THE CHENGJIANG SCIENTIFIC DRILLING PROJECT: EVIDENCE FOR THE MECHANISM OF BURGESS SHALE-TYPE PRESERVATION

Extraordinary fossilization of Cambrian Burgess Shale-type (BST) biotas worldwide followed a single major taphonomic pathway by which primary organic remains of fossil taxa were conserved as carbonaceous compressions. Their preservation must have involved taphonomic conditions unlike those present in modern marine environments, but the conditions that led to organic preservation of soft-bodied animals in marine sediments have been the subject of debate. We present data from a core drilled through the Chengjiang in July 2008 in the Haikou area, Yunnan, China. Sediment fabric and δ ³⁴S data indicate that the mechanism of Burgess Shale-type preservation in the Chengjiang was suppression of microbial activity in the sediments via oxidant restriction.

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. The event beds and intervening “background” beds were systematically sampled downcore for analysis. Event beds are characterized by heavy δ³⁴S 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 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. 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 analyzed by Hammarlund (2007) are characterized by similarly heavy δ ³⁴S values. δ ³⁴S evidence in combination with fine scale sedimentologic 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.