Nantuo Glaciation Research Articles

Active biogeochemical cycles during the Marinoan global glaciation

Termination of the Marinoan global glaciation (650–635 million years ago, Ma) was immediately followed by the diversification of eukaryotes and possible occurrence of animals, suggesting the potential linkage between biological evolution and global glaciation. It is proposed that the post-glacial eukaryote evolution might have been triggered by environmental stress imposed by the extreme pan-glacial condition. However, how life survived through the global glaciation remains speculative. Neither is known about the habitability when the ocean was completely or mostly frozen. In this study, we explore the marine carbon and sulfur biogeochemical cycles during the Nantuo (Marinoan) glaciation (650–635 Ma) in South China. Both organic carbon (δ13Corg) and pyrite sulfur isotopes (δ34Spy) show significant stratigraphic variations, suggesting active biogeochemical cycles in the pan-glacial condition. By coupling both δ13Corg and δ34Spy data, we develop numerical models to constrain marine productivity and redox conditions during the Nantuo glaciation. The marine primary productivity showed a sharp decline in the onset of glaciation and recovered right before the cap carbonate precipitation. An episodic recovery of primary productivity was observed in the non-glacial interval between the two glacial episodes. In contrast, the marine productivity was still suppressed in initial deglaciation, during which the continental weathering was intense, presumably indicating high atmospheric CO2 level and low seawater pH value. The final recovery of marine primary productivity occurred in the topmost of Nantuo Formation, when the weathering intensity was already low. Therefore, the active biogeochemical cycle implies a sustained habitability, warranting the survivorship in the pan-glacial ocean, while the persistently low and delayed recovery of marine productivity might have imposed environmental stress for tens of million years, paving the way for the evolution of animals.

Cyclic cold climate during the Nantuo Glaciation: Evidence from the Cryogenian Nantuo Formation in the Yangtze Block, South China

Geological and paleomagnetic data indicate that the ice sheets might have extended to low-latitude regions in the Cryogenian (720–635 Ma). The ‘snowball Earth’ hypothesis proposed that the Earth was completely ice-covered during global glaciation. However, the glacial sedimentary record seems to contradict with the snowball Earth hypothesis. Detailed sedimentological investigations of the glacial deposits would provide the first-order constraint on the nature of global glaciation. The Nantuo Formation (∼654–635 Ma) in the Yangtze Block of South China has been correlated with the Marinoan snowball Earth. In this study, we conducted systematic sedimentological analyses of six sections/cores of the Nantuo Formation. Three facies associations were recognized: the proximal glaciomarine, distal glaciomarine, and non-glacial marine facies associations. The vertical stacking pattern of facies associations can be correlated among the five slope and basin sections, while their correlation with the shelf section remains obscure. Our data indicate two episodes of glaciation that are separated by an interglacial interval during the Nantuo Glaciation. The first glacial episode is recorded by successions of coarse-grained facies (e.g., massive diamictite) in the lower part of the Nantuo Formation. The re-appearance of massive diamictite in the middle to upper part of the Nantuo Formation indicates onset of the second glacial episode. These two glacial episodes were separated by a siltstone/shale sequence of several 10 s m thick, suggesting an interglacial period with limited influence from glaciation. The top of Nantuo Formation consists of alternative distal-glaciomarine and non-glacial marine facies associations, representing the deglacial sequence of the Nantuo Glaciation. The sedimentary facies analysis indicates that the cold climate during the Nantuo Glaciation may be cyclic. Finally, our sedimentary analysis confirms a lag between the deglaciation and precipitation of cap carbonate.

