Biomarker and isotopic signatures of an early Cambrian Lagerstätte in the Stansbury Basin, South Australia

Abstract

The lower Cambrian (Series 2) Emu Bay Shale biota, found at Big Gully on the north coast of Kangaroo Island, South Australia, is by far the richest Burgess Shale-type (BST) fauna in the southern hemisphere. While the trace element geochemistry of the host shale shows it accumulated beneath an oxic water column, this appears difficult to reconcile with the exceptional preservation of organs such as gut glands, muscles and eyes in some of its fauna. Micro-scale sealed vessel (MSSV) pyrolysis of kerogen isolated from a solvent extracted sample of fossiliferous Emu Bay Shale provided independent confirmation of its redox status and also implicated cyanobacteria in the preservation of its fauna. Thermal extraction and pyrolysis each yielded alkanes displaying a low ratio of pristane to phytane (pr/ph = 1) and n-alkanes with a marked odd/even predominance in the <C 20 range. The latter molecular signature is diagnostic of Gloeocapsomorpha prisca and is the first indication that mats of this or a similar coccoid cyanobacterium were involved in the taphonomy of a BST deposit. Hopanes (including 2-methylhopanes) are far more abundant than steranes and the shale’s bulk organic matter has δ 13C values in the range −32‰ to −28‰, providing further evidence of the cyanobacterial affinity of its microbial biomass. Thus mat-forming cyanophytes, rather than sulfur-oxidising bacteria, may have played a key role in maintaining a sharp redox boundary at the sediment–water interface during deposition of the Emu Bay Shale Lagerstätte, thereby enabling the survival of soft body parts within its fossil fauna for long enough to permit inorganic replication of their fine detail during late diagenesis.