Semester of Graduation

Fall 2018

Degree

Master of Science (MS)

Department

Geology and Geophysics

Document Type

Thesis

Abstract

Many studies of the Mississippi River Delta (MRD) have shown historic declines in sediment load over the last few decades. Recent studies also reported that ~50% of the suspended load during floods is sequestered within the delta. While the impact of declining sediment load on wetland loss is relatively well documented, submarine sedimentary processes on the delta front during this recent period of are understudied. To better understand modern sediment dispersal and deposition across the Mississippi River Delta Front, 31 multicores were collected in June 2017 from locations extending offshore from the main river outlets in water depths of 25-280 m. Core locations were selected based on multibeam bathymetric and morphological data collected by the USGS in May 2017; the timing of collection coincided with the end of annual peak discharge on the Mississippi River. This collaborative multi-agency survey is the first to study delta-front sedimentary processes regionally with a wide suite of geophysical and coring tools. Core locations included the dominant depositional environments: mudflow lobes, gullies, and undisturbed prodelta. Nine multicores were subsampled at 2 cm intervals and analyzed for Beryllium-7 activity and 4 cores were subsampled for excess Thorium-234 activity via gamma spectrometry. Grain-size analysis and X-radiography were performed on cores as well. Results indicate a general trend of declining Be-7 activity with increasing distance from source, and in deeper water. Inshore samples near Southwest Pass show the highest Be-7 inventories and deepest penetration depth of Be-7 into the sediment (24-26 cm), which is a preliminary indicator of rapid seasonal sedimentation. Sediment focusing was determined at mudflow gully and lobe locations throughout the delta front. X-radiography revealed that the likelihood of preserving bedding layers occurs proximal to river-mouth while a trend is observed of increased bioturbation offshore. Sediment dispersal remains detectable offshore from all three major river outlets, despite overall decline of sediment load in recent decades, and pronounced declines for South Pass and Pass a Loutre. Future research should focus on relationships among changing sediment loads, dispersal patterns, and sediment transport by mudflows, which are an important process for dispersal after initial deposition.

Date

10-16-2018

Committee Chair

Bentley, Samuel

DOI

10.31390/gradschool_theses.4807

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