- #31
sylas
Science Advisor
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NileQueen said:
The literature is full of questions, proposals, and refutations of proposals. If science had all the answers, science would stop. The literature is focused on open questions, and is a great place to look for out of the box thinking that may sometime become part of the solutions. Textbooks are better for getting straightforward answers, where they exist.
My lack of interest in speculations that have failed to appear in the scientific literature is a useful filter for avoiding wasting time with ideas based on poor understanding of the problems, or what is required for a solution. There's a lot of that around. We are not going to resolve the complex issues here at physicsforums, in the sense of coming up with radical new ideas that have not occurred to working scientists, to solve significant outstanding scientific problems.
There are some people here at physicsforums who will solve significant scientific problems, or have already done so. If the problem is of general relevance, then they are usually going to be able to show us their work towards a solution with reference to their own formal publications.
What we all can realistically do here is learn. That includes learning what is unknown as well as what is known. I don't have any illusions about being able to give all the answers -- either by reference to literature or by clever outside the box thinking of my own. We can ask questions about what is described in the literature, and get answers about the problems, the evidence, and what is known, and what is so far unknown. We can get answers about what hypotheses are being explored. I'm learning a lot right now, just from reading up on all the details behind the questions.
I personally have no interest in speculations that have not appeared in the literature. That's not quite true, but it's close. There's more than enough interesting and alternative ideas in the literature to keep me reading for a lifetime; and they are almost always more worthwhile exploring.
Furthermore, there is a specific requirement in the Earth science forum especially that we stick to claims made in the scientific literature. That is because the forum is primarily for learning about conventional science, as practiced by working scientists. It is not for working out new solutions ourselves that have not been considered by scientists.
In the PF guidelines there is a provision for "outside the box" thinking of ideas that have not yet made it into the scientific literature, in a separate forum; as long as they have sufficient scholarship in their presentation. You can learn a lot by working through ideas of your own in this way. I personally don't have much interest in it, because (a) I still have far too much to learn myself before my ideas have any prospect of becoming solutions, and (b) in my experience these ideas never come to anything. The people who are going to give the real solutions are already publishing their ideas in normal scientific channels; not in web forums.
Meltwater pulse 1-A
Here is a concrete example: meltwater pulse 1-a. This is a sudden increase in sea level of about 20m, occurring around 14 to 15 thousand years ago. It is often pinned down further to 14.2 to 14.7 thousand years ago.
There are two major hypotheses for how this comes about. There is the northern source and the southern source; and each of those also has further alternative refinements. Neither is of these is known to be a true "answer". They are both hypotheses. They both involve a large "pulse" of water from accelerated melting of ice sheets; the difference is the location of the sheets involved. We can learn a lot by looking at the evidence for and against these proposals, and we get that from the ongoing scientific debate in the literature.
It has been suggested that there might be other reasons for the data. In principle, that is true, of course; but I'd like to see if anything else has actually been proposed in the literature. I've asked this, many times; and the question has always been ignored. I am pretty sure this is because the answer is no; and that working scientists now accept that the data reflects a sharp rise in sea level -- a meltwater pulse.
It has been suggested that data usually taken as indicating rising sea levels might actually be caused largely by changes in the height of land mass. I don't think this is remotely plausible, and I've suggested some reasons why. But we shouldn't be debating that here. Until it has been seriously proposed in the scientific literature as a credible solution, it belongs in the forum for new independent proposals; not in the Earth science forum.
Objections raised to the idea of a meltwater pulse
There have also been objections raised to the idea of a real pulse of meltwater. IMO it is fine to consider difficulties with solutions that are being proposed. But none of the difficulties mentioned so far in the thread actually give any reason to doubt the hypothesis.
First, a "missing water" problem was mentioned. There is no such difficulty. The amount of water bound up in the ice sheets was enormous, both before and after the meltwater pulse 1-a; much greater than what was needed to account for the pulse.
Objections have been made to the enormous volumes conventionally proposed for the Laurentide ice sheet, which suggest an ice dome of as much as 3 or 4 kilometers in height at its highest point in the last glacial maximum; and the time this would require has been raised as a difficulty. It isn't. Ice sheets can grow comparatively quickly, on the time scales being considered. For example, to get 3 kilometers of ice in ten thousand years, you need about 30cm of build up per year. In conditions where precipitation freezes and builds up the sheet, that's not lot of precipitation. There is good evidence that massive ice sheets did indeed build up and retreat, many times over the Quaternary period. Nothing in the build time required makes this particularly implausible; and the idea of massive ice sheets is now solidly established.
