Search

The supercontinent Rodinia is hypothesized to have been assembled and positioned in tropical latitudes by the early Neoproterozoic Era. Paleomagnetic data from limestones of Svalbard and basaltic dikes of South China have been interpreted to record rapid changes in paleogeography driven by true polar wander that may have rotated the supercontinent in association with the ~800 Ma Bitter Springs carbon isotope event. To further constrain early Neoproterozoic paleogeography and to test proposed rapid rotations, we have developed sequence- and chemostratigraphically constrained paleomagnetic data from the Bitter Springs Formation of the Amadeus Basin of central Australia. A new paleomagnetic pole for the post-Bitter Springs stage ~770 Ma Johnny's Creek Member (Bitter Springs Formation) provides a positive test for a long-lived history of Australia and Laurentia in a single supercontinent as its similar position to late Mesoproterozoic north Australia poles reproduces the closure of the Laurentian "Grenville Loop." This new pole also provides support for the hypothesis that there was significant rotation between north and south+west Australia at the end of the Neoproterozoic as this rotation brings the south+west Australia ~755 Ma Mundine Well pole into much closer proximity to the north Australia Johnny's Creek pole. Syn-Bitter Springs stage carbonates of the Love's Creek Member of the formation contain a well-behaved remanence held by magnetite. The direction of this remanent magnetization falls on the Cambrian portion of Australia's apparent polar wander path suggesting that the magnetite may have formed authigenically at that time. If primary, the Love's Creek direction is consistent with the true polar wander hypothesis for the Bitter Springs stage, is internally consistent with the relative sea level changes inferred from the formation, and can constrain Australia to a SouthWest North America East AnTarctica (SWEAT) fit. A remanence held by pyrrhotite in carbonates of the Bitter Springs Formation corresponds to the apparent polar wander path of Australia at ~350 Ma. This component can be used to constrain the history of the Devonian-Carboniferous Alice Springs Orogeny as it demonstrates that regional folding of basinal sediments occurred prior to ~350 Ma, but that the latest stages of tectonism in the hinterland drove fluids through the sediments that altered redox conditions to favor pyrrhotite precipitation. [ABSTRACT FROM AUTHOR]

Abstract: Sedimentologically and geochemically distinctive carbonate sequences consistently drape the glacial deposits associated with the younger Cryogenian ice age. The presence of ice-rafted debris in the basal dolostone implies that at least the lower portion of the cap sequence records deglaciation. An isochronous model proposes that cap dolostones were deposited synchronously around the world regardless of water depth, whilst a diachronous model proposes that deposition tracked glacioeustatic flooding during deglaciation. The Nuccaleena Formation of the Adelaide Rift Complex (ARC), South Australia, exhibits many of the unique sedimentary features observed in other younger Cryogenian cap dolostones around the world. Some bedforms are the product of wind-driven oscillatory flow, thus constraining post-glacial dolostone deposition to <400m. These sedimentary features are absent in the deepest basinal facies in the northern ARC, suggesting that this region was below storm wave base even at the glacial sea level lowstand. In the north Flinders Ranges, there is a distinct relationship between lateral facies variability in the pre-, syn- and post-glacial sediments and the axes of 50km scale structural folds. The northern fold limbs are characterised by basinal facies, whilst the southern limbs are associated with upper-slope facies. We interpret the abrupt facies transitions to reflect lower slope to upper slope/shelf breaks across a series of linked, south-facing half-grabens. The majority of cap dolostone carbon isotope records show monotonic declines in δ 13C of <2‰, with starting points between −0.5 and −3.5‰. An isochronous model implies a ∼3.0‰ lateral gradient from platform to lower slope that varies dramatically on a short spatial scale and non-systematically with palaeobathymetry. If pre- and syn-glacial facies are used to infer palaeobathymetry, and cap dolostones are deposited diachronously as sea level rises during deglaciation, then δ 13C values becomes progressively lighter with time, implying that cap dolostone deposition tracked the glacioeustatic sea-level rise over a series of half-grabens that deepened to the north. The carbon isotope dataset cannot rule out uniquely isochronous or diachronous models. Given the high frequency spatial variability of δ 13C values, temperature cannot be the dominant control on the isotopic variability of the cap dolostone. [Copyright &y& Elsevier]

