Cenomanian‐Coniacian Sea‐level Change and Dissolved Oxygen Fluctuations in Tethys‐Himalaya: Evidences from Benthic Foraminifera of Gamba, Tibet

Abstract:Benthic foraminifera, preserved in the Late Cretaceous organic carbon‐rich sediments of Gamba, southern Tibet, provide high‐resolution proxies for sea‐level changes and dissolved oxygen fluctuations of southeastern Tethys. The fossils were statistically analyzed and divided into three faunas of “Cenomanian fauna”, “Turonian fauna”, and “Coniacian fauna”. A middle neritic‐upper bathal environment (50–250m) was estimated considering the ratios of planktonic and epifaunal benthic foraminifera (P/(P+E)), the morphological analysis according to the studies of recent foraminifera and the abundant distributions of depth‐related species such as Alabamina creta, Laevidentalina sp., Praebulimina spp., Pleurostomella cf. naranjoensis, Pyrulina sp., Quinqueloculina spp., Haplophragmoides spp., etc. The result shows an almost parallel trend with the global transgressive and regressive cycles, but the former fluctuates more frequently at upper Cenomanian, which probably indicates tectonic instability of the continental margin. According to the benthic foraminiferal richness (BFN), Shannon‐Weiner diversity (H(s)), as well as benthic foraminiferal oxygen index (BFOI), five periods of oxygen depleted conditions (dysoxic‐anoxic) have been recognized. They correspond to the OAE2, the lower Turonian, the upper Turonian, the Turonian‐Caniacian boundary event and the probably OAE3. In addition, the oxygen fluctuations in Gamba might be controlled directly by sea‐level changes, while the paleoproductivity and oxygen conditions interacted with each other under oxygen deficiency environments.