Eiichi Tajika

The role of the vertical fluxes of particulate organic matter and calcite in the oceanic carbon cycle: Studies using an ocean biogeochemical general circulation model

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.

Role of dissolved organic matter in the marine biogeochemical cycle: Studies using an ocean biogeochemical general circulation model

A biogeochemical general circulation model which includes production and consumption processes of dissolved organic matter (DOM) is developed. The semilabile and the refractory DOM are taken into account. The vertical distribution of the dissolved organic carbon (DOC) concentration and the Δ 14 C value obtained in our model compares well with the recent observations. It is found that the double DOC maximum zone (DDMZ) extends in the east-west direction in the equatorial Pacific. Case studies, which change the decay time and production ratio constant, show that the horizontal distribution of DOC in the surface layer can be reproduced only when the decay time of the semilabile DOM is about half a year. The semilabile DOM exists only above a depth of 400 m, and its vertical and horizontal transport plays an important role in the marine biogeochemical cycle in the surface layer. However, below that depth, only the inert refractory DOM exists, and the role of the refractory DOM in the biogeochemical cycle is not important. 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.

Climate change during the last 150 million years: reconstruction from a carbon cycle model

Variations of the atmospheric CO2 level and the global mean surface temperature during the last 150 Ma are reconstructed by using a carbon cycle model with high-resolution input data. In this model, the organic carbon budget and the CO2 degassing from the mantle, both of which would characterize the carbon cycle during the Cretaceous, are considered, and the silicate weathering process is formulated consistently with an abrupt increase in the marine strontium isotope record for the last 40 Ma. The second-order variations of the atmospheric CO2 level and the global mean surface temperature in addition to the first-order cooling trend are obtained by using high-resolution data of carbon isotopic composition of marine limestone, seafloor spreading rate, and production rate of oceanic plateau basalt. The results obtained from this model are in good agreement with the previous estimates of palaeo-CO2 level and palaeoclimate inferred from geological, biogeochemical, and palaeontological models and records. The system analyses of the carbon cycle model to understand the cause of the climate change show that the dominant controlling factors for the first-order cooling trend of climate change during the last 150 Ma are tectonic forcing such as decrease in volcanic activity and the formation and uplift of the Himalayas and the Tibetan Plateau, and, to a lesser extent, biological forcing such as the increase in the soil biological activity. The mid-Cretaceous was very warm because of the high CO2 level (4–5 PAL) maintained by the enhanced CO2 degassing rate due to the increased mantle plume activities and seafloor spreading rates at that time, although the enhanced organic carbon burial would have a tendency to decrease the CO2 level effectively at that period. The variation of organic carbon burial rate may have been responsible for the second-order climate change during the last 150 Ma.

Evolution of terrestrial proto-CO2 atmosphere coupled with thermal history of the earth

Abstract The effect of volatile exchange between surface reservoirs and the mantle on the evolution of proto-CO2 atmosphere on the Earth is investigated using a global carbon cycle model coupled with thermal evolution of the mantle. Carbon is assumed to circulate among five reservoirs (atmosphere, ocean, continents, seafloor and mantle) and the carbon flux of each reservoir is calculated under varying conditions, such as an increase in solar luminosity, continental growth, and a decrease in tectonic activity with time. We consider processes such as continental weathering, carbonate precipitation in the ocean, carbonate accretion to the continents, metamorphism of carbonates followed by CO2 degassing through arc volcanism, carbon regassing into the mantle, and CO2 degassing from the mantle. The degassing rate of volatiles from the mantle is assumed to be proportional to the volatile concentration in the mantle multiplied by the mantle degassing volume. The mantle degassing volume is determined by the seafloor spreading rate and the melt generation depth in the mantle. We use a parameterized convection model to calculate the thermal evolution of the mantle, from which we estimate the seafloor spreading rate and the melt generation depth in the mantle. Numerical simulations suggest that the amount of surface carbon at the present time would be in a steady state. This is because the response time of the carbon cycle system against the perturbation for surface carbon is short, being estimated at about 900 million years under the present conditions. Thus the present amount of surface carbon would not be affected by the initial amount of surface carbon. The seafloor spreading rate would have been almost constant throughout the Earths history to explain apparent constancy of the carbon isotope ratio in the mantle after about 3.5 billion years ago. The surface carbon might probably have been circulated between surface reservoirs and the mantle once or twice after the Archean period. In any event, CO2 in the proto-atmosphere on the Earth is suggested to have decreased with the growth of continents, resulting in stabilization of the terrestrial environment against the increase in solar luminosity.