Abstract: The Neoproterozoic Akademikerbreen Group in northeastern Svalbard comprises 2 km of nearly pure carbonate section. The carbonates are generally highly 13C-enriched (δ 13C>5‰), but this trend is interrupted by an ∼325 m interval of low δ 13C values (−4 to 0‰) in the upper Grusdievbreen and lower Svanbergfjellet formations. An abrupt negative isotopic shift at the onset of this low δ 13C interval is reproduced in detail in multiple sections along the length of the outcrop belt (125 km) and everywhere coincides with a prominent sequence boundary and change in lithology. Likewise, the return to positive δ 13C values coincides with a second exposure surface. Correlation of the lower Akademikerbreen Group δ 13C record with a nearly identical isotopic profile in the Bitter Springs Formation of Central Australia suggests an age of ∼800 Ma for the low δ 13C interval and confirms that it is a global seawater signal. The coincidence of the negative and positive δ 13C shifts with major stratigraphic perturbations in the otherwise conformable succession suggests that both episodes of transient sea level change were related to global phenomena. 87Sr/86Sr ratios rise transiently from an average of 0.7063 to 0.7066 within this interval. Whereas large negative δ 13C anomalies in the Neoproterozoic are commonly associated with episodes of widespread glaciation, the Akademikerbreen low δ 13C interval precedes the oldest (Sturtian) of the known Neoproterozoic glacial events, and no other evidence suggests an ice age at this time. We propose instead that the negative δ 13C interval is related to a pair of inertial interchange true polar wander (TPW) events. [Copyright &y& Elsevier]

Thermal contraction cracking of permafrost produced sand-wedge polygons at sea level on the paleo-equator during late Neoproterozoic glacial episodes. These sand wedges have been used as evidence for high (≥54°) paleo-obliquity of the Earth’s ecliptic, because cracks that form wedges are hypothesized to require deep seasonal cooling so the depth of the stressed layer in the ground reaches ≥1 m, similar to the measured depths of cracks that form wedges. To test the counter hypothesis that equatorial cracks opened under a climate characterized by a strong diurnal cycle and low mean annual temperature (snowball Earth conditions), we examine crack formation in frozen ground subject to periodic temperature variations. We derive analytical expressions relating the Newtonian viscosity to the potential crack depth, concluding that cracks will form only in frozen soils with viscosities greater than ∼1014 Pa s. We also show numerical calculations of crack growth in frozen soils with stress- and temperature-dependent rheologies and find that fractures may propagate to depths 3–25 times the depth of the thermally stressed layer in equatorial permafrost during a snowball Earth because the mean annual temperature is low enough to keep the ground cold and brittle to relatively great depths. [Copyright &y& Elsevier]

We present new paleomagnetic data from three Middle Neoproterozoic carbonate units of East Svalbard, Norway. The paleomagnetic record is gleaned from 50 to 650 m of continuous, platformal carbonate sediment, is reproduced at three locations distributed over >100 km on a single craton, and scores a 5-6 (out of 7) on the Van der Voo (1990) reliability scale. Two >50° shifts in paleomagnetic direction are coincident with equally abrupt shifts in δ13C and transient changes in relative sea level. We explore four possible explanations for these coincidental changes: rapid plate tectonic rotation during depositional hiatus, magnetic excursions, nongeocentric axial-dipole fields, and true polar wander. We conclude that the observations are explained most readily by rapid shifts in paleogeography associated with a pair of true polar wander events. Future work in sediments of equivalent age from other basins can test directly the true polar wander hypothesis because this type of event would affect every continent in a predictable manner, depending on the continent's changing position relative to Earth's spin axis. [ABSTRACT FROM AUTHOR]

