Turbidity current processes and products in the fjords of British Columbia (Canada)

Abstract

Turbidity currents are volumetrically the most important process to transfer land-derived sediments offshore. Fast-moving, powerful turbidity currents can carve submarine canyons that rival some of the largest canyons on land. These submarine canyons feed deep-sea fans, which are the largest sediment accumulations on Earth. Such huge sediment accumulations sequester organic carbon across geologic times and are thus thought to play a role in the global carbon cycle. Additionally, turbidity currents pose a hazard for the ever-expanding seafloor infrastructure network that underpins our energy supply and global communication. Despite the global significance of turbidity currents and their deposits, the link between flow processes (e.g. triggering mechanisms, seafloor erosion) and depositional products is still poorly constrained. This is due to the difficulty in monitoring flows and sampling deposits directly resulting from flows. In this thesis I present direct measurements of turbidity currents and their deposits in two fjords of British Columbia (Canada), named Howe Sound and Bute Inlet. Glacier-fed rivers flow into both fjord heads; hence, these are excellent sites to study the transport and fate of particles from land to sea. This thesis uses these new measurements to answer three research questions. 
First how do extremely dilute river plumes generate turbidity currents? The current paradigm is that rivers need to exceed a specific sediment concentration (e.g. 1 g/L) to directly generate turbidity currents at their mouths. Chapter 2 shows that rivers with extremely dilute plumes (0.07 g/L) can generate turbidity currents under certain conditions, e.g. low tide, relatively high river discharge, a turbidity maximum being forced out on a steep delta front and the availability of loose-packed fine sediment on the seabed. These results were found using direct observations of the Squamish river plume flowing into Howe Sound. This study suggests that a much wider range of rivers than previously thought can directly generate turbidity currents on our planet. The possibility of triggering turbidity currents by a much wider selection of rivers has strong implications for understanding the fate of terrestrial particles (including organic carbon and pollutants) into the sea. 
Second, what is the diagnostic deposit facies/architecture resulting from crescentic bedforms formed by supercritical turbidity currents? Modern observations show that submarine channels can be sculpted by supercritical (i.e. thin and fast) turbidity currents. Such flows are likely to produce upstream-migrating bedforms with a crescentic planform. The depositional signature of such beforms is currently only constrained by outcrop observations and experimental models. These experiment and outcrop studies indicate different sedimentary structures form from supercritical flows. In order to reconcile this apparent discrepency, Chapter 3 presents the first observations that directly link supercritical (i.e. thin and fast) turbidity currents in the real-world with their deposits. To establish the link between flow process and resulting sedimentary deposit, I combine seafloor flow measurements, time-lapse seabed mapping and sediment cores. Using these data collected in a submarine channel on the Squamish delta (Howe Sound), I show that supercritical turbidity currents initially produce back-stepping beds of sand, as seen in experimental studies. However, the field data shows how these back-stepping beds are subsequently partially eroded by later flows, resulting in the scours filled of massive sands that are commonly observed in the depositional record. Accurate recognition of supercritical flow deposits in the depositional record is important for oil and gas reservoir characterisation, past environmental reconstruction and a general understanding of sedimentary systems. 
Third, how is young terrestrial organic carbon preserved in turbidites? Efficient burial of terrestrial organic carbon in marine sediments leads to a net CO2 removal from the atmosphere, thus regulating climate over geological timescale. Existing studies suggest that most organic carbon is buried in association with fine sediments because clay minerals are able to bind organic matter and shield it from degradation. In contrast with these studies, Chapter 4 shows that young, coarse woody debris accounts for the majority (>70 %) of terrestrial organic carbon sequestered in sandy turbidity current deposits. These coarse organic-rich layers are rapidly buried under turbidity current mud caps, reducing their direct exposure to oxygen and thereby increasing burial efficiency. These findings are based on data collected in the sandy submarine channel of Bute Inlet. The data consist of 1) flow observations, 2) deposit sampling, 3) analysis of the composition and ages of the organic carbon contained within these deposits. These results provide important insights into the effect of hydrodynamic sorting of young terrestrial organic carbon by turbidity currents. Turbidity currents dominate sediment transport in submarine environments; therefore these results suggest that previous global organic carbon burial budgets may have been underestimated. 
Finally I conclude that observations, particularly from the deep sea, are needed to test whether these scientific findings collected in fjords are applicable to the much larger deep-sea turbidity current systems.

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