So Paul Braterman was asking the other day about some advance knowledge of how the new infatuation with detrital zircons and HeFTy plots might be misread in some circles. This is really a question for a real card-carrying geochronologist, but GG will take a swing at it because he’s kind of worried a bit about this and is not staking out any particular terrain.
For the most part both of these are simple variants on classical age-dating techniques (uranium-lead dating and potassium-argon dating), and though rooted in geochronology, most of the applications are elsewhere (e.g., detrital zircons are mainly used as a means of identifying the sources (provenance) of clastic sedimentary rocks, while HeFTy (Helium-Fission Track analysis) is dominantly a means of assessing the thermal or unroofing history of some body of rock through application of multiple geochronometers). But they start pointing at things that could be misleading, so let’s look a bit more….
Not so long ago, you would get a date (one) for some igneous unit. And that was hard enough that you wouldn’t bother with two or three. Dates were so valuable one well-known scientist had an equally well-known safe to keep them in (we still live with a rule at GSA related to this fellow in that recording or photographing presentations is forbidden). Then there was recognition that some systems closed up shop at different temperatures than others. So maybe you’d see a U-Pb date and a K-Ar date. A few labs did this work, often under contract; you (the non-geochronologist) might wrap up a sample and send it on to be dated. Dates, while important, were just some numbers that were part of a geologic story.
Now, however, dates are everything (that, and chemical and isotropic analyses at the tiniest levels, which is a related outgrowth). It seems like more than half the talks at GSA involved dating detrital zircons, or dating zoning in zircons, or dating helium diffusing out of zircons. Dates are used to understand erosion, tectonics, stratigraphy, sedimentology, volcanology, paleoearthquakes, glacial action and more. Arguably this ability is utterly changing geomorphology and sedimentology and it seeps into other fields more slowly.
If you haven’t seen a pdf (probability distribution function, not portable document format) or a HEFTY thermal evolution-o-gram, you haven’t been in a geological talk in some time now.
And so it is about time for the revenge of the grumps. Not GG so much as others. For the broad application of these new techniques has excited most geologists, but history tells us that there will be a reckoning. As GG watched lots of folks who have not themselves sat in front of an LA-ICPMS machine in their lives display plot ofter plot of geochronology-derived stuff, you sense that something will come along to threaten this grand promise.
This has always been the way of new techniques. They appear, they are exciting and new, they are applied everywhere, and then discrepancies emerge. Look back in olden days and see how potassium-argon dating started; it took awhile for practitioners to recognize that sometimes crystals would lose argon and they got dates that were too young, or that certain materials would introduce an excess of argon from other minerals and a date would be too old. Some early results were discarded, the community identified situations when problems were likely to arise, and early over interpretations were scaled back.
There are hints of this already. Conflicts between U-Th/He dating and some classic geologic constraints hints at some problems in some places. Some work in the past few years indicated that fission-track thermal histories relying on track length distributions were dependent on specific laboratory practices that are not uniform. Puzzling results are emerging in some sedimentological studies where things that simply cannot be seem to be. On occasion, dates seem backwards, with younger dates from systems that should have closed well before materials yielding older dates.
None of this is really a worry. It is the shake-out that is needed. And as long as you keep in mind that there might be some landmines out there, the hazards are manageable. It is a kind of “trust, but verify” environment. But there will be reverses ahead, and some promising studies might turn out to be chimera. Don’t be surprised to see some papers saying that a certain technique is wrong when applied under certain conditions. But in the end, we will still come out with a host of tools well suited to consider geologic problems. The age of ages is upon us, like it or not.
Secretary of State Tillerson was quoted this weekend as saying “Racism is evil — it is antithetical to America’s values, it is antithetical to the American idea.” From this, a naive listener might think that racism has been opposed throughout American history, which is of course fantasy. Racism has lived within America for a long time and has been embraced at times (as in the internment of Americans of Japanese ancestry in WWII, not to mention slavery and Jim Crow laws) as formal government policy. We like to think we are better than that, and so we think that our good ancestors of course embraced our modern vision of “the American idea.” This all got GG mulling about a far more pedestrian fantasy.
