Rare earth element geochemistry of Middle Devonian reefal limestones of the Dianqiangui Basin, South China: implications for nutrient sources and expansion of the reef ecosystem.

The Givetian Age witnessed the greatest expansion of stromatoporoid-coral reefs from low to higher latitudes of the Phanerozoic. Multi-proxy seawater surface temperature reconstruction suggests the establishment of a super-greenhouse climate as a major reason for reef expansion, yet many questions remain. This article presents the results of a rare earth element and yttrium (herein referred to as REY, derived from REE + Y) geochemical study as well as mineralogy and oxygen isotope values of two well-documented Middle Givetian reefal carbonate sections (Jiwozhai and Buzhai) of the Jiwozhai Formation of South China. The nearshore Jiwozhai patch reef succession displays greater biodiversity and more abundant coral than the marginal platform Upper Buzhai reef. Reefal and micritic carbonates of the Jiwozhai section are characterized by shale-like post-Archean Australian Shale (PAAS)-normalized REY patterns, by very weak negative Ce anomaly values (Ce/Ce* 0.80-0.96; average = 0.89), slightly elevated Y/Ho values (28.9-39.1; average = 34.1), and near-unity values of (Pr/Yb)N (average = 0.87), (Pr/Tb)N (average = 0.80), and (Tb/Yb)N (average = 1.09). Moreover, REY patterns of deposits of the Jiwozhai section differ markedly from those of modern seawater. The described geochemical aspects of the Jiwozhai section and the positive correlation of REY and Th contents displayed by the section point to a terrestrial siliciclastic contribution contemporaneous with reef-building. In contrast, REY patterns of the Upper Buzhai reef section samples are similar to those of modern seawater characterized by light rare earth element (LREE) depletion (average (Pr/Yb)N = 0.76), negative Ce anomalies (average Ce/Ce* = 0.88), and average super-chondritic Y/Ho ratios (average = 45.4)). Slightly positive Eu anomalies (Eu/Eu* = 0.93-1.94; average = 1.36) of the Upper Buzhai reef section samples are attributed to the negligible effect of hydrothermal fluids. Middle REE (MREE) enrichment (average (Tb/Yb)N = 1.48) of Buzhai section carbonate samples and positive correlation of REY and Th suggest a riverine input. Combined with siliciclastic mineralogy, oxygen isotope values, and reef-building biota morphology of the studied two sections, we suggest that terrestrial nutrients delivered by rivers far outweighed upwelling as a source of nutrients supplied to the Givetian reef ecosystem of South China. Coral and stromatoporoid in tropic oceans thrived in turbid water containing abundant terrestrial sediment and the nutrient-laden water helped expand reef-builder habitats during the Givetian time.

The Upper Triassic deposits of the west Bangong-Nujiang suture zone and their paleogeographic implications.

The Bangong-Nujiang Suture Zone (BNSZ) of Tibet (Xizang) has been interpreted to represent a relic of the Bangong-Nujiang Ocean. However, the existence of this ocean during Triassic time remains a point of contention. A sedimentary succession spanning the Upper Permian through Triassic described from the central BNSZ suggests that the Lhasa and South Qiangtang terranes were contiguous thus negating the existence of a terrane-separating ocean during Triassic time. However, the apparent lack of Triassic deposits in the west BNSZ has called into question the existence of Triassic deposits in the central region of the BNSZ. Our biostratigraphic work in the Wuga Formation of the Gaize area has yielded abundant Norian conodonts thus confirming the existence of Upper Triassic deposits in the west BNSZ. The clastic deposits of the Wuga Formation are herein interpreted to be of Rhaetian age. Moreover, intercalated limestone and chert are termed the Dongnale Formation of Norian age. The Norian to Rhaetian succession can be correlated with strata of the central BNSZ as well as with deposits of the Lhasa Terrane and the South Qiangtang Terrane. Similar stratigraphies among these regions through the Late Triassic suggests a shared depositional setting and that the BNSZ was not an ocean in Norian and Rhaetian time.

Heterogenous oceanic redox conditions through the Ediacaran-Cambrian boundary limited the metazoan zonation.

Recent studies have enhanced our understanding of the linkage of oxygenation and metazoan evolution in Early Cambrian time. However, little of this work has addressed the apparent lag of animal diversification and atmospheric oxygenation during this critical period of Earth history. This study utilizes the geochemical proxy and N isotope record of the Ediacaran-Cambrian boundary preserved in intra-shelf basin, slope, and slope basin deposits of the Yangtze Sea to assess the ocean redox state during the Early Cambrian metazoan radiation. Though ferruginous conditions appear to have prevailed through the water column during this time, episodes of local bottom-water anoxia extending into the photic-zone impacted the slope belt of the basin. Heterogenous oceanic redox conditions are expressed by trace element concentrations and Fe speciation, and spatial variation of N isotopes. We propose that the coupling of ocean chemistry and Early Cambrian animal diversification was not a simple cause-and-effect relationship, but rather a complex interaction. Specifically, it is likely that animal diversification expanded not only temporally but also spatially from the shallow shelf to deep-water environments in tandem with progressive oxygenation of the extensive continental margin.