Mechanism for Burgess Shale-type preservation

Abstract

Exceptionally preserved fossil biotas of the Burgess Shale and a handful of other similar Cambrian deposits provide rare but critical insights into the early diversification of animals. The extraordinary preservation of labile tissues in these geographically widespread but temporally restricted soft-bodied fossil assemblages has remained enigmatic since Walcott's initial discovery in 1909. Here, we demonstrate the mechanism of Burgess Shale-type preservation using sedimentologic and geochemical data from the Chengjiang, Burgess Shale, and five other principal Burgess Shale-type deposits. Sulfur isotope evidence from sedimentary pyrites reveals that the exquisite fossilization of organic remains as carbonaceous compressions resulted from early inhibition of microbial activity in the sediments by means of oxidant deprivation. Low sulfate concentrations in the global ocean and low-oxygen bottom water conditions at the sites of deposition resulted in reduced oxidant availability. Subsequently, rapid entombment of fossils in fine-grained sediments and early sealing of sediments by pervasive carbonate cements at bed tops restricted oxidant flux into the sediments. A permeability barrier, provided by bed-capping cements that were emplaced at the seafloor, is a feature that is shared among Burgess Shale-type deposits, and resulted from the unusually high alkalinity of Cambrian oceans. Thus, Burgess Shale-type preservation of soft-bodied fossil assemblages worldwide was promoted by unique aspects of early Paleozoic seawater chemistry that strongly impacted sediment diagenesis, providing a fundamentally unique record of the immediate aftermath of the "Cambrian explosion."

Fig. 1.

Burgess Shale-type fossils from the Greater Phyllopod Bed of the Burgess Shale (Walcott Quarry Member), photograph taken in cross-polarized light. The fine details of soft-bodied anatomy are preserved as thin carbonaceous films. Soft-bodied fossils figured include examples of the priapulid worm Ottoia prolifica (O), the problematic medusiform animal Eldoniia ludwigi (E), and the arthropod Marrella splendens (M). Image courtesy of Jean-Bernard Caron (Royal Ontario Museum).

Fig. 2.

Sedimentary and diagenetic fabrics of BST mudstones. (A) SEM micrograph of mudstone fabric in the “Great Eldoniia Layer” (GEL) of the Burgess Shale, showing random orientations of clay mineral grains and absence of coarser grained particles. (B) Polished slab showing the GEL (arrow) with thinner beds over and underlying it. (C) SEM micrograph showing displasive growth of 5–15 μ rhombohedra of authigenic calcite (low-lying areas) within an extensively cemented bed in the Wheeler Formation. Individual clay mineral grains have been forced to the margins of the calcite crystals. The sample has been etched with HCl; clays stand out in relief. (D) Thin section micrograph of Burgess Shale (Greater Phyllopod Bed) showing concentration of authigenic calcite cements (bright) at bed tops (at both the top and bottom of image). (E) X-radiograph of polished slab of Burgess Shale containing the GEL (arrow), showing the distribution of bed-capping authigenic carbonate cements. Bright areas correspond to high wt. % CaCO₃. Extensive bed-capping cement is present at the top of the GEL as well as at the tops of the thin, millimeter-scale beds that overlie it. (F) Backscatter SEM micrograph of pyrite cluster from a Chengjiang event bed at 12.96-m depth in the Haikou core showing fine grain size (1–5 μ) and aggregative habit of octahedra and pyritohedra.

Fig. 3.

Plot of the isotopic composition of the pyrite sulfur (δ³⁴S) in the fossiliferous portion of the Chengjiang from a new drill core extracted near Haikou town, Kunming, Yunnan Province, China. Event-deposited beds (blue diamonds) are characterized by heavy values [average +9.6 ± 2.0‰ (SEM)] that display less fractionation from Cambrian seawater (ca. +30‰; ref. 31) than background sediments [orange circles; average -6.0‰ ± 0.6‰ (SEM)].