Abstract:
Prominent maxima in black shale abundance and in black shale/total shale ratio occur at 2.0-1.7 Ga, with less prominent peaks in the Late Neoproterozoic (800-600 Ma) and in the Late Archean (2.7-2.5 Ga). Peaks in chemical index of alteration (CIA) of shales at the same times suggest corresponding warm paleoclimates. The peaks in CIA and black shale abundance are correlated in time at a 94% confidence level. The black shale and CIA peaks may reflect the combined effects of mantle superplume events and supercontinent formation at 2.7 and 1.9 Ga. Mantle superplume events may have introduced large amounts of CO2 into the atmosphere-ocean system, increasing depositional rates of carbon and increasing global warming. Increased black shale deposition may reflect some combination of: (1) increased oceanic hydrothermal fluxes (introducing nutrients); (2) anoxia on continental shelves; and (3) disrupted ocean currents. The apparent absence of carbon isotope anomalies at these times reflects an increase in the deposition and burial rate of both reduced and oxidized carbon. Peaks in black shale abundance at ~2.1 Ga and 800-600 Ma correlate with peaks in δ13C in marine carbonates, increases in atmospheric oxygen, and with high CIA values in shales. These are all consistent with higher rates of organic carbon burial in black shales at these times. These peaks may record the breakup of supercontinents at 2.2-2.0 Ga and again at 800-600 Ma, which resulted in increased numbers of partially closed marine basins, disruption of ocean currents, and increased hydrothermal vents at ocean ridges, all of which led to widespread anoxia.