Cyclostratigraphic constraints on the duration of the Datangpo Formation and the onset age of the Nantuo (Marinoan) glaciation in South China

Constructing an accurate timeline is critical for reconstructing the Earth systems through critical transitions in climate, geochemistry, and life. Existing dates constrain synchronous initiation (ca. 717 Ma) and termination (ca. 660 Ma) of the Sturtian glaciation from multiple continents. The termination of the younger Marinoan glaciation is also well dated at ca. 635 Ma, but the onset of this glaciation is only roughly constrained as ≤ ca. 654 Ma (South China) and ≥ ca. 639 Ma (Namibia). To test if the Marinoan glaciation started close to ca. 654 Ma or ca. 639 Ma, we have conducted a cyclostratigraphic study on the Cryogenian non-glacial Datangpo Formation that conformably overlies and underlies Sturtian and Marinoan glacial diamictites, respectively, in a deep-water basin section in South China. A total of 28,765 magnetic susceptibility (MS) measurements from a drillcore of the 292-m-thick, muddy siltstone- and shale-dominated Datangpo Formation are used for cyclostratigraphic analysis. The results reveal significant decameter- to meter-scale sedimentary cycles of 16–12 m, 3.6–3.0 m, 1.0–0.8 m, and 0.6–0.4 m. The ratios of these cycle wavelengths match well with those of the Milankovitch cycles calibrated for the Cryogenian Period. The established astrochronologic time scale suggests that the duration of the Datangpo Formation is about 9.8 million years. Together with the radiometric age of ca. 660 Ma for the termination of the Sturtian glaciation, the cyclostratigraphic data suggest that the Nantuo (Marinoan) glaciation in South China initiated at ca. 650 Ma, which is slightly younger than but consistent with the ca. 654 Ma U–Pb age from the top of the Datangpo Formation in shelf sections. This age, however, is significantly older than the ages obtained from Marinoan-age glacial diamictites in South China (ca. 636 Ma) and Namibia (ca. 639 Ma). Given that most of the shelf sections may have suffered from glacial erosion, obtaining the onset age of the Marinoan glaciation should focus on relatively complete, deep-water successions.

New SIMS U–Pb zircon age and its constraint on the beginning of the Nantuo glaciation

A new SIMS U–Pb zircon age of 654.2 ± 2.7Ma was reported from the middle Datangpo Formation in Changyang County of Hubei Province, China. The age is identical with that of 654 ± 3.8Ma from the top Datangpo Formation in western Hunan Province, revealing that the volcanic activities (~654Ma) may have occurred widely in South China. The occurrence of the tuff beds (~654Ma) at different intervals of the Datangpo Formation indicates that the erosion process after the Datangpo deposition varied significantly in different areas. It is thus consequently suggested that the rift in South China was active until the Nantuo glaciation. In addition, the new age confirms that the Nantuo glaciation started much later than 654Ma.

Fossil microbial communities in Neoproterozoic interglacial rocks, Maikhanuul Formation, Zavkhan basin, Western Mongolia

Glaciogenic deposits are widely distributed in the Neoproterozoic successions around the world, but only few of them are associated with occurrences of authentic macroscopic fossils. Problematical circular remains recovered from intertillite beds of the Neoproterozoic Maikhanuul Formation, Zavkhan basin, Western Mongolia may be a new record of biogenic structures in ancient glacial rocks. The Maikhanuul tillites were deposited between 732 and 777Ma (the underlying volcanic rocks of the Neoproterozoic Dzabkhan Formation) and 632±14Ma (the overlying limestones of the Ediacaran – Nemakit – Daldynian Tsagaanolom Formation). The assemblage of microfossils from the Tsagaanolom Formation is comparable to the assemblage known from the 635–551Ma Doushantuo Formation of South China. The Maikhanuul glacial event may therefore be correlated with the 635Ma Nantuo glaciation. Morphological, taphonomic, biometric and geochemical observations have allowed us to interpret the problematical circular remains from the Maikhanuul tillites as microbial communities preserved in situ on the floor of an ancient glacial basin. Many discoidal fossils from Precambrian strata have been compared with extant microbial colonies that have a similar external morphology. However, ultrastructure of the hypothetical Precambrian microbial communities, to the best of our knowledge, has not previously been reported. Scanning electron microscopy of the circular remains from the Maikhanuul tillites revealed rod-shaped, filamentous and star-shaped bacteriomorph structures. Some microremains and especially the star-shaped particles are considerably enriched in iron (up to 50%) that may be attributed to bacterial activity. Furthermore, the star-shaped microremains can be compared with modern star-shaped bacteria known from freshwater and terrestrial environments and with aggregates of biogenic nanohematite. Although the presence of microbial communities could not be used for global correlation, it does provide information on prokaryotic diversity and ecology as well as on palaeoenvironmental conditions.

