Two billion year old isotopically heavy carbon: evidence from the Labrador Trough, Canada

Abstract

Forty-eight samples of bulk carbonates were studied from the Dunphy, Portage, Alder, Uvé, Denault, and Abner formations and the undivided Pistolet subgroup in the Labrador Trough, Quebec, Canada. The carbonate units occur as a number of thick beds (up to 300 m) within 100–600 m thick formations. They are interbedded with siliciclastic sediments and are composed of crystalline, sparry, micritic allochemical and stromatolitic dolostones. The carbonate rocks are recrystallized and metamorphosed (up to amphibolite grade). They are chemically dolostones, and include chemical or biochemical laminated and massive dolostones, dolorudites, dolarenites, and dololitites, which were deposited in shallow-water (Dunphy, Denault, and Abner formations) to relatively deep water (Uvé Formation) marine environments. Reef facies are present in the Denault and Abner formations. The Dunphy, Portage, Alder, and Uvé formations, deposited between 2.17 and 2.14 Ga, yield isotopically heavy values of δ13C(PDB) ranging from + 5.3 to + 15.4‰, whereas δ18O(SMOW) values range from + 16.2 to + 25.4‰, which is rather normal for Precambrian sedimentary carbonates. The average values of δ13C show a decreasing trend upwards in the stratigraphy from 14.8 ± 0.5‰ (Dunphy Formation), to 9.5 ± 0.7‰ (Alder Formation), to 8.0 ± 1.7‰ (Uvé Formation). Average δ18O values exhibit a similar tendency, 23.6 ± 1.1, 22.6 ± 1.1, and 20.0 ± 1.9‰, respectively. The Alder and Uvé formations display clustering in a δ13C histogram, while the Dunphy Formation plots as a separate offset. The Abner and Denault formations, deposited at about 1.88 Ga, show an average δ13C of 2.1 ± 0.6 and 1.3 ± 1.4‰, and an average δ18O of 21.4 ± 1.9 and 22.6 ± 2.0‰, respectively. δ13C for these two formations is similar to that of Recent marine carbonates. The isotopically heavy carbonates (13Ccarb) of the Dunphy, Portage, Alder, and Uvé formations and those of the undivided Pistolet subgroup were accumulated during approximately 30 Ma over a vast area of at least 20 000 km2. The 13Ccarb enrichment occurs in connection with diverse depositional environments, not controlled by any detectable local factors (e.g., facies, diagenetic or metamorphic alteration) and therefore considered to be part of the world-wide development of high δ13Ccarb documented at around 2.2 ± 0.1 Ga. The Canadian Shield is the fourth continent, in addition to Africa, northern Europe, and Australia, to show extensive development of isotopically heavy carbonate formations of this age.