The science of espresso, with a dash of geology

Almost every morning, I start the day with an experiment on flow in porous media. First, I generate some fine-grained sediment with a well-defined average grain size and proper sorting; then I use that sediment to fill a little basin of sort and try to mimic compaction. Finally, I use a machine to put water under pressure and force it to flow through this miniature sedimentary basin. Then I sit down to drink the fluid which is not simple water anymore, due to its interaction with the grains; and its taste and consistency tell me whether I got the grain size and the porosity right.

That’s a geologist’s view of making espresso. Unless you have a fully automated and ultra-expensive espresso machine, creating a high-quality caffeine concoction is not trivial, because the water must have the right temperature and has to spend the right amount of time in contact with the coffee grains that have the right size. The right temperature is 85–95 °C (185–203 °F), and, at least with our simple machine, the trick is to start the brewing at the right time. Better espresso machines do not use steam to generate pressure because that makes the water too hot; instead, they have a motor-driven pump that generates the ~9 bars of pressure. The correct grain size is easily achieved with a burr grinder (as opposed to a simple blender); a good espresso grind is a fine grind, because the water spends relatively little time in contact with the grains.

The duration of this contact is the most difficult bit to get right. To get a good shot with lots of crema, it cannot be less or more than 20 to 30 seconds. Not just grain size, but grain sorting as well play a role. If the coffee grinder produces a poorly sorted ‘sediment’ (and that’s what a blender does), the coffee will not be porous and permeable enough. Another factor is how well the sediment is compacted; that is, how much pressure do you apply to the coffee during tamping. This affects permeability again. Finally, it matters how much coffee you put in the coffee holder; the thicker the layer that the water has to go through, the longer the trip becomes for the same amount of water.

After using the machine hundreds of times, I still manage from time to time to produce something undrinkable. The art and science of espresso making started to make more sense once I started to think of it in terms of Darcy’s Law.

Henry Darcy was a French engineer who initially had made a name for himself by designing an enclosed and gravity-driven water-supply system for the town of Dijon. Later he had time and opportunity to do experiments of his own interest. In 1855 he measured the discharge of water under variable hydraulic heads through sand columns of different heights, and found that the discharge was directly proportional with the hydraulic head and inversely related to the height of the sand column:

Q = AK(H1-H2)/L,
where A is the cross sectional area of the sand column, L is the height of the sand column, K is the hydraulic conductivity (which is constant for the same granular material and same fluid), and H1-H2 is the hydraulic head. This is Darcy’s drawing of his experimental setup:

The hydraulic conductivity depends on both the properties of the fluids and of the granular material; these properties are the viscosity and density of the fluid, and the permeability of the sediment:

K = kρg/μ,

where K = hydraulic conductivity, k = permeability, ρ = fluid density, and μ is the dynamic viscosity.

In coffee speak, the hydraulic head is given by the pressure generated by the machine, and is fixed; one cannot change the density and viscosity of water either. The most important variable is coffee permeability, which is influenced by size, sorting, and packing (compaction) of the coffee grains. Also, it helps if you get the value of L right, that is, you shouldn’t try to save coffee.

Darcy’s Law was established with some simple experiments, and it has since then been generalized and derived from the Navier-Stokes equations, but it has a huge range of applicability, from ground-water hydrology to soil physics and petroleum engineering.

Add to that list everyday espresso making.

ps. Fantastic resource on Darcy’s work and his law here.

Liesegang bands in sandstone

Liesegang bands are poorly understood chemical structures often seen in rocks, especially sandstones. They were discovered more than a hundred years ago by the German chemist Raphael E. Liesegang, when he accidentally dropped a drop of silver nitrate solution on a layer of gel containing potassium dichromate, and concentric rings of silver dichromate started to form.

In sedimentary rocks, Liesegang bands appear well after the sediment has become a rock (that is, it got compacted and cemented). Stratification and lamination within the sansdtone are typically cross-cut by the Liesegang bands; fractures usually have a more obvious effect on the distribution and orientation of these.

The rocks shown here are turbidites of the Permian Skoorstenberg Formation, in the Karoo desert of South Africa. This Liesegang banding developed in the neighborhood of a small thrust and consists of brown bands of iron oxide that entirely ‘ignore’ the original lamination of the sandstone (not visible in the photos), but clearly like to precipitate along some of the fractures in the rock.

