Shallow Processes Research Articles

The sedimentology, ichnology and invertebrate palaeontology of the late Silurian Downton Bone Bed

The Downton Bone Bed is a Konzentrat-Lagerstätte deposit located in the Welsh Borderlands, United Kingdom. The Downton Bone Bed is late Silurian, considered to be Přídolí in age, and occurs within the Platyschisma Shale Member of the Downton Castle Sandstone Formation. The bone bed is exposed at Weir Quarry (Herefordshire), which this study proposes should become established as the type locality for this stratigraphic horizon, due to the destruction of other localities and lack of access to other sites. As Weir Quarry is one of the last remaining exposures of this unit, the objective of this study is to qualitatively describe the sedimentology, ichnology, and invertebrate palaeontology of the bone bed, to enhance the regional understanding of the palaeogeography, depositional environments and depositional processes of the Welsh Borderlands during the late Silurian. Parasequence thickness and frequency, and sedimentary structures such as hummocky cross stratification, observed within the Downton Castle Sandstone Formation, have traditionally been explained to have formed by sea-level oscillations. These are interpreted to have formed entirely by nearshore to shoreface, shallow marine autogenic sedimentary processes, such as storm events (tempestites) and tidal scour. It is interpreted that formation of the Downton Bone Bed occurred as a by-product of these autogenic sedimentary processes, through winnowing and erosion during storm-driven scour and reworking. The low diversity ichno- and invertebrate fauna observed within the Downton Bone Bed is consistent with a nearshore depositional environment and is indicative of a stressed ecosystem due to fluctuating salinity and oxygen levels.

Rapid shifting of a deep magmatic source at Fagradalsfjall volcano, Iceland

Recent Icelandic rifting events have illuminated the roles of centralized crustal magma reservoirs and lateral magma transport1–4, important characteristics of mid-ocean ridge magmatism1,5. A consequence of such shallow crustal processing of magmas4,5 is the overprinting of signatures that trace the origin, evolution and transport of melts in the uppermost mantle and lowermost crust6,7. Here we present unique insights into processes occurring in this zone from integrated petrologic and geochemical studies of the 2021 Fagradalsfjall eruption on the Reykjanes Peninsula in Iceland. Geochemical analyses of basalts erupted during the first 50 days of the eruption, combined with associated gas emissions, reveal direct sourcing from a near-Moho magma storage zone. Geochemical proxies, which signify different mantle compositions and melting conditions, changed at a rate unparalleled for individual basaltic eruptions globally. Initially, the erupted lava was dominated by melts sourced from the shallowest mantle but over the following three weeks became increasingly dominated by magmas generated at a greater depth. This exceptionally rapid trend in erupted compositions provides an unprecedented temporal record of magma mixing that filters the mantle signal, consistent with processing in near-Moho melt lenses containing 107–108 m3 of basaltic magma. Exposing previously inaccessible parts of this key magma processing zone to near-real-time investigations provides new insights into the timescales and operational mode of basaltic magma systems.

Shallow and Deep Processing – An Integrated Cognitive Architecture for Foreign Language Learning

Abstract This article aims to present an architecture for the integration of shallow and deep processing techniques in the context of foreign language learning. Since the concept of “levels of processing” has been originally proposed by Craik and Lockhart in 1972, specialized literature has been constantly tried to replicate this cognitive-based architecture in order to find its applications to different domains. In an endeavor to fill the existing gap in the literature that attempts to “marry” language learning and psycholinguistics, this study offers an added cognitive neuroscience perspective to the understanding of shallow and deep processing encoding mechanisms that facilitate effective language learning. Starting from a brief theoretical report of the role of cognitive processes activated for the acquisition and learning of a foreign language, this study will propose some integrated pedagogical suggestions for promoting learners’ effective processing of L2 data in the classroom.

The Impact of Depth of Encoding on the Transfer of Test Enhanced Learning.

