Evolution of Antarctic Tabular Icebergs

Abstract

The aim of this study was to investigate and quantify the role of the various processes affecting a large tabular iceberg during a typical lifecycle from calving to final decay. The thesis comprises three publications, each addressing a different aspect of the evolution of the tabular iceberg A-38B, which calved from the Ronne Ice Shelf in October 1998.The first publication is focussed on inherent ice dynamics. To investigate the relevance of strain thinning to iceberg evolution, a numerical ice shelf model was adapted to the iceberg case. The interaction with atmosphere and ocean was included in the model, but parameterised in a simple way and estimated on the base of measurements and external model data. A five year simulation of the evolution of iceberg A 38B showed that basal melting is the primary cause for change of iceberg geometry during drift, whereas strain thinning is only relevant in cold areas where basal melting is low. Thus, the second part of the thesis concentrates on an improved basal melting approach. In the drift period between the entering of the Scotia Sea in March 2003 until the grounding near South Georgia in 2004, freeboard changes of A 38B were observed from analysis of ICESat Laser altimeter profiles. The iceberg melt rate was then fitted by varying the turbulent exchange parameters for temperature and salt at the ice ocean boundary to match the altimeter results. The data analysis indicated, that the iceberg passed through three melting regimes during its drift, each characterised by a different magnitude of turbulent exchange between iceberg and ocean depending on drift conditions. The analysis showed also that drifting tabular icebergs export more melt water to the Scotia Sea than previously assumed. To shed light onto the final stage of iceberg evolution, the third publication is focussed on the investigation of the grounding process of A-38B and related seismic events. The characteristic spectrograms of such signals probably represent excitation of elastic modes of the ice masses by stick-slip friction. Data records from the seismic station on South Georgia Island also comprised harmonic tremor events generated by the floating iceberg A-43G. This second class of iceberg-generated tremor is probably excited by fluid flow through a major rift structure and is related to particular current regimes around the iceberg.