Water escape structures in a Cretaceous delta, Wyoming

I spent a few days in Wyoming, at a conference and field trip focusing on clinoforms, organized by SEPM (Society for Sedimentary Geology). Clinoforms are sedimentary layers with a depositional dip of a few degrees that form packages of relatively large thickness (let’s say more than a few meters; could be hundreds of meters in some cases. The point is that the foresets of ripples, sand dunes and other bedforms could be called clinoforms but they should not be). [Warning! – my definition]. After a couple of days of morning talks (many very good ones) and afternoon posters, we spent two additional days visiting some outcrops in southern Wyoming.

These photos come from exposures of the Maastrichtian Fox Hills Sandstone of the Eastern Washakie Basin, a sandstone of deltaic and fluvial origin that links through shaly clinoforms to turbidite sands and shales of the Lewis Shale, deposited in water depths of more than 400 meters (see reference below).

The photo above shows one set of smaller-scale clinoforms truncated by a cross-bedded sandstone unit above, probably of fluvial origin. This is a prograding shoreline. It was a matter of debate whether the erosional surface at the base the fluvial sands is a sequence boundary or not, and could be a blogworthy subject in itself, but I will refrain from discussing it here and now. What I think – at least visually – are more exciting are the water escape structures in the photograph below.

There are two sandy layers visible in the picture; the lower one is somewhat darker colored and more massive-looking than the upper one, which is more laminated and has an overall lighter color. The height of the rock surface covered in the photo is about 1.5 m.

A likely explanation for the structures is as follows (sorry for the arm-waving — it would be nice to put some numbers here – sedimentation rates etc., but life is too short for that right now). Soon after the first (darker) layer was deposited, another flood of the river brought more sediment to this location, and started depositing sand, mostly along a flat bed that resulted in parallel lamination. The underlying sediment was still very porous and unconsolidated, and some of its pore water was trying to get to the surface as the weight of the overlying deposit increased. Thin layers of finer-grained and therefore less permeable sediment got in the way however; and the escaping pore water had to travel laterally until it found the most vulnerable spots to go again upward. There are two of these vertical water escape conduits in the photo. As all the water coming from the lower layer had to go through a limited number of these spots, the velocity of the pore fluid must have increased significantly, until it actually was able to fully suspend the sand it encountered. In other words, some of the sand along these vertical escape zones got fluidized and carried away. The white structureless patches of sand are sedimentary intrusions. The light color suggest that these sands are much ‘cleaner’ than the rest of the rocks; the finer grains (responsible for the darker color) were washed away.

One interesting detail is that the trough cross-bedded sand between the two intrusions thickens into the depression, suggesting that the water was trying to get out in real time, that is, at the same time as the upper layer was being deposited.

Below I linked in an amateurish-looking gigapan; and here is another post on water-escape structures.

Launch full screen viewer

Reference:
Carvajal, C.R. & Steel, R.J. (2006), Thick turbidite successions from supply-dominated shelves during sea-level highstand. Geology, 34, p. 665-668.

Fossilized snake with exploded head

There is a temporary exhibit called “Geopalooza! A Hard Rock Anthology” at the Houston Museum of Natural Science. If you are in Houston this summer (until August 24), this is something absolutely worth checking out: you can see some outstanding geodes, crystals, meteorites, and fossil specimens. I have been to many natural history museums, but I rarely get as high as I did at the HMNS the other day [‘getting high’ is the right terminology here: you get to (or have to) listen to Led Zeppelin and Bob Dylan while looking at the rocks]. Even if you are not too much into rocks, minerals, fossils, and natural science in general, these pieces are so beautiful that they can simply be viewed as works of art.

Here is for example a fossil snake from the Eocene Green River Formation in Wyoming. This formation has not only one of the most significant fossil sites in the US, but it also contains the largest oil shale deposit in the country: there are 1.5 trillion barrels of shale oil within the former lake sediments. Preservation of both fossils and of organic matter requires special conditions on the lake bottom: a partial or total lack of oxygen not only prevents oxidation of organic material, but also makes life difficult for critters that otherwise would totally churn the sediment and leave no undisturbed animal remains behind.


