Geochemical Complexity in Terrestrial Hot Spring Fields: Implications for the Origin of Life.

Abstract

Prebiotic chemistry for the origin of life requires a high degree of chemical and mineralogical complexity with the potential for multiple reactions under differing physico-chemical conditions. This includes processes that can promote the condensation reactions required to form polymers and the mechanisms to concentrate trace elements that can catalyze polymerization reactions. Competing hypotheses for favorable settings for life to emerge include submerged ocean hydrothermal vents and subaerial, terrestrial hot spring fields. A key challenge for permanently submerged hydrothermal vents is the inevitable dilution that occurs when fluids are ejected from deep-sea hydrothermal vents into a relatively uniform oceanic reservoir, meaning that whatever geochemical complexity that may have developed in the subsurface conduits of such systems is rapidly lost. Open water systems also lack the ability to form polymers and concentrate the trace elements required to catalyze polymerization reactions. Terrestrial hot spring environments experience wet-dry cycling, concentrate elements through multiple processes, and can have a range of pH values, yet they are regarded by some as unfavorable sites because they are too hot (the tar problem) and typically portrayed as individual, relatively static pools (e.g., Darwin's "Warm Little Pond"). Here, we illustrate how the terrestrial hot spring field of the Taupō Volcanic Zone (TVZ) of New Zealand is much more dynamic and geochemically diverse than generally considered due to the many closely located pools with widely variable physico-chemical attributes that mix components at a variety of scales, creating a level of geochemical complexity unmatched elsewhere on Earth. The tens to thousands of diverse surface pools of the TVZ are characterized by wet-dry cycling and multiple mechanisms that can concentrate trace elements and mix fluids of very different composition that result in geochemical variability as well as mineral precipitation that can enhance the preservation of biosignatures.