Don't hold your breath, folks

I actually spent a weekend bushwalking for the first time in five years this month. On top of that, I read a paperback novel. As a result, I'm so far behind that I can't even see the tunnel from here. I'll chip in on the dinosaur migration paper when I get a chance, but I wont get a chance anytime soon. And I should probably look at this new Zinc isotope thing but I haven't even DL'ed that yet. But I have a huge backlog of work stuff, taxes, a paper to review (for an editor, not you lot), and family stuff, so it could be a while.

Orbital cycles, Australian lake levels, and the arrival of aborigines

Australia is a dry country. It is so dry, that the largest drainage basin on the continent has rivers that only occasionally carry water, and drains into a salt pan. Imagine if the Missouri only flowed every third year, or if the Zambezi was a generally a sand-filled channel that crossed a nondescript cliff at what we know as Victoria Falls.

Admittedly, the Lake Eyre basin is smaller than either of these drainages, but only slightly. But for the last 150 thousand years, it has told geologic tales which rival the best Swahili stories or Souix legends. It describes the movement of the Earth against the stars, and the coming of the first people to Australia.

The reason it can tell these stories is that, as a closed basin, the water level of lake Eyre varies dramatically with the amount of water flown in from its major tributaries. So, although the part of central Australia around the lake and the southern part of the drainage is a desert, tropical rainfall in the northern rivers fills it occasionally today, and has in the past allowed a lake many times larger than the current lakebed to exist. Magee et al. (no relation), have carefully and painstakingly reconstructed the history of the lake level over time, and it tells a fascinating tale of alternating floods and aridification over the last 150 thousand years.

What they found is that there have been five periods where a large, permanent lake replaced the current playa. Comparing the lake record to the changes in the Earth’s orbital tilt and eccentricity shows that the lake filling is consistent with wetter conditions- and a more powerful Australian monsoon, being correlated with high sea levels, low ice mass, and high northern hemisphere sunshine.

The exact reasons for this are not discussed in great detail. One is that the outflow from the Asian winter monsoon might be pushing moist tropical air towards Australia more than Australia’s modest monsoon sucks air in. Another point (made mostly in related, referenced publications) is that the warm sea north of Australia- the Gulf of Carpentaria- is shallow, and during times of low sea level, was land. So the northern edge of the Lake Eyre basin was a thousand kilometers from the sea instead of 150, due to the retreat of the Gulf shoreline.

But the other big feature is that the lake-filling events that occurred after 50,000 years ago were much smaller than those which occurred before. Climactically, the conditions 10,000 years ago should have been the same as the conditions 115,000 years ago. But the lake was only a fraction of the size. The authors find no natural causes which can explain this. So they suggest that the aridity starting around 50,000 years ago is related to the reduction in forest and increase in grasslands which occurred at this time. This vegetation change was a result of a huge increase in the frequency of fire in central Australia, which allowed fire-adapted plants to prosper at the expense of moisture-retaining forest. The increase in fire at this time is generally associated with the arrival of the first people on the Australian continent. IT is known that of Australia’s megafauna went extinct at this time, but Magee et al. (2004) show that even the tropical rains were effected by human migration, with drastic changes to the continent’s largest river basin.

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Magee, J., Miller, G., Spooner, N., & Questiaux, D. (2004). Continuous 150 k.y. monsoon record from Lake Eyre, Australia: Insolation-forcing implications and unexpected Holocene failure Geology, 32 (10) DOI: 10.1130/G20672.1

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p.s. A few Gene Expression commenters asked a month ago if I could summarize this paper. I hope this helps.

CO2 sequestration in brines: what actually happens?

“I am flying home from Europe in late August with nothing but a notebook and the 2011 Goldschmidt conference Geology giveaway issue to keep me occupied. Using the old-fashioned method of reading and writing on paper, I will blog my way through the compilation of highlighted geochemistry papers as time allows. These will then be posted via time delay to keep the blog moving while preventing paper burnout.”

We humans are perplexing beasts. In order to power the computers and airplanes and steel mills and blogs of our increasingly technological society, we are digging up and burning every source of fossilized plant and algae matte rthan we can find. In the process, we are dumping CO₂ into the atmosphere at the fastest rate since at least the Paleocene / Eocene thermal maximum 55 million years ago.

The accumulation of this gas in the atmosphere has been identified as a potential problem, so society is looking for alternative dump sites. Although some agriculturalists think that trees or soil or other surface effects can securely hold this excess carbon, geologists tend to concentrate on shoving it where the sun doesn't shine. In science talk, we replace shove with ‘sequester’ since scientists like elongated words.

