Yangtze Sea Research Articles

Oceanic environment changes caused the Late Ordovician extinction: evidence from geochemical and Nd isotopic composition in the Yangtze area, South China

AbstractThe Ordovician–Silurian (O–S) transition was a critical interval in geological history. Multiple geochemical methods are used to explore the changes in oceanic environment. The Nd isotopic compositions in the Yangtze Sea are controlled by two sources: the continental erosion and the Panthalassa Ocean. HighεNd(t) values during the Katian, late Hirnantian and Rhuddanian intervals are associated with the high sea level, which resulted in less terrestrial input based on the low Ti/Al and Zr/Al ratios. In contrast, lowεNd(t) values during the early Hirnantian interval are related to the sea-level fall; in this case, the exposure of submarine highs and the growth of Yangtze Oldlands could lead to more continental materials being transported into the Yangtze Sea based on high Ti/Al and Zr/Al ratios. In addition, the negativeεNd(t) excursion can also be attributed to the weak circulation between the Yangtze Sea and Panthalassa Ocean when sea level was low. Furthermore, the sea-level eustacy plays a significant role in the changes in redox water conditions. The redox indices, mainly UEF, Ce/Ce* and Corg/PT, across the O–S transition show a predominance of anoxic ocean over the Yangtze Sea during the Katian, late Hirnantian and Rhuddanian intervals, and an oxygenated episode was briefly introduced during the early Hirnantian period because of the fall in sea level. The Late Ordovician biotic crisis was marked by two-phase extinction events, and the change in sea level and redox chemistry may be the important kill mechanisms.

Sulfate-controlled marine euxinia in the semi-restricted inner Yangtze Sea (South China) during the Ordovician-Silurian transition

Conflicting published interpretations of marine redox conditions during the Ordovician-Silurian transition (OST) may have been linked to spatial redox heterogeneity during this interval. However, details of the pattern of redox heterogeneity and its underlying causes remain unclear. Here, we present a high-resolution geochemical study of a drillcore section (Pengye #1) from Pengshui County (Chongqing municipality, southwestern China) that was located in the semi-restricted inner Yangtze Sea during the OST. We analyzed Fe-speciation, redox-sensitive trace elements, major elements, and pyrite δ34S compositions (δ34Spy) and then compared these data with published results from coeval sections at Datianba and Shuanghe in the same basin. The integrated dataset demonstrates pronounced spatiotemporal heterogeneity of redox conditions—especially the local development of euxinic conditions in the inner Yangtze Sea during the OST. Integrated data further suggest that high primary productivity and ample Fe fluxes in the inner Yangtze Sea may have depleted dissolved sulfate through microbial sulfate reduction (MSR) and subsequent pyrite formation, except in areas with enhanced sulfate supply from continental weathering or open-ocean exchange, which varied as a function of both tectonic (i.e., the regional Kwangsian Orogeny) and eustatic changes (i.e., the global Hirnantian glaciation). Limited sulfate availability thus likely prevented the development of euxinic conditions in some regions of the inner Yangtze Sea, as reflected in spatial variation of δ34Spy. Our study highlights the potential role of sulfate availability on the development of watermass euxinia in semi-restricted marginal-marine basins during the OST.

Climatic and oceanic controlled deposition of Late Ordovician-Early Silurian black shales on the North Yangtze platform, South China

Abstract Black shales deposited across the Late Ordovician and Early Silurian transition are considered to be the most important source rock and shale gas reservoirs in the Yangtze region of South China. However, the origin of these sediments remains contentious. In this study, we investigated the systematic changes in total organic carbon (TOC), organic carbon isotopes (δ13Corg), major elements and trace elements on the Ordovician-Silurian boundary strata from Qiaoting, North Yangtze Sea, to reconstruct the climatic conditions, redox changes, primary productivity, and other factors there, promoting the understanding for the formation mechanism of the organic-rich black shales. Element compositions and their ratios suggest that the black shales studied here were dominantly sourced by the felsic igneous rock, which is similar to granodiorite in composition. Paleoclimatic proxies (CIA, CIW, δ13Corg) suggest that the global climate system experienced significant changes from warm-humid to cold-dry and then to warm-humid climates during the Ordovician-Silurian transition. Paleoredox indices (S/C, U/Th, Ni/Co and V/(V + Ni) ratios) also demonstrate fluctuating redox variations from anoxic to oxygenated, and then to anoxic states during this interval in the Yangtze Sea, which describe an oxygenated Guanyinqiao strata sandwiched between the stagnant and anoxic Wufeng and Longmaxi black shales. Paleoproductivity parameters (TOC, Babio contents and P/Al ratio) suggest that the organic-rich Wufeng and Longmaxi black shales, were deposited with a high biological productivity, while the organic-lean Guanyinqiao sediments were deposited with a low biological productivity. These data demonstrate large climatic and oceanic fluctuations during the Ordovician-Silurian transition, providing essential controlling factors on the oceanic anoxia, primary productivity, and subsequent organic-rich black shale depositions in the Yangtze region during the Late Ordovician and Early Silurian intervals.

