Ediacaran seawater temperature: Evidence from inclusions of Sinian halite |
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| Authors: | Fanwei Meng Pei Ni James D Schiffbauer Xunlai Yuan Chuanming Zhou Yigang Wang Maolong Xia |
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| Institution: | 1. State Key Laboratory for Mineral Deposits Research, Institute of Geo-Fluid Research, Department of Earth Science, Nanjing University, Hankou Road 22#, Nanjing, China;2. Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing, China;3. Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA;4. Southwest Oil and Gas Field Company Exploration and Development Research Institute, PetroChina, Chengdu, China;1. School of Geosciences, China University of Petroleum (East China), Qingdao, Shandong 266580, China;2. GLOBE Institute, University of Copenhagen, DK-1350 Copenhagen K, Denmark;3. Faculty of Science, IGN, University of Copenhagen, DK-1350 Copenhagen K, Denmark;4. Global Systems Institute, University of Exeter, Exeter EX4 4QE, UK;5. Analytical, Environmental and Geo-Chemistry Group, Division of Earth System Science, Vrije Universiteit Brussel, Brussels, Belgium;6. ET-HOME (Evolution and Tracers of the Habitability of Mars and Earth) Astrobiology Research Consortium, Belgium;7. State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China;8. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, China;9. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, Hubei 430074, China;10. Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA;11. School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA;12. School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, USA;1. School of GeoSciences, University of Edinburgh, James Hutton Road, Edinburgh EH9 3FE, UK;2. School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK |
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| Abstract: | Seawater temperatures throughout Earth's history have been suggested to illustrate a long-term cooling trend from nearly 70 °C at ～3500 Ma to around 20 °C at ～800 Ma. The terminal Neoproterozoic prior to the “Cambrian Explosion” is a key interval in evolutionary history, as complex multicellularity appeared with the advent of the Ediacara fauna. These organisms were likely the first that required higher levels of atmospheric and dissolved marine oxygen for their sustainability. It is known that most modern macroinvertebrates are intolerant of temperatures in excess of 45 °C. Perhaps more importantly, these high seawater temperatures limit the potential of dissolved oxygen, and therefore become an integral part of this evolutionary story. Previously, our understanding of seawater temperature during the terminal Neoproterozoic comes only from 18O/16O and 30Si/28Si ratios ascertained from a limited number of cherts. Isotopic ratio methods for assessing seawater temperatures are inherently indirect and have a wide range of oscillation. However, maximum homogenization temperatures (Thmax) of primary fluid inclusions in halite provide a direct means of assessing brine temperature, and have been shown to correlate well with average maximum air temperatures. The oldest halites date to the Neoproterozoic–lower Paleozoic (～700–500 Ma), and Ediacaran representatives can be found in Sichuan Province, China, which do preserve primary fluid inclusions for analysis via cooling nucleation methods. We utilized halite samples from the Changning-2 well, correlative to the Dengying Formation (551–542 Ma), to provide a direct assessment of terminal Neoproterozoic seawater temperature. Our measurements indicate that seawater temperatures where these halites formed are highly similar to tropical Phanerozoic seawater temperature estimates. From compiled paleotemperature data, the decline in seawater temperatures over the course of the Proterozoic, accompanied by the reduction of seawater salinity with the sequestration of salt in massive halite deposits in the Neoproterozoic, allowed the ocean system to accumulate more dissolved oxygen, and potentially paved the way for the evolutionary innovation of complex multicellularity. |
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