Abstract

We constructed a precise early Eocene orbital cyclostratigraphy for DSDP Site 550 (Leg 80, Goban Spur, North Atlantic) utilizing precession related cycles as represented in a high-resolution X-ray fluorescence based barium core log. Based on counting of those cycles, we constrain the exact timing of two volcanic ash layers in Site 550 which correlate to ashes +19 and −17 of the Fur Formation in Denmark. The ashes, relative to the onset of the Paleocene–Eocene Thermal Maximum (PETM), are offset by 862 kyr and 672 kyr, respectively. When combined with published absolute ages for ash −17, the absolute age for the onset of the PETM is consistent with astronomically calibrated ages. Using the current absolute age of 28.02 Ma for the Fish Canyon Tuff (FCT) standard for calibrating the absolute age of ash −17 is consistent with tuning option 2 in the astronomically calibrated Paleocene time scale of Westerhold et al. (2008) [Westerhold, T., Röhl, U., Raffi, I., Fornaciari, E., Monechi, S., Reale, V., Bowles, J., and Evans, H.F., 2008, Astronomical calibration of the Paleocene time: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 257, p. 377–403]. Using the recently recalibrated absolute age of 28.201 Ma for the FCT standard is consistent with tuning option 3 in the astronomically calibrated Paleocene time scale. The new results do not support the existence of any additional 405-kyr cycle in the early Paleocene astronomically tuned time scale.

© 2009 Elsevier B.V. All rights reserved.

]]> http://earthtime-eu.eu/earthtime/?feed=rss2&p=486 0 http://earthtime-eu.eu/earthtime/?p=464 http://earthtime-eu.eu/earthtime/?p=464#comments Mon, 06 Jul 2009 12:00:24 +0000 admin http://earthtime-eu.eu/earthtime/?p=464 Clim. Past, 5, 309-327, 2009
www.clim-past.net/5/309/2009/
© Author(s) 2009. This work is distributed
under the Creative Commons Attribution 3.0 License.

High resolution cyclostratigraphy of the early Eocene – new insights into the origin of the Cenozoic cooling trend

T. Westerhold and U. Röhl
MARUM – Center for Marine Environmental Sciences, University of Bremen, Leobener Strasse, 28359 Bremen, Germany

Abstract. Here we present a high-resolution cyclostratigraphy based on X-ray fluorescence (XRF) core scanning data from a new record retrieved from the tropical western Atlantic (Demerara Rise, ODP Leg 207, Site 1258). The Eocene sediments from ODP Site 1258 cover magnetochrons C20 to C24 and show well developed cycles. This record includes the missing interval for reevaluating the early Eocene part of the Geomagnetic Polarity Time Scale (GPTS), also providing key aspects for reconstructing high-resolution climate variability during the Early Eocene Climatic Optimum (EECO). Detailed spectral analysis demonstrates that early Eocene sedimentary cycles are characterized by precession frequencies modulated by short (100 kyr) and long (405 kyr) eccentricity with a generally minor obliquity component. Counting of both the precession and eccentricity cycles results in revised estimates for the duration of magnetochrons C21r through C24n. Our cyclostratigraphic framework also corroborates that the geochronology of the Eocene Green River Formation (Wyoming, USA) is still questionable mainly due to the uncertain correlation of the “Sixth tuff” to the GPTS.

Right at the onset of the long-term Cenozoic cooling trend the dominant eccentricity-modulated precession cycles of ODP Site 1258 are interrupted by strong obliquity cycles for a period of ~800 kyr in the middle of magnetochron C22r. These distinct obliquity cycles at this low latitude site point to (1) a high-latitude driving mechanism on global climate variability from 50.1 to 49.4 Ma, and (2) seem to coincide with a significant drop in atmospheric CO₂concentration below a critical threshold between 2- and 3-times the pre-industrial level (PAL). The here newly identified orbital configuration of low eccentricity in combination with high obliquity amplitudes during this ~800-kyr period and the crossing of a critical pCO₂ threshold may have led to the formation of the first ephemeral ice sheet on Antarctica as early as ~50 Ma ago.