Development and evolution of a euxinic wedge on the ferruginous outer shelf of the early Cambrian Yangtze sea

The early Cambrian was a critical interval in geological history and featured profound oceanic and biotic changes. To unravel the oceanic redox conditions, high-resolution analyses of iron speciation and redox-sensitive trace elements (Mo and U) within the framework of sequence stratigraphy were carried out on the Niutitang Formation (~528–521 Ma). The two examined sections, the Daotuo and Bahuang sections, were respectively located in a mid-upper slope setting and a lower slope to basin settings behind a seaward submerged sill on the middle Yangtze Block, South China. At Daotuo, the Fepy/FeHR values display moderate positive covariances with the total organic carbon contents (TOC) and the Mo/TOC ratios, notably in the basal part of the Niutitang Formation. Therefore, at this locality the euxinic water mass wedge was developed in association with a high primary organic productivity/burial rate, likely within a high-productivity zone. In contrast, at Bahuang, the Fepy/FeHR values show weak to negative covariances with the Mo/TOC ratios and TOC contents in the basal part, indicating that the biogeochemical cycles of Fe, Mo with C were decoupled in a lower-productivity, ferruginous deeper basinal setting. These spatial changes in marine redox structures and biogeochemical cycles can be reasonably explained by the existence of oceanic upwelling in the presence of strong offshore currents and a seaward submarine sill, reconciling the oxygen minimum zone (OMZ) in modern oceanic margins to some extent. This spatial redox pattern also accounts well for previous data based on which the euxinic state intermittently invaded upward onto the shelf margin and evolved into a ferruginous-dominant anoxia in the inner shelf subbasins. In addition, the euxinic wedge dynamically fluctuated upslope and downslope along the transect from the ferruginous outer shelf slope to the basin in response to rise and fall of sea-level. Consequently, the temporal evolution of the redox conditions was driven in part by eustatic variations. Moreover, the paleogeographic position of the Yangtze Block in the mid-lower latitudes suggests that the block was associated with relatively strong offshore currents induced by trade winds, which notably enhanced the oceanic upwelling near the transgression maximum. On the other hand, the dominance of deep ferruginous waters with the local occurrence of a euxinic wedge on the outer shelf slope indicates relatively low concentrations of seawater sulfate and atmospheric oxygen during the deposition of the Niutitang black shales. This redox model thus highlights the important roles of paleogeographic, paleoclimatic and eustatic sea-level changes in controlling the spatiotemporal fluctuations in marine redox conditions and biogeochemical cycling in the early Cambrian ocean.

Sponge-dominated offshore benthic ecosystems across South China in the aftermath of the end-Ordovician mass extinction

