THE EFFECT OF ORGANIC MATTER AND THE H2O2 ORGANIC-MATTER-REMOVAL METHOD ON THE δD OF SMECTITE-RICH SAMPLES

Abstract

The H2O2 organic-matter-removal method (Jackson, 1985) used routinely to remove organic matter from sediments in preparation for δD analyses, was found to alter the δD of the smectite standard, H26, up to -8%% and of Gulf Coast shale samples up to -13%%. Before the H2O2 treatment, analytical error for the δD analyses was low ranging from +/-1%% to +/-3%%. After treatment the error increased ranging from +/-4 to +/-13%% for 46% of the treated samples. This decreased reproducibility suggests the deuterium is no longer evenly distributed within the samples, and that the shift in δD may be the result of retention or removal of a hydrogen bearing phase.Total organic carbon (TOC) analyses of the shale samples before and after treatment with H2O2 show that up to 0.80 wt.% TOC remains in the samples even after the samples appear bleached, which according to Jackson (1985) indicates complete removal of the organic matter. An additional 24 h of treatment (two more additions of H2O2) or more, depending on the initial TOC content, was necessary beyond bleaching of the samples to achieve a final TOC of 0.10 wt.% or less.The δD values of the untreated Gulf Coast shale samples with 1 wt.% TOC or less, lie within a narrow range, -38 to -32%%, and within this range lack any correlation with TOC, suggesting the presence of the organic matter is not effecting the δD values. A single sample with TOC greater than 1 wt.%, PB2-4762 m with 10 wt.% TOC, has a δD value of -56%% outside of the range of the low TOC samples. Organic matter isolated from this high TOC sample was added incrementally to four of the other shale samples. δD analyses of these shale-organic matter mixtures show a decrease of 0.23%% with each mole % increase in hydrogen from organic matter. By using this slope to calculate the δD of PB2-4762 m without the organic matter gives a value of -40%%. This new value is within 2%% of the range of values for the low TOC shales, -38 to -32%%, and is within the analytical error of the δD analyses.