Chuan Yang

The Late Ordovician mass extinction (LOME) included two phases (I and II) of high species turnover that have been hypothetically linked to the Hirnantian glaciation and subsequent rapid warming, respectively. However, the timing and tempo of the LOME remain uncertain, which hinders our understanding of the feedback between the LOME and paleoclimatic change. Here, we present high-precision radioisotopic dates for the Ordovician-Silurian transition in South China that reveal the LOME began at 442.76 + 0.35/−0.22 million years ago, with the two phases lasting for 0.34 + 0.46/−0.34 and 0.06 + 0.31/−0.06 million years, respectively. The rapid switch from icehouse to greenhouse conditions, along with the higher mean rate of temperature change during LOME II, resulted in a much higher mean extinction rate during LOME II than I (71.6% versus 8.4% species loss per 100 thousand years, respectively), implying that the rate of climate change was a primary control on the tempo of the LOME.

New radio-isotopic data bracket the age of late Ediacaran fossil assemblages and two negative carbon isotope excursions.

The South China Block (SCB) has been regarded by many as an integral part of Gondwana, but proposed timing and processes for its accretion to Gondwana vary and remain contentious, largely owing to the lack of reliable Pan‐African age paleomagnetic data and tectono‐magmatic records from the SCB. Integrated in situ U‐Pb ages and Hf‐O isotope analyses of detrital zircons from geochronologically well‐calibrated Ediacaran‐Cambrian sedimentary rocks of western SCB reveal age populations of 2.51, 1.85, 1.20, 0.80, and 0.52 Ga. Detrital zircon age spectra indicate a major tectonic transition for the SCB during 0.56–0.54 Ga, interpreted to reflect the beginning of the collision between SCB‐Indochina and NW India blocks. The collisional event lasted until early Ordovician, leading to the suturing of the SCB‐Indochina to the northern margin of East Gondwana.

A new natural zircon reference material SA01 is introduced for U‐Pb geochronology as well as O and Hf isotope geochemistry by microbeam techniques. The zircon megacryst is homogeneous with respect to U‐Pb, O and Hf isotopes based on a large number of measurements by laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) and secondary ion mass spectrometry (SIMS). Chemical abrasion isotope dilution thermal ionisation mass spectrometry (CA‐ID‐TIMS) U‐Pb isotopic analyses produced a mean ²⁰⁶Pb/²³⁸U age of 535.08 ± 0.32 Ma (2s, n = 10). Results of SIMS and LA‐ICP‐MS analyses on individual shards are consistent with the TIMS ages within uncertainty. The δ¹⁸O value determined by laser fluorination is 6.16 ± 0.26‰ (2s, n = 14), and the mean ¹⁷⁶Hf/¹⁷⁷Hf ratio determined by solution MC‐ICP‐MS is 0.282293 ± 0.000007 (2s, n = 30), which are in good agreement with the statistical mean of microbeam analyses. The megacryst is characterised by significant localised variations in Th/U ratio (0.328–4.269) and Li isotopic ratio (−5.5 to +7.9‰); the latter makes it unsuitable as a lithium isotope reference material.

The Cambrian Chengjiang biota of South China provides compelling fossil evidence for the rapid appearance of metazoan phyla in Earth history (the ‘Cambrian explosion’). However, the timing of the Chengjiang biota is poorly constrained owing to lack of datable rock materials within the Maotianshan Shale that yields the fossils. Here we integrate secondary ion mass spectrometry (SIMS) and chemical ablation isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) U–Pb analyses of detrital zircons from the Maotianshan Shale to provide high-precision geochronological constraint on the Chengjiang biota. The youngest group of SIMS U–Pb detrital zircon dates yields an age peak at 520 Ma. Six zircons from this group are further dated by the CA-ID-TIMS U–Pb technique, but suggesting that they were not formed from a single zircon growth event. Thereby neither the age peak nor the weighted mean age defined by the youngest SIMS U–Pb dates could represent the maximum depositional age of the Maotianshan Shale. Instead, the youngest CA-ID-TIMS U–Pb date, 518.03 ± 0.69/0.71 Ma (2 σ , analytical uncertainty/incorporates U–Pb tracer calibration uncertainty), provides the first robust maximum age of the Chengjiang biota. This new geochronological constraint on the Chengjiang biota indicates that the Cambrian explosion reached its major phase around 518.03 ± 0.69/0.71 Ma, demonstrating a protracted process (>22 myr) for the Cambrian explosion. Supplementary material: SIMS and CA-ID-TIMS zircon U–Pb data are available at https://doi.org/10.6084/m9.figshare.c.4024849

Fossiliferous Ediacaran successions of South China, the Doushantuo and Dengying formations and their equivalents, are key to understanding bio- and geological evolution at the Neoproterozoic–Cambrian transition. However, their absolute ages, especially the upper Ediacaran successions, are poorly constrained. SIMS zircon U–Pb dating results in this study suggest that ash beds at the basal and middle parts of the Jiucheng Member (middle Dengying Formation) in eastern Yunnan Province were deposited at 553.6 ± 2.7/(3.8) Ma and 546.3 ± 2.7/(3.8) Ma, respectively. These new dates indicate that the age for the base of Dengying Formation in eastern Yunnan Province is similar to the 550.55 ± 0.75 Ma date, which is from an ash bed at the top of the Miaohe Member and has been regarded as the age for the base of Dengying Formation in Yangtze Gorges area. These dates do not permit a clear test of the two correlation models for the chronostratigraphic position of the Miaohe Member (uppermost Doushantuo Formation vs. middle Dengying Formation), implying that further integrated intra-basinal stratigraphic correlations and more high-resolution chronological data from the upper Ediacaran deposits of South China are required. New dates of the Jiucheng Member constrain the age of the fossil biotas in the middle Dengying Formation and extend the stratigraphic range ofRangea,HiemaloraandCharniodiscusto 546.3 ± 2.7/(3.8) Ma. The geochronology of the Dengying Formation implies that Ediacaran-type fossils preserved in this formation are younger than the White Sea Assemblage and temporally overlapping with the Nama Assemblage.