2019 Volume 128 Issue 4 Pages 597-623
The Earth is the only planet where liquid water and life have existed through geologic time. Therefore, it is important to investigate the surface environments of the early Earth to understand its early life. However, little is known about the environments of the early Earth because of a lack of geologic evidence. Recently, > 3.95 Ga supracrustal rocks including carbonate rocks, named Nulliak supracrustal rocks, were found in the Saglek Block, northern Labrador. The seawater composition of the early Earth and vestiges of life from the oldest supracrustal rocks are investigated. The carbonate rocks occur in four areas of the Saglek Block, and are accompanied by meta-chert and meta-pelitic rocks or metamorphosed banded iron formation and meta-basalt. They all are regionally metamorphosed under the amphibolite or the granulite facies conditions (> 500°C) to yield metamorphic minerals such as amphibole and pyroxene. The whole-rock compositions of major, trace, and rare earth elements of the carbonate rocks are assessed. The carbonate rocks are composed of CaO, MgO, FeO, and SiO2 with trace amounts of Al2O3, Zr, and Ba. Carbonate rocks with low Al2O3 (< 0.2 wt.%), Zr (< 2 ppm), and Ba (< 5 ppm) contents are selected to avoid the influences of detrital and volcanic materials on their compositions. Post-Archean Australian Average Shale (PAAS)-normalized Rare Earth Element (REE) patterns of detritus-free samples show flat to slightly light REE-depleted patterns with positive La, Eu, and Y anomalies and without a Ce anomaly. The positive La and Y anomalies indicate that the carbonate rocks originated from chemical sediments precipitated from seawater. The lack of the Ce anomaly indicates that Eoarchean seawater was anoxic. Because the carbonate rocks accompanied by the pelitic rocks have obvious Eu anomaly, even seawater near a continent had the Eu anomaly and was widely dominated by hydrothermal influx. Pelitic rocks, conglomerate, carbonate rocks, and chert nodules within the carbonate rocks in St. John's Harbour South, St. John's Harbour East, Shuldham Island, and Big Island contain graphite, whereas no graphite is found in the sedimentary rocks in Pangertok Inlet. Some of the pelitic rocks and conglomerate have low δ13Corg values with a nadir of −28.2‰, which is comparable to the minimum value of δ13C values of graphite in the Isua supracrustal belt. The δ13Corg values of graphite in the politic rocks show a weak negative correlation with total organic carbon contents. On the other hand, carbon isotope values (δ13Ccarb) of carbonate in the carbonate rocks range from −2.6‰ to −3.8‰. The δ13Corg values positively correlate with the metamorphic grade and negatively with total organic carbon contents. The correlation indicates that the variation in δ13Corg values is possibly due to carbon loss during metamorphism. It is concluded that the large fractionation between the δ13Ccarb and δ13Corg values, up to 25‰, provides the oldest evidence for organisms over 3.95 Ga.