The Sugetbrak basalts from northwestern Tarim Block of northwest China: Geochronology, geochemistry and implications for Rodinia breakup and ice age in the Late Neoproterozoic

Zircons from two samples of the Sugetbrak basalts (SB) yield weighted mean ages of 615.2±4.8Ma and 614.4±9.1Ma. These ages, interpreted as the eruption age of the SB, provide an age constraint on the timing of the Sugetbrak Formation in Sugetbrak, northwestern Tarim Block, China. These two ages suggest that the igneous activities related to the breakup of the Neoproterozoic supercontinent Rodinia lasted until 614–615Ma in at least the northwest Tarim Block. Geochemical analysis indicates that the SB was generated in an intra-continental rifting environment. Application of the dynamic melting inversion method suggests that the Sugetbrak basalts represent 7–12% partial melts. Unlike large-volume tholeiites, these low-degree transitional basaltic melts may represent the waning stage of plume volcanism during a long-lasting continental breakup. Based on the ages of the SB and its stratigraphic relationship with the Yuermeinak diamictite, the Yuermeinak glaciation in the Sugetbrak section of the northwest Tarim Block should correlate with the Tereeken glaciation in the Qurugtagh area of the northeast Tarim Block, the Nantuo glaciation in Yangtze Block and the Elatina glaciation in South Australia.

Multiple cycles of glacier advance and retreat during the Nantuo (Marinoan) glacial termination in the Three Gorges area

The Snowball Earth hypothesis suggests that the sea water had totally been kept frozen for millions of years in Neoproterozoic glaciation, followed by a rapid and catastrophic deglaciation resulting from the elevated concentration of atmospheric CO2. However, the sedimentary records are not consistent with the Snowball Earth hypothesis. The Nantuo Formation in Three Gorges area is composed of diamictites, sandstones and siltstones. The geochronology and the unconformity with underlying Liantuo Formation indicate that the Nantuo Formation in Three Gorges area may be the partial record (i.e., the final stage) of the Nantuo glaciation. Our studies on sedimentary successions of the Nantuo Formation convince the stepwise transition from the Cryogenia icehouse to the Ediacaran greenhouse, in which multiple glacier advanceretreat cycles rather than a catastrophic termination could be identified.

Chapter 32 Neoproterozoic glacial records in the Yangtze Region, China

Abstract The Nanhua rift occurred at c. 820 Ma. The last rifting stage was likely associated with the Neoproterozoic glaciations. The glacially influenced sequence, in ascending order, comprises the Jiangkou Group ( c. 720–663 Ma), the interglacial Datangpo Formation (Fm.) ( c. 663 Ma) and the Nantuo Fm. (<656–635 Ma). In deep-water environments the glacially influenced units exhibit conformable contacts with a transitional lower boundary and a sharp upper boundary. In shallow marine environments, the lower boundary is always unconformable and erosive and the upper boundary is sharp but conformable. The Jiangkou glaciogenic deposits likely comprise the waxing, the maximum and the waning stages of glaciation. The lower part of the Chang'an Fm. corresponds to the waxing stage. It initially inherited the pre-glacial deep-water environment, and later, the thickening and coarsening upward trend likely indicates a lowering of sea level and ice-sheet advance. The predominant, massive diamictite in the upper Chang'an Fm. probably highlights the maximum glacial stage, and likely resulted from an amalgamation of debris flow deposits, suspension and ice-rafted debris. The Fulu Fm. records the waning stage; its lower member is characterized by arkosic sandstone and greywacke indicative of an interstadial between glacial events, while the upper member represents the final glacial event of the Jiangkou glaciation. The carbonaceous black shale and Mn carbonate at the base of the interglacial Datangpo Fm. were likely indicative of sedimentary starvation associated with Jiangkou deglaciation. The Nantuo glaciation is characterized by an abrupt onset and termination with negligible waxing and waning stages. The sediments likely resulted from subglacial deposition, sedimentary gravity flows and ice-rafted debris with reworking by currents. The onset of Jiangkou glaciation took place around c. 720 Ma and terminated at c. 663 Ma. The Nantuo glaciation started at < c. 656 Ma and terminated at 635 Ma. The palaeolatitude during the Jiangkou glaciation is unconstrained and was 37±7° during the Nantuo glaciation. The palaeogeographic reconstruction for the Yangtze Block is controversial: it was either located between Australia and Laurentia or to the NW of Australia and to the north of India. Values of δ 34 S pyrite from the top of the Fulu and the base of Datangpo formations exhibit extraordinarily high values up to +60‰. Values of δ 13 C carb from the base of the Datangpo Fm. range from –7.4‰ to –13.0‰ and from the cap carbonate of the Doushantuo Fm. are generally lower than –5‰; however, methanogenic values as low as –48‰ have been reported. From the Datangpo Fm., dinosterane has been detected, and phytane, triterpanes and hopane-type pentacyclic triterpanes are also abundant. Biomarker data from glacially influenced rocks indicate that the photosynthetic process never ceased and that the marine environment was probably normal during glaciations. The banded iron formation (BIF) at the base of the Fulu Fm. is either basaltic, tuffaceous, siliciclastic or carbonate-hosted. Rhodochrosite at the base of the Datangpo Fm. was probably associated with submarine volcanic and hydrothermal activity, but a detrital oxide origin cannot be excluded. The difference between the Jiangkou and the Nantuo glaciations is significant. Tectonics and associated topography are speculated to be the main controlling factors on the Neoproterozoic glaciations in the Yangtze region.