One of the museum curators was around when I was checking out this snake and she explained that the reason why the head bones are in such a disarray – compared to the beautifully arranged backbone and ribs – is that, as the snake’s body started to decompose, the easiest way out for the accumulating gases was through the head.

I think this snake must belong to the species Boavus idelmani, and is probably one of the best preserved fossil snakes in North America.


The rest of the photos from Geopalooza are here.

The internal structure of the Peyto Lake delta

ResearchBlogging.org I have pledged a while ago not to blog about Peyto Lake anymore, but I have recently discovered some beautiful GPR profiles that I have to share to make the story more complete.

Derald G. Smith of the University of Calgary and Harry M. Jol of the University of Wisconsin – Eau Claire have looked at the internal structure of the Peyto Lake delta using a technology called ground-penetrating radar (GPR). The principles used in GPR data acquisition are similar to those of reflection seismology, but electromagnetic waves are used instead of acoustic energy, and reflections occur at boundaries with different dielectric constants rather than boundaries with different acoustic impedances. Compared to reflection seismic surveying (or digging a deep hole), the nice thing about GPR is that it is non-destructive: it can be used in situations where you want as little disturbance as possible. For example, it would be a bad idea to do seismic surveying of a leaking reservoir, but it is OK to look for the leaks using GPR. GPR is also used by archaeologists.


But let’s return to Peyto Lake. Although it has long been postulated that coarse-grained deltas with steep slopes (called Gilbert deltas, after the American geologist G.K. Gilbert) have a simple internal structure consisting of a topset, foreset, and bottomset, – and there are numerous ouctrop examples that show small coarse-grained deltas with such a structure -, it is not easy to prove that modern-day active deltas behave the same way. You can easily walk around on the top of the Peyto Lake delta and examine the surface morphology and characteristics in as much detail as you want; but without cool technology like GPR you can only wonder what does it look like inside. [Digging a large trench in the middle of one of the most beautiful national parks is not an option.] So Smith and Jol set out to check whether the topset-foreset-bottomset geometry is valid for this delta or it has a more complicated internal structure.

With only one day of data collection in the field (kind of unusual in the earth sciences!), they have clearly shown that the river feeding Peyto Lake is indeed building a textbook example of a simple Gilbert-type delta. The near-horizontal topset layers consist of gravels deposited by the braided river that is active today on the top; the underlying foresets are also likely to be coarse-grained and they dip at about 25 degrees toward the lake. This angle is close to the underwater angle of repose.

It is somewhat puzzling why the authors do not talk at all about the fate of finer-grained sediments (sand, silt, mud) that enter the lake; it is clear that turbidity currents directly originating from the river mouths are important agents of sediment transport in the lake (see more about this here). [They were probably focusing on presenting the results of the GPR survey.] The GPR signal is strongly attenuated in sediment finer than sand, and this might be the reason why reflections get poorly defined in the area where the bottomset is supposed to develop. In fact, I am not convinced I can differentiate the ‘bottomset facies’ the authors talk about.

So here is a representative GPR dip section showing the topset and the foreset:

It is also worthwhile looking at a section perpendicular to this; note how different the topset and foreset look like in this direction. Reflections are much more discontinuous in this image, suggesting that the spatial scale of sediment transport is more limited in a direction perpendicular to the river flow on the top of the delta (this is kind of obvious) and perpendicular to downslope processes within the lake (this is not so obvious).

Reference
SMITH, D., JOL, H. (1997). Radar structure of a Gilbert-type delta, Peyto Lake, Banff National Park, Canada. Sedimentary Geology, 113(3-4), 195-209. DOI: 10.1016/S0037-0738(97)00061-4

Crop circles of the deep sea

If ‘cereologists’ (people who seriously think that crop circles are made by aliens) knew about deep-water trace fossils, I am sure at least some of them would argue that these structures must also be the work of extraterrestrial intelligence. Many of the traces are so intricately constructed that they raise the question: how is it possible for a not-too-brainy animal to create such patterns.

