--> Does Aragonite-Dolomite Sea Exist? Widespread Marine Dolomite Precipitation in Ediacaran Period in China

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Does Aragonite-Dolomite Sea Exist? Widespread Marine Dolomite Precipitation in Ediacaran Period in China

Abstract

The extraordinary abundance of dolomite in the Ediacaran Period challenges our understanding of Precambrian marine environments. Here we show that synsedimentary marine dolomite precipitation was pervasive within Sinian carbonates from the Sichuan Basin, Southwest China. The Dengying Formation of Ediacaran was divided to 4 members. All the rocks are dolostone except the thin clastic rocks because of short regression in Member 3. The dolostone types include: stromatolititic dolostone, micritic dolostone, algal dolostone, fenestral dolostone, laminar dolostone, oolitic dolostone, oncolitic dolostone, dolostone with botryoidal structure, non-stromatolite ecologic system cyanobacteria dolostone, siliceous dolostone. Although these carbonates are composed of dolomite, textural evidence indicates an originally aragonitic mineralogy for depositional components, in common with many other Neoproterozoic carbonates. We described several new forms of fibrous marine dolomite cement from the masses that have a length-slow optical character. These fascicular slow, radial slow, and rhombic dolomite cements have finely preserved cathodoluminescent growth zones, and optical characteristics that indicate they originally precipitated as dolomite, rather than replacing calcite or aragonite cements. The low positive carbon isotope and low negative oxygen isotope data (1.59∼4.52‰, -2.82∼-4.82‰) show marine carbonate characteristics. The ordering degree is 0.645-0.832, which is lower than recrystallized dolostone. Calculated paleo-seawater temperature of Dengying Stage, Ediacaran Period is about 40.8°C. Previous mimetic dolomitization cannot explain the widespread huge thick dolostone. Abundant early marine dolomite precipitation implies a radically different seawater chemistry for the Ediacaran. In late Ediacaran Period, there was high seawater salinity, high CO2 partial pressure, anoxic and hot, evaporative condition in South China. Perhaps these aragonite-dolomite seas are associated with extreme Neoproterozoic glacial events and/or ocean anoxia.