The Evolution of Atmospheric Composition: Why Earth is a Habitable Planet

During much of its early history Earth was dominated by an oxygen-poor, CO2+CO+methane-rich atmosphere, with several thousand to tens of thousands ppm CO2, inducing high-temperature low-pH acid ocean waters, extending beyond submarine fumaroles. Compensation of the low early solar radiation by the high greenhouse gas levels and the low albedo due to low continent/ocean ratio allowed presence of liquid water at the surface. The high water temperature resulted in little sequestration of CO2 accumulated in the atmosphere from episodic volcanism, impact cratering, metamorphic release of CO2, dissociation of methane from sediments and microbial activity. The low-oxygen levels of the Archaean hydrosphere limited marine life to extremophile cyanobacteria and, locally, photosynthesizing stromatolites, with limited release of oxygen about 3.5–3.4 Ga. Temperatures declined with the development of continental cratons and recycling of crustal material through the mantle in the Proterozoic and the Phanerozoic, including lowering of oceanic salinity due to sequestering of evaporite deposits in continental settings. Microbial methanogenesis involves reactions of CO2 with H2 or acetate (CH3CO 2 ─ ) produced from fermentation of photosynthetically produced organic matter. An overall increase with time in δ18O, shown by terrestrial sediments, reflects a long term recycling of cold crustal materials through the mantle. Long-term cooling of the atmosphere and hydrosphere was related to an overall intermittent temporal decline in atmospheric CO2, as shown by plant leaf pores. An abrupt disappearance of positive sulphur (MIF-S) anomalies at ~2.45 Ga suggests atmospheric enrichment in oxygen and development of an ozone layer related to progressive photosynthesis by algal activity. The origin of banded iron formations is interpreted in terms of microbial oxidation of ferrous (Fe+2) to ferric (Fe+3) iron under oxygen-poor atmospheric and hydrospheric conditions on the early Earth and direct chemo-lithotropic or photo-ferrotropic oxidation of ferrous to ferric iron. A biological significance of dolomite is corroborated by experimental studies that indicate precipitation of low-temperature dolomite in sedimentary systems and interstices of pillow lava under unoxidizing conditions and microbial mediation.

Late Miocene climate evolution provides an opportunity to assess Earth’s climate sensitivity to carbon cycle perturbation under warmer-than-modern conditions. Despite its relevance for understanding the climate system, the driving mechanisms underlying profound climate and carbon cycle changes – including the enigmatic Late Miocene cooling from 7 to 5.4 million years ago – remain unclear. Here, we present magnetic and geochemical paleoceanographic proxies from a hydrogenetic ferromanganese crust retrieved in the northwestern Pacific Ocean. Our results indicate a striking 50% surge in deep ocean phosphorus concentrations occurred 7 – 4 million years ago, synchronous with enhanced deep ocean oxygen consumption. Employing a global biogeochemical model, we show that increased continental phosphorus weathering, without a concurrent rise in silicate weathering, contributed to the decline in atmospheric CO2 and associated cooling over the Late Miocene. This suggests a prominent decoupling of phosphorus and silicate weathering during a major carbon cycling event over the last 10 million years.

Changes in oceanic circulation and carbon storage have been implicated as important drivers of atmospheric pCO2 variability during the last glacial cycle (0 to 150 ka), though consensus on the mechanisms responsible remains elusive due to sparse marine carbonate chemistry reconstructions. Here, we present vertical profiles of carbonate ion concentration ([CO32-]) from the Southwest Atlantic to investigate oceanic carbon sequestration during key intervals of atmospheric CO2 change. During the first major decline in atmospheric CO2 (115 to 109 ka), enhanced carbon storage occurred in the abyssal Atlantic (>3,500 m water depth). During the second decline in CO2 (72 to 68 ka), enhanced carbon storage occurred throughout the deep Atlantic (>2,000 m water depth). Paired with δ13C and δ18O vertical profiles from the same locations, the [CO32-] results suggest progressively greater sequestration of biological carbon occurred in the South Atlantic as the volume of Southern Source Water expanded during glacial inception. Shoaling of the carbonate ion saturation horizon during the second decline in pCO2 resulted in seafloor calcium carbonate dissolution and enhanced alkalinity, likely contributing to further draw down of atmospheric CO2. We also document carbon release from the deep South Atlantic during glacial Terminations I and II when atmospheric CO2 was rising. Overall, our results highlight the tight linkage between pCO2 and deep Atlantic carbon storage over the last glacial cycle.

