Romain Tramoy

Integrated stratigraphy of a continental Pliensbachian–Toarcian Boundary (Lower Jurassic) section at Taskomirsay, Leontiev Graben, southwest Kazakhstan

Abstract During the Early Jurassic, major palaeoclimatic changes, associated with large carbon-cycle perturbations, occurred at the Pliensbachian–Toarcian boundary. Although the detailed marine palaeoclimatic record of this time interval and its impact on the marine biota is well recorded, much less is known about the continental realm. The current study documents new palynological and high-resolution carbon isotope data measured on bulk organic matter from the continental Lower Jurassic section of Taskomirsay in Kazakhstan, Central Asia. Both datasets allow a transition zone between the Pliensbachian and Toarcian to be identified. In addition, the spore and pollen distribution suggests a warming trend from the Pliensbachian to the Toarcian, most probably associated with a shift in floristic associations from the Siberian to the Sino-European palaeofloristic provinces, as recorded elsewhere in Central Asia during the Early Jurassic. The Taskomirsay section is thus of primary interest for palaeoclimatic studies as it is one of the very few well-dated continental section worldwide that records the Pliensbachian–Toarcian palaeoclimatic changes. Supplementary material: Table of δ13Corg data, Taskomirsay section, Karatau Graben, Kazakhstan is available at https://doi.org/10.6084/m9.figshare.c.3495645

Carbon and nitrogen dynamics in decaying wood: paleoenvironmental implications

The effect of early diagenesis on carbon and, especially, nitrogen isotopes (δ13C and δ15N) of organic matter is not well understood and is of interest for accurate paleoenvironmental reconstructions. Wood samples were incubated in distilled water and river water to assess the effects of early diagenesis on carbon and nitrogen dynamics. Elemental content and isotopic composition of carbon and nitrogen as well as mass loss of wood pieces were determined. Mass loss in river water was three times greater than in distilled water. This difference was attributed to the development of two different types of fungi characterised by various degradation rates. Carbon dynamics of wood samples showed similar patterns in both type of water: (i) a sharp increase in carbon content, possibly related to carbohydrate degradation, before it slowly returned towards initial values, and (ii) no significant changes in δ13C values. In contrast, nitrogen dynamics of samples showed complex patterns: (i) N release associated with 15N depletion in distilled water, attributed to uptake of 15N-enriched pool (i.e. proteins) by fungi, and (ii) N accumulation associated with 15N enrichment in river water. The latter pattern was attributed predominantly to microbially mediated importation of 15N-enriched nitrate from river water. Although challenging, the present results suggest that early diagenesis may average an environmental signal by integrating individual signals (woods, fungi, water) and microbial processes. Considering the non-linear behaviour of early diagenesis, this integration is probably almost instantaneous on the geological time scale, which may not preclude paleoenvironmental reconstructions.

Carbon and oxygen isotope signals from the Callovian-Oxfordian in French sedimentary basins.

High-resolution carbon and oxygen isotope data from the Paris Basin and the Subalpine Basin (France) are available in a precise biostratigraphic framework for the Callovian–Oxfordian stages. A biostratigraphically well-constrained δ13C curve, derived from bulk carbonates in the Paris Basin and the Subalpine Basin, is provided in order to document carbon-cycle evolution and to serve as a chemostratigraphic reference for the Callovian–Oxfordian in the Tethyan domain. Sea-temperature reconstructions, using diagenetically screened belemnite and oyster data, reveal major climate perturbations at the Middle–Late Jurassic transition.