Paleoenvironmental evolution and organic matter accumulation in an oxygen-enriched lacustrine basin: A case study from the Laizhou Bay Sag, southern Bohai Sea (China)

Depositional environments play important roles in the formation of lacustrine organic-rich sediments. A series of experiments (total organic carbon, Rock-Eval pyrolysis, major and trace element experiments) were performed on samples from Es-3-Es-4 in the Laizhou Bay Sag to elucidate the paleoenvironmental evolution. The results show that the source rocks from Es-3(M-L) and the top of Es(4)(U )are of excellent quality characterized by Type II kerogen, while other sections are in poor to moderate quality characterized by Type III kerogen. The samples from the Laizhou Bay Sag are enriched in CaO and specific trace elements (e.g., Sr, Mo, Ba, and U). Many inorganic proxies were employed to analyze the climate, productivity, redox condition, sedimentation rate and detrital input independently. The Mo-U covariation pattern and C-org:P index reveal the ubiquitous oxygen-rich bottom water in the Laizhou Bay Sag during the deposition of Es-4 and Es-3. The paleoenvironmental evolution in the present study can be divided into the following three stages: (1) Es-4(L) to the bottom of Es-4(U): sediments in this stage were deposited under an arid and cool climate. The paleolake was characterized by modest productivity in the surface water and oxidizing (oxic to suboxic) conditions in the bottom water, with a relatively high detrital flux. (2) The top of Es-4(U) to Es-3(M-L): sediments formed under a warm and humid climate, which resulted in the high productivity in surface water. The bottom water was under suboxic condition, and the detrital flux was relatively low. (3) Es-3(U): paleoclimate during this stage was warm and humid but exhibited a trend towards an arid and cool climate, leading to a decrease in productivity. The sediments in Es-3(U) were deposited in oxic bottom water with high detrital inputs. The mechanism for organic matter accumulation in the Laizhou Bay Sag is also discussed. Primary productivity and detrital flux are considered as the major factors controlling organic matter enrichments. Paleoclimate has an indirect impact on organic matter accumulation.