Talk:Timetable of the Precambrian

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[edit] 4592.1 Ma???

Never ever heard such a number before, every other source says Sun is about 4.6±0.5 Ga old, that oldest meteorite is about 4.56 and so on. Any source for 4592.1 Ga (please)? Or is it just an extrapolation back from 4567.17 Ma, assuming a certain contraction time and planetary formation time?? Rursus 23:09, 18 October 2006 (UTC)

OK, I'm going to explain a little myself. Now, the magical number 4567.17 Ma often cited (which is much too precize) refers to age of CAIs in meteorites. Isotopic Ages says 4567.2±0.6. Other numbers for CAI ages are 4566±2 (Allende chondrite), and 4564.7±0.6 (Acfer 059). So lot's'a solsys boulders were formed about 4565 Ma ago. (Question: is this the main value or like the oldest known?)
Rursus answers Rursus: that site explicitly searches the oldest known. Rursus 13:12, 19 October 2006 (UTC)
Now according to isotopicAges again, the CAIs where formed in a protoplanetary disk, which according to en.wikipedia itself never are older than 25 Ma. So: 4567+25=4592!! Voilá, kind of ... B-/ But I protest - this is an unjustified leap!! 25 is max age of proplyds. Let's assume proplyds slowly decay from the influence of Sun and planetesimal formation (this is kind of bold too), then the most CAIs must have been formed some time within the range of 0 Ma and 25 Ma after the formation of the proplyd. Let's boldly assume that the median of CAI ages are f.ex. 10 Ma after the proplyd formation. Then we get 4565+10=4575, but this figure is created by speculating twice. 4575 Ma is an ill grounded number to use - don't use it - 4592 Ma is far worse! Rursus 07:16, 19 October 2006 (UTC)

While nagging around and being generally inconvenient, I'll somewhat sadistically feel an urge to add another objection: the figure 4567.17 Ma, does it really refer to the age of Earth? It certainly refers to the measured age of Efremovka CAI, one of the boulders measured - but Efremovka is a carbochondrite meteorite, Earth is a planet that may or may not have been created as much as 50 Ma afterwards, if we adher to the modern theories of protoplanet "oligarchic growth" (a modern and good theory - but yet just a theory!). Let's be pessimistic, saying that 50 Ma was the delay before Earth was created, and we get 4517 Ma, which is in disaccord with the Wikipedia time for creation of Moon 4533 Ma, or the one I found on the net 4527 Ma. But either way: Earth was probably formed in the time interval 4567 to 4527, but when?? Rursus 08:06, 19 October 2006 (UTC)

Answering myself again: the isotopicAges site presumes that Sun is created about the same time as the earliest CAIs. The age of Earth isn't mentioned -- except implicitly as lesser by implication. Rursus 08:51, 19 October 2006 (UTC)

[edit] Comments

[edit] Date of the Theia impact

I took the date 4533 Ma from Giant impact hypothesis. Before that, Timetable of the Precambrian said 4537±10 Ma. Anthony Appleyard 06:15, 17 July 2006 (UTC)

Sad to say, Giant impact hypothesis doesn't source the date, and I can't find a date (other than early in the Earth's history) in the References and External links in the article. I'll look around some more when I get a chance. -- Donald Albury(Talk) 12:02, 17 July 2006 (UTC)
Dalbury said: Giant impact hypothesis doesn't source the date. Rursus replied: fixed! This link! (Ananova). It says 4527 Ma. Rursus 13:58, 19 October 2006 (UTC)
This USGS site says the oldest rocks brought back from the Moon are 4.4 to 4.5x109 years ago, and the oldest dates for Zircons found in sedimentary rocks on Earth are 4.3x109 years ago. That's certainly consistent with 4.5x109 or more years ago for the age of the Earth, but doesn't support anything more precise. -- Donald Albury 23:30, 19 October 2006 (UTC)
Acknowledged -- but much better than that I believe we cannot get, until humankind travel to Mars or Mercury, and unambiguously succeed to associate a meteorite with the Giant Impact and date it properly. Qaþ: Rursus 12:11, 20 October 2006 (UTC)
I object to the precision of the date given (6 significant digits). For something like this, I don't think we can be confident in more than 2 or 3 significant digits. I think we would be better off saying that internal evidence (the zircons) places the age of the Earth at more than 4.3x109 years, and that rocks brought back from the moon give it an age of 4.4x109 to 4.5x109 years, which supports an age for the Earth of 4.5x109 to 4.6x109 years. -- Donald Albury 01:47, 21 October 2006 (UTC)
100% agreed! More opinions on this? Otherwise User:Dalbury and me may go and cut away misleading decimals. 3 will IMHO be perfect. Rursus 15:08, 23 October 2006 (UTC)

[edit] Possible mnemonics

Possible mnemonics are:-
For the initial letters of the names of the periods:-

Some really outsize snakes can easily swallow turtles: comes easy.

For the root meanings of the names of the periods:-

Iron flowed from the mountains to the stable, covered its extension by a narrow stretch at the cold eddy.


Anthony Appleyard 18:04, 4 August 2006 (UTC)

[edit] Arrangement of the Timetable of the Precambrian page

This is the first time I've looked specifically at a page on the Earth Sciences portal. I was a little confused at first to see this page arranged with the oldest event at the top of the page. Geological convention would have the youngest at the top and the oldest at the bottom, to reflect Charles Lyell's Principle of Superposition in stratigraphy. I hesitate to suggest we should invert the order, on the basis that I might open a can of worms on numerous other pages - but one only needs to look at a stratigraphic column (see diagram in Table of geologic time section on that page) to see that it is the way a geologist would present such information. (user - dyvroeth).

It's true that the strat column forms from bottom to top. But it seems to me there's an argument for keeping the order as it is. At least in most Western languages, we write and read from the top to the bottom of the page, and this is the order that we expect the narrative to unfold from the past toward the present. We could go from the present (or in this case the Cambrian-Precambrian boundary) back to the origin of Earth, but many would find that confusing.
Cephal-odd 04:41, 15 October 2006 (UTC)

[edit] Animal tracks from 2000 Ma?

This line of the article caught my eye: "2000 Ma: Animal tracks in west Texas; first evidence of multicellular life." This would be a remarkble discovery if true. There have been claims for the discovery of burrows from over 1000 Ma, but as far as I know they're contentious. Does anyone have a reference for the west Texas tracks? Cephal-odd 04:48, 15 October 2006 (UTC)


Good question, I looked at that and did a double take myself... Jcforge 15:59, 6 December 2006 (UTC) From what I've found life seems to have 'started' only a few hundred million (to a billion) years after the earth was formed... which would be close to 2000 mya.... as for 'tracks' it's probably meaning traces of fossils that indicate life... it does seem misleading...

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