This group of trace fossils is called ‘graphoglyptids’ (don’t ask me why) and they are usually found on the soles of turbidite sandstones, layers of sand deposited in the deep sea (that is, in water depths of more or much more than a few hundred meters). Their shapes can be relatively simple meanders, can include multiple levels of meandering, meanders with bifurcations, spirals, radial patterns. The most interesting and most famous member of the group is Paleodictyon, an easy-to-recognize trace fossil with almost perfect honeycomb-like hexagonal patterns.

Many years ago I was lucky to do some work on trace fossils of the Carpathian flysch with two of the best trace fossil experts; since then I haven’t worked with trace fossils but now I wish we did more documentation of the trace-fossil-rich outcrops in the Romanian Carpathians. The Paleodictyon pictures below show turbidite sandstone soles from the Buzău Valley; I haven’t been there for a while but I hear that many of the outcrops are covered now.

The first weird thing about graphoglyptids is that they developed high diversity in an environment with limited amounts of low-quality food (lack of sunlight, hence no primary production; and stuff that sinks down from the photic zone usually has already been food for some other animal). The second weird thing is that they are not simple grazing traces like the tightly meandering patterns of sea urchins; the most widely accepted idea is that they are farming traces. In other words, these guys (whatever they might be, nobody really knows) create well aerated open burrow systems a few millimeters below the sea floor, with multiple openings to the sediment surface, so that chemosynthetic bacteria move in to get the necessary oxygen to oxidize methane and hydrogen sulphide, their favorite food.

Again, despite the abundance of these traces in turbidite successions, it is not clear what is the animal that likes to build delicate hexagonal burrows in the deep sea. It is clear however that the exact same structures have been found near the Mid-Atlantic Ridge. In 1976, Peter Rona of Rutgers University and his colleagues were looking at photos of the Atlantic seafloor and discovered some interesting geometric patterns of black dots. When Adolf Seilacher of the University of Tübingen, probably the most famous trace fossil expert, saw the pictures, he got very excited: he became convinced that it was a modern Paleodictyon. Unfortunately, no other data than the photographs with the black dots was available; no animals recovered from the sediment, and no hexagonal patterns seen below the surface. It took more than 26 years before Rona and Seilacher had the opportunity to do a new dive with the submersible Alvin and to show that the black dots on the seafloor indeed represent small shafts that belong to a hexagonal pattern a few millimeters below, a pattern identical to Paleodictyon (more details in an article by Peter Rona in Natural History Magazine; picture below is from the same article and is © of The Stephen Low Company).


This story is fascinating as it is, but it is best to see it in amazing colors and resolution, in the IMAX movie “Volcanoes of the Deep Sea“, a documentary about the black smokers of the Atlantic and the discovery of modern Paleodictyon.

The mystery of the tracemaker of Paleodictyon – and all other graphoglyptids – remains unsolved: despite the outstanding success of taking IMAX-quality pictures at the bottom and the middle of the Atlantic Ocean, no animal was ever found in the sediment samples, and we know much more about how actual crop circles are generated than we do about the behavior of Paleodictyon.

Further reading

A beautifully illustrated new book by Adolf Seilacher:
Seilacher, A. (2007) Trace Fossil Analysis. Springer, 226 p.

Paper on trace fossils in the Carpathian flysch:
Buatois, L.A., Mangano, M.G. and Sylvester, Z. (2001) A diverse deep-marine ichnofauna from the Eocene Tarcau Sandstone of the Eastern Carpathians, Romania. Ichnos, 8, 23–62.

Links to this post: Book of Barely Imagined Beings

Some questions about the ‘megatsunami chevrons’: addendum

ResearchBlogging.org A couple of months ago I have written about some coastal ‘chevron dunes’ that have been interpreted as the onshore deposits of humongous tsunamis. The subject has been on the tip of my fingers for quite some time and I thought I managed to google up most of the related papers, news articles, and blog posts, but it turns out that I missed one highly relevant discussion: in the January 2008 issue of GSA Today, there is a short paper by Nicholas Pinter and Scott E. Ishman of Southern Illinois University, entitled “Impacts, mega-tsunami, and other extraordinary claims“. [Note to self: looking things up in Google and Google Scholar is not always enough, not even for blogging.]

