Sedimentary blue carbon around decommissioned oil and gas platforms in the North Sea.

Shelf sea habitats contain significant sedimentary carbon stocks, due to their large spatial extent, and act as key ecosystems for global carbon cycling. These shelf sediments are subject to a range of anthropogenic pressures including seabed trawling, climate change and the introduction of man-made structures (MMS), such as oil and gas platforms. In the North Sea, following their production lifespan, MMS are required to be mandatorily decommissioned under OSPAR Decision 98/3. Understanding the impact of decommissioned MMS on sedimentary carbon stores is a significant research gap, as these platforms are widespread throughout many shelf seas, with many now exceeding or approaching the end of their designed lifespan. In this study we quantify carbon stocks, organic and inorganic carbon content, and sediment characteristics from shelf sediment cores taken at increasing distances (50-3200 m) away from two decommissioned North Sea oil and gas platforms, Miller and North West Hutton. Carbon accumulation rates were also determined for sediments collected around North West Hutton. Organic carbon content at Miller was highest within 50 m (24.55 ± 2.49 mg/g of sediment) of the decommissioned platform and significantly lower across all other sampling distances. Conversely, organic carbon content at North West Hutton did not vary greatly with distance; however, a significant decrease was seen at 200 m. These findings highlight that carbon dynamics around decommissioned oil and gas platforms are site-specific. Moreover, the absence of pre-decommissioning data limits our ability or disentangle the effects of decommissioning on sedimentary carbon.