Late Cretaceous paleoclimate and paleoenvironment in the Songliao Basin, China

The Songliao Basin, northeast China, possesses an excellent Cretaceous lake sediment record that provides an opportunity to investigate Cretaceous terrestrial climate and environmental conditions. We measured total organic matter (TOC), aliphatic and aromatic hydrocarbons, marine biomarkers and δ13C and δD compositions of individual n-alkanes in lacustrine sediments from the Nenjiang Formation (K2n1+2) in drill core J79, taken in the Songliao Basin. Our data indicate a punctuated continental climate and environment during the Late Cretaceous. Differences in the hydrogen isotope values between mid- and long-chain n-alkanes (ΔδDmid-long) and the abundance of retene suggest a change from a warm and humid climate during the lower part of K2n1, to a cool and arid climate during the upper part of K2n1, and then back to a humid climate during the lower part of K2n2. Presence of marine biomarkers 24-n-propyl-cholestane (0–8.0 μg/g) and 24-iso-propyl-cholestane (0–5.7 μg/g) support an inference for periodic seawater incursions into paleo Songliao Lake, which persisted under humid climate conditions. An anoxic and stratified water column existed in the lower part of K2n1, as indicated by low pristane/phytane ratios (Pr/Ph < 0.6) and high gammacerane concentrations (15.3–183.5 μg/g TOC), caused by the periodic seawater transgressions. Moreover, an environment of high evaporation and near closure of the transgression channel resulted in a declining paleolake level, higher salinity and anoxic water during the upper part of K2n1. Transgressions, however, occurred during the lower part of K2n2, followed by a massive input of fresh water that led to brackish-to-freshwater conditions in the euphotic zone, as indicated by high Pr/Ph ratios (> 1.2) and low gammacerane concentrations (8.6–89.2 μg/g TOC). Analysis of the core showed that there were three major hydrologic controls on the paleolake: marine transgressions, paleoclimate and river input.