Kent C. Condie, David J. Des Marais and Dallas Abbott
Geologic evidence for a mantle superplume event at 1.9 Ga
Geochemistry, Geophysics, Geosystems - G (super 3)(December 2000), 1(12)
Index Terms/Descriptors: alkaline earth metals; atmosphere; biogenic structures; black shale; C-13/C-12; carbon; clastic rocks; deposition; geomorphology; isotope ratios; isotopes; mantle; mantle plumes; metals; Precambrian; sedimentary rocks; sedimentary structures; Sr-87/Sr-86; stable isotopes; stromatolites; strontium; superplumes; world ocean
Both preserved and restored areal distributions of Proterozoic marine intracratonic, passive margin, and platform sediments show a prominent peak at approximately 1.9 Ga, indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time. The chemical index of alteration in shales deposited at this time was high, suggesting warm climates, possibly due to enhanced CO 2 levels in the atmosphere. High sea level and warms climate may also explain an abundance of black shale, banded iron formations, and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1.9 Ga. All of these observations are consistent with a 1.9-Ga superplume event. The occurrence of only a minor positive carbon isotope shift in marine carbonates at 1.9 Ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present. Slightly low 87 Sr/ 86 Sr isotopic ratios in seawater at 1.9 Ga reflect increased mantle input of Sr from the proposed superplume event, whereas higher ratios at 1.85-1.75 Ga may reflect increased input of continental Sr from a growing supercontinent. The first massive sulfate evaporites in the geologic record at 1.8-1.6 Ga follow the possible 1.9-Ga superplume event. This may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume-related volcanism wanes.