Controls of Atmospheric Methane on Early Earth and Inhabited Earth-like Terrestrial Exoplanets

Methane (CH4) is a primarily biogenic greenhouse gas. As such, it represents an essential biosignature to search for life on exoplanets. Atmospheric CH4 abundance on Earth-like inhabited exoplanets is likely controlled by marine biogenic production and atmospheric photochemical consumption. Such interactions have been previously examined for the case of the early Earth where primitive marine ecosystems supplied CH4 to the atmosphere, showing that the atmospheric CH4 response to biogenic CH4 flux variations is nonlinear, a critical property when assessing CH4 reliability as a biosignature. However, the contributions of atmospheric photochemistry, metabolic reactions, or solar irradiance to this nonlinear response are not well understood. Using an atmospheric photochemical model and a marine microbial ecosystem model, we show that production of hydroxyl radicals from water vapor photodissociation is a critical factor controlling the atmospheric CH4 abundance. Consequently, atmospheric CH4 partial pressure (pCH4) on inhabited Earth-like exoplanets orbiting Sun-like stars (F-, G-, and K-type stars) would be controlled primarily by stellar irradiance. Specifically, irradiance at wavelengths of approximately 200-210 nm is a major controlling factor for atmospheric pCH4 when the carbon dioxide partial pressure is sufficiently high to absorb most stellar irradiance at 170-200 nm. Finally, we also demonstrated that inhabited exoplanets orbiting near the outer edge of K-type stars' habitable zones are better suited for atmospheric pCH4 buildup. Such properties will valuably support future detection of life signatures

Anomalous negative excursion of carbon isotope in organic carbon after the last Paleoproterozoic glaciation in North America

Early Paleoproterozoic time (2.5–2.0 Ga) spanned a critical phase in Earth's history, characterized by repeated glaciations and an increase in atmospheric oxygen (the Great Oxidation Event (GOE)). Following the last and most intense glaciation of this period, marine carbonates record a large positive excursion of δ^(13)C value (termed the “Lomagundi event”) between about 2.2 and 2.1 Ga coinciding with the global appearances of red beds and sulfates, which suggest an accumulation of high levels of atmospheric oxygen. Here we report the discovery of large negative excursions of δ^(13)C in organic matter (down to −55‰) from quartzose sandstones (of the Marquette Range and the Huronian Supergroups, North America) intermediate in age between the last Paleoproterozoic glaciation and the possible onset of the Lomagundi event. The negative excursion is concomitant with the appearance of intensely weathered quartzose sandstones, which may represent hot and humid conditions. There are some interpretations that potentially explain the negative excursions: (1) redeposition of older ^(13)C-depleted kerogen, (2) later post-depositional infiltration of oil, (3) active methane productions by methanogens in shallow-marine environments, or (4) dissociation of methane hydrate. If the latter two were the case, they would provide clues for understanding the environmental change connecting the intense glaciation and an increase in oxygen

A study of the energy balance climate model with CO2-dependent outgoing radiation: Implication for the glaciation during the

Abstract. We analyze the energy balance climate model with CO2-dependent outgoing radiation, and obtain the steady-state solution for very wide range of the atmospheric CO2 partial pressure and the thermal diffusion coefficient. We propose a phase diagram of the climate on the parameter space of the atmospheric CO2 and the thermal diffusion coefficient for the latitudinal heat transport, which may be useful to understand the climate change through the history of the Earth. It is shown that the formation of polar ice caps can be caused by decrease in the atmospheric CO2 and the latitudinal heat transport. The different history of glaciation in each hemisphere through the Cenozoic might be the result of difference in the heat transport in each hemisphere. Understanding of the small ice cap instability might be important to interpret the oxygen isotope record at the Eocene-Oligocene boundary

Impact of the evolution of carbonate ballasts on marine biogeochemistry in the Mesozoic and associated changes in energy delivery to subsurface waters

Abstract. We have examined the impact of the Mesozoic algal revolution using biogeochemical simulations to analyze the energy flux into the subsurface environment. In particular, the delivery scheme of energy to the subsurface was dramatically altered by the appearance of mineralized exoskeletons, both in algal groups (e.g., coccolithophores) and in zooplanktic taxa. These biominerals, acting as ballast, accentuated the delivery of organic matter to subsurface waters. Thus, the elevated organic carbon flux associated with evolutionary developments in Mesozoic taxa caused an intense but short-lived oceanic euxinia, without an associated mass extinction event, in sharp contrast to the relatively prolonged Paleozoic euxinia that were generally coincident with mass extinctions

MODELING OF OCEANIC CARBON CYCLE

We develop an ocean general circulation model which includes biogeochemical processes (biogeochemical general circulation model, B-GCM). B-GCM can deal not only with current field, temperature, and salinity, but also with biogeochemical tracers such as phosphate, dissolved oxygen, alkalinity, total CO_2,δ^C, and Δ^C. Here, we show results of three case-studies. First, our model is driven by the wind stress, sea surface temperature, and sea surface salinity (SSS) under the present annual mean condition. The steady state obtained in our model well reproduces the following principal observed features : The phosphate concentration and the Δ^C value increase along the flow path of the deep circulation from the North Atlantic to the North Pacific. The oxygen concentration and the δ^C value decreases along the deep circulation path. Phosphate maximum and oxygen minimum are at about 1km depth, and the lysocline lies above the depth of 1km in the North Pacific. Second, our model is driven under the same condition as the first experiment except that the SSS condition in the North Atlantic is reduced by 3 psu from the present state. In this case, a weak and less saline intermediate water (North Atlantic Intermediate Water, NAIW) forms at about 1km depth instead of forming North Atlantic Deep Water (NADW). The deep water under the depth of 1km is stagnant (very weak Antarctic Bottom Water), which supports the hypothesis suggested from the Cd/Ca ratio (E.A. BOYLE; Nature, 331,55,1988). The lysocline in the North Atlantic lies at about 1km depth, which also partially supports Boyle\u27s alkalinity hypothesis. Last, transient states are calculated with alternating the flow fields in the previous two cases (we call these transition 1 : NADW on⟶off or transition 2 : NADW off→on). The following three stages are found : (1) gas exchange between the atmosphere and the sea surface layer within 1-30 years, (2) water exchange between the surface and deep layer in the Atlantic within 100-1000 years, (3) water exchange between the Atlantic and the Pacific within 1000-3000 years. The time scale of the second stage for transition 1 is 100-400 years, which is faster than the 200-1000 years for transition 2. The atmospheric CO_2 overshoots in the second stage. For example, for transition 2,the atmospheric CO_2 rapidly increases in the first stage, overshoots in the second stage, and slowly/slightly decreases in the third stage