High metal concentrations in fluid inclusions?

I've got a question for any readers with expertise in ore deposits. There's a paper in the Feb. 6 issue of Science (Wilkinson et al; discussion of paper here; warning: paywall) about anomalously high concentrations of metals found in fluid inclusions in ore minerals. I'm curious what you think, and whether the conclusions have practical implications for mineral exploration.

Here are the basics, as far as I understand them. Hot water traveling through rock is responsible for depositing many of the ore deposits that we use for metals. (There are exceptions - iron, for instance.) In previous work, the concentration of metals in the hot water has been estimated from the tiny bits of fluid trapped in quartz or other minerals in ore deposits, but not in the ore minerals themselves. (The quartz has been interpreted as having formed at the same time as the ore minerals, but the study authors argue that it's difficult to be certain.) The concentrations of the metals in those inclusions are fairly low, which means that a lot of water would need to have traveled through the rock in order to form the ore deposits.

This study measured the amount of metals in fluid inclusions in the minerals quartz and sphalerite (zinc sulfide) from two different lead-zinc deposits (one from a Mississippi-Valley-Type deposit in Arkansas, and one from a higher temperature deposit in Ireland). In both cases, the concentration of metals (especially lead) were one to two orders of magnitude higher in the sphalerite. That's a huge difference, and would mean that much smaller amounts of water (and shorter amounts of time) may be necessary to form economic mineral deposits. It also means that the processes that collect and concentrate dissolved metals may be more important in making an ore deposit than the processes that cause them to be precipitated.

But the commentary said that both experiments and theoretical models (I'm not sure which, exactly - thermodynamic models?) predicted lower concentrations than those observed, so I'm curious exactly how big of a problem these observations are for ore geochemistry. (This study dealt with lead-zinc deposits, but the commentary mentioned that other studies had found similar results for copper and gold.) This paper talked about ways that lead and zinc could be concentrated, by evaporation of brines, but I'm not sure what processes might control the concentration of copper or gold in deeper deposits. Does it change the way one might explore, or are the targets the same regardless of whether we understand the source of the fluids or not? (Would understanding this problem make it easier to guess which sites wouldn't have economic concentrations of metals?)

References:

Wilkinson, J.J., Stoffell, B., Wilkinson, C.C., Jeffries, T.E., and Appold, M.S., 2009, Anomalously metal-rich fluids form hydrothermal ore deposits: Science, v. 323, p. 764-767.

Perspective: Bodnar, Robert J., 2009, Heavy metals or punk rocks? Science, v. 323, p. 724-725.

Feb to-read list from Geology

Has Geology become open access? I've just set up RSS feed for it, and when I clicked links to several interesting articles, I discovered I could read them from home. (Cool.) Perhaps my school's subscription works from home now, which is great for me, but which would mean that these links are behind a paywall for the rest of the world.

I'm still going to post my to-read list, so I can kick myself if I don't find time to read these:

San Andreas Fault geometry through San Gorgonio Pass: southeast of LA, the San Andreas Fault is a real mess, with multiple strike-slip strands and thrust faults. Laura Dair and Michele Cooke have used numerical modeling to test whether different fault geometries, combined with known Pacific-North America plate motions, match the observed patterns of slip and uplift along the many faults. Doug Yule has a related commentary, which puts the study in context (and explains how cool and innovative it is). The results aren't just important for understanding faults - they're useful in trying to figure out just how big the southern San Andreas' Big One could be. (This one's going into my folder of possible articles to use for Advanced Structure.)

Mantle weakening and strain localization: Implications for the long-term strength of the continental lithosphere: The Jelly Sandwich and the Creme Brulee models of lithospheric strength are back. In the "jelly sandwich" model, a squishy lower crust is sandwiched between a strong upper crust and a strong upper mantle; in the "creme brulee" model, the upper mantle has no strength, and plates are held together by the top of the crust. Jacques Précigout and Frédéric Gueydan argue that a "jelly sandwich"-type lithosphere can start to behave like "creme brulee" in areas of active deformation, where high strain can result in reduced grain size and the possibility of faster deformation in small areas. (My kindergartener will be really upset if his jelly sandwich turns into creme brulee. Maybe he needs a hard-sided lunchbox to avoid rapid sandwich-squishing.)