Abstract: The limited availability and low resolution of biostratigraphic and radiometric data in the Precambrian necessitate using marine chemostratigraphy to correlate and integrate the fragmentary stratigraphic record. We present a new composite record of the strontium isotope composition of seawater during the Neoproterozoic (1000–542 Ma) Era that is linked to a high-resolution δ 13C (carbonate) compilation. The 87Sr/86Sr record consists predominantly of new and previously published data from carbonate-rich successions in northwest Canada, Svalbard, and northern Namibia. Published data from other late Neoproterozoic successions are conservatively included to fill in the Ediacaran Period (635–542 Ma). Consistent with previous compilations, the resulting strontium isotope record shows a systematic rise in 87Sr/86Sr throughout the Neoproterozoic, from as low as 0.7055 at the beginning of the era to greater than 0.7085 towards the end. Though the record remains poorly resolved and time-calibrated, it appears that declines, inflections, and rapid changes in 87Sr/86Sr all correspond to major biogeochemical or climatic events. However, this record supports neither a tight coupling between δ 13C and 87Sr/86Sr via continental erosion nor widespread mountain-building as the main driver for increasing 87Sr/86Sr compositions during the Neoproterozoic. Rather, the steady rise in 87Sr/86Sr during the Neoproterozoic corresponded to the break-up of the supercontinent Rodinia. [Copyright &y& Elsevier]

Highlights • New synthesis of late Tonian stratigraphic and isotopic data from northeastern Svalbard. • Correlation with successions in northern Canada provides new ages on Svalbard strata. • Stratigraphic data indicate a reorganization of the East Svalbard basin ca. 755 Ma. • Possibly two large negative carbon isotope anomalies during late Tonian. Abstract The Neoproterozoic stratigraphic succession in northeastern Svalbard is uniquely important in documenting the evolution of Neoproterozoic life, seawater chemistry, and paleoenvironments. This contribution focuses on the stratigraphic and geochemical records spanning from the late Tonian to the onset of Cryogenian glaciation in the Hecla Hoek Series, with the purpose of informing debates on the subdivision of the Neoproterozoic time scale, the cause of the descent into the first snowball ice age, and the tempo of Neoproterozoic oxygenation. This interval is represented by the Kinnvika Member of the uppermost Akademikerbreen Group and the Russøya Member of the Elbobreen Formation, lowermost Polarisbreen Group. These units record the demise of the long-lived, stable Akademikerbreen carbonate platform in the East Svalbard basin. The upper Russøya Member preserves a deep negative carbon isotope anomaly that has been linked mechanistically to the onset of Cryogenian cooling. This anomaly was previously correlated with the so-called Islay negative carbon isotope anomaly in Scotland. Here we propose a revised correlation of the latest Tonian section in Svalbard with equivalent-aged but better dated successions in northern Laurentia using sequence stratigraphy and carbon and strontium isotope chemostratigraphy. These correlations provide a means for calibrating the Tonian-Cryogenian transition interval in Svalbard and suggest that the upper Russøya negative carbon isotope anomaly significantly predates the onset of Cryogenian glaciation, with ca. 20 million years of missing record beneath the disconformity at the base of the overlying glaciogenic Petrovbreen Member. This anomaly may be the older of two late Tonian negative carbon isotope anomalies of similar magnitude. [ABSTRACT FROM AUTHOR]

