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‰
(2SD). The seawater is significantly higher (0.55 ± 0.18‰)
than BSE, which requires large K isotopic fractionation during continental
weathering and reverse weathering.