Chikyukagaku

Emergence of life's building blocks on the prebiotic Earth should be the fundamental step to the origins of life. Geological evidences suggest that such organic compounds accumulated at some point in the time between 4.4 to 3.8 billion years ago. During this period, the flux of extraterrestrial objects was significantly higher than the subsequent periods. Such extraterrestrial objects might have provided substantial amounts of metallic iron to the surface of the Earth. Shock-recovery experiments simulating the impact-induced reactions of such iron-bearing objects suggest that hypervelocity oceanic impacts of meteorites form nucleobases and various amino acids as well as amines and carboxylic acids. High annual mass flux of such large objects suggests that the impact-induced formation was not negligible as a source of organic compounds on the early Earth. Further investigations on the impact-induced reactions and the nature of extraterrestrial objects would elucidate the fundamental step to the origin of life.

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Chemical compositions and Sr isotopic ratio (⁸⁷Sr/⁸⁶Sr) of stream sediments collected from Akiyoshi-dai, where is underlain by a large-scale limestone bedrock, were measured to investigate the influence of limestone bedrocks on geochemical maps. The sediments were sieved with eight screen sizes, and their bulk and acetic acid soluble fractions were both analyzed. 50–100% of the total Ca was extracted by acid leaching from stream sediments in the catchment of Akiyoshi limestone, indicating that Ca exists mainly in carbonate materials. In contrast, only 5–10% and 15–45% of the total Sr were extracted from their coarse (>125 μm) and fine particles (<125 μm),respectively. Therefore, Sr exists dominantly in aluminosilicate minerals even in stream sediments originated from limestone bedrock. Because the acid extraction percentages of Ca and Sr increase exponentially below 125 μm of grain size, carbonate materials would be supplied by fine-grained particles from limestone bedrock. ⁸⁷Sr/⁸⁶Sr ratios in the acid soluble fractions were systematically lower than those of the bulk fractions and comparable to ⁸⁷Sr/⁸⁶Sr of Akiyoshi limestone. Although limestone bedrock apparently scarcely influence on Sr concentration and ⁸⁷Sr/⁸⁶Sr of stream sediments, ⁸⁷Sr/⁸⁶Sr of carbonate materials is successfully extracted from the bulk sediments using acetic acid-based extraction method.

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