Non-marine carbon-isotope stratigraphy of the Triassic-Jurassic transition in the Polish Basin and its relationships to organic carbon preservation, pCO₂ and palaeotemperature.

Abstract

New carbon-isotope data obtained from homogenous organic material (separated microfossil wood; δ13Cwood) from the upper Rhaetian and entire Lower Jurassic permit chemostratigraphic correlation of these marginal/non-marine deposits with the biostratigraphically well-constrained Llanbedr (Mochras Farm) core in N Wales and other marine profiles, supported by sequence stratigraphic correlation and biostratigraphical markers. Statistically significant (Rs = 0.61) positive exponential correlation between δ13Cwood values and continental TOC (TOCcont) concentrations occurs and can be defined empirically by equation. Changes of δ13Cwood observed in C3 plants depends on δ13CO2 of atmosphere and can be modulated by other factors such as pCO2 causing fractionation (enrichment in 12C) of C isotopes in source C3 plants and, to lesser extent, by soil moisture content. Floral remains occurring in the relatively stable palaeolatitude and climatic zone of the Polish Basin in the time interval studied lend no support for significant precipitation impact on the δ13C fractionation, although enhanced precipitation could have had a limited impact during the Toarcian Oceanic Anoxic Event (T-OAE). We argue that the observed relation between δ13Cwood values and TOCcont reflects the global carbon cycle forcing. Such correlations may develop because fluxes of of 12C-enriched methane, mobilized from near-surface carbon sources, lead to global warming, decreased δ13Cwood and enhanced (usually fungally-mediated) decomposition of the terrestrial carbon pool, while subsequent massive burial of organic carbon results in higher δ13C values in all carbon cycle reservoirs, and the attendant drawdown of atmospheric CO2 leads to global cooling and promotes sequestration of soil organic matter. In turn, this relation can be used as an indirect indicator of atmospheric temperature trends, although organic carbon isotope records are potentially subject to many different influences. Based on the δ13Cwood /TOCcont relationship, an approximate qualitative estimation of general trends in air temperature is suggested for c. 40oN paleolatitude and the warm temperate climatic zone. The observed hypothetical trends in temperature are generally in concordance with pCO2 trends calculated from stomatal index. A weak δ13Cwood and TOCcont correlation in Rhaetian deposits is explained by local environmental factors (TOC concentration dependent on a more localized fluvial plain settings), while mostly deltaic – coastal deposits contain more representative, averaged material delivered from a large catchment area.