Geological Timescales Research Articles

On the Application Limits of the Parameter in Studying Variations of the Ancient Geomagnetic Field

To describe secular geomagnetic variation on geological timescales, statistical models have been widely used in recent decades. Currently, the most popular among these is the TK03 model (Tauxe and Kent, 2004). As other statistical models, TK03 numerically characterizes the amplitude of secular geomagnetic variation and the shape of the distribution of paleomagnetic directions which are considered as directly reflecting the directions of the geomagnetic field on the considered interval of geological time. For this purpose, three main parameters are used: the scatter Sb (or S) of the virtual geomagnetic poles, the elongation E of the distribution of paleomagnetic directions, and the direction of elongation of the distribution of paleomagnetic directions. The correct application of these parameters to describe ancient secular variation requires the satisfaction of certain, sometimes rather strict conditions. These conditions for the Sb and E parameters were considered in a number of previous publications, while the limits and conditions of application of parameter have not been studied in detail so far. This paper presents the results of mathematical modeling that allow us to evaluate the stability of the calculated values of this parameter as a function of the latitude of sampling, the number of samples used for its determination, the length of the time series on which this parameter is determined, as well as inclination shallowing and the degree of averaging when is estimated in sedimentary rocks. We also consider the extent to which the parameter can be sensitive to the presence and characteristics of the equatorial dipole component in the total geomagnetic field.

Gas Migration through the Crystal Lattice of Gas Hydrates during Sedimentation in Geological Time Scale

Gas Migration through the Crystal Lattice of Gas Hydrates during Sedimentation in Geological Time Scale

The problems of the Anthropocene in the Geologic Time Scale, and beyond

The formalization of the Anthropocene in the Geologic Time Scale (GTS) is a matter of debate. An assessment of the two current options, one formalizing the Anthropocene as an Epoch and the other considering the Anthropocene as an informal event in the Earth's history that does not require formalization, are critically analyzed. The focus of the analysis is on the philosophical issues underlying these two options, which make them unsatisfactory for addressing the occurrence of humans on Earth and the evolution of the relationship between humans and nature. Essentially, the approach followed by these options is non-historical, and does not undertake a research on the structural socio-economic roots of the Anthropocene. As a result, the concept of the Anthropocene is unable to identify the particular and historical form of Anthropos underlying the ongoing environmental degradation. Some inconsistencies in the current configuration of the GTS and in the rules for formalizing units inherited from the historical tradition of the GTS are also highlighted. The current dualistic approach of the Earth system science community, including the two options analyzed for the Anthropocene, prevents a proper understanding of the relationship between humans and nature, which is fundamental to confront the Earth's habitability crisis of the Anthropocene.

Three‐Dimensional Electrical Resistivity Structure Beneath a Strain Concentration Area in the Back‐Arc Side of the Northeastern Japan Arc

AbstractIntraplate earthquakes occur more frequently in the Japanese islands than in other regions. Large intraplate earthquakes in the island arc preferentially occur in strain concentration zones detected by geological and geodetic studies. Crustal heterogeneity plays a crucial role in generating large intraplate earthquakes and strain concentrations. Thus, we elucidated the crustal heterogeneities beneath a strain concentration area on the back‐arc side of the northeastern Japan Arc based on electrical resistivity, which is sensitive to weak zones in the crust. By deploying wideband magnetotelluric surveys, we revealed the three‐dimensional electrical resistivity structure in the crust, suggesting the coexistence of two different types of strain‐concentration mechanisms in the strain‐concentration area. The shallow conductive layers and lower‐crustal conductors appear to act as low‐elastic‐modulus and low‐viscosity areas, respectively, and are responsible for the strain concentration. We found shallow and lower‐crustal conductors in the strain concentration zone revealed by geological studies. Those conductive areas are considered to act as mechanically weak zones and cause stress loading on the brittle pars of pre‐existing faults, resulting in large intraplate earthquakes. The resultant earthquakes presumably contribute to strain accumulation on a geological timescale. In addition, a spatial correlation between the epicenters of large intraplate earthquakes and edges of lower‐crustal conductors implies the contribution of the fluids in the lower crust to the generation of large earthquakes. We also identified vertical conductors ranging from the lower crust to Quaternary volcanoes, which may indicate trans‐crustal magmatic systems under these volcanoes.

