Basal Sediments Research Articles

Modeling Sediment Fluxes From Debris‐Rich Basal Ice Layers

AbstractSediment erosion, transport, and deposition by glaciers and ice sheets play crucial roles in shaping landscapes, provide important nutrients to downstream ecosystems, and preserve key indicators of past climate conditions in the geologic record. While previous work has quantified sediment fluxes from subglacial meltwater, we also observe sediment entrained within basal ice, transported by the flow of the glacier itself. However, the formation and evolution of these debris‐rich ice layers remains poorly understood and rarely represented in landscape evolution models. Here, we identify a characteristic sequence of basal ice layers at Mendenhall Glacier, Alaska. We develop a numerical model of frozen fringe and regelation processes that describes the co‐evolution of this sequence and explore the sensitivity of the model to key properties of the subglacial sedimentary system, using the Instructed Glacier Model to parameterize ice dynamics. Then, we run numerical simulations over the spatial extent of Mendenhall Glacier, showing that the sediment transport model can predict the observed basal ice stratigraphy at the glacier's terminus. From the model results, we estimate basal ice layers transport between 23,300 and 39,800 of sediment, mostly entrained in the lowermost ice layers nearest to the bed, maximized by high effective pressures and slow, convergent flow fields. Overall, our results highlight the role of basal sediment entrainment in delivering eroded material to the glacier terminus and indicate that this process should not be ignored in broader models of landscape evolution.

Reduced Injection Rates and Shallower Depths Mitigated Induced Seismicity in Oklahoma

Abstract The proximity of wastewater disposal to the Precambrian basement is a critical factor influencing induced earthquake rates in the Central United States, but the impact of reducing injection depths has not been widely demonstrated. Beginning in 2015, state regulatory efforts in Oklahoma and Kansas mandated that wells injecting into the lower Arbuckle Group, a basal sedimentary unit, be backfilled with cement (i.e., “plugged back”) so that they inject into shallower formations. This plug back activity gives us a unique opportunity to investigate the relationship between injection depth and induced seismicity rate. To evaluate the impact that decreased injection rates and plug backs had on the seismicity rates, we created a suite of rate–state earthquake models. Observed seismicity rates are best fit when only lower Arbuckle volumes are considered, suggesting the lower Arbuckle injectors were primarily responsible for the seismicity and that plug backs were effective at isolating the injected volumes to shallower formations. Our models demonstrate that if these wells had not been plugged back, seismicity rates would be multiple times larger than they are today. We find that the combination of well plug backs and injection volume decreases can be an effective strategy for reducing induced seismicity rates.

Large mammal faunas and ecosystem dynamics during the late Middle to early Late Pleistocene at Grotta Romanelli (southern Italy)

Since the beginning of the XIX century, Grotta Romanelli gained recognition in European stratigraphy as an important site for the Late Pleistocene, due to the attribution of its basal sediments to the Last Interglacial (MIS 5e). Its abundant archaeological and paleontological findings made the cave a key reference point for the study of human evolution and faunal dynamics in the larger Mediterranean area over approximately the last 130,000 years. After a century, a new chronostratigraphic reassessment of Grotta Romanelli revised the timeframe of its lower levels, predating them to the late Middle Pleistocene.In this study, we re-evaluate selected historical museum collections from the so-called “lower complex” of Grotta Romanelli and examine the new fossils excavated between 2015 and 2022. We identify three main mammal assemblages associated with three different interglacial periods spanning the last 350,000 years. Our findings offer new insights into terrestrial ecosystems in Mediterranean Europe from the late Middle Pleistocene to early Late Pleistocene. Additionally, they provide valuable information to the biochronological framework of the European large mammal faunas.

