Forwarded message from Jim Ogg (jogg@purdue.edu)
20 September 2004
To Martin Head and PaleoNet,
Your concerns on the Neogene portion of the Geologic Time Scale 2004
compilation (in press, Cambridge University Press) was forwarded to Felix
Gradstein and myself. As Martin Head surmised, the full GTS2004 text (about
500 pages) provides more documentation. We are happy to provide the following
details, and suggest that further questions on the Neogene be directed to Luc
Lourens, the coordinator of the Neogene scale, or his co-authors (Nick
Shackleton, Frits Hilgen, Doug Wilson, Jacques Laskar). Neogene ages were
mainly from cycle scaling, but with a segment in early Miocene that required
using sea-floor spreading models.
1. Foram Zone M3 – much shorter (less than 0.2 myr) than in Berggren et al.
1995 (about 1.5 myr)
Apparently true, according to some later studies. The base of M3 (mid
Burdigalian) is defined by the first occurrence of Globigerinatella insueta.
Berggren et al. 1995 estimated this as approximately base of Chron C5E.
However, a placement near base of Chron C5Dn (or, approx. 17.6 Ma) was
suggested by Shackleton et al (1999, Astronomical calibration of Oligocene-
Miocene time. Phil. Trans. Roy. Soc. London, A357: 1907-1929) and Pearson and
Chaisson (1997, Late Paleocene to middle Miocene planktonic foraminifer
biostratigraphy of the Ceara Rise. ODP Sci. Results 154: 33-68). Probably more
precise calibrations in other ocean basins might clarify the global (versus
basinal) FAD age.
2. Base of Burdigalian is confusing
Definitely. This is mainly because no global definition has been
decided. There are 3 options that have been proposed, with about a 1 myr
spread in ages – (1) top of Chron C6An (which was used on the Berggren et al
1995 scaling = 20.4 Ma on Berggren’s scale, or 20.04 Ma on new GTS2004
scaling), (2) a relatively new option of using the FAD of nannofossil
Heliscosphaera ampliaperta (MNN2a-MNN2b transition, which is near top of the
next underlying Chron C6An.2n, or about 20.4 Ma on GTS2004 scale, and which was
selected by Lourens et al in the book as their working definition), or the LAD
of foraminifer Paragloborotalia kugleri (N4-N5 transition, which is about 21.1
Ma). We apologize for the confusing indication in the GSSP summary table that
only the other two ‘traditional’ options for listed, and that the source of the
age of 20.04 from GTS2004 was not clarified. We will correct this in the GSSP
summary table, and thank you for bringing it to our attention.
3. Dinoflagellate calibrations
The assignment of the Neogene events shown in the summary graphics were
partly by James Powell, and partly by Hans Brinkhuis. Probably the
calibrations of some of these events vary among different specialists.
The detailed global calibrations, GSSP-based definitions of
international stages, and the numerical ages of events in Earth history will be
a continual ongoing process that should involve all specialists. Any ‘GTS’ is
a snapshot of our partial understanding, and it is important that everyone
contribute their critiques and knowledge to the next improved and enhanced
scale! The International Commission on Stratigraphy (ICS) will work on an on-
line version of the GTS which will be more amenable to expert adjustments and
expansions. The goal is a consensus integrated geologic time scale that
everyone can apply to understand the detailed workings of our planet.
Thank you for your careful review, comments and corrections.
-- James Ogg (secretary of ICS; Purdue Univ., USA)
-- Felix Gradstein (chair of ICS; Oslo Nat Hist Museum, Norway)
On 13 Sep 2004, at 11:37, Martin Head wrote:
Folks,
A new time scale was published this year by Gradstein et al. (2004), which I guess will replace the two very popular but aging Berggren et al. 1995 time scales. The new Gradstein et al. (2004) time scale is an abridgment of a fuller (500 p) account to be published by Cambridge University Press later this year (Gradstein et al., in press), but Gradstein et al. (2004) already gives a detailed treatment of the Neogene, including calibration of planktonic foraminiferal and calcareous nannofossil zonations, and dinocyst and radiolarian datums. This time scale is therefore ready to test drive, at least for the Neogene!
For those of you who have already begun to use Gradstein et al. (2004), what is your assessment of it? I would question some of the dinocyst datums, but I am not qualified to judge the handling of the planktonic foraminiferal and calcareous nannofossil zonations. What I do notice is that these two zonations have calibrations (to one another and to the polarity timescale) that in some cases differ substantially from Berggren et al 1995. For example Zone M3 in the upper Burdigalian is 1.5 Myr in duration according to Berggren et al. 1995, but only about 0.1 Myr in Gradstein et al. (2004). Have the planktonic foram zones really changed so much in 9 years?
I assume that some shifts in stage boundaries relative to the polarity time scale represent new research on type sections. I am, however, confused by the base of the Burdigalian, as placed in Gradstein et al. (2004). They state that the boundary is close to the top of Chron C6An (i.e. top of C6An1n), which is exactly where it is placed in Berggren et al. (1995), but the Gradstein et al. chart has the boundary drawn at the top of Chron C6An2n. This seems to represent an internal inconsistency of half a million years. Am I missing something? I wonder if there are other inconsistencies. Perhaps all becomes clear in the full version of the time scale (Gradstein et al. in press).
Finally, how does the new time scale affect the nomenclature of sequence boundaries. I notice, for example, that the sequence boundary "Lan 1" is now well into the Burdigalian.
The Gradstein et al. (2004) time scale for the Neogene is clearly an improvement on earlier efforts, and is orbitally tuned with an accuracy of 40 kyr. But should we embrace Gradstein et al. (2004) unquestioningly? And I have to ask, is there a new Berggren et al. time scale in the pipeline?
Any thoughts most welcome.
Martin
Reference:
Gradstein, F.M., Ogg, J.G., Smith, A. G., Bleeker, W., and Lourens, L.J., 2004. A new geologic time scale with special reference to Precambrian and Neogene. Episodes, 27: 83–100.
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Dr Martin J. Head
Department of Geography
University of Cambridge
Downing Place, Cambridge CB2 3EN
ENGLAND, U.K.
Phone: +44 (0)1223 339751
Fax: +44 (0)1223 333392
Email: mh300@cam.ac.uk
Home page: http://www.geog.cam.ac.uk/people/head
------------------------------------------------------------------------
Dr Martin J. Head
Department of Geography
University of Cambridge
Downing Place, Cambridge CB2 3EN
ENGLAND, U.K.
Phone: +44 (0)1223 339751
Fax: +44 (0)1223 333392
Email: mh300@cam.ac.uk
Home page: http://www.geog.cam.ac.uk/people/head