Calcium Catastrophe

Generally speaking, a sudden drastic change in the chemistry of your environment is catastrophic. From bacteria to humans, there is a range of chemistry we can tolerate, and outside that range we tend to die.

I mentioned one major geochemical event last year, when free atmospheric oxygen first became common. That was a pretty catastrophic change for the living creatures (bacteria) who were adapted to the pre-oxygen conditions of the early earth.

Some time after that, another major geochemical event happened. Some researchers now think that this led directly to the cambrian explosion and to more complex life on earth. Even so, it was a catastrophic change—from the point of view of the creatures who didn't survive it.

Just prior to the cambrian explosion, there's a huge gap in the rock record (and fossil record) known as the great unconformity. There's a chance the cambrian explosion wasn't so explosive as it seems, and it may have taken place over a very long time and we just have no fossils for that part of earth's history.

An unconformity, and the great unconformity in particular, are formed when rock is weathered down for a while and old rock is exposed, then weathering stops and new rock is deposited on top, leaving a gap in time between old rock and new. The great unconformity is a 1.2 billion year gap, so rather significant. This doesn't mean weathering was happening for 1.2 billion years, just that weathering went on long enough to expose 1.2 billion year old rock before it started getting covered again.

As I've mentioned before, rock weathering puts lots of dissolved minerals into the water. The researchers studying the rocks of that era think that the unconformity, rather than being missing information, is itself information about the conditions on earth and how they changed. The minerals dissolved as the rock weathered had to go somewhere, and that somewhere is into the ocean all the early ocean critters lived in. (At this point, there were no land plants or animals.)

While the weathering was going on, the levels of many metals and minerals in the water would be rising. Some would chemically precipitate; limestone (calcium carbonate) for example is not very soluble, but even so it would come to a steady state concentration that would be higher in calcium than it had been before the weathering started.

Cells depend on maintaining a concentration gradient of many ions inside their cell walls, and atoms can diffuse through cell walls. So what happens to creatures suddenly bathed in an excess of minerals? I'm sure many died. Others, suggest the researchers, started managing the precipitation reaction and building hard structures in their bodies, such as calcium carbonate shells, as a way to keep the dissolved mineral concentration inside their cells below toxic levels.

One theory has suggested that evolution selected for larger size, and bony structures evolved to support that, but if this new research is correct, it could well be the opposite: that bony structures evolved as a way to deal with excess minerals, then evolution took advantage of that to build larger bodies using the solid structures for support. The common biominerals all appeared about this time, and are made up of minerals that would have been washing into the ocean due to weathering of the rock: calcium carbonate in invertebrate shells, calcium phosphate in vertebrate bones, and silicon dioxide in radiolarians, an entire class of creatures I had never heard of before today, and which have shells made of biological glass!

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