Offshore benthic communities were sparse after the end-Ordovician mass extinction. The recent discovery of diverse, abundant sponges in south-eastern China (Anhui and Zhejiang provinces) that flourished during the Hirnantian (Late Ordovician) post-extinction interval raises questions over the extent and ecological significance of the sponge community at that time. This paper dramatically expands the scale of the Hirnantian sponge fauna within South China, with seven new occurrences across 2000 km. These occurrences cross the Ordovician–Silurian boundary interval in the marginal and submerged platform areas of the Yangtze Sea in Jiangsu, Hubei, Hunan, Chongqing and Sichuan provinces. In condensed sections (platform-margin facies), the occurrences are normally represented by extensive spiculite layers, but several sites yield entire, exceptionally preserved body fossils; at Ganggangshan (near Nanjing, Jiangsu Province), such preservation is abundant. The new occurrences are combined with palaeogeographic and tectonic reconstructions to produce a model for the distribution and preservation of the sponge fauna. We argue that the exceptional preservation was due largely to nepheloid layers that were generated during rapid post-glacial transgression that flooded the weathered land surface of the uplifted Cathaysia Block. Nepheloid-layer collapse led to rapid deposition of suspended sediment within the deeper parts of the intra-plate, restricted basin and on adjacent platform margins and slopes. The suspended sediment influx also introduced abundant nutrients to the water column, as indicated by a significant δ13C excursion during the sponge-bearing mudstone interval at Ganggangshan. The combined sponge assemblage was dominant over large areas of the Hirnantian offshore sea floor across South China. We predict that similar occurrences with exceptional preservation will also be found in other continental blocks: abundant articulated sponges may be encountered particularly in restricted basin successions containing rapidly deposited graptolitic mudstones near the Ordovician–Silurian boundary.

Oriented graptolites in the Uppermost Ordovician Wufeng black shale in southern China

Graptolites are elongated fossils that can be used to decipher the current direction to evaluate paleocurrent conditions. Abundant graptolite fossils are well-preserved and clearly exhibit strong alignment in the black shales of the Upper Ordovician Wufeng Formation on the southern edge of the Upper Yangtze Sea, South China. The orientations of these graptolites indicate that the remains of graptolites were deposited on the seafloor under the influence of currents, instead of under calm water conditions. Based on the measurements of the long-axis directions of graptolites, the distributions of graptolites preserved on the bedding planes display discernable orientations and exhibit systematic changes throughout the stratigraphic section. In the western region of the study area, the paleocurrent directions in the stratigraphic sections gradually change from NE–SW in the Dicellograptus complexus biozone to N–S in the Paraorthograptus pacificus and Normalograptus extraordinarius–N. ojsuensis biozones. However, in the eastern region of the study area, the paleocurrent directions are more variable, ranging from multi-directional (NNE–SSW and NNW–SSE) in the P. pacificus biozone to NW–SE in the N. extraordinarius–N. ojsuensis biozone. The changes in the orientations of graptolites in the D. complexus and the P. pacificus biozones are consistent with the southern configuration of the Upper Yangtze Sea and indicate that a coastal current existed. In addition, some of the contemporaneous deformation structures were caused by tectonic forces observed in the lower Graptolite Shale Member of the Wufeng Formation in the eastern region of the study area. Some of the changes in the orientations of graptolites in the N. extraordinarius–N. ojsuensis biozone can be interpreted to have resulted from a change in tectonic activity (the Caledonian orogeny) before the N. extraordinarius–N. ojsuensis biozone. Furthermore, Lingula brachiopods, which live on the seafloor in shallow water, have been reported in the black shale of the Wufeng Formation, which indicates that the paleoenvironment was at least partially oxic; this supported the growth, development and reproduction of planktonic graptolites and other organisms. These results support the interpretation that the remains of organisms were quickly buried and that the amount of exchange between pore water and the water column gradually decreased. H2S gas generated by the decay of organisms during deposition consequently generated a strongly reducing environment in these sediments, which resulted in the formation of black shales.

Highly heterogeneous “poikiloredox” conditions in the early Ediacaran Yangtze Sea

Although there has been a sustained effort to reconstruct ocean-redox conditions throughout the Ediacaran, a lack of high-resolution data across time-equivalent strata (shelf to basin) has hindered our ability to understand redox dynamics at the basinal scale. We measured iron species, trace- and major-element concentrations, TOC, and Corg/P ratios of black shale from six high-resolution sections from the early Ediacaran Yangtze Sea (i.e., the inner-shelf Jiulongwan, outer-shelf Zhongling, slope Siduping and Mingle, and basinal Yuanjia and Long’e sections), which were integrated with published data from these sections and seven other correlative sections from the same region for a basinwide perspective on redox tracers. These results document strong spatiotemporal redox variability during the early Ediacaran, including transitions between ferruginous, euxinic, suboxic, and oxic conditions at high frequencies and short distances (a pattern for which we coin the term “poikiloredox conditions”). Our results also suggest that multiple geochemical proxies including iron species, redox sensitive trace element concentrations and enrichments, and Corg/P ratios show different responses to poikiloredox conditions, accounting for conflicting redox interpretations in early Ediacaran studies. This study thus demonstrates the importance of multiple proxies, high stratigraphic resolution, and a basin-scale perspective in paleomarine redox studies, underlining the potential for overgeneralization of redox patterns based on analysis of a single or a limited number of proxies or sections.

