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How should geologists abbreviate time? GSA wants to know

Posted by Kim Hannula on September 21, 2009
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I confess that I didn’t know the geological conventions for abbreviating time until I started teaching the geology writing class and looked them up. (That’s despite having published a paper on argon-argon geochronology. Sometimes, just doing whatever the co-authors and reviewers say is the right thing to do…) So I don’t have any strong opinions about how to abbreviate lengths of time. But if you do, the Geological Society of America wants to know.

Short version: traditionally, geologists have used different abbreviations for ages (time before present) and duration (amount of time elapsing between two different events). Ages are abbreviated from Latin: Ga (giga-annum) is a billion years, Ma (mega-annum) is a million years, ka (kilo-annum) is a thousand years. Duration, on the other hand, has been abbreviated from “years.” (I’ve seen it abbreviated in a couple of different ways, actually, and I don’t have my USGS style guide to see what the recommendations there are.) There’s a move to use the same abbreviations for both: Ga, Ma, ka, etc., but there’s disagreement. (The main argument for using Ma to represent durations is that geologists are used to thinking about numbers meaning ages: 70 Ma refers to a time in the Cretaceous.)



My take: I don’t have a strong opinion. It takes practice to make the distinction, and my students tend to ask why we need two units, and I don’t really have a problem telling ages from lengths of time. (I can usually handle subtraction, though it’s more difficult when I’m dehydrated at the end of a long day in the field. Pretty much everything is more difficult when I’m dehydrated.) But I’m willing to be convinced.

So what do you think? (If you’ve got a really strong opinion, GSA is taking comments. But I’m happy to listen if you want to lay out your favorite argument here, too.)



(Note to physicists: 10^whatever seconds just is not going to work, or we’ll have to start working in logarithmic time.)

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Comments

  1. #1 Lab Lemming
    September 22, 2009

    That seconds comment is not funny- just try calculating ages with second-based decay constants (and everything else in years) and see what happens…

    As for the subject, if we were to adopt the IBBQWTF suggestion, and use Ma for everything, future geologists would get really confused when reading current papers, unless they were specifically told that Ma used to mean something else.

  2. #2 Robert Grumbine
    September 22, 2009

    Speaking of physicists, for your amusement, I’ll mention that they are talking about logarithmic time. The ordinary second is not fundamental in any physics sense. Fundamental would be the Planck time (about 1e-40 seconds). So then, the idea goes, base the rest of your time scale off that. As it leads to very large numbers long before you reach a second, the suggestion goes, use the base 10 log of the time and you have an ‘easy’ system to work with.

    As to the question, when I was working on Pleistocene paleoclimate (itself several eons — in net time — ago), it seemed the norm to use kya, Mya or ka, Ma for when things happened, and ky, My for durations.

  3. #3 BrianR
    September 22, 2009

    I like using “Ma” for dates and “m.y.” for duration … I don’t see it as overly confusing. Using “Ma” for both could get problematic … especially if the durations and ages are close to the same number. But, I should read the GSA comments to get a better sense of the arguments either way.

  4. #4 Kim Hannula
    September 22, 2009

    Lab Lemming:

    I hope that scientists develop a sense of units (and the need to do unit conversions as appropriate during any kind of calculation) long before they start calculating their own ages. But I guess if unit conversions can mess up rocket scientists sending a probe to Mars, they can mess up geoscientists.

  5. #5 Anne Jefferson
    September 22, 2009

    I’m one of the ones who would happily use Ma for everything if the rest of the world would let me. I think writing should be clear enough that the context should tell the reader whether an age or duration was meant. Also, I’ve found the conventions about how to shorten years (duration) to be incredibly variable from journal to journal and subfield to subfield. For example, a query of how to abbreviate radiocarbon dates in thousands of years before present resulted in 5 different answers from ecologists and geologists. That just reinforces my desire to have one simple consistent set of abbreviations (ka, Ma, Ga).

  6. #6 Lab Lemming
    September 22, 2009

    Well I spent half an hour the other day trying to figure out what the heck columbium was, so nomenclature change is definitely an issue.

    I mean, if they refuse to retire “Tertiary” for historical reasons, but do away with myr, what is up with that?

  7. #7 Robert
    September 22, 2009

    John McPhee in “Annals of the Former World” sometimes gives dates in millions of years but without any units of time. The units are implied without being stated.

    In other words: “… by 1700 or so the dockings of the arcs were complete.”

  8. #8 BrianR
    September 22, 2009

    The comment I left on the GSA site mentions that if we are going to be this strict w/ these terms then rates that used to be “cm/yr” would have to be “cm/a” … I can’t see anyone wanting to use and/or enforce “cm/a”.

  9. #9 Lab Lemming
    September 23, 2009

    Ka etc. is irrelevant to radiocarbon because radiocarbon years are not annus years- they are their own unit of measure, which need to be converted to calendar years via some other translation.

    To a letter extent, this is true of all geologic time systems; I measure things in 238U decay years. The Re, 235U, Th, Lu-Hf and Sm-Nd systems are defined relative to the 238U system (I think), while the Rb-Sr system is defined relative to K-Ar. So argon years and uranium years are not necessarily the same, although they have been well reconciled. Non-radiogenic time scales, like biostratigraphic, mag reversal, seasonal, various cosmogenics (incl 14C) are all their own systems. But none of them are absolute SI units. They are all year approximations based on the assumptions that underpin the methodology.

  10. #10 M-MML
    September 23, 2009

    Astrophysics is the other field that has to think about things happening a long time ago, and we very uniformly use Myr or Gyr for both time before present (13.7 Gyr ago) and duration (one rotation of the Milky Way takes 250 Myr).

  11. #11 Lab Lemming
    September 25, 2009

    (off topic, but cool)
    Hey Kim, seen this recently:
    http://www.minsocam.org/MSA/collectors_corner/arc/pressure_shadows.htm