High-amplitude Fluctuations Research Articles

A new strategy for reducing pressure fluctuation of Francis turbine by bionic modification of local components

Long term operation of hydroelectric units in low load conditions can induce large-scale blade vortices, swirling vortex ropes in the draft tube, and low-frequency high amplitude pressure fluctuation. These phenomena will cause adverse consequences such as excessive vibration of the unit and blade breakage of the runner. Taking inspiration from the protuberances of the leading edge of a humped whale flipper, the present study firstly proposes bionic modifications of guide vanes and draft tube to suppress high-amplitude pressure fluctuations for Francis turbine. Numerical simulations of transient flow characteristics of the prototype unit (PU), the bionic guide vane unit (BG), and the bionic draft tube unit (BDG) under two low load conditions are conducted. Results indicated that the bionic draft tube has a good effect on suppressing pressure fluctuations in the vaneless area and draft tube. Under the Q/QBEF = 0.41 working condition, BDG causes the main frequency amplitude of pressure fluctuation at the center point of the draft tube inlet to change from 8000.2 Pa to 390.9 Pa, a decrease of 95.11 %. Under the Q/QBEF = 0.57 working condition, BDG causes the maximum decrease rate of the main frequency amplitude in the draft tube to be 60.5 %. The reducing effect in BDG of monitoring points in the guide vane area has reached over 43 %. Under low load conditions, the vortices near the wall in the draft tube of BDG are intercepted by bionic structures, reducing the vortex scale and helping to prevent the generation of large swirling vortex ropes. The bionic guide vanes have a significant control effect on pressure pulsation in the the guide vane and vaneless regions, although perform poorly in the draft tube.

Geochemistry of Quaternary sediments in the northwestern South China Sea: Sediment provenance and mid-Pleistocene transition

The Quaternary sediments in the northwestern South China Sea (NW-SCS) provide valuable information about provenance, climate and sea level changes. However, the correlation between the geochemical records in marine sediments and these influencing factors remains less understood in the NW-SCS. Two wells penetrated Quaternary sediments on the shelf and shelf margin of the NW-SCS and provide an excellent dataset. In this study, the major, trace, and rare earth elements in the sediments were analyzed to reveal the Quaternary provenance changes that correlate with the climatic Middle Pleistocene transition (MPT). These results indicate that the core sediments were mainly derived from felsic source rocks and the degree of chemical weathering in the source areas is relatively low (CIA averaged at 58). The Quaternary provenance of the NW-SCS did not undergo significant changes, recording mixed sedimentation from the Red River, Hainan Island, and central Vietnamese sources. The felsic source rocks and negative Eu anomaly indicated the shelf area received sediments primarily from the Red River and Hainan Island. However, a positive Eu anomaly on the shelf margin suggests additional contributions from mafic rocks sourced from central Vietnam. During the MPT (1.3–1.4 to 0.8–0.9 Ma), the dominant periodicity of climate cycles changed from a 40 kyr obliquity cycle to a 100 kyr eccentricity cycle; the cooling climate led to a slight weaking in chemical weathering and a minor decrease in sediment supply from terrestrial sources. After the MPT, the mixed sourced sediments in the shelf and shelf margin areas of the NW-SCS likely suffered from multiple hydrodynamic forces under the influence of the Pleistocene high-frequency and high-amplitude sea level fluctuations. These climate induced changes led to significant fluctuations in element content that provide new insights into Quaternary sources and climatic events in the NW-SCS.