Abstract: Neoproterozoic successions in NE Svalbard and East Greenland host a pair of glacigenic formations, the younger of which are correlated with the terminal Cryogenian (Marinoan) glaciation based on their lithologically and isotopically diagnostic cap dolostones. A deep negative carbon isotope excursion (CIE) occurs stratigraphically beneath the older glacigenic formations in both areas, but no CIE is preserved beneath the younger glacial horizon in either area. This led to uncertainty over the age of the CIE and the paleoenvironmental significance of the units separating the glacigenic formations. 87Sr/86Sr ratios in Sr-rich limestone associated with the CIEs are ≤0.7064 in East Greenland and ≤0.7068 in NE Svalbard, consistent with early Cryogenian values globally and inconsistent with late Cryogenian ratios, which are exclusively ≥0.7071. The CIEs are tentatively correlated with the Islay excursion in the Scottish and Irish Caledonides, and potentially correlative subglacial CIEs in northwestern Canada, northwestern Tasmania and the southwestern United States. In NE Svalbard, newly-acquired δ 13Corg data covary with δ 13Ccarb across the CIE, but the organic excursion is roughly half the amplitude of the inorganic excursion. δ 13Ccarb and δ 18Ocarb do not covary, nor does δ 13Corg vary as a function of total organic carbon content. A primary origin for the CIE is supported and the accompanying proliferation of stromatolites suggests rising carbonate saturation (falling pCO2) prior to glaciation. New data suggest that the hiatus responsible for the missing Trezona CIE occurs at the top, not the bottom, of the Bråvika member sandstone in NE Svalbard. A dramatic decline in regional average thickness of older Cryogenian glacial deposits from the paleomagnetically-determined subtropics (British Isles) to the deep tropics (Svalbard) is consistent with meteoric net accumulation and sublimation zones on a snowball Earth, and inconsistent with the Jormungand climate zonation. [Copyright &y& Elsevier]

Abstract: The Neoproterozoic Era was punctuated by the ‘Sturtian’ (~710million years ago) and ‘Marinoan’ (~635million years ago) low-latitude glaciations. Carbonates preceding the younger Marinoan glacial succession record an ~18‰ negative shift in the δ13C of carbonate around the world. This ‘Trezona’ isotopic anomaly is the largest δ13C shift in Earth history and its origin and timing remain controversial. The δ13C anomaly could record a dramatic reorganization of Earth''s carbon cycle and be linked causally to the initiation of Marinoan ice-house conditions. Alternatively, the δ13C anomaly might record secondary fluid alteration following carbonate deposition. Here we document dropstones within the carbonate sediments immediately below the Marinoan glacial diamictite in South Australia. Advancing ice sheets caused soft-sediment deformation of the beds below the glacial diamictite, as well as subglacial erosion of the carbonates beneath, showing that the Trezona δ13C values must have been acquired before glaciation. Although these stratigraphic relationships do not provide a specific mechanism to explain the Trezona δ13C anomaly, they do require that the nadir of the Trezona δ13C anomaly was recorded prior to local glacier advance and long before late-stage burial diagenesis could have occurred. Furthermore, the δ13C recovery in the Trezona Formation toward 0‰ was synchronous with the appearance of icebergs in the tropics. [Copyright &y& Elsevier]

Glacial deposits of Sturtian and Marinoan age occur in the well-studied Neoproterozoic successions of northern Namibia, South Australia, and northwestern Canada. In all three regions, the Marinoan glaciation is presaged by a large negative δ13C anomaly, and the cap carbonates to both glacial units share a suite of unique sedimentological, stratigraphic, and geochemical features. These global chronostratigraphic markers are the bases of a new correlation scheme for the Neoproterozoic that corroborates radiometric data that indicate that there were three glacial epochs between ca. 750 and 580 Ma. Intraregional correlation of Neoproterozoic successions in the present-day North Atlantic region suggests that glacial diamictite pairs in the Polarisbreen Group in northeastern Svalbard and the Tillite Group in eastern Greenland were deposited during the Marinoan glaciation, whereas the younger of a pair of glacials (Mortensnes Formation) in the Vestertana Group of northern Norway was deposited during the third (Gaskiers) Neoproterozoic glaciation. Gaskiers-aged glacial deposits are neither globally distributed nor overlain by a widespread cap carbonate but are associated with an extremely negative δ13C anomaly. The chronology developed here provides the framework for a new, high-resolution model carbon-isotope record for the Neoproterozoic comprising new δ13C (carbonate) data from Svalbard (Akademikerbreen Group) and Namibia (Otavi Group) and data in the literature from Svalbard, Namibia, and Oman. A new U-Pb zircon age of 760 ± 1 Ma from an ash bed in the Ombombo Subgroup in Namibia provides the oldest direct time-calibration point in the compilation, but the time scale of this preliminary δ13C record remains poorly constrained. [ABSTRACT FROM AUTHOR]