Isotope Compositions of Century-Long Corals Reveal Significant Dissolved Cu, Zn Fluxes From Human-Accelerated Weathering Into the Ocean

Human activities have liberated substantial amounts of biologically active metals into oceans. However, identifying the sources and migration processes of these metals is challenging. Here we present first century-long records of concentrations and isotope compositions of Zn and Cu (delta Zn-66 and delta Cu-65) in two corals from the northern South China Sea. The results show dramatic enrichment in coral Zn and Cu concentrations since the 1980s, which coincide with delta Zn-66 and delta Cu-65 increases. Coupled with concurrent land use/cover changes, we suggest that the enrichment trends of Cu and Zn in corals are best explained by human-induced rock weathering and soil erosion, which transferred isotopically heavy Zn and Cu fluxes into the coastal ocean. The metal releases due to land use/cover changes relative to direct anthropogenic sources could be even more significant in the future given that anthropogenic releases have been legally controlled and started to decline in recent decades.