Browsing by Author "Kaufman, Alan J."

Now showing 1 - 11 of 11
  • Thumbnail Image
    An authigenic response to Ediacaran surface oxidation: Remarkable micron-scale isotopic heterogeneity revealed by SIMS
    Cui, Huan; Kitajima, Kouki; Orland, Ian J.; Baele, Jean-Marc; Xiao, Shuhai; Kaufman, Alan J.; Denny, Adam; Spicuzza, Michael J.; Fournelle, John H.; Valley, John W. (Elsevier, 2022-08-01)
    The Ediacaran Shuram excursion (SE) records a global decrease in carbonate carbon isotope (δ13Ccarb) values from +6‰ down to ca. –10‰, representing the largest δ13Ccarb negative anomaly in Earth history. While the SE is widely recorded in the upper Doushantuo Formation of South China, it shows highly variable δ13Ccarb profiles among correlative sections. This inconsistent expression of the SE challenges the conventional view of a homogeneous marine dissolved inorganic carbon (DIC) reservoir. A potential process that could explain δ13Ccarb variability is local mineralization of isotopically distinct authigenic carbonates near the sediment–water interface during early diagenesis. However, a direct test of such authigenic carbonates is still limited. Here, following a recent study on the SE in an intra-shelf environment, we revisited an outer-shelf section, identified and analyzed μm-scale, syn-depositional authigenic calcite cements via integrated cathodoluminescence (CL), micro-X-ray fluorescence (μXRF), scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), and secondary ion mass spectrometry (SIMS). Our new SIMS results reveal remarkable micron-scale heterogeneity of δ13Ccarb in authigenic calcite cements, including extremely negative values down to –37.5‰ (VPDB). We interpret these calcite cements as methane-derived authigenic calcite (MDAC) resulting from microbial sulfate reduction (MSR) and anaerobic oxidation of methane (AOM) during early diagenesis. Based on the new results, we propose that the heterogeneous SE in South China — manifest on micrometer, centimeter, and basinal scales — was modulated by methane oxidation under variable local redox and water depth conditions. The SE, therefore, was coupled with different degrees of methane oxidation in individual basins, and globally triggered by enhanced seawater sulfate during an atmospheric oxygenation event. In light of this study, the potential role of redox variability in methane oxidation during the SE may have been underestimated. Our study demonstrates that integrated SIMS-SEM analysis can distinguish different generations of isotopically distinct carbonates otherwise undetected by conventional analysis, and is thus an effective approach to assess the origin and diagenetic history of δ13Ccarb anomalies in the sedimentary record.
  • Thumbnail Image
    Chemostratigraphy of the Early Paleoproterozoic carbonate successions (Kaapvaal and Wyoming cratons)
    Bekker, Andrey (Virginia Tech, 2001-08-24)
    Evidence of three glaciations in Paleoproterozoic successions of North America and at least one on three other continents, suggests that these glaciations were of global extent. In common with the Neoproterozoic record, carbonates cap the glacials. However, the relationship between biogeochemical cycling of carbon and ice ages has not been fully appreciated. This research involved the sedimentology and isotope stratigraphy of carbonates and shales in Paleoproterozoic glacially-influenced successions of Wyoming and South Africa. Carbonates of the Vagner Formation cap the middle of three diamictites in the Snowy Pass Supergroup, Medicine Bow Mountains, WY. The Duitschland Formation occurs between two glacial horizons in South Africa. Limestones retain negative d13C values for over 60 m in the Vagner Formation, and for over 100 m in the lower part of the Duitschland Formation. Isotope compositions of TOC from the lower part of the Duitschland Formation reveal pronounced enrichment resulting in significantly lower fractionation between organic and inorganic carbon. This is similar to enrichment noted in Neoproterozoic cap carbonates. Combined with strongly positive carbon isotope compositions in upper Duitschland carbonates, the data from the Vagner Formation underscores strongly positive-to-negative carbon isotope trends bracketing Paleoproterozoic glaciations. These trends mimic those noted in Neoproterozoic glacial successions and possibly indicate a recurrence of global glaciations. The Slaughterhouse and Nash Fork formations significantly postdate the glacial epoch. Both the lower part of the Nash Fork Formation, Medicine Bow Mountains and the Slaughterhouse Formation, Sierra Madre contains carbonates with 13C-enrichment >+6â ° and locally up to +28%, whereas carbonates higher in the Nash Fork Formation have d13C values between 0 and 2.5%. This dramatic change in the composition of the Paleoproterozoic ocean is constrained at ca. 2.1 Ga (Karhu, 1993). Carbonates in the Rawhide Canyon section of the Whalen Group in the Hartville Uplift (the easternmost exposure of the Wyoming Craton) display δ13C values up to +8.2% suggesting correlation with the Slaughterhouse and Nash Fork formations and their deposition on continuous carbonate platform along the margin of the Wyoming Craton. These data support an open-marine, and therefore a global origin for the ca. 2.2-2.1 Ga carbon isotope excursion.
