Through the history of the Earth, surface environments had changed most dramatically on the early Earth, characterized by the formation of ocean, continents, and life. Large amounts of unclear points remained on the origin of life. Thus, this research area has large amounts of open questions to be achieved in the future. Geochemistry is one of the most powerful tool to answer the questions related to the origins of life. This special issue “Early Earth Chemistry” brings six review articles, in which the authors introduced unique aspects and recent progresses from the view of individual expertise. The contents cover the formation of oceans, formation of continents, impact events, prebiotic chemistry, and potential geological records of early life. We hope that you'll make the best from those overview contents and that this issue works to spread further novel research frameworks.

The presence of oceans and continents is one of the Earth's unique features among the planets in our Solar System. Here I discuss the formation and origin of Earth's oceans and continents by combining the results of geological, geochemical and geophysical studies. The geological and geochemical data provide evidence for the presence of Earth's oceans by ca. 3.8 Ga and probably ca. 4.3 Ga. Yet, oceans could be formed even earlier, possibly soon after the solidification of the magma ocean, especially if the Earth had gained water before the Moon-forming Giant Impact. The hydrogen isotope data for meteorites and mantle-derived terrestrial samples suggest that proto-planets and planetesimals, that accreted to form the Earth, could acquire water from solar nebular and water-rich materials delivered from outer Solar System, respectively. Furthermore, the hafnium isotope systematics of Hadean zircon constrain the timing of magma ocean solidification to as early as 4.5 Ga. The infant solid Earth would be characterized by the stagnant-lid convection regime. Meanwhile, radiogenic heat generation induced vigorous mantle plumes and stagnant plates had been cooled and hydrated. Eventually, the hydrated proto-plates were subducted beneath buoyant oceanic plateaus generated by the mantle plumes, resulting in the onset of plate tectonics. Although there is evidence that granitic rocks were locally formed on the Hadean Earth, the geochemical compositions of sedimentary rocks indicate that the emergence and growth of continents occurred during Middle and Late Archean times. This implies that the time interval from the magma ocean solidification to the onset of global plate tectonics is ~1 Gyr.

It is widely believed that the Hadean Earth suffered an intense impact bombardment. Shock vaporization/subsequent chemical reaction is one of the most curious processes on the Hadean Earth because it leads to unique chemical reactions never driven under a mean field on the Earth due to the injection of reducing materials with a significant amount of kinetic energy. Here, I review a recent research development on the nature of impactors at the terminal stage of planetary accretion. After a long discussion over a few decades, a self-consistent model referred to as “a sawtooth-like timeline” has been proposed. Then, I introduce a framework to understand shock-induced chemistry based on the entropy method. Hypervelocity impact experiments in an open system would be essential to investigate the impact-generated chemical species quantitatively. Finally, I discuss a future plan to estimate the Hadean environment on Earth from an impact laboratory. An intense and impulsive perturbation due to hypervelocity impacts might cause the release of a large amount of free energy into a prebiotic field and produce unique geochemical features, such as isotopic anomalies in oxygen and sulfur in sediments, through the recovery processes. A combination analysis with the recent model for the impact bombardment and a more detailed model for impact-induced chemical reactions will allow us to re-construct the surface environment on the Hadean Earth.

The origin of life must be studied through addressing a problem of the emergence of a free-energetically open complex system, rather than a problem of abiotic syntheses of various building blocks of life (Aono et al., 2015). One of the key aspects of the origin question is the origin of metabolism, since no (genetic) information may be preserved without ample and ordered materials supply. Thus, from the perspective of the origin of ‘proto-metabolism' we critically review currently prevailing approaches to the origin problem. Then, referring to the latest biological and geochemical findings, we will describe a scenario of the electrochemically driven emergence of ‘proto-metabolism' together with experimental proposals.

