Tracing the Global Potassium Cycle Through Its Stable Isotopes

Written by Prof. Xinyuan Zheng

Potassium (K) is a major element present at high concentrations in Earth’s crust and most surface environments. It is also a nutrient element essential for plants and animals, including humans. Investigating how K moves among different components of the Earth system, the so-called global K cycle, can provide valuable insights into some fundamental processes that regulate Earth’s climate, environment, and life, such as plate subduction, silicate weathering, and nutrient availability. Traditionally, analysis of K concentrations in natural samples has been the only tool to trace K movement. Now, Assistant Professor Xinyuan Zheng and his group is developing new mass spectrometry techniques that allow for reliable measurement of extremely small variations in stable K isotope ratios in nature, providing a new way to study the K cycle.

Zheng Group partial
Zheng and part of his research group (Left to right: Weiming Ding, Yiwen Lv, Soisiri Charin, Yuchi Zhang, Xinyuan Zheng. Not pictured:  Dylan Parmenter, Keegan Hoffer, Owen Wold, and Emily Briggs)

Potassium has three naturally occurring isotopes: 39K (~93.3%), 40K (~0.01%), and 41K (~6.7%), among which 39K and 41K are stable isotopes, and 40K is a radioactive nuclide underpinning the K–Ar / Ar–Ar radiometric dating methods. In fact, early measurements of potassium isotopes, which confirmed the existence of 40K in nature (Nier, Phys. Rev. 48, 1935), were made right in Tate Hall by Professor Alfred O.C. Nier in Department of Physics on one of many mass spectrometers he designed and built. However, the precision on stable K isotope ratio (41K/39K) measurement at the time was insufficient to resolve small K isotope variations in any samples on Earth.

Mass spec comparisons
A comparison of K isotope mass spectra on Nier’s mass spectrometer over 80 years ago (Nier, Phys. Rev., 1935) and on “Sapphire” today

More than 80 years later, using one of the most advanced mass spectrometers in the world, “Sapphire” collision-cell multicollector inductively coupled plasma mass spectrometer (cc-MC-ICP-MS), Prof. Zheng’s research group has been able to push the analytical boundary on 41K/39K measurement (Zheng et al., JAAS, 2022), achieving a precision more than 2 orders of magnitude better compared to early measurements by Nier.

Sapphire MC-ICP-MS
“Sapphire” MC-ICP-MS

This analytical advance makes University of Minnesota one of the best laboratories in the world capable of this measurement. Using this new isotope tool, Zheng’s research group is addressing a wide range of exciting science questions, ranging from understanding the fundamental controls on the long-term stability of our climate (Zheng et al., EPSL, 2022) to improving quantification of potassium fertilizer utilization in agriculture (Chen et al., ACS Earth Space Chem, 2022).

K isotope ratio measurement
Improved analytical precision in potassium isotope ratio measurement at UMN as compared to that from several other laboratories capable of such measurement in the world today (Zheng et al., JAAS, 2022).
Quantification of K fertilizer

​​​​​​Quantification of K fertilizer utilization in plants using an enriched 41K tracer (Chen et al., ACS Earth Space Chem, 2022).

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