Biogeochemical Fluxes of Nickel in the Global Oceans Inferred From a Diagnostic Model

AbstractNickel (Ni) is a micronutrient that plays a role in nitrogen uptake and fixation in the modern ocean and may have affected rates of methanogenesis on geological timescales. Here, we present the results of a diagnostic model of global ocean Ni fluxes which addresses key questions about marine Ni cycling. Sparsely available observations of Ni concentration are first extrapolated into a global gridded climatology using tracers with better observational coverage such as macronutrients, and testing three different machine learning techniques. The physical transport of Ni is then estimated using the ocean circulation inverse model (OCIM2), revealing regions of net convergence or divergence. These diagnostics are not based on any assumption about Ni biogeochemical cycling, but their spatial patterns can be used to infer where biogeochemical processes such as biological Ni uptake and regeneration take place. Although Ni and silicate (Si) have similar concentration patterns in the ocean, we find that the spatial pattern of Ni uptake in the surface ocean is similar to phosphate (P) uptake but not to silicate (Si) uptake. This suggests that their similar distributions arise from different biogeochemical mechanisms, consistent with other evidence showing that Ni is not incorporated into diatom frustules. We find that Ni:P ratios at uptake do not decrease as Ni concentrations approach 2 nM, which challenges the hypothesis of a ∼2 nM pool of non‐bioavailable Ni in the surface ocean. Finally, we find that the net regeneration of Ni occurs deeper in the ocean than for P, though not as deeply as for Si.

Quantification of grain boundary mobilities in natural olivine by annealing experiments and full-field modelling

We investigate olivine grain boundary (GB) migration in natural peridotites experimentally annealed at high pressure and high temperature, and couple the experimental observations to full-field grain growth models to provide the distribution of GB mobilities in natural olivine polycrystals. A stack of four slices of natural mylonitic peridotite (Oman ophiolite) was annealed at 1473 K for 5 h under a confining pressure of 300 MPa of argon in a Paterson press. The three sintered interfaces of the stack are characterized using scanning electron microscopy and electron backscatter diffraction (EBSD) to extract a distribution of apparent (2D) GB displacements. Full-field simulations of numerically sintered interfaces are then used to infer the GB mean mobility and distribution in olivine polycrystal allowing forward modelling exercise to reproduce the experimental GB displacement distribution.This yields widely dispersed mobilities, which can be approximated by a log-normal distribution of 10−15.85±2.58m4.J−1.s−1. Both the average and the dispersion of GB mobilities can be explained by silicon grain boundary diffusion. Finally, we demonstrate that the high variability of GB mobilities in olivine implies a decrease of the mean growth rate with time. This elucidates the difficulties of extrapolating experimental grain growth rates to geological timescales and observed microstructures in peridotites.

Volcanic Glass as a Proxy for Paleotopography Suggests New Features in Late-Miocene Oregon

Volcanic glass has been used extensively as a paleoaltimeter. Deuterium (2H) concentrations in glass have been found to be stable over geologic timescales, making δ2H (also known as δD) a reliable proxy for ancient water chemistry. However, continued work revolves around better understanding how different factors affect preserved water in volcanic ash. Here, we analyze δD in the Rattlesnake Tuff (RST), a widespread ca. 7 Ma ash-flow tuff, and create a paleoisoscape to assess variations in δD across Oregon during that time. To this end, 16 ash samples were collected across central and eastern Oregon from various flow units within the RST. Samples were analyzed for δD using a temperature conversion elemental analyzer (TC/EA) connected to a mass spectrometer and elemental composition using a scanning electron microscope (SEM). We compared the isotopic results to modern water and published ancient water proxy data to better constrain changes in climate and topography across Oregon throughout the Neogene. We also estimated wt. % H2O by calculating excess (non-stoichiometric) oxygen from SEM elemental data. We did not observe significant variations in δD among the flow units from single locations, nor was there a significant relationship between the prepared glass shard composition and wt. % H2O or δD, supporting the use of volcanic glass as a reliable paleoenvironmental indicator. Our results show significant spatial variation in δDwater values of RST, ranging from −107‰ to −154‰. δD values of ancient glass were similar to modern water near the Cascade Mountains but became relatively negative to the east near the inferred eruptive center of the RST, suggesting that a significant topographic feature existed in the vicinity of the RST eruptive center that has since subsided.

Stratigraphic evidence for a major unconformity within the Ediacaran System

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.

Evolution of groundwater system in the Pearl River Delta and its adjacent shelf since the late Pleistocene.