The Oligocene-Miocene boundary Asian climatic reorganization linked to the northward migration of the East Asian monsoon into subtropical China is a potentially important but poorly constrained atmospheric CO2 consumption process. Here, we performed a first-order estimate of the CO2 consumption induced by silicate chemical weathering and organic carbon burial in subtropical China related to this climatic reorganization. Our results show that an increase in long-term CO2 consumption by silicate weathering varies from 0.06×1012 to 0.87×1012 mol yr−1 depending on erosion flux reconstructions, with an ~50% contribution of Mg-silicate weathering since the late Oligocene. The organic carbon burial flux is approximately 25% of the contemporary CO2 consumption by silicate weathering. The results highlight the significant role of weathering of the Mg-rich upper continental crust in East China, which would contribute to the rapid decline in atmospheric CO2 during the late Oligocene and the Neogene rise in the seawater Mg content. If this climatic reorganization was mainly induced by the Tibetan Plateau uplift, our study suggests that the growth of the Himalayan-Tibetan Plateau can lead to indirect modification of the global carbon and magnesium cycles by changing the regional hydrological cycle in areas of East Asia that are tectonically less active.

Response of ocean acidification to atmospheric carbon dioxide removal

Understanding the evolution of Earth and potentially habitable Earth-like worlds is essential to fathom our origin in the Universe. The search for Earth-like planets in the habitable zone and investigation of their atmospheres with climate and photochemical models is a central focus in exoplanetary science. Taking the evolution of Earth as a reference for Earth-like planets, a central scientific goal is to understand what the interactions were between atmosphere, geology, and biology on early Earth. The Great Oxidation Event in Earth's history was certainly caused by their interplay, but the origin and controlling processes of this occurrence are not well understood, the study of which will require interdisciplinary, coupled models. In this work, we present results from our newly developed Coupled Atmosphere Biogeochemistry model in which atmospheric O2 concentrations are fixed to values inferred by geological evidence. Applying a unique tool (Pathway Analysis Program), ours is the first quantitative analysis of catalytic cycles that governed O2 in early Earth's atmosphere near the Great Oxidation Event. Complicated oxidation pathways play a key role in destroying O2, whereas in the upper atmosphere, most O2 is formed abiotically via CO2 photolysis. The O2 bistability found by Goldblatt et al. ( 2006 ) is not observed in our calculations likely due to our detailed CH4 oxidation scheme. We calculate increased CH4 with increasing O2 during the Great Oxidation Event. For a given atmospheric surface flux, different atmospheric states are possible; however, the net primary productivity of the biosphere that produces O2 is unique. Mixing, CH4 fluxes, ocean solubility, and mantle/crust properties strongly affect net primary productivity and surface O2 fluxes. Regarding exoplanets, different "states" of O2 could exist for similar biomass output. Strong geological activity could lead to false negatives for life (since our analysis suggests that reducing gases remove O2 that masks its biosphere over a wide range of conditions). Key Words: Early Earth-Proterozoic-Archean-Oxygen-Atmosphere-Biogeochemistry-Photochemistry-Biosignatures-Earth-like planets. Astrobiology 16, 27-54.

