Talk:Geochronology

From Wikipedia, the free encyclopedia

Earth sciences Portal
Geochronology is part of WikiProject Geology, an attempt at creating a standardized, informative, comprehensive and easy-to-use geology resource. If you would like to participate, you can choose to edit this article, or visit the project page for more information.
Start This article has been rated as Start-class on the quality scale.
High This article has been rated as high-importance on the importance scale.

Contents

[edit] Nanocycles

I've removed the Nanocycles method addition. It isn't a recognised 'dating method' as described in the article. It could be considered a type of incremental dating technique, but these are usually based on periodic addition of material through a continuous or correlated sequence(s). However, it would be fair to add something about variations in lunar cyclicity and the age of deposits, but this isn't usually referred to as the Nanocycles method. See Talk:Nanocycles method. Any ideas? NickW 09:04, 25 May 2005 (UTC)

If there is another name that it is well known by then it might be OK. However I am fairly sure that in the west it is indeed not well known (evidence below). However Prof Afanasiev has made numerous studies of geological formations and achieved results that look remarkable to me. I know from my cycles studies relating to other disciplines that his results are good.
One particular interesting result is the Elatina formation extensively reported by George Williams. It shows a 12 year cycle modulated by 300+ years. It was suggested to be sunspot cycle and also a lunar monthly cycle varve rather than annual varve cycle (to get the 12). However the given date of the deposit is consistent with my nanocyles accurate determination of 658.24 million years ago. Search for "Long geological cycles, nanocycles method, lunar node and Elatina cycle cause" and "Long geological cycles 2" in usenet with google for more info. The fact that no-one other than me has noticed this suggests that the method is unknown in the west. Ray Tomes 23:58, 7 Jun 2005 (UTC)
I would like to see the nanocycle method entry put back. Over the next couple of months I hope to put some translations of Afansiev's work on www in English, and include tables etc. Ray Tomes 23:58, 7 Jun 2005 (UTC)
Just want to add that it is not an incremental method. The periods in any formation have a unique pattern due to changes in the lunar orbit over geological time. There is no need to link to later patterns to get an accurate dating. Ray Tomes 11:11, 8 Jun 2005 (UTC)
Unless this method is a widely used and verified technique then I don't think it should be included in the article. Why? Because it would be difficult for a layperson to appreciate the relative prominence / usefulness of the methods listed. If nanocycles do eventually revolutionise geochronology, then fine, let's stick it in.
One point I would like to stress is that laminae within varves can reflect NUMEROUS hydrometerological processes. It seems the nanocycles method relies on an exclusive genetic link to lunar influence. While this may be the case on occasion, in the majority of cases it isn't - nature just doesn't work that way. As evidenced by the many modern studies of varve thickness variations v. hydrometeorological parameters. Consequently, the nanocycles method, even if it works for the Elatina formation, is unlikely to be universally applicable. NickW 10:53, 11 Jun 2005 (UTC)
I have had a friend who speaks Russian assisting with translation and an astronomer checking my interpretation. When I am done I will put an article on the web and post a reference to it. It will make clear the method that Afanasiev uses. I will also show several other interesting things, such as that the Elatina formation reported by G Williams and discussed extensively is in fact neither the sunspot cycle nor a monthly varve cycle, but is exactly as expected by Afanasiev's explanation and able to be dated extremely accurately. Ray Tomes 04:34, 18 Jun 2005 (UTC)

[edit] Th-Pb

Removed ref to Th-Pb dating. To the best of my knowledge Th-Pb is always done in conjunction with, and subordinately to, U-Pb dting and is very unlikely to every require an article of it's own. Unless I'm wrong, of course. Feel free to change back if so. Cheers,Rickert 04:12, 10 February 2006 (UTC)

That is not strictly true, for instance Monazite contains large amounts of Th and relatively little U and is an attractive target for so called "chemical dating" using the elemental Pb/Th ratio. But frankly there is so much to be done in the geochronology-related articles here that I think there is little to be gained by listing lots of obscure techniques when the big ones (most notable U-Pb) have not yet been addressed. I wish I had the time! Actinide 00:58, 12 February 2006 (UTC)
Hmmm, I have never done monazite EMP dating myself, but unless I am incorrect both U and Th (and Pb) abundances are measured and used to calculate an age because the decay of U provides non-trivial amounts of Pb. I understand that it is possible to do with just Th and Pb but in practice, U is used as well. For example, the methodology of the U-Mass folk (e.g., Pyle et al. 2005, Am. Min. 90 547-577) uses U abundances. I imagine that it is analagous to the fact that while it is possible to calculate 232Th-208Pb monazite ages using TIMS, generally speaking the U-Pb isotopic system is used as well. Please let me know if I am incorrect.
And I agree that the lack of a U-Pb article is a little sad...I may have some time in April to start one up.
Cheers, Rickert 02:42, 12 February 2006 (UTC)
You're right of course. I guess I was mostly responding to your "subordinately to," and was probably confusing things I'd heard about CHIME (which I have no experience with either) with Laser-Ablation ICP-MS dating of monazite in which I had understood (again, peripheral to my own experience) that the Th/U ratio was often so high as to render measurement of U isotopes irrelevant. For some multi-collector ICP-MS instruments you effectively get a choice of (U and Pb) or (Th and Pb) simultaneously, but on other (most) instrument types I guess you'd take the U as well regardless - so I'll shut up now. It's good to see another isotope geochemist in here, BTW. Cheers, Actinide 13:59, 12 February 2006 (UTC)

[edit] Merge w/Radiometric dating?

