LINKED HYDROSPHERIC AND ATMOSPHERIC CHANGES ATTENDING PROTRACTED BREAKUP OF THE LATE ARCHEAN SUPERCONTINENT

Assembly of the Late Archean supercontinent Kenorland was completed by ca. 2.42 Ga and overlapped with ca. 2.51-2.37 Ga mafic magmatic events interpreted as mantle plume breakouts. These events were followed by three global glaciations. The middle glaciation was succeeded by deposition of carbonates with negative δ¹³C values in response to high carbonate alkalinity in post-glacial ocean. From 2.32 to 2.0 Ga episodic intrusion of mafic dikes likely reflected protracted rifting and eventual breakup of Kenorland at ca. 2.0 Ga. Intracratonic rift basins formed during this period might have provided a site for enhanced burial of C_org as indicated by positive δ¹³C excursion in ca. 2.22-2.1 Ga carbonates. Sandstones deposited shortly after the last glaciation show a gradual upward increase in both compositional and textural maturity, are interlayered with aluminous shales, and contain thin carbonates with δ¹³C values ranging from 0 to +4 permil. These changes are interpreted to indicate enhanced chemical weathering as a consequence of climatic amelioration. Overlying ca. 2.22-2.1 Ga open- and restricted-marine strata include carbonates with δ¹³C values ranging from +4 to +28 permil and mature siliciclastic rocks. Both carbonate and siliciclastic rocks contain examples of early diagenetic calcium sulfate crystals and nodules. The breakup of Kenorland beginning at ca. 2.1 Ga induced relative sea level rise and enhanced ocean circulation as indicated by deposition of organic-rich shales and Mn-, Fe-, and P-rich deposits. This marine transgression coincided with the end of the ca. 2.22-2.1 Ga carbon isotope excursion. Rise of the atmospheric oxygen as indicated by mass-independent fractionation in S isotopes and other redox sensitive indicators was coupled with the Paleoproterozoic climatic changes and led to the stepwise oxidation of surface environment and atmospheric methane, a significant Archean greenhouse gas. Unstable climatic conditions continued until fully oxidized atmosphere was established in which CO₂ was the principal greenhouse gas.

Assembly, rifting, and breakup of the Kenorland between 2.5 and 2.0 Ga provided a framework in which changes in atmospheric and ocean composition and climate occurred. Precise temporal resolution of these changes will provide a valuable test for their cause and effects.