| dc.description.abstract | The Earth has experienced a complex and profound evolution since it formed 4.6 billion years ago (4.6 Ga). Based on stratigraphic characteristics and geochemical data, the Earth’s history can be divided into four Eons: Hadean, Archean, Proterozoic, and Phanerozoic. The Phanerozoic began at the Cambrian. Therefore, the period before the Cambrian is also called the Precambrian. The atmosphere not only kept the Earth’s climate warm but also fueled the metabolism of organisms. Therefore, it is crucial to understand the atmosphere, especially its compositions, at various geo-periods. The Great Oxidation Event (GOE) that happened at ~2.2 Ga surpassed the mineralization of reducing minerals, resulting in the chronological discontinuity of pyrites which is a key constraint of the atmospheric compositions. Therefore, another constraint that could be applied to reconstruct the atmospheric composition is urgently required.
Micrometeorites (MMs) are interplanetary particles with sub-millimeter radii that have passed through the atmosphere and landed on the surface after being captured by the Earth’s gravity. MMs, especially iron-type micrometeorites (IMMs), collide and react with air molecules during their trajectories. As a result, they can record atmospheric compositions. Based on such a characteristic, IMMs could be developed to constrain the atmospheric oxidants level at any geo-period as long as fossil IMMs (FosIMMs) preserved at the corresponding age are discovered.
Simulating IMMs requires an integrated understanding of four subdisciplines: atmospheric chemistry, chemical reactions and kinetics between IMMs and air, physical properties of iron and its oxides, and IMMs’ motion. However, previous IMMs modelling work did not integrate the noted four subdisciplines in one model, weakening the accuracy of their results. Based on existing work and after detailed discussions about IMMs-involved processes, the micrometeorite oxidation model including photochemistry (MOMIP) has been built in this dissertation. A major function of the MOMIP is simulating IMMs’ oxidation and their trajectories in assumed atmospheres. This function can be furtherly developed to reconstruct the early atmosphere. The detailed procedure consists of 4 steps: 1) Set a possible atmosphere (such as an Archean atmosphere with 10% carbon dioxide in mixing ratio or a proterozoic atmosphere with 1% oxygen) into a photochemical model in order to obtain realistic atmospheric compositions; 2) Simulate the motion and oxidation of pure iron particles with different initial properties in the atmosphere output from the photochemical model; 3) Compare the compositions and radii of the simulated IMMs with the excavated fossil IMMs (FosIMMs), and judge whether the assumed atmosphere is sufficiently oxidizing to reproduce the FosIMMs; 4) Repeat the former 3 steps until finding the minimum content of atmospheric oxidants required to explain the generation of FosIMMs.
Via the MOMIP, FosIMMs that were formed at 2.7 Ga and 1.625 Ga have been applied to constrain the atmospheric oxidant level at corresponding geo-ages. The main findings and conclusions are presented below:
(1) If, following previous studies, sublimation is not considered and assuming no significant radii changes, the excavated 2.7 Ga FosIMMs suggest an Archean atmosphere containing at least 32% carbon dioxide (CO2) in mixing ratio. The greenhouse effect of such a high CO2 level would be enough to offset the dimming effect of the young Sun at that time. However, in this approach, the temperature of the IMMs exceeds the sublimation point, leading to inaccuracies in the results.
(2) When sublimation is considered, only 0.2% atmospheric CO2 is sufficient to explain the oxidation of the excavated 2.7 Ga FosIMMs. Therefore, these 2.7 Ga FosIMMs are not a solid signal of high Archean atmospheric CO2 levels, and the Faint Young Sun Problem remains unsettled.
(3) Simulations of the FosIMMs formed at 1.625 Ga indicate that the atmospheric O2 level was at least 4% present atmospheric level (PAL). A Mesoproterozoic atmosphere containing >4% PAL O2 also suggests that the O2 rise is not the only critical factor triggering eukaryotes' evolution. Otherwise, there should have evidence for primitive animals existing at 1.625 Ga or near geo-age.
Because of the property of reacting with air, simulating IMMs provides another method to explore the early atmosphere. The estimated minimum level of oxidants in Precambrian atmospheres may be higher if larger FosIMMs formed at the same or similar periods are discovered. If sufficient FosIMMs formed at different Eons are found in the future, it would be possible to reconstruct the whole history of the atmospheric oxidant levels via MOMIP. | en_GB |
