Friday, 11 May 2012

The Cretaceous in Flames


Recently I blogged about the complex ecological issues regarding the abundant flora and fauna of Laramidia and how so many large dinosaurs managed to exist in such a relatively small area of land. I postulated that increased levels of CO² created a greenhouse effect that generated the vast amount of vegetation required to sustain large populations of big herbivores. High humidity, warm temperatures and abundant rainfall all increased growth levels.

 I highlighted how the dinosaurs themselves may have driven the plants to diversify and increase in quantity (coevolution) and how this may also have spurred on the rapid evolution of the angiosperms. All these suggestions, in combination with the geographical and physical barriers such as rivers and mountains, may have been enough to create and sustain an ecological explosion in both plants and animals unlike anything else we are familiar with, both today, and in the past.
And now a recent study introduces yet another player into the already complex world of the Cretaceous – wildfire. The paper by Sarah Brown et al was published in Cretaceous Research in February and reveals yet more insights into how fire added a further dimension to the terrestrial ecosystem and how the implications were often significant.
There studies reveal that, although CO² levels were indeed high and that a greenhouse effect was in place, oxygen was also increasing in volume – in fact quite significantly. This does not surprise me at all since the increased the amount of CO² taken in by a rapidly growing and expanding plant flora would have also produced large volumes of oxygen through photosynthesis. What I had not realised, however, was the effect this would have on the control of wildfires that affected the ecosystem.
The authors were able examine the records of wildfires during the Cretaceous by analysing the remains of charcoal that are dispersed in quite often large quantities in deposits throughout the world. Simply put, charcoal represents the remains of plants and trees that were caught up in these fires and when examined, using scanning electron micrographs, reveal a varied community of plants and trees – particularly the angiosperms.
Despite having to be aware of sampling bias (such as a lack of sites in the southern hemisphere), the first evidence of abundant wildfires is during the Valanginian and there appears to be a general increase in the amount of fires from that point on and they become worldwide during the Late Cretaceous. Each charcoal deposit displays unique taxanomic plant diversity and highlights the progression of the angiosperms throughout the period.
The charcoal reveals the effects of fire during high oxygen conditions. What I had not realised is that rainfall is not as effective a controller of fire as in a conventional scenario because the oxygen content enables fire to burn under wetter conditions – even wet plants would combust more readily. The sheer volume of plant fossils in the charcoal and the high oxygen content all suggest extensive wildfires – particularly in the Late Cretaceous.
There are many possible side effects of such fires and principle amongst these was post-fire erosion which has various implications for any ecosystem. Once fire has destroyed vegetation and the litter and scrub covering has been removed, the soil becomes liable to further enhanced erosion and large volume transport of sediment can occur. The lack of coverage enables, thereafter, an increase in surface runoff during rainfall and flooding events and the surface, because it is now exposed, can also be then affected by an increase in temperature which can change the soil profile leading to yet further erosion.  

An exposed charcoal bed at the Bluff
One of the after effects of such a flood event is that both charcoal deposits and dinosaur carcasses can be washed together and deposited in one accumulation. It is a fact that dinosaur remains are often found within charcoal deposits and I have first-hand experience of this at the Bluff which, coincidentally, features in this paper as does the Cuckoo’s Hole Quarry. It has been known for some years that bones and charcoal are found together but this is the first time a mechanism has been suggested to explain the cause.
As already noted, angiosperms began to proliferate during the Late Cretaceous and these wildfires may also have been instrumental in their rise to prominence. Angiosperms were much better equipped to deal with fire due to their ability to take in the increased levels of oxygen which, although enabled the plants to burn at a quicker rate, also allowed them to recover quickly and grow at elevated rates.
The study suggests the possibility that dinosaurs may have been driven to their death by these fires and that, at the very least, there would have been a form of displacement as herbivorous dinosaurs had to move off in search of fresh fodder. Another effect is that the surface runoff would have been flushed into the sea and may have caused an oceanic anoxic event whereby an algal bloom may have occurred and oxygen levels were depleted but this is only suggested and was beyond the scope of this paper. I will say that we often find the charcoal remains of trees in the Oxford Clay so we can certainly confirm that burnt remains of such plants were indeed washed into the sea although this was during the Jurassic.

A charcoal log in the Callovian Oxford Clay

I have to say that this is really a quality paper and I am impressed by the amount of data collected and the attention to detail. There are plenty of quantifications included suggesting the aforementioned sampling biases that may occur and the fact that more study is obviously needed. This study adds a further chapter to the already fascinating world of the complex ecosystems in the Late Cretaceous.  
Reference
Brown, S.A.E., et al., Cretaceous wildfires and their impact on the Earth system, Cretaceous Research (2012), doi:10.1016/j.cretres.2012.02.008

Wildfire image courtesy of SERC Media.

2 comments:

emailmark said...

Was the log not preserved as lignite (low grade coal) rather than charcoal?. Most of the vegetation I've found in the Oxford Clay had been lignitic and thus a nightmare to try to conserve.

Mark Wildman said...

I think you have a point since it is lignite that enables the clay bricks to be fired in the kilns just as it is - I understand that 5% of the clay contains lignite.

And yet it deteriorates just like charcoal. As we returned over a period of weeks, this log slowly broke up and eventually crumbled into dust. We wondered at the time how on earth you could preserve a log such as this.

Thanks for pointing this out!

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