The snowball Earth hypothesis predicts that low-latitude glaciation lasted millions of years while CO 2 built up to critical levels to culminate in catastrophic deglaciation in a supergreenhouse climate. The Gaskiers Formation of eastern Newfoundland (Canada) has been attributed to a snowball glaciation event, but the lack of robust paleomagnetic data and precise geochronological constraints has precluded tests of the hypothesis. Here we present high-precision U-Pb zircon geochronology (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) from eight tuffs from multiple distant stratigraphic sections that bracket glacial diamictites and the first appearance of large Ediacaran fossils. Including internal error, deposition of the Gaskiers diamictite on the Avalon Peninsula is constrained to have been between 580.90 ± 0.40 and 579.88 ± 0.44 Ma, and the Trinity diamictite on Bonavista Peninsula was deposited between 579.63 ± 0.15 and 579.24 ± 0.17 Ma. Assuming approximately synchronous deglaciation, these results imply a maximum duration for deposition of the Trinity diamictite of ≤340 k.y.; this is inconsistent with the multimillion year duration predicted by the snowball Earth hypothesis. Our geochronologic data also constrain the first appearance datum of Ediacaran fossils to <9.5 m.y. after the Gaskiers glaciation. Thus, despite existing paleomagnetic constraints that indicate that marine ice sheets extended to low to middle latitudes, it appears that Earth narrowly escaped a third Neoproterozoic snowball glaciation just prior to the late Ediacaran expansion of metazoan ecosystems.

[1] Among Earth sciences, paleomagnetism is particularly linked to the statistics of large sample sets as a matter of historical development and logistical necessity. Because the geomagnetic field varies over timescales relevant to sedimentary deposition and igneous intrusion, while the fidelity of recorded magnetization is modulated by original properties of rock units and by alteration histories, ''ideal'' paleomagnetic results measure remanent magnetizations of hundreds of samples at dozens of progressive demagnetization levels, accompanied by tests of magnetic composition on representative sister specimens. We present an inexpensive, open source system for automating paleomagnetic and rock magnetic measurements. Using vacuum pick-and-place technology and a quartz-glass sample holder, the system can in 1 h measure remanent magnetizations, as weak as a few pAm 2 , of $30 specimens in two vertical orientations with measurement errors comparable to those of the best manual systems. The system reduces the number of manual manipulations required per specimen approximately eightfold. Holt (2008), Rapid, precise, and high-sensitivity acquisition of paleomagnetic and rock-magnetic data: Development of a low-noise automatic sample changing system for superconducting rock magnetometers, Geochem. Geophys. Geosyst., 9, Q05Y01,

