Refining the Stratigraphic Interval of Oceanic Anoxic Event 1a (oae1a, ~120 Ma) in Gamba Area, Southern Tibet and Implications for Paleoceanographic Conditions in Eastern Tethys

The early Aptian Oceanic Anoxic Event 1a (OAE1a, ~120 Ma) represents a major perturbation to global carbon cycle and is marked by the occurrence of organic-rich shales and a pronounced carbon isotope excursion. While there are numerous OAE1a records, reliable OAE1a in the eastern Tethys is lacking. A well-exposed, nearly 2 km thick Cretaceous marine succession at the Chaqiela section in Gamba, southern Tibet provides a potential archive of OAE1a in eastern Tethys. However, the poor fossil preservation and variable stratigraphic divisions in the lower segment (e.g., Dongshan Fm, Menkadun Fm, Gucuocun Fm) of the Chaqiela section, where OAE1a intereval mostly reside, pose great chanllenges in reliably defining the stratigraphic interval of the geologically abrupt event—OAE1a. Based on the loose biostratigraphic constraint, recent efforts have relied on δ13Corg variation pattern matching with that of typical carbon isotope excursion (CIE) of well-established OAE1a records, but leading to two competing propostions of the OAE1a stratigraphic intervals in the same Chaqiela section that are ~300 m apart. To resolve this stratigraphic controversy and further investigate environmental changes during OAE1a, we conducted an integrated multi-proxy study involving detailed stratigraphic measurements, detrital zircon U–Pb geochronology, paleomagnetic invetsigation and high-resolution geochemical analyses (δ¹³Corg, δ¹⁵N, TOC, and TN). Our field campains of detailed stratigraphic measurements and meticulous stratigraphic correlations allow us to establish a unified lithological framework that integrates both new and published datasets. Also, detrital zircon U–Pb dating of sandstones between the two proposed OAE1a intervals yields a maximum depositional age of ~130 Ma, while paleomagnetic data of the sandstones indicate the occurrence of reversed polarity intervals, suggesting that the sandstone was likely deposited prior to the Cretaceous Normal Superchron (Aptian–Santonian, ~121 Ma to 84 Ma). This is consistent with the sedimentation rate estimates based on the unified stratigraphic framewor. Furthermore, we investigated the causes of organic-carbon isotope (δ13Corg) variations and find that not all δ¹³Corg variations registered global cycle features and some were influenced by regional environmental changes, particularly fluctuations in water depth. The proposed OAE1a interval below the sandstones shows large δ¹³Corg variability and low TOC in shallow-water, unstable conditions, with the negative carbon isotope excursion (CIE) corresponding to an initial shoaling of the water column followed by progressive deepening during the subsequent positive δ¹³Corg shift. The proposed OAE1a interval above the sandstones exhibits more stable isotope signatures and higher TOC, consistent with a more stable, deepening, and stratified conditions. Overall, the integrated stratigraphic, geochronological, and geochemical evidence appears to favor the proposed interval above the sandstone as the more reliable OAE1a record in eastern Tethys. Further analyses of the combined variations in δ¹⁵N, TOC, TN, and TOC/TN reveal that the eastern Tethys was characterized by relatively stable, stratified, and predominantly suboxic marine conditions during OAE1a.