Response of Lacustrine Environment and Nitrogen Cycling to the Sinemurian–Pliensbachian Boundary Event (SPBE) in Dongyuemiao Member, Sichuan Basin

The Early Jurassic Sinemurian–Pliensbachian boundary event (SPBE) was a global carbon-cycle perturbation, but its impact on lacustrine ecosystems and nitrogen cycling remains poorly understood. This study investigates fine-grained clastic rocks of the Lower Jurassic Dongyuemiao Member in the Aojiaying section, eastern Sichuan Basin, using petrology, elemental geochemistry, and nitrogen/carbon isotopes to reconstruct lacustrine environmental and nitrogen-cycle responses to the SPBE. The succession comprises an oxic shallow-lake Unit A (dominated by gray-brown siltstone and gray argillaceous bioclastic mudstone), a dysoxic–anoxic semi-deep-lake Unit B (dominated by black laminated argillaceous mudstone), and an oxic littoral–shallow-lake Unit C (dominated by gray quartzose mudstone and gray-green argillaceous mudstone). During the Pre-SPBE, the climate was generally warm–humid but internally variable, with relatively drier/hotter conditions in the middle part of Unit A, a return to more humid conditions in the upper part of Unit A, and progressive humidification through the lower–middle part of Unit B; redox conditions evolved from oxic shallow-lake conditions toward dysoxic conditions. During the Peak SPBE, the most humid interval occurred at A18–A20 in the upper part of Unit B, where Sr/Cu and Mg/Ca reached their lowest values; this humid peak coincided with intensified water-column stratification, bottom-water anoxia, and optimal organic matter preservation (average TOC 1.9%). Upward within the Peak SPBE, the climate became transitional and redox conditions gradually ameliorated. In the post-SPBE stage, relatively drier and oxic conditions returned, with low TOC (0.6%). δ15N has a background average of 3.5‰, reaches a peak of 5.0‰ at A-16 in the late Pre-SPBE, generally decreases during the Peak SPBE, rebounds to 4.9‰ at the most anoxic horizon (A-21), and then returns to background levels (average 3.7‰). The δ15N peak precedes the negative δ13Corg shift, this offset reflects both the direct responsiveness of δ15N to water-column redox changes and the faster timescale of regional hydroclimatic responses relative to global carbon-isotope transmission; thus, δ15N can serve as an effective proxy to distinguish global carbon-cycle signals from regional environmental responses.