Lower Cretaceous calcareous nannofossils and their biostratigraphic and paleoceanographic implications in Southern Tibet

Lower Cretaceous marine strata are widely distributed and well exposed in the Tingri area of Southern Tibet, which includes the Gucuocun and the overlying Gambacunkou Formations. However, the age of the boundary between these two lithologic formations is poorly constrained due to a scarcity of macrofossils. A detailed biostratigraphic study was carried out on the abundant, moderately preserved calcareous nannofossil assemblages obtained from the lower part of the Gambacunkou Formation and the underlying Gucuocun Formation in the Kangsha Section, Tingri. The first occurrence of the marker species Tranolithus orionatus (110.73 Ma) constrains the Gucuocun/Gambacunkou boundary to slightly below the lower boundary of Subzone NC8c (early Albian age), which correlates with Ocean Anoxic Event (OAE) 1b. The absence of Axopodorhabdus albianus and Eiffellithus turriseiffelii throughout the section suggests that the lower part of the Gambacunkou Formation mostly falls within nannofossil Subzone NC8c (110.73–109.94 Ma; early Albian), with a mean sedimentation rate of 12.7 cm/kyr, which is higher than that typical of the open ocean. The consistently high total organic carbon (TOC) and CaCO3 contents in Interval I (Gucuocun Fm.) indicate that productivity was higher during OAE1b, possibly due to increased terrigenous input (as evidenced by elevated C/N and nutrient/fertility indices) that carried nutrients to promote productivity during this event. Nannofossil assemblages and geochemical proxies, including TOC and elemental ratios (Ti/Al, Zr/Al, K/Al), reveal a parallel upsection increase in surface water fertility and continental weathering intensity from Interval II-1 (0–60 m) to the overlying Interval II-2 (60–102 m) in the Gambacunkou Fm., suggesting that enhanced continental weathering contributed to increase the surface water fertility and dilute the sediment carbonate content. These data also indicate that the depositional environment in the Tingri area was aerobic to hypoxic during the early Albian. Our result confirms the presence of OAE1b at the top of the Gucuocun Formation.