Our extensive field studies demonstrate that saline groundwater inland and freshened groundwater offshore coexist in the same aquifer system in the Pearl River delta and its adjacent shelf. This counterintuitive phenomenon challenges the commonly held assumption that onshore groundwater is typically fresh, while offshore groundwater is saline. To address this knowledge gap, we conduct a series of sophisticated paleo-hydrogeological models to explore the formation mechanism and evolution process of the groundwater system in the inland-shelf systems. Our findings indicate that shelf freshened groundwater has formed during the lowstands since late Pleistocene, while onshore saline groundwater is generated by paleo-seawater intrusion during the Holocene transgression. This reveals that terrestrial and offshore groundwater systems have undergone alternating changes on a geological timescale. The groundwater system exhibits hysteresis responding to paleoclimate changes, with a lag of 7 to 8 thousand years, suggesting that paleoclimatic forcings exert a significantly residual influence on the present-day groundwater system.

Bank strength variability and its impact on the system-scale morphodynamics of the upper Amazon River in Brazil

Large anabranching rivers form channels in sediments of varying strength, resulting from erosional and depositional processes that act over geological time scales. Although bank strength variability is known to affect channel morphodynamics, its impact on the migration of large sand-bed rivers remains poorly understood. We report the first in situ measurements of bank strength from an ∼100-km-long reach of the Solimões River, the Brazilian Amazon River upstream of Manaus. These show that cohesive muds in Pleistocene terraces along the river’s right margin have bank strengths as much as three times greater than Holocene floodplain deposits composing the left bank. Image analysis suggests these resistant outcrops determine channel-bar dynamics: channel widening and bar deposition are inhibited, which lowers planform curvature and reduces erosion of the opposing bank. Planform analysis of the 1600-km-long Solimões River between 1984 and 2021 shows that where the channel is associated with Pleistocene terraces, lower rates of bank erosion and bar deposition are evident. Heterogeneity in bank strength is thus a first-order control on the large-scale morphodynamics of the world’s largest lowland river.

Rapid Diagenesis and Microbial Biosignature Degradation in Spring Carbonates from Crystal Geyser, Utah

Carbonate rocks retain a well preserved record of biologically associated structures at the outcrop to millimeter scale, however microscale features such as cellular fossils are rarely represented. The lack of microscale textural information in ancient carbonates is commonly attributed to processes relating to carbonate diagenesis. However, there are relatively few examples of precisely how and when these destructive processes occur, particularly in active precipitating systems. To better understand the taphonomy of carbonate precipitating environments through early diagenesis, we investigated Crystal Geyser, an active cold water carbonate spring ( about 18 deg C) located in Grand County, Utah. Here we show that rapid precipitation is effective at initially capturing cell-like structures and forming associated microscale laminated stromatolites; however, these morphologies degrade immediately after their formation. We attribute destructive diagenetic effects to the recrystallization of metastable aragonite into the more stable polymorph calcite (i.e., inversion) and the associated textural coarsening that homogenizes and erases the original fabric (i.e., aggrading neomorphism). Despite the loss of microscale morphological information, chemical biosignatures in the form of macromolecular organics remain dispersed throughout the disrupted carbonate textures. These observations provide an example of penecontemporaneous diagenesis that obliterates primary microscale textures in carbonate rocks. Similar mechanisms and their rapid timing, as shown here, likely contributes to the observed lack of microscale morphological biosignatures in many ancient carbonates. This work further highlights that within such systems, permineralization by a more stable crystalline phase, such as chert, must occur rapidly after deposition to effectively retain these signatures over geological timescales.

Thermal tides in neutrally stratified atmospheres: Revisiting the Earth’s Precambrian rotational equilibrium

Rotational dynamics of the Earth, over geological timescales, have profoundly affected local and global climatic evolution, probably contributing to the evolution of life. To better retrieve the Earth’s rotational history, and motivated by the published hypothesis of a stabilized length of day during the Precambrian, we examined the effect of thermal tides on the evolution of planetary rotational motion. The hypothesized scenario is contingent upon encountering a resonance in atmospheric Lamb waves, whereby an amplified thermotidal torque cancels out the opposing torque generated by the oceans and solid interior, driving the Earth into rotational equilibrium. With this scenario in mind, we constructed an ab initio model of thermal tides on rocky planets describing a neutrally stratified atmosphere. The model takes into account dissipative processes with Newtonian cooling and diffusive processes in the planetary boundary layer. We retrieved, from this model, a closed-form solution for the frequency-dependent tidal torque, which captures the main spectral features previously computed using 3D general circulation models. In particular, under longwave heating, diffusive processes near the surface and the delayed thermal response of the ground prove to be responsible for attenuating, and possibly annihilating, the accelerating effect of the thermotidal torque at the resonance. When applied to the Earth, our model prediction suggests the occurrence of the Lamb resonance in the Phanerozoic, but with an amplitude that is insufficient for the rotational equilibrium. Interestingly, though our study was motivated by the Earth’s history, the generic tidal solution can be straightforwardly and efficiently applied in exoplanetary settings.

