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Units of geological time

I have an exercise in my writing course on scientific units. The last question is about units of geological time, and it always starts a debate. I favour ka, Ma, and Ga for all dates and spans of time, but I've never gone unchallenged. People like Ma BP, mya, m.y., myr, and lots of other things, and I've heard all sorts of rules for when to use which, and why. The sort of rules you can't quite remember the crucial details of.

Twitter isn't for everyone, but I think it has some real strengths — it's a great filter, a reliable connection finder, and a brilliant place to ask questions. So I asked Twitter, and compiled the responses in a storyboard:

The story exposed a useful blog postan attempt to standardize (Aubry et al., 2009, Stratigraphy 6 (2), 100–105], another attempt [Holden et al., 2011, IUPAC–IUGS recommendation], and a firm rebuttal from Nick Christie-Blick. Many thanks to all my Twitter friends — one of whom I've actually met IRL!

Bottom line — there are regional variations and personal preferences. There's no consensus. Make your choice. Write unambiguously.

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Reader Comments (7)

You know, I asked a similar question to /r/geology a while back.

My question had more to do with the definition of a year for things like dating coral and tree rings.

The consensus seem to be year is a very vague term. Are you talking about calendar years, tropical years, or sidereal years. Corals in the Devonian had ~400 daily growth rings per year. *1

Whereas Ma is a unit of time that is exactly 8.64x10^10 seconds.

*1 Wells, J. W. Coral Growth and Geochronometry. Nature 197, 948-950 (9 March 1963). [LINK — Matt]

November 20, 2012 | Unregistered CommenterToastar

@Toastar: Those papers mention this issue, at least in passing. I think we're always dealing with the current 'standard' year, which is 31.557 Ms, or the length of the year in 2000 CE. I'm not sure about your number... according to Google, that's only 2738 years.

Reminds me... I wrote a nice post about those Devonian years once. Mindbending...

November 20, 2012 | Registered CommenterMatt Hall

whooops, I left out the 365.25

In any case, I'm not sure I like the definition of the the length of the year 2000 as those papers suggest. I prefer the abstract Julian Year

It's ISO Standard, and what the IAU uses to define light years. Also you shouldn't have a unit defined by two measurements.

Also with regards to BP, I think it's ok to include it if you are using a source based on radioisotope dating, As it indicates your measurement is calibrated to 1950.

November 20, 2012 | Unregistered CommenterToastar

Let's also give props to the nifty mnemonic forthe Phanerozoic time scale:


Using the first letter for each period, you get:

Can Tom see my pants pocket? Tom Jones can. Tom's queer.

Most likely you laughed. You're welcome. Now it's burned into your memory.

November 21, 2012 | Unregistered CommenterMr. Ultramafic

I clearly go for Ma, as M is the SI prefix for 10E6, and annum is in lower case.

About "Can Tom see my pants pocket? Tom Jones can. Tom's queer.", it would probably take me longer to remember it and remembering the stages/ages of the Cretaceous!

November 22, 2012 | Unregistered CommenterJorge

@Jorge: I agree — M for 'mega'. Lowercase 'm' is metres or milli, in my book. (Don't get me started on the oilfield custom of using M for thousands!)

As for the mnemonic... It doesn't help that it doesn't work! Just one of its deficiencies...

November 22, 2012 | Registered CommenterMatt Hall

In my opinion it is Ma for million years ago and Myr for the time period X million years, where a capital M is used to signify the prefix mega.

November 22, 2012 | Unregistered CommenterJack

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