The Ediacaran Period: a new addition to the geologic time scale

The Neoproterozoic contains severe glacial intervals (750–580 Ma) including two extending to low palaeomagnetic latitudes. Paucity of radiometric dates indicates the need for chronostratigraphic tools. Whereas the marine 87 Sr/ 86 Sr signatures show a steady rise, δ 13 C fluctuates, the most reproducible variations being negative signatures in carbonate caps to glacial units, but more diagenetic work is needed. Four conceptual models for the icehouse conditions are contrasted: Zipper-Rift Earth (diachronous glaciation related to continental rift margins), High-tilt Earth (high-obliquity and preferential low-latitude glaciation), Snowball Earth (extreme glaciation related to runaway ice–albedo feedback) and Slushball Earth (coexistence of unfrozen oceans and sea-level glaciers in the tropics). Climate models readily simulate runaway glaciation, but the Earth may not be able to recover from it. The Slushball state requires more extensive modelling. Biogeochemical models highlight the lack of CO 2 buffering in the Neoproterozoic and the likely transition from a methane- to a CO 2 -dominated climate system. Relevant processes include tropical weathering of volcanic provinces, and new land biotas stimulating both clay mineral formation and P delivery to the oceans, facilitating organic C burial. Hence a step change in the Earth System was probably both facilitated by organisms and responsible for moderating Phanerozoic climate.

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“…2f), correlated with sections in China ( Fig. 2g and h) and Namibia dated at c. 635 Ma (Knoll et al 2006).…”

Section: Geological Evidencementioning

confidence: 99%

Neoproterozoic glaciation in the Earth System

Fairchild

Kennedy

2007

JGS

213

116

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“…The initial GSSPs for Phanerozoic periods were placed with reference to the first appearances of fossil animal species, a practice exported to the Proterozoic record only with difficulty. By international agreement, the GSSP for the initial boundary of the Ediacaran Period is placed with respect to major climatic and geochemical markers (Knoll et al, 2006b). …”

Section: Discussionmentioning

confidence: 99%

“…Radiometric constraints provide a sobering answer. The beginning of the Ediacaran Period is defined by a global stratigraphic section and point (GSSP) at the base of cap carbonates that directly overlie glaciogenic rocks of the Elatina Formation in the Flinders Ranges, South Australia (Knoll et al, 2006b). U-Pb zircon dates on volcanic ash beds in correlative sections from China (Condon et al, 2005) and Namibia (Hoffmann et al, 2004) suggest an age of about 635 million years (Ma) for the beginning of the period (see Calver et al, 2004, for an alternative view).…”

Section: Introductionmentioning

confidence: 99%

Neoproterozoic microfossils from the margin of the East European Platform and the search for a biostratigraphic model of lower Ediacaran rocks

Воробьева

Сергеев

Knoll

2009

Precambrian Research

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A ca. 600 m thick siliciclastic succession in northern Russia contains abundant and diverse microfossils that document early to middle Ediacaran deposition along the northeastern margin of the East European Platform. The Vychegda Formation is poorly exposed but is well documented by a core drilled in the Timan trough region (Kel'tminskaya-1 borehole). Vychegda siliciclastics lie unconformably above Tonian to lower Cryogenian strata and below equivalents of the late Ediacaran Redkino succession that is widely distributed across the platform. The basal ten meters of the formation preserve acritarchs and fragments of problematic macrofossils known elsewhere only from pre-Sturtian successions. In contrast, the upper, nearly 400 m of the succession contains abundant and diverse large acanthomorphic acritarchs attributable to the Ediacaran Complex Acanthomorph Palynoflora (ECAP). This distinctive set of taxa is known elsewhere only from lower, but not lowermost, Ediacaran rocks. In between lies an additional assemblage of relatively simple filaments and stratigraphically long ranging Manuscript Click here to view linked References 2 sphaeromorphic acritarchs interpreted as early Ediacaran in age. Bearing in mind that knowledge of late Cryogenian (post-Strurtian/pre-Marinoan) microfossils is sparse, the Vychegda record is consistent with data from Australia and China which suggest that diverse ECAP microfossil assemblages appeared well into the Ediacaran Period.Accumulating paleontological observations underscore both the promise and the challenges for biostratigraphic characterization of the early Ediacaran Period.

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“…This said, the fact that the lower boundary of the newly defined Ediacaran Period is defined as the contact between the glaciogenic Elatina Fm. (¼Marinoan) and the overlying Nuccaleena cap carbonate (Knoll et al 2006) is suggestive of the chronostratigraphic importance of at least one of these Neoproterozoic glaciations. The definition of this boundary was bolstered by several new, well placed and precise radiometric ages on basal Ediacaran cap dolostones (i.e.…”

Section: General Trendsmentioning

confidence: 99%