Post-depositional Alteration Research Articles

U–Pb zircon age and mineralogy of the St Georgen halloysite tuff shed light on the timing of the middle Badenian (mid-Langhian) transgression, ash dispersal and palaeoenvironmental conditions in the southern Vienna Basin, Austria

A geochronological study of the St Georgen tuff, favourably positioned within the Middle Miocene marine succession of the Vienna Basin, is essential to our understanding of the timing of the middle Badenian transgression. We report here new data on the separated zircon U–Pb ages/phase chemistry and the clay mineralogy of altered tuffs and then use these data to infer the provenance of the tephra and the palaeoenvironmental conditions. The ages of the tuff range between 15.78 ± 0.27 and 14.36 ± 0.31 Ma, with a weighted mean age of 14.59 ± 0.2 Ma. This defines the onset of the second Badenian transgression in the Central Paratethys region, which was the strongest transgression in the entire Miocene record of the Vienna Basin. The compositional and temporal relationships between the tuff and the neighbouring volcanism suggest that the Harsány eruption in the central Pannonian Basin is the most plausible source region for the tephra. West- and SW-directed tropospheric trade winds or easterlies were responsible for the transport of the Harsány tephra to its present location. The prevalence of halloysite, in addition to post-depositional alteration reactions (glass–smectite–halloysite and kaolinite–halloysite) suggest the fallout of tephra in a very shallow sea, which may have been affected by seasonal wetting and drying cycles at the onset of ash deposition. Supplementary material: Halloysite mineralogy, global geochemistry and U-Pb zircon age data are available at https://doi.org/10.6084/m9.figshare.c.6294944

Stromatolitic carbonates from the Middle Miocene of the western Pannonian Basin reflect trace metal availability in microbial habitats during the Badenian Salinity Crisis

Stromatolitic carbonates of the Middle Miocene Oberpullendorf Basin (Austria) provide a great opportunity to study the evolution of microbial habitats under extreme environmental changes during the Badenian (Langhian and early Serravallian) Salinity Crisis. We here present the first geochemical data for Badenian stromatolites and show in a combined approach using major, trace element, and C – O isotope compositions obtained in individual stromatolitic carbonate laminae that short-term variations of palaeo-environmental conditions within the Oberpullendorf Basin coincide with individual microbialite morphologies.The studied carbonates were affected by both detrital contamination and post-depositional alteration processes to different degrees. While fluid-mobile elements show clear evidence for post-depositional alteration processes, the rare earth element and yttrium (REY) as well as bio-essential element (Fe, Mn, Co, Zn, Mo, W) compositions of the carbonates remained unaffected. Stromatolitic carbonates that are devoid of detrital contamination (< 300 ppm Al) show typical shale-normalized seawater-like rare earth element and yttrium (REYSN) patterns with positive LaSN, GdSN anomalies, super-chondritic Y/Ho ratios, and heavy over light REYSN enrichments in the lower stromatolite units. These features suggest an open ocean seawater influenced depositional setting at the north-western margin of the Paratethys Sea. Stratigraphically upwards, pure stromatolitic carbonates show suppressed seawater-like REYSN signatures that argue for the development of a (semi)closed lagoon with restricted access to the open sea. Seawater-like REYSN patterns in the uppermost part resemble a reappearance of open marine environmental conditions. Interestingly, geochemical data of the upper part of the section contradict the ambient fossil record, showing the urge for future interdisciplinary approaches targeting the understanding and interplay of geochemistry, palaeontology, and geomicrobiology in modern and ancient microbial habitats. Enrichment factors of bio-essential trace elements that are either used as co-factors in metalloenzymes or metal-activated enzymes in biochemical reactions can be directly linked to the reconstructed environmental conditions: Sufficient element availability is ensured during marine conditions in the lower and uppermost stratigraphic sections; in contrast, continuous decreasing element availability of these elements is directly related to the temporary development of a (semi)closed lagoon.

Clumped isotopes of paired dolomite and calcite constraining alteration histories of ancient carbonate successions

Clumped isotope thermometry is an effective tool which allows to understand the post-depositional alteration, including recrystallization and solid-state reordering, of carbonate rocks. However, as both processes are controlled by elevated temperatures, it is frequently a challenge to distinguish between the relative importance of these two processes in deeply buried carbonate successions (depth > 5000 m or formation temperature > 150°C). In this paper, we have established a clumped isotopic framework of paired dolomite and calcite in order to decipher the relative influence of solid-state reordering versus recrystallization and consequently offer an improved understanding of the mechanisms involved. The material studied was derived from two deeply buried successions (Jurassic-Ediacaran) drilled in the central Sichuan Basin of South China. The calcite and dolomite minerals from these cores were separated using buffered acetic acid, and based on a two-component mixing model, the carbon, oxygen and clumped isotopic (Δ47) compositions of the two endmembers were determined. Based on an evaluation of solid-state reordering models, we conclude that micritic carbonates (dolomite and calcite) from the Jurassic to the Cambrian have been altered during burial by the geothermal gradient as determined by vitrinite reflectance. The Ediacaran micritic dolomite records a hydrothermal event in the transition between the Ediacaran and Cambrian. In contrast, the clumped isotopic temperature of Ediacaran saddle dolomites reflects subsequent hydrothermal activity during the Silurian and Devonian and shows compositional variations controlled by cooling rates. This study highlights the utility of the theoretical framework based on paired dolomite and calcite records to untangle overprinted signals of clumped isotopes in ancient carbonate rocks and gives insight to better understand the complexity of alteration processes in deeply burial carbonate successions.

