Evidence for the Mechanism of Burgess Shale-Type Preservation from the Chengjiang Scientific Drilling Project  

Cambrian Burgess Shale-type (BST) deposits occur worldwide, and provide the most important record of the initial Phanerozoic radiation of the Metazoa.  It has recently been demonstrated that the fossilization of BST biotas worldwide followed a single major taphonomic pathway by which primary organic remains of fossil taxa were conserved as carbonaceous compressions.  The preservation of BST biotas must have involved taphonomic conditions which differ from those present in modern marine environments.  The specific conditions that produced BST preservation have been the subject of debate.  Here, we present data from a core drilled through the Chengjiang deposit near Haikou, Yunnan, China.  The sediment fabric and δ ³⁴S data indicate the suppression of microbial activity within the sediment, via oxidant restriction.  This provides a possible mechanism of BST preservation in the Chengjiang.

In the Chengjiang, BST fossils occur in 1–5 cm-thick gray claystones, which were deposited in discrete depositional events, and are separated by intervals of black claystone that represent slower, pelagic sediment accumulation between depositional events.  The event beds and intervening “background” beds were systematically sampled downcore.  Event beds are characterized by heavy δ ³⁴S values that range from +46.1 to –10.1 ‰, (average +7.1 ± 3.0 ‰), whereas background beds exhibit lighter values and a narrower range of +6.3 to –12.3 ‰ (average –6.0 ± 0.6 ‰). Heavy δ ³⁴S values in the event beds indicate sulfate-restricted, “closed system” conditions during early diagenesis, relative to the lighter values of background beds that exhibit normal fractionation from seawater.  We interpret this as evidence that microbial sulfate reduction was suppressed by oxidant deprivation shortly after burial.  Sulfate deprivation may have resulted from a combination of influences which included: (1) low SO₄^- concentrations in the global ocean; (2) the influence of turbidite deposition on SO₄^- diffusion; and (3) pervasive early carbonate cements at bed tops which may have acted to suppress diffusion.  Samples of other BST deposits are characterized by similarly heavy δ ³⁴S values.  δ ³⁴S evidence, in combination with fine scale sedimentological data from the Chengjiang, Burgess, and other principal BST deposits, suggests that Burgess Shale-type preservation worldwide occurred as a result of early closure of the diagenetic system.