Paleo-environmental conditions of the Early Cambrian Niutitang Formation in the Fenggang area, the southwestern margin of the Yangtze Platform, southern China: Evidence from major elements, trace elements and other proxies

The Precambrian/Cambrian transition was a key time in Earth history, especially for marine biological evolution and oceanic chemistry. The redox-stratification with oxic shallow water and anoxic (even euxinic) deeper water in the Early Cambrian Yangtze Sea, which gradually became completely oxygenated, has been suggested as a possible trigger for the “Cambrian explosion” of biological diversity. However, for some areas in northern Guizhou where the exploration and research are lacking, identifying this pattern of redox-stratification by paleo-environmental analysis from borehole data is still in need. Here, we report a remarkable variation range in trace elements (Mo, V, U, Ni, Th, Co, Sc, Zn and Cu), molar Corg:P ratios and pyrite morphology from 27 core samples from one new drill hole (XY1, located in the Fenggang area, northern Guizhou) on the Yangtze Platform, South China.High levels of Ba (from 3242 ppm to 33,800 ppm) and total organic carbon (TOC; from 4% to 9.36%) in 15 core samples in the Lower Member (LM) of the Niutitang Formation indicated elevated primary productivity in the study area. Redox change was recorded based on enrichment factors (EFs) for RSTEs (Mo, U, and V), redox proxies (V/(V + Ni), Ni/Co, V/Sc and Th/U), Corg:P ratios and particle size of framboidal pyrite. These signatures demonstrate that the LM was deposited under anoxic conditions with sulfidic episodes, whereas the Upper Member (UM) of the Niutitang Formation was deposited under suboxic/oxic conditions with intermittently anoxic episodes. Mo/TOC ratios (from 3.72 to 39.86, mean 18.76) suggest weak-moderate water mass restriction. Mo-U covariation patterns (strong but variable enrichment of Mo and U; MoEF ranging from 31.45 to 257.97; UEF ranging from 4.68 to 39.07) in the LM show alternation of particulate shuttling and redox conditions occurred in the Early Cambrian Yangtze Sea, whereas Mo-U covariation patterns (moderate Mo enrichment but depletion or non-enrichment of U; mean MoEF: 7.29; mean UEF: 0.95) in the UM may indicate the combined influence of particulate shuttling and diagenetic diffusion of U via bioactivities, which result in low U values and an anoxic signature from frambiodal pyrite particle size (mean: 4.556 μm; median: 4.41 μm). Additionally, excess Ba (Baxs) concentration (33,800 ppm and 32,500 ppm) and association patterns of trace-metal enrichment in the LM indicate the existence of submarine hydrothermal events. In addition, during deposition of the UM, bioactivities indicated by Mo-U systematics and oxic conditions indicated by redox sensitive trace elements (RSTEs) and multiple-proxies, may be a cause of biological diversification recorded in the Early Cambrian. Finally, data in this record a progressive transition from anoxic bottom waters with euxinic episodes to overwhelming oxic conditions during Early Cambrian.

Paleogeographic evolution of the Lower Yangtze region and the break of the “platform-slope-basin” pattern during the Late Ordovician

Wide distribution of the black shales and diversification of the graptolite fauna in South China during the Late Ordovician resulted from its unique paleogeographic pattern, which was significantly affected by the paleogeographic evolution of the Lower Yangtze region. In the study, 120 Upper Ordovician sections from the Lower Yangtze region were collected, and a unified biostratigraphic framework has been applied to these sections to establish a reliable stratigraphic subdivision and correlation. Under the unified time framework, we delineate the distribution area of each lithostratigraphic unit, outline the boundary between the sea and land, and reconstruct the paleogeographic pattern for each graptolite zone. The result indicates that, with the uplift and expansion of the ‘Jiangnan Oldland’ in the beginning of the late Katian, the oldland extended into the Yangtze Sea gradually from south to north, which finally separate the Jiangnan Slope and the Yangtze Platform. Consequently, the longstanding paleogeographic pattern of “platform-slope-basin” in South China was broken. The paleogeographic change led to sedimentary differentiation among the two sides of the ‘Jiangnan Oldland’ during the Late Ordovician. This event also led to the closure of the eastern exit of the Upper Yangtze Sea, and formed a semi-closed, limited and stagnant environment for the development of the organic-rich black shales during the Late Ordovician. The major controlling factors of these paleogeographic changes in the Lower Yangtze region were not consistent from the Katian to the Hirnantian. In the late Katian, the sedimentary differentiation between the east and west sides mostly resulted from regional tectonic movement - the Kwangsian Orogeny. However, during the Hirnantian, the whole Yangtze region became shallower, which was mostly influenced by the concentration of the Gondwana ice sheet and the consequent global sea level drop.