Araguainha impact structure, Brazil: New insights into the geology of the central uplift

AbstractExtensive, new outcrops along the MT‐100 state road in the northern part of the central uplift of the 40‐km diameter, 252–259 Ma old Araguainha impact structure, Central Brazil, have become available for investigation. They offer new insight into the contact relationships between the different lithologies and the genesis of different types of impact‐related rocks, as well as the current level of erosion of the structure. Three types of impact melt rock (IMR) with different field relationships and compositions can now be distinguished: (1) Type‐I of granitic composition and occurring mainly as veins and dikes, besides a few larger pods, in the central alkali granite core of the central uplift; (2) Type‐II in the form of plastically deformed clasts of mainly highly silicious compositions in polymict impact breccia; and (3) Type‐III, derived from partially melted conglomerate or sandstone precursors, and that occurs at selected sites in (meta)sedimentary strata of the basement in the immediate environs of the alkali granite core. Both polymict lithic and melt‐bearing (suevitic) impact breccias are recognized in the 110‐m thick integrated section through impact breccia directly overlying the crater floor. This crater floor is composed of (meta)‐sedimentary basement strata with granite injections and, locally, sandstones of the Devonian sedimentary Furnas Formation of the Paraná Basin. Main breccia components are (meta)‐pelites and (meta)sandstones of the basement that is currently favored to be related to the regional Paraguay Belt and to the lower sequence of the Paraná Basin sedimentary strata. Locally, breccia contains clasts of IMR Type‐II, and only very rarely are granitic fragments observed. Clasts of IMR Type‐I have never been observed in the breccia deposits. These new observations preclude significant involvement of alkali granite in the formation of the polymict breccia or in the production of shock melts. They also reveal the major role of the (meta)sedimentary precursors in the production of IMR by shock melting and provide essential information for better understanding the cratering processes involved in the formation of an impact structure in a sedimentary target, of the size of the Araguainha impact structure.

Geochemical Evidence of Water-Fluxed Crustal Melting in the Northern Colombian Andes

Abstract The compositions of crustal magmas are powerful tools for understanding the formation and differentiation of continents. However, the geochemical fingerprints that distinguish the two dominant mechanisms of crustal melting, namely dehydration and water-fluxed melting, are still controversial. To provide new insights into this problem, we discuss the petrogenesis of the Paipa Volcanic Complex (PVC), an isolated Quaternary volcanic field in the Colombian Eastern Cordillera. The PVC is characterized by peraluminous trondhjemite-like rhyolites with exceptionally high Na2O contents (~6 wt %), super-chondritic Nb/Ta (~27), elevated Sr/Y ratios (~120), spoon-shaped REE patterns, and enriched isotopic compositions that overlap with those of the local basement. They also exhibit high pre-eruptive H2O contents (~up to 9.5 wt %) and abundant Paleozoic zircon inheritances. We demonstrate that these characteristics are inconsistent with a process of intra-crustal differentiation from a mafic or intermediate mantle-derived precursor. Instead, we propose that the origin of the PVC is best explained by melting the local (meta)sedimentary basement under H2O-saturated conditions, at middle-crustal pressures (~1.3 GPa) and relatively low temperatures (~690–740°C), following the complete breakdown of plagioclase and biotite, and the formation of reactive peritectic amphibole. This scenario differs from the high-temperature dehydration melting conditions that have been widely proposed for the Andes and globally, which result in the production of water undersaturated magmas in equilibrium with anhydrous lithologies rich in plagioclase and/or garnet. Accordingly, we speculate that an external H2O flux was ultimately sourced from a buoyant, cold, and hydrated mantle wedge that was extensively metasomatized by fluids derived from the Nazca and Caribbean flat-slab fronts. These conditions depressed the asthenospheric mantle potential temperature, likely inhibiting mantle melting. In turn, they facilitated the infiltration and ascent of mantle-derived H2O through pre-existing crustal faults and shear zones. Our results indicate that water-fluxed melting could be a plausible mechanism for generating crustal magmas in orogenic regions where the availability of free H2O has been difficult to confirm.

Joint Three-Dimensional Inversion of Gravity and Magnetic Data Collected in the Area of Victoria Mine, Nevada, Using the Gramian Constraints

Gravity and magnetic surveys have been extensively employed in various fields like regional geology studies, environmental engineering, and mineral exploration. However, interpreting the data from these surveys remains a challenge because the potential field inversion lacks uniqueness. To address this issue, combining gravity and magnetic data in a joint inversion helps to narrow down the possible solutions. This study introduces a method for jointly inverting gravity and magnetic data, specifically focusing on the sediment–basement interface. Within this framework, a 3D voxel-type inversion using joint Gramian-based techniques incorporates a depth-to-basement model as a guiding constraint. This approach was applied to gravity and magnetic data collected around the Victoria Mine area in Nevada, USA. The joint inversion successfully produced 3D models representing the density and susceptibility of both unconsolidated sediments and the basement underlying the surveyed region.

