“…warty textures 26 , microbial laminae 4 , and benthic cyanobacteria (Fig. 1e) in the Dengying limestone.Figure 5Schematic diagram illustrating the depositional environment of Dengying Formation (modified according to previous literatures 64,65 ). The O 2 level in the atmosphere remained 10–40% PAL.…”
Section: Discussionmentioning
confidence: 95%
“…Schematic diagram illustrating the depositional environment of Dengying Formation (modified according to previous literatures 64,65 ). The O 2 level in the atmosphere remained 10–40% PAL.…”
The earliest unambiguous evidence for animals is represented by various trace fossils in the latest Ediacaran Period (550–541 Ma), suggesting that the earliest animals lived on or even penetrated into the seafloor. Yet, the O2 fugacity at the sediment-water interface (SWI) for the earliest animal proliferation is poorly defined. The preferential colonization of seafloor as a first step in animal evolution is also unusual. In order to understand the environmental background, we employed a new proxy, carbonate associated ferrous iron (Fecarb), to quantify the seafloor oxygenation. Fecarb of the latest Ediacaran Shibantan limestone in South China, which yields abundant animal traces, ranges from 2.27 to 85.43 ppm, corresponding to the seafloor O2 fugacity of 162 μmol/L to 297 μmol/L. These values are significantly higher than the oxygen saturation in seawater at the contemporary atmospheric pO2 levels. The highly oxygenated seafloor might be attributed to O2 production of the microbial mats. Despite the moderate atmospheric pO2 level, microbial mats possibly provided highly oxygenated niches for the evolution of benthic metazoans. Our model suggests that the O2 barrier could be locally overcome in the mat ground, questioning the long-held belief that atmospheric oxygenation was the key control of animal evolution.
“…warty textures 26 , microbial laminae 4 , and benthic cyanobacteria (Fig. 1e) in the Dengying limestone.Figure 5Schematic diagram illustrating the depositional environment of Dengying Formation (modified according to previous literatures 64,65 ). The O 2 level in the atmosphere remained 10–40% PAL.…”
Section: Discussionmentioning
confidence: 95%
“…Schematic diagram illustrating the depositional environment of Dengying Formation (modified according to previous literatures 64,65 ). The O 2 level in the atmosphere remained 10–40% PAL.…”
The earliest unambiguous evidence for animals is represented by various trace fossils in the latest Ediacaran Period (550–541 Ma), suggesting that the earliest animals lived on or even penetrated into the seafloor. Yet, the O2 fugacity at the sediment-water interface (SWI) for the earliest animal proliferation is poorly defined. The preferential colonization of seafloor as a first step in animal evolution is also unusual. In order to understand the environmental background, we employed a new proxy, carbonate associated ferrous iron (Fecarb), to quantify the seafloor oxygenation. Fecarb of the latest Ediacaran Shibantan limestone in South China, which yields abundant animal traces, ranges from 2.27 to 85.43 ppm, corresponding to the seafloor O2 fugacity of 162 μmol/L to 297 μmol/L. These values are significantly higher than the oxygen saturation in seawater at the contemporary atmospheric pO2 levels. The highly oxygenated seafloor might be attributed to O2 production of the microbial mats. Despite the moderate atmospheric pO2 level, microbial mats possibly provided highly oxygenated niches for the evolution of benthic metazoans. Our model suggests that the O2 barrier could be locally overcome in the mat ground, questioning the long-held belief that atmospheric oxygenation was the key control of animal evolution.
“…These include negative δ 13 C fluctuations in the Mesoproterozoic (e.g., Gilleaudeau and Kah, 2013), and early Ediacaran time (e.g., Hoffman et al, 1998;Halverson et al, 2005;Lang et al, 2016), as well as at the Ediacaran-Cambrian boundary (e.g., Maloof et al, 2010a;Maloof et al, 2010b;Jiang et al, 2012), may be explained by the variable admixture of early authigenic carbonates driven by fluctuations of sulfate in the ocean. It is possible that oceanic sulfate concentration was low prior to the Ediacaran Period (Kah et al, 2004;Shen et al, 2006;Kah and Bartley, 2011;Loyd et al, 2012;Crowe et al, 2014) such that methane could have been largely released directly to the ocean and atmosphere rather than being oxidized through microbial metabolism within sediments (e.g., Halverson et al, 2002;Ader et al, 2009;Bristow and Grotzinger, 2013;Li et al, 2015;Shen et al, 2016).…”
The Ediacaran Period is characterized by the most profound negative carbon isotope (δ 13 C) excursion in Earth history, the Shuram Excursion. 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 13 C-depleted carbon. More recently, it has been suggested that global-scale precipitation of early authigenic carbonates, driven by anaerobic microbial 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 13 C -suggesting a buildup of alkalinity in pore fluids through the anaerobic oxidation of methane (AOM) -and systematically depleted in 18 O 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 13 C and 18 O 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 the way for subsequent evolution and diversification of animals. Our study highlights the significance of an integrated approach that combines petrography, mineralogy, and texture-specific micro-drilling geochemistry in chemostratigraphic studies.
“…The deglaciation of Marinoan snowball Earth was associated with intense continental weathering, recovery of marine productivity, development of oceanic euxinia, and global precipitation of cap carbonate [5][6][7] . The snowball Earth hypothesis suggests that high atmospheric CO 2 level was accumulated during the snowball Earth.…”
Geological evidence indicates that the deglaciation of Marinoan snowball Earth ice age (~635 Myr ago) was associated with intense continental weathering, recovery of primary productivity, transient marine euxinia, and potentially extensive CH4 emission. It is proposed that the deglacial CH4 emissions may have provided positive feedbacks for ice melting and global warming. However, the origin of CH4 remains unclear. Here we report Ni isotopes (δ60Ni) and Yttrium-rare earth element (YREE) compositions of syndepositional pyrites from the upper most Nantuo Formation (equivalent deposits of the Marinoan glaciation), South China. The Nantuo pyrite displays anti-correlations between Ni concentration and δ60Ni, and between Ni concentration and Sm/Yb ratio, suggesting mixing between Ni in seawater and Ni from methanogens. Our study indicates active methanogenesis during the termination of Marinoan snowball Earth. This suggests that methanogenesis was fueled by methyl sulfides produced in sulfidic seawater during the deglacial recovery of marine primary productivity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