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.

Evidence for marine redox control on spatial colonization of early animals during Cambrian Age 3 (c. 521–514 Ma) in South China

AbstractThe early Cambrian Period was a key interval in Earth history with regard to changes in both ocean chemistry and animal evolution. Although increasing ocean ventilation has been widely assumed to have played a key role in the rapid appearance, diversification and spatial colonization of early animals, this relationship is in fact not firmly established. Here, we report a high-resolution Fe-C-S-Al-Ti geochemical study of the lower Cambrian Wangjiaping section from an outer-shelf setting of the Yangtze Sea of South China. Iron speciation data document a redox transition from dominantly euxinic to ferruginous conditions during Cambrian Age 3 (c. 521–514 Ma). Interpretation of coexisting pyrite sulphur isotope (δ34Spy) records from Wangjiaping reveals relatively high marine sulphate availability at Wangjiaping. Furthermore, Wangjiaping section shows lower δ34Spy (‒2.1±5.3‰) and lower TOC (2.4±1.1%) values but higher positive correlation (R2 = 0.66, p < 0.01) between TOC and Fepy/FeHR relative to deeper sections reported previously, suggesting that euxinia developed at Wangjiaping in response to increasing marine productivity and organic matter-sinking fluxes. Our reconstructed redox conditions and fossils at Wangjiaping in comparison with previously well-studied strata in the inner-shelf Xiaotan and Shatan sections suggest that planktonic and benthic planktonic trilobites with bioturbation appeared in the oxic water columns, whereas only planktonic trilobites without bioturbation occurred within the anoxic (even euxinic) water columns during Cambrian Age 3. This finding indicates that spatial heterogeneity of redox conditions in the shelves had an important effect on early animal distribution in the Yangtze Block.

Paleo-environmental cyclicity in the Early Silurian Yangtze Sea (South China): Tectonic or glacio-eustatic control?

The Yangtze Sea of the South China Craton experienced strong environmental perturbations during the first ~5-6Myr of the Silurian. The Lower Silurian Lungmachi Formation in the Jiaoye-1 drillcore (Chongqing, China) records three sea-level cycles (stratigraphic sequences) characterized by rapid deepening at the base of each cycle followed by slow shallowing. Each deepening event was associated with shifts toward higher marine productivity, more intensely reducing bottomwater conditions, reduced watermass restriction, lower detrital input, and enhanced siliciclastic weathering intensity. We infer control by glacio-eustasy, in which deglacial sea-level rises and concurrent climatic warming triggered the observed environmental changes. Shallowing was associated with renewed glaciation and characterized by the opposite set of conditions. The results of this study thus provide evidence in support of continued continental glaciation during the Early Silurian, i.e., following the termination of the major end-Ordovician Hirnantian glaciation. The study core also exhibits a long-term shallowing trend through the entire Lower Silurian, the origin of which may have been tectono-epeirogenic uplift of the South China Craton during the Kwangsian Orogeny.

Global and regional controls on marine redox changes across the Ordovician-Silurian boundary in South China

