"What turned out to be the important key for this type of preservation is the chemistry of the global sea water," explains Gaines. "The preservation was greatly aided by enhanced calcium carbonate concentrations in the Cambrian oceans and by depletion of oxygen and sulfate. Importantly, low oxygen concentrations in the global oceans during this interval of time limited the amount of sulfate, an important microbial nutrient."
In the past, researchers have focused on the fossils themselves, rather than the details of the sediments and their chemistry. Gaines and Hammarlund found it was necessary to unlock the mystery of the strange preservationa sign that the environment was not normal.
The drill cores from the Chengjiang site were important because the heavy rains from the Himalayan monsoons in the area leach minerals, including pyrite and calcium carbonate, from the rocks that are exposed on the surface. With these cores, the team's unique collection of samples led to the recognition that unique aspects of early Paleozoic seawater chemistry that were key to the unusual Burgess-type soft-bodied fossil preservationthe low sulfate concentration, low-oxygen bottom water conditions, and the mineral carpet that aided in choking the hungry microbeswas a striking global pattern.
"I had little idea of what to expect from the geochemical data, which rarely can provide a 'silver bullet,' says Gaines. "I was literally floored. I have rarely seen geochemical data so convincing. My initial hypothesis was validated by a consistent and worldwide pattern."
|Contact: Emma Hammarlund|
University of Southern Denmark