How to Determine the Origin of Potential Cryogenian Neoproterozoic Dropstones From Death Valley

A diamictite dichotomy: Glacial conveyor belts and olistostromes in the Neoproterozoic of Death Valley, California, USA

Authors:

Le Heron et al

Abstract:

Multiple intercalations of glacially derived and slope-derived diamictites testify to the drawbacks of correlating Neoproterozoic diamictites more widely, but shed new light on the close interrelationship of these processes in the Cryogenian world. In the Neoproterozoic of Death Valley, California (USA), rifting of Rodinia occurred concomitantly with a major glacial event that deposited the Kingston Peak Formation. A new sedimentologic investigation of this formation in the Silurian Hills demonstrates, for the first time, that some diamictites are ultimately of glacial origin. Abundant dropstone textures occur in interstratified heterolithic deposits, with clasts of identical composition (gneiss, schist, granite, metabasite, quartzite) to those of boulder-bearing diamictites suggesting a common source (the glacial conveyor belt). In stark contrast, megaclast-bearing diamictites, yielding clasts of carbonate and siliciclastic preglacial strata as much as 100 m across, are interpreted as olistostromes. The occurrence of syn-sedimentary faults within the succession allows glacial versus slope-derived material to be distinguished for the first time.

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.

Was the Cryogenian Marinoan Glaciation REALLY a Snowball Earth?

Duration and nature of the end-Cryogenian (Marinoan) glaciation

Authors:

Prave et al

Abstract:

The end-Cryogenian glaciation (Marinoan) is portrayed commonly as the archetype of snowball Earth, yet its duration and character remain uncertain. Here we report U-Pb zircon ages for two ash beds from widely separated localities of the Marinoan-equivalent Ghaub Formation in Namibia: 639.29 ± 0.26 Ma and 635.21 ± 0.59 Ma. These findings verify, for the first time, the key prediction of the snowball Earth hypothesis for the Marinoan glaciation, i.e., longevity, with a duration of ≥4 m.y. They also show that the nonglacial interlude of Cryogenian time spanned 20 m.y. or less and that glacigenic erosion and sedimentation, and at least intermittent open-water conditions, occurred 4 m.y. prior to termination of the Marinoan glaciation.

The C/B Excursion During Marinoan Glaciation (Snowball Earth) may not be Meaningful

Boron during meteoric diagenesis and its potential implications for Marinoan snowball Earth δ11B-pH excursions

Authors:

Stewart et al

Abstract:

Large negative carbon (δ13C) and boron (δ11B) isotope excursions (both >6‰) within the widely distributed Neoproterozoic carbonates associated with the Marinoan "snowball Earth" event are interpreted to represent considerable perturbations of the carbon cycle and the accompanying reduction in global ocean pH. Yet this interpretation is predicated on these isotopic signals being primary in origin. Recent studies of Pleistocene carbonate platform sediments from the Great Bahama Bank (western Atlantic Ocean; Clino core, drilled by the Bahamas Drilling Project) and elsewhere demonstrate that δ13C excursions of similar magnitude and global distribution to the snowball Earth excursions are formed following eustatic sea-level fall and exposure of shelf carbonates to meteoric diagenesis. Here we present δ11B and trace element data (B/Ca, Na/Ca, Mg/Ca, and Sr/Ca) from the same Clino core carbonate sediments in order to explore the influence of this diagenetic process on the boron system. We find that within the interval of meteoric diagenesis the δ11B of bulk carbonate is reduced by ∼6‰ in conjunction with a drop in the B/Ca ratio of 90%. Our results clearly demonstrate that the boron system is impacted by meteoric diagenesis, implying that a rigorous assessment of the diagenetic history of all ancient carbonates is required to ensure any paleoceanographic interpretation based on δ11B and/or B/Ca are robust.

Carbonaceous Compression Fossils of Macroscopic Benthic Phototrophs Found From Marinoan Cryogenian NeoProterozoic South China

The survival of benthic macroscopic phototrophs on a Neoproterozoic snowball Earth

Authors:

Ye et al

Abstract:

The greatest ice ages in Earth's history occurred during the 654–635 Ma Marinoan glaciation, when glaciers reached tropical oceans and our planet approached a snowball Earth condition. Paleontological and genomic data suggest that several eukaryotic groups must have survived the Marinoan glaciation. But their fossil record is scarce and limited to microbes, whose ecological and physiological ranges are poorly constrained, thus hampering a full understanding of how and where eukaryotic life—particularly macroscopic phototrophs—survived this snowball Earth. Here we report carbonaceous compression fossils from the Marinoan-age Nantuo Formation in South China. These fossils are preserved in thin black shales sandwiched between glacial diamictites deposited in inner shelf environments of the mid-latitudinal Yangtze block. Some of these fossils are interpreted as benthic macroalgae. Thus, the Marinoan glaciation must have been punctuated by episodes of open waters where habitable benthic substrates were available in the photic zone and along the coast of mid-latitudinal continents. Such open waters may have been the refugia where macroscopic phototrophs survived the Marinoan glaciation and subsequently diversified in the early Ediacaran Period.

Evidence of Ocean Acidification After the Marinoan Glaciation From Cryogenian NeoProterozoic China

Ocean acidification in the aftermath of the Marinoan glaciation

Authors:

Ohnemueller et al

Abstract:

Boron isotope patterns preserved in cap carbonates deposited in the aftermath of the younger Cryogenian (Marinoan, ca. 635 Ma) glaciation confirm a temporary ocean acidification event on the continental margin of the southern Congo craton, Namibia. To test the significance of this acidification event and reconstruct Earth's global seawater pH states at the Cryogenian-Ediacaran transition, we present a new boron isotope data set recorded in cap carbonates deposited on the Yangtze Platform in south China and on the Karatau microcontinent in Kazakhstan. Our compiled δ11B data reveal similar ocean pH patterns for all investigated cratons and confirm the presence of a global and synchronous ocean acidification event during the Marinoan deglacial period, compatible with elevated postglacial pCO2 concentrations. Differences in the details of the ocean acidification event point to regional distinctions in the buffering capacity of Ediacaran seawater.

Marinoan Cryogenian Glaciation Ended Synchronously on a Global Scale

Globally synchronous Marinoan deglaciation

Authors:

Calver et al

Abstract:

U-Pb zircon data from the uppermost Cottons Breccia, representing the Marinoan glacial-postglacial transition on King Island, Tasmania, provide the first direct age constraint on the Cryogenian-Ediacaran boundary in Australia. Zircons in four samples from the topmost meter of the Cottons Breccia, dated by sensitive high-resolution ion microprobe, exhibit two modes ca. 660 Ma and ca. 635 Ma. The younger component predominates in the uppermost sample, a possibly volcanolithic dolomitic sandstone, apparently lacking glacially transported debris, in the transition to cap carbonate. Chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) U-Pb dating of euhedral zircons from that sample yields a weighted-mean age of 636.41 ± 0.45 Ma. Equivalence to published TIMS ash bed dates from Cryogenian-Ediacaran transitional strata in Namibia (635.51 ± 0.82 Ma, within glacial deposit) and China (635.23 ± 0.84 Ma, 2 m above glacial deposit) supports correlation of those strata to the Australian type sections and globally synchronous deglaciation at the end of the Cryogenian Period.