Origin, transport, and diagenesis of tuffs in organic-rich lacustrine mudstone: An example from the lower part of the Middle–Late Triassic Chang7 Member, Ordos Basin (NW China)

Tuffs, produced by explosive volcanic eruptions, are widely deposited throughout the Precambrian to Cenozoic stratigraphic record. Due to rapid compaction and erosion, preservation and identification of tuffs in distal environments are challenging. However, in deep lacustrine environments, tuffs may be found well-preserved as interlayers in the organic-rich mudstone. This in turn provides valuable insights into the origin, transport, and diagenesis of volcanic material. In this study, we performed an integrated sedimentological, mineralogical, and geochemical analysis of seven tuff samples collected from the lower part of the Chang7 member of the Maquan section to infer on their provenance, transport mechanisms and post-depositional alterations. Immobile element ratios, tectonic discrimination diagrams, trace element distribution patterns, and negative whole-rock εNd(t) reveal an intermediate to felsic continental arc source of the studied tuff. Supported frameworks and ash aggregates in the tuff are in favor of the transport from pyroclastic density currents and ash plumes during multiple volcanic eruptions with various degrees of eruption volume and water content. Tuff diagenesis is characterized by dissolution of plagioclase, illitization of smectite, cementation of carbonate minerals, and presence of buddingtonite. Relatively ordered illite-smectite and low vitrinite reflectance from the adjacent organic-rich mudstone indicate diagenetic evolution of studied tuff have reached mesogenetic stage.

ASSESSING THE 14C MARINE RESERVOIR EFFECT IN ARCHAEOLOGICAL CONTEXTS: DATA FROM THE CABEÇUDA SHELL MOUND IN SOUTHERN BRAZIL

ABSTRACTPrehistoric shell mounds can be useful for the quantification of the radiocarbon marine reservoir effect (MRE) and, at the same time, knowledge about the MRE allows for the establishment of robust chronologies for these sites. This creates a loop in which the archaeological setting has a dual role: it is part of both the method and the application. Therefore, it is paramount to address these sites from both archaeological and environmental perspectives, investigating their origin and diagenesis in order to overcome biases caused by post-depositional alterations. In this study, samples of bone, charcoal and shell from a Late Holocene shell mound in Southern Brazil, the Sambaqui de Cabeçuda, were analyzed following a multidisciplinary approach to disentangle the complex relationships between archaeology and the environment. We performed X-ray diffraction, radiocarbon dating, stable isotopes (δ13C, δ18O, δ15N) and anthracology analyses as well as Bayesian Chronological Models and Isotope Mixing Models to assess the local MRE and to reconstruct the diet of Cabeçuda builders. Our results reveal a negative local correction for the MRE (ΔR = –263 ± 46 14C yr), expected for the lagoon next to the site, and diets with considerable intakes of marine proteins. We examine the implications of these results for the chronology of the site and discuss a series of complications when performing MRE studies using shell mound sites.

Climatic control on detrital sedimentation in the continental margin off Chennai, western Bay of Bengal – A 42 kyr record

While the peninsular Indian rivers deliver considerable loads of detrital sediments to the Bay of Bengal, changes in their depositional patterns with time are relatively less constrained compared to those deposited by Himalayan Rivers. Here, we present a 42 kyr record of detrital sedimentation, studied using whole-core logging, environmental magnetism, and major element geochemistry from the continental margin near Chennai, in the western Bay of Bengal. The variations of magnetic mineral properties and elemental concentrations of detrital sediments combined with the published oxygen isotope record, suggest monsoonal control on detrital input to the margin. Four major periods of changes in sedimentation are identified 1) the lower section of the core representing marine isotope stage (MIS) 3 (42–24 ka) shows varying detrital input, while magnetic minerals are affected by early diagenesis, 2) the period between 24 and 15 ka exhibits increased detrital input as a result of subaerial erosion in southern India during the last glacial sea-level lowstand, 3) the deglacial period from 15 to 10 ka is marked by decreased detrital input and increased productivity, and 4) the Holocene (10–2 ka) period experienced increased detrital input and chemical weathering in the source areas. Although the post-depositional alteration of magnetic minerals in MIS 3 limits the use of environmental magnetism, major elements and δ18O record the variations in paleomonsoon and detrital input during this period. The increased detrital input during 24–15 ka, despite the glacial conditions and intensified NE winter monsoon, indicates the increased subaerial erosion of exposed continental shelf during the sea-level lowstand owing to the proximity of the sampling site to the coast. The high magnetic susceptibility (χlf), chemical index of alteration (CIA), Al and Ti content beginning at ∼10 ka marks the intensification of SW summer monsoon through the Holocene.