  • Thumbnail Image
    Deposition or diagenesis? Probing the Ediacaran Shuram excursion in South China by SIMS
    Cui, Huan; Kitajima, Kouki; Orland, Ian J.; Xiao, Shuhai; Baele, Jean-Marc; Kaufman, Alan J.; Denny, Adam; Zhou, Chuanming; Spicuzza, Michael J.; Fournelle, John H.; Valley, John W. (Elsevier, 2021-11)
    The Ediacaran Period (ca. 635–541 Ma) witnessed the earliest paleontological evidence for macroscopic animals (i.e., Ediacara biota) and geochemical observations of the largest carbon cycle anomaly in Earth history (i.e., Shuram Excursion, SE). Numerous hypotheses have been proposed for the origins of the SE, ranging from primary seawater anomaly to syn- or post-depositional diagenesis. Despite intensive geochemical and theoretical work published in the past decade, empirical evidence that is strictly based on fundamental petrographic results at the micrometer scale is still limited. To evaluate depositional compositions and diagenetic effects on samples from the SE, we investigated the EN3 interval in the Doushantuo Formation of South China via integrated cathodoluminescence (CL), secondary ion mass spectrometry (SIMS), and scanning electron microscope (SEM). Detailed petrographic observations reveal that the EN3 limestone is dominated by calcite microspar, with minor but variable amounts of disseminated zoned dolomite crystals. The former likely formed via neomorphism of depositional micrite, while the latter was the result of progressive post-depositional dolomitization. The mean values of paired SIMS δ13Ccalcite and δ13Cdolomite compositions are indistinguishable in each sample and consistent with published micro-drilled bulk-powder δ13C values, which we interpret to represent depositional “background” signals of seawater dissolved inorganic carbon (DIC). We also observed μm-scale variability of SIMS δ13C data that may reflect a variable diagenetic overprint after deposition. Our integrated petrographic and geochemical results are consistent with a depositional origin of the SE and provide little evidence for the hypothesized isotope alteration by meteoric and mixing-zone diagenesis or late burial diagenesis. In light of this study, we propose that the SE indeed represents a marine carbon cycle anomaly that bears a close temporal link to the Ediacaran surface environment.
  • Thumbnail Image
    Extensive marine anoxia during the terminal Ediacaran Period
    Zhang, Feifei; Xiao, Shuhai; Kendall, Brian; Romaniello, Stephen J.; Cui, Huan; Meyer, Mike; Gilleaudeau, Geoffrey J.; Kaufman, Alan J.; Anbar, Ariel D. (AAAS, 2018-06-20)
    The terminal Ediacaran Period witnessed the decline of the Ediacara biota (which may have included many stem-group animals). To test whether oceanic anoxia might have played a role in this evolutionary event, we measured U isotope compositions (δ238U) in sedimentary carbonates from the Dengying Formation of South China to obtain new constraints on the extent of global redox change during the terminal Ediacaran. We found the most negative carbonate δ238U values yet reported (−0.95 per mil), which were reproduced in two widely spaced coeval sections spanning the terminal Ediacaran Period (551 to 541 million years ago). Mass balance modeling indicates an episode of extensive oceanic anoxia, during which anoxia covered >21% of the seafloor and most U entering the oceans was removed into sediments below anoxic waters. The results suggest that an expansion of oceanic anoxia and temporal-spatial redox heterogeneity, independent of other environmental and ecological factors, may have contributed to the decline of the Ediacara biota and may have also stimulated animal motility.
