The “remarkable similarity” in the increase in coarse fraction in the mixed layer and the increase in G Bulloides percentage in the mixed layer suggests to me that some sort of concentrating mechanism might well be working whether or not I’m in a position to identify precisely what it is.

Actually a common mechanism is very easy to hypothesize. Suppose that the distribution of G Bulloides and G Ruber in the plus 150-micron size is coarser for G Bulloides. For example, suppose than only 10% of plus 150-micron G Ruber are plus 250-micron, while say 30% of G Bulloides are. (And it’s my understanding that the hypothesis of more large G Bulloides is not an idle hypothesis as I’m pretty sure that I’ve read somewhere that there are more large G Bulloides than G Ruber.)

Then precisely the same mechanism that generates the increased coarse fraction in the top 1.2 cm mixed layer would also generate the increased percentage of G Bulloides. Given that there’s a 0.9 correlation between pct G Bullolides and pct coarse fraction, the idea of a common mechanism is rather neat.

I don’t know what percentage of the total coarse is made up of G Bulloides – I presume that there are other constituents in the coarse fraction besides G Bulloides and that there is an increase in other coarse components besides G Bulloides: maybe you could give some estimates of what sort of distributions one would encounter.

It looks more and more to me like the pct G Bulloides series contains a spurious effect which then introduces a spurious regression with NH temperature in classic Team style. If so, it will be a rather nifty example as the series is a major contributor to the Moberg and Juckes composites – I think that it was the largest contributor to modern-medieval differences in Moberg. So it would be rather fun if it proved to be another completely spurious regression.

PS. You can start with the steady state by simply doing 1000 iterations and then chopping off the first half. I’ve edited the script a little to do this.

HAving said that, this particular mechanism would not result in huge differences in age either – for that to happen, some different mechanism would have to exist. To accomplish a substantial modification of coarse fraction age, I’d have to think up different sorts of parameters – and I’ve not explored this.

The “remarkable similarity” in the increase in coarse fraction in the mixed layer and the increase in G Bulloides percentage in the mixed layer suggests to me that some sort of concentrating mechanism might well be working whether or not I’m in a position to identify precisely what it is. As I understand it, the only other somewhat comparable data set is the Black Cariaco series, which has a completely different look to it in the top portion.

Without some replication, it seems incredibly amateur to incorporate such trial balloons into multiproxy reconstructions.

Run your code, then make this figure.
x11();par(mfrow=c(3,3))
for(i in c(1,5,10,20,50,100,150,200,500)){
plot(X[[i]][,2], main=i)
}

Does this make sense to you?

It’s a very simple simulation. I’m not saying that merely modeling the shape of a curve proves that the model is correct.

BTW are you aware of other cores in the area which show the same build-up of coarse fraction or is this information only available for these 2 cores?

The premises of the simulation were as follows:
1) all bioturbation activity originated at the surface layer in the current year, the depth of secretion had a negative exponential shape and all downward secretion were fines;
2) upward percolation balanced the bioturbation and all upward percolation was in coarse. (If upward percolation is partly fines, then I’m pretty sure that somewhat different parameters could be found to yield any shape achievable with the method here.)
3) averages were taken over 10 year intervals and plotted.

The negative exponential shape for bioturbation secretion is attested in Wheatcroft; the coarse fraction percolation in McCave; Thomson et al etc. Bioturbation is attested in the Oman OMZ, despite arm-waving by Overpeck’s associates.

This doesn’t prove that the coarse fraction profile was generated by bioturbation. However the coarse fraction profile really has an alarmingly simple shape and the fact that you can generate that shape with a simple implementation of a bioturbation mechanism should really give people some pause.

You observe that this doesn’t prove that the G Bulloides percentage are affected by this phenomenon. However, the correlation between G Bulloides percentage and coarse fraction percentage is 0.91 – a very high percentage in paleoclimate; the closing portion of the G Bulloides profile is also alarmingly untextured and this suggests to me that some nonclimatic mechanism may be involved – perhaps allied to the concentration occurring in the coarse fraction or perhaps some independent mechanism operating in a similar way.

What should concern general readers aside from this is the fact that the percentage G Bulloides result of Overpeck and associates is essentially a one-off result, which has nonetheless been inhaled into multiproxy studies (Moberg, Juckes) as though high confidence could be placed in the original study. What do we really know about G Bulloides percentages? Is there a valid relationship to temperature? The only study containing detailed information on G Bulloides over the past millennium is by Black at Cariaco and I’ll look at that in the next few days.

Yes, we need the input of a good carbonate geochemist. I was only a generalist, so I offer a general comment that I have not researched – yet. Please tell me if I should or if the answer is known.

In a column where there is simultaneous dissolution of old carbonate material and formation of new material, in strata where the conditions permit each, is there is a possibility of isotope homogenisation? That is, can the new shell isotope ratios be perturbed because some of their building block material comes from older, dissolving shells with different isotope ratios? The answer might well be that the atmosphere supplies adequate new carbon and oxygen, with whatever isotope signals are in the air at the time, but then one has to account for the calcium as well. Is it mobile up the column or does it come from airborne dust or from lateral transport? I simply don’t know the relative likelihood of these possible contributions.

I have similar conceptual problems with corals, as we have such large reefs here. We have coral of many ages decaying and producing CO2 (I presume) and thus at least some mixing of isotopes of both oxygen and carbon. How much, I do not know.