Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60615, USA.
Department of Earth Sciences, University of California, Riverside, CA 92521, USA.
Science. 2017 Sep 22;357(6357):1271-1274. doi: 10.1126/science.aan8086.
Earth exhibits a dichotomy in elevation and chemical composition between the continents and ocean floor. Reconstructing when this dichotomy arose is important for understanding when plate tectonics started and how the supply of nutrients to the oceans changed through time. We measured the titanium isotopic composition of shales to constrain the chemical composition of the continental crust exposed to weathering and found that shales of all ages have a uniform isotopic composition. This can only be explained if the emerged crust was predominantly felsic (silica-rich) since 3.5 billion years ago, requiring an early initiation of plate tectonics. We also observed a change in the abundance of biologically important nutrients phosphorus and nickel across the Archean-Proterozoic boundary, which might have helped trigger the rise in atmospheric oxygen.
地球在海拔和化学成分上呈现出大陆和海底之间的二分性。重建这种二分性出现的时间对于理解板块构造何时开始以及随着时间的推移营养物质向海洋的供应如何变化非常重要。我们测量了页岩的钛同位素组成，以约束暴露于风化作用的大陆地壳的化学成分，结果发现所有年龄的页岩都具有均匀的同位素组成。这只能解释为 35 亿年前以来，出露地壳主要是长英质（富含硅），这需要早期启动板块构造。我们还观察到在太古宙-元古宙边界处生物重要营养元素磷和镍的丰度发生了变化，这可能有助于引发大气氧气的增加。
