Correcting organic carbon accumulation rates in contaminated shelf sediments

Shelf sea sediments are integral of the global carbon cycle, accumulating vast amounts of organic carbon due to their large spatial extent and depositional areas. Accurate quantification of organic carbon accumulation rates (OCARs) is crucial for assessing sedimentary carbon stocks and their vulnerability. While lead-210 (²¹⁰Pb) geochronology is the traditional method for estimating sediment accumulation rates (SARs), and ultimately OCARs, its application can be compromised by physical and chemical disturbances, particularly the presence of industrially derived naturally occurring radioactive material (NORM) and heavy metals. This study demonstrates that enhanced levels of heavy metals released during oil extraction interfere with direct gamma spectrometric measurements, leading to a significant overestimation of OCARs. At sampling locations near the decommissioned oil and gas platform, North West Hutton, enhanced levels of heavy metals, including barium (Ba), strontium (Sr) and lead (Pb), in sub-surface sediments resulting in OCAR overestimations of over 50%. Two alternative methodologies are proposed. Firstly, the application of relative correction factors derived from bulk chemical composition; and secondly, the measurement of polonium-210, a decay product of ²¹⁰Pb, via alpha spectrometry. Our findings highlight the inappropriateness of direct gamma spectrometry for estimating OCARs via ²¹⁰Pb dating in marine environments impacted by industrially derived heavy metal and NORM discharges. This study also describes a novel approach for refining data inputs to improve the reliability of SAR and OCAR estimations. The impact of NORM and heavy metals needs to be taken into consideration when undertaking SAR/OCAR studies in potentially polluted environments to avoid over or underestimation.