Abstract

The late Quaternary deposits of southern and offshore Louisiana record a complete cycle of sea-level fluctuation which is associated with major changes in the volume of ice on the continents since the beginning of the last glacial stage. In order to date the major events of this cycle, 122 samples from both the surface and subsurface have been analyzed by the radiocarbon method.

A eustatic curve based on the age determinations of these samples supports previous estimates from geological data that the sea during the early part of the cycle fell to a position at least 450 feet below its present level. The lowest sample on this curve, which was deposited when sea level stood at −440 feet, shows that this fall took place more than 35,000 years ago. The rise of the sea during the middle part of the cycle occurred in two successive stages. The first stage is marked by a rise from more than −440 to −250 feet, about 200 feet, before 35,000 years ago, followed by a long stillstand. This period of stillstand terminated about 18,500 years ago with the beginning of the second-stage rise which brought the sea to its present position about 5000 years ago. Sea level during the last part of the cycle has remained unchanged to the present.

The eustatic curve implies that the ice sheets of the last major glacial stage not only reached their maximum extension but had begun to retreat before 35,000 years ago. Furthermore, it indicates that the final stage of retreat began about 18,500 years ago and ended about 5000 years ago. This latter estimate corresponds closely to dates obtained from glaciated areas and deep-ocean sediments for the final stage of ice withdrawal. No general agreement exists between the eustatic data and other estimates on the age of the maximum glaciation and the beginning of ice retreat.

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