Flowfield and Noise Dynamics of Supersonic Rectangular Impinging Jets: Major versus Minor Axis Orientations

The current study explores the flowfield and noise characteristics of an ideally expanded supersonic (Mach 1.44) rectangular jet impinging on a flat surface. The existing literature is primarily concentrated on axisymmetric jets, known for their resonance dominance, pronounced unsteadiness, and acoustic signatures. In contrast, non-axisymmetric jets remain relatively less understood, particularly those impinging on a ground surface. By employing Schlieren imaging, high-frequency pressure measurements using high-bandwidth transducers, and particle image velocimetry (PIV), this research comprehensively examines the flow-acoustic phenomena. Schlieren imaging revealed distinct, coherent structures and strong acoustic waves, while pressure measurements at the impingement surface exhibited high-amplitude fluctuations, peaking at approximately 186 dB. Acoustic analysis identified multiple high-amplitude tones with unique directional characteristics, suggesting the potential for multiple simultaneous modes in rectangular jets. Furthermore, the PIV data elucidated differences in the jet shear layer and wall jet development attributed to the nozzle orientation. These findings contribute to a deeper understanding of non-axisymmetric jet behavior, offering insights relevant to fundamental flow physics and practical applications such as vertical takeoff and landing aircraft.

Revealing mechanisms regulating diurnal groundwater level fluctuation in a temperate fen peatland: A spatiotemporal exploration

Diurnal groundwater level fluctuations (DGLF), closely tied to the metabolic rhythm of wetland vegetation, offer insights into direct groundwater consumption. This study is centered on exploring the spatial and temporal patterns of DGLF within the riparian aquifer of the Upper Biebrza region, motivated by the need to understand those sub-daily ecohydrological dynamics at play in this ecosystem, on a process-oriented scale. Through integrated spatiotemporal multivariate analysis, we aim to identify the key potential factors driving variations in these fluctuations across the landscape gradient, as well as, at different time scales. The study employed a comprehensive approach to gather data on groundwater heads and direct groundwater fluxes, utilizing high-temporal-resolution wells within a monitoring network. Meteorological variables from a dedicated station and high-resolution remote sensing maps were also integrated into the dataset. Analysis revealed distinct seasonal patterns and correlations in diurnal groundwater fluctuations, closely correlated with air temperature, solar radiation and subsequently vegetation phenology. Soil moisture content and summer rainfall events also influenced the intensity of these diurnal fluctuations. Dry periods intensified fluctuations, indicating an elevated reliance on groundwater by phreatophytes, while fluctuations decreased after rainfall events, signaling a shift in vegetation’s water source preference to soil moisture. Based on integrated data interpretation, a couple of potential mechanisms, reasonably forming the spatial variation pattern of DGLF, were formulated. Notably, the influence of local hydraulic gradients at transitional forest landscape edges, where higher amplitude fluctuations occurred, compared to lower fluctuations at midpoints. The proximity to the peat-mineral interface influences diurnal fluctuations, with wells closer to the interface showing sustained high-amplitude fluctuations driven by higher rates of recharge. Additionally, the influence of river proximity was explored, revealing dampened fluctuations in wells closer to the river due to rapidly changing hydraulic gradients. Further systematic experimentation, including numerical and data-driven modeling, is needed to validate these hypotheses. The study findings provide perspectives into (DGLF) patterns and distributions, enabling more accurate groundwater evapotranspiration (ETg) estimation and opening new avenues for considering ecohydrological feedback regarding carbon–water interaction, in relation to the daily-scale water table position.

Investigation of Non-Uniform Inflow Effects on Impeller Forces in Axial-Flow Pumps Operating as Turbines

Due to the existence of an inlet elbow, transmission shaft, and other structural components, the inflow of axial-flow pumps as turbines (PATs) becomes non-uniform, resulting in the complexity of internal flow and adverse effects such as structural vibration. In this paper, numerical methods were employed to explore the non-uniform inflow effects on impeller forces and internal flow field characteristics within an axial-flow PAT. The study results indicated that non-uniform inflow caused uneven pressure distribution inside the impeller, which leads to an imbalance in radial forces and offsetting the center of radial forces. With an increasing flow rate, the asymmetry of radial forces as well as the amplitude of their fluctuations increased. Non-uniform inflow was found to induce unstable flow structures inside the impeller, leading to low-frequency, high-amplitude pressure fluctuations near the hub. Using the enstrophy transport equation, it was shown that the relative vortex generation term played a major part in the spatiotemporal evolution of vortices, with minimal viscous effects.

