Junxuan Fan

The Central Guizhou and Yi- chang uplifts, Upper Yangtze region, between Ordovician and Silurian

The Central Guizhou and Yichang Uplifts are present in central Guizhou to western Hubei. Biostratigraphic evidences from more than 20 sections in 14 counties of this region provide the data about the duration of these uplifts. Although this duration differs from locality to locality, it is mainly from Ashgillian to Rhuddanian. The uplifts result from a horizontally driven movement to the South China Paleoplate from an uncertain block in southeast. Global sea-level drop during the end of the Ordovician made the uplifts obvious, particularly the Central Guizhou Uplift. It might have emerged above sea level in the short interval between the Ordovician and Silurian.

Ashgillian graptolite fauna of the Yangtze region and the biogeographical distribution of diversity in the latest Ordovician

Ashgillian graptolites have been described and recorded globally from 15 different paleoplates. The most diverse graptolite faunas are from the Yangtze region, South China, including 28 genera and 96 species. Among them, 25 genera and 73 species belong to the DDO fauna (Dicranograptidae – Diplograptidae – Orthograptidae fauna), and only three genera but 23 species belong to the N fauna (Normalograptidae fauna). Among the Yangtze graptolite fauna there are six endemic genera and 51 endemic species, which represent 21% and 52%, respectively, of the Ashgillian fauna in the region. This is an unusually high level of endemism. Endemic species are present in both the endemic and the cosmopolitan genera. A late Ashgillian stepwise extinction event has recently been recognized, based on graptolite diversity changes and graphic correlation. This begins with a major extinction from the Diceratograptus mirus Subzone to the middle Normalograptus extraordinarius-N. ojsuensis Zone, and ends with a minor extinction prior to the end of the Normalograptus persculptus Zone. Graptolite faunal replacement of the DDO fauna by the N fauna occurred throughout this interval. A comparison of Ashgillian graptolite diversity between the Yangtze region and other regions indicates that two different biogeographical realms existed in mid-Ashgillian time. A moderate-diversity graptolite fauna is present in the low-middle latitude realm, which includes South Scotland, Kazakhstan, Kolyma, Nevada, Yukon, Canadian Arctic, SE Australia. The Yangtze region was located in this realm, but was characterized by a very high-diversity fauna. Some other localities, including eastern Avalonia (Wales and England), the Argentina Precordillera, and Bohemia, which mainly represent the mid- to high-latitude realm, contain the lower-diversity mid-Ashgillian assemblages. This biogeographical distribution suggests a latitudinal diversity gradient, which may be controlled mainly by water temperature. This climate gradient becomes much less evident by late Hirnantian time in which most parts of the world have a relatively low diversity fauna totally dominated by normalograptid species, many of which appear to have been eurytopic. Throughout the Ashgillian, however, the Yangtze platform shows a high diversity and long persistence of DDO taxa the mass extinction interval. This may be a consequence of the semi-restricted nature of the basin in which conditions relatively favorable to graptolite survival and speciation existed throughout all or most of the Hirnantian.

Geobiodiversity Database: a comprehensive section-based integration of stratigraphic and paleontological data

The Geobiodiversity Database (GBDB – www.geobiodiversity.com), an integrated system for the management and analysis of stratigraphic and paleontological information, was started in 2006 and became available online in 2007. Its goal is to facilitate regional and global scientific collaborations focused on regional and global correlation, quantitative stratigraphy, systematics, biodiversity dynamics, paleogeography and paleoecology. It is unique among global, public access databases in that it is a section-based online database system, incorporating data from a wide range of disciplines of stratigraphy and paleontology, with inherent interrelationship between different kinds of data sets. It provides the capability of completely digitizing raw data, as well as integrating of different interpretations to the same paleontological and stratigraphic content. Several Windows-based visualization and analysis applications, either fully integrated with the database or supported by subset-export functions, have been developed to make the database more useful as a scientific and educational tool. The GBDB became the formal database of the International Commission on Stratigraphy (ICS) in August 2012 at the 34th International Geological Congress in Brisbane, and will produce comprehensive and authoritative web-based stratigraphic information service for global geoscientists, educators and the public.