You see, we Americans (well, mainly non-Native Americans) have this fantasy of the empty wilderness continent. The idea that there was Nature, untouched and primeval, that Euro-Americans encountered on invading the New World. Now this is utter and complete balderdash on several levels. First is the obvious presence of Native Americans in the many millions on the continent; although some rationalize away their impact as somehow treading so lightly on the land that they made no changes, this is absurd. These peoples were apex predators, and many groups farmed or managed “wild” lands through burning, harvesting, planting and so on such that their absence from many landscapes led to vast changes in ecosystems. There was no “wilderness” for them.
Last fall GG’s Western U.S. Tectonics class took on trying to evaluate the status quo challenging hypothesis of Robert Hildebrand that the western part of the U.S. (west of central Utah, roughly) was a separate ribbon continent, Rubia, prior to colliding with North America in the early Tertiary, creating the Rocky Mountains. (That status quo holds that the far west was gradually assembled from the latest Paleozoic going on to the Miocene, with an arc being present on the edge of North America from the Permian to the late Cretaceous and again in much of the Tertiary). As Hildebrand’s argument was wide ranging and published as two lengthy GSA Special Papers (457 and 495), it isn’t a casual affair to consider the question of whether Hildebrand has caught western geologists in a huge misinterpretation or not. Many workers, content with their personal knowledge, have not peered into this abyss, so the class set out to take a swing at this. Basically, has Hildebrand identified observations inconsistent with our current interpretation of the geology? And are observables more consistent with Rubia than the standard model? A “yes” to the first might show that Hildebrand has put his finger on a problem even if the answer to the second is a “no”.
The class broke the hypothesis into these elements:
- North America was subducted under Rubia in the late Cretaceous
- Mesozoic and late Paleozoic magmatism, widespread in Rubia, never extended into “true” North America
- The magmatic volumes at the end of the Cretaceous in the western arcs are far too voluminous to have been produced by subduction of oceanic lithosphere
- Much of the classic late Precambrian – Paleozoic Cordilleran miogeocline is exotic to North America (i.e., is Rubia)
- Deformation from accretionary events is limited to Rubia.
- Mesozoic thin-skinned thrusts contain too much shortening to be limited to North America and are far greater than found in backarcs of typical continental arcs
- Magmatism and uplift in the latest Cretaceous and early Tertiary was produced by the oceanic part of the subjected North American plate falling off.
You can go and read the individual assessments made by class members to particular parts of this analysis, but a summary is below.
A piece in ArsTechnica reports on some research suggesting that conservatives are more likely to respond positively to news about climate change if they are seeing that the world of the past is not the world of the present rather than being shown ideas about what the future will bring. The piece ends with the author, Cathleen O’Grady, pondering the reason for this result:
There’s also the question of why conservatives found the material more persuasive: did it tap into their desire to preserve the past, as Baldwin and Lammers suggest? Or could it be because the past-focused materials showed evidence about what has already happened, which is more persuasive than predictions about what may happen?
In a sense, the crux of the matter is twofold: showing that something is happening, and showing what the cause of that something is. This paper is addressing the first point, and heavens only knows we have lots and lots of examples now to point at, from the decline in the size of the North Polar ice cap to the decline in the volume of the Greenland ice sheet to the change in the ratio of record high to record low temperatures to the changes in hardiness zones for gardeners to changing dates when frozen lakes and rivers thaw out to the increasing incidence of non-storm related flooding of low-lying areas. In point of fact, many conservative communities have notices some of these impacts and are working to ameliorate the problem. But this level of recognition might only result in attempts to deal with a particular symptom and not the underlying disease.
So that second level, seeing the connection between the things you can see changing and the underlying cause, is also important. The climate community has leaned heavily on their climate models to make the case, but these are not compelling for many in the public, in part because of confusion between the use of retrospective models and predictive models and in part because this then seems like predicting an uncertain future. GG has harped on this before; an alternative is to look at what has happened in the geologic past. And here we can find that times when the earth was warmer were times when carbon dioxide (and/or methane) was present at higher levels. We even have an example of a moment when atmospheric carbon levels rose at a geologically rapid rate: the Paleocene-Eocene thermal maximum (PETM). We find the ocean becoming more acidic in cores of seep sea sediments, shifts in the forest trees on land, and an extinction event that defines the end of the Paleocene. We also learn that many of the environmental impacts grow more severe the shorter the time period when the carbon is added to the atmosphere: the PETM was triggered by a carbon release over a few to a couple thousand years, with many (probably most) scientists who have worked on this inclined toward the few thousand year end. Higher temperatures were achieved more gradually in the early Eocene climatic optimum, but that event was not associated with such a pronounced extinction record.
