Surface ocean nitrate-limitation in the aftermath of Marinoan snowball Earth: Evidence from the Ediacaran Doushantuo Formation in the western margin of the Yangtze Block, South China
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Snowball Earth
Continental Margin
Radiative energy‐balance models reveal that Earth could exist in any one of three discrete climate states—‘non‐glacial’ (no continental ice‐sheets), ‘glacial‐interglacial’ (high‐latitude ice‐sheets) or ‘pan‐glacial’ (ice‐sheets at all latitudes)—yet only the first two were represented in Phanerozoic time. There is mounting evidence that pan‐glacial states existed at least twice in the Cryogenian (roughly 750–635 Ma), the penultimate period of the Neoproterozoic. Consensus is lacking on whether the world ocean was fully glaciated (‘snowball’ model) or largely unglaciated (‘slushball’ model). The first appearances of multicellular animal fossils (diapause eggs and embryos in China, and sponge‐specific biomarkers in Oman), being closely associated with the last pan‐glacial state, revive speculation that environmental forces had a hand in the origin of metazoa.
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The emergence of the Ediacara biota soon after the Gaskiers glaciation ca. 580 million years ago (Ma) implies a possible glacial fuse for the evolution of animals. However, the timing of Ediacaran glaciation remains controversial because of poor age constraints on the ∼30 Ediacaran glacial deposits known worldwide. In addition, paleomagnetic constraints and a lack of convincing Snowball-like cap carbonates indicate that Ediacaran glaciations likely did not occur at low latitudes. Thus, reconciling the global occurrences without global glaciation remains a paradox. Here, we report that the large amplitude, globally synchronous ca. 571-562 Ma Shuram carbon isotope excursion occurs below the Ediacaran Hankalchough glacial deposit in Tarim, confirming a post-Shuram glaciation. Leveraging paleomagnetic evidence for a ∼90° reorientation of all continents due to true polar wander, and a non-Snowball condition that rules out low-latitude glaciations, we use paleogeographic reconstructions to further constrain glacial ages. Our results depict a 'Great Ediacaran Glaciation' occurring diachronously but continuously from ca. 580-560 Ma as different continents migrated through polar-temperate latitudes. The succession of radiation, turnover and extinction of the Ediacara biota strongly reflects glacial-deglacial dynamics.
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Snowball Earth
Extinction (optical mineralogy)
Last Glacial Maximum
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To understand the processes controlling production, accumulation, and preservation of organic matter in the Lower Oxford Clay (LOC), we determined the hydrogen index (HI), the oxygen index (OI), the Tmax (from Rock-Eval), the content of total organic carbon (TOC), total carbon and total sulfur, and the carbon isotopic composition of bulk organic matter from 160 samples collected from 6 different quarries and one continuous core. With concentrations of TOC varying between 0.5% and 16.6%, the LOC is an organic-rich shale. For samples dominated by organic matter of phytoplanktonic origin, the hydrogen and oxygen indices and the Tmax (~418°) indicate low levels of maturity, and, thus, the shallow burial of the LOC through geologic time. Two main sources of organic matter can be distinguished: a major phytoplanktonic source with high HI and low OI and a minor terrestrial source with low HI and high OI. A third group, represented by samples with low HI and low OI, consists mainly of altered materials from the Middle Oxford Clay and the LOC. Selection of samples for chemical analysis was based on the macrofaunal assemblages defined by Duff (1975). These various biofacies are characterized by specific organic geochemical features indicating the relationship between conditions affecting faunal assemblages and those controlling accumulation and preservation of organic matter. For example, Duff's ‘deposit feeder shales', which are dominated by epifaunal bivalves and are depleted in infaunal organisms, exhibit the highest concentration and best preservation of marine organic matter, with an average TOC of 6.8% for 56 samples analyzed. The preservation of such organic matter requires a dysaerobic water column and a high sedimentation rate. Carbon isotopic compositions within the ‘deposit feeder shale’ biofacies (−27.6 to −23.2±) appear to have been controlled by the intensity of primary productivity. The highest-TOC, marine-dominated, 13 C-rich samples reflect photosynthetic drawdown of dissolved-CO 2 level, and, thus, originated in highly productive environments. On the other hand, variations in the carbon isotopic composition of organic matter in shell beds (−27.5 to −26±) probably reflect heterotrophic reworking of the organic matter, winnowing of the sediments, and mixing with a source of organic matter enriched in 13 C, such as wood (δ 13 C from −25 to −23±). Such mixing phenomena may also explain the high variability of the carbon isotopic compositions of TOC-depleted and altered samples from the Middle and Upper Oxford Clay. The environment of deposition of the LOC would be characterized by the alternation of two major conditions: 1) periods of high productivity, dysoxic water column and high sedimentation rate leading to the development of organic-rich shales dominated by phytoplanktonic organic matter, and 2) periods of low productivity, oxic water column and high current activity implying winnowing and alteration of organic matter, and leading to the formation of shell beds where marine and terrestrial organic matter are mixed.
Carbon fibers
Sedimentary organic matter
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Carbon fibers
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Snowball Earth
Diamictite
Deglaciation
Wisconsin glaciation
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