Temporal and spatial distribution of the Wufeng Formation black shales (Upper Ordovician) in South China

The Wufeng Formation black shales (upper Katian to lower Hirnantian) are widely distributed on the Yangtze Platform in South China. Data from 389 sections that span the stratigraphic interval of the Wufeng Formation were captured through the GBDB online database. By using the GBDB online analytical functions and the ArcGIS software, we have visualized the dynamic changes of the geographic distribution of the Wufeng Formation black shales and calculated their sedimentary variables, such as distribution area, rock volume and mean thickness. The distribution area of the Wufeng Formation decreased through the study interval, which likely represents the effect of the global sea-level fall (resulting from the growth of the Gondwana ice sheets) and the gradual uplift of the study area. The extremely low sedimentary rate on the Yangtze Platform reflects a very restricted terrigenous supply during the Late Ordovician. To the south of the Yangtze Platform, there are two areas in which coeval sediments are over 1000 m thick, which probably represent environments with sustained subsidence of the underlying crust and more proximal terrigenous sediment sources.

Geographic dynamics of some major graptolite taxa of the Diplograptina during the Late Ordovician mass extinction in South China

The study of graptolite paleobiogeography is important for understanding the relationship between geography and evolutionary dynamics in graptolites. However, there have been few detailed studies of graptolite paleobiogeography during the Late Ordovician, or the role that geography played in the Hirnantian Mass Extinction (HME). In this study, in order to investigate the geographic dynamics of major graptolite taxa during the HME, a data-set of graptolite occurrences in the late Katian–Hirnantian from 61 localities in South China was compiled using the GBDB online database. The minimum polygon method was used to reconstruct 14 distribution maps of 5 major taxa belonging to the Diplograptina. The geographic dynamics of most diplograptine taxa show a small reduction in their geographic ranges from the Dicellograptus complexus Zone to the Paraorthograptus pacificus Zone, probably because of the contractions in the distribution areas of the coeval marine sediments. A substantial reduction in the distributions of most diplograptine taxa from the P. pacificus Zone to the Metabolograptus extraordinarius Zone mainly represents the effect of the major extinction event. Our study also demonstrates a reduction in the geographic ranges of each taxon to the center of the Yangtze epicontinental sea during the HME. The changes in geographic ranges are mirrored by a similar reduction in species-level total diversity during the study interval.

Geobiodiversity Database (GBDB) in stratigraphic, palaeontological and palaeogeographic research: graptolites as an example

The Geobiodiversity Database (GBDB) Project is dedicated to the construction and maintenance of a web-enabled taxonomic, stratigraphic and geographic database for information gathered from the fossil record. Its goal is to facilitate regional and global scientific collaborations focused on studying the history, diversity, geography and environmental context of life on Earth. After 7 years of development, its abundant data resources and diverse integrated functions make it a useful platform for stratigraphic, palaeontological and palaeogeographic analysis. Graptolites are the first major fossil group that has been the focus of our efforts to compile, standardize, digitize and analyse over the past several years. Here, we use the graptolite group as an example to introduce the major data resources and functionality of the GBDB.

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.

A Graphic Correlation and Diversity Analysis of the Upper Darriwilian to Lower Katian Graptolites

A graphic correlation of the upper Darriwilian to Lower Katian graptolites from Northwest China is conducted to produce a composite sequence, which is used to precisely estimate Darriwilian to Lower Katian graptolite biodiversity. In total, 122 species or subspecies from seven sections, which include the Dawangou (Kalpin, Xinjiang, taken as the reference section), Sishichang (Aksu, Xinjiang), Subashigou (Kalpin, Xinjiang), Guanzhuang (Pingliang, Gansu), Longmendong (Longxian, Shaanxi), Dashimen (Wuhai, Inner Mongolia), and Gongwusu (Wuhai, Inner Mongolia) are analyzed for a composite sequence of graptolites spanning early Darriwilian to early Katian age. In the composite sequence, eight graptolite biozones, the Cryptograptus gracilicornis , Pterograptus elegans , Didymograptus murchisoni , Jiangxigraptus vagus, Nemagraptus gracilis , Climacograptus bicornis , Diplacanthograptus caudatus , and Diplacanthograptus spiniferus biozones, in ascending order are defined and identified. Based on the composite sequence and the graptolite occurrences, the temporal distribution of the graptolite richness of the study interval is presented. The pattern displays two peaks, one in the basal D. murchisoni Biozone and the other in the basal N. gracilis Biozone.