•The Miaohe Member is equivalent to the upper part of the Doushantuo Member IV.•The δ13C record in the upper Doushantuo Formation is incomplete in the Jiulongwan section.•The EN3/Shuram excursion may ...be significantly older than 551Ma.
The middle Ediacaran Shuram excursion represents the most pronounced negative carbon isotopic shift in Earth history, and has been considered as evidence for a profound disturbance to the global carbon cycle and proposed as a key chemostratigraphic marker for Ediacaran stratigraphic subdivision and global correlation. Previous study has revealed a pronounced negative δ13C shift (EN3) in the upper Doushantuo Formation of South China, which has been interpreted as an equivalent of the Shuram excursion. Detailed δ13C investigation of multiple sections of the Ediacaran Doushantuo Formation around the Huangling Anticline, western Hubei Province, South China, indicates that the δ13C variation in the upper Doushantuo Formation is more complex than previously reported. In the western region of the Huangling anticline, a moderately positive δ13C excursion (here termed the Diaoyapo positive excursion) bisects two strongly negative δ13C excursions (here termed the Jiuqunao negative excursion and the younger Miaohe negative excursion), with a ∼551Ma ash bed capping the Miaohe negative excursion. In the central region, there is only one negative δ13C excursion, whereas in the eastern region, the negative δ13C excursion is absent. The variable stratigraphic expression of these δ13C excursions is most parsimoniously interpreted as reflecting local facies variation, diagenesis, and/or a cryptic unconformity. The more complete but more complex δ13C chemostratigraphic record in the western region of Huangling Anticline implies that, depending on how this record is correlated with the Shuram excursion, the latter may be significantly older than 551Ma if it does represent a global chemostratigraphic marker.
•Two high-precision U-Pb zircon ages are reported from the Doushantuo Formation.•A cryptic unconformity with a >44 Myr gap in sediment accumulation was revealed.•The unconformity may record ...glacio-eustatic sea-level changes in the mid-Ediacaran.
The subdivision and correlation of the Ediacaran System within the geologic timescale is desirable, but challenging. The Ediacaran Doushantuo Formation (∼635–550 Ma) in South China is one of the most important sedimentary successions for studying the co-evolution of Earth's environment and life at that time. However, its stratigraphic continuity is confusing and deserves scrutiny. In this study we evaluate stratigraphic continuity of the Doushantuo Formation from a key section deposited in an alleged deep-water “basinal” setting. Using litho-, chemo-, and chrono-stratigraphic data anchored by two new CA-ID-TIMS U-Pb zircon dates of 629.7 Ma and 556.4 Ma from two tuff layers, we reveal that this section contains a cryptic subaerial unconformity with a >44 Myr gap in the sedimentary record. The unconformity occurs immediately prior to the EN3/Shuram δ13C excursion in the mid-Ediacaran, and can be traced regionally and may be correlative to other mid-Ediacaran unconformities globally. We suggest that its development is related to glacio-eustatic sea-level changes during the mid-Ediacaran. This finding provides a critical stratigraphic boundary for future subdivision and correlation of the Ediacaran System in South China, and we advocate for a reconsideration of the completeness of the Ediacaran stratigraphic record in this area.
The Labrador Trough in northern Quebec and Labrador is a 900 km long Rhyacian-Orosirian orogenic belt containing mixed sedimentary-volcanic successions. Despite having been studied intensively since ...the 1940s, relatively few chemostratigraphic studies have been conducted. To improve our understanding of the Labrador Trough in the context of Earth history, and better constrain the local record of the Lomagundi-Jatuli carbon isotope excursion, high-resolution sampling and carbon isotope analyses of the Le Fer and Denault formations were conducted. Carbonate carbon isotopes (δ13C) in the Le Fer Formation record a large range in values from -4.4 ppm to +6.9 ppm. This large range is likely attributable to a combination of post-depositional alteration and variable abundance of authigenic carbonate minerals; elemental ratios suggest that the most 13C-enriched samples reflect the composition of the water column at the time of deposition. Cumulatively, these data suggest that the Lomagundi-Jatuli Excursion was ongoing during deposition of the Le Fer Formation, approximately 2 km higher in the stratigraphy than previously recognised. However, the possibility of a post-Lomagundi-Jatuli Excursion carbon isotope event cannot conclusively be ruled out. The directly overlying Denault Formation records a range in δ13C values, from -0.5 ppm to +4.3 ppm, suggesting that it was deposited after the conclusion of the Lomagundi-Jatuli Excursion and that the contact between the Le Fer and Denault formations occurred sometime during the transition out of the Lomagundi-Jatuli Excursion, ca. 2106 to 2057 Ma.