Analysis of chemical weathering trends across three compositional dimensions: applications to modern and ancient mafic-rock weathering profiles

Chemical weathering indices (one-dimensional/1D index values) and accompanying ternary plots (two-dimensional/2D compositional space) facilitate quantitative comparison of whole-rock and mineral major-element data, and empirical chemical trends with predicted weathering vectors. However, data analysis in ternary plots is restricted by poles grouping elements that are hosted in different minerals or that are influenced selectively by later alteration (e.g., diagenesis/metasomatism). Tetrahedral plots (three-dimensional/3D compositional space) offer enhanced analytical utility of major-element data by shifting elements across four poles and (or) incorporating additional proxy elements. Tetrahedral space can better reveal combined effects on major-element compositions from independent mineralogical controls and post-depositional alteration via curvilinear trends that are otherwise simplified and linear in ternary space. This study focuses on mafic-rock weathering and first reviews applications and limitations of the 1D mafic index of alteration (MIA) and index of lateritization/bauxitization (IOL/IOB) that integrate into molar Al2O3–CaO*–Na2O–K2O–(FeO(T)/Fe2O3(T))–MgO and SiO2–Al2O3–Fe2O3(T) ternary compositional space, respectively. Analysis in tetrahedral space is then demonstrated with Phanerozoic weathering profile and Precambrian paleosol data in two plots of the molar Al2O3–CaO*–Na2O–K2O–(FeO(T)/Fe2O3(T))–MgO system (A–CN–K–FM and AF–CN–K–M plots) and one plot of the molar Al2O3–CaO*–Na2O–K2O–(FeO(T)/Fe2O3(T))–MgO–SiO2 system (A–L–F–S plot). Common chemical weathering indices are integrated into these 3D tetrahedral spaces or onto some of their 2D ternary faces. However, the tetrahedral compositional space is a key to (1) assessing integrative effects from labile element loss while accounting for the variable, redox-dependent behaviour of Fe, (2) better exposing, and correcting for, overprinting effects of diagenesis/metasomatism, and (3) tracking Si loss across all stages of chemical weathering.

δ18O of O2 in a Tibetan Ice Core Constrains Its Chronology to the Holocene

AbstractDating of the Tibetan ice cores is often challenging with great uncertainties. The significant age difference between the Guliya ice core (∼760 ka [thousand years]) and other Tibetan ice cores (Holocene origin) necessitates new approaches to resolve the inconsistency. Here, we present the atmospheric O2 δ18O (δ18Oatm) profile of the 216.6 m Chongce ice core. Our data suggest that post depositional alterations significantly changed δ18Oatm. We reject samples with melt layers, and reconstruct the δ18Oatm values with two extreme correction approaches: a minimum isotopic correction based on gas loss fractionation, and a maximum isotopic correction based on respiration fractionation. We consequently estimate the age range of the Chongce ice core to be 2.763–7.192 ka at the depth of 215.14 m, and ∼3–9.799 ka at the depth of 215.61 m. Our work provides an independent line of evidence for the Holocene origin of the Chongce ice core.

A new aspect of post-depositional alterations of lithic artefacts: the case of Medzhibozh Lower Palaeolithic assemblages

Flint artefacts have been identified in the materials of the Lower Palaeolithic sites of Medzhibozh, which have experienced the effects of post-depositional alterations (PDA). We suggest a particular combination of anthropogenic and natural effects on lithic artefacts. In the vicinity of Medzhibozh in the Khmelnytskyi region in the West of Ukraine, several multilayered stratified Lower Palaeolithic sites are currently known. Artefacts and accompanying fauna have been identified in subaerial buried soils, initial hydromorphic soils, and alluvial deposits. The ages of the artefact-bearing sediments are preliminarily estimated to be between 1.2 and 0.4 Ma. The lithic assemblages of all horizons contain technologically and typologically archaic artefacts, the industry being referred to as Mode 1. Various types of rock were used as raw materials, including locally occurring flints in the form of pebbles and flattened nodules. Flint products from many assemblages are characterized by rounding and sheen due to their extended exposure to water-permeable horizons. Most of the flint artefacts belonging to a particular assemblage are of a similar type of preservation. The assemblages also contain artefacts with surfaces of different states of preservation, suggesting that they are of different ages. These could either be products of reutilisation or natural alterations. However, more recent surfaces tend to be isolated and do not form a removals system. Moreover, most of the individual PDA pieces show a technological and morphological consistency of differently preserved surfaces, indicating that a force initiating detachment was applied simultaneously. We, therefore, consider artefacts showing a variate state of scar preservation to be PDA products. We assume we are dealing with a new aspect of post-depositional alterations of lithic artefacts. Natural alteration had subjected artefacts, which received internal damage to the structure earlier when processed by man. The bipolar-on-anvil technique was actively used at Medzhibozh. Many artefacts show signs of multiple attempts of knapping, accompanied by a change in the position of the segmenting item. Such signs are impact pits, visible cracks, and Herzian cones disturbing the rock's texture. We believe that the internal cracks that appeared in the area of the not detached spall contribute to the accelerated natural alteration of the item in this area and, ultimately, to the detachment of the flake. The probability of spalling depends on the intensity of the internal distress, the structure of the rock, the climatic conditions, and the post-depositional environment. The most responsible factor for the effect of the “time-delayed spalling” seems to be repeated cycles of temperature changes. A comprehensive study of post-depositional modifications of lithic artefacts is essential in studies of Lower Palaeolithic sites, particularly the technological Mode 1 (Oldowan). This is due to the highly heterogeneous raw materials used, the bipolar-on-anvil knapping technique, and a long period of post-discard behaviour of artefacts. Key words: Lower Palaeolithic, Medzhibozh, stone artefacts, post-depositional alterations, time-delayed spalling, taphonomy.