토석류 예측 모델을 이용한 재해취약 지역에서의 토사재해에 관한 연구

Torrential rains occurring during the unpredictable climate change cause many mountain disasters in vulnerable areas. This study is to analyze the debris flow mechanism by applying the multiplier of the vulnerable area debris flow prediction model presented by USACE to the erosion or sedimentation model. The higher the typhoon category, the higher the debris flow peak value, and the frequency of peaks of flow discharge with long durations increased. Analysis of changes in vegetation density showed that the initial peak value of the debris flow reaching the bottom was highest due to dense vegetation, and high-amplitude fluctuations appeared due to energy dissipation. In the case of heavy rainfall, the peak value of debris flow increased by more than 22%, and fluctuations of flow discharge occurred earlier at the elapsed time. In addition, debris flow fluctuations with high wave heights appeared, and the high-amplitude band width also appeared to be quite long. The analysis of present study will provide information for developing and prevention disaster reduction measures in vulnerable areas.

Numerical Investigation of the Effect of the Wall Properties on Downward Mercury Flow and Temperature Fluctuations in a Vertical Heated Pipe under a Transverse Magnetic Field

The work is motivated by the need to study the effect of abnormal quasi-periodic high-amplitude temperature fluctuations in a liquid metal flow in pipes and channels, which arise under conditions close to those specific for liquid metal (PbLi alloy, etc.) hydrodynamics and heat transfer in blankets of tokamak fusion reactors. Such temperature fluctuations in the cooling channels of the fusion reactor blanket pose a threat to the strength of the channel walls. The influence of the physical properties of the channel walls and potential fouling of their inner surface on the generation of these fluctuations has not been studied as yet. Therefore, this problem has been numerically investigated for a MHD mixed convection of a liquid metal in a round tube for a downward flow of mercury with the one-side heating condition under a strong transverse magnetic field using conjugate problem statement by the LES (Large Eddy Simulation) method. Numerical simulation was performed for four cases: (i) the effect of thermophysical and electrophysical properties of the steel pipe wall is neglected, (ii) the effect of wall material properties is included, (iii) the effect of the moderate contact electrical resistance caused by a layer of fouling deposits or oxides on the inner surface of the pipe is accounted for, and (iv) the resistance of the thin fouling layer is assumed to be very high. The predictions for the cases (i) and (iv) are in good agreement with the experimental data and the results of the direct numerical simulation. They demonstrate the existence of quasi-periodic abnormal high-amplitude temperature fluctuations in the fluid with a frequency of approximately 0.14 Hz. With a relatively low electrical resistance of the fouling film (case iii), the frequency of high-amplitude temperature fluctuations was considerably lower (0.07 Hz). In the absence of electrical contact resistance on the inner surface of the steel pipe (case ii), high-amplitude velocity and temperature fluctuations were not revealed in the fluid. Thus, it was shown for the first time that the physical properties of a wall and the electrical resistance between the fluid and the electrically conducting wall were responsible for the development of abnormal temperature fluctuations in the liquid and the wall and control of their amplitude and frequency. The causes of the revealed effects are discussed.

High-amplitude pressure fluctuations of a pump-turbine with large head variable ratio during the turbine load rejection process

High-amplitude pressure fluctuations of a pump-turbine with large head variable ratio during the turbine load rejection process

Magnetoconvection in a horizontal duct flow — A parametric study

Mixed convection flows in a horizontal duct with a heated bottom wall and a magnetic field imposed in the horizontal transverse direction are analyzed numerically using the two-dimensional approximation valid in the limit of strong magnetic field effect. A parametric study is carried out to understand the effect of the Reynolds, Hartmann, and Grashof numbers on the flow. The focus is on the convection instability of the quasi-two-dimensional states that occur under strong magnetic fields. The range of existence of such states is established. Linear stability analysis and simulations of nonlinear regimes demonstrate that high-amplitude low-frequency magnetoconvective fluctuations are generated by the instability in a broad range of parameters and that their strength is not diminished as the magnetic field becomes stronger. Good agreement with experimental data is found.

