Post-depositional Alteration Research Articles

Early Diagenetic Imprint on Temperature Proxies in Holocene Corals: A Case Study From French Polynesia

Coral-based reconstructions of sea surface temperatures (SSTs) using Sr/Ca, U/Ca and d18O are important tools for quantitative analysis of past climate variabilities. However, post-depositional alteration of coral aragonite, particularly early diagenesis, restrict the accuracy of calibrated proxies even on young corals. Considering the diagenetic effects, we present new Mid to Late Holocene SST reconstructions on well-dated (U/Th: ca. 70 yr to 5.4 ka) fossil Porites sp. collected from the Society Islands, French Polynesia. For few corals, quality pre-screening routines revealed the presence of secondary aragonite needles inside primary pore space, resulting in a mean increase in Sr/Ca ratios between 5-30%, in contrast to the massive skeletal parts. Characterized by a Sr/Ca above 10 mmol/mol, we interpret this value as the threshold between diagenetically altered and unaltered coral material. At a high-resolution, observed intra-skeletal variability of 5.4 to 9.9 mmol/mol probably reflects the physiological control of corals over their trace metal uptake, and individual variations controlled by CaCO3- precipitation rates. Overall, the Sr/Ca, U/Ca and d18O trends are well correlated, but we observed a significant offset up to +/- 7°C among the proxies on derived palaeo-SST estimates. It appears that the related alteration process tends to amplify temperature extremes, resulting in increased SST-U/Ca and SST-Sr/Ca gradients, and consequently their apparent temperature sensitivities. A relative SST reconstruction is still feasible by normalizing our records to their individual mean value defined as 1SST. This approach shows that 1SST records derived from different proxies agree with an amplitudinal variability of up to +/- 2°C with respect to their Holocene mean value. Higher 1SST values than the mean SSTs (Holocene warm periods) were recorded from ca. 1.8 to ca. 2.8 ka (Interval I), ca. 3.7 to 4.0 ka (Interval III) and before ca. 5 ka, while lower 1SST values (Holocene cold periods, Interval II and IV) were recorded in between. The ensuing SST periodicity of ca. 1.5 ka in the Society Islands record is in line with the solar activity reconstructed from 10Be and 14C production (Vonmoos et al., 2006), emphasizing the role of solar activity on climate variability during the Late Holocene.

Geochemistry and mineralogy of the Late Pleistocene loess-palaeosol sequence in Złota (near Sandomierz, Poland): Implications for weathering, sedimentary recycling and provenance

In order to determine the provenance of material, the Late Pleistocene loess-palaeosol sequence in Złota (Poland) was analysed geochemically (ICP-MS, ICP-ES) and mineralogically (QEMSCAN®), investigating simultaneously the weathering signature and effect of sedimentary sorting. A total number of 39 loess and soil samples were tested for the geochemical composition, and 5 for the mineralogy. The investigated samples, compared to upper continental crust, are enriched with elements such as Si, Zr, Hf, Nb, Ta, Th, U and rare earth elements (especially the heavy rare earth elements) related rather to felsic igneous rocks. The enrichment of only two mafic elements (Ti and Cr) can be explained by heavy minerals admixture. Loess in Złota is strongly mixed and homogenized during transportation and/or sedimentary recycling. For each sample the uniform shape of the chondrite-normalized rare earth elements patterns were found, characterized by steep light rare elements (LaN/SmN = 3.45 – 4.08, avg. 3.81), constant negative Eu anomalies (Eu/Eu* = 0.50–0.63; avg. 0.56) and distinctly flat heavy rare earth elements (GdN/YbN = 1.17–1.45, avg. 1.28). The material is not significantly affected by chemical weathering and the post-depositional modifications are not particularly important for the trace element composition both in loess and soils (e.g. Th/U = 3.1–4.65, avg. 3.64; Ce/Ce* = 0.96–1.09, avg. 1.01; LaN/YbN = 6.22–7.93, avg. 6.94). The research reveals that relatively “fresh”, felsic-related parent material has been well-mixed during transportation, but post-depositional alterations (related only to the pedogenesis) have not been intense enough to mask their chemical signature. The investigated material has been probably originated in Scandinavia (likely Norway), but deflated directly from the glacial drift covering the terrain of Poland.