Significant Ediacaran paleoenvironmental changes recorded in the mixed carbonate-siliciclastic Bambuí Basin, east-central Brazil, outcrop in one of the best-preserved sites, the Januária paleo-high. ...From a detailed stratigraphic and chemostratigraphic approach (carbon and oxygen isotopes, total organic carbon, total sulfur and selected elements abundances), we identified transgressive-regressive intervals and depositional settings within the Bambuí Group stratigraphy and developed a new model for the origin of the extremely positive Middle Bambuí Excursion (MIBE) present in the middle section of the basin. A post-Marinoan cap carbonate interval is recorded in the first ~10 m, preserving aragonite pseudomorph fans, barite-rich layers and negative δ13C anomaly. A hiatus separates the cap carbonate from the late Ediacaran succession that makes up the remaining Bambuí Group. The younger intervals preserve both siliciclastic (middle Serra de Santa Helena Formation) and carbonate (middle/upper Sete Lagoas and Lagoa do Jacaré formations) shallow-water depositional settings, requiring tectonic influence or climatic changes in source areas. The MIBE yields δ13C values as high as +14‰ and extends for about 350 m, from the upper Sete Lagoas Formation to the lower Serra da Saudade Formation. We suggest a model of a restricted basin setting that favored local carbon isotopic signals in the δ13C record of sedimentary carbonates through preferential weathering of ancient carbonate platforms on the continent and higher burial rate of authigenic carbonate. This scenario caused the global δ13C budget to be affected by an offset of, at least, +4‰ in sedimentary carbonates deposited on the São Francisco craton and along its margin during the terminal Ediacaran. Many previous studies suggested a restricted nature for the middle Bambuí Basin, which probably resulted from the central position of the São Francisco paleocontinent within the mosaic of collisional blocks during the SW Gondwana amalgamation.
•A complete stratigraphic and chemostratigraphic data of the Ediacaran Bambuí Basin of Brazil.•Peritidal carbonates and sandstones to deep-water shales make up the mixed carbonate-siliciclastic sedimentation.•The MIddle Bambuí Excursion (MIBE) yields extremely heavy δ13C values as high as +14‰.•Paleogeography in the terminal Ediacaran probably had an important role in basin restriction and positive isotope excursion.
Today's geomagnetic field can prevent energetic particles, including solar energetic particles and galactic cosmic rays, from directly hitting the Earth's atmosphere. However, when the geomagnetic ...field strength is significantly decreased during geomagnetic field excursions or reversals, the geomagnetic field shielding effect becomes less prominent. Geomagnetic cutoff rigidity, as a quantitative estimation of the shielding effect, can be calculated using trajectory tracing or theoretical equations. We use a recent high‐resolution continuous geomagnetic field model (LSMOD.2) to study the geomagnetic cutoff rigidity during the Laschamps excursion. Global grids of the geomagnetic cutoff rigidities are presented, in particular for the excursion midpoint when the geomagnetic field is weak and not dipole‐dominated anymore at Earth's surface. We compare the cutoff rigidity calculation results between a trajectory tracing program and theoretical equations and we find that the influence of the non‐dipole component of the geomagnetic field cannot be ignored during the excursion. Our results indicate that the exposure of Earth's atmosphere to energetic particles of cosmic and solar origin is high and nearly independent of latitude in the middle of the Laschamps excursion. Our results will be useful for future studies associated with cosmic radiation dose rate and cosmogenic isotope production rate during the Laschamps excursion.
