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.
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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.
Donnelly and Woodruff data, top 100 cm.
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.
Reference:
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.
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31 Comments
‘Their resolution at the top of the core is about 5 years, increasing to about 30-40 years at the base.’
Don’t you mean the resolution is decreasing to 30-40 years?
#1, grammatical pickiness, eh? Then while we are at it, how about
Here are some pictures of the researchers at the lagoon.
It seems to me that the guy drilling the core is standing in the sediment while he’s drilling. Wouldn’t that tend to stir up the sediment? Do they actually consider the part of the core where the researcher is standing as valid data?? Am I missing something?
#3-Patrick M.
They’re going to find an untrampled spot, sink the corer, and then maybe move closer as they try to drive it deeper. Nobody would sample where they’ve walked.
Can’t bioturbation occur in an environment with no oxygen, vial anaerobic organisms, which would release methane? Just like the local landfill.
RE 4 dogwood.
If bioturbation-driven sediment sorting were to move the coarse upward, could coarse layers further down in the core indicate a lull in bioturbation?
As granulated materials settle, the large particles sift to the top. In a garden, rocks will appear in the soil after you work it up. Essentially, the small particles can squeeze into small openings below, that the larger particles cannot, so that the the larger particles appear to rise out of the soil after a time.
It doesn’t take biological action to mix the soil and cause larger particulates to rise. It will happen all by itself.
Seems like seismic activity could produce sorting as well.
@Bill
Sounds reasonable, but how do you get the layers of coarse material from time to time in the core?
If the process, (whatever process), that lifts the coarse stuff is ongoing then we should not see layers of coarse stuff in the core, it would all be on top.
Harry E:
I read this as 5-years per cm at the top of the core and 30-40-years per centimeter at the base. Sediments compact with depth due to the pressure of the dirt piled above.
Patrick@10,
no it wouldn’t, it’s a cyclical process, and the compaction of the fines will introduce barriers that stop the upwards progression. It’s not appropriate to infer sub century accuracy to the resulting core given the processes that the settlement goes through.
MDM says: November 27th, 2007 at 3:02 pm
Yes I agree and in such a seismically active area I’m surprised it’s even considered as a proxy for hurricane activity as it would be quite a serious confounding factor
From the paper:
I think this sums up the issue. By magnitude I think they mean the change in size of coarse fraction.
A geologist would look at the grain size distribution as a simple regressive beach sequence, meaning that the beach is getting closer to where you took the core and therefore the grain size increases. Since you are in the lagoon, the strongest winds will be onshore and therefore a lot of the fine sand/silt might be wind-blown. Closer beach means more coarse sand. Bioturbation in this type of setting would tend to homoginize the package of sediment and remove the coarse-up trend. The peaks need to be extracted from the “background” grain size, which is dependent on the proximity to the barrier beach berm shown in the photo, and normalized to remove the trend.
This is a similar issue to the Iceberg Lake paper, which assumed a grain size relationship to “climate change” and makes numerous assertions based on that assumption. A number of cores with correlating “events” would settle the proximity to beach proxy.
BTW one hurricane direct hit could fill the entire lagoon with sediment, so what they are measuring are obviously major “surge” events and not direct hits. Is there a record of recent hurricane activity at this location to tie the recent peaks to?
You also get periodic flooding type deposits especially in lagoonal sediments. Under normal depositional conditions, the rate of deposition is slow and the fines can filter down and the coarse fraction can accumulate at the surface. If you have a large flood event, large storm, or other unusual event, it is possible to deposit a thick layer of sediment over the top of the existing surface (coarse fraction falling out first) that in effect resets the depositional surface and then the normal depositional processes takeover again.
#13, I’d agree with you, but this particular study discovered that the spikes at Vieques that they attributed to intense hurricane landfalls matched up with other studies done in New England, New York and the US Gulf Coast. This is actually a study I’ve read and a topic I’ve been very, very interested in for some years now.