When the first biosphere was formed on the early earth has been actively discussed recent years. Oldest biogenic graphite and microfossils have been repeatedly tested from early 2000s, due to unclear geological settings and discovery of abiotic production of organic compounds in native environments. Here, I introduce our discovery of novel graphite-rich metasedimentary rocks in the northwest of ca. 3.8 billion-years-old Isua Supracrustal Belt, western Greenland by detail geological investigation. Isotopic and nano-scale structural signatures of graphite show that graphite is originated in biogenic organic matter, providing further oldest evidence of biological activity in the ca. 3.8 Ga ocean. Evidence of early biosphere could be found outside of carbonaceous materials in old rocks. Banded iron formations, chemical marine sediments including Fe-bearing minerals, have been a subject of active research connecting redox state to biosphere on early earth's surface. Previous studies indicate that atmospheric oxygen increased at ca. 2.45 Ga (Great Oxidation Event: GOE),suggesting that oxygenic photosynthesizing bacteria have been flourished and emitted oxygen. However, more recent geological and geochemical records reveal that shallow part of ocean and atmosphere have been slightly oxidized before GOE, which evokes that oxygenic photosynthesizing bacteria was active earlier than that anticipated.

Morphologically preserved microfossils are direct evidences of existence of life on Earth. However, it is usually insufficient to identify microfossils using morphological analysis alone because of their simple morphology and small size. Chemical analysis of the carbonaceous matter is necessary in order to fully understand its origin and characteristics. In situ analysis of Precambrian microfossils and carbonaceous matter preserved in petrographic thin sections is a powerful tool for analyzing their chemical composition. Interpretation of analytical techniques reveals elemental, isotopic, and molecular compositions on nanometer-to-micrometer scales, and enables us to examine spatial relationships between individual morphological structures and the surrounding minerals. This chemical information is used to assess the syngenicity and biogenicity of the individual microfossil-like structures. This review explores examples of in situ analytical techniques applied to Precambrian microfossils, microfossil-like structures, and carbonaceous matter.

Understanding the origin and early evolution of life is fundamental to improve our knowledge on ancient living systems and their environments. Information about the environment of early Earth is sometimes obtained from fossil records. However, no fossil records of ancient organisms that lived more than 3,500 million years ago have been found. Instead, we can now predict the sequences of ancient genes and proteins by comparing extant genome sequences accumulated by the genome project of various organisms. A number of computational studies have focused on ancestral base contents of ribosomal RNAs and the amino acid compositions of ancestral proteins, estimating the environmental temperatures of early life with conflicting conclusions. On the other hand, we experimentally resurrected inferred ancestral amino acid sequences of nucleoside diphosphate kinase that might have existed 3,500–3,800 million years ago. The resurrected proteins are stable around 100℃, being consistent with the thermophilic ancestry of life. Our experimental data do not exclusively indicate the thermophilic origin of life; rather, our conclusion is compatible with the idea that the hyperthermophilic ancestor was selected for increased environmental temperatures of early Earth probably caused by meteorite impacts.

I have been studying origin and evolution of extraterrestrial organic matter on the basis of experimental kinetic approaches that enable us to predict changes in its molecular structure and chemistry during parent body processes. Here I summarize experimental studies of organic solid formations starting from formaldehyde and ammonia with the presence of liquid water, and decreasing in aliphatic C–H bonds as observed in infrared spectra with heating experiments of Murchison meteorite. The related works are also reviewed and the applications for deciphering thermal histories of asteroids are discussed.

High pH values of 9.65 and 10.1 were observed in the Amano River, Osaka Prefecture in 2013 and 2015. On the other hand, the high pH values were not recorded in 2014. This basification was observed under conditions of high water temperature and high dissolved oxygen concentration. From the results of pH, dissolved oxygen, nitrate+nitrite, phosphate and total alkalinity, we confirmed that this basification might be due to decrease of carbon dioxide by photosynthesis in the Amano River. The changes of pH value in the river water might be affect the speciation of trace metals. Concentrations of dissolved species of aluminum, chromium and copper were increased with increase of pH value, while those of dissolved species of nickel and zinc were decreased with increase of pH value. From the positive correlation between the relative fluorescent intensity of dissolved organic matter and the concentrations of nickel and zinc we confirmed that these elements might be dissolved as metal-fulvic acid complex within the observed pH range.