Discovery of columnar jointing on Mars: The title tells the story. There are old volcanoes and lava flows on Mars, and HiRISE has found columnar joints. The pictures alone are amazing, but the authors argue that the type of columnar jointing is typical of fractures formed when lava comes into contact with water.

Porphyroblast rotation versus nonrotation: conflict resolution! Comment and reply. It turns out that the conflict is not, in fact, resolved, and the garnets are left spinning. Or not. The commenters and repliers focus on different models to make their arguments, and make some rather pointed comments. ("Fay et al. (2008) present a few numerical simulations (with unspecified code or boundary conditions) as evidence for nonrotation of porphyroblasts during non-coaxial flow.") It's a good example of how scientists write when they disagree on something (especially when their disagreement is around 20 years old, at this point).

Ophiolites and ocean crust: slow tectonic shifts in textbook science

For more than fifteen years, I've been making my Plate Tectonics students read Assembling California by John McPhee. In part, I'm just using my dastardly professorial powers to inflict one of my favorite writers on helpless students. In part, I just want students to read lines like "You need a new geologist. You need a Californian." But mostly, I want students to think about ophiolites and the ways that scientists change their minds.

All of the textbooks that I've used simplify the ophiolite sequence in the same basic way. It's oceanic crust, stranded on land where we can see it and touch it and measure its structures and sample its minerals. Sediments, pillow lavas, sheeted dikes, massive gabbro, layered gabbro, layered peridotite, deformed mantle rock. The field geologist's answer to the layers recognized by seismologists on the ocean floor. The record of sea-floor spreading, one of the many pieces that fell into place as plate tectonic theory came about.

Getting that ocean floor onto continents where geologists can study it without submersibles or seismic waves is tricky. I remember arguing about California's Coast Range Ophiolite with my officemates (probably at Friday Beer after the high-pressure-metamorphism seminar). It's inland of the old subduction complex of the Franciscan Formation, so did that mean that it represented the closure of an old ocean basin? Or was it shoved up onto the continent as some kind of flake? Or, well, how do you do that mechanically, anyways? (Big picture tectonic arm-waving works better after beer. Increase pore fluid pressure and all that.)

Sometime in those nineteen years during since that seminar, the consensus understanding of ophiolites has changed. It hasn't been something that makes headlines, but in study after study, it's turned out that the trace element geochemistry has the wrong fingerprints for the ocean floor. Most ophiolites - even the famous ones like Troodos on Cyprus or the Semail ophiolite in Oman - most ophiolites formed above subduction zones.

I had heard rumors about this from igneous geochemists before, but an article in last month's GSA Today went further. If the geochemistry says that ophiolites aren't rocks that formed at mid-ocean ridges, well, we really shouldn't be using them to study what happens at mid-ocean ridges. We can't even use the sheeted dike complexes - sheets of cooled magma that intruded one another so that only half of any original dike is left - to talk about sea-floor spreading. At mid-ocean ridges, the rates of magma production and spreading are tied together, but above subduction zones, they're the result of two different processes. It's possible for the plate above a subduction zone to spread, if the downgoing plate sinks faster than the over-riding plate slide across it. (That's happening, for instance, near the Mariana Trench. And it's one of those complicating bits of tectonics that doesn't match the stories that get told in introductory classes, and that can serve as a GOTCHA! for partly-informed skeptics. But we know about trench rollback, and it's been incorporated into geologists' understanding of tectonics for a couple decades, even if we don't explain it that well to students.) The spreading of the over-riding plate might look like a mid-ocean ridge in the rock record - it's all extensional tectonics, after all - but the relationship to magmatism is different. Not the same relationship, not the same process. Still interesting... but not a way to study the way that oceans grow. Sorry.