Major, trace element, and Nd isotopic compositions of banded iron formation and shales from the Sirsi shelf, Dharwar Craton, India: Implications for paleo-seawater chemistry, post-depositional alteration, and provenance

We report geochemical and Nd isotopic data for ca. 2.5 Ga old Banded Iron Formation (BIF) and associated clastic rocks which include red- and purple-coloured shales, mudstone, and sandstone from the Sirsi shelf of the Dharwar Craton. Based on lanthanide and non-lanthanide (yttrium) rare earth element (REE) concentration patterns, the Sirsi BIF samples are sub-divided into two groups. The group-1 BIF samples show low shale-normalized (SN) ratios of (La/Yb)SN (0.37–0.53) and (Pr/Yb)SN (0.29–0.34), and superchondritic Y/Ho (41–52); these samples also show insignificant negative Ce anomaly ((Ce/Ce*)SN = 0.83–1.22) compared to modern seawater and seawater-derived precipitates. In contrast, the group-2 BIF samples show strong light REE (LREE) enrichment with high (La/Yb)SN (1.73–18.48) and (Pr/Yb)SN (1.50–7.58), negative Ce anomaly ((Ce/Ce*)SN = 0.02–0.16), and mostly subchondritic Y/Ho (19–39). The group-1 BIF displays modern-day seawater-like REE patterns and their initial εNd at 2.5 Ga overlaps with contemporaneous global BIF, suggesting their precipitation from Archean seawater. The REE compositions of the group-2 Sirsi BIF cannot be explained by contribution of detritus and are best explained by alteration by a LREE-enriched and Ce-depleted fluid. The Sm–Nd isotopic compositions suggest that this alteration event took place approximately 0.5 Ga ago. The timing of the alteration event coincides with the Pan-African orogeny, which had regionally affected the Greater Dharwar Craton. Among the associated clastic rocks, red and purple shales show group-2-BIF-like REE signatures with negative Ce anomaly, and display high Chemical Index of Alteration (CIA) values (78–98), which suggest high degree of chemical alteration. The enrichment factors (calculated as [(Xelement/XTi)sample/(Xelement/XTi)reference-1] relative to the Archean Upper Continental Crust) of La, Pr, and Ce, in the Sirsi red shales further confirm post-depositional modifications by a LREE-enriched and Ce-depleted fluid, and indicate that negative Ce anomaly in ancient sedimentary rocks could be generated by post-depositional fluid overprint. Geochemical compositions of the Sirsi clastic rocks suggest a felsic provenance, while Nd isotopic compositions of selected shale samples indicate that these sediments were most likely derived from granitoids in the Goa-Dharwar Sector.

Decoupling of physical and chemical erosion in the Himalayas revealed by radiogenic Ca isotopes

Determining the rate of CO2 consumption associated with the Himalayan uplift is an essential prerequisite to understanding climate evolution throughout the Cenozoic. The riverine fluxes of dissolved calcium can be used to quantify uptakes of atmospheric CO2 by chemical weathering, but this approach requires deciphering the silicate-derived Ca flux from the generally dominant carbonate-derived Ca flux. Here we present high-precision radiogenic calcium (40Ca) analyses of bank sediments and dissolved loads from a network of rivers in central Nepal Himalaya and Bangladesh, to constrain the sources and relative contributions of carbonate and silicate-derived calcium to the dissolved loads of Himalayan rivers. Calcium isotope analyses were performed in multidynamic mode by thermal ionization mass spectrometry, yielding an external precision of ±0.4 ε-units (2 S.D.). Our results show that silicate catchments exposed in the Himalayan range have variably radiogenic ε40Ca compositions relative to seawater, ranging from +0.7 in the TSS to +14 in the LH. In contrast, sedimentary carbonates, including metamorphosed dolomites with variably radiogenic 87Sr/86Sr, exhibit uniform ε40Ca identical to modern seawater. The homogeneous ε40Ca composition of sedimentary carbonates confirms the relative resistance of radiogenic Ca signatures to post-depositional alteration and provides a robust baseline against which the contribution of silicate weathering to the riverine Ca flux can be evaluated. Dissolved load compositions of rivers draining LH catchments exhibit ε40Ca values ranging from 0 to +11, reflecting the relative contribution of unradiogenic carbonates and highly radiogenic metapelitic units of the LH sequence. HHC rivers show a more moderate range, from 0.5 to +2, reflecting the less radiogenic composition of HHC paragneisses. Himalayan front rivers draining all three lithotectonic units of the Himalayan range exhibit moderately radiogenic ε40Ca, ranging from +0.5 to +1.4 and overall similar to the average composition of the Ganga mainstream in Bangladesh. Using an approach combining conventional alkalinity budgets with isotopic mass balance calculations, we show that the dissolved silicate Ca flux of major Himalayan front rivers is primarily derived from the weathering of metapelitic rocks of the lesser Himalaya. In contrast, regions of the high range that generate the bulk of the detrital flux to the Gangetic floodplain represent a subordinate source of silicate-derived Ca. This result demonstrates that the chemical weathering of Ca-silicates in the Himalayan system is decoupled from physical erosion. The extensive erosional activity observed in the high range may thus play a limited role in promoting CO2 consumption and global cooling by silicate weathering.

