The long history of the Earth includes periods of warming and cooling, including times with no ice and at least one “snowball Earth”, with ice covering the whole surface. If nothing else, the geological record should teach us that in the long run, things can change a whole lot.
One of the interesting large scale events was the Cambrian explosion, a bloom of life 500 some million years ago. At the time, the atmosphere was dominated by CO2 levels much higher than today, followed later by increased levels of Oxygen, presumably the byproduct of the spreading life. Some of the oldest lifeforms known were cyanobacteria which breath CO2 and emit O2 as waste.
These long term trends have reversed several times over hundreds of millions of years. The levels of CO2 are, of course, linked to global temperatures. But CO2 and other chemicals dramatically affect the chemistry of the oceans, which, in turn, favors different life forms depending on conditions. (Today we are seeing a trend toward higher CO2, with concomitant effects on the oceans and global temperatures.)
The evolution of the atmosphere is no trivial matter of breathing out. As we know today, chemicals from the atmosphere react with rocks and water, pulling constituents out of the air. In the case of Oxygen, it reacts strongly with many elements, not least Iron, forming many oxides. It also mixes with ocean waters, changing the acidity of the liquid and then reacting with yet other materials.
This summer a group of European researchers report a new analysis of yet another factor: the activities of sea bottom burrowers . Today, these little critters have a surprisingly large role in the chemistry of the ocean and atmosphere. They only burrow a few centimeters, but this mixes up the sediments, letting in water, letting out chemicals, and generally causing a lot of biochemistry.
The new research pulls together many geochemical studies of the Earth’s past with a model of “burial, weathering and degassing processes, which transport chemical species between the atmosphere, oceans and sediments over geological timescales” (p. 4 ) The basic idea is to explore alternative scenarios with different assumptions about the amount and intensity of burrowing.
The fossil record indicates the evolution of ocean bottom burrows in the Cambrian period, with increasing evidence in the ensuing hundreds of millions of years. The research uses different assumptions about how active these diggers might have been, and then computes long term estimates of the global abundance of various chemicals.
The results suggest that even the early “moderate levels of shallow bioturbation have a large impact on sediment geochemistry”. (p. 8 ) Even the smallest entity can change the course of the future.
The authors also speculate that these animals may have a role in some of the complicated events in geological history. As areas the ocean bottom became strongly anoxic, this could have led to extinction of burrowing species, which in turn dampened their effects, amplifying the climate change.
“There is a strong correlation between ocean anoxia, positive δ13Ccarb excursions and extinction events, and it is possible that the interactions between burrowing macrofauna, biogeochemical cycling and ocean anoxia may have contributed to these patterns.”
To paraphrase Lady Galadriel in the movie, ‘Even the smallest entity can change the course of the future.’
- Sebastiaan van de Velde, Benjamin J. W. Mills, Filip J. R. Meysman, Timothy M. Lenton, and Simon W. Poulton, Early Palaeozoic ocean anoxia and global warming driven by the evolution of shallow burrowing. Nature Communications, 9 (1):2554, 2018/07/02 2018. https://doi.org/10.1038/s41467-018-04973-4