So touching an ophiolite isn't touching a bit of old ocean floor, after all. And we don't need to argue about how the rocks got on top of the subduction complex if they usually are formed there. And a lot of tectonic arm-waving was for naught.

And the textbook science changes.

Reference:

Robinson, P.T., Malpas, J., Dilek, Y., and Zhou, M., 2008, The significance of sheeted dike complexes in ophiolites: GSA Today, v. 18,. p. 4-10. (Available for free online here.)

Misleading headline of the day: sea level is falling!

Headline from National Geographic: Sea Levels to Plunge Long Term, Study of Dino Era Says.

It paints a scary view of the Cretaceous: forget T. Rex. Did you know that sea levels were 560 feet higher than they are now? That would make the tip of the Washington Monument barely above sea level! (And, for that matter, the high points of Florida, Delaware, and Louisiana would be only accessible by scuba diving!) That's right... and it's all natural - it's due to the sea floor pulling apart!

Umm. Actually, I did know that. In fact, where I live, at 6500 feet above sea level, I can go see the shoreline of the Cretaceous Interior Seaway that flooded North America. The high sea levels of the Cretaceous are hardly big news to geologists, and the role of mid-ocean ridges in controlling global sea levels was accepted by the 1980's, at least.

The story is about an article coming out in Science tomorrow. Science is supposed to published ground-breaking, cutting-edge research. They wouldn't publish something that's been accepted for decades, would they?

Of course they wouldn't.

The paper discussed by National Geographic (Muller, R.D., Sdrolias, M., Gaina, C., Steinberger, B., Heine, C. (2008). Long-Term Sea-Level Fluctuations Driven by Ocean Basin Dynamics. Science, 319(5868), 1357-1362. DOI: 10.1126/science.1151540) deals with a far more subtle question. We know sea level was high during the Cretaceous, but how high was it, exactly? Different methods of estimation give different answers. Estimations of mid-ocean ridge volume gave numbers in the neighborhood of 250 meters (820 feet - above the high point of Rhode Island). Sedimentary rocks in New Jersey, on the other hand, imply that the highest Cretaceous sea level was only 40 meters (131 feet) - less than the sea level rise that would occur if we melted all the ice on the planet. Why the discrepancy?

Muller and colleagues started by assuming the sea level rise was mostly the result of tectonics. The young rocks at mid-ocean ridges ride higher than the old, cold rocks of the abyssal plains, so the first thing they had to do was figure out just how old the ocean floor was in the Cretaceous. That's not that easy, because several oceanic plates in the present-day Pacific have gone down subduction zones and disappeared. Then the authors had to account for hot spots (like Hawaii), and for sediments, and for changes in the area of oceans versus continents (such as that caused by the widening of Nevada during formation of the Basin and Range). And finally, yes, they calculated how much sea level would rise if our ice caps all melted.

They conclude that Cretaceous sea levels were 170 meters (~560 feet) above present-day sea level.

So why do the sedimentary rocks from New Jersey give such a different answer?

They blame the mantle. Actually, they blame the Farallon plate - the oceanic plate that subducted beneath North America during the Mesozoic (and whose remnants are still sliding under the Pacific Northwest, Central America, and South America). The old Farallon slab is still down there, sinking... and apparently it is dragging New Jersey (and presumably the rest of the East Coast of the US) down with it.

I'm not sure how to evaluate the explanation for New Jersey's low elevation. If it's correct, it implies that the mantle drives major changes in relative sea level. Now, heat in the mantle is blamed for the high elevations in rift zones (such as the East African Rift), but I hadn't heard of ancient subducting plates dragging their overriding continents down. (Is there a similar effect in eastern South America? Is northern Asia affected by subduction of the Indian plate?)

