There was Significantly Less Sediment Laid Down During the Cryogenian NeoProterozoic (Snowball Earth) Glaciations

Slow net sediment accumulation sets snowball Earth apart from all younger glacial episodes

Authors:

Partin et al

Abstract:

During the Sturtian and Marinoan "snowball Earth" episodes, ice cover is thought to have extended from polar to tropical latitudes. We test the supposition that such an extreme glacial climate, not repeated in the subsequent ∼635 m.y. of Earth history, would have reduced the vigor of the hydrologic cycle and thus diminished sediment flux to the oceans. With >500 sediment accumulation rates to characterize Sturtian and Marinoan deposits, we find median accumulation rates at least four to 15 times slower than expected for Phanerozoic glaciomarine deposits as characterized by >10,000 rates. Our comparison is conservative with respect to time span, latitude, and distance from the ice margin. Phanerozoic accumulation rates decrease systematically when averaged over longer time spans. Comparisons were drawn, therefore, at 5 and 57 m.y. time spans to match minimum Marinoan and Sturtian durations, respectively. Cenozoic glaciomarine accumulation also slows with increasing latitude from temperate to polar climates and with increasing distance from the ice margin. After accounting for time span, snowball Earth deposits at low latitude are found to be thinner than would be expected either for high-latitude Cenozoic glacial deposits or for very distal glaciomarine abyssal muds with ice-rafted debris. The rate discrepancy is not readily attributed to overestimates of the total Marinoan or Sturtian durations. If sediment fluxes during warm melt intervals did approach Phanerozoic rates, these intervals must have occupied a much smaller proportion of snowball Earth episodes than in younger glacial climates.

Explaining the Iron Drop Stones of the Sturtian Glaciation (Snowball Earth)

Sub–ice shelf ironstone deposition during the Neoproterozoic Sturtian glaciation

Authors:

Lechte et al

Abstract:

The Neoproterozoic Sturtian glaciation is considered to be among the most severe glaciations in Earth history, possibly encompassing the entire planet and lasting for more than 50 m.y. Iron formations are globally associated with Sturtian glacial successions, although the influence of glaciation on the genesis of these iron formations remains contentious. Here we examine the Sturtian iron formations of Namibia and Australia that feature finely laminated ironstones containing up to 55% total iron. These ironstones are repeatedly interbedded with massive diamictites, yet dropstones and other clastic input are nearly absent in the laminated ironstone facies. Intercalated diamictites are variably ferruginous and characterized by a strong glacial influence with evidence of glaciotectonism. The ferruginous facies are laterally discontinuous and commonly occupy paleobathymetric depressions. Rare earth element signatures from these iron formations are similar to those from modern seawater but lack cerium anomalies. The paradox of dropstone-free, laminated sediments intimately interlaminated with massive ice-proximal diamictites can be resolved by deposition under an ice shelf. Polynya activity and the mixing of cold, oxygenated glacial fluids with ferruginous seawater via an ice pump mechanism may explain the deposition of these iron formations and their restriction to Sturtian glacial successions globally

Was Venus Habitable Until the the Start of Earth's Sturtian Glaciations (Snowball Earth)

Was Venus the First Habitable World of our Solar System?

Authors:

Way et al

Abstract:

Present-day Venus is an inhospitable place with surface temperatures approaching 750K and an atmosphere over 90 times as thick as present day Earth's. Billions of years ago the picture may have been very different. We have created a suite of 3D climate simulations using topographic data from the Magellan mission, solar spectral irradiance estimates for 2.9 and 0.715 billion years ago, present day Venus orbital parameters, an ocean volume consistent with current theory and measurements, and an atmospheric composition estimated for early Venus. Using these parameters we find that such a world could have had moderate temperatures if Venus had a rotation period slower than about 16 Earth days, despite an incident solar flux 46-70% higher than modern Earth receives. At its current rotation period of 243 days, Venus's climate could have remained habitable until at least 715 million years ago if it hosted a shallow primordial ocean. These results demonstrate the vital role that rotation and topography play in understanding the climatic history of exoplanetary Venus-like worlds being discovered in the present epoch.

