Distributions of chemical tracers in the world ocean are well reproduced in an ocean general circulation model which includes biogeochemical processes (biogeochemical general circulation model, B‐GCM). The difference in concentration of tracers between the surface and the deep water depends not only on the export production but also on the remineralization depth. Case studies changing the vertical profile of particulate organic matter (POM) flux and the export production show that the phosphate distribution can be reproduced only when the vertical profile of POM flux observed by sediment traps is used. The export production consistent with the observed distribution of phosphate is estimated to be about 10 GtC/yr. Case studies changing the vertical profile of calcite flux and the rain ratio, a ratio of production rate of calcite against that of particulate organic carbon (POC), show that the rain ratio should be smaller than the widely used value of 0.25. The rain ratio consistent with the observed distribution of alkalinity is estimated to be 0.08 to approximately 0.10. This value can be easily understood in a two‐box model where the difference of remineralization depth between POC and calcite is taken into account.

The possibility of low but nontrivial atmospheric oxygen (O 2 ) levels during the mid‐Proterozoic (between 1.8 and 0.8 billion years ago, Ga) has important ramifications for understanding Earth's O 2 cycle, the evolution of complex life and evolving climate stability. However, the regulatory mechanisms and redox fluxes required to stabilize these O 2 levels in the face of continued biological oxygen production remain uncertain. Here, we develop a biogeochemical model of the C‐N‐P‐O 2 ‐S cycles and use it to constrain global redox balance in the mid‐Proterozoic ocean–atmosphere system. By employing a Monte Carlo approach bounded by observations from the geologic record, we infer that the rate of net biospheric O 2 production was Tmol year −1 (1σ), or ~25% of today's value, owing largely to phosphorus scarcity in the ocean interior. Pyrite burial in marine sediments would have represented a comparable or more significant O 2 source than organic carbon burial, implying a potentially important role for Earth's sulphur cycle in balancing the oxygen cycle and regulating atmospheric O 2 levels. Our statistical approach provides a uniquely comprehensive view of Earth system biogeochemistry and global O 2 cycling during mid‐Proterozoic time and implicates severe P biolimitation as the backdrop for Precambrian geochemical and biological evolution.

The evolution of different forms of photosynthetic life has profoundly altered the activity level of the biosphere, radically reshaping the composition of Earth's oceans and atmosphere over time. However, the mechanistic impacts of a primitive photosynthetic biosphere on Earth's early atmospheric chemistry and climate are poorly understood. Here, we use a global redox balance model to explore the biogeochemical and climatological effects of different forms of primitive photosynthesis. We find that a hybrid ecosystem of H2-based and Fe 2+ -based anoxygenic photoautotrophs-organisms that perform photosynthesis without producing oxygen-gives rise to a strong nonlinear amplification of Earth's methane (CH4) cycle, and would thus have represented a critical component of Earth's early climate system before the advent of oxygenic photosynthesis. Using a Monte Carlo approach, we find that a photosynthetic hybrid biosphere widens the range of geochemical conditions that allow for warm climate states well beyond either of these metabolisms acting in isolation. Our results imply that the Earth's early climate was governed by a novel and poorly explored set of regulatory feedbacks linking the anoxic biosphere and the coupled H, C, and Fe cycles, with important ramifications for the sustained habitability of reducing Earth-like planets hosting primitive photosynthetic life.

Earth-like planets in the habitable zone (HZ) have been considered to have warm climates and liquid water on their surfaces if the carbonate-silicate geochemical cycle is working as on Earth. However, it is known that even the present Earth may be globally ice-covered when the rate of CO 2 degassing via volcanism becomes low. Here we discuss the climates of Earth-like planets in which the carbonate-silicate geochemical cycle is working, with focusing particularly on insolation and the CO 2 degassing rate. The climate of Earth-like planets within the HZ can be classified into three climate modes (hot, warm, and snowball climate modes). We found that the conditions for the existence of liquid water should be largely restricted even when the planet is orbiting within the HZ and the carbonate-silicate geochemical cycle is working. We show that these conditions should depend strongly on the rate of CO 2 degassing via volcanism. It is, therefore, suggested that thermal evolution of the planetary interiors will be a controlling factor for Earth-like planets to have liquid water on their surface.

The global export production due to the particulate organic matter (POM) and DOM at a depth of 100 m is estimated to be about 8 Gt C/yr and about 3 Gt C/yr, respectively. The vertical transport below 400 m is due almost entirely to POM.