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

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.