2nd Cryogenian NeoProterozoic Sturtian Glaciation Started Synchronously Around the World

Global synchronous initiation of the 2nd episode of Sturtian glaciation: SIMS zircon U–Pb and O isotope evidence from the Jiangkou Group, South China

Authors:

Lan et al

Abstract:

Depositional age of the Xieshuihe Formation in the Nanhua Basin, South China has been controversial in the past two decades, which hampers our understanding of the synchroneity of the 2nd episode of Sturtian glaciation as well as regional stratigraphic correlation. In this study, a new SIMS zircon U–Pb age of 691.9 ± 8.0 Ma (MSWD = 1.3) was obtained from a tuffaceous siltstone bed within the upper Xieshuihe Formation in the Nanhua Basin, which is within errors consistent with the ID-TIMS U–Pb zircon age of 688.6 + 9.5/−6.2 Ma from Yukon, Canada, the SHRIMP U–Pb zircon ages of 685 ± 7 and 684 ± 4 Ma from central Idaho, the SHRIMP U–Pb zircon age of 686 ± 4 Ma from southeastern Idaho, and the SHRIMP U–Pb age of 703 ± 6 Ma from northern Utah, North American Cordilleran margin. All these dated formations conformably underlie the upper Sturtian glaciogenic diamictite. Hence new ages suggest a maximum initiation age of ca. 690 Ma for the 2nd episode of Sturtian glaciation, which is synchronous in at least two geographically separated palaeocontinents. The commonly low δ18O values (3–5‰) from the zircon grains of Xieshuihe Formation suggest an influence of glaciation during their formation, probably by means of an ice–fire interaction process as previously proposed. Additionally, new age data suggest the Xieshuihe Formation is not correlative with the Liantuo Formation/Banxi Group as previously assumed. Rather, it could be correlated with the Liangjiahe Member, Fulu Formation in South China, which is feasible in terms of sedimentology and sequence stratigraphy.

Oxygen-17 Evidence of the PaleoClimate of Snowball Earths Past

Revealing the climate of snowball Earth from Δ17O systematics of hydrothermal rocks

Authors:

Herawartz et al

Abstract:

The oxygen isotopic composition of hydrothermally altered rocks partly originates from the interacting fluid. We use the triple oxygen isotope composition (17O/16O, 18O/16O) of Proterozoic rocks to reconstruct the 18O/16O ratio of ancient meteoric waters. Some of these waters have originated from snowball Earth glaciers and thus give insight into the climate and hydrology of these critical intervals in Earth history. For a Paleoproterozoic [∼2.3–2.4 gigayears ago (Ga)] snowball Earth, δ18O = −43 ± 3‰ is estimated for pristine meteoric waters that precipitated at low paleo-latitudes (≤35°N). Today, such low 18O/16O values are only observed in central Antarctica, where long distillation trajectories in combination with low condensation temperatures promote extreme 18O depletion. For a Neoproterozoic (∼0.6–0.7 Ga) snowball Earth, higher meltwater δ18O estimates of −21 ± 3‰ imply less extreme climate conditions at similar paleo-latitudes (≤35°N). Both estimates are single snapshots of ancient water samples and may not represent peak snowball Earth conditions. We demonstrate how 17O/16O measurements provide information beyond traditional 18O/16O measurements, even though all fractionation processes are purely mass dependent.

Marine Conditions Before, During and After the Cryogenian NeoProterozoic Sturtian Glaciations

The Marine Environments encompassing the Neoproterozoic glaciations: Evidence from C, Sr and Fe isotope ratios in the Hecla Hoek Supergroup in Svalbard

Authors:

Tahata et al

Abstract:

The Neoproterozoic Era records several important events in Earth history. The associations between BIF deposition, Snowball Earth events and the redox state of seawater are the key to explain the re-appearance of banded iron formations (BIF) during the Neoproterozoic. Unraveling ancient iron cycles provides important information about the linkage between the precipitation of BIF and the redox condition in the ocean, but current iron isotopic data are limited to sediments deposited only during the Sturtian glaciations and Ediacaran.

We conducted a detailed geological survey of the Tonian to Ediacaran sedimentary succession in Nordaustlandet island, Svalbard. Our chemostratigraphies of δ13Ccarb from pre-Sturtian to post-Marinoan sedimentary successions in the Polarisbreen Group are consistent with those in coeval sediments.

We first analyzed iron isotope ratios (δ56/54Fe) of individual pyrite grains with diverse shapes in carbonates, black shales, quartz arenites and diamictites in order to constrain the redox condition in seawater. We found large variations in iron isotope ratios; from ca. -2 to ca. +4 ‰. Positive δ56/54Fe values, over +1.0 ‰, are only preserved in euhedral and aggregated pyrite grains in black shales of the Backlundtoppen Formation and the MacDonaldryggen Member of the Elbobreen Formation. A partial oxidation of ferrous iron in seawater is necessary to explain such high δ56/54Fe values, which means that deep seawater was ferruginous (ferrous, iron-rich) before and after the Sturtian glaciation. This stands in opposition to the traditional idea that a glacial cover on an ocean is necessary for the accumulation of ferrous iron in Neoproterozoic seawater. This study presents the first evidence from iron isotopes that supports a deep ferruginous ocean around the Sturtian glaciation. If the deep ocean was intrinsically iron-rich, it is likely that Neoproterozoic BIFs formed by thermohaline circulation during the glaciation and by massive oxidation due to the expansion of oxic oceans after the glaciation.

