The effect of organic matter and the H2O2 organic-matter-removal method on the δD of smectite-rich samples

The H2O2 organic-matter-removal method (Jackson, 1985) used routinely to remove organic matter from sediments in preparation for deltaD analyses, was found to alter the deltaD of the smectite standard, H26, up to - 8 parts per thousand and of Gulf Coast shale samples up to - 13 parts per thousand. Before the H2O2 treatment, analytical error for the deltaD analyses was low ranging from +1 parts per thousand to 13 parts per thousand. After treatment the error increased ranging from +/-4 to +/-13 parts per thousand 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 deltaD 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 deltaD values of the untreated Gulf Coast shale samples with I wt.% TOC or less, he within a narrow range, -38 to -32 parts per thousand, and within this range lack any correlation with TOC, suggesting the presence of the organic matter is not effecting the deltaD values. A single sample with TOC greater than 1 wt.%, PB2-4762 m with 10 wt.% TOC, has a deltaD value of -56 parts per thousand 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. deltaD analyses of these shale-organic matter mixtures show a decrease of 0.23 parts per thousand with each mole % increase in hydrogen from organic matter. By using this slope to calculate the deltaD of PB2-4762 m without the organic matter gives a Value of -40 parts per thousand. This new value is within 2 parts per thousand of the range of values for the low TOC shales, -38 to -32 parts per thousand, and is within the analytical error of the deltaD analyses. Copyright (C) 2000 Elsevier Science Ltd.