Whether their explanation for New Jersey is correct or not, however, the National Geographic article completely misses the point. Yes, I know that "Discrepancy Resolved Between Cretaceous Sea Level Estimates" doesn't have the same ring. But the Science article does not say anything about future drops in sea level. It certainly doesn't try to extrapolate the trend 80 million years into the future, as the National Geographic article does:

When this trend is extrapolated out 80 million years from now, it suggests that even if all of today's ice caps were to melt, sea levels would be 230 feet (70 meters) lower than they are today.

This sounds like the advice of a bad stock analyst in the 90's: if it's going up now; it will always go up. And if it's going down now, it... will continuing decreasing forever.

My advice: if you are hoping for subduction of mid-ocean ridges to save your city from flooding due to global warming, don't hold your breath. Even if tectonics causes global sea levels to fall, the ocean floor isn't going to change very fast.

Edit: Reuters has an equally misleading headline. It sounds from the news release like the authors of the paper are claiming that the paper predicts the future. Hmmm. I guess it's possible to predict the future subduction of the East Pacific Rise and the Gorda/Juan de Fuca/Explorer Ridges. But is it possible to predict future hot spots/large igneous provinces, especially given that there's a debate about their origin? And do we know exactly why the Pacific plate appeared at the triple junction between three other oceanic plates? (Is it the result of some kind of triple-junction instability?)

Meanwhile, Ole has a good discussion of long-term versus short-term sea level changes. I can't comment on his site (it requires registration), so I'll say here: read it. It's a good discussion.

Does subduction quit every now and then?

Callan Bentley at NOVA Geoblog has a good discussion of an article published in Science by Paul Silver and Mark Behn about the possibility that plate tectonics has been intermittent, rather than a continuous feature of Earth’s geology. I was intrigued by the press releases, but the paper itself seems to have problems, and I’m curious whether I’m simply being dense, or whether these problems really exist.

The basic premise of the Silver and Behn paper is this: when continents collide, a subduction zone goes out of existence. Cartoons of pre-Mesozoic* collisions (such as the Appalachian collisions that Callan shows on his blog) usually show subduction initiating someplace in a nearby ocean, to balance sea-floor spreading somewhere else on the planet. But what if that doesn’t happen? And what if there aren’t major subduction zones on the opposite sides of the colliding continents to take up the motion? Is it possible that world-wide plate motions could simply stop, until enough heat built up in the mantle to force continents to rift (through initiation of mantle plumes, if they really exist)?

It’s an intriguing idea. It’s true that subduction zones don’t seem to regularly start in places where oceanic crust is particularly old, and that collisions don’t always result in the formation of new subduction zones. There are only two dinky little subduction zones in the Atlantic (the Caribbean and Scotia arcs), and there is no new subduction zone south of India. On the other hand, there is a new subduction zone forming on the north side of Papua New Guinea, after the northern continental shelf of Australia clogged the subduction zone. And the idea that supercontinents could insulate the mantle beneath them, leading to increased heat flow, could explain one of the mysterious features of the Precambrian basement in my neck of the woods: the ~1.4 billion-year-old “anorogenic” granites, and the pervasive high temperature/low pressure metamorphism that can be found even far from the contact of any exposed granite.

But the proposal implies a particular sequence of events... and the data used to support the model suggests that things actually happened in a different order.

The order ought to be this:

1) A Pacific-type ocean basin (in which the ocean basin existed before breakup of the previous supercontinent) closes, creating a supercontinent.

2) The majority of worldwide subduction zones are caught in the middle of these collisions, and disappear.

3) The lack of subduction causes mid-ocean ridges to slow, as well, and plate motion decreases (or, possibly, stops).

4) Heat builds up in the mantle until plate tectonics resumes (presumably driven by the rising of hot material and some kind of rifting; subduction would have to begin at the same time to keep the Earth the same size).