The composition of heavy minerals of the sandy lands, Northeast China and their implications for tracing detrital sources.

Comprehending heavy mineral composition of the sandy land in Northeast China (NESL) is of great significance for interpreting generation, pathways, source and geochemistry of sediments in this area. To this end, the fine-grained (<63 μm) aeolian-fluvial sediments and loess deposits, which were taken from the Onqin Daga Sandy Land, the Horqin Sandy Land, the Hulun Buir Sandy Land and the Songnen Sandy Land, and from the downwind loess section (L1), respectively, were analyzed to construct the heavy mineral data set of NESL source and sink and to evaluate feasibility of the heavy mineral method in tracing the source of aeolian dust in Northeast China. Additionally, the <63 μm, 63–125 μm and 125–250 μm fractions of the fluvial sands from the different Balan River reaches having a same source, were analyzed to valuate the impact of the river transport-sedimentation process on the heavy mineral composition. The results show that the NESL shows moderate similarities in the heavy mineral composition, with ilmenite, epidote, zircon and amphibole as the primary minerals. In the source-to-sink system in the NESL, limited by sedimentary differentiation, post-deposition alteration and similar source material composition, the heavy mineral composition of the loess and that of sandy-land sources does not well correlate, indicating single heavy mineral method is incapable of unequivocally detecting loess sources when not considering the physical geographical conditions. The sediments in the different Balan River reaches show clear diversities and grain-size dependency in heavy minerals composition, indicating the river transport-deposition processes exert a clear control on the heavy-mineral composition in the sediment downstream. Both a wide grain-size window and more numbers of samples are needed to obtain a complete heavy-mineral picture in the source area.

A geochemical and mineral chemical assessment of sediment provenance and post-depositional alteration of auriferous conglomerates in the Singhbhum Craton

The Mesoarchean Dhosrapahar Group and the Phuljhari Formation in the Singhbhum Craton host so far subeconomic gold occurrences in metaconglomerate beds, similar to the world-famous conglomerate-hosted gold deposits of the Mesoarchean Witwatersrand Basin in South Africa. In the studied metaconglomerates from India, gold occurs as invisible inclusions in pyrite and as native gold grains associated with detrital chromite, quartz, zircon and metamorphic chlorite. Unlike most Archean paleoplacer deposits, the inferred detrital gold here appears unrelated to detrital pyrite, thereby highlighting the decoupling of the sources for the various placer components. To this end, we attempt to delineate the possible source rocks and assess the extent of post-depositional alteration to better understand the controls on gold accumulation in these conglomerates. Elevated Ni, Co and As, to a lesser extent V, Cu, and Se contents in Au-bearing inclusion-rich detrital pyrite are suggested to be derived from Paleoarchean banded cherts in the Gorumahisani Greenstone Belt. Relatively immobile trace element ratios, as recorded by the conglomerate matrix, reflect a predominantly felsic source with minor sediment input from mafic/ultramafic sources. Fractionated chondrite-normalized light rare earth and flat heavy rare earth element patterns, along with a distinct negative Eu-anomaly, obtained for the samples from the Phuljhari Formation point to a differentiated felsic source, most likely the c. 3.05 Ga K-rich Mayurbhanj Granite. In contrast, depleted heavy rare earth element patterns and a weak negative Eu-anomaly in the conglomerates from the Dhosrapahar Group indicate derivation from a felsic source that had experienced garnet fractionation, the obvious candidate being the underlying c. 3.37 Ga Rairangpur tonalite-trondhjemite-granodiorite (TTG) suite. Detrital rutile is rich in Cr and V, and Zr-in-rutile geothermometry yielded crystallization temperatures between ~800 and 1000 °C. The most likely source of this rutile are 3047±17 Ma gabbro-anorthosite bodies in the basement. Post-depositional modification of the conglomerates is dominantly manifested as greenschist-facies metamorphic overprint, and is reflected also by detrital chromite texture and chemistry, and the occurrence of secondary rutile, euhedral pyrite, and secondary gold with metamorphic chlorite. Our provenance interpretations do not indicate any likely point-source of detrital gold, suggestive of the entire Archean hinterland, irrespective of composition, as a potential source, as already indicated for the Mesoarchean auriferous conglomerates of the Witwatersrand Basin. This contention is further substantiated by the evidence presented here for moderate-to deep chemical weathering of the Archean hinterland of the Singhbhum Craton.

