Coupling of paleoenvironment and biogeochemistry of deep-time alkaline lakes: A lipid biomarker perspective

Studies of alkaline lakes have critical biological-environmental-economic properties, but deep-time alkaline lakes are challenging to investigate. Lipid biomarkers can provide valuable insights into such lakes and their biogeochemical significance. This paper reviews and compares typical examples of ancient alkaline lakes across the world. Lipid biomarker evidences, including C-30-steranes, Pr/Ph, Pr/n-C-17-Ph/n-C-18, (beta+gamma-carotane)/nCmax,and gammacerane/C-30 alpha beta H values, suggest these alkaline lakes were reducing, hypersaline, and stratified. The n-alkanes, steranes/hopanes, C-28-St/C27-29-St%, and C-28/C-29-St values indicate that the preserved biomass of the alkaline lakes were dominated by algae and bacteria, with less input from higher plants. The algae were mainly halotolerant green algae, rather than cyanobacteria. The different alkaline lakes have some subtle differences in their sedimentary environments. The paleoenvironmental setting and biomass of the alkaline lakes co-vary systematically. The ratio of algae/bacteria is positively correlated with increasingly reducing and saline conditions, because the increase in salinity improves the competitiveness of halotolerant green algae. The changes in these extreme alkaline environments are too small to cause obvious variations in the proportion of green algae/total algae, and the abundance of cyanobacteria, photoautotrophs, and/or type I methanotrophic proteobacteria. Lipid biomarker data show that the primary controlling factor on the biomass of saline and alkaline lakes is their geologic age and, to a lesser extent, their salinity. The abundance of organic matter in these sediments varies greatly, and the types of organic matter are generally good for hydrocarbon generation. The formation of oil and gas is controlled by factors such as abundance of organic matter, thermal maturity, size of lake basin, and thickness of the organic-rich sediments.

Automated summary

Lipid biomarkers indicate that ancient alkaline lakes were reducing, hypersaline, and stratified, with biomass dominated by algae and bacteria, especially halotolerant green algae. Different alkaline lakes have subtle differences in their sedimentary environments, and the ratio of algae/bacteria is positively correlated with increasing reducing and saline conditions in the lakes.