Pinter and Ishman criticize both the idea that several impact-related megatsunamis occurred during the last 10,000 years and the hypothesis that a 12,900 year old impact caused “Younger Dryas climate event, extinction of Pleistocene mega-fauna, demise of the Clovis culture, the dawn of agriculture, and other events”. The first idea is promoted by Dallas Abbott (at Lamont Doherty Earth Observatory) and her co-workers, but there is no real peer-reviewed publication yet; the second has been put forward by Richard Firestone (of the Lawrence Berkeley National Laboratory) et al. in Proceedings of the National Academy of Sciences and a book. The evidence for the 12.9-ka impact includes magnetic grains, microspherules, iridium, glass-like carbon, carbonaceous deposits draped over mammoth bones, fullerenes enriched in 3He, and micron-scale “nanodiamonds”. Pinter and Ishman suggest that

the data are not consistent with the 4–5-km-diameter impactor that has been proposed, but rather with the constant and certainly noncatastrophic rain of sand-sized micrometeorites into Earth’s atmosphere.

But I am going to focus here on the ‘chevron dune’ part of the story. Pinter and Ishman have essentially the same main issue that I have blogged about: landforms that are morphologically identical to the so-called chevron dunes are well-known in the literature and they are called parabolic dunes. There is no need to introduce a new term:

We suggest that these Holocene features are clearly eolian, and that the term “chevron” should be purged from the impact-related literature.

The eolian origin of the alleged megatsunami deposits is difficult to deny taking into account that wind direction measurements at two sites perfectly match the orientation of the sand dunes (figure from Pinter and Ishman):

Dallas Abbott and her co-workers address Pinter and Ishman’s criticisms in a short reply. This is what they have to say about the chevron dunes:

Pinter and Ishman also claim that chevron dunes in Madagascar and on Long Island are aeolian in origin. We visited both locations and found many features that seem incompatible with an aeolian origin. First, parts of the chevrons in both locations contain fist-sized rocks. These rocks are too large to be transported by the wind. Second, the orientations of the chevrons do not match the current prevailing wind direction. In both areas, some of the thicker sand deposits are being reworked into classic windblown dunes. The direction of movement of these dunes differs 8° to 22° from the long-axis of the chevrons. Third, the degree of roundness of the grains in the chevrons is not characteristic of wind transport over long distances. In both locations, sand grains on the distal ends of the chevrons are not well sorted or well rounded. Sand moved by the wind obtains an aeolian size and sorting distribution after only 10–12 km of saltation transport (Sharp, 1966); however, at Ampalaza in Madagascar, the chevron is >40 km long and rises to 63 m above sea level. At its distal end, the chevron is 7.2 km in a direct line from the coast and contains unbroken, unabraded marine microfossils and conchoidally fractured sand grains. It is impossible to transport unabraded marine microfossils to this location via wind-generated saltation. The site is too far above sea level for storm waves, and there is no local agricultural activity. The chevron was deposited by a tsunami.

Well, these seem valid counterarguments at first sight — but it would be nice and it would be time to see actual data: images, measurements, grain size distributions. Which parts of the chevrons are reworked into eolian dunes? What is the difference in the morphology of tsunami dunes and eolian dunes? How does this relate to flow dynamics? What are those “rocks” that occur within the dunes? Where do they occur exactly? And so on. The fact that the authors end their reply with an unqualified strong statement like “The chevron was deposited by a tsunami” suggests that they are unwilling to admit that not every piece of evidence favors their interpretation and that some legitimate questions can be raised. They do this after first admitting that parts of the dunes were indeed reworked into classic wind-blown dunes.

So, at least as far as the ‘chevron dunes’ are concerned, I have to concur with Pinter and Ishman’s harsh conclusion:

Both the 12.9-ka impact and the Holocene mega-tsunami appear to be spectacular explanations on long fishing expeditions for shreds of support. Both stories have played out primarily in the popular press, highlighting how successful impact events can be in attracting attention. The desire for such attention is understandable in an environment where science and scientific funding are increasingly competitive. The National Science Foundation now emphasizes “transformative” research, and few events are as transformative as an impact. In an era when evolution, geologic deep time, and global warming are under assault, this type of “science by press release” and spectacular stories to explain unspectacular evidence consume the finite commodity of scientific credibility.