Quantitative evaluation of the biostratigraphic distribution of acanthomorphic acritarchs in the Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China

Doushantuo acanthomorphic acritarchs are large morphologically complex organic-walled microfossils broadly constrained between the ∼635 Ma Nantuo glaciation and the ∼551 Ma Miaohe Biota. They are potential biostratigraphic tools for subdivision and correlation of the Ediacaran System in South China. However, major variations in sedimentary facies and stratigraphic thickness present challenges in understanding the spatial and temporal distribution of these acritarchs. Further, the distribution of acritarchs in the Doushantuo Formation is associated with the presence of early diagenetic chert and phosphate, implying a certain degree of preservational bias and/or environmental control. The purpose of this paper is to document the stratigraphic distribution of Doushantuo acritarchs and to quantitatively evaluate their biostratigraphic significance and possible taphonomic–environmental biases, based on high-resolution paleontological data from six sections over ∼100 km in the Yangtze Gorges area, South China. A total of 1082 acritarch fossils were recorded from 84 chert horizons in the six sections of the study area. These chert horizons are not uniformly distributed throughout the Doushantuo Formation, thus the presence/absence of early diagenetic chert does play a role in controlling the distribution of acritarchs. The sampled chert horizons can be grouped into two stratigraphic intervals in the lower and upper Doushantuo Formation, respectively, based on regional stratigraphic correlation. Quantitative analysis shows that the two intervals are distinct taxonomically and largely independent of taphonomic or facies controls. Thus, the two intervals can be regarded as assemblage biozones. The lower biozone (biozone 1) is numerically dominated by Tianzhushania spinosa ( n = 587; 68.3%) and Meghystrichosphaeridium magnificum ( n = 74; 8.6%), whereas the upper biozone (biozone 2) is dominated by Ericiasphaera rigida ( n = 104; 47.1%) and Tianzhushania spinosa ( n = 34; 14.1%). The three most common genera, Meghystrichosphaeridium, Tianzhushania, and Ericiasphaera, have been identified in all sections. Correspondence analysis (CA), detrended correspondence analysis (DCA), discriminant analysis (DA), and pairwise comparisons of samples (Spearman rank coefficient and Jaccard–Chao index), all consistently support biostratigraphic zonations. Thus, the distribution of the Doushantuo acanthomorphs is primarily controlled by biostratigraphic position of samples, with facies and taphonomic differences playing a secondary role. Our case study suggests that acanthomorphic acritarchs can offer a viable tool for regional correlation of the Ediacaran System.

