To explore how seawater chemistry might influence exceptional Burgess Shale-type preservation, freshly-killed shrimp and annelids were covered in clay and exposed to high and low sulfate concentrations for up to six weeks of anaerobic decay. Decay was monitored by carbon mass balance calculations and non-destructive imaging. Decay rates and visual distortion of shrimp cuticle and muscle appear slower in environments without sulfate, compared to environments with normal marine sulfate concentrations. By means of X-ray and neutron tomography, the carapace and tail muscle of shrimp was observed to pass from seemingly intact after three weeks of decay to distorted after six weeks of decay. The distortion of the annelids was more rapid. Preservation of detailed structures must occur within this short time span, in order to produce exceptional fossils. When sulfate is absent, methanogenesis is the dominant pathway of carbon re-mineralization. We argue that a slight inefficiency of methanogenic carbon oxidation, also indicated in other studies, could widen the time frame for initial preservation and enhance the likelihood of labile tissue being preserved, as well as play a role in the formation of Burgess Shale-type carbonaceous compressions.