Changes to depositional palaeoenvironments within the Qikou depression (Bohaiwan Basin, China): carbon and oxygen isotopes in lacustrine carbonates of the Palaeogene Shahejie Formation

摘要

This paper reports the carbon and oxygen isotope compositions of lacustrine carbonate sediments from the Palaeogene Shahejie Formation, Qikou depression, Bohaiwan Basin, with the aim of determining the palaeoenvironmental conditions in the region. Results from E_(s2), the second member of the Shahejie Formation, showed values of δ~(13)C and δ~(18)O from -1.2‰ to +2.4‰ (average +0.6‰) and from -6.8‰ to -4.7‰ (average -5.7‰), respectively, suggesting a relatively hot climate attending deposition. The slightly closed nature of the lake, which contains brackish water, resulted in higher carbonate δ~(13)C and δ~(18)O values than in a meteoric environment. The values of δ~(13)C and δ~(18)O preserved within the carbonates of the overlying lower Shahejie I (Es_1) varied between +1.3‰ and +4.9‰ (average +3.2‰) and from -4.4‰ to -1.8‰ (average -3. l‰), respectively, indicating that the climate became colder at that time. Subsequently, a marine transgression caused the salinity of the lake water to increase. The values of δ~(13)C and δ~(18)O were controlled by salinity. The high δ~(13)C values were also influenced by the rapid burial of the lake organisms and by algal photosynthesis. Values of δ~(13)C and δ~(18)O from carbonates in upper Esi ranged from- 8.0‰ to +11.0‰ (average +10.1‰) and from -5.0‰ to- 1.5‰ (average - 3.4‰), respectively, indicating a slight increase in the temperature over time. In the closed and reducing environment, extremes in 8'3C values resulted from biochemical fermentation. The positive δ~(13)C excursion recorded in the carbonates of the Shahejie Formation in the Qikou depression indicates that the palaeoclimate underwent a significant transformation during the Eocene and the Oligocene.