The age of the Nantuo Formation and Nantuo glaciation in South China

AbstractA U–Pb sensitive high‐resolution ion microprobe (SHRIMP) age of 654.5 ± 3.8 Ma from an ash bed immediately below the Nantuo Formation in South China provides the lowest age constraint for the Nantuo glaciation, which has been correlated with the global ‘Marinoan’ glaciation. A U–Pb SHRIMP age of 636.3 ± 4.9 Ma from a fallout tuff within the basal layer of the Nantuo Formation, along with the existing age of 635.2 ± 0.6 Ma from its overlying Doushantuo cap carbonate, suggests that the traditionally defined Nantuo Formation may have been deposited in a short time period at the end of the Nantuo glaciation. In combination with available ages globally, the data support a relatively short duration and rapid termination of ‘Marinoan’ glaciations.

Carbon isotope evidence for widespread methane seeps in the ca. 635 Ma Doushantuo cap carbonate in south China

Distinctive sedimentary structures, textures, and extremely negative δ 13 C carb values from the Ediacaran Doushantuo cap carbonate (ca. 635 Ma) in south China were taken as evidence for a methane hydrate destabilization event, but existing data for a methane-derived δ 13 C carb signature were reported from a single locality in the Yangtze Gorges area. Here we report negative δ 13 C carb values as low as −48‰ (Vienna Peedee belemnite) from two additional sections ~6 km and 55 km from the original locality. These negative δ 13 C carb values were obtained from isopachous cements that fill stromatactis-like cavities, sheet cracks, and fractures, and from partially recrystallized carbonate crusts, consistent with carbonate precipitation at cold methane seeps. The new data support a widespread methane release event following the Nantuo glaciation in south China (ca. 635 Ma), which may have contributed to the origin of unusual sedimentary and isotope features of cap carbonates.

The diversification and extinction of Doushantuo‐Pertatataka acritarchs in South China: causes and biostratigraphic significance

AbstractThe Ediacaran Period immediately follows the last Cryogenian glaciation—the ∼635 Ma Marinoan or Nantuo glaciation, and it is also punctuated by another brief glaciation—the ∼582 Ma Gaskiers glaciation. It is possible that these glaciations may have had significant impact on Ediacaran biological evolution (e.g. the appearance or disappearance of Doushantuo‐Pertatataka acritarchs). Alternative hypotheses propose that the diversification of Doushantuo‐Pertatataka acritarchs was caused by the Acraman bolide impact or by emerging eumetazoans. To test these hypotheses, high‐resolution geochronological and biostratigraphic data are required. The Doushantuo Formation in South China, radiometrically constrained between ∼635 and ∼551 Ma, has the potential to clarify the global picture of early‐middle Ediacaran evolution. Here we present preliminary biostratigraphic data from the Doushantuo Formation in the East Yangtze Gorges area and new δ13C chemostratigraphic data from the Doushantuo Formation at Weng'an. These and previously published palaeontological data, aided by the tool of δ13C chemostratigraphy, indicate that the biostratigraphic record of the Doushantuo Formation is locally sensitive to the availability of specific taphonomic windows. In the East Yangtze Gorges area, Doushantuo‐Pertatataka acritarchs first appeared shortly after the termination of the Nantuo glaciation and gradually diversified throughout the Doushantuo Formation. At Weng'an, however, such acritarchs first appear—abruptly and in much greater diversity—in phosphorite of the upper Doushantuo Formation, immediately above a subaerial exposure surface. Thus, the biostratigraphic pattern in the East Yangtze Gorges area permits, whereas that at Weng'an apparently disallows, a causal relationship between the Nantuo glaciation and the diversification of Doushantuo‐Pertatataka acritarchs. We conclude that the biostratigraphic record is incomplete at Weng'an, where the early Ediacaran evolutionary history is not preserved. The South China data indicate that special attention has to be paid to taphonomic and palaeoenvironmental analysis before extrapolating local and regional biostratigraphic ranges to make global interpretations.It is less clear when Doushantuo‐Pertatataka acritarchs disappeared. Previous investigators have variously suggested that they disappeared before, at, or after, the Gaskiers glaciation. These hypotheses are difficult to test because of the lack of sedimentary evidence for the Gaskiers glaciation in South China and other regions (e.g. South Australia) where Doushantuo‐Pertatataka acritarchs are abundant. In Australia, Doushantuo‐Pertatataka acritarchs are thought to have experienced a sharp decline after the Egan glaciation, which may well be equivalent to the Gaskiers glaciation. If true, then Doushantuo‐Pertatataka acritarchs are largely restricted to the interval between the Nantuo and Gaskiers glaciations. This conclusion allows us to place constraints on the possible causes of the diversification and extinction of Doushantuo‐Pertatataka acritarchs and has important implications for the biostratigraphic significance of Doushantuo‐Pertatataka acritarchs. Copyright © 2006 John Wiley & Sons, Ltd.