The mediator effectiveness hypothesis states the benefit of retrieval practice is a consequence of the activation of mediators linking cue and target items during review. Evidence has found that mediators are more effective at prompting recall of target words than words not associated with the original cue, a pattern that is larger following testing than restudy. The benefit of testing for unstudied cues at the final test is referred to as transfer of test-enhanced learning. The current study examined whether the activation of mediators is moderated by the depth of processing completed at encoding. During an initial study of weakly related word pairs (e.g., Mother-CHILD), participants completed an encoding task that directed deep, shallow, or no specific depth of processing. During review, participants restudied the pairs or attempted to recall the target given the original cue (e.g., Mother). On the final test, participants were presented with unstudied cues that were related to either the original cue (mediators, e.g., Father) or the target (target-related cues, e.g., Baby). The results found mediator generation during review to be greatly impaired by shallow processing tasks completed during encoding. In contrast, the effectiveness of target-related cues was not affected by depth of processing tasks.

The age and origin of the Balleny and Scott volcanic provinces, Ross Sea, Antarctica

The Balleny and Scott volcanic provinces are two isolated and remote volcanic occurrences located along the rifted margin of Western Antarctica in the Ross Sea. They include oceanic plateaus, seamounts, and volcanic islands. Although volcanic activity has been documented through remote sensing, there is no geochronological data based on radio-isotope methods constraining the duration of the volcanic activity in this region. As a consequence, hypotheses, based on the interpretation of chemical characteristics of the lavas, that suggested the volcanism to be either the result a deep mantle plume or a large-scale shallow melting anomaly have not yet been tested decisively.In order to tackle this issue, we have now dated lavas dredged from seamounts from both Balleny and Scott provinces using the 40Ar/39Ar technique. Our new age dates suggest that the Balleny and Scott volcanic provinces were active since at least 2.8 Ma and 2.4 Ma respectively. This suggests that these volcanic provinces were active simultaneously to the magmatism of the McMurdo volcanic group in the Western Antarctica Rift System that comprises Mount Erebus and Mount Melbourne stratovolcanoes. Our new dates suggest that there is no obvious age trend across the Balleny and Scott provinces. Combined with plate reconstructions since the end of Mesozoic, these data suggest that the Scott province does not coincide with any potential hot-spot track, thus negating a deep mantle plume origin for this volcanism. The case of the Balleny province is more ambiguous. The hypothesis of this province representing the end of the Tasman seamounts chain formed by a hot-spot track with decreasing-age trend is not confirmed by the new geochronological data, since available dates for the region are rather scarce and unreliable for the Tasman seamounts.Therefore, we suggest that the Balleny and Scott provinces originated from shallow mantle melting processes, a hypothesis that seems to be consistent with our re-interpretation of the SrNdPb isotope data of the lavas from the Balleny and Scott provinces. We argue that these provinces were derived from a heterogeneous subcontinental lithospheric mantle metasomatized during ancient subductions and which is now scattered in the shallow mantle of the southwest Pacific realm. This scattered mantle source component has been melting since the beginning of Cenozoic, and was then progressively depleted in the most fusible components. This new working model can explain the presence of several magmatic provinces in the southwest Pacific-Antarctica realm which show similar chemical signatures.

Multi-array analysis of volcano-seismic signals at Fogo and Brava, Cape Verde

Abstract. Seismic arrays provide tools for the localization of events without clear phases or events outside the network, where the station coverage prohibits classical localization techniques. Beam forming allows the determination of the direction (back azimuth) and horizontal (apparent) velocity of an incoming wavefront. Here we combine multiple arrays to retrieve event epicentres from the area of intersecting beams without the need to specify a velocity model. The analysis is performed in the time domain, which allows selecting a relatively narrow time window around the phase of interest while preserving frequency bandwidth. This technique is applied to earthquakes and hybrid events in the region of Fogo and Brava, two islands of the southern chain of the Cape Verde archipelago. The results show that the earthquakes mainly originate near Brava, whereas the hybrid events are located on Fogo. By multiple-event beam stacking we are able to further constrain the epicentral locations of the hybrid events in the northwestern part of the collapse scar of Fogo. In previous studies, these events were attributed to shallow hydrothermal processes. However, we obtain relatively high apparent velocities at the arrays, pointing to either deeper sources or complex ray paths. For a better understanding of possible errors of the multi-array analysis, we also compare slowness values obtained from the array analysis with those derived from earthquake locations from classical (local network) localizations. In general, the results agree well. Nevertheless, some systematic deviations of the array-derived back-azimuth and slowness values occur that can be quantified for certain event locations.

