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Rubia Slippers

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.

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Can Geology Overcome Climate Denialism?

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…

Guns and Climate and Mammoths, Oh My!

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.

Eocene Telephone

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.

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Subtly Siccar

One of the key differences between geology and most other science is that later workers can see the exact same “experiment” as the original workers.  In other fields, replication is the standard; while you cannot see the original experiment, you should be able to reproduce it.  For us in geology, though, we can usually see the exact features interpreted before.

And so in visiting Siccar Point, GG could look at the same rocks that James Hutton saw and used in arguing for the great depth of time needed for geological relationships.  Although Hutton found many unconformities in Scotland, this was the one that has impressed geology more than the others.  The neat thing in visiting is appreciating some of the less obvious characteristics of the place. Here is the classic view found in most geology textbooks:

The "Classic" view of Siccar Point with the angular unconformity  between the Old Red Sandstone (dipping gently to the left) and the underlying Devonian turbidites (vertical beds at right). CH Jones July 2015.

The “Classic” view of Siccar Point with the angular unconformity between the Late Devonian Old Red Sandstone (dipping gently to the left) and the underlying Silurian turbidites (vertical beds at right). CH Jones July 2015.

Texts often trim away that righthand side and sometimes zoom in even closer to the unconformity. But if you come to visit, that righthand side stands out: those Silurian graywackes are rising up above the unconformity.  Keep that in mind; we’ll come back to it.

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People v. Megafauna (Book Review)

One of the more interesting intersections in earth science is at the end of the Pleistocene some 10,000 years ago in North America.  Trying to untangle the climate and human changes at this time is challenging in no small part because you are dealing with experts in archeology, paleontology, and paleoclimate (there is even a small role for solid earth geophysics if you look carefully) as well as cultural clashes over ethnic heritage and, occasionally, legal battles.  It is where GG’s Historical Geology class really starts, so a book really digging in to this material is always welcome.

David Meltzer’s First Peoples in a New World: Colonizing Ice Age Americais a rarity as the author is a working archeologist and the text doesn’t try to gloss over disagreements but instead fleshes out arguments too often presented as disagreements between personalities. Although the book is now five years old and so missing some recent discoveries, by and large this covers the big issues in the original peopling of North America as they still stand today.

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When Observations Collide…A Grand Canyon Story

Two stories are out there about the Grand Canyon: one says the canyon is young (cut in the past 5 million years), one says it is old (cut by about 70 million years ago). Why is this?  Fundamentally it is because one group is tied to one observation and another to another. This sort of thing happens in earth science (most intractably in the controversy over the Cretaceous location of British Columbia) and can lead to immense frustration.

Here, the idea that the canyon is young is fairly longstanding and largely based on the absence of detritus from the upper Colorado River in sediments deposited in the vicinity of Lake Mead (the Muddy Creek Formation, should you wish to look it up).  There are workarounds that have been (and continue to be suggested), so let’s not focus on that particular problem now.  There are two pieces of evidence that are close together and whose interpretations are mutually incompatible: a new-fangled radiometric age date and a classic old school geologic outcrop.

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