Numerical modelling of sediment delivery from tidewater glaciers to the marine environment

• Main Author: Mugford, Ruth Isobel
• Other Authors: Dowdeswell, Julian
• Published: University of Cambridge 2008
• Subjects: 550
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612173

Glaciated fjords are dynamic sedimentary environments with high deposition rates, thus providing high-resolution sedimentary records of deglaciation. This thesis introduces two numerical models, SedBerg and SedPlume, developed to simulate marine sedimentation from tidewater glaciers for the dominant transport processes of icebergrafting and glacial meltwater plumes. Icebergs calved from tidewater glaciers contain sediment, which is released as the icebergs melt. The SedBerg Model utilises Monte Carlo based techniques to simulate the formation, drift and melt of a population of icebergs, and the subsequent deposition. Subaqueous and subaerial melt of the icebergs result in a continuous rainout of sediment with occasional sediment dumping from overturning events. Underlying the model are a number of parametric probability distributions to describe the stochastic behaviour of iceberg formation and dynamics. Parametric values of the probability density functions are found using maximum likelihood estimation from field observations. Turbid meltwater emerging from beneath a glacier into a fjord rises as a buoyant forced plume due to salinity and temperature contrasts with the ambient fjord water. The SedPlume Model utilises an integral model formulation for the conservation of volume, momentum, buoyancy and sediment mass along the path of a turbulent, entraining plume injected into stably stratified ambient fluid. The resulting system of non-linear ordinary differential equations are solved numerically using an adaptive stepsize fourth-order Runge-Kutta method. When the plume reaches the surface, it is treated as a radially spreading surface gravity current, for which exact solutions exist for the mass flux of sediment deposited. A case study is examined for each model. The SedBerg Model is applied to Kangerdlugssuaq Fjord in East Greenland, as an example of an iceberg-dominated depositional environment. The sedimentation rate due to iceberg rafting is simulated for Kangerdlugssuaq Fjord over the last 1500 years, with climatic, oceanographic and glacial conditions estimated from a mixture of present day measurements and inferences from the sedimentary record. The SedPlume Model is applied to McBride Inlet in Alaska, as an example of a glacial meltwater-dominated sedimentary environment, with input data and ice-proximal sedimentation rates taken from published data. The influence of various parameters on sediment deposition is investigated for each of the two models. For the SedBerg Model, the parameters considered are: iceberg size; temperature and depth of thermocline; and seeds of the random number generator for water and wind velocities. For the SedPlume Model, the parameters studied are: subglacial conduit radius and initial meltwater velocity; initial sand fraction; and ambient density gradient. The SedBerg and SedPlume Models enable quantitative assessments of the role and importance of different environmental and climatic conditions on the rate of deposition and the formation of distinctive marine deposits by tidewater glaciers.