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Abstract

Geologic Problem Solving with Microfossils: A Volume in Honor of Garry D. Jones

SEPM Special Publication No. 93, Copyright © 2008

SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-137-7, p. 83–92.

Beds of benthic macro-vegetation are important in shallow-water, carbonate-producing regimes. Seagrass blades baffle currents, leading to deposition of fine sediment, and grass root systems effectively bind the sediment, preventing erosion. Seagrass and benthic macroalgae are substrates for a variety of epibionts, including foraminifera. Due to their higher preservation potential, foraminifera can be used as proxies in studies of ancient vegetation-bed environments. In this actualistic study, we investigated the potential of using foraminifera to reconstruct the taxonomic composition and density of the vegetation, water depth, and a range of other environmental variables.

At each of the six localities selected, vegetation densities were recorded in 50 cm x 50 cm grids located at 10 m intervals along transects. A sediment sample from the top 1 cm of the seafloor was taken at each station, and samples of each of the major genera of vegetation algae, seagrass, and macroalgae were collected at each locality. In the laboratory, vegetation was examined, and relatively large foraminifers were picked from the 2.0–0.5 mm sediment fractions. Total foraminiferal density was recorded as the number of individuals per plant and per gram of sediment picked. Dead foraminifers were categorized by taphonomic condition: pristine, good, altered, and extremely altered.

Sorites and Planorbulina are the dominant large foraminifers living on the preferred substrates, Thalassia and Halimeda, in proportions that vary according to locality. However, sediment assemblages are dominated by Archaias, Cyclorbiculina, and Valvulina, with very few live individuals. In vegetation beds dominated by seagrasses, foraminiferal density in the sediment appears to reflect the standing Thalassia density to a degree, but maximum test density occurs in moderately dense seagrass beds. The most reliable proxy for the presence and density of seagrass beds is the relatively high quality of test preservation as assessed by the quality of preservation index (QPI), the percent of live, pristine, and good tests in an assemblage. Medium- to high-density seagrass beds consistently rank in the range of 70–85%, whereas sparse beds are much lower. In addition, processes of alteration, such as abrasion versus encrustation and cementation, are promising areas for further study. These preliminary results indicate strongly that taphonomic state and mode of alteration should be included in future carbonate-platform foraminiferal studies

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