Gamma ray bursts (GRB's) and the Ordovician Mass Extinction

[jim mcnamara]

99% of all marine fossil generating species on Earth that ever existed have gone
extinct. There are periods in the fossil record that show massive extinction
rates. For example, the K-T (now the K-Pg) boundary marks the extinction of
dinosaurs in the fossil record along with marine animals, at the Cretaceous-
Paleogene boundary.

The cause of the major mass extinctions varies. And there has always been
debate because geologic evidence can be subducted by plate tectonics, for
example. Things change over 250 million years. So naming a culprit is difficult
even when there is geologic evidence. In the case of K-Pg, current thinking has
the smoking gun as a large meteor impact, preceeded by a long period of a major
volcanic outflow at the Deccan Traps. A double whammy.

My question is about the Ordovician-Silurian extinction event ~425mya, two major
events very close together, so it's another possible double whammy:
http://en.wikipedia.org/wiki/Ordovician–Silurian_extinction_event

(Note the graphic at the top of the article. Off topic: hidden in there is
a ~62my cycle of recurring ME's.)

The primary cause was an ice age, but there is a proposed extra-terrestrial
culprit: a gamma ray burst.
http://en.wikipedia.org/wiki/Gamma-ray_burst
http://arxiv.org/abs/astro-ph/0309415

This is posited because the distribution of extinctions was very largely in
genera that lived near the surface of the ocean. And it happened quickly
geologically. IMO, it is also interesting to consider. I believe the Mellott
paper started consideration of GRB's for the culprit.

GRB's apparently occur only for a short period after some types of
massive stars implode.

Let's assume the culprit object was within 6000ly of Earth 425mya. The
search radius will have increased over time.

What kind of object would a viable candidate now be?

Is it currently possible to analyze existing Chandra survey results for possible
candidate objects?
http://www.nasa.gov/mission_pages/chandra/main/index.html#.Uim9sTaUSwA

Is this even a remotely feasible path to take to provide some sort of evidence for the GRB hypothesis?

 

[mfb]

Typical relative velocities for stars are ~20km/s or more. An object that has been close to Earth 400 million years ago can be on the other side of the galaxy now (20km/s*400 million years = 27000 light years).
Apart from observing the object (which is challenging by itself), you would need a precise measurement of its 3-dimensional velocity and an estimate for its age. I don't see how this could work.

Local evidence (means: in the solar system) is probably a better approach.

 

[jim mcnamara]

What local evidence does a GRB leave behind? Its effect is surficial. It damages the atmosphere, outright kills exposed organisms, and effectively removes most of the ozone cover on the cooked side of Earth, allowing lethal UV penetration to the surface. It also oxidizes free nitrogen, altering the albedo.

I personally do not see a way to establish the GRB hypothesis. Why then propose a non-verifiable hypothesis?
Thanks.

 

[mfb]

Why then propose a non-verifiable hypothesis?

It has an impact on Earth - the mass extinctions are certainly an effect humans can (and do) study. Variations in the chemical composition of things might be detectable. I would also expect that GRBs can produce some radioactive isotopes, but I have no idea if some of them are long-living enough to get detected today.

Do you have a better hypothesis? Maybe even so good that the GRBs are not considered any more? Publish it.

 

[phyzguy]

Since even the "long" gamma ray bursts last only seconds to minutes, it is hard to see how they could cause mass extinctions over the entire surface of the Earth. The strong correlation of mass extinction events with flood basalt events (like the Deccan traps and Siberian traps) has always seemed to me to be too strong a coincidence to ignore, so I think these events must play a role in the mass extinctions. However, as the extinction events get older and older (like the Ordovician event 425 mya), it gets harder and harder to pin down causes.

 

[jim mcnamara]

The current hypothesis is glaciation, for which there evidence. (I assume) Mellott proposed GRB's because of the dramatic short duration second spike, which happened ~5 million years after the initial glaciation.

