All posts tagged geology

I just finished Don Canfield‘s book Oxygen, a sweeping exploration of the history of Earth’s most volatile atmospheric constituent.

Canfield starts by exploring how cyanobacteria generate oxygen during photosynthesis and how the process evolved. For instance, chloroplasts, the photosynthetic power plants of cyanobacteria, were once free-floating cyanobacteria themselves that took up residence in and eventually merged with eukaryotes that evolved into present-day cyanobacteria.

Subsequently, Canfield discusses Earth’s oxygen removal and renewal processes – decomposition of organics burns up oxygen, while their rapid burial preserves it.

The next several chapters present the geochemical evidence for changes in oxygen throughout Earth’s history, including variations in the ratios of different isotopes, sensitive to biological and abiological processes. Since I teach a class on astrobiology at Boise State, I focused a lot of attention on these parts, trying the piece together the interplay between biology and geology betokened by the isotopic variations.

One element of that story especially relevant to my class: variations in the carbon-13 isotope. As it turns out, one enzyme in cyanobacteria, RuBisCO, helps convert atmospheric C02 into organic carbon compounds, but it preferentially selects the lighter carbon-12 isotope 2.5% more often than the carbon-13 isotope. The organic compounds built using RuBisCO are therefore slightly depleted in carbon-13 relative to the atmosphere and when they are later incorporated into geological strata, the slight depletion gives a measure of how much life was around when the stratum was laid down.

Zooplankton salp pellets. From

The book contained lots of other appealing details. For instance, it’s not exactly clear what caused an enormous variation in oxygen on Earth 580 million years ago, a sea change in Earth’s history matter-of-factly called the Great Oxygenation Event. But one explanation has to do with the evolution of a new kind of poop:

The idea is that zooplankton [newly evolved 580 million years ago] produce fast-sinking fecal pellets. These would decompose less in the upper layers of the ocean as they sink […] when compared to the smaller, slowly settling microbial biomass [that had previously predominated].  (pp. 135-136)

Since the old sinking biomass took a long time to sink to the ocean floor, it had a long time for bacteria to decompose it, using up a lot of oxygen in the process. But the new, faster-sinking poop made it to the ocean floor before it decomposed much and so left the oxygen dissolved in the ocean instead.

I did have to spend a lot of time reading and re-reading the discussions of geochemical cycles and signals because it’s been a long time since high school chemistry for me, but I was willing to struggle through these parts because I found the underlying story so interesting.

So a really fascinating and challenging read about the complex (and poop-filled) evolution of Earth’s bio-geo-atmosphere.

Artist's depiction of ancient snowball Earth. From

Artist’s depiction of ancient snowball Earth. From×395/s/sn/snowball_earth/snowball_earth_1.jpg.

I sat in on a fascinating seminar today in the Geosciences Dept. given by Prof. Carol Dehler of Utah State University about statigraphic and isotopic evidence for a snowball Earth event.

Over its 4.5 billion year history, the Earth has probably experienced at least one, maybe more, severe glaciation events, during which ice covered more or less the whole Earth’s surface.

These events were probably very challenging for ancient Earth life, and some of the best documented events occurred at the end of the Neoproterozoic, about 540 million years ago, probably driving mass extinctions and possibly kicking off the Cambrian explosion in animal life.

In her presentation today, Prof. Dehler discussed her extensive field and laboratory studies of statigraphic deposits in the western US, laid down about this time in Earth’s history, deposits she calls ChUMPs.

While the ChUMP layers were building up, the Earth was going through dramatic geological and climatic variations. The ancient supercontinent of Rodinia was breaking up, and Dehler can see evidence of volcanism that accompanied this rifting as huge excursions in carbon isotopes locked up in the ChUMP deposits.

The carbon dioxide injected by these large eruptions probably also drove climate change, raising global temperatures, which caused rapid weathering of rocks that locked up the excess carbon dioxide in the rocks.

With so much CO2 sucked out the atmosphere, it’s possible the Earth then underwent rapid cooling, and this is what could have driven Earth into a snowball state during the Neoproterozoic.

However, the story told by the rocks and their isotopic compositions is complicated, and it’s not entirely clear whether the enhanced volcanism preceded the snowball Earth event: The ages attributed to the rocks still have uncertainties that are too large to be sure.

In any case, Dehler presented a compelling, complex story about the Earth’s ancient history, meticulously pieced together, one mineral grain at a time.

Found some beautiful basalt columns around Lucky Peak State Park just east of Boise. A quick google search doesn’t turn up any previous surveys, so these could make a good spot for some follow-on studies to our field work back in 2011.