Emma HAMMARLUND1,3, Robert GAINES2, Don E. CANFIELD3, Xian-Guang HOU4 and Stefan BENGTSON1
1Department of Palaeozoology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden
2Geology Department, Pomona College, Claremont, CA 91711, USA
3NordCEE, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark
4Yunnan Key Laboratory for Paleobiology, Yunnan University, Kunming 650091, China
The Chengjiang Lagerstätte preserves a record of ocean chemistry during a time of rapid metazoan diversification on Earth. A core drilled near Haikou, Kunming in Yunnan, China, penetrated the Helinpu Formation, which is comprised of the Yu’anshan Shale and the underlying Shiyantou Siltstone. The Yu’anshan Shale contains the Chengjiang biota, which occurs stratigraphically 30 m above the base of the first occurrence of trilobites in the lowest part of the Yu’anshan Member. Geochemical data from this core reveal dynamic changes in the paleoenvironment before and during the interval bearing exceptionally preserved fossils. Iron speciation results from the interval containing Chengjiang biota are inconclusive (FeHR/FeT 0.26 ±0.08), but indications of anoxia occur in the base of this interval, and decrease upcore. Non-mineralized fossils of the Chengjiang biota appear to have been deposited below an increasingly oxic water column. Bioturbation is completely absent from this interval of the core, however, and evidence favours fluctuating water column redox conditions, and possibly bottom-water anoxia. The latter would account for the lack of bioturbation in this interval and is consistent with models for Burgess Shale-type preservation. In the lower part of the Chengjiang interval, pyrite is the dominant iron phase (FePY/FeHR 0.65 ±0.13 when FeHR/FeT>0.30) and the isotopic composition of the pyrite shows a consistent fractionation of about 40‰, indicative of active sulfate reduction.
Below the Chengjiang interval, iron extraction results indicate either one or two euxinic events, which occurred during the transgression that characterizes the lowermost Yu’anshan. Our results seem to indicate a stratified water column with an oxic upper layer, a ferruginous middle zone and euxinic deep water. The water column overlying the Yu’anshan Shale may have gone through all three states in a couple of million years.
In a portion of the lowermost Shiyantou, during a sea level highstand, iron enrichment is absent (FeHR/FeT 0.22 ±0.09), and iron oxides occur as a dominant iron phase. This is interpreted as an early pulse of oxic ventilation of the deeper water column. Moderate bioturbation (ichnofabric index 3) occurs throughout the Shiyantou, but body fossils are not preserved. The δ34S values become increasingly heavy upcore within the Shiyantou siltstone (7.4 ±11.2‰), a trend that is stronger than the assumed shifts in oceanic sulfate composition. Oxic respiration in the uppermost sediments might have pushed the zone of sulfate reduction deeper into the sediment, with slow diffusion driving these values heavy, but this trend is also clear in anoxic intervals of the Shiyantuo. We speculate that sufficient background sedimentation rates, low oceanic sulfate concentrations, and carbonate precipitation in pore spaces have all influenced the geochemical signals present in this core.
Emneord: Cambrian explosion
|Period||6. Aug 2009|
|Event title||Walcott 2009 - An International Conference on the Cambrian Explosion: null|
|Location||Banff, Alberta, August 3–8, 2009, Canada|
- Cambrian explosion