Richard T and I had an interesting discussion here about the bioturbation and coarse fractions in the Arabian Sea G Bulloides proxy that is very influential in Moberg and not used in Loehle (as it was not calibrated to temperature.)
I had observed that the increase in G Bulloides percentage in the very top part of the RC2730 core was almost exactly matched by what paleoclimatologists would call a “remarkably similar” increase in coarse fraction percentage. The G Bulloides series from this core is a huge contributor to the 20th century-MWP accounting in Moberg.
I also discussed some literature which pointed out that bioturbation was an established phenomenon which caused an upward percolation of coarse fraction, leading to a concentration of coarse fraction in the top mixer layer (which was a transient phenomenon). I observed that bioturbation had been summarily rejected by the Arabian Sea G Bulloides series authors as it was in an oxygen-minimum zone, but other literature (including Arabian Sea literature) reported bioturbation even in an oxygen-minimum zone. We discussed (without any agreed resolution) whether this phenomenon could have accounted for the G Bulloides percentage increase or whether this increase was independent of the coarse fraction phenomenon. Richard T signed off as follows:
A prediction: If bioturbation-driven sediment sorting (rather than homogenisation) is important, it should be a general pattern, observable in most cores. Whereas, if the increase in coarse fraction is climatically driven, it should only occur in a few sites. Coarse fraction is measured fairly often, so it should be possible to test your hypothesis.
Donnelly and Woodruff (2007) is a very recent Nature study that applies lagoon sediments from Laguna Playa Grande, Vieques, Puerto Rico (17N, 65W) to reconstruct hurricane activity for the past 5,000 years. Their proxy is the bulk mean grain size(in microns). I see some parallels between increasing coarseness here (although measured differently) and the Arabian Sea situation (but am just thinking out loud in this instance.)
The Donnelly and Woodruff core is sampled at 1 cm intervals over a core length of 396 cm. Their resolution at the top of the core is about 5 years, increasing to about 30-40 years at the base. The topmost segment is assigned a data of 2000 AD. So this qualifies as a high-resolution data set covering the past 2 millennia.
Reviewing Arabian Sea Coarse Fraction
In the Arabian Sea case, the increase in coarse fraction in the mixed layer could be easily modeled with a negative exponential curve (I used the same function as I use in tree ring standardization). On the left below is a plot of the coarse fraction for Arabian Sea core RC2730. I proposed a simple model for how a bioturbation mechanism using known concepts could yield a coarse fraction with this type of shape (shown at the right) as follows:
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 Im 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.
Left: RC2730 coarse percentage; right – simulation of coarse fraction fraction using bioturbation mechanism. There are a several free parameters and this could be easily tuned to the left curve.
Donnelly and Woodruff Coarse Fraction
The Donnelly and Woodruff data for mean grain size is plotted below. It shows a remarkable increase in mean grain size in the 20th century. Indeed, one might say that the mean grain size is unprecedented. Just how unprecedented? Since 1940, there have been 12 readings with a mean grain size exceeding 150 microns and in the preceding 4000 years, there weren’t any.
Plot of Donnelly and Woodruff data from here
Donnelly and Woodruff argue that their proxy measures past hurricane activity. However some of these very high recent values occur in periods when hurricane activity wasn’t very high e.g. the 1970s-1980s. So there must be some other factor confounding the recent values. Here’s a plot of the top 100 cm of the core in a format similar to the Arabian Sea coarse fraction plot.
Is it possible that we’re seeing a mixing phenomenon over the top 20 cm or so of the core? This would be a considerably thicker mixed layer than at RC2730 where I hypothesized only about 2 cm. But this core is taken from surface and not from an oxygen-minimum zone at 600 m or so. Without knowing much about bioturbation mechanisms, I don’t see any reason why a mixed layer of 20 cm or so would be precluded in this case. If this is the case, then the high coarse fraction in the mixed layer would be a transient phenomenon (in the sense that the mixed layer is always on top). On this model, the coarse fraction in the “final” layer that a paleoclimatologist in 2200 would date to the 20th century would revert back to something like the mean as fines are mixed downward by bioturbation.
Donnelly, J.P. and J.D. Woodruff. 2007. Intense hurricane activity over the past 5,000 years controlled by El Nino and the West African monsoon. Nature, Vol. 447, pp. 465-468, 24 May 2007. doi:10.1038/nature05834
Donnelly website http://www.whoi.edu/science/GG/coastal/publications/jeff.html
Donnelly, J.P. and J.D. Woodruff. 2007. Laguna Playa Grande, Puerto Rico Grain Size/Paleohurricane Data.IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series # 2007-068. NOAA/NCDC Paleoclimatology Program, Boulder CO, USA.