Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Tuesday, September 21, 2010

Pushing up daisies

Life is opportunistic. About a 150 million years ago flowering plants did not exist on the Earth, today we are positively tripping over the things, so what happened? While there are many factors involved, a particularly interesting one has come up for recent discussion - and relates to a previous post on these pages.

A couple of weeks ago Bond & Scott published a paper in the rather wonderfully named scientific journal 'New Phytologist' that discusses how flowering planets, or angiosperms, spread during the Cretaceous some 65 to 145 million years ago. The novel aspect to this work is the suggestion that critically during this period, because of an elevated atmospheric oxygen level compared to today - perhaps to 25% rather than our paltry 21% by volume, surface fires were much more pervasive. I talked about this general phenomenon a while back.

So, the picture goes like this. Wildfires (well, I guess every fire was 'wild' during the Cretaceous) would have been significantly more frequent with higher atmospheric oxygen. This would have posed a significant challenge to surface plant life. Long-lived and slow growing species, like larger conifer trees - which have an ancient lineage - would have a hard time regenerating their populations fast enough. Imagine a cosy little spot, a fire rips through, everything burnt to a crisp. Seeds arrive, new plants grow, but sure enough another fire comes tearing across the land. Only those plants that had grown fast enough to mature and dump out the next round of seeds (carried off by wind and newly minted mammals and birds) would stand a chance at producing another generation. It's a vicious cycle, the fast growing angiosperms (one presumes helped along by insect and animal pollination) not only outrun the fire cycle, but they quickly produce the next round of fuel.

The upshot is that flowering plants don't get as much competition for resources from the previously dominant types of vegetation - in essence the weedy daisies win the day. The evidence for all this combustible carnage lurks in the remarkable charcoal deposits, and charcoal fossils from this geological period.

It's another example of the incredibly intertwined nature of life on a planet, and another great example of the constant 'what ifs' of evolution. Would an Earth that had always kept a low oxygen level have ended up with flowering plants - and the particular effect this implies on continental albedo and biosignatures?


Eniac said...

I am not sure if it is correct to say more oxygen means more fire means different plants. If there really was more fire, it would use up the oxygen and keep it from running high. The amount of fire, then, should be determined by the flux of oxygen, not its concentration.

My feeling is that the oxygen level regulates itself to be at exactly the level where plants just start burning, but never beyond. Substantial historical changes in oxygen levels should then be taken as indications of change in the flammability of land biomass. There are presumably many factors in that, but the most relevant is likely humidity. A humid climate would lead to more oxygen, because fires not burn as easily.

The amount of fire should actually be relatively constant, because the supply of oxygen is given by the efficiency of photosynthesis, insolation, and available land area, all of which should be fairly constant.

Caleb Scharf said...

Some great points. I think the global oxygen levels would be little impacted by fires unless they were enormous, continent spanning, storms. The amount of O2 used up by combustion is tiny compared to the huge atmospheric pool of O2 - and marine photosynthesis is certainly going to carry on producing oxygen regardless. So I'm not sure that fire would be the main feedback keeping oxygen at a particular level.

Humidity, as you say, is indeed a factor, and it's interesting that during much of the Cretaceous global temperatures were higher and the hydrological cycle of the planet would have been different.

Eniac said...

If not fire, what would be the limiting factor for O2? Is consumption by animals larger, comparable, or smaller than that by fire? Is there a geological sink? Time to hit the books, I guess...

Or Wikipedia: http://en.wikipedia.org/wiki/Oxygen_cycle

Fire is not mentioned here, nor could I find reference to it anywhere else as an oxygen sink. This is hard to believe for me, as wildfires seem like a formidable sink of oxygen, and should be at least comparable with controlled industrial combustion. Maybe I am wrong on that? It would be good to have some actual numbers, though.

Fascinating subject!