They (and others done in the Atlantic basin, and other studies using different proxies around other tropical ocean basins) kind of cast a doubt over the assertions of Holland, Webster, Emmanuell, et al. that the current active Atlantic and elsewhere is unprecedented. Ok, “kind of” is being kind.
tpguydk:
In the Long Island study (second link on Donnely’s website ), they refer to “vibracore” Have you used this? My experience with this type of “direct push” (GeoProbe) in land soil sampling applications is that recoveries in the softer horizons can be as low as 50%. Is there a good description in the topic paper about the specific sampling technique employed? Do they discuss percent recovery? Are they using clear plastic liners, etc?
Thanks.
#17 I’ve never actually done the work. Paleotempestology is just a topic I’m interested in. I became interested in it after Katrina. As for the rest I’d have to dig the actual paper out of my files to read their methodology.
I will get back to you ASAP.
Here’s is a chart of the storm tracks 1851-2006 within 1 degree of the D&W location.
Here’s a map of the location. Link
The map leaves a skewed impression of the intensity in the area due to overlaying higher speeds on top of slower speeds. Here is actual distribution.
SS TS Cat 1 Cat 2 Cat 3 Cat 4 Cat 5 Total
7 49 23 15 7 10 3 114
I use both geoprobe and vibracore technologies in my line of work, and they are actually somewhat different. Geoprobes basically use a hydraulic hammer to drive the casing into the ground, and while you generally can get good recovery even in very soft sediments if you use the proper tools, the sample you get will not be undisturbed in the classic sense. Vibracore technology uses vibration to loosen the soil/sediment along the length of the drillstem and allows the drillstem to pass through without necessarily being “driven” with great force like the geoprobe. The vibracore will also tend to disturb the sample to some degree and will cause a form of compaction due to the setlling induced by the vibration, but it doesn’t necessarily forcibly compact through the application of the driving force the sample the way a geoprobe will tend to do.
tpguydk says:
November 27th, 2007 at 4:42 pm
I’d probably agree that it’s a proxy in New England, New York and the US Gulf Coast but years watching, whilst calibrating a new system, coarse grains of coffee, soup granules and the like rising to the top when vibrated to compact the product makes me question it’s use in the Vieques location. It’s siting above a tectonic vibrator this is consistent also with (Steve Mc):
and see the plot below.
Thanks Bill F. I got off my a$$ and googled vibracore and found this tutorial from the USGS. I have not looked through it all, but it looks like a nice overview of how soft sediments are sampled. In general, I would look at the top portion of the core with much skepticism due to the interaction of the core catcher and the top soft and loose muck..
Thanks Bill F. I got off my a$$ and googled vibracore and found this tutorial from the USGS. I have not looked through it all, but it looks like a nice overview of how soft sediments are sampled. In general, I would look at the top portion of the core with much skepticism due to the interaction of the core catcher and the top soft and loose muck. Here is the link (I hope).
Link
Does the sediment know the difference between hurricanes and thunderstorms?
overwash (which would create the sediment layers) doesn’t occur with thunderstorms.
Named storms that struck Puerto Rico:
Year Direct hits / cat. (within 50 miles) / cat.
2000: 0 1 / 1
1999 0 1 / TS
1998: 1 / 2 1 / 1
1997 0 0
1996 1 / 1 1 / 1
1995 1 / 2 1 / TD
1994 0 0
1993 0 1 / TS
1992 0 0
1991 0 0
1990 0 1 / TS
1989 1 / 4 0
1988 0 1 / TD
1987 0 1 / 3
1986 0 0
1985 0 1 / TD
1984 1 / TS 1 / TD
1983 0 0
1982 0 0
1981 1 / TS 1 / TS
1980 0 0
Source: http://weather.unisys.com/hurricane/atlantic/
Above is the first 20 years of their hurricane period.
That didn’t work too well.
Nevertheless, the following hit the island in the last 20 years of their study:
1 category 4
2 category 2
2 category 1
2 Tropical Storms
Storms that came within 50 miles of the island during the same 20 years:
1 category 3
3 category 1
4 Tropical Storms
4 Tropical Depressions
It is hard for me to believe that only 6 hurricanes have hit Puerto Rico over the past 20 years! Is that definitely accurate?
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