Burgess Shale-type fossil preservation

By Rachel Berkowitz

Many paleontologists have a love affair with the Burgess Shale Formation in the Canadian Rockies. The area is famous for the exceptional fossil record of the Cambrian explosion, when complex life flowered from single-celled ancestors. But it’s not just shells, teeth, and bones that are well preserved: The soft tissues of animals are also imprinted in the shale, and that’s remarkable for remains of things that lived 505 million years ago.

When Charles Walcott discovered the Burgess Shale in 1909, nobody could explain why the fossils were so well preserved. Later hypotheses proposed that the animals were swept downslope in mudflows and buried under anaerobic conditions. That’s true but it’s not enough: Tissues can degrade just as rapidly under anaerobic conditions as under aerobic ones.

In 2008 it was shown that fossils in Burgess Shale-type deposits around the world were all preserved primarily as carbonaceous remains under a layer of calcium carbonate cement. That finding is extraordinary because carbonaceous fossilization requires a completely different means of preservation than occurs in today’s oceans.

After a decade of investigation into the Burgess Shale sediments and their chemistry, Robert Gaines and his team at Pomona College in California have provided a mechanism to explain the unusual preservation process. Their theory is published in the March 2012 edition of the Proceedings of the National Academy of Sciences.

“The sediments of these deposits are especially cryptic,” says Gaines. They are unusually fine-grained and lack sedimentary structures that give clues to the processes involved in their deposition. They are further complicated by the “bizarre and anomalous fabrics” produced by carbonate cement overgrowths. “It turned out that a microscale understanding of the rocks in combination with their geochemistry was key to unlocking this puzzle.”

The Pomona researchers propose that the soft tissues are so well preserved because microbial activity in the sediments was inhibited early on by means of oxidant deprivation. Fossils were entombed rapidly in fine-grained sediments sealed by carbonate cements that restricted oxidant flux and built a permeability barrier. Low sulfate concentrations in the Cambrian global ocean and low-oxygen bottom water conditions at sites of sediment deposition resulted in reduced oxidant availability.

Gaines had hypothesized that carbonate sealing of the seafloor would have deprived microbial communities that rely on nitrates, sulfates, and other oxidants from access to overlying seawater. Without that access, they could not decompose organic tissues. He tested his hypothesis by analyzing sulfur isotope composition of the sediments.

“Sulfate reduction is the primary pathway for degradation of organic matter in the absence of oxygen,” explains Gaines. The sulfur isotope composition of the sediments, which form as a byproduct of microbial respiration of sulfate, shows how much sulfate was available.

The sulfur isotope signature in the upper sediment layer is expected to be lighter than that of seawater because microbes prefer to respire the lighter isotope. As sulfate becomes limiting, they’ll take whatever they can get. Thus the “heavy” signature in samples of Burgess-type shale all over the world means that microbial activity was severely impeded in fossil-bearing layers. Experiments indicate that microbial activity is greatly reduced at sulfate concentrations that produce such low fractionation.

The fossils’ fate was sealed by a calcium carbonate layer that precipitated directly from the seawater onto the seafloor. It built a permeability barrier that prevented diffusion of ions into or out of the sediments. Thus microbes that usually decompose organic tissues fully in two to four weeks were literally choked. These bed-capping cements resulted from the unusually high alkalinity of Cambrian oceans.

The Burgess Shale documents the relatively rapid appearance of most major animal phyla that led to the diversity of life as we know it today. Gaines’s interpretation shows just how important the unusual Cambrian seawater chemistry was to this priceless paleontological record.