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Hyperalkaline waters display unusually high productivity, which makes them prime targets in the search for life elsewhere in the solar system. However, the formation mechanisms of alkaline waters are not well understood, because the response of biogeochemical proxies to these conditions is poorly constrained. To address this issue, we assessed the influence of hydrothermal fluids on the salinity and pH of alkaline lakes based on a case study of an early Permian paleo-alkaline lake (~290 Ma; Fengcheng Formation) in the Mahu Sag, northwestern Junggar Basin, China. Multiple proxies indicate that hydrothermal fluids in the central salt rock and marginal tuff-mudstone areas of the Fengcheng Formation were affected by deep and shallow hydrothermal fluids, respectively. A small part of the transitional area was affected by hydrothermal fluids with a hybrid nature. The hydrothermal fluid activity gradually weakened up-section in all areas while salinity (inferred from carbon and oxygen isotopes) increased and pH (inferred from nitrogen isotopes and mineralogy) decreased from hyperalkaline (> 9.25) to moderately alkaline conditions. These trends suggest that hyperalkalinity was largely driven by hydrothermal processes. In contrast, evaporation, which dominated towards the end of the lake's lifetime, maintained an elevated pH but did evidently not have a similarly strong effect as hydrothermal fluids. Our data suggest that hydrothermal activity and evaporation in closed lacustrine basins have the potential to create extreme conditions for the formation of alkaline lakes. The evolution of salinity and pH may not necessarily be synchronized.

It is established that green algae and land plants progressively colonized freshwater and terrestrial habitats throughout the Paleozoic Era, but little is known about the ecology of Paleozoic saline lakes. Here, we report lipid biomarker and petrographic evidence for the occurrence of a green alga as a major primary producer in a late Paleozoic alkaline lake (Fengcheng Formation; 309–292 Ma). A persistently saline and alkaline lacustrine setting is supported by mineralogical and lipid biomarker evidence alongside extremely enriched δ15Nbulk values (+16 to +24‰) for the lake depocenter. The prominence of C28 and C29 steroids, co‐occurring with abundant carotene‐derived accessory pigment markers in these ancient rocks, is suggestive of prolific primary production and elevated source inputs from haloalkaliphilic green algae. The high C28/C29‐sterane ratios (0.78–1.29) are significantly higher than the typical marine value reported for late Paleozoic rocks (<0.5) and thus are associated with certain groups of chlorophytes. Adaptation to such extreme lacustrine environments, aided by enhanced biosynthesis of certain cell membrane lipids, likely played an important role in the evolution and physiological development of ancient green algae.