Abstract
Fjords are widely recognised as hotspots for carbon burial and those in New Zealand are no exception. The vulnerability of these systems in a warming world is yet to be tested, but must be understood to help predict how the Earth’s climate will evolve. This thesis provides a framework from which controls on the burial process can be better determined.
Better understanding of controls on carbon burial, such as regional precipitation, wind patterns and water circulation has resulted from significant research. Fiordland, in southwestern New Zealand, represents an important site for studying these processes as it has minimal human-induced modification and precipitation is primarily controlled by the Southern Hemisphere Westerly Winds, a wind field circulating around Antarctica. To date, sediment cores have been collected around Fiordland, providing insight into sedimentation processes and enabling measurement of organic carbon accumulation rates during the late Pleistocene and Holocene. Organic carbon accumulation rates have been used to provide an estimate for modern carbon burial rates in New Zealand’s fjords of 35.8kt/yr.
Describing carbon burial processes on a longer time scale requires understanding deeper and older sediments than those that have already been sampled. It has been proposed to obtain long cores - in the hundreds of metres - to constrain these processes better. This requires understanding the sediment package along the length of the fjords to select a site for these long cores. In this thesis, short sediment cores are analysed in conjunction with multi-channel seismic data and bathymetry in order to describe the sediments of Dusky Sound.
Seismic data were collected along the axes of main basins in Dusky Sound, supplemented by crossing lines at sites that were shown to have thick sediments from existing single-channel seismic data. Multibeam bathymetric data were used to identify seafloor features indicating the prevalence of different processes occurring in different parts of Dusky Sound. Seismic data were interpreted with the assistance of a novel seismic reflection simulation software that has been extended here to be compatible with migrated seismic reflection data. Piston cores were non-destructively scanned, measuring down-core elemental composition and physical properties. Elemental ratios and physical properties were used as proxies to characterise processes occurring in the vicinity of the core location. Organic carbon and carbonate samples were radiocarbon dated to calculate linear sedimentation rates and to provide a chronology over which events in the core occurred.
Synthesis of the different datasets has revealed several key findings. Glacial extent in Dusky Sound 13kyr ago was found to be more significant than currently understood in literature. A relative sea level curve for Fiordland has been amended and extended and found to relate to existing eustatic sea level data through a linear uplift rate with no correction for glacioisostatic rebound. Additional relative sea level constraints representing the marine incursion of two basins in Dusky Sound have been considered but not incorporated due to poor constraints. Basins in Dusky Sound were found to exhibit trade-offs between carbon burial potential, records of glacial extent and sediment thickness. Consequently two complementary locations have been identified for sediment drilling. Additional sites are suggested to obtain supplementary short cores which will allow for testing of hypotheses made in this thesis as well as better dating of constraints on sea level and glaciation events.
Long cores obtained from these sites will allow for direct measurement of carbon burial rates and provide paleoclimate information in Fiordland since and during the last glaciation. Understanding carbon burial processes in Fiordland will provide crucial data for understanding how the Earth’s climate will evolve in the future.