Non-Linear Dimensionality Reduction With a Variational Encoder Decoder to Understand Convective Processes in Climate Models.

Deep learning can accurately represent sub‐grid‐scale convective processes in climate models, learning from high resolution simulations. However, deep learning methods usually lack interpretability due to large internal dimensionality, resulting in reduced trustworthiness in these methods. Here, we use Variational Encoder Decoder structures (VED), a non‐linear dimensionality reduction technique, to learn and understand convective processes in an aquaplanet superparameterized climate model simulation, where deep convective processes are simulated explicitly. We show that similar to previous deep learning studies based on feed‐forward neural nets, the VED is capable of learning and accurately reproducing convective processes. In contrast to past work, we show this can be achieved by compressing the original information into only five latent nodes. As a result, the VED can be used to understand convective processes and delineate modes of convection through the exploration of its latent dimensions. A close investigation of the latent space enables the identification of different convective regimes: (a) stable conditions are clearly distinguished from deep convection with low outgoing longwave radiation and strong precipitation; (b) high optically thin cirrus‐like clouds are separated from low optically thick cumulus clouds; and (c) shallow convective processes are associated with large‐scale moisture content and surface diabatic heating. Our results demonstrate that VEDs can accurately represent convective processes in climate models, while enabling interpretability and better understanding of sub‐grid‐scale physical processes, paving the way to increasingly interpretable machine learning parameterizations with promising generative properties.

A Dual-Encoder-Single-Decoder Based Low-Dose CT Denoising Network.

Generative adversarial networks (GAN) have shown great potential for image quality improvement in low-dose CT (LDCT). In general, the shallow features of generator include more shallow visual information such as edges and texture, while the deep features of generator contain more deep semantic information such as organization structure. To improve the network's ability to categorically deal with different kinds of information, this paper proposes a new type of GAN with dual-encoder- single-decoder structure. In the structure of the generator, firstly, a pyramid non-local attention module in the main encoder channel is designed to improve the feature extraction effectiveness by enhancing the features with self-similarity; Secondly, another encoder with shallow feature processing module and deep feature processing module is proposed to improve the encoding capabilities of the generator; Finally, the final denoised CT image is generated by fusing main encoder's features, shallow visual features, and deep semantic features. The quality of the generated images is improved due to the use of feature complementation in the generator. In order to improve the adversarial training ability of discriminator, a hierarchical-split ResNet structure is proposed, which improves the feature's richness and reduces the feature's redundancy in discriminator. The experimental results show that compared with the traditional single-encoder- single-decoder based GAN, the proposed method performs better in both image quality and medical diagnostic acceptability. Code is available in https://github.com/hanzefang/DESDGAN.

Modest volcanic SO2 emissions from the Indonesian archipelago

Indonesia hosts the largest number of active volcanoes, several of which are renowned for climate-changing historical eruptions. This pedigree might suggest a substantial fraction of global volcanic sulfur emissions from Indonesia and are intrinsically driven by sulfur-rich magmas. However, a paucity of observations has hampered evaluation of these points—many volcanoes have hitherto not been subject to emissions measurements. Here we report new gas measurements from Indonesian volcanoes. The combined SO2 output amounts to 1.15 ± 0.48 Tg/yr. We estimate an additional time-averaged SO2 yield of 0.12-0.54 Tg/yr for explosive eruptions, indicating a total SO2 inventory of 1.27-1.69 Tg/yr for Indonesian. This is comparatively modest—individual volcanoes such as Etna have sustained higher fluxes. To understand this paradox, we compare the geodynamic, petrologic, magma dynamical and shallow magmatic-hydrothermal processes that influence the sulfur transfer to the atmosphere. Results reinforce the idea that sulfur-rich eruptions reflect long-term accumulation of volatiles in the reservoirs.

Does depth of processing affect temporal contiguity?

