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August 4, 2021 10:00 am

'Totally New' Idea Suggests Longer Days On Early Earth Set Stage For Complex Life

"A research team has proposed a novel link between how fast our planet spun on its axis, which defines the length of a day, and the ancient production of additional oxygen," reports Science Magazine. "Their modeling of Earth's early days, which incorporates evidence from microbial mats coating the bottom of a shallow, sunlit sinkhole in Lake Huron, produced a surprising conclusion: as Earth's spin slowed, the resulting longer days could have triggered more photosynthesis from similar mats, allowing oxygen to build up in ancient seas and diffuse up into the atmosphere." From the report: As a postdoc at the University of Michigan, Ann Arbor, Klatt had studied microbial mats growing on sediments in the Middle Island Sinkhole in Lake Huron. There, the water is shallow enough for the cyanobacteria to get enough sunlight for photosynthesis. Oxygen-depleted water and sulfur gas bubble up from the lake floor, creating anoxic conditions that roughly approximate conditions of early Earth. Scuba divers collected samples of the microbial mats and in the lab, Klatt tracked the amount of oxygen they released under various day lengths simulated with halogen lamps. The longer the exposure to light, the more of the gas the mats released. Excited, Klatt and Arjun Chennu, a modeler from the Leibniz Center for Tropical Marine Research, set up a numerical model to calculate how much oxygen ancient cyanobacteria could have produced on a global scale. When the microbial mat results and other data were plugged into this computer program, it revealed a key interaction between light exposure and the microbial mats. Typically, microbial mats "breathe" in almost as much oxygen at night as they produce during the day. But as Earth's spin slowed, the additional continuous hours of daylight allowed the simulated mats to build up a surplus, releasing oxygen into the water. As a result, atmospheric oxygen tracked estimated day length over the eons: Both rose in a stepped fashion with a long plateau. This "elegant" idea helps explain why oxygen didn't build up in the atmosphere as soon as cyanobacteria appeared on the scene 3.5 billion years ago, says Timothy Lyons, a biogeochemist at the University of California, Riverside. Because day length was still so short back then, oxygen in the mats never had a chance to build up enough to diffuse out. "Long daytimes simply allow more oxygen to escape to the overlying waters and eventually the atmosphere," Lyons says. Still, Lyons and others say, many factors likely contributed to the rise in oxygen. For example, Fischer suspects free-floating cyanobacteria, not just those in rock-affixed mats, were big players. Benjamin Mills, an Earth system modeler at the University of Leeds, thinks the release of oxygen-binding minerals by ancient volcanoes likely countered the early buildup of the gas at times and should be factored into oxygen calculations. Nonetheless, changing day length "is something that should be considered in more detail," he says. "I'll try to add it to our Earth system models."

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