Quantifying the effects of offshore infrastructure on shelf sediment blue carbon dynamics

The effect of offshore infrastructure on shelf sediment blue carbon dynamics, is a key knowledge gap for blue carbon science, and shelf systems’ ability to mitigate climate change. This thesis addresses this research gap, by quantifying sediment type, organic carbon stocks, accumulation rate, source and vulnerability in sediments surrounding two decommissioned oil and gas platforms (North West Hutton and Miller) in the North Sea. Overall, organic carbon stocks and sediment type varied distinctly between sites, with higher carbon stocks closer to Miller, but no relationship with distance at North West Hutton. Accumulation rates were refined to account for heavy metal contamination, which provides a new methodological framework of correcting for this issue in future blue carbon assessments. Using a novel binary mixing model application and hydrocarbon analysis, both sites presented organic carbon enrichment from anthropogenic sources, namely hydrocarbons, within close proximity (50 m) to the decommissioned sites. This highlights that sources of organic carbon other than marine may be present within shelf sediment stocks. A key recommendation from these studies is to take these measurements throughout the lifecycle of infrastructure; from pre and post construction, during operation, and pre and post decommissioning to better assess the varying impact of infrastructure. Finally, this thesis presents a methodological exploration to determine organic carbon mineralisation rates from acute disturbance events in sediments using oxygen consumption rates as a proxy. This provides a baseline for addressing the key question of carbon vulnerability from increasing infrastructure induced disturbance in a warming climate.