Abstract
Current understanding of carbon dioxide draw down over Fiordland, New Zealand does not align with atmospheric observations and modelling. The region appears to be sinking significantly larger quantities of carbon than expected and the mechanisms driving this sink are unknown. Mass wasting (landslides and turbidity currents) has been hypothesised to account for the discrepancy. This thesis conducted ramped pyrolysis oxidation (a novel radiocarbon dating technique) across two mass wasting events preserved in the fjord sedimentary record of Dusky Sound.
It was found that under inter-event conditions, organic material is transported longitudinally through the fjord catchment over 400-500 radiocarbon years (yr) prior to deposition in the submarine basin. Stable isotope and thermochemical stability data highlighted material is relatively homogeneous in composition, suggesting preferential transformation and mobilisation attributed to hydrodynamic sorting and dissolved organic carbon/particulate organic carbon (DOC/POC) leaching. In comparison, the mass wasting sediment appears to have been laterally transported into the fjord from the adjacent alpine catchment and submarine fan. This material compositionally heterogeneous and thermochemically stable relative to the sediment underlying the mass wasting deposit due to wholesale transport of the soil package, eroding carbon that is resistant to mobilisation under inter-event conditions.
The ages of the organic matter within the mass wasting deposit relative to the underlying inter-event sediment appear to depend on the timing of the previous event. Most of this material is homogeneous in age and directly post-dates the timing of the underlying mass wasting event. This suggests the entire organic profile was flushed during the previous mass wasting and rejuvenation occurred rapidly, re-establishing soil stocks on the landscape. There is a much older fraction present within these horizons which is hypothesised to be reworked material derived from much older mass wasting events.
The ramped pyrolysis results demonstrate the rapid growth of rejuvenating slopes following mass wasting which likely drive the anomalous draw down of carbon dioxide over the region. Ultimately, this research provides a unique application of the ramped pyrolysis oxidation technique and highlights that carbon cycling is episodic with fluxes driven by mass wasting within Fiordland.