资源环境科技发展态势分析平台

As reverse weathering has been shown to impact long-term changes in atmospheric CO2 levels, it is crucial to develop quantitative tools to reconstruct marine authigenic clay formation. We explored the potential of the beryllium (Be) isotope ratio (Be-10/Be-9) 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 Be-10/Be-9 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 Be-10/Be-9 ratio of this fraction. Be-10/Be-9 ratios increase fourfold from riverine to marine sediment. We attribute this increase to the incorporation of Be high in Be-10/Be-9 from dissolved biogenic opal, which also serves as a Si-source for the precipitation of marine authigenic clays. Be-10/Be-9 ratios thus sensitively track reverse-weathering reactions forming marine authigenic clays.

Plain Language Summary Clay minerals can form on land by the chemical breakdown of rock-forming minerals, but clays can also form in the ocean. When clay formation takes place in the ocean, CO2 is released. To date, there is no method that can easily measure the amount of clay minerals formed in the ocean. We used two isotopes of the same element, beryllium (Be), with the atomic mass of 9 and 10 to test whether this isotope system can be used to measure marine clay formation. The abundance of these isotopes differs majorly on land and in the ocean. We measured beryllium isotopes in river sediment and ocean-bottom sediment offshore the Chile coast and compared the ratios of the isotopes (Be-10/Be-9). The ratio is four times higher in ocean sediment, when compared to river sediment. We interpret this increase to be due to the formation of clay minerals in the ocean, which include the high Be-10/Be-9 ratio during their formation. We conclude that the beryllium-isotope system can be used to measure the formation of even very small amounts (less than 2%) of marine clay minerals. This is important, as the clay-forming chemical reactions release CO2 which has a long-term effect on global climate.