Evidence for high salinity of Early Cretaceous sea water from the Chesapeake Bay crater

摘要

High-salinity groundwater more than 1,000 metres deep in the Atlantic coastal plain of the USA has been documented in several locations most recently within the 35-million-year-old Chesapeake Bay impact crater Suggestions for the origin of increased salinity in the crater have included evaporite dissolution, osmosis and evaporation from heating associated with the bolide impact. Here we present chemical, isotopic and physical evidence that together indicate that groundwater in the Chesapeake crater is remnant Early Cretaceous North Atlantic (ECNA) sea water. We find that the sea water is probably 100-145 million years old and that it has an average salinity of about 70 per mil, which is twice that of modern sea water and consistent with the nearly closed ECNA basin. Previous evidence for temperature and salinity levels of ancient oceans have been estimated indirectly from geochemical, isotopic and palaeon-tological analyses of solid materials in deep sediment cores. In contrast, our study identifies ancient sea water in situ and provides a direct estimate of its age and salinity. Moreover, we suggest that it is likely that remnants of ECNA sea water persist in deep sediments at many locations along the Atlantic margin.%在估计古海洋的温度和盐度时，地质学家和海洋学家一般都要依靠来自对深层沉积芯中固体材料所做分析的间接证据。但现在，来自美国地质调查局的一个小组发现了一处地下水，它实际上是过去一个海洋水体的残留物，即早白垩纪北大西洋的一处海水。在Chesapeake Bay碰撞坑所做的深度钻探，发现了具有同位素和化学组成特征的地下水。该特征再加上模型分析表明，它是在发生在距今3500万年前左右的碰撞之前被束缚到沉积层中的。这些水保持未受扰动状态的时间可能要长得多：它的平均盐度大约为千分之70，是现代海水的两倍，并且可能已有1亿～1.45亿年了。