ABSTRACT: 500 billion mollusk shells washed up onto Colorado delta beaches:
Spatial and temporal mixing in coquinas and the nature of paleoecological data.

Kowalewski, M., Flessa, K.W.
Department of Geosciences, University of Arizona, and
Goodfriend, G.A. Geophysical Lab, Carnegie Institution.

Geological Society of America Abstracts with Programs. Vol. 28, p. A-365.

Bioclastic beach ridges currently forming along the shoreline of the Lower Colorado River Delta (Baja California, Mexico) are 4m thick, 40m wide, and continuously extend over a distance of 60km. They represent about 10 million cubic meters of mollusk-dominated shell material. with the average concentration of 50,000 shells per cubic meter (estimated by 61 bulk samples, 0.005 cubic meters each), the ridges contain total of about 500 billion shells.

Radio carbon/amino-acid dating indicates that the ridges consist primarily (80% or 400 billion specimens) or mollusks that have lived during the last 500 years. If we consider that only a small fraction of the shells (~10%) have accumulated in the ridges--with the rest scattered around the macrotidal zone--the actual number of shells in the area is 4,000 billion. Assuming a population turnover rate of 2.5 years, a benthic population of at least 20 billion individuals must have continuously existed in the area for the last 500 years. Even allowing for a very high population density (100 per square meter), the minimum area to sustain such a population is 200 square kilometers. Thus, the Colorado Delta ridges are both time- and space-averaged: a typical sample may contain specimens that lived hundreds of years apart and at least kilometers away from each other.

All fossil coquinas, regardless of their origin, are similar to the Colorado Delta ridges in that they contain huge quantities of bioclastic material. These quantities alone suggest that coquinas are a product of extreme temporal and/or spatial mixing. Coquinas in areas of lower productivity will suffer even greater temporal and spatial mixing than the Colorado Delta deposits. In sum, coquinas--a primary source of information for marine paleoecologists--provide data that may be too coarse to test for the paleontological consequences of models that call for "ecological locking" in ancient communities.

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