http://www.clim-past.net/5/309/2009/cp-5-309-2009.html

To the Editor
The need for much improved knowledge of the durations and ages of climatic and geological events, such as the Palaeocene–Eocene Thermal Maximum (55 million years ago), has become urgent within the Earth science and climate modelling communities. The exact dating and timing of fluxes into and out of the marine carbon reservoir can differentiate between competing hypotheses of climatic change. Highly detailed reconstructions of the Earth’s history allow us to assess whether past climatic change can be used as an analogue for the current and future change of ocean acidification and climate. The Earthtime project is an international effort with the goal to further this quest for a well calibrated and stable timescale that will allow more precise dating of rock layers and minerals1.
Radioisotopic dating methods have small but significant errors that hinder our ability to assess geologically short-lived climate events. For instance, the most widely used method for the Cenozoic era is 40Ar/39 Ar, which has an error of up to 2.5% and few tie points of known age. Yet, over the last two decades much progress has been made in exploiting the imprint of the Earth’s orbital variations in palaeoclimatic records. This has dramatically increased the potential age resolution of approaches like cycle-counting and pattern matching, to less than 40,000 years throughout much of Cenozoic time (the past 66 million years, Fig. 1).
Unfortunately, there have been a number of inconsistencies and discrepancies between ages and durations derived from radioisotopic and astronomical dating. What is now needed is a more systematic and coordinated approach to provide a detailed intercalibration of radioisotopic clocks (U–Pb, Ar–Ar methods), the rock standards that are used for these methods, and geological tie-points with astronomical ages. At the same time, Cenozoic palaeoclimatic compilations need to be improved by closing existing gaps, verifying data from single sites and supplementing the database of magneto- and biostratigraphy so we can improve the accuracy of existing age calibrations.
In particular, a major advance towards a fully astronomically calibrated geological timescale needs to be accomplished in the middle Eocene epoch (40 to 50 million years). Very few suitable sections have been acquired so far for this period, presumably because the calcite compensation depth was very shallow during this time, which would have prevented the preservation of carbonate material in the deep-ocean marine sediments.
The Earthtime projects are open community efforts aimed at improving intercalibration between astronomical and radioisotope dating methods by finding ash layers that can be dated with radioisotopes within astronomically age-calibrated sections. The immediate aim is to arrive at a highly accurate and stable Cenozoic timescale.
REFERENCES
1. http://earthtime-eu.eu; http://www.earth-time.org
2. Lourens, L. J. et al. in Geologic Time Scale 2004 (eds Gradstein, F. M., Ogg, J. G., Smith, A. G.) 409–440 (Cambridge Univ. Press, 2004).
3. Raffi, I. et al. Quat. Sci. Rev. 25, 3113–3137 (2006). | Article |
4. Billups, K. et al. Earth Planet. Sci. Lett. 224, 33–44 (2004). | Article |
5. Pälike, H. et al. Science 314, 1894–1898 (2006). | Article |
6. Pälike, H., Shackleton, N. J. & Röhl, U. Earth Planet. Sci. Lett. 193, 589–602 (2001). | Article |
7. Röhl, U., Norris, R. D. & Ogg, J. G. Spec. Pap. Geol. Soc. Am. 369, 576–589 (2003).
8. Lourens, L. J. et al. Nature 435, 1083–1087 (2005). | Article |
9. Westerhold, T. et al. Palaeogeogr. Palaeocl. 257, 377–403 (2008) . | Article |
10. Kuiper, K. F. et al. Science (in the press).
11. Zachos, J. C., Dickens, G. R. & Zeebe, R. E. Nature 451, 279–283 (2008). | Article |
Please note correction:
Reference 7 should be:
Röhl, U, Brinkhuis, H, Fuller, M, Schellenberg, S A, Stickley, C E, Wefer, G., Williams, G L (2004) Cyclostratigraphy of middle and late Eocene sediments drilled on the East Tasman Plateau (Site 1172)- In Exon, NF, Malone, M, and Kennett, JP (Eds.), Climate evolution in the Southern Ocean and Australia’s Cenozoic flight northward from Antarctica. Am. Geophys. Union, Geophys. Monogr., 151, 127-152.

Reference 10 is published:
RESEARCH ARTICLE in SCIENCE:
Synchronizing Rock Clocks of Earth History
K. F. Kuiper, A. Deino, F. J. Hilgen, W. Krijgsman, P. R. Renne, and J. R. Wijbrans (25 April 2008)
Science 320 (5875), 500. [DOI: 10.1126/science.1154339]

Figure 1 Caption:
The upper part illustrates a selection of previous work that resulted in detailed age calibrations for the Neogene and Quaternary2, 3, and parts of the Palaeogene5, 6, 7, 8, 9, 10. The uppermost horizontal line indicates a fairly stable and accurate astronomical age calibration with multiple site coverage (solid line), and more tentative or unverified age calibrations (dashed line). There is a significant gap in the middle Eocene (approximately 42 to 53 Myr ago). Age calibrations are shown in the context of an updated multi-site compilation from the Cenozoic of benthic foraminiferal oxygen isotope data11, supplemented by the most recent age compilation for magnetic reversals during the Cenozoic2. The labels C1n, C2n, and so on, and respective black bars, correspond to the geomagnetic polarity timescale defined by Cande and Kent using the revised ages from the Geological Time Scale 20042.

Also see commentary by Richard Kerr:
Science 25 April 2008:
Vol. 320. no. 5875, pp. 434 – 435
DOI: 10.1126/science.320.5875.434
NEWS OF THE WEEK
GEOCHEMISTRY:
Two Geologic Clocks Finally Keeping the Same Time
Richard A. Kerr

Additional commentary from Science Daily:
http://www.sciencedaily.com/releases/2008/04/080424140400.htm

and the Berkeley group:
http://www.berkeley.edu/news/media/releases/2008/04/24_argondating.shtml

Abstract:

Recent developments in integrated high-resolution stratigraphy and astronomical tuning of continuous deep marine successions invalidate arguments against the designation of unit stratotypes for global stages, the basic building blocks of the standard Global Chronostratigraphic Scale (GCS). For the late Neogene, Global Stratotype Section and Point (GSSP) sections may also serve as unit stratotypes, covering the interval from the base of a stage up to the level that–time-stratigraphically–correlates with the base of the next younger stage in a continuous and well-tuned deep marine succession. The added value of such sections as unit stratotype lies in the integrated high-resolution stratigraphy and astronomical tuning, which combined, provides an excellent age control with an unprecedented resolution, precision and accuracy within the entire stage. As such they form the backbone of the new integrated late Neogene time scale and provide the basis for reconstructing Earth’s history. In this way a stage is also defined by its content and not only by its boundaries. Our unit stratotype concept strengthens the importance of time-rock units by allowing the introduction of astronomically defined chronozones as formal chronostratigraphic units, thereby arguing against the elimination of the dual classification of chronostratigraphy and geochronology.

Extending this concept to older time intervals requires that well-tuned, continuous deep marine sections are employed, thus necessitating the employment of multiple hole (I)ODP sites for defining (remaining) stages and stage boundaries in at least the Cenozoic and Cretaceous and possibly the entire Mesozoic. Evidently the construction of the Geological Time Scale (including the GCS) should be based on the most appropriate sections available while, where possible, taking the historical concept of global stages into account.