Plain Language Summary
The geomagnetic field is a natural shield, prohibiting that solar energetic particles and galactic cosmic rays directly hit the Earth's atmosphere. However, the geomagnetic field is not steady, and even polarity reversals or excursions have occurred numerous times in the geological past. During the geomagnetic field transitions, the morphology of the geomagnetic field is highly complex and the geomagnetic field strength is significantly decreased so that energetic particles can much easier access the Earth's atmosphere. The knowledge of the geomagnetic field variation during excursions has been greatly improved by global models of the paleomagnetic field. A recent high‐resolution continuous geomagnetic field model (LSMOD.2) provides a view of the characteristics of the Laschamps excursion all over Earth's surface. Using the LSMOD.2 model, we analyze the geomagnetic field shielding effect during the Laschamps excursion. Our results suggest that the exposure of Earth's atmosphere to energetic particles, the cosmic radiation dose rate, and cosmogenic isotope production rate increased strongly during the excursion.
Key Points
The global distribution of geomagnetic cutoff rigidities during the Laschamps excursion is presented
Both a trajectory tracing program and theoretical equations are utilized to calculate the cutoff rigidity using the LSMOD.2 model
At the midpoint of the Laschamps excursion, the cutoff rigidities are lower than 4 GV globally and nearly independent of latitude
To gain a better understanding of the oceanic environment of the transitional Lower-Middle Cambrian boundary and provide additional data for the global ROECE correlation, we studied the carbon ...isotope records of shallow-water platform carbonates of the Cambrian Stage 4 Longwangmiao Formation in the Bandenggou section in Chongqing, South China, and correlated with previously reported contemporaneous δ13Ccarb records across a range of depositional environments on the Yangtze Platform and other basins. The δ13Ccarb curve of the Longwangmiao Formation fluctuates frequently approximately 0‰ with multiple excursions at its lower sequence, exhibits a conspicuous negative δ13C excursion with a minimum peak value of −4.76‰ at the middle sequence, and then possesses a stable positive distribution upward. The correlation across the platform-to-basin transect on the Yangtze Platform shows that the Toyonian sections, from slope to basin, have similar shapes of δ13C curves, with a depleted trend upward and consistent timings of the largest-amplitude negative δ13C excursion, termed ROECE, while contemporaneous shallow-water platform sections have different δ13C profile shapes and timings of their largest-amplitude negative δ13C excursions, as do sections at a range of depositional environments from a multitude of continents. We infer that the carbon isotopic compositions from the Bandenggou section in the shallow-water environment layer may retain a regional signal. The negative δ13C distribution of the lower sequence is primarily due to a local, long-term riverine influx and low relative sea level at the early stage, which amplified organic matter decay and diminished primary productivity. Eventually, this elevated the oxidation and eutrophication of the shallow water, while the stable positive δ13C distribution upward is attributed to the integrated influence of a long-term scant riverine flux and high relative sea level. Additionally, the location of ROECE in the shallow-water sections still needs substantiation at the global scale.
•Carbon isotope records of Toyonian through Yangtze Platform is first established.•δ13C excursions are mainly due to the coeval riverine flux and relative sea level.•δ13C excursion of −4.76‰ at the Middle Toyonian should be a candidate for ROECE.•ROECE's precise position needs substantiation from other shallow-water sections.
Investigated sediment cores from the southeastern Black Sea provide a high-resolution record from mid latitudes of the Laschamp geomagnetic polarity excursion. Age constraints are provided by 16 AMS ...14C ages, identification of the Campanian Ignimbrite tephra (39.28±0.11ka), and by detailed tuning of sedimentologic parameters of the Black Sea sediments to the oxygen isotope record from the Greenland NGRIP ice core. According to the derived age model, virtual geomagnetic pole (VGP) positions during the Laschamp excursion persisted in Antarctica for an estimated 440yr, making the Laschamp excursion a short-lived event with fully reversed polarity directions. The reversed phase, centred at 41.0ka, is associated with a significant field intensity recovery to 20% of the preceding strong field maximum at ∼50ka. Recorded field reversals of the Laschamp excursion, lasting only an estimated ∼250yr, are characterized by low relative paleointensities (5% relative to 50ka). The central, fully reversed phase of the Laschamp excursion is bracketed by VGP excursions to the Sargasso Sea (∼41.9ka) and to the Labrador Sea (∼39.6ka). Paleomagnetic results from the Black Sea are in excellent agreement with VGP data from the French type locality which facilitates the chronological ordering of the non-superposed lavas that crop out at Laschamp–Olby. In addition, VGPs between 34 and 35ka reach low northerly to equatorial latitudes during a clockwise loop, inferred to be the Mono lake excursion.
