Evidence for Earth's early atmosphere
Chemistry of the atmosphere • Evolution of the Earth's atmosphere
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Composition of the early atmosphere
The early atmosphere contains high levels of carbon dioxide, water vapour and nitrogen produced by volcanic outgassing. Low free oxygen dominates because oxygen-producing photosynthesis has not yet become widespread, so oxidised minerals form only where local oxygen exists. Volcanic gases supply CO2 and H2O, causing a greenhouse effect that maintains warmer surface temperatures despite a fainter young Sun. The presence or absence of oxygen controls the chemical reactions in surface waters and minerals, creating diagnostic evidence in sediments and rocks.
Banded iron formations (BIFs)
Banded iron formations form when dissolved iron in seawater oxidises and precipitates as iron-rich layers alternating with silica or carbonate. The cause is rising oxygen levels in local surface waters produced by photosynthetic organisms; the effect is deposition of alternating iron and silica bands that preserve episodes of oxygen availability. The timing and distribution of BIFs indicate large-scale changes in ocean oxygenation. A decline in widespread BIF formation marks the rise of persistent atmospheric oxygen because most dissolved iron then precipitates earlier or becomes limited.
Stromatolites and microfossils
Stromatolites form by layers of microbial mats trapping sediments and hint at early photosynthetic life. The cause is microbial activity, often cyanobacteria-like organisms, producing oxygen as a by-product; the effect is laminated structures preserved in ancient rocks. Microfossils and stromatolite abundance indicate biological oxygen production before large atmospheric oxygen increases. Presence of such fossils documents biological sources of oxygen but not immediate global oxygenation because oxygen can remain locally consumed by reduced materials.
Isotope evidence (carbon and sulfur isotopes)
Isotope ratios change because biological and chemical processes fractionate isotopes differently. The cause is preferential uptake or reaction of lighter isotopes by biological or chemical reactions; the effect is measurable shifts in 12C/13C and 32S/34S ratios preserved in ancient rocks. Variations in carbon isotopes indicate shifts between organic carbon burial and atmospheric CO2 levels. Large sulfur isotope anomalies in very old rocks indicate low atmospheric oxygen because mass-independent fractionation requires an oxygen-poor atmosphere.
Red beds, paleosols and oxidation indicators
Red beds and oxidised paleosols form when iron in soils oxidises in the presence of free atmospheric oxygen. The cause is sustained atmospheric oxygen interacting with surface materials; the effect is red or rusty colouring in sedimentary rocks and soil horizons. Appearance of red beds in the rock record marks the establishment of appreciable atmospheric oxygen. The absence of red beds before a certain age implies limited atmospheric oxygen or continuous consumption of oxygen by reduced surface reservoirs.
Trapped gases and mineral inclusions
Gas inclusions in minerals and fluid inclusions in ancient sediments trap samples of ancient atmospheres and waters. The cause is entrapment of gas during mineral formation; the effect is direct chemical and isotopic information about past gas compositions when inclusions remain unaltered. Limitations arise because most very-old rocks undergo metamorphism that can modify or destroy inclusions. Only rare, well-preserved samples provide reliable trapped-gas evidence.
Limitations and uncertainties in interpretation
Post-depositional alteration, metamorphism and chemical exchange change original signals in rocks and fossils. The cause is burial, tectonism and chemical weathering; the effect is possible misinterpretation of original atmospheric conditions if alteration is not recognised. Dating uncertainties and local environmental effects create ambiguity. Accurate interpretation requires multiple, independent lines of evidence (stratigraphy, isotopes, fossils) and assessment of possible alteration.
Key notes
Important points to keep in mind