We present new geochemical evidence of changes in oxygenation of the deep Antarctic Pacific over the last 770 kyr. Our data are derived from redox-sensitive metals and export production proxies extracted from gravity core ANT34/A2-10 at 4217 m water depth. Our results show that oxygen levels in the deep Antarctic Zone (AZ) varied in line with the release of deeply sequestered remineralized carbon to the atmosphere during glacial–interglacial (G–IG) cycles, with lower oxygen concentrations and more carbon storage during glacial periods. Subsequent reductions in the amount of carbon stored at depth were closely associated with improved ventilation during glacial terminations. The systematic and repeated glacial-to-interglacial increases in export production in the AZ region indicate a robust pattern of enhanced Southern Ocean (SO) ventilation during interglacial periods. In addition to the decline in atmospheric CO2 caused by iron fertilization in the Subantarctic AZ (SAZ) during the latter half of the glacial progression, decreases in productivity in the central AZ suggest that the weakening of SO ventilation induced deep AZ carbon sequestration and that this might have made a continuous additional contribution to the CO2 decline from each interglacial peak to glacial maximum. Observed variations in the degree of deep oxygenation and “organic carbon pump” efficiency in the central AZ might be driven primarily by physical “ventilation” processes (i.e., overturning circulation, mixing, and/or air–sea gas exchange). Our records of abyssal oxygenation in the central AZ, which vary in concert with atmospheric CO2 levels over the last several G–IG cycles, provide strong evidence that SO ventilation plays a significant role in controlling variations in both the amount of respired carbon sequestered in the deep ocean and atmospheric CO2 concentrations on G–IG timescales. Specifically, we suggest that the “organic carbon pump” (OCP) in the SAZ and the physical ventilation processes in the AZ (the “carbon venting valve”) acted together synergistically, but dominated at different intervals over G–IG cycles, to repeatedly switch the SO between carbon sink and carbon source, thereby modulating the atmospheric CO2 over the last 770 kyr. These findings provide new insights into the role of the AZ in controlling deep SO carbon sequestration and atmospheric CO2 levels in G–IG cycles.

The crustal Urey cycle of CO2 involving silicate weathering and metamorphism acts as a dynamic climate buffer. In this cycle, warmer temperatures speed silicate weathering and carbonate formation, reducing atmospheric CO2 and thereby inducing global cooling. Over long periods of time, cycling of CO2 into and out of the mantle also dynamically buffers CO2. In the mantle cycle, CO2 is outgassed at ridge axes and island arcs, while subduction of carbonatized oceanic basalt and pelagic sediments returns CO2 to the mantle. Negative feedback is provided because the amount of basalt carbonatization depends on CO2 in seawater and therefore on CO2 in the air. On the early Earth, processes involving tectonics were more vigorous than at present, and the dynamic mantle buffer dominated over the crustal one. The mantle cycle would have maintained atmospheric and oceanic CO2 reservoirs at levels where the climate was cold in the Archean unless another greenhouse gas was important. Reaction of CO2 with impact ejecta and its eventual subduction produce even lower levels of atmospheric CO2 and small crustal carbonate reservoirs in the Hadean. Despite its name, the Hadean climate would have been freezing unless tempered by other greenhouse gases.

Mammals have dominated the Earth for the last ~55 Myr. Mammals have shown remarkable adaptation and resilience to climate change. However, it is unknown how long the Earth will be able to continue to sustain mammalian life. Estimates suggest the ultimate demise of all life will be in a ‘Venusian’ style runaway greenhouse climate ~1.5 billion years where increasing solar luminosity (L☉) will raise temperature beyond that able to sustain life. However, conditions may develop sooner that will render the Earth naturally inhospitable to mammals. In ~250 million years all the continents of the world come together to form the Earth's fourth supercontinent, Pangea-Ultima. A natural consequence of the creation and decay of Pangea-Ultima will be extremes in pCO2, both low (silicate weathering) and high (volcanic degassing). Here we show that variations in pCO2, increased solar luminosity (~2% greater than now), and extreme continentality will lead to extreme climate states that are inhospitable to mammalian life. We assess the impact of these climate states on mammalian physiological limits using dry-bulb, wet-bulb, and Humidex stress indicators as well as planetary habitability index. Although low pCO2 states will increase habitability, snowball Earth conditions may occur if the silicate weathering-pCO2 burial feedback becomes too strong (resulting in low pCO2 values <280ppm) under increased L☉. Likewise, small short-term spikes in pCO2 (≥1120ppm) outgassing will lead to extremes in heat. Under such conditions, thermal tolerances of endotherms will exceed physiological limits leading to mass extinction. The results reported here also show that global landmass configuration, pCO2, and solar luminosity play a critical role in planetary habitability.

