Integrating sequence and carbon isotope stratigraphy in the lower Cambrian (Stage 3), Tarim Basin: Implications for stratigraphic correlation and environmental-biotic evolution

Abstract

Carbon isotope stratigraphy is a strong tool for stratigraphic correlation and for understanding carbon cycling and biotic-environmental interactions. During the Cambrian Age 3 (ca. 521–514 Ma), metazoan (e.g, archaeocyathid, trilobite) diversification and subsequent first extinction (e.g., archaeocyaths) are marked with distinctive global carbon isotope excursions. However, correlating the carbon isotope records from the Xiaoerbulak (XBL) Formation (Stage 3) in the Tarim Basin with those from other regions has been challenged, hindering establishment of a reliable stratigraphic framework. To solve this dilemma, a sequence stratigraphic framework made of four and a half sequences was established for this formation, further revealing a long-term depositional hiatus in the platform interior. In this light, two continuous sections of deeper slope-basinal settings at Sugaitebulak (SG) and Kungaikuotan (KG) were selected for high-resolution δ¹³C_carb analysis. Three prominent negative (N1−N3) with two unnamed subordinate ones and two positive (P1, P2) excursions with an additional minor positive one were identified. Of these, N1 and N3 mark the base and top of XBL Formation, respectively. Combined with biostratigraphic constraints, the P1, P2, and N3 excursions correlate with the global Cambrian carbon isotope events CARE, MICE, and AECE, respectively. Notably, N2 and P2 perturbations were absent in the platform interior, confirming occurrence of the sedimentological hiatus. Coupled sequence and carbon isotope stratigraphic analysis reveals that the negative excursions are present in transgressive systems tracts, while the positive excursions in the highstand systems tracts dominated by metazoan-microbial buildups. This suggests that transgression-enhanced upwelling of ¹²C-rich, oxygen-depleted deep waters onto the shallow water regimesmay have caused negative δ¹³C_carb excursions and biotic declines, subsequently, the increased primary productivity and organic burialcould have drawn down the CO₂ levels in atmosphere/seawater, leading to the positive δ¹³C_carb excursion and eclogical amelioration, as seen in the HST successions.