Silver and Behn argue that Pangea shouldn’t fit this model, because when the Caledonian/Appalachian/Variscan/Ural collisions occurred to form Pangea, the oceans that closed were young. (And there were many active subduction zones around the edges of Pangea, off the west coasts of present-day North and South America, among other places.) Instead, they focus on two older supercontinents: Rodinia, which formed as a result of the Grenville orogeny around a billion years ago, and Pannotia/Gondwana, which formed slightly later.

Their primary way of testing the model is to look at the geochemical evolution of the mantle as a way of measuring the amount of subduction that occurs. They use two different measures of mantle chemistry: the Nb/Th ratio (which should increase in mantle-derived rocks through time, because Th is preferentially incorporated into subduction zone melts), and the ratio between two isotopes of He.**

The test is summarized in their figure 1, which I can’t show because it’s behind a paywall. So I’ll draw some sketches to show why I’m confused. The approximate shape of their estimated subduction flux looks like this:


There are two peaks, one in the late Archean (around 2.5 billion years ago) and one in the Phanerozoic. And there’s one minimum, around 1 billion years ago.

If you add the times during which Rodinia and Pannotia/Gondwana formed by closure of Pacific-type basins, the diagram looks approximately like this:


Do you see why I’m confused? Yes, there appears that global subduction decreased (if the proxies are correct) between 2.5 and 1.0 billion years ago. But the minimum subduction rate occurred just before Rodinia formed – exactly the opposite order than one should expect if the formation of Rodinia caused subduction zones to shut down. In fact, the formation of Rodinia occurred just before the worldwide subduction rate began to increase, as if the formation of supercontinents actually caused new subduction zones to form in other parts of the world.

There is another, even older supercontinent (Nuna) proposed, but it isn’t clear exactly what kind of ocean basin closed to form it. And it formed during the middle of the decline in subduction rates.


In fact, the only supercontinent whose formation correlates to a decrease in the proxies for subduction rate is Pangea – and we know that subduction occurred before, during, and after Pangea existed. Pangea was the one supercontinent they excluded from consideration.

So... interesting idea. But their measures of subduction flux don’t seem to support it.

I have no trouble with the idea that plate tectonic rates may have varied through time (though I would love to see, say, a compilation of paleomag data as an independent test of the geochemical proxies). But their explanation – that the formation of supercontinents causes subduction to halt – doesn’t fit the data.

If this study is correct, it would have interesting implications for things like the driving mechanism of plate tectonics. But the data doesn’t seem to fit the conclusions. Am I missing something critical about their reasoning – some reason why the decline in subduction should precede the collisions?

I’m not teaching plate tectonics again until 2010, so there’s time for comments, replies, and discussions before I would discuss this in class.

Reference: Silver, Paul G., and Behn, Mark D., 2008, Intermittent plate tectonics?: Science, v. 319, p. 85-88, doi: 10.1126/science.1148397.

*Plate movements from the Jurassic to the present are much better constrained than pre-Mesozoic motions, because magnetic lineations in the ocean floor provide a quite detailed record. So the changes in plate motions after the Himalayan collision can be examined worldwide.

**I don’t know how quickly mantle chemistry should respond to the amount of subduction – that gets into questions about how quickly the mantle mixes, and I can’t even say how readily the mantle mixes vertically, let alone how easy it is for an aesthenospheric wedge above a long-lived subduction zone to mix with the wider mantle. But we’re talking about lots of time in the Precambrian, so maybe that isn’t really an issue.

Serpentine rheology and subduction zone earthquakes

There’s something about subduction zones. Maybe it’s the potential for magnitude 9+ earthquakes and major tsunamis... and for silent earthquakes, and slow slip events, and postseismic deformation. Maybe it’s the explosive volcanoes. Maybe it’s the accretion of stuff scraped off the downgoing plate. Maybe it’s my old fondness for minerals that look purple under a microscope. Maybe I’ve just been seduced by the name.

