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Whoops, These Old Rocks Are Really Quite Young

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  • Chris Ashcraft
    Whoops, These Old Rocks Are Really Quite Young 10/01/2003 A classic example of ancient rock, long assumed to have contained organic compounds from the
    Message 1 of 1 , Oct 1, 2003
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      A classic example of ancient rock, long assumed to have contained organic compounds from the earliest life-forms, has been reinterpreted as very young.  In the October issue of Geology1, Donald R. Lowe (Stanford) and Gary R. Byerly (Louisiana State) looked at some South African ironstone pods that �have been interpreted as deposits of Archean seafloor hydrothermal vents and have provided what are arguably key observations about surface environments on early Earth.�  Up till now, these deposits �yielded what are putatively the oldest-known complex organic compounds and have been used to estimate Archean surface temperature, ocean depth and volume, and seawater composition and to deduce relationships between hydrothermal activity and seafloor sedimentation.�

          But Lowe and Byerly wondered why the beds were so undeformed if they were really three billion years old, especially in the vicinity of deformed rocks of much younger age.  They also wondered how the material they were made of, a thermally unstable hydrated form of iron oxide, could have survived intact for so long.  In addition, they found cavities filled with dripstone that seem to have formed around the same time.  This formation begged for reinterpretation.

          Lowe and Byerly have a new story.  They argue that these rocks are in fact Quaternary deposits �formed as spring and shallow subsurface deposits of young (Quaternary) groundwater and/or low-temperature hydrothermal systems.�  (The Quaternary Period is the most recent in the geologic column, estimated to be 10,000 to 1.8 million years old, whereas Archean rocks are assumed to be the very oldest, up to 3.5 billion years old.)  The ironstone may contain some rare remnant veins of Archean quartz, but otherwise, �The presence of a well-preserved modern iron oxide spring terrace confirms that these are deposits of young subaerial springs and contain no record of Archean life or environments.�

      1Donald R. Lowe and Gary R. Byerly, �Ironstone pods in the Archean Barberton greenstone belt, South Africa: Earth's oldest seafloor hydrothermal vents reinterpreted as Quaternary subaerial springs,� Geology Vol. 31, No. 10, pp. 909�912, DOI: 10.1130/G19664.1.

      Notice that this deposit had been considered Earth�s oldest seafloor by earlier storytellers.  Notice how it was used as a foundation for other tall tales about the ancient ocean, including its depth, temperature and chemistry.  And notice the new date differs by three orders of magnitude or more, undermining the whole pyramid of stories built on top of one faulty assumption.  Anyone think geological dating is an exact science?  Anyone think the new story and its date estimates are the final truth?
          Remember this article next time you read a confident national park sign explaining how such and such a formation was formed by oopty-oop processes 4.28523 kajillion years ago.

      Next headline on: . � Next headline on: Dating Methods.


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      Christopher W. Ashcraft

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