  • Thumbnail Image
    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.
  • Thumbnail Image
    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.
  • Thumbnail Image
    Recent advances in understanding the terminal Ediacaran Earth-life system in South China and Arctic Siberia
    Cui, H.; Kaufman, Alan J.; Xiao, S.; Grazhdankin, D. V.; Peek, S.; Martin, A. J.; Bykova, N. V.; Rogov, V. I.; Liu, X. M.; Zhang, F.; Romaniello, S. J.; Anbar, A. D.; Peng, Y.; Cai, Y.; Schiffbauer, J. D.; Meyer, M.; Gilleaudeau, Geoffrey J.; Plummer, Rebecca E.; Sievers, N. E.; Goderis, S.; Claeys, P. (2019-11-22)
    The terminal Ediacaran contains dramatic changes in biogeochemical cycles, many of which are closely coupled with evolutionary transitions in the corresponding fossil records. Dynamic redox conditions may have caused a profound impact on early animal evolution. Our work highlights the significance of integrated bio-, litho-, and chemo-stratigraphy in geobiology research of the deep time.
  • Thumbnail Image
    Sedimentology and chemostratigraphy of the terminal Ediacaran Dengying Formation at the Gaojiashan section, South China
    Cui, Huan; Xiao, Shuhai; Cai, Yaoping; Peek, Sara; Plummer, Rebecca E.; Kaufman, Alan J. (2019-11)
    The terminal Ediacaran Dengying Formation (c. 551.1-538.8 Ma) in South China is one of two successions where Ediacara-type macrofossils are preserved in carbonate facies along with skeletal fossils and bilaterian animal traces. Given the remarkable thickness of carbonate-bearing strata deposited in less than 12.3 million years, the Dengying Formation holds the potential for construction of a relatively continuous chemostratigraphic profile for the terminal Ediacaran Period. In this study, a detailed sedimentological and chemostratigraphic (delta 13C(carb), delta O-18(carb), delta C-13(org), delta S-34(pyrite), and Sr-87/Sr-86) investigation was conducted on the Dengying Formation at the Gaojiashan section, Ningqiang County of southern Shaanxi Province, South China. Sedimentological results reveal an overall shallow-marine depositional environment. Carbonate breccia, void-filling botryoidal precipitates and aragonite crystal fans are common in the Algal Dolomite Member of the Dengying Formation, suggesting that peritidal facies were repeatedly karstified. The timing of karstification was likely early, probably soon after the deposition of the dolomite sediments. The presence of authigenic aragonite cements suggests high alkalinity in the terminal Ediacaran ocean. Geochemical analysis of micro-drilled samples shows that distinct compositions are registered in different carbonate phases, which should be considered when constructing chemostratigraphic profiles representative of true temporal variations in seawater chemistry. Integrated chemostratigraphic data suggest enhanced burial of organic carbon and pyrite, and the occurrence of extensive marine anoxia (at least in the Gaojiashan Member). Rapid basinal subsidence and carbonate accumulation during a time of elevated seawater alkalinity and increased rates of pyrite burial may have facilitated the evolutionary innovation of early biomineralizing metazoans.
  • Thumbnail Image
    Transient marine euxinia at the end of the terminal Cryogenian glaciation
    Lang, Xianguo; Shen, Bing; Peng, Yongbo; Xiao, Shuhai; Zhou, Chuanming; Bao, Huiming; Kaufman, Alan J.; Huang, Kangjun; Crockford, Peter W.; Liu, Yonggang; Tang, Wenbo; Ma, Haoran (Springer Nature, 2018-08-01)
    Termination of the terminal Cryogenian Marinoan snowball Earth glaciation (similar to 650-635 Ma) is associated with the worldwide deposition of a cap carbonate. Modeling studies suggest that, during and immediately following deglaciation, the ocean may have experienced a rapid rise in pH and physical stratification followed by oceanic overturn. Testing these predictions requires the establishment of a high-resolution sequence of events within sedimentary records. Here we report the conspicuous occurrence of pyrite concretions in the topmost Nantuo Formation ( South China) that was deposited in the Marinoan glacial deposits. Sedimentary facies and sulfur isotope data indicate pyrite precipitation in the sediments with H2S diffusing from the overlying sulfidic/euxinic seawater and Fe (II) from diamictite sediments. These observations suggest a transient but widespread presence of marine euxinia in an ocean characterized by redox stratification, high bioproductivity, and high-fluxes of sulfate from chemical weathering before the deposition of the cap carbonate.
