Bioturbation

From: bivalve (bivalve@mail.davidson.alumlink.com)
Date: Tue May 27 2003 - 12:26:54 EDT

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    >Are you saying that layered sediment, as seen throughout the geologic record, is evidence of rapid burial - burial that was too rapid for bioturbation to occur?<

    Hi Bill,

    It might help folks who are not geologists (if any read this) to define bioturbation. It is the activities of organisms that stir up sediment-digging, burrowing, walking, etc.

    Fine layering suggests that bioturbation was very limited. This could happen for any of several reasons:

    Bioturbators were globally rare or absent. This seems to be the case for the Precambrian into the early Paleozoic, and immediately after some mass extinctions.

    The environment was inimical to bioturbators. This includes anoxic or dysoxic environments (the latter often anoxic in the sediment), tidal flats, extremely unstable sediments (e.g., dunes or sand waves), or hypersaline water.

    Net sedimentation was extremely slow, so that only a small amount was deposited before the layer was lithified.

    Sedimentation was faster than bioturbation. This can happen in a storm or a local flood, for example. Or, in environments that are not especially conducive to high levels of bioturbation, this may not require as much sedimentation. Deep lakes, even if they are not anoxic, tend to have a relatively low number of large, active bioturbators, and distinct annual layers easily accumulate.

    Bioturbation may not necessarily totally obliterate all layering. If sediment is accumulating more or less continually, with some variation in those sedmients that can be detected as layers, bioturbation will mix things around a bit but eventually they get buried deep enough to no longer get frequent mixing. Thus, although the layering is smeared, it may not be totally gone.

    There's a whole range in the degree of bioturbation observed in sediments. You can look up various references. I think Sue Kidwell and Mary Droser are some of the names to look for.

    Remember also that, although fine lamination is found here and there throughout the geologic column, extensively bioturbated beds are common. You can't honestly claim that the whole geologic column is depauperate in bioturbation. As I am generally looking for beds with lots of shells, my collecting emphasizes well-bioturbated facies, but I do see a lot of places with plenty of bioturbation. I sometimes encounter layers with good lamination. Very many involve dark layers, which indicates high organic content. High organic content implies low oxygen levels.

    Also, each lamination takes some amount of time to form. With about 200,000 laminations in the Castille Formation in western Texas, you would have to form one organic rich-organic poor couplet every two to three minutes to fit it all into one year. That's just one formation.

    Trying to explain any substantial portion of the geologic column as resulting from the Flood requires extremely high sedimentation rates, and the fact that organisms frequently had plenty of time to burrow around poses a problem for this. On the other hand, occasional deposits being deposited rapidly in no way poses a problem for an old earth view. The young-earth view is at a disadvantage, as anything taking a long period of time is incompatible with it, whereas lots of things can happen quickly within an old earth model.

        Dr. David Campbell
        Old Seashells
        University of Alabama
        Biodiversity & Systematics
        Dept. Biological Sciences
        Box 870345
        Tuscaloosa, AL 35487-0345 USA
        bivalve@mail.davidson.alumlink.com

    That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa

                     



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