Carbon isotope record across the Lower-Middle Triassic transition in the Holy Cross Mountains (Poland): chemostratigraphic and palaeoenvironmental significance

The carbon isotope signature of carbonates across the Lower-Middle Triassic transition was investigated to reconstruct environmental change and carbon cycle dynamics in the Holy Cross Mountains (HCM, Poland). This area was located in the southeastern part of the semi-enclosed Germanic Basin, intermittently connected to the western Tethys Ocean through tectonically controlled marine gateways.

The dataset is derived from the uppermost Olenekian to upper Anisian interval of the Piekoszów IG-1 borehole. This approximately 240 m thick succession comprises mudstones, marly limestones, dolostones and limestones, locally nodular, classified as wackestones to packstones and subordinate grainstones. These deposits document the carbonate ramp setting ranging from restricted marginal to open-marine conditions. A distinctive red mudstone horizon in the lower Anisian records short-lived emersion within the ramp. The biostratigraphic framework of this succession is constrained by conodonts, bivalves, echinoderms, brachiopods and miospores.

The δ¹³C curve across the Olenekian-Anisian transition ranges from -3.16‰ to +0.70‰ VPDB. A positive shift recorded close to the Olenekian-Anisian boundary broadly corresponds to the Spathian-Anisian carbon isotope event and represents a potential regional chemostratigraphic marker. A pronounced negative excursion (down to -3.82‰) is observed within a lower Anisian dolostone interval. Miospores identified above this interval indicate the Perotrilites minor palynozone of the Lower Muschelkalk, allowing precise stratigraphic calibration of the isotope record. A second significant positive shift (+3.06‰) occurs near the Bithynian-Pelsonian boundary and correlates with coeval carbon isotope excursions documented elsewhere in the Germanic Basin and the western Tethys. This is followed by a gradual decline culminating in a negative excursion in the upper Anisian.

The δ¹³C fluctuations observed in the late Olenekian and early Anisian are interpreted to reflect local palaeoenvironmental conditions within a shallow, partly restricted epicontinental sea. Restricted circulation and episodic freshwater input likely modified the dissolved inorganic carbon pool through mixing with isotopically light terrestrial carbon and reduced exchange with open-marine reservoirs. In contrast, more positive δ¹³C values during the middle and late Anisian coincide with the development of open-marine carbonate ramp conditions, enhanced carbonate production and improved basin connectivity.

The δ¹⁸O values display a wide scatter consistent with variable early diagenetic conditions and facies-controlled overprint in a restricted to shallow-marine setting. The absence of systematic δ¹³C-δ¹⁸O covariance suggests that the main carbon isotope trends are not dominated by late-stage diagenetic alteration.

Overall, the isotope record from the HCM reflects regional palaeoenvironmental dynamics in the Polish segment of the Germanic Basin related to tectonically driven changes in basin connectivity, water circulation and third-order transgressive-regressive cycles, supporting the use of δ¹³C trends as tools for regional chemostratigraphic correlation across the Lower-Middle Triassic transition.