  • Thumbnail Image
    A unifying model for Neoproterozoic–Palaeozoic exceptional fossil preservation through pyritization and carbonaceous compression
    Schiffbauer, James D.; Xiao, Shuhai; Cai, Yaoping; Wallace, Adam F.; Hua, Hong; Hunter, Jerry; Xu, Huifang; Peng, Yongbo; Kaufman, Alan J. (Nature, 2014-12-17)
    Soft-tissue fossils capture exquisite biological detail and provide our clearest views onto the rise of animals across the Ediacaran–Cambrian transition. The processes contributing to fossilization of soft tissues, however, have long been a subject of debate. The Ediacaran Gaojiashan biota displays soft-tissue preservational styles ranging from pervasive pyritization to carbonaceous compression, and thus provides an excellent opportunity to dissect the relationships between these taphonomic pathways. Here geochemical analyses of the Gaojiashan fossil Conotubus hemiannulatus show that pyrite precipitation was fuelled by the degradation of labile tissues through bacterial sulfate reduction (BSR). Pyritization initiated with nucleation on recalcitrant tube walls, proceeded centripetally, decelerated with exhaustion of labile tissues and possibly continued beneath the BSR zone. We propose that pyritization and kerogenization are regulated principally by placement and duration of the decaying organism in different microbial zones of the sediment column, which hinge on post-burial sedimentation rate and/or microbial zone thickness.
  • Thumbnail Image
    Was the Ediacaran Shuram Excursion a globally synchronized early diagenetic event? Insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China
    Cui, Huan; Kaufman, Alan J.; Xiao, Shuhai; Zhou, Chuanming; Liu, Xiao-Ming (2017-02)
    The Ediacaran Period is characterized by the most profound negative carbon isotope (δ13C) excursion in Earth history, the ShuramExcursion. Various hypotheses – including the massive oxidation of dissolved organic carbon (DOC) in the oceans, the weathering of terrestrial organic carbon, or the release and oxidation of methane hydrates and/or expelled petroleum from the subsurface – have been proposed as sources of the 13C-depleted carbon. More recently, it has been suggested that global-scale precipitation of early authigenic carbonates, driven by anaerobicmicrobial metabolism in unconsolidated sediments, may have caused the Shuram Excursion, but empirical evidence is lacking. Here we present a comprehensive analysis of a Shuram-associated interval from the uppermost Doushantuo Formation in South China. Our study reveals petrographic evidence of methane-derived authigenic calcite (formed as early diagenetic cements and nodules) that are remarkably depleted in 13C – suggesting a buildup of alkalinity in pore fluids through the anaerobic oxidation of methane (AOM) – and systematically depleted in 18O relative to co-occurring dolomite. Early authigenesis of these minerals is likely to be driven by increased microbial sulfate reduction, triggered by enhanced continental weathering in the context of a marked rise in atmospheric oxygen levels. In light of the finding of methane-derived authigenic carbonates at Zhongling, and based on our basin-scale stratigraphic correlation, we hypothesize that the marked 13C and 18O depletion (including their co-variation noted worldwide) in the Shuram Excursion may reflect an episode of authigenesis occurring within a sulfate–methane transition zone (SMTZ). If true, the Shuram Excursion was then a global biogeochemical response to enhanced seawater sulfate concentration in the Ediacaran ocean driven by the Neoproterozoic oxidation of surface environments. This paleo-oceanographic transition may have therefore paved theway for subsequent evolution and diversification of animals. Our study highlights the significance of an integrated approach that combines petrography, mineralogy, and texture-specificmicro-drilling geochemistry in chemostratigraphic studies.