Would bringing these geologically relevant examples to the fore help in convincing folks that the core problem here is our increase in CO2 levels? It sure deserves a chance…
The BBC has a piece recapping arguments over whether humans are responsible for the megafauna extinction at the end of the Pleistocene. There really isn’t anything fresh there, though it does name advocates on both sides.
Frankly, that this dispute continues puzzles GG, but perhaps the issue is more in the word “cause”. Are we talking proximate cause or ultimate cause? Are we identifying the particular events that pushed a species over the edge, or a unique link in a chain of events?
Consider an analogy: when a person is murdered by gunshot, you can say that the gun was the proximate cause. If there were no guns, some argue, there would be no murders. But a pile of guns in a room doesn’t result in deaths; the unique element in there is somebody willing to pull a trigger. Probably some of those people would not use a knife or baseball bat or poison, but probably some would; removing guns might reduce the death toll but not end murder altogether.
OK, how does this compare with the Pleistocene megafauna extinction? It is possible that the proximate cause of extinction for some species was a change in climate, or perhaps climate was only just removed because of its change impacting food sources. And in those instances you might argue that if there was no climate change, that species might be here today. Does that finger the changing climate as the ultimate cause?
GG’s view is that the scientific dispute actually is quite misleading. If humans do not invade a continent, there is no massive extinction event; the presence of humans is the ultimate cause of the megafauna extinction, the unique link in the causal chain that, if removed, breaks the whole chain. GG gets the distinct impression that the scientific argument is over proximate causes–what was the murder weapon? Spears? Fire? Competition for food? Disease? Warming climate? While there is a great deal of value to be learned about just exactly how these animals left the face of the earth (which is why these scientists argue), this should not be confused with the basic bottom-line truth: human involvement is the unique element, the ultimate cause. No humans, no massive extinction event.
We, all of us, have forebears who contributed to the extinction of these large Pleistocene animals. Pretending that the last deglaciation was so different from the dozens that preceded that it was the sole cause of extinction is an act of delusion. We should, as a species, accept our culpability even though those ancestors were not intending (so far as we know) to wipe out these species. And we should therefore accept a special responsibility to not let it happen again, deliberately or inadvertently. Let us accept our history, learn from it, and be the better for it.
Recently, GG concurred in the observation that myths can persist in the scientific community and added his own story of the “ignorant sheepherder” comment supposedly directed by Whitney at Muir. Some readers might have said so what, these are innocent little pieces of color commentary independent of the march of science. So for those skeptics, a more significant example.
A lot of recent work has been done on the Auriferous Gravels. These papers pretty uniformly assign a middle-late Eocene age to these rocks. For instance, Cassel et al. (2009, Int. Geol Rev.) said “Middle – late Eocene flora from within the upper half of the sequence are the only dateable material in the prevolcanic gravel (MacGinitie 1941).” A later paper gets a bit more precise (Cassel and Graham, 2011, GSA Bull):
The “Chalk Bluffs flora,” from the auriferous gravels at You Bet Diggings (Fig. 1), has been used to estimate the depositional age. Originally described as Capay stage and interpreted as middle Eocene by MacGinitie (1941), the Chalk Bluffs flora is now considered to be early Eocene (48.6–55.8 Ma; Wing and Greenwood, 1993; Wolfe, 1994; Fricke and Wing, 2004), which is consistent with comparable floral assemblages in other recently dated sections (Meyer, 2003; Retallack et al., 2004; Prothero, 2008).
Hren et al. (2010, Geology) similarly date these rocks: “Plant fossils are classified as Chalk Bluffs Flora after their best-preserved occurrence, and are dated at 52–49 Ma by faunal and floral correlation (MacGinitie, 1941; Wing and Greenwood, 1993).” It would seem that these sediments are pretty firmly dated to 49-52 Ma.
Except that in fact there is no firm floral date for these rocks.