Potassium cycling in seawater and teleosts: Insights from stable potassium isotopes (41K/39K)

Abstract

Potassium is the most abundant intracellular cation in all animal cells and a major cation in seawater. However, the behavior of stable potassium isotopes (41K/39K) in these systems has been largely unexplored. Here we use high-resolution, inductively coupled (cold) plasma multi-collector mass spectrometry to measure 41K/39K ratios in a wide range of geological and biological samples. Measurements of potassium isotopes in rivers, high-temperature hydrothermal fluids, siliciclastic sediments, deep-sea pore-fluids, and altered oceanic crust permit a preliminary characterization of the potassium isotope mass-balance in seawater. Our results suggest that K isotope fractionation during (1) continental silicate weathering, (2) marine sedimentary diagenesis and (3) low-temperature oceanic crust alteration all likely contribute to the high 41K/39K ratio of seawater (d41Kseawater ~ 0‰) compared to bulk silicate Earth (d41KBSE = -0.54‰). We propose that this seawater isotopic enrichment results from both clay formation (when 39K is preferentially removed) and chemical diffusion (since 39K has a higher diffusion rate than 41K in liquid water). Additionally, we here consider diffusive K fractionation in biological systems. Measurements of potassium isotopes in both freshwater and marine teleost indicate that different strategies for osmoregulation result in internal pools of K that may be either enriched or depleted in 41K compared to potassium in the environment. Results from simple 1D model simulations of K transport through these marine and freshwater teleost suggest an important role for diffusion in setting the 41K

composition of muscle tissues. Overall, our results demonstrate that high-precision measurements of 41K/39K ratios can provide new constraints on the relative importance of different sources and sinks within the global potassium cycle in seawater, and show promise for understanding the mass transport of potassium associated with osmoregulation and potassium homeostasis.