Precambrian superplumes and supercontinents: a record in black shales, carbon isotopes, and paleoclimates?

Prominent maxima in black shale abundance and in black shale/total shale ra tio occur at 2.0-1.7 Ga, with less prominent peaks in the Late Neoproterozo ic (800-600 Ma) and in the Late Archean (2.7-2.5 Ga). Peaks in chemical ind ex of alteration (CIA) of shales at the same times suggest corresponding wa rm 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 refle ct the combined effects of mantle superplume events and supercontinent form ation at 2.7 and 1.9 Ga. Mantle superplume events may have introduced large amounts of CO2 into the atmosphere-ocean system, increasing depositional r ates of carbon and increasing global warming. Increased black shale deposit ion may reflect some combination of: (1) increased oceanic hydrothermal flu xes (introducing nutrients); (2) anoxia on continental shelves; and (3) dis rupted 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 similar to 2 .1 Ga and 800-600 Ma correlate with peaks in delta C-13 in marine carbonate s, increases in atmospheric oxygen, and with high CIA values in shales. The se are all consistent with higher rates of organic carbon burial in black s hales 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 incre ased hydrothermal vents at ocean ridges, all of which led to widespread ano xia. (C) 2001 Elsevier Science B.V. All rights reserved.