Hydrogen Diffusion in the Lower Mantle Revealed by Machine Learning Potentials

AbstractHydrogen may be incorporated into nominally anhydrous minerals including bridgmanite and post‐perovskite as defects, making the Earth's deep mantle a potentially significant water reservoir. The diffusion of hydrogen and its contribution to the electrical conductivity in the lower mantle are rarely explored and remain largely unconstrained. Here we calculate hydrogen diffusivity in hydrous bridgmanite and post‐perovskite, using molecular dynamics simulations driven by machine learning potentials of ab initio quality. Our findings reveal that hydrogen diffusivity significantly increases with increasing temperature and decreasing pressure, and is considerably sensitive to hydrogen incorporation mechanism. Among the four defect mechanisms examined, (Mg + 2H)Si and (Al + H)Si show similar patterns and yield the highest hydrogen diffusivity. Hydrogen diffusion is generally faster in post‐perovskite than in bridgmanite, and these two phases exhibit distinct diffusion anisotropies. Overall, hydrogen diffusion is slow on geological time scales and may result in heterogeneous water distribution in the lower mantle. Additionally, the proton conductivity of bridgmanite for (Mg + 2H)Si and (Al + H)Si defects aligns with the same order of magnitude of lower mantle conductivity, suggesting that the water distribution in the lower mantle may be inferred by examining the heterogeneity of electrical conductivity.

Technical note: A comparison of methods for estimating coccolith mass

Abstract. The fossil record of coccolithophores dates back approximately 225 million years, and the production of their calcite platelets (coccoliths) contributes to the global carbon cycle over short and geological timescales. Variations in coccolithophore parameters (e.g. community composition, morphology, size and coccolith mass) are a key factor for ocean biogeochemical dynamics (e.g. biological carbon pump) and have been used as a palaeoproxy to understand past oceanographic conditions. Coccolith mass has been frequently estimated with different methods with electron microscopy being the most applied. Here, we compared the electron microscopy (EM) method with the Coulter multisizer (CM) (i.e. electric field disturbance) and bidirectional circular polarization (BCP) methods to estimate coccolith masses (pg CaCO3) in controlled laboratory experiments with two ecotypes of Emiliania huxleyi. Average coccolith mass estimates were in good agreement with literature data. However, mass estimates from the CM were slightly overestimated compared to EM and BCP estimates, and a correction factor (cf=0.8) is suggested to compensate for this discrepancy. The relative change in coccolith mass triggered by morphotype-specific structures and environmental parameters (i.e. seawater carbonate chemistry) was suitably captured by each of the three techniques.

Assessing the performance of a radioactive waste repository over geological timescales: past experience and the way forward

The successful implementation of plans to develop a repository for radioactive waste in the United Kingdom will require an assessment of the ability of the facility to meet appropriate regulatory safety criteria at all stages during its construction, operation and after closure. As part of this assessment, an environmental safety case (ESC) will be presented and this will contain a collection of claims, arguments and evidence which collectively demonstrate that long-term safety can be achieved and maintained. Natural analogues (NA) can be helpful in demonstrating understanding of aspects of repository performance by, for example, providing evidence that certain materials can survive for long periods. Appropriate NAs can be critical to providing long-term practical demonstrations to support the theoretical and mathematical arguments of the ESC and they may have a significant role in the overall process understanding. As part of the ongoing repository development programme in the UK, an updated version of an existing catalogue of NA studies (focussed on the requirements of the ESC) has been produced and the background to NA studies in general, and to the catalogue in particular, are presented here. Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive

Stable Isotopes Constrain Water Seepage From Gnammas Into Bare Granitic Bedrock

AbstractSubtle bedrock depressions called gnammas allow the water in ephemeral pools to maintain contact with bare rock, thus serving as natural rock‐weathering experiments. Following filling by precipitation, evaporation is often assumed to be the sole process of water loss from gnammas. We evaluated this assumption by monitoring evolving stable isotope compositions of gnamma waters hosted in granite of Colorado's Front Range. Surprisingly, we found that a significant fraction of the water was lost by seepage through the underlying bedrock. Seepage dominated, with only 10%–20% loss by evaporation. We propose a conceptual model of gnamma formation in which enhanced weathering of the bedrock beneath the gnamma increases the water holding capacity and permeability of the underlying rock that in turn promotes efficient water loss through seepage and further weathering of the surrounding rock. This model has implications for bare‐rock weathering and hence the evolution of landscapes over geologic timescales.

