10 Be/ 9 Be Ratios Reveal Marine Authigenic Clay Formation

As reverse weathering has been shown to impact long‐term changes in atmospheric CO 2 levels, it is crucial to develop quantitative tools to reconstruct marine authigenic clay formation. We explored the potential of the beryllium (Be) isotope ratio ( 10 Be/ 9 Be) recorded in marine clay‐sized sediment to track neoformation of authigenic clays. The power of such proxy relies on the orders‐of‐magnitude difference in 10 Be/ 9 Be ratios between continental Be and Be dissolved in seawater. On marine sediments collected along a Chilean margin transect we chemically extracted reactive phases and separated the clay‐sized fraction to compare the riverine and marine 10 Be/ 9 Be ratio of this fraction. 10 Be/ 9 Be ratios increase fourfold from riverine to marine sediment. We attribute this increase to the incorporation of Be high in 10 Be/ 9 Be from dissolved biogenic opal, which also serves as a Si‐source for the precipitation of marine authigenic clays. 10 Be/ 9 Be ratios thus sensitively track reverse‐weathering reactions forming marine authigenic clays.