The theoriticians like to daydream about ‘sequestering’ their carbon in al sorts of fanciful places, but a perennial favorite is the deep, dark, hot, salty brines of saline aquifers. Often, these aquifers underlie current (or former) oil and gas reservoirs, in which case a fair amount is learned about them in the petroleum extraction process. However, carbon dioxide is supercritical at the pressures and temperatures of these deep reservoirs, and becomes highly reactive as a result. Brine can also be quite chemically reactive. In a nutshell, the supercritical CO₂ dissolves into the brine to form carbonic acid. But predicting the details is tricky.

In order to stop the theoriticians from talking smack about CO₂-brine reactions, 1600 tons of CO₂ was injected into a brine in an abandoned oil well. Carefully monitored aqueous geochemical hijinks ensued.

By measuring the composition of the reservoir fluids both before and after CO₂ injection, Kharaka et al. are able to quantify these hijinks.

In short, the pH plummets as the HCO₃- skyrockets, and dissolved alkali earths, transition metals, and base metals increase as a result. This is predicted to be a result of dissolution of carbonate and iron hydroxide cement. Obviously, dissolving the intragranular cement should increase the porosity and permeability, making it easier for the fluids to migrate. Despite this, no leakage was observed into the overlying sandstone unit.
Another disturbing observation was the increase in dissolved organic molecules, some of which are quite toxic. This observation was unexpected, and not fully understood.

The last experiment was to use the d₁₈O values of the brine and CO₂, which were initially quite different, to calculate mixing and residual supercritical CO₂ which had not dissolved into the brine.

My only complaint is that they did not look at the behavior of sulfur. Sulfur can be present in brines and co-existing residual hydrocarbons in either oxidized or reduced forms, and can also form a variety of minerals. Sulfur oxidation is what generates acid mine drainage, and it is an important constraint on both the acidity of the fluids present and on the solubility of various metals.
Kharaka, Y., Cole, D., Hovorka, S., Gunter, W., Knauss, K., & Freifeld, B. (2006). Gas-water-rock interactions in Frio Formation following CO2 injection: Implications for the storage of greenhouse gases in sedimentary basins Geology, 34 (7) DOI: 10.1130/G22357.1

Early Earth awesomeness and middle Earth magic

One of the problems with making illustrated linear geologic timescales is that the middle 80% of the timescale generally looks fairly boring. Of course, all sorts of things were happening in the Archean and Proterozoic, but they ren't always as easy to sketch cartoons of as a trilobite. I'm currently doodling a cartoon illustrated timescale, and I was wondering. do any of you have any favorite Precambrian events that can take up the timeline space that would otherwise be white? If so, and you don't mind me stealing your favorites, please share.

Viewing imaginary spacecraft from the ground

I read and watched a lot of science fiction when I was young. I don’t much any more, mostly because I’m too busy, but every now and then I have a relapse. Also, for the most part, real science is more fun these days. But they aren’t necessarily mutually exclusive.

For example, this evening, I was thinking about the International Space Station. Under optimal viewing conditions, the ISS is the brightest thing in the sky, aside from the sun and moon. But while the station is surprisingly large (about the size of a football field), it is generally smaller than most science fiction spacecraft which are capable of interstellar travel.

Science fiction generally depicts people walking around on the ground, or starships floating close above a planet, but with little connection between the two; The only time I can recall people on the ground seeing spacecraft above are when the Death Star explodes in Return of the Jedi, and when the remains of the Enterprise re-enter the atmosphere in Star Trek 3. But if you can see the ISS from here on Earth, then surely a larger science fiction (or alien) spacecraft would be brighter still.

Figure 1. Since you can see the ISS from your backyard, you don’t need the force to detect something much bigger in the same orbit. Click for larger image.

Thanks to Jeff Russell’s Starship dimensions, scaled profiles of most major starships can be easily compared to the ISS. That’s all well and good, and we can estimate areas and visual magnitudes in the -7 to -10 range for various popular starships. But since there isn’t anything of that brightness in our skies, it doesn’t mean much, except to tell us that they would be easily visible from the back yard of anyone looking for them (assuming they aren’t in Earth’s shadow). But there is a useful celestial yardstick.

Figure 2. relative apparent sizes of various spacecraft (and the ISS) when directly overhead in a 350 km low Earth orbit, when compared to the apparent size of moon. The moon is of course 1000 times larger and 1000 times farther away. Click for larger image.

The Moon, which has a radius of about 1740 km, is about 1000 times farther away than low earth orbit. So a spacecraft 1000 times smaller- say, a flying saucer with a 1.7 km radius- would have the same apparent size when directly overhead. Thirty degrees above the horizon, it would appear half a big. In other words, the shape of kilometer-scale spacecraft, such as a Star Destroyer, or a Babylon 5 capital ship, would easily be discernable to people below. The 24 km-wide flying saucers from Independence Day would appear to be seven times larger than the moon, and would blot out the sun for up to 4 seconds as they passed in front of the sun.

So forget all the garbage you hear about radar jamming and government cover-ups. When the alien invasion fleet comes for us, we’ll be able to watch it from the back yard.