The Ordovician-Silurian (O-S) transition coincided with significant environmental and biological changes. In South China, the Yangtze Platform experienced both global and regional events at this time, including sea-level fluctuations, tectonic movements, volcanic eruptions, mass extinction, and widespread anoxia. The O-S transitional strata of the Yangtze Platform comprise organic-rich black shales that are an important oil source rock. To explore the evolution of watermass chemistry and its relationship to organic matter accumulation, we conducted an integrated Fe-S-C geochemical study (i.e., Fe-speciation, δ13Corg, δ34Spy, pyrite-S and TOC) of the O-S boundary sections at Datianba (Chongqing Municipality) and Shuanghe (Sichuan Province). These sections were located in the restricted inner Yangtze Sea, in contrast to the previously studied Wangjiawan section, which was deposited in a more open setting of the outer Yangtze Sea. In contrast to the well-oxygenated conditions at Wangjiawan, the Datianba and Shuanghe sections record persistently anoxic, and episodically euxinic, conditions during the Hirnantian, which can be explained by the unique paleogeography and restricted hydrography of the inner Yangtze Sea. We propose that the regional Kwangsian Orogeny, which was driven by collision of the Yangtze and Cathaysia blocks, played a key role in basin development, watermass chemistry changes, and accumulation of Wufeng Formation black shales within the Yangtze Sea. The more widespread black shales of the Lower Silurian Lungmachi Formation were linked to the post-Hirnantian global marine transgression.

Yellow Sea Transform Fault (YSTF) and the developemnt of Korean Peninsula

The Yellow Sea Transform Fault (YSTF), the boundary between the Korean Peninsula and the South China Plate was a repeatedly reactivated ancient fault, extant since 1 Ga or more ago. Similarly polycyclic continental collisions along the Qinling-Dabie-Sulu (QDS) suture dispatched the lateral (vectorial) collisional effects eastwardly toward the Korean Peninsula across YSTF, the east end of the QDS belt. The Korean Peninsula, coexisted with YSTF, has been a promontory of the Sino-Korean Plate (SKP) at least since the Rodinia assembly, ca 1 Ga. The Jarly Paleozoic rift origin of the Okcheon Trough, a major aulacogen developed within the Korean Peninsula of the Sino-Korean Plate is attributed to the transform role of the YSTF. Euring the Middle Paleozoic, the Yangtze Plate, an inherent component of the South China Plate, collided SKP so mildly and enduringly that SKP had to develop the Late Ordovician-Early Carboniferous ‘great hiatus’ over the cratonic SKP. Contemporaneously, the clustered aulacogens were formed over an area near the YSTF. It is envisioned that during the middle Paleozoic, the compressed part of the SKP by the eastward-pushing Yangtze Plate formed an extensional upper crust where the aulacogens formed. The Yangtze sea invaded the aulacogens where the clastic sediments of the mixed environment were dominated by the supply from the Yangtze Plate as witnessed by the clastic zircon grains showing the Yangtze-akin isotope dates. The development of both the middle Paleozoic ‘great hiatus’ and the clustered aulacogens represents the Caledonian tectonic phase though scarcely accompanied deformations or an orogeny. The Carboniferous-Permian metamorphism recorded in the Middle Paleozoic aulacogens represents the Hercynian (Variscan) phase, but without obvious structural deformations. The deepest subduction and the most intensive collision of the Yangtze Plate along the Paleotethyan suture was made in the late Permian-mid-Triassic time, the Indosinian phase. The coeval Songnim Orogeny in Korea was similarly intensive, though it was a derived, secondary, orogeny propagated ultimately from the QDS collision belt. Because of the eastward compression derived then from the QDS collision belt, YSTF was so deformed and considerably pushed eastward that it now occurs as a deformed-dislocated fault zone called the West Marginal Fault of Korean Peninsula (WMF in Fig. 1). The location of the mid-Triassic Korean Peninsula was inserted between the eastwardly compressing marginal Yangtze Plate and the counter balancing Permian-Triassic subduction-metamorphic-accretionary complex of the Japanese Pacific. Such a sandwich tectonics effectively intensified the Indosinian Songnim Orogeny of Korea.

Changes in marine productivity and redox conditions during the Late Ordovician Hirnantian glaciation

