Sunday, October 30, 2011

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.

Thursday, October 13, 2011

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

ResearchBlogging.orgAustralia 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.

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

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

Monday, October 03, 2011

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.”

ResearchBlogging.orgWe 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