Laser ablation behavior and mechanism of Cf/C–SiC composites under different laser energy densities

Ceramic matrix composite (CMC) is typical difficult-to-machine material, laser assisted machining (LAM) is an effective method to solve the problem of poor machinability, and a deep understanding of the interaction between laser and material can lay a foundation for LAM. Aiming at the unclear mechanism of microstructure formation, crack propagation and material hardness evolution of laser-irradiated CMC, laser ablation behavior and mechanism of Cf/C–SiC composites under different laser energy densities were studied in this paper. The results show that Cf/C–SiC presents two different states of modification and ablation under irradiating. In modified state, fiber interface in central area is oxidized, and crystal transformation occurs. Amorphous Si–O–C, Si and spherical SiO2 exist in edge deposition area, and continuous phase disappears in heat affected zone. In ablative state, carbon nanosheets and clusters are formed in ablation groove. Surface of convex and deposition area are composed of sedimentary basement and clusters, and there are recast layer and heat affected zone in cross section. When thermal stress exceeds interfacial bond strength and critical matrix stress, the fiber interface debonding and matrix cracking occur, and both transgranular and intergranular fracture exist in the matrix. When laser scanning direction is perpendicular to fiber extension, crack propagation can be inhibited. The propagation and intersection of cracks and coarsening of SiC crystal phase in heat affected zone lead to the decrease of matrix microhardness. The results of this study can be used to provide references for parameters selection and theoretical support for LAM of CMC.

Crustal structure of Borneo, Makassar Strait and Sulawesi from ambient noise tomography

SUMMARY Borneo and Sulawesi are two large islands separated by the Makassar Strait that lie within the complex tectonic setting of central Indonesia. The seismic structure beneath this region is poorly understood due to the limited data availability. In this study, we present Rayleigh wave tomography results that illuminate the underlying crustal structure. Group velocity is retrieved from dispersion analysis of Rayleigh waves extracted from the ambient noise field by cross-correlating long-term recordings from 108 seismic stations over a period of 8 months. We then produce a 3-D shear wave velocity model via a two-stage process in which group velocity maps are computed across a range of periods and then sampled over a dense grid of points to produce pseudo-dispersion curves; these dispersion curves are then separately inverted for 1-D shear wave velocity (Vs), with the resultant models combined and interpolated to form a 3-D model. In this model, we observed up to ± 1.2 km s−1 lateral Vs heterogeneities as a function of depth. Our models illuminate a strong low shear wave velocity (Vs) anomaly at shallow depth (≤ 14 km) and a strong high Vs anomaly at depths of 20–30 km beneath the North Makassar Strait. We inferred the sediment basement and Moho depth from our 3-D Vs model based on iso-velocity constrained by the positive vertical gradient of the Vs models. The broad and deep sedimentary basement at ∼14 ± 2 km depth beneath the North Makassar Strait is floored by a shallow Moho at ∼22 ± 2 km depth, which is the thinnest crust in the study area. To the east of this region, our model reveals a Moho depth of ∼45 ± 2 km beneath Central Sulawesi, the thickest crust in our study area, which suggests crustal thickening since the late Oligocene. Moreover, the presence of high near-surface Vs anomalies with only slight changes of velocity with increasing depth in southwest Borneo close to Schwaner Mountain confirm the existence of a crustal root beneath this region.