Anyway, although I’ve worked in rocks that have been down in subduction zones, I don’t have a very good idea about the nature of the faults that generate the immense earthquakes. I work on land, and on rocks that have made it to the earth’s surface; the subducting plate starts on the ocean floor, and goes down from there. So I don’t have a mental image of what’s involved – not like I have of, say, the Wasatch normal fault, or the Snake Range decollement.

An article in last week’s Science by Nadege Hilairet and co-authors, about experimental deformation of serpentine, gives me a better idea.

Minerals can deform in a number of different ways, depending on the temperature, the stress they experience, and the rate of deformation. At low temperatures, things deform elastically - they change shape in response to the stress until either the stress goes away, or they break. At higher temperatures, chemical bonds break and reform within the crystals, allowing the crystals to change shape without physically breaking. This can lead to some major differences in behavior when the rocks are stressed – at low temperatures, the release of elastic energy when rocks break and slip gives us earthquakes. At high temperatures, rocks behave more like Silly Putty – they’re elastic at short time scales (and can transmit seismic waves), but over longer times, they change shape at a rate that’s proportional to the stress they feel.

Different minerals deform by different mechanisms. Some change shape ridiculously easily – rock salt, for instance, flows so easily that it is used to equalize pressures in rock deformation experiments. Others, such as olivine, are strong even at quite high temperatures – that is one reason why the uppermost part of the mantle, the mantle lithosphere, is the strongest part of most tectonic plates.

And then there’s serpentine, the hydrous magnesium-iron-aluminum silicate that forms from adding water to mantle rock. It’s the mineral that dominates the state rock of California (and the rock found in the core across the San Andreas Fault). And it is, presumably, a mineral that forms in the mantle wedge above the subducting plate, as the downgoing plate loses its water. (Some subduction zones also have evidence that it really is there – seismic waves traveling through the overriding plate are slowed by a material that has the right seismic velocity and other elastic properties to be serpentinite.)

Serpentine is soft and slippery at surface conditions, so one might expect it to be weak in subduction zones, as well. But serpentine is brittle in low-pressure experiments (below 0.7 GPa, which would be equivalent to a depth in the neighborhood of 20 to 25 km) – near the surface, it breaks. The authors, however, managed to do deformation experiments at pressures of 1 to 4 GPa (equivalent to around 33 to 120 km – for a subduction zone beneath a continent, we’re talking about depths from the average base of continental crust to the base of the mantle lithosphere – though at a subduction zone, I’m not sure what is typically found at those depths).

They found that serpentine behaves differently at different pressures, presumably because different deformation mechanisms are at work. At both pressures, the behavior could be modeled as power-law creep. Power-law creep means that the strain rate is related to stress taken to some power n – that is, the higher the stress, the faster the strain rate, and the higher the value of n, the more sensitive strain rate is to increases in stress. Power-law creep is typical of deformation mechanisms involving movement of dislocations (defects in a crystal lattice). But at lower pressures, the stress exponent was higher – increasing stress really made deformation happen more quickly – which is typical of sliding along grain boundaries. At higher pressures, the behavior was closer to that of a viscous material (though still with viscosities that change with stress).

Ok, nice. So why, exactly, is this interesting? Well, for one thing, I usually think about the effect of temperature on deformation mechanisms, but I don’t usually see discussions of the role of pressure. Temperature’s important because intracrystalline deformation involves breaking chemical bonds, and higher temperatures generally make it easier to break and re-form bonds. In fact, I had always thought about the great depths of subduction-zone earthquakes as a function of the cold temperatures in the subducting slab. (The rock descends faster than heat can flow into it – among other things, that makes high pressure/low temperature metamorphic rocks possible.) So it’s interesting to see rheology controlled by pressure as well.

For another, the experiments show that, at depths in the range of approximately 30 to 100 km, serpentine is much weaker than most other likely minerals (pyroxene, plagioclase, and wet and dry olivine – the likely constitutents of oceanic crust and mantle). Is the likelihood of large earthquakes on subduction zones governed by the presence or absence of serpentine, rather than age of subducting crust or rate of plate movement? This would seem difficult to test, but if the seismic velocity and Poisson ratio of serpentinites are unusual, maybe tests are possible.