References

Pinter, N., Ishman, S.E. (2008). Impacts, mega-tsunami, and other extraordinary claims. GSA Today, 18(1), 37. DOI: 10.1130/GSAT01801GW.1

Abbott, D.H., Bryant, E.F., Gusiakov, V., Masse, W., Breger, D. (2008). Impacts, mega-tsunami, and other extraordinary claims: COMMENT. GSA Today, 18(6), e12.

Firestone, R.B., West, A., Kennett, J.P., Becker, L., Bunch, T.E., Revay, Z.S., Schultz, P.H., Belgya, T., Kennett, D.J., Erlandson, J.M., Dickenson, O.J., Goodyear, A.C., Harris, R.S., Howard, G.A., Kloosterman, J.B., Lechler, P., Mayewski, P.A., Montgomery, J., Poreda, R., Darrah, T., Hee, S.S., Smith, A.R., Stich, A., Topping, W., Wittke, J.H., Wolbach, W.S. (2007). Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences, 104(41), 16016-16021. DOI: 10.1073/pnas.0706977104

Climbing Ripples I.

Ripples, dunes, cross bedding and cross lamination have always been some of the sexiest subjects in sedimentary geology. They are certainly responsible (in part) for my choice of a certain walk of life that consists of studying dirt. You might say that everything has been already said about ripples and dunes, and you clearly get that feeling if you read some of J.R.L. Allen’s work on the subject (and that can be a lot of reading, by the way) or look at the fantastic multimedia material that David Rubin at the USGS put together. [Of course, there are numerous other authors who have written great papers on the subject, but it is not my purpose here to write a history of bedform sedimentology. Although that would be an interesting subject, if somebody had the time for it.]

However, little of this material gets into the standard sedimentology and stratigraphy textbooks. Maybe rightly so: after all, textbooks are not supposed to include all the details about any particular subject. And maybe there are higher-density issues out there, like whether we should call something a turbidite or a debrite. [Sorry, I could not refrain from typing that].

Take for example climbing ripples. They form when several trains of ripples are superimposed on each other and they seem to ‘climb’, by generating stratigraphic surfaces that are tilted in an upcurrent direction. [Note however that these surfaces are *not* topographic – or time – surfaces; more on that later]. Numerous textbooks and many papers mention climbing ripple cross lamination, but often the explanation is something like “they indicate high rates of deposition”, or “the steepness of the climb and stoss-side preservation are a function of the ratio between suspended-load and bedload”. The question is, what do we *exactly* mean by ‘high rates of deposition’? If we cannot put numbers on it, it is not that informative. Also, by ‘suspended load’, do we mean suspended load concentration? Or deposition from suspended load and bedload, respectively? Those statements are not necessarily wrong, but they do not do justice to the models that have been published many years ago, models that actually have some numbers and equations behind the “conclusion” section.

The key paper that I am talking about is “A quantitative model of climbing ripples and their cross-laminated deposit“, by J.R.L. Allen, published in 1970 in the journal Sedimentology.

The most important relationship that Allen has derived links the angle of climb ζ (see the sketch below) to the rate of deposition M (measured in units of mass over unit time and area), the rate of bedload sediment transport j, and the ripple height H:

tanζ = MH / 2j

This is simply based on decomposing the sediment flux to and through the bed into vertical and horizontal components (plus a relationship between the horizontal sediment transport rate in ripples and the horizontal migration rate of the bedforms). Note that the quantity j refers to the sediment mass that moves through a cross section perpendicular to the general current direction, and does this by being part of the ripples themselves. In other words, there is no direct equivalence between M and suspended load deposition, and j and bedload deposition. Although it is possible that in general suspended load contributes more to M than deposition from bedload, it says nowhere that grains transported within the bedload cannot be deposited on the stoss side of the ripples and thus contribute to the vertical growth of the bed.

Obviously, if the angle of climb is smaller than the dip of the stoss side, there will be no stoss side preservation and the resulting cross lamination will look like in the sketch below (which, by the way, was quite an effort to generate in Matlab; you can easily do this and much-much more with David Rubin’s Matlab code, but I wanted to understand things a little better by coding something simple myself):

This is often called ‘A-type’ (or subcritical) climbing ripple cross lamination, but everybody knows what you are talking about if you “simply” call it climbing ripple cross lamination with no stoss-side preservation.