In situ hydrodynamic observations on three reef flats in the Nansha Islands, South China Sea

Waves and currents are responsible for sediment movement around and off coral reefs, affecting reef growth at both geological and modern timescales. Given the wide distribution and limited hydrodynamic information of reefs in the South China Sea, we carried out observations on tidal-cycle hydrodynamics in the Nansha Islands with tripod stationary instruments on the seafloor in order to fill the gap in our understanding of these processes. It was found that the magnitudes of near-bed orbital velocity were comparable with that of the mean tidal current, despite generally calm wave conditions. Waves dominated the combined wave-current skin-friction shear velocities acting on reef sediment, which were significantly higher than those generated by currents alone. Due to the large physical roughness of reef, drag coefficient and hydrodynamic roughness length estimated from logarithmic velocity profiles were two orders of magnitude higher than that in macro-tidal-estuary or inner shelf areas covered with siliceous muds or sands. The combined sinusoidal wave and asymmetric tidal current, along with the physical reef roughness, shaped velocity profile structures in the bottom boundary layer, which exhibited a logarithmic profile during the flood tide and a potential flow during the ebb. In absence of wave breaking, strong turbulence dissipation was observed across the rough reef, promoting strong mixing of water, which is crucial for delivering nutrients for coral growth. These findings imply the need to consider the unique characteristics of rough reef structure and combined effects of waves and currents to model the hydrodynamics in reef environment correctly. This understanding is critical for predicting energy and material transport in reef environments, which is essential for maintaining healthy coral ecosystems, and opens new paths for managing and preserving coral reefs in the face of environmental change.

Space Geodetic Insights to the Dramatic Stress Rotation Induced by the February 2023 Turkey‐Syria Earthquake Doublet

AbstractThe February 2023 Turkey‐Syria Earthquake doublet ruptured multiple segments of the East Anatolian Fault (EAF) Zone. Dominating seismicity focal mechanism shifted dramatically from strike‐slip to normal‐faulting after the doublet. To better understand this shift, here we derived a comprehensive 3D co‐seismic displacement field and performed the stress analysis. Abundant space geodetic data were used to generate high‐resolution 3D surface displacement, which provide tight constraints on fault geometry, slip distribution and stress field. Together with stress inversion from aftershock focal mechanisms, we show that the principal stress direction rotation in the region with the most normal‐faulting aftershocks is the staggering 29°. The induced heterogenous stress may explain the shift of the dominant focal mechanism toward normal faulting. We suggest that the extensional horsetail splay faults, likely formed through geologic time scale related to the releasing bend on the EAF, are the hosts of most of the normal faulting aftershocks.

Geochronological constraints on the Hangenberg Event of the latest Devonian in South China

The Hangenberg Event was one of the most significant mass extinctions accompanied by large-scale climate changes at the end of the Devonian period. Lack of precise and accurate geochronological constraints in the eastern Paleotethys has hindered a high-resolution global correlation of the Hangenberg Event. Here, we report the U–Pb dates of zircons from the bentonite samples intercalated with strata recording the Hangenberg Event in the Muhua region, Guizhou, South China, using both sensitive high-resolution ion microprobe (SHRIMP) and chemical abrasion - isotope dilution - thermal ionization mass spectrometry (CA-ID-TIMS) methods. A bentonite within the Hangenberg Event strata in the Muhua area was dated at 360.47 ± 0.68 Ma by CA-ID-TIMS, which is more precise than the results obtained by SHRIMP methods (361.5 ± 2.9 Ma and 359.8 ± 1.5 Ma). Thus, we propose that the Hangenberg Event took place at 360.47 ± 0.68 Ma in South China, based on the first precise CA-ID-TIMS zircon age from the Daposhang section. The age obtained from South China for the Devonian–Carboniferous boundary overlaps with that in Geologic Time Scale 2020, within the total uncertainties.

Ratification of the base of the ICS Geological Time Scale: the Global Standard Stratigraphic Age (GSSA) for the Hadean lower boundary

Ratification of the base of the ICS Geological Time Scale: the Global Standard Stratigraphic Age (GSSA) for the Hadean lower boundary