Memory tends to be better when items are processed for their meaning (deep processing) rather than their perceptual features (shallow processing). This levels of processing (LOP) effect is well-replicated and has been applied in many settings, but the mechanisms involved are still not well understood. The temporal contiguity effect (TCE), the finding that recalling one event often triggers recall of another event experienced nearby in time, also predicts memory performance. This effect has given rise to several competing theories with specific contiguity-generating mechanisms related to how items are processed. Therefore, studying how LOP and the TCE interact may shed light on the mechanisms underlying both effects. However, it is unknown how LOP and the TCE interact—various theories make differing predictions. In this preregistered study, we tested predictions of three theoretical explanations: accounts which assume temporal information is automatically encoded, accounts based on a trade-off between item and order information, and accounts which emphasize the importance of strategic control processes. Participants completed an immediate free recall task where they either engaged in deep processing, shallow processing, or no additional task while studying each word. Recall and the TCE were highest for no-task lists and greater for deep than shallow processing. Our results support theories which assume temporal associations are automatically encoded and those which emphasize strategic control processes. Both perspectives should be considered in theory development. These findings also suggest temporal information may contribute to better recall under deeper processing with implications for determining which situations benefit from deep processing.Supplementary InformationThe online version contains supplementary material available at 10.3758/s13423-022-02112-1.

Encoding tasks moderated the reward effect on brain activity during memory retrieval

Previous studies have explored the effects of retrieval reward and depth of processing in encoding on recognition, but it remains unclear whether and how reward and depth of processing during encoding influence recognition. We investigated the effect and neural mechanisms of encoding reward and processing depth on recognition using event-related potentials (ERPs) in this study. In the study phase, participants were asked to perform two encoding tasks: congruity-judgment (deep processing) and size-judgment (shallow processing) in reward and no-reward conditions. The test phases included object (item) and background (source) tests. The results of item retrieval showed that the accuracy of rewarded items was higher than that of unrewarded items only in the congruity-judgment task, and the reward effect (the average amplitudes in the reward condition were significantly more positive than those in the no-reward condition) in the 300–500 and 500–700 ms were greater in the congruity-judgment task than in the size-judgment task. The results of source retrieval showed that the accuracy of rewarded items was higher than that of unrewarded items, that the difference in the size-judgment task was significantly larger, and that the reward effect in the 300–500 and 500–700 ms were greater in the size-judgment task than in the congruity-judgment task. In conclusion, the encoding task moderated the reward effect in item and source memory.

Reflection residual statics estimated using a high-resolution neural network

Performing reflection statics corrections for improving seismic imaging is an important step in land and shallow marine data processing. The reflection residual statics solution is commonly derived from stack power maximization with hyperbolic assumptions that are valid for normal moveout (NMO) correction and stacking. We have developed a deep learning method for deriving surface-consistent reflection residual statics directly from common-shot, common-receiver, or common-midpoint gathers without velocity analysis and NMO correction. To mitigate overfitting and improve generalization ability, we augment training data by using only a few gathers but massive synthetic statics values. Because the magnitude of residual statics is often small, we apply a high-resolution neural network as a backbone to learn detailed features. We also design the head of the neural network to allow multiscale training and multiscale testing, which enables different acquisition geometries for prediction. Training with labeled samples from synthetic or real data has been tested with new real data as prediction inputs. In both cases, residual statics help improve stacked sections. If the NMO assumption is valid, our method produces results comparable to stack power maximization with much less computation time. If the NMO assumption is invalid, our method produces better results than stack power maximization.

The processing of ambiguous pronominal reference is sensitive to depth of processing

