Report

U-Pb Ages from the Neoproterozoic Doushantuo Formation, China

See allHide authors and affiliations

Science  01 Apr 2005:
Vol. 308, Issue 5718, pp. 95-98
DOI: 10.1126/science.1107765

Abstract

U-Pb zircon dates from volcanic ash beds within the Doushantuo Formation (China) indicate that its deposition occurred between 635 and 551 million years ago. The base records termination of the global-scale Marinoan glaciation and is coeval with similar dated rocks from Namibia, indicating synchronous deglaciation. Carbon isotopic and sequence-stratigraphic data imply that the spectacular animal fossils of the Doushantuo Formation are for the most part younger than 580 million years old. The uppermost Doushantuo Formation contains a pronounced negative carbonate carbon isotopic excursion, which we interpret as a global event at circa 551 million years ago.

The Neoproterozoic Doushantuo Formation (South China), deposited after a global-scale glaciation, contains fossil embryos, algae, achritarchs, and small bilaterians that are purportedly the Earth's earliest animals. This formation also contains a chemostratigraphic record of environmental change during its deposition with distinct δ13C anomalies occurring at its base and top. Here, we report previously undetermined U-Pb zircon ages for volcanic ash beds from the lower and uppermost Doushantuo Formation (Yangtze Gorges area, South China), each of which is associated with globally correlated negative δ13C anomalies.

In its type area along the Yangtze Gorges, the Doushantuo Formation comprises a succession of carbonate, shale, and phosphatic shale that is about 100 m thick. It can be correlated over 8000 km2 to the south and west (1). The Doushantuo Formation is characterized by two transgressive sequences. The lower sequence begins above a distinctive Lower Dolomite Member (cap carbonate), which is overlain by black shale and phosphorite that grade upward into increasingly dolomitic micrite and grainstone. The upper sequence is characterized by a basal black shale and phosphorite unit that grades into dolomitic micrite. These two sequences are separated by an interbasinal sequence boundary, which in the more proximal Weng'an successions occurs as a karstic unconformity (2, 3). The well-preserved animal embryos, algae, and small bilaterians of the Doushantuo Formation are from the phosphorites of the Upper Sequence (2, 3) at Weng'an, Guizhou Province. The uppermost part of the Doushantuo Formation (Miaohe Member) is considered to be correlative to the lower part of a third transgressive cycle grading into carbonates of the Dengying Formation. Abundant macroscopic multicellular algae fossils known as the Miaohe biota are found in the black shale (4).

The Lower Dolomite Member overlies the Nantuo Tillite and is equated with other Marinoan-type cap carbonates worldwide because of its distinctive carbonate facies and associated negative δ13C values (5). Fossils from the overlying Dengying Formation consist of large horizontal traces and the weakly mineralized metazoan Cloudina considered characteristic of the <10 million years (My) preceding the Ediacaran/Cambrian boundary (6). An ash bed below the Nantuo Formation (i.e., Datangpo Formation) has a U-Pb zircon age (±SEM) of 663 ± 4 million years ago (Ma) (5). The overlying Cambrian Meishucunian phosphorite unit (the Zhongyicun Member), which contains abundant phosphatized small shelly fossils, provides a minimum age of 538 ± 1.5 Ma (7). Barfod et al. (8) reported Lu-Hf and Pb-Pb whole-rock isochron ages on phosphorites from the lower part of the Upper Sequence of the Doushantuo Formation of 602 ± 48 Ma [mean square of weighted deviates (MSWD) = 2] and 599 ± 4Ma (MSWD = 2.9), respectively. Chen et al. (9) reported Pb-Pb isochron dates of 598 ± 26 Ma (MSWD = 2.7) and 576 ± 14 Ma (MSWD = 0.4) from the lower and upper parts, respectively, of the same sequence.

We collected three volcanic ash samples from the Doushantuo Formation in the type sections of the Yangtze Gorges area. Sample YG-04-15 comes from a 1-cm-thick clay-rich ash bed 2.3 m above the base of the Doushantuo Formation (Wuhe-Gaojiaxi Section) within the Lower Dolomite Member. Zircons from this sample are variably discordant and define a linear array on a concordia diagram anchored by three concordant points (fig. S2). All points (n = 11) yield a weighted mean 207Pb/206Pb date of 635.4 ± 1.3 Ma (MSWD = 0.31), and the four concordant points yield a U-Pb “concordia age” (10) of 635.23 ± 0.57 Ma (MSWD of concordance and equivalence = 0.55) (11). The Lower Dolomite Member is the cap carbonate to the Nantuo Tillite (5, 1214). More specifically, ash bed YG-04-15 occurs at the transition from laminated limestone-dolostone (displaying sheet-crack and tepee-like structures) to laminated limestone-dolostone micrite [transition from C2 to C3 facies in the sense of (13)]. Most models for cap carbonate precipitation invoke high accumulation rates (15). Therefore, we consider the age of sample YG-04-15 to date to the end of the Nantuo glaciation.

