Temperature and salinity history of the Precambrian ocean: Implications for the course of microbial evolution

The temperature and salinity histories of the oceans are major environmental variables relevant to the course of microbial evolution in the Precambrian, the "age of microbes". Oxygen isotope data for early diagenetic cherts indicate surface temperatures on the order of 55-85°C throughout the Archean, so early thermophilic microbes (as deduced from the rRNA tree) could have been global and not just huddled around hydrothermal vents as often assumed. Initial salinity of the oceans was 1.5-2× the modern value and remained high throughout the Archean in the absence of long-lived continental cratons required to sequester giant halite beds and brine derived from evaporating seawater. Marine life was limited to microbes (including cyanobacteria) that could tolerate the hot, saline early ocean. Because O ₂ solubility decreases strongly with increasing temperature and salinity, the Archean ocean was anoxic and dominated by anaerobic microbes even if atmospheric O₂ were somehow as high as 70% of the modern level. Temperatures declined dramatically in the Paleoproterozoic as long-lived continental cratons developed. Values similar to those of the Phanerozoic were reached by 1.2 Ga. The first great lowering of oceanic salinity probably occurred in latest Precambrian when enormous amounts of salt and brine were sequestered in giant Neoproterozoic evaporite basins. The lowering of salinity at this time, together with major cooling associated with the Neoproterozoic glaciations, allowed dissolved O₂ in the ocean for the first time. This terminated a vast habitat for anaerobes and produced threshold levels of O₂ required for metazoan respiration. Non-marine environments could have been oxygenated earlier, so the possibility arises that metazoans developed in such environments and moved into a calcite and silica saturated sea to produce the Cambrian explosion of shelled organisms that ended exclusive microbial occupation of the ocean. Inasmuch as chlorine is a common element throughout the galaxy and follows the water during atmospheric outgassing, it is likely that early oceans on other worlds are also probably so saline that evolution beyond the microbial stage is inhibited unless long-lived continental cratons develop.