Rare earth elements and neodymium and strontium isotopic constraints on provenance switch and post-depositional alteration of fossiliferous Ediacaran and lowermost Cambrian strata from Arctic Norway

The Digermulen Peninsula in northeastern Finnmark, Arctic Norway, comprises one of the most complete Ediacaran–Cambrian transitions worldwide with a nearly continuous record of micro- and macrofossils from the interval of the diversification of complex life. Here, we report on the provenance and post-depositional alteration of argillaceous mudstones from the Digermulen Peninsula using rare earth elements and Sm–Nd and Rb–Sr isotopic systematics to provide an environmental context and better understand this important transition in Earth’s history. The studied sections comprise a mid-Ediacaran glacial–interglacial cycle, including the Nyborg Formation (ca. 590 Ma) and Mortensnes Formation (related to the ca. 580 Ma-old Gaskiers glaciation), and the Stáhpogieddi Formation (ca. 560–537 Ma), which yields Ediacara-type fossils in the Indreelva Member and contains the Ediacaran–Cambrian boundary interval in the Manndrapselva Member and basal part of the informal Lower Breidvika member (ca. 537–530 Ma). Three sample groups, (1) Nyborg and Mortensnes formations, (2) the lowermost five samples from the Indreelva Member and (3) the remaining samples from the Indreelva as well as from the Manndrapselva and Lower Breidvika members, can be distinguished, belonging to distinct depositional units. All samples have negative εNd(T) values (−6.00 to − 21.04) indicating a dominant input of terrigenous detritus with an old continental crust affinity. Significant shifts in Sm–Nd isotope values are related to changes in the sediment source, i.e. Svecofennian province vs Karelian province vs Svecofennian province plus in addition likely some juvenile (late Neoproterozoic volcanic) material, and probably reflect palaeotectonic reorganisation along the Iapetus-facing margin of Baltica. The combined Rb–Sr isotopic data of all samples yield an errorchron age of about 430 Ma reflecting the resetting of the Rb–Sr whole-rock isotope systems of the mudstones during the Scandian tectono-metamorphic event in the Gaissa Nappe Complex of Finnmark. Preservation of palaeopascichnids coincides with the sedimentation regimes of sample groups 2 and 3 while other Ediacara-type fossils, e.g. Aspidella-type and frondose forms, are limited to the sample group 3. Our results are similar to those of earlier studies from the East European Platform in suggesting oxic seafloor conditions during the late Ediacaran.

Geodynamic seawater-sediment porewater evolution of the east central Atlantic Paleogene ocean margin revealed by U-Pb dating of sedimentary phosphates

Emerging evidence suggests that U-Pb and Lu-Hf ages of sedimentary apatite group minerals are often younger than their biostratigraphic ages. However, U-Pb dating of exquisitely preserved carbonate fluorapatite (CFA) is rare. The Upper Cretaceous/Paleogene marine sedimentary rocks of the Moroccan High Atlas host phosphate-rich sediments bracketed by calcareous nannofossil Zones (NP4-NP9) of late Danian to Thanetian age. Here, we use a laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to decipher whether CFA minerals are suitable for U-Pb chronostratigraphy and whether they can reveal the sedimentary and seawater history from which they formed. U-Pb dating of the CFA grains yields ages of 42.9 ± 1.3 Ma (MSWD = 2.3) and 35.7 ± 2.8 Ma (MSWD = 1.3) from three distinct phosphate-rich beds, being &amp;gt;15 million years younger than the expected biostratigraphic age. Combined scanning electron microscopy, X-ray diffraction, and infrared spectroscopy analyses, associate the Mg-rich clay minerals sepiolite and palygorskite, with micro-CFA crystals, while LA-ICP-MS trace element, rare earth element, and yttrium content for primary CFA grains, collectively point to long-term early diagenetic adsorption from oxygenated seawater-dominated porewater fluids. Authigenic clay minerals display a seawater-like pattern, with negligible U concentrations suggesting limited clay mineral influence on U-Pb dating of the CFA crystals. Considering the absence of extensive post-depositional alteration, we propose that because of their large surface area, the µm-sized CFA crystallites facilitated real-time surface adsorption and desorption of elements and diffusion processes. These conditions generated long-term open system connection of sediments with overlying seawater, enabling continuous U-Pb exchange for 15–25 Myr after phosphate precipitation. The data suggest that system closure was potentially associated with sediment lithification and the Atlas orogeny, pointing to stable oxygenation of shallow marine waters along the eastern passive margin of the central Atlantic Ocean in the Paleogene.

Life Histories of Theban Tombs: Transdisciplinary Investigations of a Cluster of Rock-Cut Tombs at Sheikh ‘Abd al-Qurna Edited by Andrea Loprieno-Gnirs (AUC Press Archaeological Reports). Cairo: American University in Cairo Press 2021. Pp. 200. $79.95. ISBN 9781617979910 (paper).

<i>Life Histories of Theban Tombs: Transdisciplinary Investigations of a Cluster of Rock-Cut Tombs at Sheikh ‘Abd al-Qurna</i> Edited by Andrea Loprieno-Gnirs (AUC Press Archaeological Reports). Cairo: American University in Cairo Press 2021. Pp. 200. $79.95. ISBN 9781617979910 (paper).

