Two Ordovician Papers

I read a couple of papers dealing with Ordovician period geology and paleontology.

Evidence suggesting that earth had a ring during the Ordovician, Andrew G Tomkins, Erin L Martin, Peter A Cawood, Earth and Planetary Science Letters, 646 (2024)118991

Understanding the Great Ordovician Biodiversity Event (GOBE): Influences of paleogeography, paleoclimate, or paleoecology?, Thomas Servais, David A.T. Harper, Jun Li, Axel Munnecke, Alan W Owen, Peter M Sheehan, GSA Today, April/May 2009

I didn’t read these as closely as I have some other papers, so I’ll combine my thoughts on the two papers in one post.

A ring?

Tomkins, Martin and Cawood created a splash with their paper This is the paper that suggests the earth might have had a Saturn-like ring of space debris.

The starts with something that’s really not disputed, the L-Chondrite parent body breakup about 466 MYA. The resulting increase in dust infall to the earth has been proposed as a trigger for the Hirnantian Glaciation, which might have caused the end Ordovician extinction

The next steps are the interesting parts.

First they claim there’s a spike in impacts during the L-Chondrite dust infall period. They did a lot of work on how much of earth’s crust could show craters and so forth from the Ordovician, and count how many craters exist to try to show such a spike.

Then, the authors consider multiple models of earth’s continents during Ordovician and tally up the ages of crater and impact structures. They decide that almost all craters and impact structures of the right age hit the earth near the equator around 460 MYA. Good, there’s an increase in L-Chondrite dust, and a boatload of impact structures that got formed by material arriving from at or near an equatorial orbit.

Next, they calculate a probability that the impacts occurred by meteorite arrivals from an equatorial plane, or randomly. They assume the latter would be how asteroid fragments from the L-Chondrite parent body would arrive. They find that the probabilities indicate arrival from equatorial orbits.

The fact that the Moon and Mars don’t show evidence of a similar Ordovician meteor storm is very interesting, and to me, more convincing than the probability calculations. On the other hand, this may also invalidate the Ordovician “impact spike” this paper’s authors worked so hard to validate.

The existence of material in an equatorial orbit around the earth is hypothesized to have resulted from a very near miss between the earth and a larger fragment of the L-Chondrite parent body. If this fragment came with the earth’s Roche Limit, tidal forces would tear up the fragment. I buy the Roche Limit stuff. This is just stress analysis of a large body. It’s now known that many asteroids have the tensile strength of cotton candy.

What I don’t quite get is how much of a large asteroid fragment, further ripped apart by tidal forces inside earth’s Roche Limit, would end up in earth orbit. Does enough kinetic energy get absorbed by the ripping up that at least some sub-fragments get captured by the earth?

All considered, an intriguing paper, that supports its hypothesis, and has fascinating speculation.

This paper also has an interesting bibliography, mixing paleogeography, geology of impact structures and orbital mechanics.

Great Ordovician Biodiversity Event - GOBE

Understanding the Great Ordovician Biodiversity Event (GOBE): Influences of paleogeography, paleoclimate, or paleoecology? is a summary paper appearing in GSA Today. “GSA” stands for Geological Society of America. This is a survey of more specialized work for somewhat less specialized geologists.

This paper makes a whole lot of sense, tying together paleogeography and the fossil record to show that a biodiversity event isn’t simple to define, or cause.