In contrast, aggradation is much more prominent if the angle of climb is larger than the slope of the stoss side, and in this case deposition takes place on the stoss sides as well, resulting in ‘S-type’ (or supercritical) lamination:

Of course, it says nowhere that the rate of deposition M or the bedload transport rate j must stay constant through time. If the ratio of these quantities changes, the angle of climb will change as well. This sketch shows an example where the rate of deposition M increases through time:

One of the main points of the paper is that there is a fundamental difference between the rate of deposition M and the bedload sediment transport rate j. A rate of deposition larger than zero means that the sediment transport rate within the flow must decrease from an upcurrent position to a downcurrent position; a simple mass balance tells us that this change in the sediment transport rate has to equal the rate of deposition. In other words, the rate of deposition M is a derivative of the sediment transport rate, and as such, does not belong in the same drawer of physical quantities as the bedload transport rate.

Along the same line of thought, Allen emphasizes that climbing ripple lamination says something about flow uniformity and steadiness. A uniform and steady flow can only form a single train of ripples; either non-uniformity or unsteadiness is needed to have climbing-ripple deposition.

That’s it for now; to be continued. It’s time to do my taxes.

Further reading: Brian has a Friday Field Photo and a Geopuzzle on climbing ripples. Here are some pictures and a movie of climbing ripples generated by a turbidity current in a flume.

Some questions about the ‘megatsunami-chevrons’

The allegedly tsunami-related chevron-shaped nearshore deposits are back in the center of attention, at least among geo-bloggers. And rightly so: the idea that lots of coastal sand sheets all over the world were generated by relatively recent, impact-related mega-tsunamis of unimaginable scale is a fascinating hypothesis and, if proven true, it would revolutionize not only our understanding of frequency of meteorite impacts, but also change the predominant views about coastal geomorphology.

However, I have to confess that I find the geomorphologic and sedimentologic side of the argument fairly weakly constructed and documented, at least in the papers I was able to google up. Here are a few questions that could be asked to clarify some issues.

Many of the aerial photographs of the so-called chevrons could be used as textbook examples of parabolic dunes. Parabolic dunes are U-shaped wind-blown dunes usually fed by coastal sand deposits. Their nose points in a downwind direction, that is, the opposite way from barchan dunes. While wind strengths, direction, and sediment source are the main players in the generation of barchan dunes, vegetation plays a key role in the development of parabolic dunes. The ‘arms’ of a parabolic dune are left behind because they have a lower migration rate than the main body and the nose. The lower migration rate is due to plant growth in areas of lower sedimentation and/or erosion. There is a strong correlation between vegetation cover and dune migration rate or activity. This image comes from a recent paper by Duran et al.,


and it shows active (on the left) and inactive (on the right) parabolic dunes. Here is a sketch (source):


So the question is: if ‘chevrons’ are indeed different from classic eolian parabolic dunes, what is this difference, both geomorphologically and sedimentologically? Are they internally stratified? Do they show large-scale cross-bedding? What are the typical grain size distributions? Are they different from typical wind-blown sand? (One of the arguments is that large boulder fields occur in several places; however, my impression is that the boulders described here do not occur within, below, above, or right next to any chevron deposits. This short report claims that large pieces of rock are all over the place in the Madagascar chevrons, but no actual data is presented). The Madagascar chevrons featured in the New York Times show the signatures of typical parabolic dunes: U-shape, vegetated back sides and sandy crest plus nose. Why would an old tsunami deposit be vegetated only in certain places? The simple fact that these dunes are not entirely covered by vegetation suggests that their sandy parts do consist of wind-blown sand. Of course, tsunami deposits can be reworked by the wind, but why would the tsunami-related morphology be so well in tune with the eolian signature?

Second, if the chevrons are indeed produced by tsunamis, the tsunamis must have been humongous. Dune heights are related to flow depth, and a rule of thumb is that flow depth has to be around 6 times the dune height. So a 50 m high dune would require a 300 m high wave. Is that really possible? How big an impact do you need to generate such humongous waves? Also, what about the backwash? Why are most of the chevron dunes pointing systematically in one direction, and do not seem to be altered by any seaward oriented flow?