Integrated stratigraphy of the upper Neoproterozoic succession in Yunnan Province of South China: Re-evaluation of global correlation and carbon cycle

This study provides a chemostratigraphic and sequence stratigraphic framework of a relatively unfossiliferous sub-Cambrian marine sedimentary succession in Yunnan Province of South China. Our results allow its global stratigraphic correlation and provide an improved understanding of marine carbon cycle during the late Neoproterozoic. The succession has a basal glacial diamictite (“Nantuo” Formation) and comprises two large-scale depositional cycles towards the Precambrian-Cambrian (PC-C) boundary, each of which changes from siliciclastic to carbonate sediments with environments ranging from shallow subtidal to evaporitic supratidal. In the absence of biostratigraphic control, attribution of the tillite has remained equivocal between the ∼710 Ma Sturtian and ∼635 Ma Marinoan glaciations. A compilation of known paleomagnetic and other geochronologic constraints suggests that the Nantuo glaciation in South China was at least diachronous, in which the “Nantuo” tillite in Yunnan belongs to the older glaciation. Stratigraphic patterns of the diagenetically-screened δ 13C variation and depositional sequences during the carbonate-dominated interval show a striking similarity with those of the upper Cryogenian and entire terminal Proterozoic carbonates at widely-separated other localities rather than the terminal Proterozoic alone, supporting the view that deposition of the “Nantuo” tillite in Yunnan reflects the Sturtian rather than Marinoan glaciation. Further considerations of the δ 13C results provide three important implications for the late Neoproterozoic marine carbon cycle. First, comparison of different δ 13C profiles suggests considerable variability in the absolute values of carbonate on both intra- and inter-basinal scales, which differs profoundly from their relative homogeneity in marine Phanerozoic counterparts and may therefore reflect lateral isotopic heterogeneity in the oceanic carbonate reservoir during the late Neoproterozoic. Second, an empirical correlation between the δ 13C and relative sea-level changes was applied to evaluate hypothesized explanations for δ 13C temporal variation in surface sea, yielding short-term inverse and long-term positive relationships. The short-term trend marks periods of the negative δ 13C excursions and is compatible to most of the existing arguments for the δ 13C events. On the other hand, mechanisms causing the long-term trend remain enigmatic in the absence of comparable data from other localities. Our observation, however, suggests that at least two different mechanisms operated marine carbon cycle during the late Neoproterozoic. Third, the negative δ 13C anomaly, characterizing the period following the Marinoan glaciation worldwide, was not coincident between kerogen and carbonate but occurs as a lagged drop in Yunnan, where a ∼7‰-depletion in kerogen preceded an ∼3‰-depletion in carbonate. Their lagged trends likely reflect different sources, as kerogen and carbonate signals are considered to monitor surface and deeper waters, respectively. The negative δ 13C event would therefore have occurred in surface water, followed by a smaller 13C-depletion through the entire water column due to vertical mixing between surface and deeper waters at the studied site. This mechanism supports the proposed hypothesis that injection of the 13C-depleted atmospheric CO 2 into the surface sea, deriving ultimately from quantitative dissociation of methane hydrate in terrestrial permafrost during the post-glacial warming, contributed to the negative δ 13C anomaly.

