Seawater Chemistry and Hydrothermal Controls on the Cenozoic Osmium Cycle

Osmium isotope ratios are a key tool to track changes in global weathering and carbon cycle evolution through time. Long‐term changes in seawater Os isotope records over the Cenozoic have been used to argue for changes in weathering from increased uplift, leading to long‐term global cooling. Sulfide mineral precipitation during hydrothermal circulation traps the majority of mantle‐derived Os within mid‐ocean ridge (MOR) hydrothermal systems. Building from evidence that the amount of sulfide mineral precipitation in the subseafloor of MOR systems is related to the amount of sulfate and calcium in seawater, we show that as seawater chemistry changed through time, so too has the extent of hydrothermal sulfide formation and the global Os cycle. With currently estimated changes in seawater Ca/SO4 ratios, the observed progressive increase in seawater Os isotope values through the Cenozoic can be linked to changes in seawater chemistry—instead of a major shift in continental weathering processes. Plain Language Summary Osmium (Os) isotopes have been used to track continental weathering, which plays a significant role in controlling climate and shifts in the global carbon cycle throughout Earth history. Tectonic drivers, such as increased uplift, are often invoked to explain the long‐term rise in Os isotope composition recorded from 65 million years (Myr) to present. Here, we offer a contrasting perspective by providing evidence that the increase in Os isotope composition may be controlled by variations in the flux of Os coming from high‐temperature fluids venting from mid‐ocean ridge hydrothermal systems, which can be directly tied to changes in global ocean chemistry, such as the amount of calcium and sulfate in seawater. A steady decrease in this hydrothermal flux from 65 Myr to present provides a simple explanation for the evolution of the seawater Os curve over the past 65 Myr without requiring changes in continental weathering fluxes, which has been the traditional interpretation. This unconsidered player in the Os system has major implications for how we interpret Os data throughout Earth’s history and the conclusions drawn from them. Key Points We use geochemical and mass balance modeling to show that changing seawater chemistry has unrecognized influence on the Cenozoic Os record We question if Cenozoic Os isotope records can be used to support link between increased continental weathering and major global cooling Our framework suggests the largest change