Third, are any features of the chevrons consistent with what we know about well-documented recent and ancient tsunami deposits? Tsunami-related sandy layers tend to be comparable to turbidites: largely unstratified, normally graded units suggesting rapid deposition from flows of decreasing velocity; they do not show large-scale cross bedding that requires relatively steady flow over longer time scales. In contrast, the chevron morphologies suggest that they are internally well-stratified, as the result of stoss-side avalanches. Here is a sand layer from the 1998 Papua New Guinea tsunami (source):

Obviously, it may turn out that there are tsunami-related coastal dunes and wind-blown parabolic dunes, with very similar morphologies, but fundamentally different origins. At the moment, the geomorphologic and sedimentologic evidence for this extraordinary hypothesis seems quite preliminary. And, needless to say, extraordinary claims require extraordinary evidence.

ps. 1: Lots of information on tsunami deposits here.
And a paper arguing for wind-blown origin of some deposits from the Bahamas, previously interpreted as tsunami-related: Kindler, P. & Strasser, A. (2000) Palaeoclimatic significance of co-occurring wind- and water-induced sedimentary structures in the last-interglacial coastal deposits from Bermuda and the Bahamas. Sedimentary Geology 131, 1-7.

ps. 2: More good stuff on megatsunamis and their deposits at Highly Allochthonous.

Teaching of evolution in Romania: an endangered species

Romania is one of those countries that, after the fall of supposedly atheistic communist governments, are still struggling with the place of religion in public life and in education. The new Romanian constitution goes beyond guaranteeing freedom of religion and explicitly endorses state support for religious organizations (“Religious cults shall be autonomous from the State and shall enjoy support from it, including the facilitation of religious assistance in the army, in hospitals, prisons, homes and orphanages.” – article 29). Yes, that is right: religious cults are autonomous but they enjoy state support. In other words, they do what they want with taxpayer money. Historically established religious denominations get government recognition; this is a major issue, because in practice only those religions enjoy ‘religious freedom’ who are recognized by the government. In other words, “Recognized religions have the right to establish schools, teach religion in public schools, receive government funds to build churches, pay clergy salaries with state funds and subsidize clergy’s housing expenses, broadcast religious programming on radio and television, apply for broadcasting licenses for denominational frequencies, and enjoy tax-exempt status.” (source). Note that the majority of Romanians see absolutely no problems with the government giving money to religious organizations, including funding for teaching religion in public schools. Religious institutions enjoy almost unlimited trust from the public (as opposed to the senate, the parliament, or universities), and if you dare to criticize a priest or a religious organization, you will quickly find yourself under a flood of attacks from people of all walks of life.

In parallel with the state-supported resurgence of religious life, the boundaries between secular and religious education are getting blurred. At the end of 2006, the secretary of state for research and education at that time, Mihail Hardau, signed a ruling that eliminated virtually all references to evolution from the science standards for public schools. In the meantime, 73% of the Romanian high-school students already think that the universe and humans were created by God. Scientific literacy is so low in the country that very few people see this as a negative development; even some biology teachers say that Darwinism does not necessarily contradict creationism and it is out of date anyway. Most journalists and politicians who express an opinion on the subject only prove that they did not even take the time to look up the words “Darwinism” and “evolution” in a dictionary.

This is sad news for me. I learned basic biology in communist Romania, in the eighties, and at that time there was no place for God and creationism in biology classes.[Of course, that was about the only good thing about communism — so I am delighted it is a thing of the past, do not get me wrong]. Although my understanding of evolution largely comes from popular science books rather than those old biology lectures, at least you could not finish high school without hearing about Darwin and evolution. Now it is different: it has become difficult to get through the public education system without being indoctrinated (on taxpayer money) with the dogma of your favorite religion, and you might only hear about Darwin in the context of outdated atheistic thinkers who are not relevant any more.

If you want to help, here is the email address of the Romanian Ministry of Education: informare.publica@medu.edu.ro; more info here. Also, if you have a blog or website, feel free to spread the word. More people in Romania and outside Romania need to realize that the integrity of science education in one of the largest countries in Europe is at stake here.