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

The reported new U-Pb age by sensitive high-resolution ion microprobe (SHRIMP II) on zircon was obtained from a tuff sample at the basal Doushantuo Formation in the Jiuqunao section, which situated at the western limb of the Huangling anticline in the Yangtze Gorges in Zigui, Hubei, South China. Eighteen spots of zircons were analyzed and they form two clusters: one includes three spots, with an inherited age of 784±15 Ma (MSWD=0.05); the other consists of 15 spots and gives a weighted mean 206Pb/238U age of 628.3±5.8 Ma (MSWD=0.86). It is the first SHRIMP U-Pb zircon age obtained nearly the base of the Doushantuo Formation of Ediacaran and represents a maximum age of the Doushantuo Formation It also forms an age constraint on the upper limit age of the Nantuo (Marinoantype) glaciation.

New constraints on the ages of Neoproterozoic glaciations in south China

The most complete Neoproterozoic successions in south China contain three diamictite intervals in the Changan, Tiesiao, and Nantuo Formations. The youngest and most widespread Nantuo glacial deposit overlies the Datangpo Formation and underlies the fossil- rich Doushantuo Formation. Previous authors have correlated the Nantuo diamictite to either Sturtian or Marinoan glacial deposits elsewhere. Here we report sedimentary and δ13C chemostratigraphic data of the Doushantuo cap dolostone, which overlies the Nantuo Formation. Facies-dependent variation in δ13C is interpreted as evidence for spatial heterogeneity in δ13C and/or temporal diachroneity in the initiation of cap carbonate sedimentation. Sedimentary and chemostratigraphic data are indicative of a Marinoan age for the Nantuo glaciation. This inference is supported by a new U-Pb zircon age of 663 ± 4 Ma from a tuffaceous bed in the Datangpo Formation. The new date and other isotopic ages from south China constrain the age of the Changan and Tiesiao glaciation(s) as between 761 ± 8 Ma and 663 ± 4 Ma, and the Nantuo glaciation as between 663 ± 4 Ma and 599 ± 4 Ma.

History of Neoproterozoic rift basins in South China: implications for Rodinia break-up

Abstract We present here a first-order analysis of the Neoproterozoic continental rift basins in South China, focusing on the north-south trending Kangdian basin in the western part, and the Yangtze-side of the northeasterly-trending Nanhua basin. The Yangtze-side of the Nanhua basin is subdivided into the Hunan-Guangxi, the Jiangnan Ridge, and the northern Zhejiang sub-basins. The Neoproterozoic successions in these basins consist of four major sequence-sets, representing four phases of rifting. The first phase occurred at ca. 820 Ma after the 830–820 Ma bimodal magmatism and rapid crustal unroofing, and is represented by the Shiqiaopu Formation in the Hunan-Guangxi sub-basin and the Luojiamen Formation in the northern Zhejiang sub-basin. The second phase at ca. 800 Ma is represented by the Hongchicun Formation in the northern Zhejiang sub-basin. The third, a major rift phase, occurred at ca. 780–750 Ma, represented by the Sanmenjie Formation in the Hunan-Guangxi sub-basin, the Luokedong and Jingtan formations in the Jiangnan Ridge sub-basin, and the Shangshu Formation in the northern Zhejiang sub-basin. The final phase recorded the rift–drift transition during the Nantuo glaciation, estimated at ca. 750–690 Ma, with no volcanism. This was followed by a sag phase to the remainder of the Neoproterozoic, which saw the entire Yangtze Block covered with either platform carbonate deposits or basinal carbonaceous and cherty deposits. The basin history in South China prior to ca. 600 Ma can be well correlated with that in the Adelaide rift system in southeastern Australia, suggesting that South China could have been adjacent to eastern Australia in the supercontinent Rodinia, but broke away from Australia by the end of the continental rifting at ca. 750–690 Ma.