Carboniferous sills and related metasomatic activity in the area of Asaritsa Peak, West Stara Planina Mountain

Two diorite to monzodiorite sills are recently found among the Silurian sedimentary succession near to Asaritsa peak, West Stara Planina Mountain. Several levels with diagenetic metalliferous nodules occur within shale interval in the sediments. The magmatic rocks are affected by strong propylitic to sericitic alteration (chlorite, sericite, albite and quartz) and only relicts of the rock-forming minerals (feldspar, biotite and clinopyroxene) are observed. The bodies are crosscut by thin quartz veins and disseminated pyrite is observed. The chondrite normalized spidergrams show patterns similar to orogenic rocks, with very high peaks of Th and U and depletion in Sr and Ti. The zircon U-Pb geochronology reveals that the magmatic crystallization is during the Carboniferous period – 326.4±3.2 Ма, with low content of xenocryst most probably from the sedimentary basement. The magmatic activity coincides with the Variscan orogeny formed during the late Paleozoic and most probabaly is related to the evolution of the alkaline Buhovo-Seslavtsi Pluton which is situated just to the south, at a direct proximity. The different composition of the pyrite from the sills and the diagenetic nodules from the sediments is most probably due to their different genesis. While the alterations of the nodules can be attributed to metasomatic processes related to the magmatic activity.

Application of Horizontal-to-vertical Spectral Ratio with a Dense Linear Array on Prospecting Urban Underground Active Faults: a Case in The Mingguang City, Anhui Province

Reliable knowledge of sedimentary structures is beneficial for the study of active faults covered by sediments and can also provide important information for seismic hazard assessment, earthquake emergency response and urban planning. Based on the dense seismic array with a spacing of ∼ 60 m between stations and a length of ∼ 8 km, the horizontal-to-vertical spectral ratio (HVSR) curves were obtained at 133 stations with the ambient noise, and the two-dimensional sedimentary structures of the Quaternary and Neogene strata were imaged over two main faults of the Tanlu fault zone in the eastern Jiashan Basin by frequency-depth conversion. The variations (∼ 130 m) in the depth of the Quaternary and Neogene sedimentary basement directly beneath the fault zone agree well with the spatial characteristics of the fault revealed by the shallow seismic reflection profile with active sources at the same locations.

Mapping of the Structural Lineaments and Sedimentary Basement Relief Using Gravity Data to Guide Mineral Exploration in the Denizli Basin

The Aegean Graben System is a complex tectonic structure in Western Anatolia and the Denizli Graben is a member of this system that hosts many geothermal springs, ore deposits, and travertine areas. In this study, the gravity data were analyzed to determine the subsurface geological structures and the depth model of the basin. The Bouguer gravity anomaly has a NW–SE pattern that is consistent with the general trend of the Denizli Basin. The pre-Neogene basement depths range from 0.1 km to 2.3 km. The Denizli Basin is composed of the Çürüksu Basin and the Laodikia sub-basin. The basins have undulated structures with many depressions; the deepest depression region is in the northern part of the Çürüksu Basin, which is close to the Pamukkale Fault Zone. In addition, the new gravity lineament map was obtained by using new-generation edge detection techniques: the tilt angle of the horizontal gradient amplitude (TAHG), and fast sigmoid-edge detection (FSED) of gravity data. The new proposed lineament map shows that the Denizli Basin has complex structures consisting of NW–SE, E–W, and NE–SE trending lineaments, and the major NW–SE trending faults and NE–SW trending lineaments control the main structural configuration. The uplifts and depressions in the basin deposit and the intersection area of lineaments are promising prospective areas for mineral deposits and have energy resource potential.

Controls on upstream-migrating bed forms in sandy submarine channels

Abstract Submarine channels parallel river channels in their ability to transport sediment. However, in contrast to rivers, sediment transport and bed-form development in submarine channels are less well understood. Many steep (>1°), sandy submarine channels are dominated by upstream-migrating bed forms. The flow conditions required to form these upstream-migrating bed forms remain debated because the interactions between turbidity currents and active bed forms are difficult to measure directly. Consequently, we used a depth-resolved numerical model to test the role of flow parameters that are hypothesized to control the formation of upstream-migrating bed forms in submarine channels. While our modeling results confirmed the importance of previously identified flow parameters (e.g., densiometric Froude number), we found that basal sediment concentration in turbidity currents is the strongest predictor of upstream-migrating bed-form formation. Our model shows how locally steep gradients enable high sediment concentrations (average >5 vol%) in the basal parts of flows, which allow the development of cyclic step instabilities and their associated bed forms. This new insight explains the previously puzzling observation that upstream-migrating bed forms are abundant in proximal, steep, sandy reaches of submarine channels, while their occurrence becomes more intermittent downslope.

