Periodic Reporting for period 2 - EARTHBLOOM (Earth’s first biological bloom: An integrated field, geochemical, and geobiological examination of the origins of photosynthesis and carbonate production 3 billion years ago)
Période du rapport: 2018-08-01 au 2020-01-31
The biosphere has been dominated by microbial life for most of Earth’s four and a half billion-year history. Geological records testifying to the presence of microbes on Earth date back to at least three and a half billion years ago. Around three billion years ago, photosynthetic microbes began precipitating large amounts of carbonate minerals, generating entire carbonate platforms similar to the modern day Bahamas. However, instead of coral, these platforms were made of the organo-mineral structures called stromatolites that primitive microbes precipitated as the result of their photosynthetic activity. The capture of atmospheric carbon dioxide, its precipitation into carbonate minerals, and its eventual subduction back into the mantle, is a critical process in the regulation of Earth’s long-term carbon cycle. Understanding how this process began, and what role primitive microbes played in this process, is of paramount importance for understanding how habitable planets establish and maintain a stable and life-sustaining climate state.
Around three billion years ago, photosynthesis was in its infancy. There exist several different kinds of photosynthesis on Earth, but it is overwhelmingly one kind of photosynthesis, so-called oxygenic photosynthesis, that fuels Earth’s modern biosphere. Oxygenic photosynthesis involves the oxidation of water and the release of free oxygen, and this process has played a central role in the evolution of Earth’s surface from oxygen-starved to oxygen-replete conditions over billions of years. However, it is debated when exactly this process began on Earth. The EARTHBLOOM project is focused on a period in time, three billion years ago, when this fundamental metabolism may have first evolved or was in its infancy. Indeed, it may be that the history of carbonate production on Earth is intimately related to evolutionary milestones in the evolution of photosynthesis.
In short, the EARTHBLOOM project seeks to better understand how Earth began making large amounts of carbonate minerals around three billion years ago – and what role Earth’s primitive photosynthetic biosphere played in this important revolution in the chemistry of Earth’s surface.
The EARTHBLOOM project is using multiple diamond drill cores, donated by a gold mining company, to study the architecture and chemistry of one of Earth’s oldest carbonate platforms, preserved in the Red Lake area, Ontario. These drill cores have now been carefully examined and a detailed stratigraphic log established that describes the nature and thickness of the carbonate rocks and their relationships to other lithologies in the cores. These drill cores have also been extensively sampled and analyzed for chemical signatures that provide insight into carbonate mineral precipitation processes, the chemistry of the seawater in which they were precipitated, in the availability of nutrients in ancient seawater, and whether there was free oxygen being produced at the time of carbonate precipitation. Data from the drill cores has been supplemented with additional samples obtained from geological field work in the Red Lake area, where mapping and surface observations provide additional insights into the composition and architecture of this unique deposit. The EARTHBLOOM project is also examining other sites of similar age in Northwestern Ontario where stromatolitic carbonates are found in abundance, in order to better understand the full extent of carbonate precipitation at this time, and the diversity of habitats and the primitive photosynthetic life that occupied them.
To date, the EARTHBLOOM project has obtained several important results. These include (1) confirmation that the carbonate occurrences in Northwestern Ontario really do hold one of Earth’s most ancient thick occurrences of carbonate minerals, and host an exceptional abundance of fossilized photosynthetic microbial communities (stromatolites), (2) that these communities were diverse, forming a variety of carbonate structures in a wide range of sedimentary settings, (3) that abundant sulfur minerals found in association with the ancient photosynthetic communities were derived from surface reservoirs and show chemical signatures of both atmospheric and biological processing, (4) that chemical records of carbon cycling are rather unusual in these communities, and indicate that some of these microbes may have acquired their carbon using somewhat atypical biological pathways, and (5) that free oxygen was sometimes present in shallow seawater, but was generally scarce. This latter point is especially interesting considering that on the modern Earth, only oxygenic photosynthesis is generally considered an important driver of carbonate mineral precipitation, while alternative forms of photosynthesis that don't produce oxygen, so-called anoxygenic photosynthesis, is not thought to have been important for carbonate mineral precipitation.
The EARTHBLOOM project has now identified, using sophisticated chemical tracers, some of the oldest evidence for the accumulation of free oxygen in shallow waters on what are amongst Earth’s oldest shorelines. Going forwards, these chemical tracers will be further examined using novel methods that will permit us to directly date the chemical signals of oxygen – and thereby resolve a major ongoing debate as to whether such ancient signals of oxygen are truly reflecting oxygenic photosynthesis around three billion years ago, or whether such signals are a later occurrence arising from interaction with younger fluids in contact with Earth’s later, more oxidizing atmosphere.
It is expected that by the end of the EARTHBLOOM project, we will have a complete understanding of what triggered the onset of massive carbonate precipitation on Earth and how the primitive microbial photosynthetic communities that controlled carbonate precipitation were functioning – how they were acquiring their carbon, how they were metabolizing, and how they were influencing their chemical environment. These results will speak directly to how Earth’s most important metabolisms evolved, and how the primitive biosphere set the Earth on a three-billion-year trajectory of clement climatic conditions and sustained habitability.