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.

本研究将钾同位素体系提出为大陆风化与全球钾循环的新型示踪剂。目前在主要储库间观测到的最大钾同位素分馏，发生于海水与整体硅酸盐地球（bulk silicate Earth, BSE）之间。相较于BSE，海水显著富集重同位素，是地球上重同位素丰度最高的储库。受限于现有分析数据量，目前仍无法明确海水在垂直、横向及全球尺度上的钾同位素组成是否均一。此外，诸如热液输入等过程对海水重同位素富集的贡献机制与贡献程度，尚未得到较好限定。为更深入认识现代海水的钾同位素组成，并探究海底热液喷口对海水钾同位素组成的潜在影响，我们对取自大西洋与太平洋两处海域的46个海水样品开展了钾同位素组成分析。两处采样点均采用成对深度剖面的采样方案，其中一处位于活跃热液喷口场（ASHES，轴海山，胡安·德·富卡脊）。研究结果显示，在当前分析不确定度范围内，所有海水样品的钾同位素组成均保持一致，不受采样地点、水深、钾浓度及盐度的影响。结合本次新获得的分析数据与既往研究数据，我们确定现代海水最具代表性的钾同位素组成为+0.12 ± 0.07‰（2SD）。海水钾同位素组成较BSE显著偏高（差值为0.55 ± 0.18‰），这表明大陆风化与反向风化过程中存在显著的钾同位素分馏。