Oil sands in glacial till as a driver of fast flow and instability in the former Laurentide Ice Sheet: Alberta, Canada

AbstractReconstructions of the southwestern Laurentide Ice Sheet (LIS) from geomorphology have revealed complex cross‐cutting ice streams, indicative of surging and flow switching. This flow pattern and behaviour is distinct from the rest of the ice sheet where ice streams flowed radially to the ice sheet margin. Many ice streams in the southwestern sector originate around the Alberta Oil Sands (AOS). Previous reports have detected oil sands material in surficial and glacial sediments south of the AOS, demonstrating potential glacial mobilisation of the oil sands. In this study, we use geochemical fingerprinting to systematically investigate surficial sediments from the former Central Alberta Ice Stream (CAIS) flow track. We compare the geochemical signatures of 82 sediments from within and outside the CAIS limits with those of the AOS (from mines and natural exposures), using gas chromatography–mass spectrometry oil–oil correlation techniques. Our results provide geochemical evidence of glacial erosion and long‐distance mobilisation of AOS producing contamination signatures in sediments throughout the CAIS flow track. The strength of the AOS signature is particularly strong in sediments along the terminating margins, to the east of Calgary and in the Cooking Lake area to the southeast of Edmonton. These results inform theoretical models of enhanced slipperiness, inspired by slippery liquid infused porous surfaces (SLIPS), whereby oil lubrication of the basal sediment influences the degree of sliding and basal deformation in the ice stream. We hypothesise that naturally occurring hydrocarbons at the basal interface exerted control on the location of the onset zone of the CAIS and surrounding ice streams in Alberta. The enhanced slipperiness caused by oil contamination may also explain ice streaming and surging in other ice sheets, such as the Barents Sea Ice Sheet, where hydrocarbons are known to have been driven to the sedimentary interface during glaciation.

Quantifying the Structure and Extension Rate of the Linfen Basin, Shanxi Rift System Since the Latest Miocene: Implications for Continental Magma‐Poor Rifting

AbstractThe formation of magma‐poor continental rifts is an enigmatic process, as the weakening mechanism(s) for cratonic lithosphere remains uncertain in the absence of elevated lithospheric temperature. One view links weakening to melts hidden at depth, while another ascribes it to pre‐existing weaknesses. Long‐term extensional rates also influence lithospheric strength and rift evolution. We target the Linfen Basin (LB) in the magma‐poor Shanxi Rift System (SRS) in the North China Craton to understand these components. We apply cosmogenic 26Al/10Be burial dating on 14 core samples at different depths from three deep boreholes in the basin and obtain six valid burial ages ranging from 2.37+1.18/−1.21 to 5.86+inf/−1.37 Ma. We further re‐interpret a seismic reflection profile and quantify the geometry and amount of extension by forward structural modeling with multiple constraints based on extensional fault‐bend folding theory. The timing of the basal sedimentation is estimated to be ∼6.1 and ∼4.2 Ma in the southern and northern portions, respectively, indicating diachronous, northward‐propagating rifting. The amount and mean rate of extension are ∼3.6 km and ∼0.9 km/Myr, respectively. The basin depths increasing northward indicates the clockwise rotation of the basin. We propose a basin‐scale non‐rigid transtensional bookshelf faulting model to explain the rotation patterns of the circum‐Ordos basins. We argue that the inherited structures weaken the cratonic lithosphere of the SRS, and the low extension rate contributes to its magma‐poor nature. We propose a lithospheric‐scale evolution model for the LB, invoking the inherited crustal weakness, low extension rate, and lower lithosphere counterflow.

