THE ECOSYSTEMS IN SUBMARINE HYDROTHERMAL ENVIRONMENTS OF THE CANADIAN SHIELD 2.7 BILLION YEARS AGO

We have carried out a systematic and multidimensional geochemical investigation on 2.7 Ga shales from the Kidd Creek area in the Abitibi district, Canada. We recognized correlations among the following parameters of samples: (a) carbon isotope ratios of kerogen, (b) organic C contents, (c) occurrences of organic matter, (d) C/P ratios, and (e) Zn contents. We also compared the relationships between parameters (a) and (b) in 2.7 Ga shales from other parts of the Superior Province, Canada. Quantitative analyses of these correlations led us to present the following suggestions for the biosphere, hydrosphere, and atmosphere 2.7 Ga ago: (1) the dominant primary producers in oceans were probably cyanobacteria with carbon isotope ratios around –28permil; (2) the kinetic isotope effect during photosynthesis was about 8permil greater than today; (3) the normal open oceans were oxic; (4) anoxic submarine depressions were created locally, especially in regions with active tectonics and volcanism; (5) many submarine depressions became sites of hydrothermal-fluid discharge; (6) hydrothermal-fluid plumes enhanced the recycling of phosphate stored in the depressions, thereby enhancing cyanobacteria activity in the surface zone; (7) the increased flux of cyanobacteria remnants enhanced the activity of heterotrophic microbial communities in the depressions; (8) the microbial communities that developed under the influence of Zn-rich low-temperature hydrothermal-fluids were probably composed of fermentative bacteria, methanogens, and sulfate-reducing bacteria with carbon isotope ratios of –55 to –35permil; and (9) microbial communities under the influence of Cu+Zn-rich high-temperature hydrothermal-fluids were probably composed of thermophiles with carbon isotope ratios of –20 to –10permil. These suggestions imply: (10) contrary to a current popular theory, the production of organic matter with very light carbon isotope ratios (<-35permil) did not occur globally but were restricted in local depressions under the influence of low temperature hydrothermal-fluids; and (11) because the oxygen content of deep ocean water depends on the atmospheric pO2 level, suggestion (3) proposes the pO2 level at 2.7 Ga was already greater than ~0.5 PAL.