Previous studies on the processing of ambiguous pronominal reference have led to contradictory results: some suggested that ambiguity may hinder processing (Stewart, Holler, & Kidd, 2007), while others showed an ambiguity advantage (Grant, Sloggett, & Dillon, 2020) similar to what has been reported for structural ambiguities. This study provides a conceptual replication of Stewart et al. (2007, Experiment 1), to examine whether the discrepancy in earlier results is caused by the processing depth that participants engage in (cf. Swets, Desmet, Clifton, & Ferreira, 2008). We present the results from a word-by-word self-paced reading experiment with Dutch sentences that contained a personal pronoun in an embedded clause that was either ambiguous or disambiguated through gender features. Depth of processing of the embedded clause was manipulated through offline comprehension questions. The results showed that the difference in reading times for ambiguous versus unambiguous sentences depends on the processing depth: a significant ambiguity penalty was found under deep processing but not under shallow processing. No significant ambiguity advantage was found, regardless of processing depth. This replicates the results in Stewart et al. (2007) using a different methodology and a larger sample size for appropriate statistical power. These findings provide further evidence that ambiguous pronominal reference resolution is a flexible process, such that the way in which ambiguous sentences are processed depends on the depth of processing of the relevant information. Theoretical and methodological implications of these findings are discussed.

Embracing New Techniques in Deep Learning for Estimating Image Memorability

Various works have suggested that the memorability of an image is consistent across people, and thus can be treated as an intrinsic property of an image. Using computer vision models, we can make specific predictions about what people will remember or forget. While older work has used now-outdated deep learning architectures rooted in shallow visual processing to predict image memorability, innovations in the field have given us new techniques to apply to this problem. Here, we propose and evaluate five alternative deep learning models which exploit developments in the field from the last 5 years, largely the introduction of residual neural networks, which are intended to allow the model to use semantic information in the memorability estimation process. These new models were tested against the prior state of the art with a combined dataset built to optimize both within-category and across-category predictions. Our findings suggest that the key prior memorability network had overstated its generalizability and was overfit on its training set. Our new models outperform this prior model, leading us to conclude that residual networks outperform simpler convolutional neural networks in memorability regression. We make our new state-of-the-art model readily available to the research community, allowing memory researchers to make predictions about memorability on a wider range of images.

Application of Noise Attenuation on 2D Shallow Offshore Seismic Reflection Data: A Case Study from the Baltic Sea

Noise is always present in offshore seismic data, as there isn't a single method that could eliminate all the forms of noise. In this study, noise removal techniques were applied to attenuate different noises in 2D shallow-marine seismic data from the Baltic Sea area. Amplitude recovery should be applied before the noise attenuation stage as a preconditioning process for showing all noises in the deeper part of the seismic data. Frequency filters (notch filter and low-cut filter), frequency-wavenumber (FK) filter, and swell noise attenuation (Deswell) were applied as robust noise attenuation techniques. The method of directly modifying the amplitude spectrum of the seismic data is known as frequency filtering. A notch filter can be used to remove the harmonic noise of the power line harmonic noise (mono-frequency noise). A low-cut filter can be used to remove the low-frequency noises due to the influence of hydrostatic pressure variations. The linearly correlated events, such as tail-buoy and operational noise, were removed using the FK filter. Incoherent noise, such as swell noise, can be attenuated by swell noise attenuation (Deswell). The seismic results are displayed before and after the applied noise attenuation techniques to prove the validity of the applied filters. This study aimed to show the importance of shallow offshore seismic data processing in removing different types of noise, as it increases the value of data for seismic data interpretation and marine geohazard assessment.

High-temperature oxidation of proximal basaltic pyroclasts, 1886 Tarawera, New Zealand

Microlite crystallization in erupting basalt can occur in the conduit, in flight, or in situ after deposition. Distinguishing the products of primary versus secondary (post-fragmentation) crystallization can be challenging in near-vent environments, but is vital for interpreting shallow conduit conditions from pyroclast textures. Here, we examine pyroclasts of the 1886 basaltic Plinian eruption of Tarawera volcano, New Zealand, to assess the roles of primary versus secondary crystallization of microlites. Lapilli and ash were selected from (a) an ultra-proximal section (T47), < 100 m from vent, which is dominated by pyroclasts derived from the Plinian jet and column margin, and (b) a medial fall deposit section (T43), 2.5 km from the fissure, which contains products from the umbrella cloud. Strong contrasts in pyroclast groundmass crystallinities exist between sections, from near-holocrystalline (90–97% void-free corrected; VFC) in T47 pyroclasts to highly crystalline (77–83% VFC) in T43 pyroclasts. Subhedral-euhedral Fe–Ti microlites (< 3 μm) are ubiquitous and abundant in T47 pyroclasts, whereas they are virtually absent in T43 pyroclasts. Olivine is present in both T47 and T43 pyroclasts but evidence of its subsolidus transformation is only seen in T47 clasts, whereas in T43 clasts olivine is fresh. Near-complete crystallization of the groundmass and subsolidus transformation of olivine in the ultraproximal T47 clasts are evidence that post-depositional modification of primary pyroclast textures occurred in ultra-proximal environments at Tarawera as a response to high residual temperatures and oxidizing conditions, aided by short transport times of relatively coarse ejecta at high accumulation rates, likely supplemented by intense vent-derived heat. Ultra-proximal basaltic eruption products can continue to crystallize after deposition and are thus unlikely to be faithful indictors of shallow conduit magmatic processes; we recommend the use of medial and distal products instead.

