Palynology of the surface sediments of Great Bahama Bank, as related to water movement and sedimentation

Abstract

Approximately 130 samples of surface sediment from a series of traverses on the Great Bahama Bank and from other locations in the Bahamas were analyzed palynologically in this study. The investigation was undertaken as an aid to geologic studies of the Bahamas, especially with reference to sedimentation patterns. Fossil pollen and other maceration-resistant microfossils are present in fair abundance in the Bahama surface sediment. The distribution of pine pollen has proved especially significant. This large, buoyant pollen type, once delivered to the water, remains suspended therein for long periods and is eventually deposited in the sediment as a rather sensitive response to hydrographic features on the Bank, especially water turbulence. Analysis of residues from centrifuged sea water west of Andros Island has shown that in July, 1959, five months after the flowering time of pine, samples of the water contained as much as 2,000 pine-pollen grains/100 1 of water. Pine pollen in the water northwest of Andros Island was most abundant at the very edge of the Bank, showing an apparently inverse relationship to distribution of pine pollen in the sediment. The fact that proximity of source is not a prime factor in pine-pollen sedimentation on the Bank is shown by concentrations of pine pollen in sediment near Eleuthera Island, which has no pines, and by the concentration of high values northwest of Andros, in a zone limited on the margins by low values. This distribution is obviously explicable in terms of sedimentation patterns, not source. Pollen highs are regularly found in the sediment from the leeward sides of the larger islands, where water turbulence is minimal and the sediment is prevailingly fine grained. “Microforaminiferal” chitinous inner tests do not show this limitation but are found in abundance in sediment at some stations where pollen highs are not found. The suggested explanation is that the Foraminifera before our maceration treatment are calcareous and are typically bottom dwellers, and hence they are deposited more or less in situ rather than being swept about freely on the Bank. Charts are presented which show distribution of pine pollen, other types of pollen, “microforaminiferal” inner tests, hystrichosphaerids, and HCl-insoluble (organic) residues. The possibility is pointed out that analysis of maceration-resistant microfossils in ancient carbonate banks might yield valuable information about water movements and sedimentation trends during deposition of the bank sediment.