Determination of Blue Carbon accumulation rates in sediments impacted by legacy oil and gas extraction activities

Continental shelf sediments are vast areas, some regions of which accumulate and store organic carbon. However, these regions are increasingly impacted by anthropogenic pressures, particularly from infrastructure related to energy extraction. While determining the standing stock of carbon within sediment provides a useful snapshot for current carbon budgets, directly measuring the accumulation potential allows for assessments of how inputs which drive carbon stocks will vary temporally and defines the climate regulation service (aka ‘blue carbon’) in space. However, estimating carbon accumulation potential of sediments can be confounded by anthropogenic activity, particularly around oil and gas extraction activities.

This study describes carbon stocks, sediment type and carbon accumulation rates (CARs) in sediment surrounding the North West Hutton decommissioned platform in the northern North Sea. By accounting for heavy metals derived from ICP-MS to create correction factors, previous estimates of CARs from gamma spectrometry have been corrected and CARs close to North West Hutton, which were previously undetermined, have been calculated. Results show that CARs determined by gamma spectrometry alone are consistently lower than those corrected by heavy metal attenuation factors and also using polonium-210 measurements from alpha spectrometry analysis. This work indicated that CARs could be underestimated in regions directly impacted by heavy metals associated with the extraction activity.

This novel method provides an opportunity to determine CARs in other marine areas impacted by similar chemical pollution pressures. Many coastal regions across Europe are directly affected by legacy or operational industrial discharges of waste containing enhanced levels of heavy metals and natural radioactivity, also known as NORM, in particular from mineral mining, extraction and processing activities. This work highlights  the potential underestimation in the offshore environment, but if this approach is applied to coastal sites where accumulation rates are generally considerably higher, and impacted by industry, this could have widespread implications for service assessments and blue carbon accounting.