A second ash bed (YG-04-2) occurs 9.5 m above the base of the Doushantuo Formation (Jijiawan Section) and 5 m above the top of the Lower Dolomite Member (Nantuo Cap Carbonate). Zircons from this ash bed yield variably discordant data (n = 9) that define a linear array on a concordia diagram anchored by three concordant data points (fig. S2). The U-Pb concordia age of 632.50 ± 0.48 Ma (MSWD of concordance and equivalence = 0.38) is indistinguishable from the weighted mean 207Pb/206Pb age of 632.4 ± 1.3 Ma (MSWD = 0.36). Comparing this age with YG-04-15 constrains the time interval between these two beds to between 1.7 and 3.8 My, and in the Jijiawan section this interval is represented by 3 m of dolomite overlain by about 5 m of black shale (16).

The age of the uppermost Doushantuo Formation (Miaohe Member) is constrained by the age of a third ash bed, JIN-04-2 (Jijiawan Section) that occurs at the top of the black shale member containing the Miaohe biota. Zircons from this ash bed yield variably discordant dates (n = 10) that define a linear array on a concordia diagram anchored by two concordant analyses (fig. S2). All data yield a weighted mean 207Pb/206Pb date of 550.55 ± 0.75 Ma (MSWD = 0.48). The two concordant analyses yield a U-Pb concordia age of 551.07 ± 0.61 Ma (MSWD of concordance and equivalence = 0.48). This ash bed occurs about 85 cm below the base of the Dengying Formation (within the Miaohe Member) at the interface between a black shale unit and overlying carbonates that record a progressive increase in δ13C values [from –4 to +0.5 per mil (‰)] over a thickness of 2.3 m. Negative δ13C values occur both above and below the sequence boundary separating the Upper Sequence and Miaohe Member (fig. S1). Below this boundary a pronounced δ13C excursion (with values as low as –8‰) occurs; in the Wuhe section it is characterized by invariant δ13C values of –8‰, whereas in the Jijiawan section there is a return to positive δ13C values before the base Miaohe Member sequence boundary is encountered. This local variation is most likely related to variable preservation below the sequence boundary and/or lateral variation in sediment accumulation and preservation. In both sections, the lowermost Dengying Formation dolomites have values of –3 to –1‰, which increase to about 3‰ over about 5 m. This trend is interpreted + as being the top of the pronounced δ13C excursion (with values as low as –8‰) that characterizes the top of the Doushantuo Formation (Fig. 1), thus constraining the age of the sustained negative excursion to (just) older than 551.1 ± 0.7 Ma, the time at which δ13C values increase to positive values. This interpretation assumes that any period of nondeposition across this sequence boundary has a duration that is less than the duration of the negative δ13C excursion itself.

Fig. 1.

Correlation of Doushantuo Formation (Yangtze Platform) with the Nama Group (Nama Basin) and Nafun/Ara Groups (Oman) successions. Three distinct sections are identified in each succession on the basis of magnitude of δ13C and nature (increasing, decreasing, or invariant) of stratigraphic trend. (1) Correlation of cap carbonate excursion (δ13C starting at –2‰, going to a nadir of –5‰, and returning to +2‰). (2) Invariant negative δ13C excursion in the range of –7‰. (3) Transition from negative to positive (about 1‰) values after the upper Doushantuo/Shuram/Kuibis anomaly. Nama Group (Kuibis Formation) carbonates record values as low as –4‰. However, correlation with the Doushantuo Formation is based on stratigraphic proximity to the 549 Ma ash bed. Internal stratigraphy of the Doushantuo Formation: A, Lower Dolomite Member; B, Lower Sequence; C, Upper Sequence; and D, Miaohe Member. Superscripted numbers next to dates correspond to reference numbers.

The Doushantuo Formation contains two prominent δ13C features, a negative (–5‰) to positive (up to 2‰) excursion in the Lower Dolomite Member (5, 13) and a pronounced negative (down to –8‰) excursion in the uppermost Doushantuo Formation (Fig. 1 and fig. S1). The Nantuo Tillite–Lower Dolomite Member has been correlated with other Marinoan-type glacial successions recorded worldwide (5, 17). Our age for the termination of the Marinoan glaciation is nearly identical to a concordant U-Pb age of 635.51 ± 0.54 Ma (18, 19) marking the top of glacial deposits in the Ghaub Formation, Namibia. Termination of the Marinoan glaciation and cap carbonate deposition was thus essentially synchronous, within the age uncertainty, worldwide. These data support the hypothesis that cap carbonates were deposited very quickly after deglaciation (20).

It has been commonly assumed that the negative δ13C anomaly at the top of the Doushantuo Formation was related to the Gaskiers glaciation (21, 22), albeit with no sedimentological evidence for glaciation. If the lowermost Dengying Formation with its negative-to-positive δ13C trend represents the top of this anomaly, this correlation is no longer viable because the 551.1 ± 0.7 Ma age from the uppermost Doushantuo ash bed (JIN-04-2, Jijiawan Section) postdates the circa 580 Ma Gaskiers glaciation. This age does reveal that much of the Upper Sequence and associated isotopic excursion occurred during a time when macroscopic Ediacaran Fauna are preserved in other successions worldwide [e.g., Mistaken Point Fauna (23); White Sea Fauna, and inferentially Australian Fauna (24)] and that their absence in the Doushantuo Formation is likely an artifact of paleo-ecological context (25).

