Mixing It Up: Burrowing Animals Contribute to Regulated Oxygen Levels
Have you ever built a tower of blocks? If so, then you know the importance of starting with a stable base. Without such a base, the weight of the blocks, small nudges, or even the vibrations from a passer-by can cause the tower to come crashing down. A stable base is also essential to support diverse ecosystems. For large-bodied animals, a regulated oxygen supply in the water and atmosphere provides part of that stability. And, as it turns out, some of those animals play a key role in regulating the oxygen supply.
Fossil record evidence suggests that soft-bodied animals existed prior to the Cambrian explosion (which marks the abrupt appearance around 542 million years ago of a diverse community of large-bodied organisms with hard skeletons). Specifically, tracks formed by what look to be burrowing creatures first appear in the fossil record before the Cambrian explosion,1 but the number of tracks increases dramatically afterward. These animals mixed sediments from different layers in a process called bioturbation, resulting in well-mixed, post-Cambrian sedimentary records that contrast sharply with the stratified layers of earlier times.
The enhanced bioturbation during the Cambrian era also increased the amount of ocean water (with a growing oxygen content) exposed to sediments on the ocean floor. Modern experiments reveal that such conditions cause the bacteria in those sediments to store more phosphate, thus removing phosphorus from the marine environment. Since phosphorus serves to limit marine productivity—less phosphorus results in less carbon burial and a corresponding decrease in available oxygen. So, the burrowing animals—which require oxygen for survival—contribute to conditions that remove oxygen from the environment.
Stated another way, bioturbation provides negative feedback on the amount of free oxygen in Earth’s oceans and atmosphere. An increase of oxygen will trigger more bioturbation that will then lower oxygen levels.2 Remarkably, the animals that burrow through the sediments to regulate the supply of oxygen appeared as part of the animals that need a stable supply of oxygen.
Studies of Earth’s history continue to reveal evidence of design and fine-tuning. The well-designed feedback on oxygen resulting from bioturbation adds to the stable base that supports Earth’s long history of biodiversity. Other foundation components include the Sun’s increasing luminosity, the formation of continents, active plate tectonics, a few well-timed increases in Earth’s oxygen reservoir, and more.
- Zhe Chen et al., “Trace Fossil Evidence for Ediacaran Bilaterian Animals with Complex Behaviors,” Precambrian Research 224 (January 2013): 690–701.
- A. Boyle et al., “Stabilization of the Coupled Oxygen and Phosphorus Cycles by the Evolution of Bioturbation,” Nature Geoscience (published electronically August 3, 2014): DOI:10.1038/ngeo2213, http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2213.html.