Semester of Graduation

Fall 2020

Degree

Master of Science (MS)

Department

Geology and Geophysics

Document Type

Thesis

Abstract

Offshore sand deposits on the Louisiana Continental Shelf, such as inner shelf shoals and buried paleo-river channels, can be excavated to restore beaches and barrier islands that are rapidly deteriorating due to subsidence, sea-level rise and deficits in coastal sediment supply. Presented here is grain size, x-radiograph, and Beryllium-7 (7Be) derived sedimentation rates from multicores (~ 50 cm depth) retrieved from borrow areas (BAs) in contrasting depositional settings, all of which have implications for management of water quality, seafloor sedimentology, and biogeochemistry in proximal areas. Multicores were retrieved in fall 2018 at Caminada BA — a sandy energetic site 25 km offshore of central Louisiana excavated from 2013 to 2016 — and in fall and spring 2019 at Sandy Point BA — a muddy site located 20 km west of Southwest Pass of the modern Mississippi River excavated in 2012.

Results at Caminada for fall 2018 reveal 2 – 6 cm (0.02 – 0.04 cm day-1) of 7Be-laden sediment deposited, significantly less than the 8 – 16 cm (0.05 – 0.15 cm day-1) reported for spring 2018, indicating substantial seasonal variability. There was little difference in median grain size of cores between the two seasons within Caminada BA (12 – 45 µm or 4.5 – 6.5 Φ), with multiple layers of coarser silts seen within x-radiographs. Results at Sandy Point for fall 2019 reveal 2 – 6 cm (0.02 – 0.03 cm day-1) of 7Be-laden sediment deposited, significantly less than the 14 – 34 cm (0.1 – 0.5 cm day-1) reported for spring 2019, indicating greater seasonal variation compared to Caminada BA. There is little difference in median grain sizes of 6.0 – 8.5 µm (~ 7 Φ) between the two seasons at Sandy Point BA, although cores retrieved in fall 2019 have very few to no coarse silt laminations.

Estimates of volumetric and mass infill for Caminada BA reveal that bathymetric low areas are currently infilling at ~ 78,000 m3 yr-1, with a predicted 100% infill time of 60 years. Sandy Point BA is infilling more rapidly at ~ 235,000 m3 yr-1, with a predicted infill time of 16 years. Analysis of wind/pressure data in tandem with grain size and 7Be activity reveal that winter storms contribute ~ 66 - 73 % of annual infill at Caminada BA in spring 2018 and ~ 10– 20 % at Sandy Point BA in spring 2019. Further, Sandy Point BA cores retrieved in fall 2019 revealed 0.3 – 0.5 m of 7Be-dead sediment with a fining-upward pattern, interpreted as rapid deposition following Hurricane Barry. These results are in contrast with numerical models, as infill rates are slower than predicted for both sandy and muddy settings.

Committee Chair

Wilson, Carol

DOI

10.31390/gradschool_theses.5210

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