A 6200-Year Environmental History of the Pearl River Marsh, Louisiana.

An 8.81 m sediment core from the fresh marsh area of the Pearl River delta was examined for pollen, diatom, and organic content, to reconstruct the history of the late-Holocene environmental changes. The data show that prior to 5900 years B.P., the coring site was occupied by a swamp forest. Afterwards, the swamp was replaced by a marsh vegetation as a result of postglacial transgression. Since 5900 years B.P. the marsh has remained brackish for most of the time, characterized by abundant marine diatoms and brackish marsh pollen. However, the brackish water environment was interrupted at least two times when the local salinity became low enough to support oligohaline communities. The two regressions correspond to C-14 dates of 3400 to 2200, and 1400 to 1100 years B.P. The most prominent regression is the one from 3400 to 2200 years B.P., which is correlatable with a major neoglacial event. This regression is also compatible with the results of sea level studies in other areas of the U.S. The study results indicate that there were more large fluvial discharge layers during the time of the mid-Holocene than during the late-Holocene. This probably corresponds to a climatic regime of increased precipitation during the mid-Holocene. These results support the climate modeling prediction of increased precipitation during the mid-Holocene in the southeastern U.S. The study indicates that the climatic response to Hypsithermal warming in the Gulf coastal region was probably different from the other regions of the U.S. such as the Midwest. Temperature depression during the early part of the Little Ice Age probably was severe enough to affect hurricane formation as indicated by the extreme low percentages of marine and brackish water diatoms in the sediment core from the Pearl River Marsh. The hypothesis is further substantiated by the study of a lake sediment core from Horn Island, Mississippi