And what about those weird not-really-earthquake events that take place? Silent earthquakes, like the ones along the Cascadia subduction zone beneath Vancouver Island? Or afterslip – slow movement after a major earthquake, like occurred after the 2004 Boxing Day earthquake off Sumatra? (GPS data suggests that, during the month after the earthquake, slow slip equivalent to a M 8.7 earthquake occurred (Subarya et al., 2006).) The authors calculate that stresses in serpentine should relax on the same time scales described for slow earthquakes and afterslip, and imply that serpentine could explain some of the things that have been observed. There are some problems with that explanation, though. Modeling of afterslip following the Sumatra earthquake (Subarya et al., 2006), placed the slip in the same area as the initial earthquake, at depths shallower than 25 to 50 km, depending on the model they used to invert their ground deformation data. And Hilairet et al. cite studies saying that Sumatra has no evidence of serpentinization. So...the pieces don’t all fit together. Not perfectly.

This study, plus the recent study that found talc in the San Andreas Fault, are interesting because they tie together mineral behavior, rock mechanics, and seismology. The combination seems fruitful. I’ll be interested to see how the modelers who study triggered earthquakes work with these data.

References:

Hilairet, N., Reynard, B., Wang, Y., Daniel, I., Merkel, S., Nishiyama, N., and Petitgirard, S., 2007, High-pressure creep of serpentine, interseismic deformation, and initiation of subduction: Science, v. 318, p. 1910-1913.

Subarya, C., Chlieh, M., Prawirodirdjo, L., Avouac, J-P., Bock, Y., Sieh, K., Meltzner, A.J., Natawidjaja, D.H., and McCaffrey, R., 2006, Plate-boundary deformation associated with the great Sumatra-Andaman earthquake: Nature, v. 440, p. 46-51, doi:10.1038.

Misleading headline of the day: groundwater and climate change

From ScienceDaily and the US Department of Agriculture: ”Climate Change Likely To Help With Groundwater Recharge”.



Image source: http://www.usgcrp.gov/usgcrp/images/ocp2003/WaterCycle-optimized.jpg
I get my water from a well that runs dry every so often. And I’ve heard Jonathan Overpeck talk about how global warming may affect the southwestern US - in particular, how it may affect our pattern of snowfall and snowmelt, and therefore exacerbate our current drought. So I was particularly curious about where, and under what circumstances, increased CO2 would lead to increased groundwater recharge. (The groundwater situation depends, after all, on many things. What’s the nature of the aquifer – is it shallow or deep, spatially restricted or regional? What kind of rocks is it in, and how and where do they connect with the surface? What’s the land use, the vegetation, the rainfall, the snow pack?)

So I checked out the actual paper. It is focused on modeling the effects of increased rainfall on two areas in Australia with different seasonal rainfall patterns. The study’s particular contribution was modeling the effects of plant growth on recharge – previous studies had left out the effects of transpiration.

Probably the most important point left out of the press release had to do with the assumptions: in these two areas, global climate models predict rainfall to increase (14% in one area, and 37% in the other). The study therefore shows that increased evaporation and transpiration shouldn’t use up the increased rainfall, and some of the water should go into the groundwater. And that’s an important result, and the study demonstrates an approach that could be used for other scenarios.

But as the authors themselves say in their conclusion:

The direction and magnitude of the change in local recharge depended on the combination of climate scenarios, vegetation, and soil properties. It is not possible to make generic conclusions about climate change impacts based on this limited sample of climatic zones in Australia.

(Their words; my emphasis.)

Contrast the conclusion of the press release from ScienceDaily:

In both locations, changes caused to soil, precipitation and plant transpiration by simulated climates with twice the existing CO2 led to significant changes to the rate of groundwater recharge. Water recharged from 34 percent slower to 119 percent faster in the Mediterranean climate, and from 74 to 500 percent faster for the subtropical climate.