The Neoproterozoic Era witnessed a succession of biological innovations that culminated 23 in a wide range of animal body plans and behaviours during the Ediacaran-Cambrian 24 radiations. Intriguingly, this interval is also marked by perturbations to the global carbon 25 cycle, as evidenced by extreme fluctuations in climate and carbon isotopes. The 26Neoproterozoic isotope record has defied parsimonious explanation because sustained 27 12 C-enrichment (low 13 C) in seawater seems to imply that substantially more oxygen was 28 consumed by organic carbon oxidation than could possibly have been available. Here we 29 propose a solution to this problem, in which carbon and oxygen cycles can maintain 30 dynamic equilibrium during negative 13 C excursions when surplus oxidant is generated 31 through bacterial reduction of sulfate that originates from evaporite weathering. 32Coupling of evaporite dissolution with pyrite burial drives a positive feedback loop 33 whereby net oxidation of marine organic carbon can sustain greenhouse forcing of 34 chemical weathering, nutrient input and ocean margin euxinia. Our proposed model is 35 particularly applicable to the late Ediacaran 'Shuram' isotope excursion that directly 36 preceded the emergence of energetic metazoan metabolisms during the Ediacaran-37 Cambrian transition. Non-steady state sulfate dynamics are likely to have contributed to 38 climate change, episodic ocean oxygenation and opportunistic radiations of aerobic life 39 forms during the Neoproterozoic Era. 40 41The Neoproterozoic Era (1000c.540 Ma) marks a turning point in Earth history when groups 42 of morphologically complex multicellular eukaryotes, including algae and animals, attained 43 ecological dominance, irrevocably changing Earth System dynamics 1 . These biological 44 radiations took place amid fluctuating climate, including two prolonged episodes of global 45 glaciation during the Cryogenian Period (c.715c.660 and c.650c.635 Ma) and short-lived, 46 regional ice ages during the Ediacaran Period (e.g. c.580 Ma), interspersed with warmer 47 intervals. The world's oceans also became episodically more oxygenated during the 48 Neoproterozoic with the extent of oxygenated seafloor reaching near-modern levels at times 49 during the early Cambrian 2 . Both climate and oxygenation are regulated by Earth's long-term 50 carbon cycle, and so perhaps unsurprisingly this interval is characterised by extreme carbon 51 isotope instability 3 (Fig. 1). Since their discovery over 30 years ago 4-7 , the uniquely high 52 amplitudes of Neoproterozoic 13 C excursions have defied conventional interpretation 3,[8][9][10] . 53Here we relate the largest of these anomalies to the transfer of oxidant from the evaporite rock 54 reservoir to the surface environment via the coupling of sulfate weathering and pyrite burial. 55 Such pulses of evaporite weathering are predicted to have occurred during the Ediacaran Period 56 (see detailed account in SI 4), in particular, as Rodinia's passive margins underwent tectonic 57 uplift during ...

The role of sediment melting in Earth's mantle remains controversial, as direct observation of melt generation in the mantle is not possible. Geochemical fingerprints provide indirect evidence for subduction-delivery of sediment to the mantle, however sediment abundance in mantle-derived melt is generally low (0-2%), and difficult to detect. Here we 1 provide evidence for bulk melting of subducted sediment in the mantle through isotopic analysis of granite sampled from an exhumed mantle section. Peraluminous granite dikes that intrude peridotite in the Oman-United Arab Emirates ophiolite have U-Pb ages of 99.8±3.3 Ma that predate obduction at ca. 85 to 90 Ma. The dikes have unusually high oxygen isotope (δ 18 O) values for whole rock (14-23‰) and quartz (20-22‰), and yield the highest δ 18 O zircon values known (14-28‰; values relative to Vienna standard mean ocean water). The extremely high oxygen isotope ratios uniquely identify the melt source as high δ 18 O marine sediment (pelitic and/or siliciceous mud), as no other source could produce granite with such anomalously high δ 18 O. Formation of high δ 18 O sediment-derived (S-type) granite within peridotite requires delivery of sediment to the mantle by subduction, where it melted and intruded the overlying mantle wedge. The granite suite described here contains the most evolved oxygen isotope ratios reported for igneous rocks, yet intruded mantle peridotite below the Mohorovičić seismic discontinuity, the most primitive oxygen isotope reservoir in the silicate Earth. Identifying the presence and quantifying the extent of sediment melting within the mantle has important implications for understanding subduction recycling of crust and mantle heterogeneity over time.

Hotspot tracks represent plate motions relative to mantle sources, and paleomagnetic data from magmatic units along those tracks can quantify motions of those mantle anomalies relative to the Earth's magnetic field and rotational axis. The Ediacaran Period is notable for rapid and large paleomagnetic apparent polar wander (APW) for many continents. Whereas magmatic units attributed to the "Sutton" mantle plume suggest a practically stationary hotspot track, paleolatitudes of Laurentia for that interval vary dramatically; geologic and paleomagnetic data are at odds unless true polar wander (TPW) is invoked to explain a majority of APW. Here we test the plume-TPW hypothesis by generating the predicted Sutton hotspot track for a stationary plume under a moving plate along the Laurentian margin during the interval from 615 to 530 Ma. Our model is the first to provide a kinematic framework for the extensive large igneous province associated with opening the Iapetus Ocean.