Princeton University researchers report in the journal Nature that rocks preserved in Earth's crust reveal that a steep declinein the intensity of melting within the planet's mantle -- the hot,heat-transferring rock layer between the crust and molten outercore -- brought about ideal conditions for the period known as theGreat Oxygenation Event (GOE) that occurred roughly 2.5 billionyears ago. During the GOE -- which may have lasted up to 900 million years --oxygen levels in the atmosphere exploded and eventually gave riseto our present atmosphere. Blair Schoene, a Princeton assistant professor of geosciences, andlead author C. Brenhin Keller, a Princeton geosciences doctoralstudent, compiled a database of more than 70,000 geological samplesto construct a 4-billion-year geochemical timeline. |
Their analysisuncovered a sharp drop in mantle melting 2.5 billion years ago thatcoincides with existing rock evidence of atmospheric changesrelated to the GOE. Based on this correlation, the researchers suggest in Nature thatdiminished melting in the mantle decreased the depth of melting inEarth's crust, which in turn reduced the output of reactive, ironoxide-based volcanic gases into the atmosphere. A lowerconcentration of these gases -- which react with and remove oxygenfrom the atmosphere -- allowed free oxygen molecules toproliferate. The Princeton research offers the strongest data-driven correlationyet between deep Earth processes and the GOE, Schoene said.Previous hypotheses are largely based on qualitative observationsof the rock record and computational models that simulate how thisrapid oxygenation might have occurred.
The Princeton research,however, is based on a statistical analysis of the geologic recordand the chemical traces of deep-Earth activity it has preserved,Schoene said. "The perspective behind past efforts to connect geologicprocesses to the Great Oxygenation Event has been hypothetical,saying that 'If the Earth had been X, there would have beenreaction Y,'" Schoene said. "But these ideas cannot betested experimentally because they are largely notional. In ourpaper, we have the evidence to say, 'The Earth was like this,' andthen propose a hypothesis that can be tested by examining the samerich database of mantle and deep-crust changes we used in ourwork." A change in subsurface activity around the time of the GOE has beennoted before, Keller explained.
But evidence of that shift isgeochemically subtle, especially after billions of years. Thedatabase he and Schoene created allowed them to show more preciselyhow the geochemical makeup of the crust changed through time,resulting in a more detailed hypotheses about how this would affectthe atmosphere, Keller said. "Research in this area has been largely qualitative, but withthis much data, we can pick up finer features in the geologicrecord, particularly a level of detail related to this suddenchange 2.5 billion years ago that people had not seen with suchclarity before," Keller said. A missing piece of the GOE puzzle? Woodward Fischer, an assistant professor of geobiology at theCalifornia Institute of Technology who specializes in the GOE, saidthat the Princeton research could help shed more light on animportant factor in Earth's oxygenation that is not wellunderstood. Fischer is familiar with the paper but had no role init.
The dominant theory of oxygenation is that an abundance ofphotosynthetic life emerged some hundreds of millions of yearsbefore the GOE and began producing oxygen via photosynthesis,Fischer said. The problem is that this output would not have beenenough to overcome "sinks" that were absorbing moreoxygen from the atmosphere than was being put into it. So, alingering question is what happened to those sinks to bring aboutoxygenation. Keller and Schoene show how one of the primary sinks -- volcanicgases -- might have suddenly been neutralized, Fischer said. Theexact effect this would have had on atmospheric oxygen levels isdifficult to know -- even recent fluctuations are hard to gauge, hesaid.
Nonetheless, the clear and objective data the researchers usestrongly suggests that a quick reduction in volcanic gases broughtabout by a drop in mantle-melt intensity was an important precursorto oxygenation, Fischer said. "This paper offers a really striking assessment of changesoccurring in the solid Earth that greatly helped set the stage forone of the most marked environmental transitions in Earthhistory," Fischer said. "And their methodology precludes a strong tendency thatresearchers, as humans invested in our work, have to look foranecdotal geological evidence and conclude based on coincidencethat events co-occurring in time must have been related,"Fischer said. "The statistical approach taken by the authorsin this paper really lets the data shine and reveals that therewere important secular changes in the way the Earth made igneousrocks, and that these changes were possibly part of an interplaybetween life and deep-Earth processes." Keller and Schoene fashioned their expansive database frompreviously reported rock and trace element analyses, which areincreasingly available through online databases.
They focused onchanges in the chemical composition of basalt, a byproduct ofmelting in Earth's mantle. When melting in the mantle is high, Keller said, basalt containsgreater concentrations of "compatible" elements such aschromium and magnesium that are ordinarily found in the mantle.Less intense melting, on the other hand, results in basalt with ahigher content of incompatible elements such as sodium andpotassium that are found closer to Earth's surface. From their examination, Keller and Schoene saw that Earth's mantlehas undergone a gradual cooling since the planet's early history,which is consistent with scientists' expectations based on heatloss at Earth's surface. Around 2.5 billion years ago, however, thelevels of compatible elements in the sampled basalt plummeted,indicating that the magnitude of melting deep in the mantle droppedoff suddenly. Keller and Schoene confirmed their findings by checking themagainst existing analyses of crust-level "felsic" rockssuch as granite, which form when hot basalt merges with otherminerals.
Heightened melt activity in the mantle leads to deepermelting in Earth's crust, and felsic rocks can indicate theintensity of mantle melting, Keller said. The researchers conclude that when melting happens at a great depthin the crust then the concentration of the iron-oxide gases inmagma increases. When emitted into the air by volcanoes, thesegases bond with free oxygen and essentially remove it from the air.On the other hand, when crust melting becomes shallower, as theyobserved, atmospheric levels of those volcanic gases drop and freeoxygen molecules can flourish. Connecting the Earth's systems In a broader sense, said Schoene, his and Keller's research depictsa close interaction between Earth's geologic and biological systemsthat is becoming more apparent.
"In science, it is becomingincreasingly obvious that seemingly different systems act togetherand the question is how," Schoene said. "Overall, this analysis strengthens emerging arguments thatinteraction between the solid Earth and biosphere are very intimateand important," he said. "This is strong evidence of howbiological and geological systems might work together, and itsuggests that important planetary change is not simply the resultof life dragging the rest of the planet along." Fischer of Caltech added that this interplay of systems applies tovarious events in the planet's history -- such as mass extinctions-- that are the result of multiple factors both above and belowEarth's surface. Decidedly more difficult is tracing how theseevents influenced one another and ultimately led to a greaterplanetary change, he said.
"Because of the complicated questions of how solid Earthchanges lead to biological innovations, scientists now have tostart thinking deeply and working across the boundaries of whathave traditionally been pretty rigid subdisciplines in the Earthsciences," Fischer said. "It's clear from research like this," he said, "thatthere is hay to be made by interdisciplinary efforts to connectprocesses and mechanisms from the solid to the fluid Earth, and tounderstand that interplay with an ever-evolving biology.".
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