Sequestration of Carbonate Shell Material in Coastal Dunes on the Gulf of California (Baja California Sur, Mexico)

Atmospheric and oceanographic conditions stimulate high productivity of marine organisms in the Gulf of California that are bulk producers of calcium carbonate. Due to prevailing winter winds, north-facing beaches receive vast amounts of shell debris derived from offshore clam banks above the 50-m isobath. As shells undergo mechanical abrasion, the smaller particles of carbonate material are transferred from beaches by the wind and sequestered in coastal dunes. This study reports on two dune fields from the midriff zone of the Gulf. Sieve analysis is used to describe the grain size and sorting characteristics of seven samples from dunes at Cerro El Gallo near Mulegé and 12 samples from dunes farther south near San Nicolás. Dune sediments also were impregnated with epoxy to simulate rock samples from which thin sections were made to determine composition and relative abundances of constituent grains. The dunes at Cerro El Gallo are carbonate poor (5–15%) compared with those near San Nicolás (26–51%) and possible factors contributing to regional variation are explored. Another part of this study appraises the fecundity necessary to produce any carbonate fraction integrated by coastal dunes from seashells. One of the region's more abundant clams, Megapitaria squalida, was used as a model to estimate the number of individuals of a given size and age class required to generate a cubic meter of pure carbonate sand. Satellite images of the dune fields at El Gallo and San Nicolás were viewed to map surface coverage. With input on the thickness of dune deposits and their composition, it is possible to roughly estimate the number of mature Megapitaria equal to the carbonate fraction of these dunes. The method may be applied to all coastal dunes on the Gulf of California and could be used to assess a part of the region's overall carbon budget heretofore unappreciated.

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