Sequencing the Neoproterozoic: The importance of
high-precision geochronology
by Sam Bowring & Dan Condon

Understanding the details of Neoproterozoic climate fluctuations and links with biological evolution center around our ability to precisely correlate and sequence disparate stratigraphic sections. Relative ages of events can be established within single sections or by regional correlation using litho-, chemo- and/or biostratigraphic markers. However, such chronologies do not allow testing of the synchroneity of units, the validity of correlations or determining rates of change/duration of events. At present, the major limitation to our understanding of the Neoproterozoic, and the number and duration of “snowball” glaciations, is the dearth of high-precision dates. However, the increase in geochronological constraints over the past five years indicates that progress is possible.

The determination of 'absolute' age constraints for Neoproterozoic successions can be achieved using a variety of geochronological techniques. These include whole rock approaches using Re-Os, Pb-Pb and Lu-Hf decay schemes as well as U-Pb dating of zircon from volcanic rocks to directly date the horizon sampled, and detrital zircon to constrain the maximum depositional age. While the database of geochronological constraints for the Neoproterozoic is growing the data are of variable precision and accuracy and typically subject to multiple assumptions and interpretations. For example, whole rock dates depend on the assumptions that a suite of samples all have the same initial isotopic ratios and evolved through time only as a function of different parent/daughter ratios. In the cases of Pb-Pb, U-Pb and Lu-Hf dating, calculated dates are interpreted to reflect either primary precipitation or the time of early diagenesis/fluid flow of carbonate and phosphate (Pb-Pb, Lu-Hf) or enrichment of parent isotope during deposition/early diagenesis (Re-Os). These assumptions are difficult to evaluate in many cases and it is possible to produce statistically significant linear arrays on isotope correlation diagrams that are mixing lines and have no geological significance (e.g. Schaeffer and Burgess). However, new developments in Re-Os dating of black shales show considerable promise. U-Pb geochronology data are derived from Isotope Dilution Thermal Ionization Mass-Spectrometry (ID-TIMS), ion-probes (SHRIMP, Cameca 1270-1280’s), and Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICPMS) and there are important differences between these techniques. While it is tempting to use all available geochronological data in compilations irrespective of decay scheme and precision, caution is urged.

The ID-TIMS approach can yield single grain U-Pb dates with a precision of better than about 0.1 percent; however, it is a time consuming and expensive approach. In contrast, ion-probe single spot analyses have typical analytical uncertainties of about 2-5 percent and are reliant upon the pooling of datasets with lots of samples in order to calculate dates with uncertainties of about 1 percent. Although in-situ analyses afford high-spatial resolution combined with rapid throughput of analyses, the low precision of a single analysis does not permit identification of the subtle effects of Pb-loss/inheritance (see Figure above). In the end, high precision dates are required to test synchroneity/diachroneity of glacial deposits.

What do we know?
Considerable progress has been made in the past decade on the calibration of Neoproterozoic time. Although the number, timing, duration and possible synchroneity of ‘Cryogenian’ glacial episodes still remains poorly constrained, there is growing evidence for at least two glacial-cap carbonate sequences during the 760 to 700 Ma interval, one at about 635 Ma, and a final one at about 582 Ma. The base of the Ediacaran period is formally defined at the base of the Nuccaleena (Marinoan) cap-carbonate as exposed in Enorama Creek, Flinders Ranges, South Australia. Correlation of its distinctive cap sequence coupled with high–precision U-Pb (zircon) ages from Namibia (within the glacial Ghaub Formation) and Southern China (within the cap carbonate to the Nantuo tillite) indicate synchronous termination of the Marinoan glaciation at about 635 Ma (See photo). The top of the Ediacaran Period/base Cambrian Period is not dated at its type locality. However U-Pb zircon dates on ash beds from Oman and Namibia constrain it to be about 542 Ma. During the Ediacaran Period the short-lived Gaskiers glaciation occurred about at 582 Ma and the oldest known Ediacaran fossils first appear within 4 Ma of deglaciation. When all well-dated sequences containing Ediacaran fossils are considered in the context of global chemostratigraphic correlation schemes, a number of major conclusions can be drawn. At about 560-551 Ma, the global carbon cycle underwent a major reorganization consistent with progressive oxidation and remineralization of the organic reservoir. At about same time, and suggestive of a link, the first complex trace fossils as well as the stem group mollusc, Kimberella, are found in White Sea sections. Weakly calcified metazoans, such as Cloudina and Namacalathus appear at about 548 Ma and continue to the Ediacaran/Cambrian boundary where they are inferred to have become extinct. It is clear that our understanding of the relationships/feedback loops between biology, the carbon cycle, and climate will require a much more highly calibrated record.