Shallow-ocean and atmospheric redox signatures preserved in the ca. 1.88 Ga Sokoman iron formation, Labrador Trough, Canada

Detailed mineralogical and geochemical analysis of drill core samples from three previously unstudied localities (Sheps Lake, Lac Ritchie, Hayot Lake) of the ca. 1.88 Ga Sokoman continental margin-type iron formation (IF) was undertaken to better understand tectonically stable, shallow-marine environments and surface redox conditions during the late Paleoproterozoic. Suboxic (Fe-oxide-rich including paragenetically early hematite) and anoxic (Fe-silicate/carbonate-rich) mineral paragenetic pathways operated during IF deposition. Post-depositional alteration beyond late diagenesis/metamorphism was negligible, based on petrographic examination and analysis of bulk Fe(III)/Fe(II) ratios. High-precision trace element (TE) data of the Sokoman IF, in the context of new analyses of IF/iron ore reference materials (IOC-1, FeR-3, FeR-4), reveal similarities to contemporaneous continental margin-type IF. However, both the analytical approach and integration of chemostratigraphic variations in detrital element, rare earth element and yttrium (REE + Y), and other TE (Cr, V, U, Ni, Co, Zn) parameters with a previously published sequence-stratigraphic framework provides refined insight into the ca. 1.88 Ga marine surface environment. Specifically, this study dissects new details on the effects of base-level fluctuations, terrigenous input, basin redox stratification, and microbial activity that are collectively captured within the mineralogically and texturally complex units of the Sokoman IF.The REE + Y signature of the Sokoman IF is confirmed to have developed during deposition/early diagenesis through a comparison of geochemical signatures of chert (jasper) intraclasts and surrounding bulk IF. Furthermore, the Sokoman IF REE + Y data show patterns reminiscent of modern seawater (LREE depletion, small negative Ce anomalies, small positive La, Gd, and Y anomalies), but in some cases also strong positive Ce anomalies. Modelling of hyperbolic trends in Ce/Ce*-Pr/Pr* plots, preserved despite varying detrital admixtures, provides supporting evidence for interaction of dissolved REE + Y with marine Fe- and Mn-(oxyhydr)oxides, and quantitatively constrains the amount of detritus required to overprint Ce anomalies. The co-existence of positive and negative bulk-rock Ce anomalies, similar to those of other ca. 1.88 Ga IF, implies the presence of a shallow marine redoxcline at that time. However, the absence of any strong covariations between these Ce anomalies and (1) Mn- or Fe-enrichments, (2) Y anomalies, (3) LREE/HREE ratios, or (4) tetrad coefficients (τ) is best explained by the separation of a shallow Mn-redoxcline from a slightly deeper and more diffuse Fe-redoxcline inferred here to be controlled by cyanobacteria and photoferrotrophs, respectively.Combined plots of chemostratigraphic and TE/ΣFe vs. enrichment factors highlight variable input/scavenging of different TEs within the Sokoman IF; authigenic TE enrichment is more readily captured in deeper, suboxic to anoxic units relative to shallower, nearshore units where even low amounts of continental detritus can obscure low-magnitude, authigenic redox signatures. This approach confirms the low magnitude and limited range of authigenic enrichments in redox-sensitive and nutrient-type TEs in the Sokoman IF as being similar to those of other ca. 1.88 Ga IF localities, but reveals which depositional environments best capture specific authigenic signatures (e.g., Cr-V-U-P enrichments). Detritus-poor samples record highly fractionated Nb/Ta and Zr/Hf ratios (m/m; Nb/Ta: median 56.4, range 15.5–680; Zr/Hf: median 97.8, range 42.9–409) that exceed those observed in the modern hydrosphere, and are interpreted to reflect greater interaction of the “dissolved” load of these elements with abundant marine Fe/Mn colloids/fine-particulates. Detritus-rich samples have Nb/Ta and Zr/Hf ratios converging towards crustal values similar to those of shales within the Sokoman basin; both datasets support a model for a predominantly felsic (Archean plutonic/metamorphic rock) source.New inferences from our data on the Sokoman IF support a close link between atmosphere–ocean oxygenation and microbial ecosystems via continental weathering under an oxygen-poor atmosphere (aided locally by arid conditions). In this model, such conditions limited the terrestrial supply of redox-sensitive and nutrient-type elements (most notably P) into the ocean, largely restricting the spatial extent of primary productivity to the photic zone of coastal regions. These processes are consistent with collective evidence from other ca. 1.88 Ga IF deposits that suggest low-O2 and nutrient-limited Earth surface conditions relative to preceding time intervals in the Paleoproterozoic.

Data-Driven Predictive Modeling of Lithofacies and Fe In-Situ Grade in the Assen Fe Ore Deposit of the Transvaal Supergroup (South Africa) and Implications on the Genesis of Banded Iron Formations