► Laschamp geomagnetic excursion occurred at 41ka. ► It was characterised by a full reversal lasting about 440yr. ► Reversed phase was associated with a significant recovery in field strength. ► Virtual geomagnetic pole movement was in the range of half a degree in latitude per year. ► Comparison with globally distributed Laschamp records indicates non-dipolar excursional field behaviour.
A full‐vector paleomagnetic record, comprising directional data and relative paleointensity (rPI), was derived from 16 sediment cores recovered from the southeastern Black Sea. The obtained data were ...used to create a stack covering the time window between 68.9 and 14.5 ka. Age models are based on radiocarbon dating and correlations of warming/cooling cycles monitored by high‐resolution X‐ray fluorescence (XRF) elementary ratios and by ice‐rafted debris (IRD) in Black Sea sediments to the sequence of “Dansgaard‐Oeschger” (D‐O) events defined from the Greenland ice core oxygen isotope stratigraphy. The reconstructed prominent lows in paleointensity at about 64.5, 41.2, and 34.5 ka are coeval with the Norwegian‐Greenland Sea, the Laschamps, and the Mono Lake excursions, respectively. For a further analysis, the stacked Black Sea paleomagnetic record was converted into one component being parallel to the direction expected from a geocentric axial dipole (GAD) and two components perpendicular to it (EW, inclined NS), representing definitely only non‐GAD components of the geomagnetic field. Discussions of the field configurations at the Black Sea site are focused on the three excursional events. The Norwegian‐Greenland Sea excursion was dominated by a decaying axial dipole and persisting weak nondipole field, with directional variations still within the range of normal secular variations. The Laschamps excursion comprises two full polarity transitions and a short stable interval of reversed polarity in between. The Mono Lake excursion was mostly dominated by a nondipole field, though with a less pronounced weakening of the axial dipole component.
Key Points
Reconstructed paleosecular variation record between 68.9 and 14.5 ka from Black Sea sediments
Increasing axial dipole component with reversed polarity during the midpoint of the Laschamps excursion
Comparable levels of dipole and nondipole contributions during the Mono Lake excursion
Display omitted
•Laschamp, Norwegian-Greenland Sea (I & II) excursion events are observed in the Mahanadi basin.•The observed inclination anomalies are not associated with the changes in magnetic ...minerals/depositional environment.•The excursions are often associated with a drop in paleointensity.•The VGP path traced by Laschamp excursion shows a clockwise loop.
High resolution paleomagnetic investigations were performed on a 50.08 m long sediment core (MD161/20) from Mahanadi basin, Bay of Bengal. Core yielded reliable paleomagnetic results for top 20 m below seafloor (mbsf) which spans about 80 ka. Based on the analysis of rock magnetic data, the core is subdivided into five distinct Zones: Zone 1 and Zone 2 cover top 20 mbsf and do not show any abrupt change in magnetic mineralogy, concentration and grain size. Zones 3 and 5 show significant reduction in χLF, χARM and SIRM due to dissolution of magnetic minerals. Zone 4 shows moderate values of χLF and SIRM. The low value of χARM suggests that magnetic signal is mostly carried by magnetic grains in PSD/MD state. The paleomagnetic data for the top 20 mbsf show four prominent geomagnetic excursions at ∼9 mbsf, ∼13.5 to 15 mbsf, ∼16.3 mbsf and ∼18 to 18.2 mbsf. The age-depth relationship is established using stratigraphic correlation between well-dated sedimentary core NGHP-01-19B and the core MD161/20. The ages of the observed excursions correspond to ∼18 to 20 ka, ∼42 to 49 ka, ∼54 to 57 ka and ∼69 to 70 ka. The excursions at ∼42 to 49 ka, ∼54 to 57 ka, and ∼67 to 70 ka is similar to the known excursions the Laschamp and the split Norwegian-Greenland Sea events (NGS-I and NGS-II). The excursion at 18–20 ka is not observed globally and may be related to lithological/sedimentological changes occurring during last glacial maxima (LGM). The virtual geomagnetic path (VGP) of Laschamp excursion traces clockwise loop. All excursions identified in present study fall in the periods of relatively low paleointensity.
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