Implications for Chondrule Formation Regions and Solar Nebula Magnetism from Statistical Reanalysis of Chondrule Paleomagnetism

Converging lines of evidence show that protoplanetary disks are complex environments hosting spatial and temporal variability at multiple scales. Here we reanalyze paleomagnetic estimates of solar nebula magnetic field strengths using a Bayesian framework that tests for recording bias due to chondrule motion and explicitly accounts for time-varying ambient fields. We find that LL and CO group chondrule paleointensities likely rotated during cooling (p = 0.79–0.99), validating assumptions in previous paleomagnetic studies. Chondrule rotation also suggests low gas density formation environments beyond 2 and 4 au for LL and CO chondrules, respectively. Our recomputed paleointensities for LL and CO chondrules imply either: (1) temporally constant magnetic fields of μT and μT, respectively; or (2) time-varying magnetic fields with peak amplitudes between μT and μT. Considering the known mechanisms for sustaining magnetic field gradients and high-amplitude temporal magnetic fluctuations in the solar nebula, we find that magnetic field flux concentrations in disk gaps or time-varying magnetic fields, for example due to the Hall shear instability, are most compatible with the existing data. Using this statistical framework, future paleointensity studies of chondrules can be used to directly test for the variability of magnetic fields in the solar system protoplanetary disk and to distinguish between these scenarios.

Migratory behavior in the enigmatic Late Pleistocene bovid Rusingoryx atopocranion

For many animals, migration is an important strategy for navigating seasonal bottlenecks in resource availability. In the savannas of eastern Africa, herds of grazing animals, including blue wildebeest (Connochaetes taurinus), Thomson's gazelle (Eudorcas thomsonii), and plains zebra (Equus quagga), travel hundreds of kilometers annually tracking suitable forage and water. However, we know nearly nothing about migration among the extinct species that often dominated Late Pleistocene communities. Using serially sampled 87Sr/86Sr and δ13C, we characterize the prehistoric movement and diet of the enigmatic wildebeest Rusingoryx atopocranion from two localities (Karungu and Rusinga Island) in the Lake Victoria Basin of western Kenya. We find clear evidence for migration in all four individuals studied, with three 87Sr/86Sr series demonstrating high-amplitude fluctuations and all falling outside the modeled isoscape 87Sr/86Sr ranges of the fossil localities from which they were recovered. This suggests that R. atopocranion exhibited migratory behavior comparable to that of its closest living relatives in the genus Connochaetes. Additionally, individuals show seasonally-variable δ13C, with a higher browse intake than modern and fossil eastern African alcelaphins indicating behavioral differences among extinct taxa otherwise unrecognized by comparison with extant related species. That this species was highly migratory aligns with its morphology matching that of an open grassland migrant: it had open-adapted postcranial morphology along with a unique cranial structure convergent with lambeosaurine dinosaurs for calling long distances. We further hypothesize that its migratory behavior may be linked to its extinction, as R. atopocranion disappears from the Lake Victoria Basin fossil sequence coincident with the refilling of Lake Victoria sometime after 36 ka, potentially impeding its past migratory routes. This study characterizes migration in an extinct eastern African species for the first time and shapes our ecological understanding of this unique bovid and the ecosystems in which Middle Stone Age humans lived.

Harnessing Instability Mechanisms in Airfoil Flow for Data-Driven Forecasting of Extreme Events

