熱水系物質循環と海水組成の進化

抄録

Geochemical data on the evolution of seawater chemistry during the past one billion years (Sr/Ca, Li/Ca, Ge/Si, ¹⁸⁷ Os/¹⁸⁶ Os, δ³⁴ S, 87Sr/⁸⁶ Sr) were summarized. These data indicate that the variations in seawater chemistry have been influenced by the circulation of ydrothermal solution at mid-oceanic ridges. The circulation rate of hydrothermal solution is related to ocean floor spreading rate. Thus, it is considered that tectonics is important factor in controlling the chemical evolution of seawater.
Tectonics affects not only seawater chemistry, but also CO ₂ content in atmosphere and consequently climate. A correlation between δ³⁴ S and ⁸⁷ Sr/⁸⁶ Sr variations of seawater indicates that hydrothermal solution and seawater-rock interaction was important for causing the variations in seawater chemistry before 1 billion years ago. Geochemical data (REE etc.) on BIF (banded iron formations) and sedimentary rocks indicate that the hydrothermal solution-oceanic crust interaction controlled Archean seawater chemistry. Influence of this interaction on the seawater chemistry decreased from Archean to Proterozoic, while weathering and biogenic activity became important.