Semester of Graduation

Fall 2022

Degree

Master of Science (MS)

Department

Geography and Anthropology

Document Type

Thesis

Abstract

Terrestrial ecosystems along exposed continental shelves during times of relatively low sea level and glaciation are rarely preserved due to the mechanically erosive nature of marine transgression and regression. In this study, I investigate the geochemical and palynological characteristics of two well-preserved sites in the northern Gulf of Mexico (NGOM) to determine the ecologic settings and subsequent mechanisms of preservation in this region. Both sites contain preserved terrigenous sediments of previously exposed coastal margins; the first is located at the Alabama Underwater Forest (~13 km south of Gulf Shores, AL, at ~15 mbsl), and optically stimulated luminescence dates to 72–56 ±8 ka, 2σ (Marine Isotope Stage 3–5a; Late Pleistocene). The second is located ~22 km south of Horn Island, MS, at ~25 mbsl and 14C dates up to 10–11 ka (Early Holocene). The terrestrial sections of three sediment cores across these two sites were analyzed for palynomorphs and stable isotopes (bulk organic δ 13C for C3 vs. C4 vegetation, δ 15N for nutrient cycling, and δ 34S for freshwater vs. marine signals) to reconstruct paleoenvironmental conditions. Stable isotopes of AL sediments indicate a swamp to saltmarsh transitional series, whereas MS sediments indicate a swamp to freshwater marsh transitional ecosystem. Palynological analyses confirm these environmental interpretations at both sites. A linear discriminant analysis (LDA) was also performed to predict the type of past NGOM environments using both the isotope values of this study and a legacy data set of previously reported isotopic compositions. The LDA identifies swamp and marsh conditions at both sites, further confirming interpretations from geochemical and palynological analyses. Swamp and marsh environments are typically euxinic; these sulfur-rich anoxic conditions promote the preservation of buried organic material. I suggest that the near-pristine preservation of these shelf environments is not solely due to a physical mechanism, such as rapid burial, but is coupled to euxinia-driven biogeochemical reactions that are innate to swamp and marsh environments, such as those identified in the NGOM. This multiproxy approach can be used as a framework for studies of similarly well-preserved coastal environments to further investigate the ecological and chemical properties that contribute to preservation.

Date

11-1-2022

Committee Chair

DeLong, Kristine

DOI

10.31390/gradschool_theses.5677

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