The subject matter in this article that deals with radiometric decay is reproduced in radiometric dating and can be merged. The downside is that merging with radiometric dating would leave dendro, varves, ice cores and lichenometry out in the cold. This article, however, doesn't really do them justice and really just provides a list of links. Any thoughts? Rickert 05:11, 10 February 2006 (UTC)

I had thought it was all a bit of a mess too. "Geochronology" perhaps should ultimately be a meta-page pointing to more detailed pages for radiometric dating, stratigraphic correlation (incl. micropal, Sr isotopes etc, maybe also paleomag), and Quaternary geochronology (includes your orphaned topics above), maybe one or two others. I'm not sure it should be merged with Radiometric dating though primarily because in the Quaternary other methods are very important, even dominant. Ultimately I think this page should contain a similar level of detail to what is currently present for radiometric dating, for each broad sub-category as (or vaguely similar to) above. I guess it looks a bit silly right now but ultimately it should make sense with the current structure. Actinide 13:59, 12 February 2006 (UTC)

[edit] Geochronological units

Geochronological units - examples:

You can write: Tyrannosaurus rex lived in Late Cretaceous, but if you write: a T-rex was found in Late Cretaceous, you mean that a timetraveller found it 67 mya.

See also Chronostratigraphy.

This text is inaccurate and irrelevant. Firstly, the name of eons and aeons, etc are derived by reference to the classical bostratigraphic nomenclature. For instance, the Proterozoic is the age before multi-cellular animals. The Palaeozoic s the bit before vertebrates; etc etc. These divisions n time were derived before the 'ages of which they relate to were known; biostratigraphy (when done well) is absolute in that a faunal assemblage places it in the correct Period, but has no absolute age information.

Hence, deriving an age date from radiometric dating or other geochonological method does nothing to give a rock a biostratigraphic age; in fact in the case of some age date information, it relates more to a geologic event (metamorphic event, etc) than to any real rock stratum or igneous event. Hence, it is not sufficient nor accurate to start quoting "geochrnological units" and subdivisions.

We must gain an age date number from the process and methods of geochronology; radiometric dating. We then assign this significance based on an interpretation of what the data means. For instance if we derive a Rb-Sr isochron of a granite, the isotopic data doesn't lie; that is the Rb-Sr composition of the granite when it cooled below the closure temperature. But we have to say whether it is the age of emplacement, metamorphism, etc.

We can then relate this to any biostratigraphic information only when it crosscuts and when the age of the biostratigraphy is known. This is an inference, not a fact, and hence, it is imporoper to say a granite is Dalradian, for example. We must give geochronological dates in ka, Ma, Ga.

A case in point is the Permo-Triassic boundary. This is a date unfixed in time because of a laack of conclusive cutting of the floral and faunal successions by a known igneous rock. We know it is around 250 Ma, and there are plenty of rocks of around that age, but the age date changes a few Ma each way each time a new age date is given and rigorously proven to define and bracket the fossil assemblages of the Permian and Triassic.

If, for argument's sakes, you called a granite Permian, and the biostratigraphic column's chronostratigraphy was altered, it could then in hndsight be shown to be Triassic if the goalposts are moved. Hence, i reisterate, do not confuse biostrat with chronostrat, with geochron.

Cheers, Rolinator 07:05, 30 October 2006 (UTC)

According to "Glossary of Geology, 4th. edition, American Geological Institute publications 1997, ISBN 0-922152-34-9": Geochronological units are divisions of time based on chronostratigraphic units. It is time spans corresponding to the chronostratigraphic material units. Geochronological units in order of decreasing rank are eon, era, period, epoch and age. Names of periods and units of lower rank are the same as those of the corresponding chronostratigraphic units; the names of some eras and eons are independently formed.
My contribution: Therefore IMHO the deleted text is accurate. Most of the boundaries are determined by first or last appearances of important fossils or a geologic event, but not the Proterozoic — geologists have voted that it began at 2.5 Ga.
Many people confuse geochronological and chronostratigraphic units: A T-rex was found in Late Cretaceous (time travellers?) or Tyrannosaurus rex lived in Upper Cretaceous (Tyrannosaurus lithofagus?) Therefore IMHO it is highly relevant. Necessary Evil 16:25, 30 October 2006 (UTC)

[edit] Cretaceous Incorrect / Correct Example

I can't for the life of me figure out what this example means or why it's here. Could someone clarify it or delete it if it serves no purpose? —Preceding unsigned comment added by 74.12.71.34 (talk) 02:30, 28 January 2008 (UTC)

I've seen reporters, high school students and others mixing up geochronologic and chronostratigraphic units. The examples are here because non-geologists are also using Wikipedia.
  • [Late Cretaceous] is a geochronologic unit - meaning a time span. It's correct to write "Tyrannosaurus lived in Late Cretaceous" but "A Tyrannosaurus was found in Late Cretaceous" means that a time traveller found it more than 65 mya. However, "A Tyrannosaurus was found in deposits from Late Cretaceous" is correct.
  • [Upper Cretaceous] is a chronostratigraphic unit - meaning the rocks or deposits from [Late Cretaceous]. "A Tyrannosaurus was found in Upper Cretaceous" is correct but "Tyrannosaurus lived in Upper Cretaceous" means that it was living inside the rock.
Professional geologists may never confuse Late C./Upper C. but others could. --Necessary Evil (talk) 12:13, 28 January 2008 (UTC)