The Ancient Earth

Much of Chap. 5 focused on the fluid envelope of various surface environments in the modern Earth, where Fe redox changes, organic complexation, and mineral precipitation produce large changes in Fe isotope compositions of fluids and minerals. As we step into the ancient Earth, we lose the ability to measure fluids directly, yet the fluid envelope remained a key component of the Fe biogeochemical cycle in Earth’s past, especially in an anoxic Earth when large quantities of \( {\text{Fe}}_{{\,\,\,\,{\text{aq}}}}^{ 2+ } \) existed in the oceans.