Paper in press: Tracking thallium on tidal timescales

A new study published by Chad Ostrander in Geochimica et Cosmochimica Acta examining thallium cycling in Siders Pond, a salt-stratified, low-oxygen, sulfide-rich body of water right here in Falmouth!

The thallium isotope composition of marine sediments is a valuable tool for studying historical oxygen dynamics on Earth. This proxy relies on the sensitivity of thallium isotope fractionation to the presence of molecular oxygen, which is essential for manganese oxide burial. However, few studies have delved into how changes in biological activity or short-term fluctuations in manganese oxide production impact thallium isotope cycling. To address this, our research examined the cycling of thallium and its isotopes between dissolved and particulate forms over different timeframes, spanning from days to years.

Our study revealed three key findings. First, we noted substantial variations in thallium concentrations in the surface of Siders Pond over short periods (days to weeks). We also observed significant biological uptake of thallium, often favoring the lighter thallium isotope. Second, while manganese oxide levels in the pond’s surface seemed to have little impact on thallium concentrations, when dissolved thallium encountered free sulfide in the deeper parts of the pond, it rapidly partitioned into particulates without any detectable isotopic fractionation. Third, our observations showed that sediments in the pond effectively recorded the long-term (years) thallium cycle in the pond. Essentially all sediments deposited below the oxycline mirrored the thallium isotope composition predicted for the pond’s waters.

Our study clarifies various short-term processes influencing thallium cycling and demonstrates how thallium isotopes can contribute to understanding long-term burial of manganese oxides. If you want to learn more, check out the study here, or you can view near real-time chemical and physical conditions in Siders Pond by going to SidersPond.com.

Full citation: Ostrander, C. M., Nielsen, S. G., Gadol, H. J., Villarroel, L., Wankel, S. D., Horner, T. J., Blusztajn, J., & Hansel, C. M. (2023). Thallium isotope cycling between waters, particles, and sediments across a redox gradient. Geochimica et Cosmochimica Acta, 348, 397-409, doi:10.1016/j.gca.2023.03.028.