16:15 〜 16:30
[MIS20-09] 地球初期海底熱水系におけるアンモニア生成の化学と同位体効果の実験的研究
キーワード：冥王代、非生物学的窒素サイクル、アンモニア
Abiotic fixation of atmospheric dinitrogen to ammonia is important in prebiotic chemistry and biological evolution in the Hadean and Archean oceans. Though it is widely accepted that nitrate (NO3−) was generated in the early atmospheres, the stable pathways of ammonia production from nitrate deposited in the early oceans remain unknown. We conducted the laboratory experiments in a series of physicochemical conditions (temperature and fluid compositions) simulating high-temperature, high-pressure reactions between nitrate and komatiite to find stable chemical pathways to deliver ammonia to the vent–ocean interface of seafloor hydrothermal systems and the global ocean on geological timescales. The fluid chemistry and mineralogy of both the komatiite–H2O–NO3––CO2 and komatiite–H2O–NO3– systems showed iron-mediated production of ammonia from nitrate with yields up to 70% for temperatures from 250 C to 350 C, with pressure of 500 bars.
The komatiite–H2O–NO3––CO2 system generated H2-rich fluids, a well-known prerequisite for prebiotic and primordial metabolisms, only at higher temperatures than the komatiite–H2O–NO3– system. This suggests that CO2 suppresses the H2 production coupled with iron oxidation through the incorporation of ferrous iron into carbonate minerals. In contrast, CO2 stimulated the production of ammonia at temperatures examined in this study. Further, the resultant ammonia was isotopically fractionated from nitrate with implications for interpretations of nitrogen isotope records in the early earth. Our results imply that komatiite-hosted hydrothermal activity in the Hadean and Archean oceans might have been important not only for the development of primordial chemosynthetic ecosystems at the vent-ocean interface but also for that of photosynthetic ecosystems on the early Earth.
The komatiite–H2O–NO3––CO2 system generated H2-rich fluids, a well-known prerequisite for prebiotic and primordial metabolisms, only at higher temperatures than the komatiite–H2O–NO3– system. This suggests that CO2 suppresses the H2 production coupled with iron oxidation through the incorporation of ferrous iron into carbonate minerals. In contrast, CO2 stimulated the production of ammonia at temperatures examined in this study. Further, the resultant ammonia was isotopically fractionated from nitrate with implications for interpretations of nitrogen isotope records in the early earth. Our results imply that komatiite-hosted hydrothermal activity in the Hadean and Archean oceans might have been important not only for the development of primordial chemosynthetic ecosystems at the vent-ocean interface but also for that of photosynthetic ecosystems on the early Earth.