Effects of Bioturbation on Sediment Properties
Published:January 01, 1984
The texture, composition and stability of sediment may be altered significantly as a result of burrowing activities of organisms. Sediment texture can be changed by (A) mixing together sedimentary layers with different textural characteristics, (B) deposit-feeding, either by sorting the sediment durinq ingestion or by breakdown or aggregation of grains during digestion and excretion, (C) sorting of grains during burrow construction, either by selecting certain size classes for the wall structure or by simply moving certain size fractions of the sediment out of the way, and (D) sediment production from bioerosion of rock. Sediment composition can be changed with respect to its (A) organic content, (B) trace element concentration and (C) redox potential. Sediment stability can be increased by the baffling effect of rigid dwelling tubes, or it can be decreased by the disturbance of grain packing (and thus increasing the porosity, permeability and volume of interstitial water) caused by organisms churning the sediment.
Biogenic graded bedding is a vertical gradient in the sediment texture resulting directly from bioturbation. For example, Rhoads and Stanley (1964) studied the sedimentologic effects of burrowing by the deposit-feeding polychaete Clymenella torquata in intertidal habitats in Barnstable Harbor, Cape Cod Bay, Massachusetts. The animal lives in a vertical, 20 cm-long, agglutinated tube and feeds with its head down and anus up. At the lower end of the tube it ingests mud and fine sand, which makes its way through the worm's gut and is excreted as uncompacted feces at the sediment surface. Because grains larger
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Ichnology: The Use of Trace Fossils in Sedimentology and Stratigraphy
Ichnology is a fascinating field of endeavor. As with science in general, it is a process of solving mysteries–in this case, mysteries of fossil behavior. In a very real sense the ichnologist is Sam Spade or Sherlock Holmes–following footprints, searching for traces of dastardly deeds, studying artifacts, attempting to reconstruct a sequence of events from subtle clues, pursuing the identity of someone (or something) long dead. Who was the culprit? What was he/she doing? Where was he/she living, working or going? Not only intellectually intriguing, ichnology also has practical application and economic importance. In today’s frenzied quest for energy and mineral resources, exploration geologists value every tool that aids their search. Ichnologic observations and analyses can help the sedimentologist reconstruct ancient depositional environments, help the stratigrapher correlate sedimentary strata, help the paleontologist determine the nature of fossil communities, and help the geochemist determine the effect of organisms on sediment composition. This publication was written to serve as a comprehensive and intelligible introduction to ichnology for anyone with even rudimentary geologic training, whether or not that person enrolls in a formal course on the subject. The book emphasizes sedimentologic, stratigraphic and paleoecologic al aspects of ichnology.