Hydrothermal influence on seawater composition: First direct observation from potassium isotopes





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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 the BSE, and seawater represents the heaviest reservoir on Earth. Due to 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 influence of seafloor hydrothermal vents on the K isotope composition of seawater, we analyzed the K isotope composition of 46 seawater samples collected in four depth profiles from both the Atlantic and Pacific Oceans, including two near an active hydrothermal vent field (ASHES, Axial Seamount, Juan de Fuca Ridge). We found that within the current analytical uncertainty, all seawater samples that are not directly affected by hydrothermal plumes have the same K isotope composition regardless of their location, depth, K concentration, and salinity. We also observed that the input from the hydrothermal vents, which most likely have lighter K isotopic compositions than seawater, has a measurable effect on seawater sampled adjacent to the vent. Finally, combining our new analyses with literature, we define the best representative K isotope composition of modern seawater as +0.13±0.01‰.



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