Carbon and nitrogen cycling in response to global environmental change during the Carnian Pluvial Episode (late Triassic)

The Carnian Pluvial Episode (CPE) (∼234 Ma) was marked by recurrent climate shifts, changes in the hydrological cycle, in particular precipitation pattern, which in turn affected biogeochemical cycles and nutrient supply to shelf and epicontinental seas. Among biogeochemical cycles, the nitrogen cycle represents a fundamental process governing the transformation and distribution of nitrogenous species, including dinitrogen (N₂), ammonium (NH₄⁺), nitrite (NO₂⁻), and nitrate (NO₃⁻). However, due to the lack of detailed studies, the response of the nitrogen cycle to the environmental changes during the CPE remains poorly understood. This study presents organic carbon (δ¹³C_org) and nitrogen isotopes (δ¹⁵N) along with bulk elemental geochemical data from the Carnian Quemo Co section in the Qiangtang Basin, China. The δ¹³C_org chemostratigraphy indicates that the sediment succession investigated spans the onset of the CPE, which is marked by successive short-term negative carbon isotope excursions (NCIEs). Lithological observations, coupled with elevated Si/Al and Al/K ratios suggest an increased input of silt and chemically altered clay minerals, in which may related to increased hydrological cycle during the CPE. A positive δ¹⁵N excursion during the NCIE interval suggests increased nutrient fluxes to marine environments, possibly driven by increased terrestrial organic matter input resulting from enhanced erosion and weathering, stimulating nitrogen uptake by marine phytoplankton and partial nitrate assimilation.