Elemental and stable isotope variations of organic matter from a terrestrial sequence containing the Cretaceous/Tertiary boundary at York Canyon, New Mexico

Abstract

Elemental and isotopic composition of organic matter from a terrestrial sequence including the palynological Cretaceous/Tertiary (K-T) boundary together with an Ir anomaly at York Canyon, New Mexico, record information about paleoclimatological and environmental conditions. Six layers of coal, carbonaceous shale and mudstone with high contents of organic material were selected for analysis. A Late Cretaceous coal bed 10 m below the K-T boundary and an Early Tertiary coal bed containing the K-T iridium anomaly at its base were sampled intensively. In the lower bed, the isotopic ratios 13C/ 12C, 15N/ 14N,andD/H and the elemental ratios C/N andN/H, all varied sympathetically with one another over depth. In contrast, in the upper coal layer, only the D/H,C/N,andN/H ratios showed some coupling. Immediately above the K-T boundary, the δ 13C values displayed a long-term shift of 1.8‰ to more negative values, while the hydrogen isotope ratios in these samples did not change significantly. We interpret the covariations in both coal layers as sympathetic responses of the isotopic and elemental ratios to climatic and environmental changes, as have been observed for younger sedimentary organic matter. The long-term δ 13C shift during the early Tertiary is similar to the trend observed for δ 13C values of marine carbonates. Our data thus support the proposal that the carbon cycle was perturbed globally by the effects of a drastically decreased marine bioproductivity along the K-T transition. The uncoupling of variations in the climatically sensitive isotopic and elemental ratios seen in the Early Tertiary coal bed provides evidence for major geochemical and environmental changes in the York Canyon area at the end of the Mesozoic. On the other hand, the constancy of δD values in the organic matter deposited at and above the K-T boundary gives no indication of significant changes in the hydrologic/climatic regimes as recorded in the D/H ratios at the site for several thousand years following the event which produced the high Ir concentrations. Our results provide constraints on models that have been advanced to explain that event and its consequences.