Publication Date

2014

Document Type

Honors Thesis

Department

Geosciences

Keywords

Sand-Bahamas-San Salvador Island, Sediments (Geology)-Bahamas-San Salvodor Island, Calcium carbonate, Biochemical markers, Ooids, Biomarkers, Bahamas, San Salvador, Pigion Creek

Abstract

Ooids are small spherical to ellipsoidal coated grains, characterized by concentric layers of calcium carbonate. Despite the ubiquity of these grains in the geologic past, their formation, including their accretion and shaping, is still the subject of debate. In 2013, a targeted collection of sand containing ooids was performed in subtidal and beach settings along Pigeon Creek Delta on San Salvador Island, The Bahamas. Thirteen samples were taken in the tidal channel at different locations and it was determined that ooids were most abundant at localized sites along the tidal delta, suggesting that ooids are actively forming in the tidal channel and transported onto beachshores. To characterize the organic matter associated with ooids, samples were examined for lipid biomarkers at MIT. As in previous work, our results showed characteristically long-chain normal fatty acids (FA) and a distinguishable even-over-odd carbon number compounds with very small abundances of iso— and anteiso—FA. These results suggest that organic matter in Pigeon Creek ooids is dominated by bacterial communities like cyanobacteria and sulfate reducing bacteria, suggesting that microbes are present on ooid surfaces as well as in the water column. To assess the role of abrasion in ooid formation, preexisting ooid sand collections from Cat Island, Bahamas, were used in accretion and erosion experiments where ooids were rolled under high wave conditions from 6 days to 2 months both with and without cyanobacteria. Despite producing different amounts of carbonate mud during erosion experiments, ooid surfaces were not broken in half or extremely eroded. This multidisciplinary project has revealed that: 1) ooids are actively forming and accumulating in shallow portions of the tidal channel (sites S7 and S8), and are being transported to the beach through flood tides; 2) cyanobacteria are colonizing ooid surfaces, but whether they contribute to ooid formation is uncertain; and 3) high wave agitation causes the outer layers of ooids to erode (breakdown) and get rounded as carbonate mud is produced. Further work will require additional multidisciplinary approaches to understand the mechanics of ooid formation, and explore the relationship between bacterial communities and ooids.

Language

English

Comments

67 pages : color illustrations. Honors Project-Smith College, 2014. Includes bibliographical references (pages 45-48)

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