Methodologies Developed to Identify the Source of Mobile Boom Clay Organic Matter

The Boom clay formation is studied in the TRANCOM -II project and contains relatively large levels of organic matter (ca. 2 -5 %). The largest fraction of this organic matter (OM) consists of kerogen of terrestrial and marine origin. Previous measurements based on piezometer waters indicated that only a small fraction (0.01%) of the total OM is present in the interstitial solution. Piezometer pore water was sampled at specific stratigraphic levels within the clay formation and contains around 100 to 200 mgC/L, but might be up to 400 mgC/L. Up to now, the Dissolved Organic Carbon (DOC) can not be linked to the neighbouring Total Organic Carbon (TOC) as no correlation exists in their concentrations. Leaching experiments of the bulk solid Boom clay phase with inorganic electrolyte solution resulted in the dissolution of part of the organic matter pool. This soluble fraction contains a higher amount of total organic carbon than the Boom clay pore water, and the UV280 absorbance is linearly correlated to TOC. Leached organic matter is predominantly composed of humic acids (HA) and (just as the pore water organic matter) exhibits a bimodal distribution. Upon equilibrating {sup 14}C containing solutions with different amounts of Boom clay, distribution coefficients were observed to be governed by similar reaction equilibria, but were about 200 times smaller, than the distribution of the organic matter calculated on a weight basis. This result indicates that part of the organic matter pool is not involved in the {sup 14}C distribution. Apart from chemical leaching, small dissolved organic matter molecules are also brought into solution by applying thermic stress to the kerogen fraction. These molecules are predominantly hetero - aromatic products (oxygenated and oxygenated/nitrogenated compounds), e.g. fatty acids. (authors)