Paleo-oceanographic Conditions Of The Mid-Cretaceous Marine Organic Matter Accumulation In Gamba, Southern Tibet

As a background of Oceanic Anoxic Events (OAEs), the research that concerns biological effects and environmental conditions of marine organic matter accumulations has been an important paleoceanographic subject for decades. During the Cenomanian-Turonian boundary event (~93Ma, OAE2), several layers of mid-Cretaceous carbon-rich sediments, containing abundant microfossils (especially foraminifera), developed in Gamba, Southern Tibet. As a widely used paleoenvironmental proxy, benthic foraminifera have great significance in restoring the ancient water depth, bottom water oxygen concentration, the oceanic productivity and so on. The benthic foraminifera across the C/T boundary can be divided into three evolution stages, and both the main advantage species (Dorothia, Anomalinoides, Lenticulina, Conobina etc.) and plankton/benthos ratios (P / (P + B) are of 50% -80%) indicate that the habitat type isâ…£1-2 (outer shelf - shelf edge environment) with the water depth approximately 100-200m. The benthic foraminiferal oxygen index (BFOI) ranges from -6 to -25, corresponding to the dissolved oxygen content of about 0.7~1.4ml/L which belongs to suboxic condition. The benthic foraminiferal microhabitat (deep infauna/epifauna) shows a higher primary productivity in this area. On the other hand, the primary productivity (PP) has been calculated using the biogenic barium (Babio). Its value is generally in the 36-206 g.m-2.a-1 range, and the C/T boundary has a higher productivity of 124-206 g.m-2.a-1, which has closed to the level of modern ocean upwelling zone. High marine productivity is the most important factor in controlling organic carbon deposition in Gamba. Primary productivity and OAEs are coupling factors which affect each other. Short-term increase of marine productivity is an important reason for the depleted dissolved oxygen, but a series of environmental events is the main reason leading to OAEs. Anoxic events can cause mass extinction of marine life and thus, to some extent, have certain influences to productivity. However, the impact is very limited. During the OAE2, a strongly reducing environment probably never occurred,which may be one of the most significant reason why the high organic carbon content of marine source rocks had not formed under the background of high productivity.