Andre has objected that there should be evidence of glacial retreat at this time. But if you read the papers I have cited (like Simms et al, 2007) the major cause being proposed is THINNING of the ice sheet. So it doesn't actually follow that there has to be a sudden retreat of the margins at this precise time. The main proposal as I understand it involves collapse of the parts of the ice dome, rather than retreat at the margins. We can't prove all this in detail of course; the ice is gone. But as a refutation of the hypothesis, this fails.
Also important is that we don't actually know the chronology of the retreat of the ice margins with the precision required. Others have noted that this region is worked over by many glaciations. Sorting out the margins at a specific time is not easy at all. The evidence for retreat of the margins is there, and I previously asked Andre directly if he had a problem with the reconstructions of the retreat illustrated as follows:
Note the retreat of the margins apparent here between 16.5 and 13.75 thousand years ago -- contradicting the notion that all we have over this time is a "readvance" of glaciers. My understanding is that the fine details of the chronology for this retreat are nowhere near good enough make it "simple" to test the idea of a retreat at the precise time of the meltwater pulse. It's good enough to confirm a significant retreat of the Laurentide sheet between times before the pulse and after the pulse; but not good enough to confirm or to refute an acceleration of retreat at the margins precisely at the same time as the pulse.
NileQueen said:
Exactly. Also, the evidence for more recent glaciations can sometimes be confused as well, as it is not always clear which glaciations to associate with different features. The last is the easiest to sort out, but we don't actually have such a precise chronology that we can clearly identify accelerations of the margin retreat on small time scales. If you agree with this, and if Andre can agree as well, then this would be progress.
Note please that I am not the one arguing that it is simple to sort these things out! My position is that there are different ideas being proposed for the sizes of ice sheets and the timing of their retreat since the LGM, and for the source of the meltwater pulse; and that the literature does NOT give definite answers on these; neither a firm confirmation nor a strong refutation. These are open questions. They are not merely speculations without evidence; they are being considered using empirical evidence. But it is unfinished business, and the evidence is currently not sufficient to definitely confirm or reject either hypothesis.
Andre has suggested that the timing of the retreat is so well known that it should be simply to test whether or not a meltwater pulse was associated with an accelerated retreat at the margins of the LIS.
Andre said:
Andre, I have not seen you or anyone show that "very detailed evidence of the glacial retreat and advance" is available at such a great resolution that it would be "very easy" to show evidence of glacial retreat at the "right period" -- being between 14.2 and 14.7 thousand years ago. I don't think this is simple at all! The special issue of the Quaternary Science Reviews that I cited previously (Vol 21, Iss 1-3, 2002) suggests all kinds of unknowns about ice sheet margins during their retreat after the LGM.
Note that the retreat or advance of mountain glaciers is in general not synchronized with the retreat of the large ice sheets; and that the meltwater pulse source is proposed to be the ice sheets.
I don't think that there actually has to be an accelerated retreat at the margins, given that the proposal I have cited as an example (Simms et al 2007) is primarily about thinning of the sheet.
Be that as it may, the diagram I asked you about previously, of the LIS retreat, does show a large retreat of the LIS between 16.5 and 13.75 ka. I don't see any reason whatever to think it would be easy to test the idea of an accelerated margin retreat precisely at the time of the meltwater pulse, and I do not see this being raised as an objection by scientists who publish in support of the Southern source hypothesis. I think this is because the data cannot constrain the margins at anything like the resolution you require.
In summary, the source of the meltwater pulse is not definitely known. There are different proposals considered in the literature, and it remains an open question. I do think that the only credible proposal is an actual pulse of meltwater, and I don't think you've given any good reason at all to reject that idea.
There's nothing wrong with coming up with radical new ideas for the data. I remain genuinely interested to know if there are other proposals considered in the literature. So far -- apparently not. My understanding is that you can still use the "independent ideas" forum for ideas you are proposing on your own behalf. I'm not personally interested, but that shouldn't stop you.
Reasoned criticism of existing ideas here in Earth science is fine, of course. I have provided rebuttals, which is also fine.
Cheers -- sylas