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1
Manuscript received, October 17, 1959. In addition to support from the American Petroleum Institute, Project 51, assistance was given by a grant-in-aid from The American Association of Petroleum Geologists, and the National Science Foundation, NSF G-5419.
2
Scripps Institution of Oceanography, University of California.
The generous assistance of all of the staff of A.PJ. Project 51 is gratefully acknowledged and particularly that of Joseph R. Curray, Fred B Phleger, Gene A. Rusnak, and Francis P. Shepard. The illustrations were prepared by James R. Moriarty.
Individuals and institutions providing suggestions and data are-the staff of the Marine Laboratory, Texas Game and Fish Commission, Rockport, Texas, especially William Guest, Robert Hofstetter, Howard.T. Lee, and Ernest Simmons; Gordon Gunter and Donald R. Moore of the Gulf Coast Research Laboratory, Ocean Springs, Mississippi; the staff of the Institute of Marine Science, University of Texas, Port Aransas, Texas; Thomas E. Pulley of Rice Institute, Houston, Texas; Frederick M. Marland, Texas Agricultural and Mechanical College, College Station, Texas; and R. F. Rutsch, University of Berne, Switzerland.
Identifications of many of the mollusks were checked by Harald Rehder and William J. Morrison, U. S. National Museum; William J. Clench and Ruth D. Turner, Museum of Comparative Zoology, Harvard University; R. Tucker Abbott, Academy of Natural Sciences of Philadelphia; William K. Emerson, American Museum of Natural History; and Thomas E. Pulley.

Abstract

The macro-invertebrate assemblages in the northern Gulf of Mexico can be separated into those occurring in the shallow lagoons and estuaries and those on the continental shelf and slope. Distributional patterns of faunal assemblages in lagoons and estuaries are considerably modified by prevailing climates. Climatic control is exerted to a lesser degree in the shallow waters on the continental shelf.

Eleven macrofaunal assemblages are recognized in the lagoons and estuaries along the northern coast of the Gulf of Mexico. Within each climatic zone certain of these assemblages are predominant, whereas others may be completely absent. Those assemblages .which tolerate extended periods of low salinity, such as fresh-water marsh, river-influenced, and low-salinity oyster-reef assemblages, cover extensive areas in the humid zone. The number of species is small and populations are high in these environments. High-salinity- assemblages characterize the semiarid zone, and here, also, large populations and a small number of species are typical. Within the subhumid zone, and its corresponding wide range of physical factors, most of the eleven macrofaunal assemblages are present. The extent of these assemblages in the subhumid zone is dependent entirely upon the prevailing climate at the time of sampling; during droughts, high salinity, open bay and sound assemblages predominate, and during wet periods, oyster reef and inter-reef assemblages are the most extensive.

Eight macrofaunal assemblages occur on the continental shelf and upper continental slope to 500 fathoms. Although six of these assemblages appear to be confined to rather narrow depth ranges, they are considerably modified by sediment type and average bottom-water temperature conditions. The general limits of the six level-bottom assemblages are (1) surf zone, (2) 2–12 fathoms, on the open coast, (3) 2–12 fathoms off the Mississippi Delta, (4) 12 to 30–40 fathoms, (5) 40–65 fathoms, and (6) 65–500 fathoms. One assemblage is restricted to rocky or calcareous bottom regardless of depth, and another consists of pelagic mollusks which indicate offshore surface waters.

Changes in climate and associated shallow-water environments are demonstrated for the Holo-cene transgression across the continental shelf of the northwestern Gulf of Mexico by using carbon-14-dated shells of species with restricted depth and environmental range, and temperature restrictive geographic ranges. At about 11,000 and again 9,000 years before present, the distribution of dated mollusk shells indicates that the climate along the now submerged shore lines changed from warm-temperate to subtropical and tropical. These conditions were synchronous with major circulation changes in the northern Gulf of Mexico as deduced from other evidence. The presence of extensive deposits of low salinity and surf-zone shells on the continental shelf also permits an interpretation of the location of large lagoons and estuaries and extensive stretches of barrier beaches during various periods of the Holocene transgression.

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