Subglacial hydrology from high-resolution ice-flow simulations of the Rhine Glacier during the Last Glacial Maximum: a proxy for glacial erosion

Abstract. At the Last Glacial Maximum (LGM), the Rhine Glacier complex (Rhine and Linth glaciers) formed large piedmont lobes extending north into the Swiss and German Alpine forelands. Numerous overdeepened valleys there were formed by repeated glaciations. A characteristic of these overdeepened valleys is their location close to the LGM ice margin, away from the Alps. Numerical models of ice flow of the Rhine Glacier indicate a poor fit between the sliding distance, a proxy for glacial erosion, and the location of these overdeepenings. Calculations of the hydraulic potential based on the computed time-dependent ice surface elevations of the Rhine Glacier lobe obtained from a high-resolution thermo-mechanically coupled Stokes flow model are used to estimate the location of subglacial water drainage routes. Results indicate that the subglacial water discharge is high and focused along glacial valleys and overdeepenings when water pressure is equal to the ice overburden pressure. These conditions are necessary for subglacial water to remove basal sediments, expose fresh bedrock, and favor further erosion by quarrying and abrasion. Knowledge of the location of paleo-subglacial water drainage routes may be useful to understand patterns of subglacial erosion beneath paleo-ice masses that do not otherwise relate to the sliding of ice. Comparison of the erosion pattern from subglacial meltwater with those from quarrying and abrasion shows the importance of subglacial water flow in the formation of distal overdeepenings in the Swiss lowlands.

Thermally Significant Fluid Seepage Through Thick Sediment on the Juan de Fuca Plate Entering the Cascadia Subduction Zone

AbstractWe use heat flux measurements colocated with seismic reflection profiles over a buried basement high on the Juan de Fuca plate ∼25 km seaward of the deformation front offshore Oregon to test for the presence of hydrothermal circulation in the oceanic crust. We also revisit heat flux data crossing a buried basement high ∼25 km seaward of the deformation front ∼150 km north, offshore Washington. Seafloor heat flux is inversely correlated with sediment thickness, consistent with vigorous hydrothermal circulation in the basement aquifer homogenizing temperatures at the top of the basement. Heat flux immediately above the summit of the basement highs is greater than expected solely from conduction. Fluid seepage at rates of ∼2.6–5.4 cm yr−1 in a 1–1.5 km‐wide conduit through ∼800–1,300 m thick sediment sections above these basement highs can explain these observations. Observations of thermally significant fluid seepage through sediment >225 m thick on oceanic crust are unprecedented. High sediment permeability, high fluid overpressure in the basement, or a combination of both is required to drive fluid seepage at the observed rates. We infer that rapid seepage occurs because the basement highs rise above the low permeability basal sediment with their tops protruding into the base of high permeability Nitinat or Astoria Fan sediment. Seepage from basement highs penetrating into the submarine fans can affect the thermal state of crust entering the subduction zone.

Structure of the Gonghe Sedimentary Basin in the northeastern Tibetan Plateau: evidence from teleseismicPwaves recorded by a dense seismic array

SUMMARYThe Gonghe Basin in the northeast Tibetan Plateau presents significant potential for hot dry rock (HDR) geothermal resources. A 1990 Mw 6.4 earthquake in the basin furthers the need for an improved understanding of its sedimentary structure. In this study, we utilize data from a dense seismic array of 88 short-period seismometers deployed at an interstation spacing of approximately 3 km to scrutinize the sedimentary structure of the Gonghe Basin. By analysing teleseismic P waveforms, we identify P-to-S converted waves (Ps wave) originating from the sedimentary basement. We then determine the delay time between the Ps waves and the direct P waves (P wave) through waveform cross-correlation. By integrating this delay time with empirical velocity structure models, HDR borehole data and results from teleseismic receiver function analysis, we derive a sediment thickness model of the Gonghe Basin for the Qabqa geothermal area. Our findings reveal a gradual increase in sediment thickness from around 500 m in the east to approximately 3000 m in the west, which is consistent with other geophysical surveys and borehole data. The thick sediments in the basin could potentially serve as an excellent thermal storage cover for HDR. The strong ground motion simulation using our sediment thickness model shows that thick sediments can amplify seismic waves, increasing the risk of seismic hazards. Moreover, our study indicates that the clear Ps waves can be effectively extracted to construct a dependable sediment thickness model using teleseismic P waves recorded by a short-period dense seismic array.

