Ground penetrating radar surveying and sediment coring analysis of a post-glacial lake, Eastern Newfoundland

Chen, Jianguang (2018) Ground penetrating radar surveying and sediment coring analysis of a post-glacial lake, Eastern Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Ground penetrating radar (GPR) is a relatively newly developed high-frequency electromagnetic technique that has been widely used in the shallow subsurface investigation for the last few decades. Recently, a GPR survey of a small organic-rich post-glacial lake (Grassy Pond) in Eastern Newfoundland shows significant continuous laminations within the lake sediments in the GPR profiles. Since there have been very few GPR stratigraphy studies of lacustrine sediments, the main focus of this project is on the correlation between the sediment stratigraphy and the GPR sub-bottom profiles. Secondary interests are: to estimate the carbon content of a typical small inland lake to help assess how such bodies have contributed to the carbon budget since the last glaciation; and to investigate chemical variability within the sediments. The work in this project includes GPR surveying, sediment coring, and sediment physical, geochemical and chronostratigraphic data acquisition, calibration and correlation. First of all, 50 and 100 MHz GPR surveys were completed on Grassy Pond when the lake surface was frozen in the winter. Bathymetric and depth-to-bedrock maps were created from the GPR profiles. Based on these two maps, a sediment distribution map was also created and this was used to choose sediment coring locations. Four sediment cores were collected by using a rod-driven piston corer, and additional GPR profiles were collected over these core locations. The cores were then scanned by a Multi-Sensor Core Logger (MSCL) to determine the physical properties. After that, the cores were sub-sampled and geochemically analyzed by ICP-OES. Selected sediment samples were also analyzed for C and N contents and isotopes, and radiocarbon dated. Lastly, the linkage was made between the geophysical and geochemical data, and a simple GPR forward model was created based on the sediment physical properties to enhance the data interpretation and correlation. The results show that the lake sediments of Grassy Pond are highly-organic and water-rich. Forward models of EM wave reflections show that the laminated GPR reflections within the sediments are caused by variations in water content. The geochemical analysis shows that the water content is anti-correlated to the lithic elemental concentrations. Since the lithic inputs can reflect past climate changes, we suggest that paleoclimatic changes may ultimately be responsible for the laminations seen in the GPR profiles. The carbon budge of Grassy Pond is calculated based on the sediment volume and average carbon content of the sediments, and it is estimated as 29 kg/m², which is significantly higher than forest soils. Besides these major results, one of our basal sediment samples is dated back to 8.6 radiocarbon years ago, which corresponds to the end of last glaciation in the same region of Newfoundland. The sediments of Grassy Pond are found to be highly enriched in arsenic (As) and molybdenum (Mo), likely associated with the erosion of iron oxide minerals in the surrounding land, as Grassy Pond overlies the alteration zone of a gold prospect. In the deepest sediment core, many elements show a concentration peak near 6.3k years ago when the regional climate started to become drier, and erosion rates increased.

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