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Abstract

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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