Productivity-climate coupling recorded in Pleistocene sediments off Prydz Bay (East Antarctica)

Human impacts prior to the Industrial Revolution are not well constrained. We investigate whether the decline in global atmospheric CO2 concentration by 7–10 ppm in the late 1500s and early 1600s which globally lowered surface air temperatures by 0.15∘C, were generated by natural forcing or were a result of the large-scale depopulation of the Americas after European arrival, subsequent land use change and secondary succession. We quantitatively review the evidence for (i) the pre-Columbian population size, (ii) their per capita land use, (iii) the post-1492 population loss, (iv) the resulting carbon uptake of the abandoned anthropogenic landscapes, and then compare these to potential natural drivers of global carbon declines of 7–10 ppm. From 119 published regional population estimates we calculate a pre-1492 CE population of 60.5 million (interquartile range, IQR 44.8–78.2 million), utilizing 1.04 ha land per capita (IQR 0.98–1.11). European epidemics removed 90% (IQR 87–92%) of the indigenous population over the next century. This resulted in secondary succession of 55.8 Mha (IQR 39.0–78.4 Mha) of abandoned land, sequestering 7.4 Pg C (IQR 4.9–10.8 Pg C), equivalent to a decline in atmospheric CO2 of 3.5 ppm (IQR 2.3–5.1 ppm CO2). Accounting for carbon cycle feedbacks plus LUC outside the Americas gives a total 5 ppm CO2 additional uptake into the land surface in the 1500s compared to the 1400s, 47–67% of the atmospheric CO2 decline. Furthermore, we show that the global carbon budget of the 1500s cannot be balanced until large-scale vegetation regeneration in the Americas is included. The Great Dying of the Indigenous Peoples of the Americas resulted in a human-driven global impact on the Earth System in the two centuries prior to the Industrial Revolution.

Geochemistry and mineralogy of Paleoproterozoic metasediments in the Imandra-Varzuga Greenstone Belt: Implications for sediment provenance, tectonic settings and weathering intensity at the transition to oxygenated surface environments

Investigating the Paleoproterozoic glaciations with 3-D climate modeling

The globally averaged calcite compensation depth has deepened by several hundred metres in the past 15 Myr. This deepening has previously been interpreted to reflect increased alkalinity supply to the ocean driven by enhanced continental weathering due to the Himalayan orogeny during the late Neogene period. Here we examine mass accumulation rates of the main marine calcifying groups and show that global accumulation of pelagic carbonates has decreased from the late Miocene epoch to the late Pleistocene epoch even though CaCO3 preservation has improved, suggesting a decrease in weathering alkalinity input to the ocean, thus opposing expectations from the Himalayan uplift hypothesis. Instead, changes in relative contributions of coccoliths and planktonic foraminifera to the pelagic carbonates in relative shallow sites, where dissolution has not taken its toll, suggest that coccolith production in the euphotic zone decreased concomitantly with the reduction in weathering alkalinity inputs as registered by the decline in pelagic carbonate accumulation. Our work highlights a mechanism whereby, in addition to deep-sea dissolution, changes in marine calcification acted to modulate carbonate compensation in response to reduced weathering linked to the late Neogene cooling and decline in atmospheric partial pressure of carbon dioxide. A redistribution of marine calcifiers along with a reduction in weathering led to increased seafloor carbonate deposition during the late Neogene, according to a global compilation of carbonate mass accumulation rate records from sediment cores.

On the basis of a synthesis of recent progress in belowground ecology, we advance and discuss a hypothesis that relates root trait evolution to the increased dominance of angiosperms into dry upland habitats and the decline of atmospheric CO2 concentration that began in the Cretaceous. Our hypothesis is built from examining patterns of fine root adaptations during the Cretaceous, when angiosperms dramatically diversified in association with arbuscular and ectomycorrhizal root-fungal symbionts. We then explore the potential effects of root adaptations and mycorrhizas on the geochemical carbon cycle. On the basis of phylogenetic analyses of root traits among extant plant species, we suggest that angiosperm taxa, which diversified since the early Cretaceous, evolved thinner roots with greater root length per unit of biomass invested (i.e., specific root length [SRL]) than earlier diverging taxa. We suggest that these changes in root morphology were facilitated by a decline in atmospheric CO2, which likely caused water to become more limiting and nutrients more bound to organic matter. Under these conditions, we suggest that thin roots with long SRL would have allowed plants to more efficiently forage for soil water and nutrients. This assertion is supported by the observation that SRL correlates with greater root length density in soil and increased root capacity to take up water. Simulations indicate that the evolution of angiosperm root systems with greater SRL and ectomycorrhizas during the Cretaceous and Cenozoic substantially increased mineral weathering rates, with a fourfold increase in SRL, equivalent to a quadrupling of atmospheric CO2 concentration. The hypothesis presented here raises the possibility that plant hydraulic status and nutrient balance together shaped whole-plant growth strategies, with important consequences for the evolution of the biosphere.