Influence of the Cultivator Working Body Parameters on the Soil Crumbling Quality

It was shown that soil is a specific type of natural resource that can be renewed when reasonably cultivated by various tillage methods. It requires the availability of working bodies that meet the qualitative agrotechnical indicators of the technological process. (Research purpose) To improve the design of the working body meeting the qualitative agrotechnical indicators of the shallow tillage technological process. (Materials and methods) The main agrotechnical indicators of the cultivator's working body were determined. (Results and discussion) It was proved that the working bodies developed at the Agrarian Scientific Center "Donskoy" meet the agrotechnical requirements for the indicators of the shallow tillage technological process in terms of the crumbling quality: the content of fractions less than 25 millimeters in size in the treated soil layer is 81-92 percent in terms of quantitative composition. It was found out that the highest quality of seam crumbling during the shallow tillage process with a predominant content of fractions less than 25 millimeters in size (on average 90-91 percent) was made possible with the help of a working body with a 30-degree rack sharpening angle. It was determined that at the same time the share of erosive dust-like particles increased by 1.3-3.0 percent, which is unacceptable due to the agrotechnical requirements for shallow tillage. (Conclusions) Working bodies with a rack sharpening angle of 50 and 70 degrees and a paw opening angle of 94, 104 and 114 degrees compile with agrotechnical requirements for all qualitative indicators of the shallow tillage technological process and can be applied as anti-erosion ones when used in the combined units equipped with a device for mulching the surface soil layer on stubble backgrounds without preliminary processing.

Persistent shallow micro-seismicity at Llaima volcano, Chile, with implications for long-term monitoring

Identifying the source mechanisms of low-frequency earthquakes at ice-covered volcanoes can be challenging due to overlapping characteristics of glacially and magmatically derived seismicity. Here we present an analysis of two months of seismic data from Llaima volcano, Chile, recorded by the permanent monitoring network in 2019. We find over 2000 repeating low-frequency events split across 82 families, the largest of which contains over 200 events. Estimated locations for the largest families indicate shallow sources directly beneath or near the edge of glaciers around the summit vent. These low-frequency earthquakes are part of an annual cycle in activity at the volcano that is strongly correlated with variations in atmospheric temperature, leading us to conclude that meltwater from ice and snow strongly affects the seismic source mechanisms related to glacier dynamics and shallow volcanic processes. The results presented here should inform future assessments of eruptive potential at Llaima volcano, as well as other ice-covered volcanoes in Chile and worldwide.

The origin of early continental crust: New clues from coupling Ge/Si ratios with silicon isotopes

