Main content area

Are there ancient soils in the 3.7 Ga Isua Greenstone Belt, Greenland?

Retallack, Gregory J., Noffke, Nora
Palaeogeography, palaeoclimatology, palaeoecology 2019 v.514 pp. 18-30
carbon, carbon dioxide, chemical analysis, clay, color, crystals, distillation, grains, models, organic matter, oxygen, paleoclimatology, paleosolic soil types, quartzite, sand, sandstone, silt, weathering, Greenland
A lens of black schist within 3.7 Ga quartzites of Greenland may be Earth's oldest known alluvial paleosol. The suspect metamorphic rock is a lens in orthoquartzite of berthierine schist with crystals of ripidolite, but it has a truncated top above dark gray grading down to gray color, ptygmatically folded surface cracks filled with silt grains, and large sand crystals, unusual for sedimentary or metamorphic rocks. The paleosol hypothesis was tested with thin sections showing plausible mineral weathering trends, and by chemical analysis showing molar weathering ratios and REE distribution like those of soils. The schist is deeply weathered and at the culmination of weathering trends from analysis of other metasediments of the Isukasia area. The protolith can be reconstructed as a saponite clay with a salt-rich horizon of kieserite, like other acid-sulfate paleosols of the early Earth. Models for proton and electron consumption of paleosols applied to the profile reveal an atmosphere with only 36 ± 510 ppm O2 and 820 ± 201 ppm CO2, and humid, cool temperate paleoclimate. The profile has organic δ13CPDB consistently of −24.2 to −27.4‰, and modest Raleigh distillation near the top. Similar consistent values and trends are produced by decay of organic matter in living soils, but biotic carbon isotopic composition of sediments is erratic from bed to bed, and abiotic carbon compounds of meteorites differ dramatically for each kerogen particle. Thus life in this very ancient soil is not precluded by our analyses, but ultrastructural and geochemical testing of carbon particles would further test this hypothesis.