We report here on a new paleomagnetic (directions and intensities) and coupled K/Ar and 40Ar/39Ar analysis of 35 different flows, emplaced in the Chaîne des Puys during the 75 to 10 kyr interval, ...which contains the Mono Lake and Laschamp excursions. There is a remarkable agreement between the new set of absolute volcanic intensities and published sedimentary (GLOPIS-75) and cosmogenic (10Be and 36Cl) records. The Laschamp and Mono Lake excursions are clearly revealed by a very significant intensity drop at 41.2±1.6 ka and 34.2±1.2 ka respectively. The duration of the Laschamp excursion is ∼1500 yr and about 640 yr when the drop of paleointensity or the directional change are considered respectively. The intensity drop at the Mono Lake is twice as short. In the ∼7 ka interval separating the two excursions, the field intensity recovers to almost non-transitional values. The rate of decrease of the field intensity during these excursions attains 18 nT/yr for the Laschamp and even greater value (33 nT/yr) for the Mono Lake. This figure is, for the Laschamp excursion, similar to the present field intensity decrease in the last two centuries so that one may wonder whether such a high rate of change may be characteristic of an impending geomagnetic event (reversal or excursion). We suggest that the name Auckland excursion should be used for the present-day called Mono Lake.
•New lava flows recording the intensity low of the Mono Lake.•New flows recording the Laschamp excursion with a mean age of 41.3±0.6 ka (2σ).•∼1500 yr (intensity), 640 yr (directions) duration for Laschamp excursion.•Full intensity recover between the well distinct Laschamp and Mono Lake excursions.•Rate of decrease for the Laschamp excursion similar to that observed since 1840.
This paper summarizes the pattern of paleomagnetic secular variation (PSV) and excursions within MIS 6–7 (130–243 ka) from the western North Atlantic Ocean – ODP Sites 1060–1063. Composite ...high-resolution PSV records (both directions and relative paleointensity) have been developed for each site and inter-compared. There are 65 correlatable inclination features, 67 correlatable declination features, and 15 correlatable paleointensity features in the four PSV records. We have used the results of Grutzner et al. (2002), who compared the calcium carbonate records of ODP Leg 172 sediments and tuned them with Milankovich cyclicity, to date our records. We also dated our PSV records by comparing our paleointensity variability with the study the PISO-1500 global paleointensity record. The two methods agree well. We use the Grutzner phase-shifted chronology to date and analyse our cores because it has lower age uncertainties (~±2 ka). We noted two excursions in our PSV records – Excursions 7α and 7β. These are equivalent to the Iceland Basin and Pringle Falls Excursions. Our revised age estimate for the Iceland Basin Excursion is 196 ± 3 ka. Our revised age estimate for the Pringle Falls Excursion is 215 ± 2 ka. Both are Class I excursions noted by 2–3 cycles of high-amplitude oscillatory inclinations and declinations. We note a strong similarity in the oscillatory kinematic pattern of the two excursions. We also note evidence for similar but lower amplitude directional PSV between and after the excursions. This complex pattern suggests that the dynamics of the regional dynamo process responsible for these excursions is a continuing process for more than 35 ky. We have carried out a statistical study of the PSV records over the interval 120–250 ka after removing all true excursional directions. We have averaged directions and paleointensity in 3 ka and 9 ka overlapping intervals to evaluate the space/time character of PSV and its geodynamo cause(s). We see significant PSV directional variability over 104 yr time scales that is regionally correlatable among the four sites. There is a notable pattern of angular dispersion variability with most time spent with low (~10°) dispersion, with four shorter intervals of high (~25°) dispersion. There are also four notable intervals of low paleointensity in all four records. There is a one-to-one correspondence between the four low paleointensity intervals and the four intervals of high angular dispersion. The Blake Event (123 ± 3 ka) and Excursion 8a (238 ± 2 ka) occur in two of the low intensity/high dispersion intervals that bound MIS 6/7. The two magnetic field excursions within MIS 7 (Iceland Basin and Pringle Falls Excursions) occur in one distinctive long (~35 ka) interval of low paleointensity/high dispersion interval.
•First reproducible records of directional secular variation for MIS 7•First statistical study of PSV in MIS 6–7•First evidence for repeating excursional waveforms
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.
The late Ediacaran Shuram Excursion (SE) records the most prominent negative δ13C excursions (δ13C = −12‰) during Earth’s history. It has been hypothesized to have resulted from oxidation of ...dissolved organic matter, diagenetic or authigenic precipitates. However, the origin of the SE remains enigmatic; current models face challenges regarding the significant amount of atmospheric oxygen needed to balance such extensive oxidation and sustained inputs of light carbon with extremely negative C isotope compositions. Here, we show that the Doushantuo Formation at the Jiulongwan section in South China, a key stratum recording the SE event, contains mineralogical and geochemical signatures related to igneous processes. Both the occurrence of ankerite, feldspar, moissanite and euhedral quartz in the SE samples and the relatively consistent Ce anomalies of carbonate and O isotopes of quartz indicate a contribution from an igneous source. In particular, the SE samples have trace element and C isotope compositions similar to those of recycled carbonatites formed by decarbonation and melting of sedimentary carbonate rocks. These observations suggest that the deep cycle of ancient carbonate rocks, which were subjected to decarbonation during subduction, melting and eruption related to the breakup of the Rodinia supercontinent, contributed to the SE. This igneous model for the SE may provide a connection between the deep and shallow carbon cycles of the Earth.
