Carbon and Nitrogen Cycle Dynamic in Continental Late-Carboniferous to Early Permian Basins of Eastern Pangea (Northeastern Massif Central, France)

Abstract

Late Carboniferous to early Permian organic-rich sedimentary successions of late-orogenic continental basins from the northeastern Massif Central (France) coincide with both the Variscan mountain dismantling and the acme of the long-lasting Late Paleozoic Ice Age. Here, we investigate the carbon and nitrogen cycles in the newly dated sedimentary successions of the Decize–La Machine and Autun basins during these geodynamic and climate upheavals. The sedimentary organic matter has been analyzed through Rock-Eval pyrolysis, palynofacies and elemental and isotope geochemistry along cored-wells and outcropping sections, previously accurately defined in terms of paleo-depositional environments. Rock-Eval and palynofacies data have evidenced two origins of organic matter: a phytoplanktonic/bacterial lacustrine origin (Type I organic matter, organic δ13C values around −23.5‰), and a terrestrial origin (vascular land plants, Type III organic matter, organic δ13C values around −20‰), mixed in the deltaic-lacustrine sediments during background sedimentation (mean organic δ13C values around −22‰). Episodes of high organic matter storage, reflected by black shales and coal-bearing deposits (total organic carbon up to 20 and 70%, respectively) are also recognized in the successions, and are characterized by large negative organic carbon isotope excursions down to −29‰. We suggest that these negative isotope excursions reflect secondary processes, such as organic matter remineralization and/or secondary productivity varying under strict local controls, or possibly larger scale climate controls. At times, these negative δ13C excursions are paired with positive δ15N excursions up to +10‰, reflecting water column denitrification and anammox during lake-water stratification episodes. Together, these isotopic signals (i.e., low sedimentary organic δ13C associated with high bulk δ15N values) indicate periods of high primary productivity of surface waters, where nitrogen and carbon cycles are spatially decoupled. These local processes on the sedimentary isotope archives may partially blur our ability to directly reconstruct paleoclimate variations in such continental settings using only C and N isotopes. At last, we explore an organic δ13C-based mixing model to propose ways to disentangle autochthonous versus allochthonous origin of organic matter in lacustrine continental settings.

Decoding the drivers of deep‐time wetland biodiversity: insights from an early Permian tropical lake ecosystem

“…However, the nature of these open‐water microorganisms remains unknown, and their future recognition requires methods used in palynology, organic geochemistry and organic petrology, such as fluorescence microscopy and carbon‐isotope spectrometry (see Clausing 1992; Clausing & Boy 2000; Mercuzot et al . 2021). However, next to the autochthonous primary production, we also note that the biotas of the co‐existing terrestrial environments contributed to the trophic structure of the lake ecosystem.…”

Reconsidering Carboniferous–Permian continental paleoenvironments in eastern equatorial Pangea: facies and sequence stratigraphy investigations in the Autun Basin (France)

“…Therefore, this facies association is attributed to a profundal lake environment in its most distal part, with minimal sediment supply preventing the homogenization of the water column and favoring OM preservation when dysoxic to anoxic conditions are reached (P2a facies association, Table 2, Fig. 7), as evidenced by Mercuzot et al (2021b).…”

“…The Autun Basin has been extensively investigated for its paleontological content (flora and fauna) and its mineral resources (coal, oil and uranium) since the 18 th century, as described by Pruvost (1942), Elsass-Damon 6 (1977) and Marteau (1983). More recently, studies on OM have attempted to characterize the current alteration of fossil-bearing shales (Odin et al 2015a, b), as well as to describe the types of OM found in the sediments, their variations in the stratigraphy, particularly in the main OM-rich levels of the basin, as well as the dynamics of the carbon and nitrogen cycles (Garel et al 2017;Mercuzot et al 2021b).…”

U–Pb age constraints on the Carboniferous–Permian transition in continental basins of eastern equatorial Pangaea (France): implications for the depositional history and correlations across the late Variscan Belt

“…Ongoing efforts to document the basin evolution, palaeogeography, depositional environments, and local to regional climate signals have been hampered by a lack of highprecision radioisotopic ages, preventing the construction of a robust chronostratigraphic framework, and hence precise regional and global correlations (e.g. Müller et al 2006;Izart et al 2012;Garel et al 2017;Lützner et al 2021;Mercuzot et al 2021b). Many correlations across the late Carboniferous to Permian (C-P) European basins still rely on the use of regional stages based on continental biostratigraphy.…”

“…The structural and palaeoenvironmental settings of C-P basins of the northern Massif Central (France, Fig. 1b) have been re-evaluated and documented in a number of recent studies (Beccaletto et al 2015;Ducassou et al 2019;Garel et al 2017;Mercuzot et al 2021aMercuzot et al , 2021bMercuzot et al , 2022Luccisano et al, 2022).…”