Geochemical anomalies at the Cenomanian-Turonian boundary, northwest New Mexico

Abstract

A high-resolution section of 64 carbonates and calcareous shales from northwest New Mexico, spanning the Cenomanian-Turonian (C—T) boundary, is examined for selected elemental concentrations as well as carbon isotopic and molecular distributions. Spatial and temporal sampling resolutions for the entire sample set are 1 spl/19 cm and 1 spl/33 ka, respectively. In proximity to the C—T boundary, respective spatial and temporal resolutions increase to 1 spl/10 cm and 1 spl/17 ka. Results show the occurrence of several geochemical anomalies, all with coincident maxima at about 0.5 m below the C—T boundary. Stable carbon isotope ratios of the kerogen and the aliphatic and aromatic hydrocarbon fractions increase by approximately 4% within 20 m of the boundary; this entire C—T boundary event isotope anomaly occured over approximately 2.5 Ma. A 5-fold increase in the concentrations of U and Th occurs synchronous with the isotope excursion, although this [U + Th] spike occurs over a period of only 0.2–0.3 Ma. Elevated U and Th concentrations at this spike co-occur with (a) the highest 20S/(20S + 20R) sterane epimer ratios, (b) C 27/C 27–30 sterane homolog ratios and (c) the lowest n-alkane yields in kerogen pyrolyzates. These observations suggest an effect of radiogenic heat production on the organic matter in the section. The origin of the carbon isotope excursion is attributed, as in previous studies, to atmospheric CO 2 drawdown in latest Cenomanian time, caused by rapid burial of organic matter due to a global marine transgression. The U and Th spike is probably secondary, perhaps resulting from adsorption of these elements onto clays concentrated at the C—T boundary. Although the occurrence of coeval maxima for these isotopic and elemental anomalies suggests a common cause, extrapolation of the CO 2 drawdown model to explain the [U + Th] anomaly is unwarranted at this time. Ongoing mineralogical analysis of clays and carbonates close to the C—T boundary in the Red Wash section may ultimately explain the occurrence of this U and Th spike.