Tightening up the Cryogenian Glaciation Chronology

A Cryogenian chronology: Two long-lasting synchronous Neoproterozoic glaciations

Authors:

Rooney et al

Abstract:

The snowball Earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion-year time scale. Geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons. Here we present four new Re-Os geochronology age constraints on Sturtian (717–660 Ma) and Marinoan (635 Ma termination) glacial deposits from three different paleocontinents. A 752.7 ± 5.5 Ma age from the base of the Callison Lake Formation in Yukon, Canada, confirms nonglacial sedimentation on the western margin of Laurentia between ca. 753 and 717 Ma. Coupled with a new 727.3 ± 4.9 Ma age directly below the glacigenic deposits of the Grand Conglomerate on the Congo craton (Africa), these data refute the notion of a global ca. 740 Ma Kaigas glaciation. A 659.0 ± 4.5 Ma age directly above the Maikhan-Uul diamictite in Mongolia confirms previous constraints on a long duration for the 717–660 Ma Sturtian glacial epoch and a relatively short nonglacial interlude. In addition, we provide the first direct radiometric age constraint for the termination of the Marinoan glaciation in Laurentia with an age of 632.3 ± 5.9 Ma from the basal Sheepbed Formation of northwest Canada, which is identical, within uncertainty, to U-Pb zircon ages from China, Australia, and Namibia. Together, these data unite Re-Os and U-Pb geochronological constraints and provide a refined temporal framework for Cryogenian Earth history

Demosponges Diverged From Other Animals During the Cryogenian Sturtian Glaciations (Snowball Earth?)

Early divergence dates of demosponges based on mitogenomics and evaluated fossil calibrations

Authors:

Ma et al

Abstract:

Demosponges are among the most primitive biomineralized metazoans to appear first in the fossil record with hard skeletons; their confirmed earliest fossils are from the lower Cambrian rocks about 520 Ma, with putative demosponge biomarkers reported from 713∼635 Ma sediments. In this study, we use mitogenomic data to approach the early divergence timescale of demosponges using relaxed molecular clock techniques and likelihood-evaluated fossil calibration strategies. We found that among various molecular dating models, the correlated rate model yielded time estimates of demosponges in this analysis which is most congruent with the fossil appearance dates of demosponges. Our dating analyses show that crown groups of Demospongiae appeared at about 704 (674∼741) Ma, and the silicification in demosponges (divergence of spicular sponges) began about 633 (616∼648) Ma indicating a gap of over 100 million years between the origin of silicification and their first unequivocal appearance of siliceous spicules in the fossil record (520∼525 Ma); demosponges with tetraxon-type spicules (Tetractinellida) are dated here at about 514 (498∼530) Ma, an estimate comparable with the earliest tetraxial megasclere fossil records (510∼520 Ma, Ordian Age, middle Cambrian).

Evidence Sturtian Glaciations of the Cryogenian NeoProterozoic Were From a Snowball Earth Scenario

A rapid and synchronous initiation of the wide spread Cryogenian glaciations

Authors:

Lan et al

Abstract:

The Neoproterozoic glaciations represent a remarkable milestone in Earth evolution due to their major influence on atmosphere, biosphere, and hydrosphere. The early Cryogenian, ‘Sturtian’ glaciations, evidence for which has been recorded worldwide, may have lasted for about 50 million years, but the precise timing and synchroneity of these glaciations have been disputed. Jiangkou glacigenic strata in South China can be attributed to the Sturtian glacial episode as they share sedimentological, stratigraphic and lithological similarities with Sturtian strata in South Australia. As such, dating the onset of the Jiangkou glaciation would contribute to testing the synchroneity of the worldwide Sturtian glaciation. In this paper, we demonstrate by means of SIMS U-Pb zircon dating that the Jiangkou glaciation in South China was initiated at 715.9 ± 2.8 Ma. The new age is, within uncertainty, synchronous with dates obtained from the lower parts of glacigenic successions in Laurentia and Oman, thereby indicating thereby indicating a synchroneity and global extent to the Sturtian glaciation. The synchronous occurrence of the Sturtian glaciation in mid- to low-latitude regions implies rapid ice sheet advancement in response to ice-albedo feedback, as supported by numerical climatic modelling.