Local Moho distribution in the eastern Mediterranean region from gravity inversion: eastern Mediterranean Sea

This study investigates the variations of surface Moho inverted from Bouguer gravity data beneath the northwestern Arabian plate. The results were obtained by applying the 3DINVER program developed in a MATLAB environment using the Parker–Oldenburg method. The calculation results show that the maximum Moho depth observed in the western Arabian shield approaches more than 40 km, while the minimum values of 16–20 km are mainly concentrated beneath the Mediterranean Sea. A key innovation of this study is its implementation of exponential density contrast decrease in the sediment–basement interface in the Syrian part of the Arabian plate. The gravity anomaly of the sediment is computed using a density contrast which varies exponentially with depth, leading to a more accurate representation of the subsurface structure and offering valuable insights into the geodynamic processes and seismogenic potential of the area. To validate the inverted results from the 3DINVER program, the study compares these findings with seismic results that show good agreement with our results from the same region. This leads to a better understanding of the crustal structure and provides insights into its implications for the geodynamic and seismogenic nature of the northwestern Arabian plate—critical for assessing the potential for earthquakes and other geological hazards. The density contrast between the Earth’s crust and mantle can be related to variations in the composition, temperature, and pressure of the rocks. These factors influence the mechanical properties of the crust, ultimately affecting the tectonic processes and seismic activity in the region.

Formation of crevasse‐squeeze ridges at Trygghamna, Svalbard

AbstractCrevasse‐squeeze ridges (CSRs) are landforms that have been unequivocally linked to surge‐type glaciers. The formation of CSRs has been discussed since they were first defined in the mid‐1980s. Here, we describe geometric CSR networks from the terrestrial glacier forefields of two glaciers in Trygghamna, Western Svalbard. No glacier surges have been observed in Trygghamna; however, the presence of the CSRs signifies past surge activity. Detailed geomorphological maps were constructed, and the spatial context of these landforms described. Cross‐sections of several CSRs highlight ridge architecture, structure and relationships to surrounding landforms and sediments. Most CSRs are symmetrical in cross‐profile, orientated perpendicular or oblique to the ice‐flow direction. Like previous investigations, we observe these ridge networks on top of till and flutes. Additionally, we, for the first time, document CSRs deposited directly on non‐glaciogenic subsurfaces, namely, beach gravels and bedrock. Our findings confirm previous CSR formation theories; basal sediments are squeezed into bottom‐up crevasses during surges, which are subsequently transported englacially until surge termination and are finally released by melt out from stagnant ice. Consequently, a network of CSRs is the product of a significant reorganisation and down‐glacier transport of basal sediment, exemplifying how single surges are agents of glacial sediment redistribution. These formation processes are illustrated in a refined schematic model. The results further contemporary understanding of CSRs in terrestrial surge‐type glacier settings and may also apply to landforms and sediments in certain marine settings and palaeoglacial environments.

Basal debris of the NEEM ice core, Greenland: a window into sub-ice-sheet geology, basal ice processes and ice-sheet oscillations

AbstractWe present new data from the debris-rich basal ice layers of the NEEM ice core (NW Greenland). Using mineralogical observations, SEM imagery, geochemical data from silicates (meteoric10Be, εNd,87Sr/86Sr) and organic material (C/N, δ13C), we characterize the source material, succession of previous glaciations and deglaciations and the paleoecological conditions during ice-free episodes. Meteoric10Be data and grain features indicate that the ice sheet interacted with paleosols and eroded fresh bedrock, leading to mixing in these debris-rich ice layers. Our analysis also identifies four successive stages in NW Greenland: (1) initial preglacial conditions, (2) glacial advance 1, (3) glacial retreat and interglacial conditions and (4) glacial advance 2 (current ice-sheet development). C/N and δ13C data suggest that deglacial environments favored the development of tundra and taiga ecosystems. These two successive glacial fluctuations observed at NEEM are consistent with those identified from the Camp Century core basal sediments over the last 3 Ma. Further inland, GRIP and GISP2 summit sites have remained glaciated more continuously than the western margin, with less intense ice-substratum interactions than those observed at NEEM.