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The Precambrian recorded dramatic atmospheric and geochemical fluctuations, such as the “Snowball Earth,” the most severe glaciation in geologic history, and the Earth’s most significant negative carbon isotope excursion, the Shuram Excursion, which has been identified in multiple, globally-disparate locations. The Excursion recorded a rapid negative δ13C shift to approximately -12‰ and recovery over hundreds of meters in some units, which are predominantly shallow-marine carbonates. The excursion’s poor age constraints currently imply a maximum of some forty million years, between 635 Ma and approximately 591 Ma. This requires a large input of carbon-12 over a significant period of geologic time, and while debate exists over whether it is primary or diagenetic, evidence now leans in favour of a primary origin. This study presents δ13C data from a deep-water turbiditic carbonate in the Canadian Rockies that has produced a precise Re-Os date of 607.8 ± 4.7 Ma, known as the Old Fort Point Formation. Significantly, this unit records a δ13C excursion of -11.5‰ before the Re-Os date. The distinctive δ13C signal record implies it is correlative to the Shuram Excursion, and moves the Shuram Excursion farther back than previously estimated. The presence of multiple excursions between the Shuram Excursion and the Precambrian-Cambrian boundary indicate that the carbon cycle was unstable following the meltdown of the Marinoan glaciation. The OFPF helps to constrain the Shuram Excursion but more work to is needed to determine if current stratigraphic correlations have mis-correlated multiple excursions or not.

The Lomagundi Event (LE) was a period in Paleoproterozoic Earth history during which carbon isotopic composition of carbonates deposited globally record highly positive isotopic values (averaging at +8 ‰ VPDB). Current geochronological constraints from basins worldwide have bracketed the LE between ~2.22 and 2.06 Ga (Karhu and Holland, 1996; Bekker et al., 2003; Bekker and Holland, 2012; Bekker, 2014). The termination of the LE is commonly associated with the deposition of organic-rich lithologies, a return of carbon isotope composition of carbonates to ~0 ‰ VPDB.

The East Arm basin of the southeastern Slave craton contains an extensive Paleoproterozoic cover sequence of sedimentary and volcanic strata overlying the Archean granitic basement. Conventionally, it has been inferred that the 1.93 Ga Wilson Island Group is the base of the Paleoproterozoic succession based on the higher degree of metamorphism compared to the structurally isolated Union Island Group (Sheen et al. 2018; Bowring et al. 1984). Recent U-Pb zircon dating of the lower basalt unit of the Union Island Group has constrained its depositional age to 2045.8 ± 1.0 Ma (Sheen et al., 2018), placing this group at the base of the Paleoproterozoic supracrustal succession of the East Arm basin.

The Union Island Group contains carbonates, mafic volcanics, and mudrocks unconformably overlying an Archean granitic basement of the Slave craton with locally developed quartz pebble conglomerate and paleosols. The five stratigraphic units (inferred from regional mapping) in ascending order are: 1) lower massive dolostone locally underlain by quartz pebble conglomerate and paleosols developed on the Archean granitic basement, 2) organic-rich black shale with carbonate interbeds, 3) lower basalt package composed of alkaline to sub-alkaline basalt flows, flow breccia, pillows, and pillow breccia, 4) finely laminated, upper dolostone with thin red mudstone beds and locally present sub-alkaline pillow basalts, and 5) a laminated red and green mudstone (Thorstad, 1976; Hoffman et al., 1977; Sheen et al., 2018). Carbon isotope values of carbonates from the lower unit are highly positive (with the average of +7.4 ‰ VPDB), while the upper dolostone unit C isotopic values are near 0 ‰ (with the average of +0.5 ‰ VPDB). The highly positive C isotope values of carbonates from the lower dolostone unit are similar to those of carbonates deposited globally during the LE whereas the near-to-zero C isotope values of the upper dolostone unit are similar to those of carbonates deposited after the LE. 87Sr/86Sr values are highly radiogenic, with the lowest values of 0.7084 and 0.7095 for the lower and upper dolostone units, respectively, suggesting that the carbonates of the Union Island Group do not record a primary marine signal. This could be an indication of deposition in a closed to semi-closed basin, where a lacustrine setting is intermittently subjected to marine incursions. Organic carbon isotope values for the black shale unit separating the dolostone units are highly negative (with values as low as -43.1 ‰ VPDB), indicating oxidative methane cycling, while nitrogen isotopic values are positive (averaging +5.0 ‰), consistent with nitrification-denitrification. Carbon isotope values of carbonates associated with the black shale are slightly negative (with values averaging -1.9 ‰ VPDB) indicating diagenetic origin. Deposition of the Union Island Group black shales is proposed to have occurred in a redox-stratified water column beneath the redoxcline, on the basis of Fe-speciation data which suggests deposition under anoxic ferruginous conditions.

Deposition of the lower dolostone unit of the Union Island Group might correspond to the final stage of the LE, or a short-lived highly positive carbon isotope excursion in the aftermath and preceding the 2.03 Ga (Bekker et al., 2016) Wooley Dolomite excursion. However, the 2045.8 ± 1.0 Ma age seems to contradict with the accepted age for the end of the LE at 2.06 Ga. Combined with the earlier inferred similarly short-lived carbon isotope excursion at ~2.03 Ga (Bekker et al., 2016) in the Wooley Dolomite, the Union Island Group record points to a volatile state of the biogeochemical carbon cycle in the aftermath of the LE, which until recently was not appreciated.