Browsing by Author "Jiang, Ganqing"

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    Integrated High-resolution Stratigraphy of the Doushantuo Formation, South China
    McFadden, Kathleen Anne (Virginia Tech, 2008-09-26)
    The Ediacaran Period (635-542 Ma) just preceded the radiation of animals, yet witnessed profound changes in biological innovation, including the first appearance of large spiny acritarchs called the acanthomorphic acritarch, followed by the radiation of the Ediacara biota (575-542 Ma), and earliest recognizable bilaterally symmetrical animals (~550 Ma). It has been proposed that key environmental events, such as the termination of the Cryogenian glaciations, the Neoproterozoic Acraman impact event, and oxygenation of the deep oceans may have played an integral role in the evolution of Ediacara organisms and early animals. However, the extent to which these events shaped biological evolution remains elusive. The Doushantuo Formation in South China, radiometrically constrained between 635.2±0.6 and 551.1±0.7 Ma, is ideal for high-resolution interdisciplinary research, and has the potential to clarify the relationship between environmental and biological events. Research in this dissertation aims to address the following questions: (1) was the Doushantuo Formation deposited in an open marine or a (partially) restricted basin; (2) are Doushantuo paleoenvironmental and biostratigraphic proxies consistent with an Ediacaran oxidation event; and (3) can the Doushantuo acanthomorphic acritarchs be useful biostratigraphic tools for the Ediacaran Period? Detailed (sub-meter) sampling of six sections in the Doushantuo Formation in the Yangtze Gorges area of South China reveal a complex depositional history. Eight broad lithostratigraphic facies and 6 cycles packaged into 3 sequences can be identified and potentially traced into basinal sections. It is likely that the deposition of the Doushantuo Formation occurred under open marine conditions and became increasingly restricted with the development of thick carbonate accumulations at the platform margin. Geochemical analysis shows extreme isotopic variability in the Doushantuo Formation that may be the result of pulsed oxidation of a deep oceanic organic carbon reservoir. Oxidation events may have had further implications on the radiation of early animals. Distinct assemblage biozonation of the Doushantuo acanthomorphic acritarchs is concurrent with isotopic variability, suggesting an ecological and/or evolutionary response during the early Ediacaran. Further efforts in refining the internal geochronology of the Doushantuo Formation is needed in order to test competing hypotheses on the radiation of important taxonomic groups.
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    Lithium isotope evidence for a plumeworld ocean in the aftermath of the Marinoan snowball Earth
    Gan, Tian; Tian, Meng; Wang, Xi-Kai; Wang, Shijie; Liu, Xiao-Ming; Jiang, Ganqing; Gill, Benjamin C.; Nolan, Morrison; Kaufman, Alan J.; Luo, Taiyi; Xiao, Shuhai (National Academies of Science, 2024-11-05)
    The snowball Earth hypothesis predicts that continental chemical weathering diminished substantially during, but rebounded strongly after, the Marinoan ice age some 635 Mya. Defrosting the planet would result in a plume of fresh glacial meltwater with a different chemical composition from underlying hypersaline seawater, generating both vertical and lateral salinity gradients. Here, we test the plumeworld hypothesis using lithium isotope compositions in the Ediacaran Doushantuo cap dolostone that accumulated in the aftermath of the Marinoan snowball Earth along a proximal–distal (nearshore–offshore) transect in South China. Our data show an overall decreasing δ⁷Li trend with distance from the shoreline, consistent with the variable mixing of a meltwater plume with high δ⁷Li (due to incongruent silicate weathering on the continent) and hypersaline seawater with low δ⁷Li (due to synglacial distillation). The evolution of low δ⁷Li of synglacial seawater, as opposed to the modern oceans with high δ⁷Li, was likely driven by weak continental chemical weathering coupled with strong reverse weathering on the seafloor underneath silica-rich oceans. The spatial pattern of δ⁷Li is also consistent with the development and then collapse of the meltwater plume that occurred at the time scale of cap dolostone accumulation. Therefore, the δ7Li data are consistent with the plumeworld hypothesis, considerably reduced chemical weathering on the continent during the Marinoan snowball Earth, and enhanced reverse weathering on the seafloor of Precambrian oceans.
