Department or Program

Geology

Abstract

Seagrass beds are important ecosystems in nearshore environments. They provide nutrients and habitat for commercially important fish species, buffer against storm erosion, and are effective at sequestering carbon. Globally, seagrass beds are in a general state of decline due to human activities such as pollution, climate change, and dredging in the nearshore zone. Little is known about natural fluctuations in seagrass distribution through geologic time because long and unequivocal records of paleo-seagrass have yet to be produced. The purpose of this study is to use organic geochemical techniques (lipid biomarker concentrations and isotope composition) to determine if seagrass organic matter can be detected in sediment cores from Maquoit Bay, Casco Bay, Maine. Maquoit Bay has extensive beds of the seagrass Zostera marina (hereafter referred to as eelgrass) and is located off of Casco Bay in the Gulf of Maine. Seven sediment cores (ranging between 20-50 cm in depth) were taken from Maquoit Bay using a livingstone corer. These cores were subsampled for organic geochemistry (bulk and higher plant leaf wax lipid carbon isotope composition), 239+240Pu chronology, and grain size determinations. Plutonium results indicate that the sediment cores represent the last 50 years of deposition. Within all of the analyzed cores, the δ13C of the bulk sediments ranged from -17‰ at the coretop to -22‰ deeper in the core. Given the multiple sources of carbon with varying δ13C values in the system, it is impossible to determine, with certainty, the degree to which eelgrass contributes to the total organic pool. Analysis of the lipid biomarker data indicates that eelgrass does contain higher plant leaf wax lipids (C24, C26, C28 fatty acids), as do the sediments in the core. High values of HPLWs have also been found at the surface of the core, which decrease with depth. Isotope analysis shows high C24 and C26, indicating a high even-over-odd predominance typical of HPLW lipids. Using a 2-end-member mixing model in conjuction with a source contribution plot, the organic carbon throughout the sediment core was characterized to determine that eelgrass (33%), phytoplankton (41%), bacteria (14%), and other C3 plants (12%) all contribute to the deposition of organic carbon in Maquoit Bay. This final analysis may provide an important proxy for eelgrass in other nearshore environments.

Level of Access

Open Access

First Advisor

Beverly Johnson

Date of Graduation

Spring 5-2012

Degree Name

Bachelor of Science

Number of Pages

88

Components of Thesis

1 PDF file

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