While the opportunity for decreased recharge rates exists, the general trend is towards increase. Future research will investigate whether those changes would benefit or harm those ecosystems.

The press release never mentions that the simulation included an increase in rainfall... which is probably the single most important factor in increasing groundwater recharge.

And the headline. That headline. “Climate Change Likely to Help With Groundwater Recharge.” The headline does exactly what the authors of the study warn against doing: it makes generic conclusions from a limited study of particular places.

Well. I don't live in Australia, and climate models predict less rainfall for my corner of the world. So regardless of the headline, I'm not going to count on my well recovering any time soon.

talc and the "weak fault problem"

While I was out with my Advanced Structure class, looking at fault and shear-zone rocks, there was a fascinating article published in Nature about the surprise finding of talc in the San Andreas fault zone.

The San Andreas both makes wonderful sense – it’s a strike-slip plate boundary, THE strike-slip plate boundary that everyone knows about – and it’s mysterious. A year or two ago, the Teaching Structural Geology mailing list put out a call for current enigmas in structure and tectonics, and right there at the top of the list is “the weak fault problem.” The San Andreas fault may be capable of a pretty impressive shake, but it’s not as tough as one might expect. For one thing, the principle stress directions around it are wrong. A strike slip fault should theoretically have its most and least compressive principle stresses at an angle to it, somewhere in the neighborhood of 30 and 60 degrees in an ideal world where all rocks break like in laboratory experiments and nothing ever rotates. But even in a non-ideal world, you shouldn’t get principle stresses parallel and perpendicular to a fault. Those imply that the fault is a free surface, that it has no strength, that it can deform continuously, like water or air. Water could solve the problem, but the evidence for the necessary fluid pressures just doesn’t seem to be there.

Now, a major fault zone like the San Andreas isn’t composed so much of rock as it is of rock that has been broken, fragmented, crushed, cataclasized. But even powdered rock has frictional strength – too much strength for the stress orientations along the San Andreas. Even serpentinite, California’s slippery state rock, found along the eastern side of the fault where it follows the old Coast Range ophiolite, is too strong to explain the fault’s behavior.

So they dug a hole into the San Andreas fault, just north of Parkfield. (As an aside: I think it’s wild that the long-expected Parkfield earthquake happened while they were starting to drill.) And they sent the cuttings to Diane Moore, who studies the metamorphism of the mafic and ultramafic rocks in California. And she found...



Ok, no, not that. But she found talc. Softest mineral known. And, better than that, a mineral whose behavior in deformation seems to fit some of the behavior of the San Andreas. Talc can deform constantly, but only when deformation is slow. Push it too hard, and it becomes stronger. And it’s a common alteration product of ultramafic rocks in continental crust (at least) – just add quartz to serpentine, and you’ve got talc. (In fact, this is why American baby powder makers have switched from talc to corn starch. Talc mines also frequently contain serpentine... and serpentine is one of the minerals that can form asbestos.)

There wasn’t much talc in the cuttings, and it doesn’t solve every problem with the behavior of the San Andreas, but the thought is intriguing. And... well, could there be talc in other faults through ultramafic rock? Are there subduction zones that have managed to incorporate hydrated mantle rock and then added quartz... and become too weak for magnitude 9 earthquakes? I don’t know. I don’t even know how such ideas could be tested, or how anyone could tell if a fault is soft and slippery at depth. (I guess that maybe GPS monitoring would be able to pinpoint faults that deform without major earthquakes. But how to tell if they’ve got talc along them... drilling holes into them all probably isn’t feasible.) But it’s a neat idea.

Reference: Moore, Diane E. and Rymer, Michael J., 2007, Talc-bearing serpentinite and the creeping section of the San Andreas fault: Nature, v. 488, p. 795-797. (It's behind a paywall, so I can't directly link to the paper.)