In a restricted part of an area intensively sampled by box-coring (approximately 50 samples from a 2 x 4-km rectangle), a stiff, muddy glacial gravel ("till") is encountered at a depth of 30-40 cm. X-radiographs of the upper 15 to 20 cm of the cores reveal gravel in overlying foraminiferal mud only from the area where there is the deeper "till" layer. The localization strongly suggests that the material in the top of the cores has its source in the deeper layer. I postulate that burrowing organisms move gravel upward by pushing aside the particles that are too large to be eaten. Because the strength of the enclosing mud decreases upwards, the gravel particles move preferentially upward. The upward decrease in shear strength of the sediment is rapid at an interface occurring 4 cm below the surface on average. Thus hydrostatic pressures of 20 to 200 g/cm 2 exerted by vermiform-burrowing animals act within an environment of asymmetrical sediment property, permitting an asymmetrical response to the uniformly directed body pressure. This asymmetrical response, a pumping mechanism affecting material that does not pass through the gut, is contrasted with the mixing of finer sediments that are eaten and excreted, documented by many other workers. Dimensional arguments suggest the flux of material is related to the ratio of biological mixing rate to sedimentation rate. The mechanism is also applicable to maintenance of Mn nodules and their nuclei at the surface. If macrofossils have behaved like gravel, some shell beds may have been generated and some stratigraphic anomalies caused by this means.

You do not currently have access to this article.