Phosphatized biotas from the Neoproterozoic Doushantuo Formation on the Yangtze Platform

The Doushantuo Formation in South China was deposited after the Nantuo glaciation but before the evolution of complex Ediacaran metazoans. It contains multiple taphonomic windows, in its cherts, carbonaceous shales, and phosphorites, onto the late Neoproterozoic biosphere. The phosphatic window is unusually clear; Doushantuo phosphorites at Weng’an (Guizhou Province) are known to exquisitely preserve a multitude of single-celled eukaryotes, multicellular algae, and microscopic animals. Our recent survey reveals that, in addition to the now famous Weng’an locality, Doushantuo phosphorites at Baokang (Hubei), Chadian (Shaanxi), and Shangrao (Jiangxi) also contain diverse eukaryotes preserved at the cellular level. All these phosphorites were deposited in shallow-water environments, typically above fair weather wave base and close to ancient islands. Along with Doushantuo cherts and shales, these phosphorites give us a clearer and more complete picture of late Neoproterozoic biological evolution: there is a remarkable diversification of multicellular eukaryotes shortly after several Neoproterozoic glaciation events.

A high-quality mid-Neoproterozoic paleomagnetic pole from South China, with implications for ice ages and the breakup configuration of Rodinia

Neoproterozoic (Sinian) sediments are exceptionally well preserved in the Three Gorges region (western Hubei Province) of the South China block. We report new paleomagnetic results, obtained independently by two separate laboratories, from a total of 157 samples of the 748±12 Ma, basal Sinian Liantuo Formation at its type locality. Detailed thermal demagnetization procedures and least-squares line analyses reveal three distinct magnetic components among the suite of samples. Two overprint components can be distinguished from each other by their laboratory unblocking temperatures. The first to be removed (‘C’), always annihilated below 600°C, is common throughout the dataset but is amenable to least-squares line-fitting in only 37 samples. It yields a pole which in present coordinates resembles Mesozoic overprints identified from previous studies in the Three Gorges region (75.7°N, 174.3°E, d p=6.0°, d m=8.3°, Q=4). The higher unblocking-temperature overprint (‘B’), always subsidiary to the ‘A’ component, is more prevalent than ‘C’ and was identified by line-fitting in 67 samples. The ‘B’ direction is very steep and generates a paleopole whose in situ coordinates do not resemble the Mesozoic–Cenozoic apparent polar wander path for South China, and whose tilt-corrected coordinates (20.3°N, 106.2°E, d p=7.2°, d m=7.3°, Q=5) bear no resemblance to any reliable Phanerozoic paleopoles from the South China block. The steep ‘B’ direction, if an unbiased representative of an ancient geomagnetic dipole field, was probably acquired some time in the 200 m.y. interval between deposition of the Liantuo Formation at ∼750 Ma and Cambrian time. The most stable component is a two-polarity remanence, removed at temperatures predominantly >630°C, which we infer to reside in hematite. A change in polarity of this component occupies a similar stratigraphic position (within 5 cm) among three outcrops separated by ∼100 m lateral distance. We calculate a mean paleomagnetic pole from each of the laboratories' datasets and combine these with a previously determined pole from correlative rocks in Yunnan [‘N1’ of Zhang and Piper, Precambrian Res. 85 (1997) 173–199], to obtain an overall weighted mean paleomagnetic pole (04.4°N, 161.1°E, A 95=12.9°, Q=7) for the South China block at 748±12 Ma. The combined ‘Z1’ pole is considered to be primary based on its thermal stability, its magnetostratigraphic consistency, and a soft-sediment fold test determined by previous work. Results from individual sampling areas constrain the depositional paleolatitude of the Liantuo Formation and equivalent Sinian rocks to 30–40°. This result applies to one or both of the stratigraphically adjacent Chang'an and Nantuo glacial deposits; unfortunately, it is neither high nor low enough to refute any of the conceptual models for the enigmatic Neoproterozoic glaciations. The new basal Sinian paleopole, in the context of recent paleomagnetic and geochronological results from Australia, suggests that the Nantuo glaciation is pre-Marinoan. The new ‘Z1’ pole may also provide constraints on the various proposed reconstructions of South China's position in Rodinia. In particular, a paleoposition adjacent to northwestern Australia at ∼750 Ma requires a specific relative orientation between the two blocks. Likewise, if Rodinia were still intact by 750 Ma, South China may have lain between Australia and Laurentia only in an orientation different from that originally proposed in the ‘missing link’ hypothesis. As a final alternative, the new paleomagnetic data could be used to position South China, Australia, and Laurentia in an immediately post-Rodinian paleogeography around the nascent Pacific Ocean.