Time constraints for the Lomagundi excursion and the earliest known Paleoproterozoic macroscopic organisms from the Francevillian Group, Gabon

Abstract

The Francevillian Group, deposited during the Lomagundi carbon isotope excursion (LE), recently revealed evidence for complex and diverse Paleoproterozoic biota. This biota is preserved as pyritized and non-pyritized macrofossil structures hosted in black shale deposited in an oxygenated, open-marine environment. However, the timing of the LE, and the time when these macroscopic organisms evolved is still poorly constrained. Here, we present U-Pb ages for zircons separated from coeval volcaniclastic sandstone, ²⁰⁷Pb/²⁰⁶Pb model ages for pyrite preserving the macrofossils, and ⁴⁰Ar/³⁹Ar dates for K-rich clay minerals from the fossiliferous black shales. The youngest group of zircons yields a weighted average ²⁰⁷Pb/²⁰⁶Pb age of 2132 ± 4 Ma, which is considered as a maximum depositional age for the strata that record the LE and host the earliest known, macroscopic multicellular organisms. By contrast, the ²⁰⁷Pb/²⁰⁶Pb dates of pyritized fossils, scattering between ca. 2085 and 2070 Ma, with a weighted average of 2077 ± 17 Ma, reflect early diagenesis in the Francevillian basin and thus provide a minimum age for fossiliferous strata. This range of ages overlaps with reproducible ⁴⁰Ar/³⁹Ar dates and the ages for other open-marine sedimentary successions that record the LE in both shallow- and deep-marine environments. Taken together, the data demonstrate that the synchronicity of the record of the Lomagundi carbon isotope excursion in shallow- and deep-marine carbonates worldwide is consistent with a global biogeochemical signature of the Paleoproterozoic oceans, with its latest stage, between ca. 2.13 and 2.08 Ga, being preserved in the fossiliferous Francevillian Group strata. Further, the results also suggest that ⁴⁰Ar/³⁹Ar dating of K-rich clay minerals extracted from black shale can be used to constrain the depositional and/or early diagenetic age of more than 2-billion-years old sedimentary strata not affected by high-temperature hydrothermal and metamorphic overprint.