The Danakil desert in Ethiopia with its acidic lakes gives an mpression of how Earth may have looked ... [+] around 4 billion years ago.gettyPockets of liquid water existed on Earth already 4.1 billion years ago, with the first oceans forming around 4 billion years ago. But these oceans were highly acidic, preventing life on Earth from developing for the planets first 500 million years."To understand the origin of life, it becomes important to understand when and how Earth began hosting an ocean with a more neutral pH," says Meng Guo, a former Yale graduate student who is now a presidential postdoctoral fellow at Nanyang Technological University in Singapore and first author of a new study addressing this problem.An acidic pH lower than 7 can inhibit the synthesis of organic molecules and damage living tissue; modern-day seawater has a neutral pH of about 8."But modeling the long-term evolution of ocean pH is a notoriously difficult problem, as it involves almost all of the components of the Earth system: the atmosphere, the ocean, the crust, and the mantle," explains Guo."This is a tour-de-force theoretical endeavor, bridging a longstanding gap between surface processes and processes deep in the Earth," adds co-author Jun Korenaga, a professor of Earth and planetary sciences in Yales Faculty of Arts and Sciences.For example, ocean pH depends to a large extent on atmospheric carbon dioxide, which, in turn, is influenced by a variety of other factors. Early Earth was covered by a dense atmosphere composed mostly of water vapor and carbon dioxide, a result of volcanic degassing of the still partially molten planet. Only over time did the concentration of carbon dioxide decrease thanks to various geological processes.For their study, Korenaga and Guo carefully calibrated and set parameters for how each of these processes functionedand then had them interact."I think the main reason why we are able to do this modeling now is that our understanding of early Earth tectonics has been drastically improved in the last few years," so Korenaga.Rocks like the 4.01-billion-year-old Acasta Gneiss suggest that plate tectonics was already active on Earth at the time. When two plates collide, the resulting mountain building process typically accelerates both weathering and erosion. When a tectonic plate is pushed beneath another plate during subduction, sediments containing carbon dioxide, like limestone, are buried underground. Both processes gradually remove carbon dioxide from the atmosphere and the oceans.Using their new model, Guo and Korenaga estimated that it would have taken Earth 500 million years to neutralize ocean acidity enough to support life.So far, the oldest verified signs of life on Earth are 3.5 billion-year old dome-like sedimentary structures formed by microbial activity. But some experts believe that iron and carbon concretions found in hydrothermal veins of the Nuvvuagittuq Greenstone Belt in Quebec, Canada, could be the remains of microbes. This succession of former sediments and volcanic rocks is over 4 billion years old. The new model makes it seem unlikely that the observed concretions are indeed fossils.The researchers conclude that their findings can shed light not only on early Earth processes, but also on the role those processes play in modern day climate and its long-term evolution.The study, "Rapid rise of early ocean pH under elevated weathering rates," was published in the journal Nature Geoscience.Additional material and interviews provided by Jim Shelton, Yale University.