For certain Reynolds numbers, airfoils are subject to sporadic high-amplitude fluctuations in the aerodynamic forces. These extreme excursions may be seen as prototypical of the kind of unsteady and intermittent dynamics relevant to the flow around airfoils and wings in a variety of real-world applications. Here we investigate the instability mechanisms at the heart of these extreme events, and how they may be harnessed for efficient data-driven forecasting. Through a wavelet and spectral analysis of the pressure and vorticity, we find that the extreme events arise due to the instability of a specific frequency component distinct from the vortex shedding mode. During these events, this extreme event frequency draws energy from the energetically dominant vortex shedding flow and undergoes an abrupt transfer of energy from small to large scales. We propose a preprocessing algorithm to extract this extreme event frequency from the surface pressure data, which in conjunction with an extreme event-tailored loss function, allows us to avoid the commonly used long short-term memory architecture in favor of a simple feed-forward network—a significant reduction in cost over the previous state-of-the-art. Our model requires only three pressure sensors, and it is robust to their location—showing promise for the use of our model in dynamically varying applications. Finally, we show that relying solely on the statistics of the pressure and drag data for optimal sensor placement fails to improve model prediction over uniform or random sensor placement.

A novel rolling bearing fault diagnosis method based on continuous hierarchical fractional range entropy

Recognizing rolling bearing faults heavily relies on effective feature extraction methods. In practical engineering, the collected vibration data contains significant levels of noise and amplitude fluctuations, impeding the successful extraction of fault features. To address these challenges, a fault extraction method known as continuous hierarchical fractional range entropy (CHFRE) is proposed. First, range entropy (RE) is introduced and derived to the fractional domain to quantify the fault characteristics. This can overcome the effects of high noise and amplitude fluctuations. Second, the fault features hidden in the low- and high-frequency components are extracted in the multiscale domain by improved hierarchical analysis. Finally, a model for rolling bearing faults identification is established. This paper explores the CHFRE parameters, and a crucial finding is presented: the fractional order contains several invalid intervals within its defined range. The validity of the method and the existence of the invalid intervals are verified by two experiments.

RNA-binding domain of SARS-CoV2 nucleocapsid: MD simulation study of the effect of the proline substitutions P67S and P80R on the structure of the protein

The nucleocapsid component of SARS-CoV2 is involved in the viral genome packaging. GammaP.1(Brazil) and the 20 C-US(USA) variants had a high frequency of the P80R and P67S mutations respectively in the RNA-binding domain of the nucleocapsid. Since RNA-binding domain participates in the electrostatic interactions with the viral genome, the study of the effects of proline substitutions on the flexibility of the protein will be meaningful. It evinced that the trajectory of the wildtype and mutants was stable during the simulation and exhibited distinct changes in the flexibility of the protein. Moreover, the beta-hairpin loop region of the protein structures exhibited high amplitude fluctuations and dominant motions. Additionally, modulations were detected in the drug binding site. Besides, the extent of correlation and anti-correlation motions involving the protruding region, helix, and the other RNA binding sites differed between the wildtype and mutants. The secondary structure analysis disclosed the variation in the occurrence pattern of the secondary structure elements between the proteins. Protein-ssRNA interaction analysis was also done to detect the amino acid contacts with ssRNA. R44, R59, and Y61 residues of the wildtype and P80R mutant exhibited different duration contacts with the ssRNA. It was also noticed that R44, R59, and Y61 of the wildtype and P80R formed hydrogen bonds with the ssRNA. However in P67S, residues T43, R44, R45, R40, R59, and R41 displayed contacts and formed hydrogen bonds with ssRNA. Binding free energy was also calculated and was lowest for P67S than wildtype andP80R. Thus, proline substitutions influence the structure of the RNA-binding domain and may modulate viral genome packaging besides the host-immune response. Communicated by Ramaswamy H. Sarma

Numerical study on hydrodynamic performance of an underwater propulsive wing propulsor

The propulsive wing has an excellent aerodynamic performance due to boundary layer control. When propulsive wings are applied to unmanned underwater vehicles (UUVs), high hydrodynamic forces in both the horizontal and vertical directions significantly enhance UUV maneuverability. To investigate the hydrodynamic performance of a propulsive wing propulsor (PWP), the underwater flow field of the PWP is numerically simulated. The hydrodynamic performance of the PWP is subjected to comprehensive investigation in terms of its hydrodynamic efficiency, unsteady force, and wake evolution. A three-dimensional simulation is undertaken to briefly analyze spanwise effects on PWP hydrodynamic performance. The results show that the PWP delivers an excellent hydrodynamic performance with high hydrodynamic coefficients, where power load and propulsive efficiency before stall reach 4.4 and 0.23, respectively. The convergence of blade-shedding vortices at the bottom of the deflector provides vortex-induced lift but leads to high-frequency and high-amplitude fluctuations in PWP hydrodynamic forces. The amplitudes of the fluctuations are closely correlated to the vorticities of the blade-shedding vortices. The three-dimensional simulation results show that, before stall, spanwise effects on PWP hydrodynamic forces and the wake structure are limited.