Length of stationary Gaussian excursions Chakrabarty, Arijit; Pandey, Manish; Chakraborty, Sukrit
Proceedings of the American Mathematical Society,
March 1, 2023, 2023-3-00, Letnik:
151, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Given that a stationary Gaussian process is above a high threshold, the length of time it spends before going below that threshold is studied. The asymptotic order is determined by the smoothness of ...the sample paths, which in turn is a function of the tails of the spectral measure. Two disjoint regimes are studied – one in which the second spectral moment is finite and the other in which the tails of the spectral measure are regularly varying and the second moment is infinite.
SUMMARY
We present palaeo- and rock magnetic results from a well-dated, 21 m-thick, Late Pleistocene continental sedimentary section located in southern Germany. Rock magnetic measurements reveal a ...complex magnetic mineralogy dominated by low coercivity minerals likely related to single domain biogenic magnetite and biogenic or early diagenetic greigite. In the lower part of the section also detrital haematite is present. The stable remanence shows predominantly normal polarity with two marked deviations at ∼1280-1200 cm and at 886 cm profile depth. Whereas the lower excursion is well established by several samples and documented also by detrital haematite, the upper one is only represented by a single sample and revealed by magnetite and greigite. Using the radiocarbon-based age model for the section, the lower excursion yields an age of 42.8–41.3 ka cal BP and is interpreted to represent the Laschamps geomagnetic excursion. The increased abundance of greigite in the upper part of the section, especially in the sample responsible for the upper anomalous direction, renders the interpretation of an actual excursion problematic due to the reducing environment necessary for the greigite formation.
The initial appearance of animals in the fossil record during the Ediacaran period (635 to 539 Ma ago) has been classically linked to a dramatic increase in marine oxygen levels. Indeed, some ...geochemical trends broadly overlapping in time with these fossils can be interpreted as tracking widespread marine oxygenation. Complications and inconsistencies linger, however. It is not clear that Ediacaran animals required any dramatic change in marine O2 levels. There exist multiple alternative interpretations of the geochemical trends that allegedly track this O2 change. And emerging geochronological constraints reveal temporal misalignments. The timing therefore seems right to mull, and maybe even null, the coeval rise of Ediacaran oxygen and animals.
•Emerging datasets complicate the alleged unified rise of Ediacaran animals and oxygen.•Coeval marine oxygenation, if it did occur, was probably less severe than envisioned.•One presently viable hypothesis invokes no dramatic marine oxygenation whatsoever.
The uncertain demeanour from wind generators and loads adversely affect the grid operational stability. Various control approaches have been explored to remedy the system uncertainties while ...maintaining generation and load demand balance. This study proposes a fuzzy-based proportional–fractional integral–derivative with filter controller to sustain frequency stability in wind integrated power systems having different configurations. The controller parameters have been tuned using a recently developed coyote optimisation algorithm (COA). The proposed control approach is executed and validated on three distinct configurations of two-area power systems. All test models are integrated with a doubly fed induction generator (DFIG) type wind turbine units (WTUs). Different case scenarios have been considered to analyse the efficacy of the proposed control strategy in the presence of WTU. Furthermore, the impact of inertial support delivered by the DFIG-WTU and higher penetration of wind energy in the power system has been studied. The analysis reveals that the control scheme in coordination with WTU support reduces the stress on a wind turbine during the inertial control scheme and maintains the grid frequency stability under unexpected load disturbances. Stability and robustness analysis are also conducted to verify the validity of the introduced control approach.
No basis for controversy exists in the naming of a global, strongly negative, uppermost Cambrian carbon isotope (δ13C) excursion. The HERB Event (HERB) has met the standards for chemostratigraphic ...units (i.e. consistent biostratigraphic brackets, content and concept) since 1992. By comparison, the TOCE excursion morphed through four temporally distinct δ13C events with spike-like nadirs that shifted temporally through the uppermost Cambrian until its synonymization with HERB (2006–12). In 2018, TOCE became a prolonged interval with very early onset and enveloped HERB – meaning five TOCE homonyms have been unambiguously defined and figured. HERB lies in the high-diversity ptychaspid biomere (trilobites) and below the ptychaspid extinction. But, data on it were used in TOCE’s 2006 proposal and in later iterations (2008, 2012) to show it (1) higher, both at and above the ptychaspid extinction; (2) at the level of HERB (2012, 2018); and (3) even extending well below HERB (2018). TOCE fails the recommendations for a formal chemostratigraphic unit. Its relationship to latest Cambrian biotic turnover includes equation with extinction and high-diversity intervals. One TOCE homonym is a synonym, albeit junior, of HERB.
