Regional Heterogeneity in Marine Nitrogen Cycling During the Carnian Pluvial Episode (late Triassic)

The Carnian Pluvial Episode (CPE) was a major climatic perturbation of the Late Triassic, marked by intensification of the hydrological cycle, collapse and reorganization of carbonate platforms, increased influx of terrigenous detritus, and repeated negative carbon-isotope excursions. Nitrogen isotope signatures preserved in sedimentary rocks provide important constraints on the evolution and functioning of the nitrogen cycle through geological time. Yet the response of the marine nitrogen cycle to this event remains poorly constrained because high-resolution nitrogen-isotope records are still scarce, particularly across contrasting depositional settings. Here we present new organic nitrogen-isotope data (δ15Norg), together with total organic carbon (TOC) and total nitrogen (TN), from the Chenjiabao section in the Nanpanjiang Basin, Guizhou Province, deposited in a semi-deep-water platform-margin setting. We use these data to reconstruct nitrogen-cycle dynamics through the CPE and compare them with the Quemo Co section in the Qiangtang Basin, a shallow marine shelf-lagoon succession reported by Ke et al. (2026). The Chenjiabao section is characterized by persistently low δ15Norg values (-0.1‰ to 2.7‰) and several negative excursions, indicating a long-term depleted marine nitrate reservoir and repeated enhancement of biological N2 fixation. We infer that, during the CPE, volcanically enhanced phosphorus input stimulated N2 fixation, whereas expansion of marine anoxia promoted denitrification and anammox, progressively reducing nitrate availability. In contrast, the Quemo Co section displays a much wider δ15N range (-5.2‰ to 10.6‰) and is dominated by positive excursions, consistent with stronger control by terrigenous nutrient supply, partial nitrate assimilation, and recycling of isotopically heavy nitrogen from remobilized organic matter. Comparison of the two sections reveals pronounced regional heterogeneity in nitrogen cycling during the CPE. The proximal Quemo Co setting mainly records continental input and weathering effects, whereas the more distal Chenjiabao setting more directly records marine redox-driven nitrate depletion and compensatory N2 fixation. These results show that the marine nitrogen cycle during the CPE was shaped by strongly spatially variable local controls, while still responding to broader environmental forcing.