Meteorite Monday: The Middle Ordovician Meteor Shower

No, I didn’t make that name up nor is it the name of some 50’s sci-fi show. The name refers to a time period from nearly 460 million years ago when the earth did look quite alien. It was a time when the continents as we know them weren’t in their present positions and life was dominated by trilobites, snails, clams and other seafaring creatures. The Ordovician occurred right on the heels of the Cambrian period when life first started to make it’s appearance in the fossil record.

The position of the continents during the Ordovician.

The position of the continents during the Ordovician.

At around the same time, another important event was occurring in the solar system: the break-up of the L-chondrite parent body. An L-chondrite is a type of stony meteorite that is relatively common and has a low abundance of iron. Most of the L chondrites are heavily shocked and share other chemical commonalities. This has led researchers to hypothesize that L chondrites came from the same source. With this break up of the parent body came an influx of meteorites that would impact the earth; some of which would land in shallow water among the sea creatures of the Ordovician. Those creatures would die off and become the limestone layers in which those meteorites would be encased.

A fossil meteorite and nautiloid encased in limestone. (Image from Schmitz et al., EPSL 194. 2001. P. 4)

A fossil meteorite and nautiloid encased in limestone. (Image taken from Hawaii Institute of Geophysics and Planetology. Originally found in Schmitz et al., EPSL 194. 2001. P. 4)

This particular image came from a group of researchers that examined a limestone outcrop from a quarry in Kinnekulle, Sweden. They analyzed the distribution and size of meteorites in multiple layers of limestone in order to constrain the influx rate of the meteorites from the parent body break-up. In total, the researchers retrieved 40 meteorites from about 3 meters limestone, or about 10 feet.

What’s really interesting is that not all layers yielded an equal amount of meteorites, nor were they of equal size. Some layers only had one, while another had six. One layer, the Arkeologen, put out 26 meteorites! What this tells us is that we had multiple falls from possibly the same event. And keep in mind, these fossil meteorites were found in a relatively small area. Of the known 3300 m^2 of the Arkeologen,only about 2700 m^2 of it has been searched for meteorites.  Stratigraphically, we can assume that more meteorites could be recovered from the formations across a broader area. They were able to use this information to determine that there was an increase in meteorites delivered to the earth at that time.

One of the difficulties with pairing meteorites from this location is the extreme calcification that occurs from being trapped in limestone. Meteorites tend to crack and weather easily on the earth. This can cause internal alterations, such as oxidation, that chemically alter the meteorites. However, Schmitz et al., were able to use relict chromite grains as a way to possibly link them to the same parent body.

 

Sources:

  • A rain of chondritic meteorites in the early Ordovician. Schmitz, Birger., Tassinari, Mario., Peucker-Ehrenbrink, Bernhard. Earth and Planetary Science Letters. Vol. 194. Issue 1-2. P. 1-15. DOI-  http://dx.doi.org/10.1016/S0012-821X(01)00559-3
  • The Ordovician Period- University of California Museum of Paleontology.
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