Figure 1 is from STS 118 and Return of the Jedi.

Isotopic wins the 2011 Acrtic sea ice minimum competition

The winner of the third annual Arctic sea ice prediction pool is “Isotopic”, with a winning guess of 4567 +/- 100 thousand square kilometers.

As is usual, Isotopic’s prize is the chance to nominate a blog topic upon which I will try to write.

Congratulations, and thanks to all who played.

Real scientists study climate

There is often an argument, usually heard from the math/ engineering wing of the global warming skeptic industry, which suggests that climate scientists are a separate and distinct group of researchers. A cabal who don’t do real science, and who train and study in isolation, cut off from the rest of the scientific endeavor.

This is generally not the case. Most of the climate scientists I know started out doing something else. Some worked in the gold mining or oil & gas industries. Some studied the formation of continents, or the origin of granite. Some were volcanologists, or modeled deep mantle convection. A few were not even geoscientists at all, but came from disciplines such as chemistry, or nuclear physics. There are some people who go the other way, and move from climate science into archeology, or astrobiology.

There are several reasons for this. First of all, the analytical tools used to study non-climatological processes can often be applied to climate questions. And more importantly, when scientific discoveries of all types are first made, it is not necessarily clear where that discovery will have the most impact. It is not unusual for something in a seemingly unrelated field to get picked up by climate research.

There is also the funding aspect. Here in Australia, there has been an increasing reluctance to fund basic research. Most climate science is considered applied study, not basic science, so there has been a real trend for researchers chasing the funding dollar to go into areas like climate, mining, or forensics, where funds are easier to obtain.

But the point is that as professional scientist wind their way through various scientific inquiries that lie on their career paths, they don’t turn off the analytical parts of their brains when there is a climatological implication to their studies. Ultimately, climate science is just like any other sort of science, and it is studies using many of the same tools and methods as the rest of Earth science.

How long as the Atacama been dry?

“I am flying home from Europe in late August with nothing but a notebook and the 2011 Goldschmidt conference Geology giveaway issue to keep me occupied. Using the old-fashioned method of reading and writing on paper, I will blog my way through the compilation of highlighted geochemistry papers as time allows. These will then be posted via time delay to keep the blog moving while preventing paper burnout.”

The Atacama desert, on the west coast of South America, is the driest desert on Earth. The high Andes mountains block moisture transport from the Amazon basin, and the cold Humboldt current offshorehttp://www.blogger.com/img/blank.gif provides little evaporative moisture.

Dunai et al. (2005) attempt to determine whether the hyperarid conditions are ancient (early Miocene) or more recent (late Miocene) by looking at the cosmic ray exposure ages of easily eroded sediment.

Cosmic rays are extremely high energy protons which are generated beyond our solar system (ask an Astronomer for details). The are energetic enough to penetrate the atmosphere and the first few meters of rock when they strike the Earth. When they do hit rock, they can create nuclear reactions between the atoms in the rock. One of the products of these reactions, ²¹Ne, can be measured using noble gas mass spectrometry. So the amount of excess ²¹Ne a rock has is proportional to how long it has been close to the earth’s surface, and the cosmic ray flux.

Dunai et al. (2005)’s sample sites were specifically chosen to exclude areas where the outwash from the high Andes east of the desert would erode or cover the local rocks. Only local rainfall could erode the selected areas, so only local, medium elevation, near-shore precipitation (or lack thereof) was relevant to the erosion rates.

Their results show that most of the rocks they sampled have been at or near the surface for 20-30 million years. These are among the oldest exposure ages in terrestrial rocks. The implication is that there has been negligible erosion since that time.

On the other hand, I wish the paper made more of an effort to explain why the results given were not within error of each other. Call me old fashioned, but a data table would be nice as well.

The other question that they ask is which came first, the aridity or the uplift? It is easy to see how uplift causes aridity- the rain shadow gets stronger. How aridity causes uplift is less obvious, and the reference given is not available on this aircraft. But the general idea (based on context) seems to be that with no fluvial input to the subduction trench, it accumulates very little sediment. Without sediment, the rocks are stronger, and can withstand more stress, pushing the mountains higher.

The problem with this conclusion is that it requires knowing the sediment flux from the entire drainage area. Presumably the sediment transport would be controlled mainly by erosion of the high (and higher precipitation) Andes.

Dunai et al. (2005) specifically chose a site that did not record the sediment flux from the eastern, mountainous part of the drainage basin. Instead they chose to focus on local conditions. By excluding the most important potential sediment source, they put themselves in the worst possible position to answer questions about sediment transfer in the rest of the Atacama desert, including total transport to the trench.

Dunai, T., González López, G., & Juez-Larré, J. (2005). Oligocene–Miocene age of aridity in the Atacama Desert revealed by exposure dating of erosion-sensitive landforms Geology, 33 (4) DOI: 10.1130/G21184.1