Assessing the influence of consolidation in marine sediment cores for Blue Carbon quantification

Abstract

Blue Carbon traditionally refers to carbon buried and stored in coastal or terrestrial environments such as mangrove forests and seagrass meadows. However, marine sediments, like clays and sands, found on continental shelves, are increasingly being recognised as important Blue Carbon settings and are being included in national marine management plans.  To fully understand the importance of such environments and its potential to mitigate against climate change, a thorough understanding of the quantification of the carbon stored is required. This is performed through the analysis of marine sediment cores.  Marine sediment cores are typically extracted by forcing a PVC pipe into the seafloor through varied methodologies. During the extraction process, the method itself can have adverse effects  on the sediment, including causing changes in profile integrity resulting in a shortening of the core profile through consolidation. This can increase the dry bulk density of the sediment, which is an important parameter in calculating carbon stock, resulting in an overestimation of results. There is a paucity of studies regarding the impacts these effects can have on the quantification of Blue Carbon in marine sediments.  In this study, a set of cores were gathered from the same geographical location using three different offshore coring techniques, namely: gravity coring, vibro coring and box coring. These techniques are standard in offshore site investigations. Samples from these cores were used to assess the extent of the impact of consolidation on quantifying carbon stock measurements in marine sediments through several geotechnical techniques. These include evaluating parameters that directly influence consolidation in marine sediments, such as moisture content, Atterberg limits and particle size. Additionally, compressibility measurements, through oedometer testing, can help elucidate to what degree compaction may have taken place. Carbon stocks are calculated using total organic carbon and loss on ignition measurements, which will be compared across profiles from different coring techniques. Similarly, accumulation rates calculated using gamma spectrometry (e.g. 210Pb) allow for comparison across core profiles. These tests were performed across the three offshore coring techniques to determine which method of core extraction is optimal for Blue Carbon quantification.