Antiphase hydrogen and oxygen isotope periodicity in chert nodules

摘要

Oxygen and hydrogen isotope analyses were made of Jurassic-age chert nodules from the Holy Cross Mountains, SE Poland, along radial transects at high spatial resolution. There is a radial "sigmoidal" periodicity for both isotope ratios, but the two are out of phase, with high deltaD values corresponding to low delta(18)O values. Periodicity for a 100- to 120-mm diameter nodule is approximately 16 mm, increasing slightly toward the rim, with amplitudes approaching 20 and 3.0%o for hydrogen and oxygen, respectively. The combined hydrogen-oxygen isotope data for one nodule fall on a published curve for chert forming in equilibrium with seawater (Knauth and Epstein, 1976); the range of delta values corresponds to temperature variations of similar to10degreesC. Data for a second chert fall on a subparallel deltaD-delta(18)O line with deltaD values that are almost 50%o lower. The deltaD-delta(18)O patterns for the nodules cannot be explained by periodic mixing of meteoric and ocean water because the hydrogen and oxygen isotope data are out of phase. Two possible explanations for the antiphase periodicity are (a) cyclical temperature variations, perhaps related to an unstable convection system (e.g., Bolton et al., 1999), and (b) self-organizing catalytic precipitation (e.g., Wang and Merino, 1990). The systematic isotopic variations are difficult to explain by diagenesis and strongly suggest that primary isotopic compositions are preserved. The isotopic data provide important information on the thermal history of the sedimentary basin, if temperature variations are the cause of the isotopic periodicity. Copyright (C) 2002 Elsevier Science Ltd. [References: 28]