Earth's atmospheric composition continues to change through natural geochemical processes
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Peer-reviewed literature and reference materials report that Earth's atmospheric composition and geochemical processes continue to evolve through natural geological and biological interactions.
ABSTRACTAnalyses of the chemistry of the earth's sedimentary shell (and the crystalline crust) reveal evolutionary trends in sedimentary rock composition, depositional environments, and the intensity and relative importance of geochemical processes. Successive changes in the mode of crustal movement, the intensity of volcanic and intrusive activity, and in the structural plan of continents (reduction of the area of active geosynclines, increase of the area of platforms) caused a regular transformation of the petrographic composition of ancient erosion areas and corresponding changes in the composition of terrigenous material that at different stages was supplied to depositional areas (gradually lower amounts of greywacke, decrease of average Na, Mg, Fe and Al content, inverse development of K and Ca). Sediment‐composition and lithological proportion in the sedimentary shell changed in the course of time: the abundance of volcanogenic rocks decreased, the abundance of carbonates and evaporites increased, the types of prevailing carbonate accumulation changed (siderite→dolomite→calcite), the degree of compositional differentiation of terrigenous sediments increased. These changes were accompanied by regular evolutionary changes in the chemical composition of the most important sedimentary rock types: the lowering of average Na and Fegen contents, the increase of Corg, Ssulf and Spyr concentration, the inverse K development, the increase of the Ca/Mg ratio, the rise of the iron oxidation degree, the displacement of isotope ratios of oxygen and sulfur, etc.A definite trend in the development of the main types of sedimentary geochemical processes is established. The processes of clastogenesis and hydrolysis developed inversely: the importance of the first process decreased, but that of the second increased in the course of time. The intensity of oxidation and precipitation processes also increased through time; chemical precipitation was predominant during the early development stages, and biogenic precipitation predominated during the later stages. Characteristic boundaries in the evolution of sedimentary rock composition and geochemical processes are traced, which correspond to the boundaries A and Pt1, Pt1 and Pt3, Rf3 and V, Pz3 and Mz. Discontinuous variations in the concentrations and ratios of elements in sediments and in the intensity of some geochemical processes are confined to these boundaries. They were determined by changes in the composition of erosion areas, by the increasing importance of platform types of sedimentary processes, by large changes in the biosphere, by the evolution of the atmosphere and ocean composition, and by changes in the types of surface equilibria in the dynamic system: atmosphere‐ocean‐stratisphere.
Carbon dioxide (CO2) is released from the Earth’s interior into the atmosphere through both volcanic and non-volcanic sources in a variety of tectonic settings. A quantitative understanding of CO2 outgassing fluxes in different geological settings is thus critical for decoding the link between the global carbon budget and different natural processes (e.g., volcanic eruption and earthquake nucleation) and the effects on the climate evolution over geological time. It has recently been proposed that CO2 degassing from non-volcanic areas is a major component of the natural CO2 emission budget, but available data are still sparse and incomplete. Here, we report the results of a geochemical survey aimed at quantifying CO2 emissions through cold and thermal springs of the tectonically active Pollino Massif and Calabrian arc (Southern Italy). The chemical ad isotopic (He and C) composition of fifty-five dissolved gas samples allows to identify two different domains: 1) a shallow system dominated by gas components of atmospheric signature (helium, hereafter He) and biogenic origin (C), and 2) a deeper system in which crustal/deep fluids (CO2 and He) are dominant. The measured He isotope ratios range from 0.03 to 1.1 Ra (where Ra is the He isotopic ratio in the atmosphere) revealing a variable atmospheric contamination. Furthermore, the He isotopic data indicate the presence of traces of mantle He contributions (2%–3%) in the thermal groundwater. The prevailing low R/Ra values reflect the addition of crustal radiogenic 4He during groundwater circulation. Using helium and carbon isotope data, we explore the possible sources of fluids and the secondary processes (dissolution/precipitation) that act to modify the chemistry of pristine volatiles. For the thermal springs, we estimate a deep C output of 2.3 x 107 to 6.1 x 108 mol year−1. These values correspond to deep CO2 fluxes per square km comparable with those estimated in several active and inactive volcanic areas and in continental regions affected by metamorphic CO2 degassing (e.g., the southern margin of the Tibetan Plateau).
The long term evolution of Earth’s atmosphere and climate has been an active topic of investigation for at least the last 60 years. My own participation in this investigation goes back more than 45 years, and this monograph relates that story from my personal perspective.
One major thread concerns the rise of atmospheric O2 from near-zero levels initially to the 21 percent mixing ratio that we observe today. Photochemical models developed by me and my students, along with some close colleagues, have helped to better constrain the prebiotic O2 concentration and to interpret the constraints imposed by the record of mass independent fractionation of sulfur isotopes. Most geochemists now agree that a so called Great Oxidation
Event (GOE) occurred between 2.4 and 2.2 Ga and that the atmosphere has been O2-rich since that time. However, the exact level of O2 during the ensuing Proterozoic Era remains controversial, as do the timing and magnitude of subsequent O2 increases. The corresponding development of the ozone layer is also of interest because of its moderating influence on surface solar UV fluxes and their effect on biological evolution. This can also be studied with photochemical models.
A second thread concerns the gradual decline in atmospheric CO2 in response to slowly increasing solar luminosity. The early Earth would have been frozen had the atmosphere not contained high concentrations of greenhouse gases, most importantly CO2. A negative feedback in the carbonate-silicate cycle that controls CO2 over long time scales has ensured that Earth’s surface has remained habitable during most of Earth’s history, despite occasional forays into Snowball Earth conditions. Evidence from palaeosols provides support for this hypothesis. CH4 is an additional greenhouse gas that may have supplemented surface warming prior to the GOE. The increase in O2 at that time may have caused CH4 to decrease, possibly triggering the Huronian glaciations. The same feedback mechanism that controls long term CO2 evolution on Earth could operate on Earth-like planets orbiting other stars, increasing the probability that some of them may harbour life. Large direct imaging space telescopes currently under development may eventually allow us to test this hypothesis and to learn whether we have company in this part of our galaxy.
Earth is the third planet from the Sun and the only astronomical object known to harbor life. This is made possible by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all of Earth's water is contained in its ocean, which covers 70.8% of Earth's crust. The remaining 29.2% of Earth's crust is land, which is predominantly located within Earth's lan
The main part of Earth's magnetic field is generated in the core, the site of a dynamo process that converts the kinetic energy of thermally and compositionally driven convection into electrical and magnetic field energy. The field extends outwards from the core, through the mantle, and up to Earth's surface, where it is approximately a dipole.…
Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km (6.8 mi) of the surface; this lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower-density air then rises and is replaced by cooler, higher-density air. The result is atmospheric circulation that drives the weather and climate through redistribution of thermal energy.
The atmosphere is composed of 78.084% nitrogen, 20.946% oxygen, 0.934% argon, and trace amounts of carbon dioxide and other gaseous molecules. Water vapor content varies between 0.01% and 4% but averages about 1%. Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 Gya, forming the primarily nitrogen–oxygen atmosphere of today. This change enabled the proliferation of aerobic organisms and, indirectly, the formation of the ozone layer due to the subsequent conversion of atmospheric O2 into O3. The ozone layer blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature. This last phenomenon is the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the surface, thereby raising the average temperature. Water vapor, carbon dioxide, methane, nitrous oxide, and ozone are the primary greenhouse gases in the atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C (−0.4 °F), in contrast to the current +15 °C (59 °F), and life on Earth probably would not exist in its current form.
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