 

[phyzguy]

The current hypothesis is glaciation, for which there evidence. (I assume) Mellott proposed GRB's because of the dramatic short duration second spike, which happened ~5 million years after the initial glaciation.

But how does a gamma ray burst wipe out the organisms which are on the half of the Earth which is not exposed to the burst? This is what I have trouble understanding.

 

[mfb]

Did you check the linked wikipedia article? GRB can change the chemical composition of the atmosphere, and that will affect the whole earth.

 

[phyzguy]

Did you check the linked wikipedia article? GRB can change the chemical composition of the atmosphere, and that will affect the whole earth.

I read it. The hypothesized chain of events is, in my opinion, highly speculative and highly unlikely. I doubt that depleting 25% of the ozone layer would be deleterious enough to cause mass extinctions, and I doubt that the hypothesized photochemical smog could be dense enough to cause extended darkness.

 

[Chronos]

I tend to agree with phyzguy, a GRB would not instantly sterilize the entire surface of the Earth [albeit the after effects might]. A nearby supernova would appear to be a much more likely suspect. SNI supernova typically leave no remnants so the logical thing to look for would be SNII remnants - i.e., neutron stars. Calvera, estimated to possibly be as near as 250 light years, is is one example and at least 7 other neutron stars are known to exist within 500 light years. 450 My is a long time and neutron stars tend to move more quickly than most stars, so any of these could have been neighbors 450 My ago. It is guesstimated an SNII supernova would need to be around 30 light years distant to pose a serious mass extinction threat.

 

[phyzguy]

This correlation between flood basalts and mass extinctions is what I was referring to. It doesn't go as far back as the Ordovician event, but as we go further back in time our knowledge gets poorer. There could have been a flood basalt associated with the Ordovician event that we just don't know about.

As an example of how difficult it is to piece together what happened >400 Mya, consider the fact that basically all of the ocean floor from this period, which is ~75% of the surface area of the Earth, has been recycled since then, and is no longer available for study. So some major event, like an asteroid impact or a flood basalt, could have taken place in what was then the ocean floor, and been subsequently subducted so that we would have no evidence of it.

 

[jim mcnamara]

You have hit on the whole problem. Here is an example discussion on the 62 million year mass extinction cycle.

http://www.lpi.usra.edu/meetings/lpsc2005/pdf/1009.pdf

Summary:

Over the past 500 million years,
great terrestrial cataclysms forced global mass extinctions. A detailed study of postulated extinction mechanisms has lead to insight on both timing and cause.
Two primary mechanisms, Oort cloud comet impacts
and nearby supernova events, are believed responsible.
A dual cycle of extinctions is observed and well ordered in geological time. Both mechanisms are synchronized to the passage of the solar system through
the spiral arms of the Milky Way galaxy.

The perpsective ignores ice ages, and volcanism. The above perspective Phyzguy cites ignores deep impacts from extraterrestrial objects. And the issue is, IMO, you cannot ignore possibilities or dismiss them with some weasel words. You have to test if possible.

For example. The late heavy bombardment clearly would have caused the destruction of any existing biotic/prebiotic environment. But there there counterexamples, indicating that the LHB is a sampling anomaly from Apollo missions samples in a small lunar surface area, which data contributed to a badly skewed LHB model.

The Sudbury Ontario impact ~2bya certainly would have been an environment altering event.

For fun, here is a an impact database, you can find the data on the Sudbury impact in there.
http://www.passc.net/EarthImpactDatabase/index.html

The point of all of this: ice ages, volcanism, plate tectonics, and extraterrestrial impacts all occurred.
We have debate on these. The other proposed events can be modeled but finding evidence is really not feasible now. Maybe never will be. However I see discussions in on PF about testability of various models. This falls into the same category.

 

[Jon B]

My understanding is that gamma bombardment and atmospheric conditions have their own global climate change effects that last far longer than geological and extraterrestrial disturbances. 3 years of winter in Europe is nothing compared to the shifting of average global temperatures by as much as 20 degrees over the millions of years.