Changes in marine productivity and redox conditions during the end-Ordovician (Hirnantian) glaciation and Ordovician–Silurian transition were investigated through Mo-isotope and major- and trace-element analyses of the Wangjiawan (Hubei Province) and Nanbazi (Guizhou Province) sections from the Yangtze Platform of South China. Katian shales of the Wufeng Formation, which yield the graptolites Dicellograptus complanatus, Dicellograptus complexus, and Paraorthograptus pacificus, were deposited under euxinic conditions at both localities, as shown by high MoEF, UEF, and δ98Mo values. A major sea-level regression during the Hirnantian glaciation resulted in shallowing and a shift toward better-oxygenated conditions within the Yangtze Sea, as well as deposition of thin-bedded siliceous sediments, calcirudite debris flows, and limestone turbidites of the Kuanyinchiao Formation, the base of which correlates with the first phase of the end-Ordovician mass extinction. The termination of the Hirnantian glaciation at the top of the Kuanyinchiao Formation was associated with a major sea-level transgression, a rapid expansion of euxinia in the Yangtze Sea (as documented by a return of high MoEF, UEF, and δ98Mo values), and the second phase of the mass extinction, during which the cool-adapted Hirnantian Fauna went extinct.The long-term cooling trend of the Middle and Late Ordovician, which culminated in the Hirnantian glaciation, was driven by enhanced burial of organic carbon, as documented by δ13Ccarb, δ13Corg, and δ34Spy records. Increased organic carbon burial was linked to high rates of marine productivity, as shown by high TOC and biogenic Ba concentrations especially at the deeper, less-restricted Wangjiawan locale, producing the HICE (Hirnantian Isotopic Curve Excursion) δ13Ccarb excursion. The locus of organic carbon burial during the Hirnantian crisis shifted to deeper-water environments that were located outside the study region. The relatively rapid onset and termination of the Hirnantian glaciation were probably due to crossing of tipping points in the Late Ordovician climatic–oceanic system.

Organic matter accumulation of Late Ordovician sediments in North Guizhou Province, China: Sulfur isotope and trace element evidences

The organic-rich Upper Ordovician sediments (Wufeng and Guanyinqiao Formations) on the Yangtze platform are considered to be one of the main source rocks. Here we present geochemical proxies, including redox indicator (S/C ratios and sulfur isotopes) and productivity indices (TOC, Mo and Ba contents), from Nanbazi section in North Guizhou province, South China, in order to investigate the mechanism of organic matter accumulation. The geochemical data suggest a stagnant and anoxic environment predominated the Yangtze Sea during the Wufeng period, whereas ventilated and oxygenated marine conditions pervaded the Yangtze Sea during the Guanyinqiao period. Variations in the concentration of TOC, Mo and Ba indicate that higher organic carbon export in the Wufeng intervals than those in the Guanyinqiao intervals. These variations in redox and productivity during the late Ordovician were associated with different mechanisms and forcing processes. The abrupt change from anoxic to oxygenated condition at the beginning of the Guanyinqiao was concomitant with the global glacial period, likely resulted from the glacio-eustatic sea-level fall and subsequent circulation of cold, dense oxygenated waters upon the shelf seabed. The productivity variations were related to the change of nutrient supply, which is consistent with volcanic activities and runoff to the Yangtze Sea. Redox changes, together with primary productivity fluctuations could have played a significant role on the variation of organic matter accumulation during the late Ordovician in South China.

The correlation between macroscopic algae and metazoans in the Ediacaran: a case study on the Wenghui biota in northeastern Guizhou, South China

After the Marinoan glaciation, macroscopic organisms thrived in the Yangtze Sea, South China, during the Ediacaran period. The Wenghui biota, which is found from the upper Doushantuo Formation black shales (>551 Ma) in northeastern Guizhou, South China, includes macroscopic algae, metazoans and ichnofossils. Most macroalgae in the Wenghui biota bear a holdfast to secure them onto seafloor and have a thallus of various lengths extending into the water column. This biota can be divided into Globusphyton, Sectoralga–Longifuniculum, Cucullus, Beltanelliformis and Baculiphyca–Gesinella assemblages. A fossil-barren interval containing a thin layer of feldspathic sandstone separates the macroscopic organisms into two distinct parts. From the Globusphyton assemblage through the Sectoralga–Longifuniculum assemblage to the Cucullus assemblage, metazoans show a positive correlation with the abundance and diversity of branching macroalgae at both metre and millimetre scales. Nevertheless, both Beltanelliformis and Baculiphyca–Gesinella assemblages, in which the number and diversity of macroscopic algae and metazoans, especially the shorter branching macroalgae, are obviously decreasing or even lacking, might be related to a special environment and a fragile ecosystem. In addition, the ratios of Ni/Co, U/Th and V/(V + Ni) display zigzagged profiles at millimetre scales indicating frequent redox fluctuations. Variations in macrofossils and trace elements at both millimetre and metre scales indicate that the oxygen content in the northeast Guizhou Sea fluctuated frequently during the middle–late Ediacaran period and the Wenghui biota possibly lived in the redox buffering zone. Moreover, the abundance and diversity of macroalgae, especially the branching macroalgae, could have significantly influenced the redox conditions in water column. The increase in oxygen may have improved the environment for the growth and reproduction of macroalgae and metazoans.