Outstanding issues center on the number, synchroneity and durations of glacial deposits, the exact age of the spectacular animal embryos preserved in the Doushantou Formation of southern China, the cause of large δ13C excursions that are seemingly unrelated to glaciation, the validity of molecular clock estimates for the timing of animal evolution, and the global significance of the Gaskiers glacial event and the first appearance of megascopic Ediacaran fossils. At present the age of the only glacial deposits to yield unequivocal equatorial paleolatitudes, the Elatena, is unknown but the rocks have been correlated with dated rocs that are as old as 635 Ma and as young as 580 Ma. Future work will focus on using the highly calibrated record to understand developmental and environmental controls on evolution that preceded the Cambrian explosion, including a precise and accurate temporal framework for the period from about 1000 Ma until 750 Ma in order to integrate proxy records (isotopic, lithostratigraphic and paleomagnetic) with evaluated causal relationships and rate-dependent effects responsible for the transition into the Cryogenian.

Selected References
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Barfod, G. H., Albarede, F., Knoll, A. H., Xiao, S. H., Telouk, P., Frei, R., and Baker, J., 2002, New Lu-Hf and Pb-Pb age constraints on the earliest animal fossils: Earth And Planetary Science Letters, v. 201, p. 203-212.

Bowring, S. A., and Schmitz, M. D., 2003, High-precision U-Pb zircon geochronology and the stratigraphic record: Zircon, v. 53, p. 305-326.

Bowring, S., Myrow, P., Landing, E., and Ramezani, J., 2003, Geochronological constraints on terminal Neoproterozoic events and the rise of metazoans: Geophysical Research Abstracts, v. 5, p. 219.

Brasier, M., McCarron, G., Tucker, R., Leather, J., Allen, P., and Shields, G., 2000, New U-Pb zircon dates for the Neoproterozoic Ghubrah glaciation and for the top of the Huqf Supergroup, Oman: Geology, v. 28, p. 175-178.

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Condon, D., Zhu, M. Y., Bowring, S., Wang, W., Yang, A. H., and Jin, Y. G., 2005, U-Pb ages from the neoproterozoic Doushantuo Formation, China: Science, v. 308, p. 95-98.

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Halverson, G. P., Hoffman, P. F., Schrag, D. P., Maloof, A. C., and Rice, A. H. N., 2005, Toward a Neoproterozoic composite carbon-isotope record: Geological Society Of America Bulletin, v. 117, p. 1181-1207.

Hoffman, P. F., Hawkins, D. P., Isachsen, C. E., and Bowring, S. A., 1996, Precise U-Pb zircon ages for early Damaran magmatism in the Summas Mountains and Welwischia Inlier, northern Damara Belt, Namibia: Communications of the Geological Survey of Namibia, v. 11, p. 21-31.

Hoffmann, K.-H., Condon, D. J., Bowring, S. A., and Crowley, J. L., 2004, U-Pb zircon date from the Neoproterozoic Ghaub Formation, Namibia: Constraints on Marinoan glaciation: Geology, v. 32, p. 817-820.

Kendall, B. S., Creaser, R. A., Ross, G. M., and Selby, D., 2004, Constraints on the timing of Marinoan "Snowball Earth" glaciation by Re-187-Os-187 dating of a Neoproterozoic, post-glacial black shale in Western Canada: Earth And Planetary Science Letters, v. 222, p. 729-740.

Martin, M. W., Grazhdankin, D. V., Bowring, S. A., Evans, D. A. D., Fedonkin, M. A., and Kirschvink, J. L., 2000, Age of Neoproterozoic bilatarian body and trace fossils, White Sea, Russia: Implications for metazoan evolution: Science, v. 288, p. 841-845.

Yin, C., Tang, F., Liu, Y., Gao, L., Liu, P., Xing, Y., Yang, Z., Wan, Y., and Wang, Z., 2005. U-Pb zircon age from the base of the Ediacaran Doushantuo Formation in the Yangtze Gorges, South China: constraint on the age of Marinoan glaciation. Episodes 28, 48-49.

Zhang, S. H., Jiang, G. Q., Zhang, J. M., Song, B., Kennedy, M. J., and Christie-Blick, N., 2005, U-Pb sensitive high-resolution ion microprobe ages from the Doushantuo Formation in south China: Constraints on late Neoproterozoic glaciations: Geology, v. 33, p. 473-476.

Zhou, C. M., Tucker, R., Xiao, S. H., Peng, Z. X., Yuan, X. L., and Chen, Z., 2004, New constraints on the ages of neoproterozoic glaciations in south China: Geology, v. 32, p. 437-440.

Sam Bowring is a professor at Massachusetts Institute of Technology and Dan Condon is with the NERC Isotope Laboratory at the British Geological Survey. Both laboratories are partipating in EARTHTIME, the international scientific initiative to calibrate geological time to one tenth of one percent.