Global Average Potassium Isotope Composition of Modern Seawater

The potassium isotope system was proposed as a new tracer in continental weathering and global K cycling. The largest K isotope fractionation observed among major reservoirs is between the ocean and bulk silicate Earth (BSE). Seawater is significantly enriched in heavy isotopes compared to BSE, and seawater represents the heaviest reservoir on Earth. Because of limited analyses, it is still unknown whether seawater is homogeneous in terms of K isotopes vertically, laterally, and globally. In addition, what processes (e.g., hydrothermal inputs) and to what degrees these processes have contributed to this heavy isotope enrichment in seawater are still not well constrained. To better understand the K isotopic compositions of modern seawater and to examine the possible influence of seafloor hydrothermal vents on the K isotope composition of seawater, we analyzed the K isotope composition of 46 seawater samples collected as two pairs of depth profiles in two locations from the Atlantic and Pacific oceans, including one near an active hydrothermal vent field (ASHES, Axial Seamount, Juan de Fuca Ridge). We found that within the current analytical uncertainty, all seawater samples have the same K isotope composition regardless of their location, depth, [K] concentration, and salinity. Combining our new analyses with data from previous studies, we define the best representative K isotope composition of modern seawater as +0.12 +/- 0.07 parts per thousand (2SD). The seawater is significantly higher (0.55 +/- 0.18 parts per thousand) than BSE, which requires large K isotopic fractionation during continental weathering and reverse weathering.