Coupled nonlinear dynamics of acoustic modes in a quasi 1-D duct with inhomogeneous mean properties and mean flow

This study investigates the coupled nonlinear evolution of acoustic modes in a quasi 1-D duct with axially inhomogeneous mean velocity and mean thermodynamic properties. A novel modification of the Krylov–Bogoliubov method of averaging (KBMA) is developed to analytically solve the modal-amplitude equations that are linearly and nonlinearly coupled with both quadratic and cubic nonlinearities. Approximate analytical solutions based on the method of multiple scales (MMS) are also derived for the coupled modal-amplitude equations with cubic nonlinearities. The KBMA and MMS solutions are presented for the cases with and without internal resonance among the acoustic modes. Furthermore, novel initial conditions are derived for the KBMA and MMS modal amplitude and phase equations that are essential to validate the analytical solutions using numerical solutions. While both the analytical solutions are seen to be in good to excellent agreement with the numerical solutions, only KBMA captures the low-frequency oscillations in the peak amplitude of the limit cycle oscillations. The role of the order of nonlinear terms in the evolution to limiting oscillations is elucidated by successively considering coupled modal equations with both quadratic and cubic nonlinearities, only cubic nonlinearities and only quadratic nonlinearities. It is seen that while the cubic nonlinearities determine the asymptotic temporal behavior, the quadratic nonlinearities give rise to complex oscillation patterns characterized by a range of non-integral frequencies. For coupled oscillators with only quadratic nonlinearities, it is observed that for mean temperature gradients below a threshold, the pressure fluctuations are characterized by intermittent bursts of high-amplitude fluctuations, as well as by a frequency spectrum that is essentially continuous. The effects of linear coupling of the modal-amplitude equations on the limit-cycle behavior of the solutions are also illustrated.

Active control of coupling and its effect on near-field pressure fluctuations in supersonic rectangular twin jets

Supersonic rectangular twin jets (SRTJ) are of interest for current and future generations of tactical aircraft. However, the adverse effects of screech-loop coupling have previously been documented to significantly increase the near-field (NF) pressure fluctuations. These high-amplitude pressure fluctuations have the potential to fatigue and damage nearby aft components of the aircraft. Previous studies have documented that the NF pressure fluctuation level depends on the coupling of the jets: in-phase coupling along the twin jets’ minor axes produces stronger NF pressure fluctuations than that of out-of-phase coupling. The objective of this work is to further investigate the effects of coupling modes on NF pressure fluctuations in SRTJ and to mitigate the adverse effects of coupling using active flow control. Localized arc filament plasma actuators are employed to alter the SRTJs’ coupling mode by leveraging natural flow instabilities with minimal power input. A NF microphone array is used for simultaneous coupling and NF pressure fluctuation measurements. Schlieren imaging and spectral proper orthogonal decomposition are used to assess the effects of control on the flow field. The effect of excitation at various frequencies and actuation patterns on coupling and NF pressure fluctuations in different flow regimes are explored and discussed.

Reconstructing midge consumer-resource dynamics using carbon stable isotope signatures of archived specimens.

