Saturday, April 17, 2010

Apocalypse past

Freezing over and burning up ... it's all there in the geological record.

A recent issue of science had an interesting pair of geology articles, one reviewing knowledge about the dinosaur-killing asteroid of 65.5 million years ago, and the other reviewing whole-earth glaciations of ~700 million years ago, in an era now dubbed the Cryogenian. In short, fire and ice.

I don't have much new to write about either event, but want to bring up interesting points of each and try to increase appreciation of the dynamic earth we live on, whose future livability can not be taken for granted.

The first article, on the 10 km diamter asteroid that hit the Yucatan peninsula, is a review that ties various pieces together to solidify the consensus that the dramatic extinctions of this time were indeed caused by the impact, as were a very distinctive set of geologic layers all over the world. Alternately, extensive vocanism occurring in India around this time over about one million years might have begun the environmental destruction. Yet the authors point out that the asteroid lofted a thousand times the sulfur and other debris into the atmosphere that volcanism did on a yearly basis. There simply is no comparison.

They present a fabulous pair of figures, one mapping sampled sites all over the world, with diagrams of typical geological profiles, and a second with one closely annotated core taken from the western Atlantic.

(Click to blow this up)

The green wave on each profile in B is the iridium signature of this asteroid impact- an element that is extremely rare on Earth and is presumed to have come directly from the asteroid and been deposited all over the world in a diagnostic geologic layer.

The "graded clastic unit" of the first panel in B is debris from the impact, pure and simple, quickly overlain with the dust and ashes that were lofted and gradually settled back down. The latter is all one sees in the farther-away sites (distal) which feature the "ejecta spherules", "Ni-rich spinels", and "shocked minerals", each of which are further impact signatures. The spherules are typically tiny sprays of molten rock from the impact site, the nickle is directly from the asteroid, and the shocked minerals are typically very hard minerals (quartz) from the impact site that were not melted but only scarred and turned into dust by the impact.

(Click to blow this up)

This second figure shows one core from drilling point 207 in the first figure, off Guyana, South America. The first thing to notice is the total break between the bottom and top parts of the sedimentary record. Usually, this would be due to a break in the geologic time line, where the bottom unit had been raised up and eroded for some time before being buried again and overlaid with a different type of sediment. Not here! The break represents continuous stratigraphy during a real change in materials being deposited, from calcium rich before the inpact, through the impact episode, to low-calcium shale above. They do not comment, unfortunately, about the details of this macroscopic difference.

In the middle is 1 cm or so of material from the impact event, mostly rubble and other dust (with spherules, "SP"), topped with a lighter melange blown up in B. This image is a doubtless an X-ray spectrum micrograph false-colored with various elemental compositions (purple- calcite, blue- dolomite, and red, quartz). Each of these minerals comes from the impact site, and panel C shows the cracked nature of the "shocked" quartz "Q". Unfortunately, I don't have the expertise to know what the tan matrix is here.. probably sandstone composed mostly of feldspar- various silicon oxides that make up garden-variety rocks.

This paper would not shock most people, but there have been persistent controversies in the field as various critics tried to come up with alternative scenarios. Catastrophism vs gradualism continues to cause tension in studies of Earth's history! One scare in the literature sampled many organisms through the event horizon and concluded that most of them were declining precipitously well prior to the asteroid impact. Later criticism showed, however, that this conclusion was false, due to statistical errors in sampling that didn't properly account for a sudden boundary of zero fossils combined with spotty deposition up to that point- a boundary that the typical paleontologist (and her software) would interpret as gradual decline of organisms up to that point.



The other paper of interest describes better dating of events around the period of "snowball Earth"- very roughly from 600 to 750 million years ago- just before the rise of eukaryotes / animals when the Earth was apparently covered with ice sheets down to the equator. Whether the oceans were iced-over as well is still controversial, but the idea is that once glaciers cover a certain proportion of the land, increased solar reflection drove the process to completion at least on land, resulting in a totally locked-in condition for millions of years.

That condition was only reversed when volcanic emissions drove enough CO2 into the atmosphere to cause a run-away greenhouse effect. CO2 is usually eaten up on geological time scales by mineral erosion- combining with calcium and other elements into carbonates (limestone, marble, dolomite, etc.). With the earth covered with glaciers, there was no air-exposed erosion, and CO2 built up without hindrance. The cold temperatures would have inhibited carbon fixation by oceanic photosynthesizers as well. This part of the process is quite well-understood.

Less understood is why the glaciations happened in the first place. The going theory is that the tectonic plate organization at the time happened to be propitious, with all the land masses at middle latitudes. This made erosion particularly active, driving CO2 levels down, (there's that CO2 again- always a problem!), and also increasing the reflectivity of the planet once snow began to accumulate on the land. Having so much land near the equator made the climate very unstable, allowing icy conditions to reach a critical tipping point. I'd also note that at this time, land was not clothed with plants and greenery as it is today, leaving it uniformly open to erosion.


Model of paleogeography circa 750 million years ago.

In detail, these authors work to more closely pin down two separate episodes of long-term, global glaciation, using a lot of uranium/lead mineral dating. The Sturtian was around 716 million years ago, and they synthesize evidence from other workers to say that it lasted at least 5 million years. That is quite a long time! Catastrophic for anything living at the time. This was repeated in the very similar Marinoan glaciation about 635 million years ago.


The figure gives their interpretation of the timing of this era, combined with estimates of when various eukaryotic lineages arose on the right side. All of the lineages before 700 million years ago are microscopic. The metazoa/animals only arrive in yellow at the far top right (indeed, the closest linage to us of all these is the fungi). The dashes symbolize missing data- the time between the two great glaciations is practically devoid of fossils, for instance. Only afterwards do eukaryotic macro-organisms begin to occur, in the Ediacarian epoch.

Some of the largest organisms from ~750 million years ago, predating the Cryogenian. Eukaryotic protists of some sort.

The key data they offer is the carbon isotope ratio between carbon 13 and carbon 12 in the middle graph, which flips wildly between high prior to glaciation and super-low during glaciation. This ratio, drawn from studies on carbonate minerals, (absent from the period of the glaciation itself, understandably enough), is a bit difficult to interpret, unfortunately. It seems to track biological productivity, since organisms tend to fix light carbon (12) in their metabolism in preference to carbon 13. High levels in geologic deposits correspond to periods when a great deal of carbon was biologically removed from the atmosphere, leaving higher ratios of 13C to 12C, then incorporated into carbonate minerals. Deeply negative values, as seen here at the boundaries of these glaciations, indicate low biological fixation and perhaps release of stored 12C-rich carbon from organic burial/storage, as is happening again today as the atmospheric 13C ratio is declining steeply.

The asteroid impact is well-known, but the whole-earth ice ages prior to the rise of multicellular life are not as well known, making them (possibly) interesting to readers. Both show the dynamic nature of earth's geologic and climate system. The ice ages especially accentuate the role of CO2 as the controlling greenhouse gas. With the current anthropogenic CO2 emissions and climate change, we are not going anywhere the earth has not been before at some point, but we are going to places where humanity has never been before, and going there at (geologically) breakneck speed.

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