Comparison of Pretreatment Methods for Organic-matter Removal and their Effects on the Hydrogen Isotope (d2H) Composition of Kaolinite

The hydrogen isotopic composition (delta H-2) of authigenic clay minerals has been used extensively in paleoclimate studies. The separation of clay minerals from sediments/soils, using various chemicals, is a prerequisite for isotope ratio measurements, where carbonate, Fe-(oxyhydr)oxides, and organic matter are removed successively from the sediments for a greater clay yield. The commonly adopted organic matter-removal method using hydrogen peroxide (H2O2) is thought to either alter directly the pristine delta H-2 values of the smectite clay minerals or to introduce organic hydrogen-bearing impurities through the ineffective removal of organic matter. The objective of the present study was to test whether H2O2 treatment can alter the delta H-2 values of kaolinite (Kln) by comparing two organic matter-removal methods, namely, H2O2 and disodium peroxodisulfate (Na2S2O8) combined with a neutral buffer. In doing so, kaolinite-rich, old (similar to 56 Ma) sediment samples and pure kaolinite internal laboratory reference materials were used to understand the effectiveness and suitability of the above-mentioned methods in clay-sample preparation for delta H-2 measurements. The delta H-2 values of the H2O2-treated aliquots show smaller delta H-2 values than those for the Na2S2O8-treated aliquots. Estimated ambient water delta O-18 values (-4 parts per thousand ) from the Na2S2O8-treated aliquots agreed well with the bio-phosphate (fish vertebrae) based environmental water delta O-18 estimation (-3.3 parts per thousand ). The present study indicated, therefore, that delta H-2 values obtained after Na2S2O8 treatment are likely to be more realistic for paleoclimate reconstruction.