The Assen Fe ore deposit is a banded iron formation (BIF)-hosted orebody, occurring in the Penge Formation of the Transvaal Supergroup, located 50 km northwest of Pretoria in South Africa. Most BIF-hosted Fe ore deposits have experienced post-depositional alteration including supergene enrichment of Fe and low-grade regional metamorphism. Unlike most of the known BIF-hosted Fe ore deposits, high-grade hematite (> 60% Fe) in the Assen Fe ore deposit is located along the lithological contacts with dolerite intrusions. Due to the variability in alteration levels, identifying the lithologies present within the various parts of the Assen Fe ore deposit, specifically within the weathering zone, is often challenging. To address this challenge, machine learning was applied to enable the automatic classification of rock types identified within the Assen Fe ore mine and to predict the in-situ Fe grade. This classification is based on geochemical analyses, as well as petrography and geological mapping. A total of 21 diamond core drill cores were sampled at 1 m intervals, covering all the lithofacies present at Assen mine. These were analyzed for major elements and oxides by means of X-ray fluorescence spectrometry. Numerous machine learning algorithms were trained, tested and cross-validated for automated lithofacies classification and prediction of in-situ Fe grade, namely (a) k-nearest neighbors, (b) elastic-net, (c) support vector machines (SVMs), (d) adaptive boosting, (e) random forest, (f) logistic regression, (g) Naïve Bayes, (h) artificial neural network (ANN) and (i) Gaussian process algorithms. Random forest, SVM and ANN classifiers yield high classification accuracy scores during model training, testing and cross-validation. For in-situ Fe grade prediction, the same algorithms also consistently yielded the best results. The predictability of in-situ Fe grade on a per-lithology basis, combined with the fact that CaO and SiO2 were the strongest predictors of Fe concentration, support the hypothesis that the process that led to Fe enrichment in the Assen Fe ore deposit is dominated by supergene processes. Moreover, we show that predictive modeling can be used to demonstrate that in this case, the main differentiator between the predictability of Fe concentration between different lithofacies lies in the strength of multivariate elemental associations between Fe and other oxides. Localized high-grade Fe ore along with lithological contacts with dolerite intrusion is indicative of intra-basinal fluid circulation from an already Fe-enriched hematite. These findings have a wider implication on lithofacies classification in weathered rocks and mobility of economic valuable elements such as Fe.

Origin and Provenance of Archean Keewaywin Formation clastic rocks at Sandy Lake, NW Ontario, Canada: Constraints on Archean weathering and depositional processes

Mesoarchean supracrustal sedimentary and volcanic rocks reflect provenance, depositional, tectonic and metamorphic conditions in Archean cratons. In the Sandy Lake Greenstone Belt within the North Caribou Terrane of the Superior Province, the Mesoarchean (∼2900 Ma) Keewaywin Formation is a metamorphosed siliciclastic deposit of mudstone, quartz-rich arenite, and volcaniclastic feldspathic litharenites and schists. Analysis of the petrography, mineralogy, and whole-rock and mineral geochemistry of sandstones and pelites indicate that precursor kaolinitic muds, dissolved iron, and quartz-rich sands were developed in soils by moderate to intense, long term weathering of felsic and mafic basement rocks in a tectonically stable environment. The detritus, half or more pelitic, was eroded from those soils and deposited as mud, some aluminous kaolinitic, some ferruginous, and as quartz-rich sands in a low relief setting. Volcaniclastic detritus from contemporaneous felsic volcanism was deposited with continually available kaolinitic mud and quartz- rich sediment. Kaolinitic aluminous muds were variously metasomatised at surface or near surface conditions with K, yielding muscovite-rich schists (as much as 95% muscovite); Ca and Na metasomatism yielded mm-scale, plagioclase rich laminae (up to 89% normative plagioclase) indicative of evaporite conditions. Ferruginous muds yielded chlorite rich schists (as much as 95 modal % chlorite). Reduced iron suggests the original sediments were deposited in an anoxic environment. New detrital zircon U/Pb age for a litharenite sample is 2930–2890 Ma is indicative of the age of an unconformity recognized in the North Caribou Core of the North Caribou Terrane. Detailed analysis of metamorphosed greenstone belt sedimentary rocks enables interpretation of their provenance, climate, weathering, depositional settings and post-depositional alteration. The shale compositions are particularly sensitive indicators of weathering conditions, hence climate. With age data, correlations between greenstone belts may allow recognition of regional large scale climate, weathering and sedimentation events.

Source Characteristics of the Carboniferous Ortokarnash Manganese Deposit in the Western Kunlun Mountains

The specific source of ancient sedimentary manganese (Mn) deposits is commonly complex. Here we use systematic major and trace element data with strontium (Sr) and neodymium (Nd) isotopic analyses of the Ortokarnash Mn(II) carbonate ores and associated carbonate rocks from the Upper Carboniferous Kalaatehe Formation (ca. 320 Ma) in order to constrain the Mn source. This formation consists of three members: the first member is a volcanic breccia limestone, the second member is a sandy limestone, and the third member is a black marlstone with the Mn(II) carbonate interlayers. Petrographic observations in combination with low Al2O3 (&lt;3.0 wt%) and Hf (&lt;0.40 ppm) contents and the lack of correlations between the Al2O3 and 87Sr/86Sr ratios as well as εNd(t) values demonstrate a negligible influence of terrigenous detrital contamination on both Sr and Nd isotopic compositions of the Mn(II) carbonate ores. The Sr isotopes of Mn(II) carbonate ores are most likely affected by post-depositional alteration, while Nd isotopes remain unaltered. The initial 87Sr/86Sr ratios in the associated carbonate rocks are likely the result of a mixture of the chemical components (i.e., seawater) and the Al-rich components (e.g., volcanoclastic material), while the detrital effects on Nd isotopes are negligible. In addition, both Sr and Nd isotopes in these non-mineralized wall rocks remained unchanged during post-depositional processes. The relatively low Th/Sc ratios and positive εNd(t) values suggest that the aluminosilicate fraction in the calcarenite and sandy limestone was mainly derived from the weathering of a depleted mafic source, representing the riverine input into the seawater. Given that the Mn(II) carbonate ores are characterized by negative εNd(t) values, these suggest that seafloor-vented hydrothermal fluids derived from interaction with the underlying old continental crust mainly contribute to the source of the Mn(II) carbonates.