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    Systematic paleontology, acritarch biostratigraphy, and delta C-13 chemostratigraphy of the early Ediacaran Krol A Formation, Lesser Himalaya, northern India
    Xiao, Shuhai; Jiang, Ganqing; Ye, Qin; Ouyang, Qing; Banerjee, Dhiraj M.; Singh, Birendra P.; Muscente, A. D.; Zhou, Chuanming; Hughes, Nigel C. (Cambridge University Press, 2022)
    Acritarch biostratigraphic and delta C-13 chemostratigraphic data from the Krol A Formation in the Solan area (Lesser Himalaya, northern India) are integrated to aid inter-basinal correlation of early-middle Ediacaran strata. We identified a prominent negative delta C-13 excursion (likely equivalent to EN2 in the lower Doushantuo Formation in the Yangtze Gorges area of South China), over a dozen species of acanthomorphs (including two new species-Cavaspina tiwariae Xiao n. sp., Dictyotidium grazhdankinii Xiao n. sp.), and numerous other microfossils from an interval in the Krol A Formation. Most microfossil taxa from the Krol A and the underlying Infra-Krol formations are also present in the Doushantuo Formation. Infra-Krol acanthomorphs support a correlation with the earliest Doushantuo biozone: the Appendisphaera grandis-Weissiella grandistella-Tianzhushania spinosa Assemblage Zone. Krol A microfossils indicate a correlation with the second or (more likely, when delta C-13 data are considered) the third biozone in the lower Doushantuo Formation (i.e., the Tanarium tuberosum-Schizofusa zangwenlongii or Tanarium conoideum-Cavaspina basiconica Assemblage Zone). The association of acanthomorphs with EN2 in the Krol Formation fills a critical gap in South China where chert nodules, and thus acanthomorphs, are rare in the EN2 interval. Like many other Ediacaran acanthomorphs assemblages, Krol A and Doushantuo acanthomorphs are distributed in low paleolatitudes, and they may represent a distinct paleobiogeographic province in east Gondwana. The Indian data affirm the stratigraphic significance of acanthomorphs and delta C-13, clarify key issues of lower Ediacaran bio- and chemostratigraphic correlation, and strengthen the basis for the study of Ediacaran eukaryote evolution and paleobiogeography. UUID: http://zoobank.org/5289fdb2-0e49-4b3b-880f-f5b21acab371.
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    A transient peak in marine sulfate after the 635-Ma snowball Earth
    Peng, Yongbo; Bao, Huiming; Jiang, Ganqing; Crockford, Peter; Feng, Dong; Xiao, Shuhai; Kaufman, Alan Jay; Wang, Jiasheng (National Academy of Sciences, 2022-05-10)
    A series of dramatic oceanic and atmospheric events occurred in the immediate aftermath of the Marinoan “snowball Earth” meltdown ∼635 My ago. However, at the 10-to 100-ky timescale, the order, rate, duration, and causal-feedback relationships of these individual events remain nebulous. Nonetheless, rapid swings in regional marine sulfate concentrations are predicted to have occurred in the aftermath of a snowball Earth, due to the nonlinear responses of its two major controlling fluxes: oxidative weathering on the continents and pyrite burial in marine sediments. Here, through the application of multiple isotope systems on various carbon and sulfur compounds, we determined extremely 13C-depleted calcite cements in the basal Ediacaran in South China to be the result of microbial sulfate reduction coupled to anaerobic oxidation of methane, which indicates an interval of high sulfate concentrations in some part of the postmeltdown ocean. Regional chemostratigraphy places the 13C-depleted cements at the equivalent of the earliest Ediacaran 17O-depletion episode, thus confining the timing of this peak in sulfate concentrations within ∼50 ky since the onset of the deglaciation. The dearth of similarly 13C-depleted cements in other Proterozoic successions implies that the earliest Ediacaran peak in marine sulfate concentration is a regional and likely transient event.