Improving Marine Sediment Carbon Stock Estimates: The Role of Dry Bulk Density and Predictor Adjustments

Abstract. Continental shelves are critical for the global carbon cycle, storing substantial amounts of organic carbon (OC) over geological timescales. Shelf sediments can also be subject to considerable anthropogenic pressures, offshore construction and bottom trawling for example, potentially releasing OC that has been sequestered into sediments. As a result, these sediments have attracted attention from policy makers regarding how their management can be leveraged to meet national emissions reductions targets. Spatial models offer solutions to identifying organic carbon storage hotspots; however, data gaps can reduce their utility for practical management decisions. Regional spatial models of OC often use global scale predictors which may have biases on regional scales. Moreover, dry bulk density (DBD), an important factor in calculating OC stock from sediment OC content, has comparatively few data points globally. We compared two spatial models of OC stock in the Irish Sea, one using unadjusted predictors and a previously used method to estimate DBD, and another incorporating bias-adjusted predictors, from in situ data, and a machine learning-based DBD model, to assess their relative performance. The adjusted model predicted a total OC reservoir of 46.6 ± 43.6 Tg within the Irish Sea, which was 31.4 % lower compared to unadjusted estimates. 70.1 % of the difference between adjusted and unadjusted OC stock estimates was due to the approach for estimating DBD. These findings suggest that previous models may have overestimated OC reservoirs and emphasizes the influence of accurate DBD and predictor adjustments on stock estimates. These findings highlight the need for increased in situ DBD measurements and refined modelling approaches to enhance the reliability of OC stock predictions for policy makers. This study provides a framework for refining spatial models and underscores the importance of addressing uncertainties in key parameters to better understand and manage the carbon sequestration potential of marine sediments.