Comparison of Pretreatment Methods for Organic-matter Removal and their Effects on the Hydrogen Isotope (δ2H) Composition of Kaolinite

The hydrogen isotopic composition (δ 2 H) 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 (H 2 O 2 ) is thought to either alter directly the pristine δ 2 H 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 H 2 O 2 treatment can alter the δ 2 H values of kaolinite (Kln) by comparing two organic matter-removal methods, namely, H 2 O 2 and disodium peroxodisulfate (Na 2 S 2 O 8 ) combined with a neutral buffer. In doing so, kaolinite-rich, old (~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 δ 2 H measurements. The δ 2 H values of the H 2 O 2 -treated aliquots show smaller δ 2 H values than those for the Na 2 S 2 O 8 -treated aliquots. Estimated ambient water δ 18 O values (−4‰) from the Na 2 S 2 O 8 -treated aliquots agreed well with the bio-phosphate (fish vertebrae) based environmental water δ 18 O estimation (−3.3‰). The present study indicated, therefore, that δ 2 H values obtained after Na 2 S 2 O 8 treatment are likely to be more realistic for paleoclimate reconstruction.