Given that the top of the Doushantuo is not correlative with the Gaskiers glaciation, we suggest that the sequence boundary separating the Lower and Upper sequences (Fig. 1) within the Doushantuo reflects glacio-eustatic sea-level fluctuation associated with it. In the Yangtze Gorge region, there is no distinct evidence of a sea-level fluctuation. However, the more proximal succession at Weng'an, the corresponding surface, is karstic with evidence for subaerial exposure (2, 3). An age of circa 580 Ma for this surface is at odds with the data from Barfod et al. (8), but given the complexity of diagenesis in phosphorites and the use of leached whole-rock samples that span several meters of section, it is difficult to evaluate the accuracy of the calculated dates. This interpretation would imply that the animal embryos and small bilaterian of the Weng'an Phosphorite Member (2, 3) postdate the last extensive glaciation (circa 580 Ma) of the Neoproterozoic (26).

In Southern Namibia, carbonate rocks of the Schwarzrand and Kuibis subgroups (Southern and Northern Nama Basin) display a δ13C excursion above a sequence boundary (6, 27, 28) and 500 m below an ash bed dated at 548.8 ± 1.0 Ma (6). Estimates of sediment accumulation rates in the Nama basin are about 100 to 200 m per 106 years, so the shift in δ13C values is estimated to be about 551 Ma. In Oman, a distinct negative δ13C anomaly in the lower half of the Shuram Formation (2931) lies about 200 m below ash beds in the Ara Group dated at 542 Ma (29) and above the Fiq Formation, which is interpreted as a Marinoan-type glacial deposit (32). Given that in Namibia, Oman, and China, these similar sequences are overlain by rocks containing the Ediacaran/Cambrian boundary and their δ13C curves display similar trends, we suggest correlation of the Shuram, Kuibis, and upper Doushantuo excursions. Halverson and colleagues (17) also correlated the Kuibis/Shuram anomalies with those of similar magnitude and stratigraphic position (i.e., post-Marinoan and pre-Cambrian) in Australia [the Wonoka Formation (33)] and the western United States [Rainstorm Member, Johnnie Formation (34)] but chose to equate it with a postulated Gaskiers-related δ13C excursion from circa 580 Ma (35). Instead, our data indicate that this globally correlated negative δ13C excursion is not related to any known glaciation (36). The duration of this excursion is unconstrained; however, given that it is captured within over 100 m of section in Oman (Fig. 1) combined with approximate sediment accumulation rates calculated with our constraints, we suggest a duration of >1 and <10 My.

The Doushantuo and correlative strata record a fundamental shift from an interval of large carbon isotopic anomalies corresponding to glacial episodes (750 to 580 Ma) to an interval of anomalies unrelated to obvious glacial episodes (i.e., the anomalies from circa 551 and 542 Ma), as well as subsequent large fluctuations in the lower Cambrian. These new geochronological data allow us to calibrate that shift as being synchronous with the appearance of larger and more complex metazoans; this suggests possible feedback relationships between evolutionary innovation and seawater chemistry (Fig. 2).

Fig. 2.

Schematic diagram comparing fossil occurrences, evolutionary developments, and globally correlated δ13C excursions within the context to a temporal framework constrained by U-Pb ages from sequences in Namibia, Oman, China, and Newfoundland. Newfoundland carbonate carbon δ13C data are from Myrow and Kaufman (35). The absolute age of the –8‰ nadir is unconstrained except that it must be older than 551.1 ± 0.7 Ma. Superscripted numbers next to dates correspond to reference numbers.

Our ages indicate that the Doushantuo Formation spans more than 90% of the Ediacaran Period. These constraints are consistent with the upper Doushantuo/Shuram/Kuibis excursion being broadly coincident with the first appearance of complex trace fossils and mollusk-like bilaterian Kimberella (37), dated as slightly older than 555.1 ± 1.0 Ma (24). The advent of large pelagic bilaterians with unidirectional guts would have increased the flux of organic carbon to the deep ocean (38). Additionally, the radiation of algae containing resistant biopolymers in cell wall and cysts (i.e., Miaohe Biota) and the advent of biomineralization (Namacalathus and Cloudina, >549 Ma) would have also resulted in an increased organic carbon and carbonate carbon flux (39). These changes would have resulted in a downward flux of organic carbon with a possible coupled oxidation of the organic reservoir (38, 39) driving the negative δ13C excursion. This feedback loop would lead to an increase in marine oxygen levels and stimulate productivity and inferentially predation. It may be no coincidence that the first reefs inhabited by abundant weakly calcified and rare fully calcified metazoans appeared at about the same time as the isotopic anomaly [i.e., before 549 Ma in Namibia (6, 40)].

Supporting Online Material

www.sciencemag.org/cgi/content/full/1107765/DC1

Materials and Methods

Figs. S1 and S2

Tables S1 and S2

References

References and Notes

View Abstract

Navigate This Article