Reconstruction of the mid-Hirnantian palaeotopography in the Upper Yangtze region, South China; pp. 329–334

Reconstruction of the Hirnantian (Late Ordovician) palaeotopography in South China is important for understanding the distribution pattern of the Hirnantian marine depositional environment. In this study, we reconstructed the Hirnantian palaeotopography in the Upper Yangtze region based on the rankings of the palaeo-water depths, which were inferred according to the lithofacies and biofacies characteristics of the sections. Data from 374 Hirnantian sections were collected and standardized through the online Geobiodiversity Database. The Ordinary Kriging interpolation method in the ArcGIS software was applied to create the continuous surface of the palaeo-water depths, i.e. the Hirnantian palaeotopography. Meanwhile, the line transect analysis was used to further observe the terrain changes along two given directions. The reconstructed palaeotopographic map shows a relatively flat and shallow epicontinental sea with three local depressions and a submarine high on the Upper Yangtze region during the Hirnantian. The water depth is mostly less than 60 m and the Yangtze Sea gradually deepens towards the north.

Tracking shallow marine red beds through geological time as exemplified by the lower Telychian (Silurian) in the Upper Yangtze Region, South China

Marine red beds occur frequently in China through geological time. Despite their complex environments, the red beds are found in three depositional settings: 1) oceanic, deep water, as in the Upper Cretaceous of southern Tibet; 2) outer shelf, deeper water, as in the Lower-Middle Ordovician of South China; and 3) inner shelf, shallow water, as in the Silurian and Triassic in South China. The Silurian marine red beds are recurrent in the lower Telychian, upper Telychian, and upper Ludlow. This paper is to document the marine nature of the lower Telychian red beds (LRBs) in the Upper Yangtze Region and to discuss the spatial and temporal distribution of the LRBs and their depositional environments. The LRBs are best developed on the north side of the Cathaysian Oldland, which can be interpreted as the source area. It is inferred that they were deposited during a marine regression, characterized by the lack of upwelling, low nutrition and organic productivity with a decrease of biodiversity and a high rate of sedimentation. The iron-rich sediments may have been transported by rivers on the oldland into the Upper Yangtze Sea, as rates of deposition were rapid enough to counteract normal reducing effect around sediment-water interface. The LRBs are different from the off-shore, deeper water red beds of lower Telychian in Avalonia and Baltica and further from the oceanic, deep water red beds of Upper Cretaceous in southern Tibet chiefly in palaeogeographic settings, biotic assemblages and marine environments.

Predominance of stratified anoxic Yangtze Sea interrupted by short-term oxygenation during the Ordo-Silurian transition

The Ordo-Silurian transition was a critical interval in geological history, during which profound biotic, climatic and oceanic changes occurred. In order to explore the oceanic palaeoredox changes, multiple geochemical proxies, including ratios of S/C, FeHR/FeT, FeP/FeHR and DOP values, are presented here from three sections (Wangjiawan, western Hubei, Sanjiaguan, northern Hunan, and Nanbazi, northern Guizhou) across the Ordo-Silurian boundary on the Yangtze Platform. These palaeoredox data indicate a predominance of stratified, anoxic (ferruginous) ocean on the Yangtze block during this transition, which was interrupted by a brief episode of oceanic oxygenation in the early Hirnantian. This oxygenation, temporally coinciding with the end-Ordovician glaciation and global glacio-eustatic sea level fall, likely resulted from enhanced circulation of polar cold, dense oxygen-rich water onto the low-latitude shelf. The prior and subsequent longer-term episodes of anoxic ocean, particularly the later one which started in the late Hirnantian, occurred in parallel with the rapid climatic warming and sea level rise, which could have slowed down oceanic circulation, thereby enhancing oceanic stratification, anoxia and organic preservation. Oceanic redox changes, together with rapid climatic and sea-level fluctuations, were likely responsible for the stepwise massive demise of the Ordo-Silurian biotic crisis.