Faint young Sun and the carbon cycle : Implication for the Proterozoic global glaciations

The Earth may have been globally ice-covered several times during the Proterozoic. While the Neoproterozoic and the Paleoproterozoic glaciations may have been ‘snowball’ Earth events, there is no evidence for such glaciation during the Phanerozoic. It might be hypothesized that a dimmer Sun earlier in Earth’s history may have made the Earth more susceptible to global glaciation. In this paper, the roles of solar flux and soil biological activity in the carbon cycle and the climate during the Proterozoic are investigated using a simple carbon geochemical cycle model with a one-dimensional energy balance climate model. The results indicate, perhaps counterintuitively, that the Proterozoic Earth, with its dimmer Sun, was not more susceptible to ‘snowball glaciation’. Metamorphic and volcanic CO2 fluxes accumulate in the atmosphere and ocean until such time that those inputs are balanced by silicate weathering followed by carbonate precipitation and net organic carbon burial. Because of the dependence of weathering rates on climatic conditions, changes in geologic CO2 inputs have a large influence on climatic conditions. In contrast, slow variation in solar flux has relatively little long-term impact on climate, because of large compensating changes in atmospheric CO2 level. A reduction in CO2 inputs lowers atmospheric CO2 level, which finally initiates global glaciation. The atmospheric CO2 level at the critical condition for a globally ice-covered state would have been high during the Proterozoic. However, roughly the same amount of CO2 flux reduction is required for both the Proterozoic and the Phanerozoic. This is essentially because the temperatures at the critical condition are very low, hence the silicate weathering rate (which should balance with a net CO2 input rate in a steady state) is also very low, regardless of the variation in solar flux. Furthermore, the effect of the lower solar flux on the CO2 input rate at the critical condition would have been largely canceled by a lower efficiency of the silicate weathering rate due to lower soil biological activity during the Proterozoic. As a result, CO2 flux conditions for initiating the global glaciation may be similar during both the Proterozoic and the Phanerozoic. Therefore, the explanation for the susceptibility of the Proterozoic Earth to ‘snowball’ conditions cannot hinge simply on the dimmer Sun; we must look to other differences in behaviors of the carbon cycle and the climate between these two ages.

Generation and propagation of a tsunami from the Cretaceous-Tertiary impact event

We have studied mechanism of tsunami generation by meteorite impact on a shallow ocean at 65 Ma and modeled the propagation of that tsunami in the Gulf of Mexico. We found that the water flow into and out of the crater cavity causes most of the tsunami. The height of the wave coming out of the crater is controlled by the depth of the shallow-water region surrounding the crater. We show that the lower the flow velocity in the shallow-water region, the lower the wave height, and the longer the oscillation period. If the depth of the sea above the Yucatan platform was 200 m at the end of the Maastrichtian, the maximum tsunami wave height and period at the rim of the crater are estimated to be 50 m and 10 h, respectively. Using these results, we simulated the propagation of the K-T impact-generated tsunami in the Gulf of Mexico. There are two types of tsunami; the receding wave and the rushing wave. The receding wave traveled across the entire gulf within 10 h of the impact. Tsunamis attacked the coast as a leading negative wave. The rushing wave flowed with a height of more than 200 m and reached the coastal area of North America. It ran up over the land and crossed the Mississippi embayment, a distance of more than 300 km. The averaged runup was more than 150 m, but it reached a height of 300 m near the Rio Grande embayment.

Complex tsunami waves suggested by the Cretaceous-Tertiary boundary deposit at the Moncada section, western Cuba

The Moncada Formation in western Cuba is an 2-m-thick weakly metamorphosed complex characterized by repetition of calcareous sandstone units that show overall upward fining and thinning. The Moncada Formation contains abundant shocked quartz, altered vesicular impact-melt fragments, and altered and deformed greenish grains of possible impact glass origin. In addition, a high iridium (0.8 ppb)

Degassing history and carbon cycle of the Earth: From an impact-induced steam atmosphere to the present atmosphere

Abstract The recent theoretical studies on the formation and evolution of the atmosphere and oceans of the Earth are reviewed. Impact degassing during accretion of the Earth would probably generate an impact-induced steam atmosphere on the proto-Earth. At the end of accretion, the steam atmosphere became unstable and condensed to form the proto-ocean with almost the present mass of ocean. The steam atmosphere would have thus evolved to the proto-CO2 atmosphere during the earliest history of the Earth because CO in the proto-atmosphere may be photochemically converted to CO2. However, CO2 in the proto-atmosphere has decreased with time through the global carbon cycle which may have stabilized the terrestrial environment against an increase in the solar luminosity. The continental growth during Hadean and Archean would therefore have a significant influence on the carbon cycle and the surface temperature. It is also suggested that the continental growth is a necessary condition for the terrestrial environment to evolve to the present state. Both the impact degassing and the subsequent continuous degassing are suggested to have played a major role in the formation and evolution of the atmosphere and ocean. In particular, most of N2 may have been produced by the impact degassing during accretion, and the contribution of the subsequent continuous degassing is at most 10% for N2. As a consequence, after the CO2 level decreased to less than 1 bar, the atmosphere may have been at about 1 bar and composed mainly of N2 for most of the subsequent history of the Earth.

Evolution of seafloor spreading rate based on 40Ar degassing history

A new degassing model of 40Ar coupled with thermal evolution of the mantle is constructed to constrain the temporal variation of seafloor spreading rate. In this model, we take into account the effects of elemental partition and solubility during melt generation and bubble formation, and changes in both seafloor spreading rate and melt generation depth in the mantle. It is suggested that the seafloor spreading rate would have been almost the same as that of today over the history of the Earth in order to explain the present amount of 40Ar in the atmosphere. This result may also imply the mild degassing history of volatiles from the mantle.

CONDITIONS FOR OCEANS ON EARTH-LIKE PLANETS ORBITING WITHIN THE HABITABLE ZONE: IMPORTANCE OF VOLCANIC CO2 DEGASSING

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 CO2 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 CO2 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 CO2 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.