Granite rocks formed from melted sediments are revealing how much ancient plant and animal matter was buried over billions of years—and that record could rewrite what we know about Earth’s changing atmosphere. The problem is a stubborn gap in knowledge. Scientists know that burying organic carbon removes it from the air and boosts oxygen levels, which allowed complex life to evolve. But after decades of work, no one agrees whether biomass burial has stayed constant or increased more than tenfold over the past 3.5 billion years. The early sedimentary rock record is mostly gone, destroyed by plate tectonics. This project uses granite—a tougher survivor of that destruction—as a proxy. The team has already found that nitrogen levels in granite jumped after the rise of complex life. Now they need to understand what happens to nitrogen when sediments melt into granite. If successful, this research will provide the first quantitative link between plate tectonics, biomass burial, and atmospheric composition over deep time. It is fundamental science: it will not directly change daily life, but it will answer a long-standing question about how Earth’s air, life, and rocks co-evolved. Similar fundamental work on ancient atmospheres has underpinned climate models and our understanding of planetary habitability.
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The chemistry and mass of Earth's atmosphere have changed markedly over the past 4.5 billion years, but quantitative constraints are rare. Earth's environmental history is traditionally read from sedimentary rocks that formed at Earth's surface. This approach has yielded important insights into oxygenation events that facilitated the rise of complex life. A key process is the removal of organic carbon which led to an increase in atmospheric O2. However, despite over half a century of effort to quantify changes in biomass burial there is no agreement concerning the scale of change in biomass burial through time. For example, recent predictions range from no significant change to >10-fold increase over the past 3.5 billion years. One major issue is that there is no sedimentary rock record for most of early Earth's history because sedimentary rocks are metamorphosed and/or removed by the relentless churn of plate tectonics. However, when tectonic plate collide sediments are partially melted and produce a robust rock which dominates the oldest sections of Earth's rock record - granite. We have shown that granites produced via the melting of sediments show statistically significant enrichment in nitrogen contents since the end of the pre-Cambrian, which is coeval with the rise of complex life. The key known-unknown required to convert the data from the granite record into the volume of biomass buried in sediments is what happens to sedimentary-hosted nitrogen during crustal melting? The answer contains the variables required to quantitively evaluate the effect changes in biomass burial have had on the N2/O2 and CO2/O2 ratios of Earth's atmosphere. We have designed a project to answer this question which will enable us to quantitively assess how plate tectonics and changes in biomass burial have co-contributed to shaping Earth's present-day environmental conditions, thus charting the co-evolution of Earth's atmosphere, biosphere, and geosphere.
Eva Stueeken (Co-Investigator)Nicholas Gardiner (Co-Investigator)Paul Savage (Co-Investigator)Richard White (Co-Investigator)Sami Mikhail (Principal Investigator)
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