The recent discovery of heavy Si isotopic compositions in both high-Na Tonalite-Trondhjemite-Granodiorite (TTG) and High-K Granite-Monzonite-Syenite (GMS) suites of early continental crust requires that a notable seawater-derived silica-rich component had been added to their respective protoliths prior to melting. Here we use the Ge/Si ratio as a complementary tracer to δ30Si in order to delineate the exact role of modal quartz and silicified basalts from the Archean seafloor among the primary controls of the early appearance of felsic melts on Earth. We have approached the question by (1) specifying the Ge/Si signatures of various Archean and post-Archean rock types by compiling the Ge-SiO2 data stored within the GEOROC database; (2) coupling Ge/Si investigation to silicon isotopes on a large selection of silicified and unsilicified altered mafic and ultramafic greenstones, felsic volcanics, TTG and GMS granitoids and mineral separates from TTGs of the Barberton Greenstone Belt (BGB) of South Africa. The GEOROC compilation demonstrates that Archean TTGs and granites display much lower Ge/Si (1.15±0.10 and 1.13±0.11 μmol/mol, respectively) than post-Archean adakites, granites, tonalites and granodiorites (with average Ge/Si in the range of 1.64 to 1.85 μmol/mol). This result is corroborated by the Ge/Si ratios we report from BGB rocks that formed prior to Kaapvaal craton stabilisation, including 3.5 Ga Theespruit Formation felsic volcanic rocks, 3.5-3.2 Ga TTGs and 3.2-3.1 Ga GMSs, all of which exhibit low ratios (0.68±0.23; 0.92±0.17; 1.05±0.19 μmol/mol, respectively). Based on their low TTG-like Ge/Si, pre-cratonic GMSs are dismissed as being derived from melting of both TTG and metasedimentary sources. Instead, TTGs and GMSs originated from similar Ge-depleted sources. Low Ge/Si ratios, coupled to heavy Si isotopic signatures (−0.14‰<δ30Si<+0.27‰) are also a characteristic feature of mafic and ultramafic BGB greenstones subjected to low-temperature hydrothermal seafloor alteration, especially at the interface between silicified (0.2<Ge/Si<1.2 μmol/mol) and unsilicified (1.8<Ge/Si<3.1 μmol/mol) portions of the altered Archean seafloor. We infer that both Na-rich and K-rich Archean felsic melts are derived from a unique class of protoliths: Ge-depleted metabasalts containing a significant modal proportion of supracrustal quartz generated by the silicification of the Eo-Paleoarchean basaltic seafloors. The transition from Na-rich to K-rich felsic melts in the BGB is assumed to be connected to a gradual increase of potassium as a key element associated with seafloor silicification. In contrast, younger (3.07-2.69 Ga), post-cratonic BGB granites have higher Ge/Si (1.93±0.23 μmol/mol) and were generated through the reworking of a TTG-like basement, by incongruent melting of biotite and hornblende (1.79<Ge/Si<2.97 μmol/mol) leaving an oligoclase-rich (0.64<Ge/Si<0.72) residue. The Earth changed from a prevalent Ge-depleted felsic crust in early-middle Archean times to a widespread Ge-enriched post-Archean crust, which emphasizes the importance of late Archean changes in the Earth dynamics. In particular, our data suggest the likelihood of generating primitive felsic continents by rather shallow melting processes, without the need for inducing high pressures by subduction, as long as Archean ocean floors were silicified.

Indosinian Supergene Karsts in the Qaidam Basin, NW China: Evidence from Paleo‐tectonics, Paleoclimates and Paleo‐fluids

AbstractIn the Qaidam basin, Northwest China, karstic fissures and caves are developed in many places of the Lower Carboniferous Chengqianggou Fm. and Huaitoutala Fm. and the Upper Carboniferous Keluke Fm., in which are found typical karstic marks. Several suites of transgressive carbonate rocks formed during the Late Paleozoic epoch are preserved as reservoir properties in the Qaidam basin, with paleokarst‐related cave‐fractured diagenetic structures, which provided the basis for cave‐fractured development. Carbonate and clastic samples covering or infilling caves and fractures in Carboniferous strata were collected in the eastern Qaidam basin to focus on petrological and geochemical analyses of karst and infilling materials. Based on this study of the Caledonian diagenetic sequence, and bitumen‐infilled inclusion temperature and burial history, there existed a continuous atmospheric freshwater leaching process, which played a critical role in the construction of the Carboniferous supergene paleokarsts. Investigation and survey of the tectonic setting, plate paleolatitude and river development intensity also proves that there was abundant rainfall during the Indosinian period, after which the strata undergo a shallow burial process. Because Indosinian period is earlier than the key timing of hydrocarbon‐generation of the Carboniferous source rocks, this kind of karst reservoir has potential significance for oil‐gas resources in this area.