This week we're going to talk about the changing composition of the early atmosphere, and one of the most profound events in that changing composition. In fact one of the most profound events in Earth evolution, was the oxygenation of the atmosphere beginning about 2.4 billions years ago, when the atmosphere essentially contained no oxygen to about 2.1 billion years. and in that interval, the oxygen content of the atmosphere increased to approximately several tenths of a percent, or perhaps a percent. That's known as the Great Oxygenation Event. And, what we're going to do is spin out a theory that the oxygen that that caused this increase came from photosynthesis. from photosynthesizing organisms such as cyanobacteria, that appeared in the geologic, or that appeared on, on the planet perhaps a billion years before. The primary evidence being in fossils known as stromatolites. So the question is, well, what do we really know? And that's what I want to explore today. And to begin that exploration, I'm going to pose two questions. The first question is, was the GOE, in other words, the Great Oxygenation Event, primarily due to biological innovation? And by that we're talking about photosynthesis. Or was there some fundamental change in the solid earth that contributed to or was the primary driver of GOE, of the GOE? In other words, is there some other cause, or something else that combined with the production of photo the production of oxygen, by photosynthesis that caused this increase in oxygen? And then second question is that if photosynthesis was important, did photosynthesis actually emerge just before the Great Oxygenation Event at the end of the Archean? Or did photosynthesis begin earlier in Earth history? And did oxygen just was oxygen just slowly added to the Earth's system to some point where it, there, there was a threshold that reached, and all of a sudden it increased dramatically? So let's investigate those two questions. To begin our investigation, let's consider the evidence for the earliest life. And to do that, we'll go to West Greenland, in a sequence of rocks known as the Isua Complex. And these rocks crop out near the capital of Greenland, which is the town of Nuuk. they crop out near the ice cap. And the, the Isua rocks are essentially among the oldest surface deposits that we know of on Earth. They consist mainly of highly metamorphosed lavas, and highly metamorphosed other kinds of rocks, that in some cases, we really can't recognize what they are. But in some places, what one finds are little enclaves, that one can actually recognize what the rocks were before they were metamorphosed. And this photograph right here is is of one such outcrop. And I think you can see the bedding in that photograph, suggesting that that layer, layering suggest that it was sedimentary. these rocks date to 3800 million years ago. And they hold the evidence for the first life. So what is that evidence? As I said, these are metamorphic rocks, and these metamorpha sediments contain the mineral garnet. we know from dating techniques, that that garnet formed at about 3.8 billion years. But as you can see in this particular slide, the garnet contains small inclusions. And these inclusions form trains, that run through the garnet. And many of those inclusions are made of carbonaceous material. So, this is a map of the distribution of carbon over this garnet grain. And you can actually map inclusions with these high concentrations of carbon. It turns out that the ratio of carbon 13 to carbon 12 in those inclusions, is consistent with a biogenic origin for these carbons. That is the primary evidence that life existed on Earth at 3.8 billion years ago. This is not very strong evidence, we have to admit. it's not very strong evidence, because we don't really know what happened to this carbon during the metamorphism. Metamorphic processes can actually change the carbon 13 to carbon 12 ratio. so we don't actually know what happened, prior to the entrapment of the carbon in the garnet. But what we do know, is that these carbons these inclusion carbons, have not changed their composition since they were trapped at 3.8 billion years ago. Now as I mentioned the evidence for photosynthetic organisms also exist in the form of stromatolites. The first evidence of which appears on the planet about 3.4 billion years ago. But a very important piece of evidence about the nature of early life, occurs at about 2.7 billion years. When we find bio-markers, or in other words, molecular fossils, that provide evidence that bacteria and eukaryotes existed on Earth at that time. Molecular fossils, that is to say bio-markers, are these relatively refractory organic compounds. they're derived mainly from lipids. lipids are compounds that make up cellular membranes, for example. And these compounds become incorporated in the sediments, and of course they do break down with time and, and lithification. But absent extreme metamorphism, certain structural components of these compounds survive, and we find them in these rocks. So they've survived for billions of years. Some of these structural components are diagnostic of bacteria such as cyanobacteria. Some other structural components, are also diagnostic of eukaryotes. And if that's true, eukaryotes certainly must have indicated the presence of photosynthesis. So there's good evidence, that photosynthesis existed at least by 2.7 billion years, and probably earlier. So what is the significance of these observations that life may have existed on Earth at 3.8 billion years? And that by at least 2.7 billion years fairly complex forms of life existed. Well, it's embodied in in this statement here, the Archean saw the emergence of quote, diverse metabolic pathways, photosynthesis, carbon fixation, nutrient assimilation, and lipid biosynthesis. It was the cardinal epoch of biochemical innovation. And that's a quote from Waldbauer et al., in 2009. That's the significance of these observations. The Archean was this cardinal epoch of biochemical innovation. But now let's think about photosynthesis. Here's a model reaction. Carbon dioxide plus water goes to the organic compounds that make up the organism plus oxygen, and of course we need energy for that, in the form of sunlight. Well, I think you can see by this model reaction, what I mean by a model reaction, is a simple reaction that illustrates a general process. I think you can see that the oxygen actually isn't going to build up in the system unless the oxygen and the oxygen and the organic material Is separated. Because if the organic material is just sitting around on the surface, it's going to oxidize with any excess oxygen to form carbon dioxide, and oxygen isn't going to build up in the system. So we need to figure out a way to get rid of the organic material. And one way to do that is to bury it. And now this brings us to another question, how are we going to bury organic compounds? And it brings us to the matter of the role of tectonics of this whole affair. Here's a map. This is a map of all the present day continents, not in their present position of course. And the black areas are the areas of exposed Archean crust. The gray areas of exposed Proterozoic crust, and the white areas the you know, the younger Phanerozoic crust. That is crust that's less than about 640 billion years old. so there's extensive Archean crust. But what we observe is that most Archean crust is really made of igneous rocks and metamorphic equivalents of igneous rocks. While the Proterozoic crust, is characterized as well by extensive sedimentary basin. So at the end of the Archean, just before the Great Oxygenation Event, it appears that that was the time of the beginning of the development of large sedimentary basins, and erosion of the continents. So in other words this is a time also of extensive shallow seas. Which could have been you know, provided expanded habitat for stromatolites. the continents must have eroded at this time, to produce all this sediment that we see in the rock record. erosion implies that it was rock weathering, and rock weathering implies that that, or, or is a process rather that we expect to influence both ocean composition, and atmosphere composition. And the presence of the sedimentary basins is one way to rapidly very organic material. More generally, plate tectonics must have existed at this time. And that also means that there was biochemical recycling, between the surface of the planet and the mantle. So maybe this burial involved injection of carbon by subduction and water by subduction into the mantle. So all of these processes could help oxidize the Earth. So let's return to the original questions. you know, what was the cause of the Great Oxygenation Event? And, you know, what was the role of photosynthesis? And, I think fundamentally, we just don't know the answers to those questions. They remain unanswered.
