Unusual Intraclast Conglomerates in a Stormy, Hot-House Lake: The Early Triassic North China Basin

Early Triassic temperatures were some of the hottest of the Phanerozoic, sea-surface temperatures approached 40°C, with profound consequences for both the sedimentology and faunal distributions in the oceans. However, the impact of these temperatures in terrestrial settings is unclear. This study examines shallow lacustrine sediments from the Lower Triassic succession of North China. These consist of diverse fluvial to shallow lacustrine sandstones and also spectacular, coarse conglomerates composed of diverse, intraformational clasts reworked from the interbedded sediments. The conglomerate beds can show inverse grading and high angle, flat-pebble imbrication in their lower part and vertically orientated flat pebbles in their upper part. The cobbles include cemented and reworked conglomerate intraclasts and sandstone concentrically-laminated concretions that record multi-step histories of growth and reworking, pointing to rapid cementation of the sandy lake bed (likely facilitated by high temperatures). The conglomerates record frequent, high-energy events that were capable of brecciating a lithified lake bed and transporting cobbles in wave-influenced sediment-gravity flows. Initially, powerful oscillatory flows brecciated and deflated the lake bed and subsequently helped to sustain turbulence during short-distance lateral flow. It is possible that hurricanes, originating from the adjacent hyper-warm, Palaeo-Tethyan Ocean travelled into the major lakes of the North China continent during the Early Triassic

Early Triassic wrinkle structures on land:stressed environments and oases for life

Wrinkle structures in rocks younger than the Permian-Triassic (P-Tr) extinction have been reported repeatedly in marine strata, but rarely mentioned in rocks recording land. Here, three newly studied terrestrial P-Tr boundary rock succession in North China have yielded diverse wrinkle structures. All of these wrinkles are preserved in barely bioturbated shore-shallow lacustrine siliciclastic deposits of the Liujiagou Formation. Conversely, both the lacustrine siliciclastic deposits of the underlying Sunjiagou Formation and the overlying Heshanggou Formation show rich bioturbation, but no wrinkle structures or other microbial-related structures. The occurrence of terrestrial wrinkle structures in the studied sections reflects abnormal hydrochemical and physical environments, presumably associated with the extinction of terrestrial organisms. Only very rare trace fossils occurred in the aftermath of the P-Tr extinction, but most of them were preserved together with the microbial mats. This suggests that microbial mats acted as potential oases for the surviving aquatic animals, as a source of food and oxygen. The new finds suggests that extreme environmental stresses were prevalent both in the sea and on land through most of the Early Triassic

Integrated Sr isotope variations and global environmental changes through the Late Permian to early Late Triassic

New 87Sr/86Sr data based on 127 well-preserved and well-dated conodont samples from South China were measured using a new technique (LA-MC-ICPMS) based on single conodont albid crown analysis. These reveal a spectacular climb in seawater 87Sr/86Sr ratios during the Early Triassic that was the most rapid of the Phanerozoic. The rapid increase began in Bed 25 of the Meishan section (GSSP of the Permian–Triassic boundary, PTB), and coincided closely with the latest Permian extinction. Modeling results indicate that the accelerated rise of 87Sr/86Sr ratios can be ascribed to a rapid increase (>2.8×) of riverine flux of Sr caused by intensified weathering. This phenomenon could in turn be related to an intensification of warming-driven runoff and vegetation die-off. Continued rise of 87Sr/86Sr ratios in the Early Triassic indicates that continental weathering rates were enhanced >1.9 times compared to those of the Late Permian. Continental weathering rates began to decline in the middle–late Spathian, which may have played a role in the decrease of oceanic anoxia and recovery of marine benthos. The 87Sr/86Sr values decline gradually into the Middle Triassic to an equilibrium values around 1.2 times those of the Late Permian level, suggesting that vegetation coverage did not attain pre-extinction levels thereby allowing higher runoff

Recurrent biotic rebounds during the Early Triassic: biostratigraphy and temporal size variation of conodonts from the Nanpanjiang Basin, South China

Eleven conodont zones are established for the Lower Triassic of the Motianling section in the Nanpanjiang Basin, South China. Detailed size measurements on 2244 P1 conodont elements demonstrate dynamic size variation patterns during the Early Triassic that are calibrated to the Early Triassic timescale and conodont zonation. Correlation of conodont size variation with various environmental indicators shows that these size changes are primarily controlled by rapidly changing and episodic environmental conditions and ecological recovery in the aftermath of the end-Permian mass extinction. Decreasing size trends occur in the early Griesbachian, late Griesbachian and at the Smithian-Spathian boundary. Episodes of size increase (rebounds) occur in the middle Griesbachian, Induan–Olenekian boundary and middle–late Spathian. The observed temporal size variations indicate that conodont animals were significantly affected by rapidly changing environmental conditions (mainly temperature and redox conditions) during the Early Triassic and recurrent size increases (rebounds) occurred during periods of environmental and climatic amelioration (climatic cooling and oxygenation), demonstrating a complicated recovery history of marine ecosystems in the aftermath of the largest Phanerozoic biotic crisis