Cod (Siders Pond) show that it is feasible to generate quality Tl isotope data for water, particles, and sediments in an environment with very low Tl abundances.

Furthermore, early results from this work provide important new information about the links that connect Tl isotopes to Mn oxides, and Mn oxides to O2 – links that were probably also present in ancient marine environments. It is the PIs’ plan to upscale their work in Siders Pond and extend it to two freshwater lakes in Minnesota (Deming and Steel lakes).

The results of this work will vastly improve our understanding of the modern Tl isotope cycle, in-turn allowing for more confident reconstructions of Earth’s past ocean oxygenation using Tl isotopes. This research will be led by a postdoctoral investigator and will further the education of several undergraduate summer interns. K-12 outreach efforts associated with this research will introduce students and teachers in the greater Boston area to biogeochemistry and Earth science.

Measurements of Tl isotope ratios in ancient marine sedimentary rocks have rapidly accelerated over the past half-decade because there is reason to think they can track changes in past ocean oxygenation. Unfortunately, the modern Tl isotope investigations necessary to guide and hone interpretations have not kept pace with the ancient applications. The PIs’ preliminary data from a redox-stratified and brackish pond (Siders Pond, Cape Cod) show that it is feasible, even under very low Tl concentrations, to generate quality Tl isotope data for waters, particles, and sediments in a natural setting.

Moreover, these preliminary data identify a strong and temporally dynamic link connecting Tl isotopes to local manganese (Mn) oxide cycling. These results highlight the direct role that Mn oxide minerals – and not O2 – play in driving Tl isotope fractionation effects. The PIs are proposing to expand their Tl isotope investigation of Siders Pond, and also to target two additional geochemically distinct freshwater settings in Minnesota (Deming and Steel lakes).

Guided by the preliminary data, the PIs predict that Tl isotope cycling in these additional settings will also be most directly coupled to local Mn cycling, with comparatively little to no effects being driven by other Tl interactions. Objectives guiding this research are (1) to better understand and quantify how Tl and its isotopes are partitioned between waters and particles under different redox conditions, and (2) to better understand and quantify how Tl and its isotopes are retained in sediments under different redox conditions.

These objectives will be addressed via a combination of fieldwork, trace metal and isotope ratio measurements, and synchrotron-based techniques. An improved understanding of modern Tl isotope cycling will only serve to strengthen interpretations of ancient sedimentary Tl isotope ratios – and their connections to past ocean oxygenation.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.