Population cycles can be caused by consumer-resource interactions. Confirming the role of consumer-resource interactions, however, can be challenging due to an absence of data for the resource candidate. For example, interactions between midge larvae and benthic algae likely govern the high-amplitude population fluctuations of Tanytarsus gracilentus in Lake Mývatn, Iceland, but there are no records of benthic resources concurrent with adult midge population counts. Here, we investigate consumer population dynamics using the carbon stable isotope signatures of archived T. gracilentus specimens collected from 1977 to 2015, under the assumption that midge δ13 C values reflect those of resources they consumed as larvae. We used the time series for population abundance and δ13 C to estimate interactions between midges and resources while accounting for measurement error and possible preservation effects on isotope values. Results were consistent with consumer-resource interactions: high δ13 C values preceded peaks in the midge population, and δ13 C values tended to decline after midges reached high abundance. One interpretation of this dynamic coupling is that midge isotope signatures reflect temporal variation in benthic algal δ13 C values, which we expected to mirror primary production. Following from this explanation, high benthic production (enriched δ13 C values) would contribute to increased midge abundance, and high midge abundance would result in declining benthic production (depleted δ13 C values). An additional and related explanation is that midges deplete benthic algal abundance once they reach peak densities, causing midges to increase their relative reliance on other resources including detritus and associated microorganisms. Such a shift in resource use would be consistent with the subsequent decline in midge δ13 C values. Our study adds evidence that midge-resource interactions drive T. gracilentus fluctuations and demonstrates a novel application of stable isotope time-series data to understand consumer population dynamics.

Experimental study of submerged liquid metal jet in a rectangular duct in a transverse magnetic field

A liquid metal flow in the form of a submerged round jet entering a square duct in the presence of a transverse magnetic field is studied experimentally. A range of high Reynolds and Hartmann numbers is considered. Flow velocity is measured using electric potential difference probes. A detailed study of the flow in the duct's cross-section about seven jet's diameters downstream of the inlet reveals the dynamics, which is unsteady and dominated by high-amplitude fluctuations resulting from the instability of the jet. The flow structure and fluctuation properties are largely determined by the value of the Stuart number ${{N}}$ . At moderate ${{N}}$ , the mean velocity profile retains a central jet with three-dimensional perturbations increasingly suppressed by the magnetic field as ${{N}}$ grows. At higher values of ${{N}}$ , the flow becomes quasi-two-dimensional and acquires the form of an asymmetric macrovortex, with high-amplitude velocity fluctuations reemerging.

Early Eocene carbon isotope excursions in a lignite-bearing succession at the southern edge of the proto-North Sea (Schöningen, Germany)

Abstract. Situated at the southern edge of the proto-North Sea, the lower Eocene Schöningen Formation of the Helmstedt Lignite Mining District, Lower Saxony, Germany, is characterized by several lignite seams alternating with estuarine to brackish interbeds. Here, we present carbon isotope data for bulk organic matter (δ13CTOC), total organic carbon content (%TOC), and palynomorphs from a 98 m thick sequence of the Schöningen Formation in combination with a new robust age model, which is based on eustatic sea level changes, biostratigraphy, and a correlation with existing radiometric ages. Based on the δ13CTOC data we observe six negative carbon isotope excursions (CIEs) reflecting massive short-term carbon cycle perturbations. A strong CIE of −2.6 ‰ in δ13CTOC values in the Main Seam and the succeeding marine interbed can be related to the Paleocene–Eocene Thermal Maximum (PETM). The subsequent CIE of −1.7 ‰ in δ13CTOC values may be correlated with the Eocene Thermal Maximum 2 (ETM2) or slightly older events preceding the ETM2. High-amplitude climate fluctuations including at least four minor CIEs with a maximum negative shift of −1.3 ‰ in δ13CTOC in the upper part of the studied section are characteristic of the Early Eocene Climatic Optimum (EECO). Palynological analysis across the Main Seam proved that shifts in δ13CTOC values are correlated with changes in the peat-forming wetland vegetation, specifically the change from a mixed angiosperm and gymnosperm flora to angiosperm-dominated vegetation at the onset of the PETM. The PETM-related CIE shows a distinct rebound to higher δ13CTOC values shortly after the onset of the CIE, which is recognized here as a common feature of terrestrial and marginal marine PETM records worldwide and may reflect a stepwise injection of carbon into the atmosphere.