Elemental and isotopic compositions of trench-slope black shales, Bohemian Massif, with implications for oceanic and atmospheric oxygenation in early Cambrian
Abstrakt
This study examines a lower Cambrian pyrite-bearing black shale-graywacke succession in the Czech Republic interpreted as infill of a deep-marine, extensional trench-slope basin on top of an accretionary wedge that developed during subduction of an oceanic plate beneath the northern margin of Gondwana. The new U-Pb detrital zircon geochronology of the graywacke constrains the maximum age of deposition to 533 +5/-6 Ma, whereas the Re-Os dating of pyrite yielded a younger age of 507 +/- 16 Ma.
Distribution of major and trace elements and calculated enrichment factors (EF) indicate that this succession was deposited under changing redox conditions over a short time span and that the basin was presumably controlled by tectonic subsidence and varying supply of terrigenous arc-derived material. This depositional setting is reflected by largely variable EFs and δ(98)Mo and δ(53)Cr values.
Euxinic conditions detected at the base of the sampled black shale interval are documented by the highest values of EFs of redox-sensitive metals (e.g., Mo, U, V, Ni, Co, As) and also the C(org)/P (>1000) and DOP(T) (>0.7) values. Black shales show lower δ(56)Fe values due to the excess of authigenic pyrite-Fe with a mean δ(56)Fe value of -0.02 promile over detrital Fe in graywacke with a mean δ(56)Fe value of +0.25 promile.
However, the Fe isotopic signatures of the black shales are not consistent with iron shuttling, mixing of authigenic and detrital sources, or hydrothermal metal enrichment. Instead, they most likely resulted from partial oxidation of pyrite through the syndepositional oxidizing hydrothermal fluids (Si-Ba enrichment), which resulted in precipitation of isotopically heavy Fe-oxyhydroxides.
We propose that our maximal recorded δ(98)Mo value (+ 0.98 promile) might represent the best estimate for the ancient local seawater Mo composition at around 533 Ma and argues against deep-ocean oxygenation in the early Cambrian. On the other hand, the Se/Co ratios of synsedimentary pyrite indicate a mean atmosphere O2 value of similar to 27%.