[Article] Environmental effects of ashfall

[El Moose Monstero]

(This thread author has asked that the opening thread be expunged for reasons which are rather complicated. - SCRawl)

[Ziggy Stardust]

Since I have a SpringerLink account I can access the full paper.

I am reading through it, albeit rather quickly because I'm at work. Some pretty interesting stuff in here, good job.

This isn't my field, and I am reading it pretty quickly, so I apologize if I am missing something, but how quickly or readily does tephra cement after deposition? You note that the exact causes are not known, but roughly what proportion of deposits DO cement? Once again, maybe this is addressed in a part of the paper I've missed, but it seems that the ecological impacts of tephra are hugely dependent upon the degree to which it hardens, and that proper understanding of that process could potential have huge ramifications for dealing with these events.

[El Moose Monstero]

Thanks for the compliment!

To be honest, I'm not sure if there is any sense on the timescale for cementation. Someone did an experiment in our lab with some ash coated in secondary minerals - they weren't looking at cementation per se, but they were passing water through an ash covered soil. They found that overnight, infiltration had basically stopped in that sample.

As to the fraction of deposits that cement, it's tricky - it's probably a combination of surface properties, particle size distribution and aridity. Deposits in Iceland stay pretty unconsolidated, for example, and get blown around by wind. It's probably dependent on a lot of other environmental factors too, but as far as I know, it's not really been looked at. However, you're right in that as it dictates the deposit residence time on a lot of surfaces, it's worth looking at.

[Ziggy Stardust]

After reading a little more (and doing some Google research) I came up with another question.

In your paper, you talk about the depressive effect of tephra layering on underlying soil temperature. If I may quote you, here:

Increasing soil albedo decreases net radiation, which in turn may affect the thermal regime of the soil. Soil
temperatures measured beneath a few dozen millimetres of undisturbed tephra were up to 10 °C lower than those in
soils not covered by tephra (Black and Mack 1986; Cook et al. 1981). A drop in the soil surface temperature may negatively
affect microbial activity and, hence, organic matter decomposition rate. Conversely, a dark-coloured tephra deposit
will decrease the surface albedo, which may be accompanied by a rise in soil temperature.

In this book on ecosystem restoration (page 320), the authors have the following claims (they are citing studies done on volcanic eruptions in the arenales of Central America):

In our system, the main barrier to plant establishment ... is the high temperature reached by tephra exposed to direct sunlight during the dry season ... Furthermore, substrate temperature can be considered as a "secondary" controlling variable that is itself the result of one physical property (black color) of the "primary" controlling variable (the tephra layer) that interacts with solar radiation during the dry season. The depth of the tephra layer is not relevant for the effect of surface temperature because what limits plant establishment and survival are high temperatures in the first 5cm that kill seedlings and small plants by damaging stems in contact with hot material.

Certainly, the cooling of the substrate will have an adverse effect, but do you know anything about the heating effects of surface tephra? That is, from your research, which seems to be a bigger factor? They are interrelated factors, to be sure.

[El Moose Monstero]

That's an interesting find, it's a shame we didn't find that during writing as it's definitely worth mentioning.

I'm not too sure I can answer your question to the extent that you would like. There haven't been that many studies which really set out to resolve the contributing effects of different variables, and it's a bit of a lame excuse but the paper was not so much about ecosystem recovery but more on the immediate impacts, so we weren't always looking in detail at longer term recovery issues.

I don't think you would have a competition between a surface heating and a soil cooling effect beneath a dark coloured tephra - as you point out from our quote, darker tephras (common to low-Si eruptions) absorb more light and induce warming, this warming should also be transmitted into the underlying soil.

In terms of whether it is the heating of the soil or direct 'burning' of the plants by contact with darker coloured tephra which is most damaging, I'd anticipate that the two are relevant on different timescales. Newly rooting vegetation might still make it through the tephra deposit irrespective of soil temperature, but not if it is burned at the immediate surface at a vulnerable moment in its development. I'd also point out, I'm not sure if we mentioned it in the paper or not, that sharp-edged tephra abrades plants as they root through - this was observed at Mt St Helens, so is not specific to ash colouring. All in all, I think if you're talking specifically about rooting through the deposit, I can imagine that it's very much these sorts of things that are the most problematic. However, if it penetrates the deposit and survives, the plant might then be growing in a warmer soil might experience different water use requirements, stresses, and I'm speculating here, but perhaps even different pathogens and disease, than it would have done in the pre-eruptive environment. This may result in more long term changes, but I'm not really sure what those would be. We'd need a plant biologist to say what the response to a long-term stress would be on an affected ecosystem.

Is that any help?

[Ziggy Stardust]

That's an interesting find, it's a shame we didn't find that during writing as it's definitely worth mentioning.

Heh, I know the feeling. Almost every time I have to write a paper or prepare a poster I find great information/sources in the days after it is already too late.

I'm not too sure I can answer your question to the extent that you would like. There haven't been that many studies which really set out to resolve the contributing effects of different variables, and it's a bit of a lame excuse but the paper was not so much about ecosystem recovery but more on the immediate impacts, so we weren't always looking in detail at longer term recovery issues.

Fair enough. Especially since it is a review paper. I was mostly curious if you had any "educated guesses," so to speak, or anecdotal evidence from your work that would lead you to have a theory one way or the other, even if it isn't solid/empirical enough for publication.

I don't think you would have a competition between a surface heating and a soil cooling effect beneath a dark coloured tephra - as you point out from our quote, darker tephras (common to low-Si eruptions) absorb more light and induce warming, this warming should also be transmitted into the underlying soil.

I guess here is where the depth of the tephra layering would come in. Theoretically, there is some depth at which the surface heating would only transfer negligible heat to the substrate, though I don't know what that thickness might be, or whether that thickness occurs commonly enough in natural conditions for it to even worry about.

All in all, I think if you're talking specifically about rooting through the deposit, I can imagine that it's very much these sorts of things that are the most problematic. However, if it penetrates the deposit and survives, the plant might then be growing in a warmer soil might experience different water use requirements, stresses, and I'm speculating here, but perhaps even different pathogens and disease, than it would have done in the pre-eruptive environment. This may result in more long term changes, but I'm not really sure what those would be. We'd need a plant biologist to say what the response to a long-term stress would be on an affected ecosystem.

Is that any help?

Yes, quite a bit. This field is nowhere near my areas of expertise, but I find it interesting nonetheless.