Oxygen and stable carbon isotopes from a nautiloid from the middle Pennsylvanian (Late Carboniferous) impregnation Lagerst?tte 'Buckhorn Asphalt Quarry' - Primary paleo-environmental signals versus diagenesis

摘要

Two shell fragments, one of an undetermined cephalopod and one of an orthoconic nautiloid (Pseudorthoceratidae gen. et. sp. indet.), from the Pennsylvanian Buckhorn Asphalt Lagerst?tte (Oklahoma) were studied to reconstruct paleotemperatures and to explore possible diagenetic alterations. For the first time, not only parts of the shell of an orthoconic nautiloid, but also its cameral deposits were sampled in high resolution. The mineralogy, geochemistry, and microstructure of both shell remains, the pseudorthoceratid and the undetermined cephalopod, were investigated in detail before high-resolution transects for oxygen and stable carbon isotopes were sampled.The shell fragment of the undetermined cephalopod showed no or very little diagenetic alteration. Therefore, the measured δ~(18)O-variation (δ ~(18)O-range of 0.68‰) and the inferred temperature range of 28 to 31°C are interpreted to represent an original signal. The temperatures indicate that the specimen lived in shallow, warm tropical waters. The δ ~(13)C-signal, with a δ ~(13)C-range of 3.43‰ is interpreted to be the consequence of a strong metabolic effect.A septum of the phragmocone of the pseudorthoceratid nautiloid was only slightly altered by diagenesis and a largely original δ ~(18)O-signal could be obtained. The δ ~(18)O-values exhibit an inferred temperature range of 14 to 15°C indicating that this species lived in deeper and cooler waters. The cameral deposits were investigated in detail by energy dispersive X-ray (EDX) and by XRD-GADDS (X-ray diffraction - general area detector diffraction system) analyses. Part of the cameral deposits in the orthoconic nautiloid turned out to originally consist of high-Mg calcite (HMC) instead of the estimated aragonite. Anyhow, the massive cameral deposits show signs of strong small scale alteration resulting in a distinct spreading of oxygen-values. The oxygen and carbon isotope plots show a linear trend for the oxygen-carbon-correlation. This is interpreted as the mixing of two end-members with the lightest end-members suggesting a meteoric diagenesis with a diagenetic fractionation of the aragonite to low-Mg calcite (LMC) respectively the HMC to LMC and dolomite.The calculated temperatures of the pseudorthoceratid specimen and the undetermined cephalopod indicate two different paleoenvironments. Additionally, the pseudorthoceratid nautiloid displays the narrow range of temperatures in the deeper parts of the sea and the undetermined cephalopod the more fluctuating temperatures in the shallow areas. It is also shown how important a detailed investigation of microstructures and mineralogy of samples with an according cautious interpretation of the achieved dataset is. The importance of an intensive examination of samples is pointed out, to avoid small scale alterations to be overlooked, since this would lead to a misinterpretation of the results.