Petrographical characteristics and post-depositional alteration affecting porosity and permeability of Oligocene sandstones, block 15-1/05, Cuu Long basin

Petrographical characteristics and post-depositional alteration studies of sandstones are the two important factors to reservoir rocks, which affect oil and gas storage and permeability of reservoir rocks. This study revealed petrographical characteristics, post-depositional alteration, and their influence on the porosity and permeability of Oligocene sandstones, including C, D, and E and F sequences, block 15-1/05, Cuu Long Basin. The results show that most of the sandstones were arkose, lithic arkose, and sporadically interbedded by feldspathic greywacke. The post-depositional alteration was progressively increasing following the burial depth from early diagenesis of sequence C, to intermediate diagenesis of sequence D and advanced diagenesis of sequence E and F. The post-depositional alterations significantly influenced on the porosity of the Oligocene sandstone were the cementation and mechanical compaction. They reduced the porosity and permeability of the sandstone. Additionally, authigenic clay minerals have a negative effect on permeability in which sandstones were rich illite and illite-smectite clay minerals, and the permeability tended to decrease stronger than others. Our results showed that the potential reservoir rocks of Oligocene sandstones, block 15-1/05 were sequence E and F sandstones that are in well grain sorting, well grain roundness shape, and contained a small number of cement, particularly the absence of illite and illite-smectite.

Detection of flow units of Quseir Formation (Lower Campanian) as a potential reservoir using experimental correlations of capillary pressure derived parameters, Gebel el-Silsila, Egypt

Sandstone surface samples were collected from Quseir Fm., a member of Nubian Sandstone at Gebel el-Silsila and subjected to petrographical investigation using polarizing microscope. X-Ray diffraction was carried out to determine mineralogy. Scanning electron microscope also was utilized for the morphological characterization of the rock components. This sandstone petrofacies is mainly quartz arenite. The depositional environment of the studied sandstones is fluvial to fluvial-marine, and they suffered post-depositional alterations. Alterations include diagenesis (clay infiltration and original detrital grains disintegration), weathering, deterioration phenomena as erosion, voids existence due to dissolution and absence of bound materials. Mineralogical characterization indicated that quartz is the main constituent of sandstone in addition to kaolinite clay mineral as a cementing material. Petrophysically, the studied samples subjected to porosity and permeability measurements. The measurements showed that the samples are highly porous and permeable. Permeability–porosity relation displayed two groups (A and B). Although porosity values are close to great extent, permeability is quite different for the two groups due to the different distributions of pore throats, perhaps as a result of uneven clay distribution within the two groups of samples. Many capillary pressure-derived parameters were calculated using experimental correlations. The results confirmed the presence of two flow groups, macroport group (group A) and megaport group (group B) that is the best in terms of reservoir quality. The two groups reflect the excellent properties for the studied potential reservoir.

Significance of granite-greenstone terranes in the formation of Witwatersrand-type gold mineralisation – A case study of the Neoarchaean Black Reef Formation, South Africa

Geological characterisation and U-Pb geochronology of detrital zircon grains has proven valuable in finding potential source terranes of siliciclastic sediments. Sediment transport and reworking from gold-bearing Archaean/Palaeoproterozoic granite-greenstone rocks are significant in the formation of conglomerate-hosted (Witwatersrand-type) gold deposits. We further support this link with an example from the Neoarchaean Black Reef Formation, in the Carletonville Goldfield, South Africa. In this study, the detrital and hydrothermal pyrite grains are petrographically distinguishable. Association of gold with solely detrital heavy minerals such as pyrite and not with hydrothermal minerals is similar to other Witwatersrand-type examples, which were formed during reducing atmospheric conditions from granite-greenstone protosource rocks. Textural evidence, together with gold occurrence data, support gradual homogenisation of primary sedimentological and mineralogical features due to post-depositional alteration. Uranium-Pb dating of the Black Reef Formation detrital zircon grains yields an age between 3149 and 3061 Ma, indicating that that the provenance of Black Reef Formation sediments could have been derived from a mixture of the hinterland proto granite-greenstone rocks, and from the reworking of Witwatersrand auriferous conglomerates. We further infer a favourable palaeo-drainage for primary sedimentary gold deposition from an isopach model and palaeocurrent data. The fluvial origin of the Black Reef Formation basal conglomerate provides an ideal setting for gold mobilisation from the sediment-source rocks. Notwithstanding the contribution of hydrothermal activity, sedimentary processes are interpreted to have been primary in the initial concentration of gold through reworking of the Witwatersrand auriferous conglomerates. This has important implications for the genesis of the younger Witwatersrand-type gold mineralisation. The fact that the Witwatersrand-type gold deposits, a target for exploration, occur spatially and temporally within the Archaean/Palaeoproterozoic geological record demonstrates that the Witwatersrand Basin is not unique in terms of composition and paragenesis, but is unique in terms of physical size and magnitude of its gold and uranium endowment.

Talking stones: Taphonomy of the lithic assemblage of Pirro Nord 13 (Apricena, FG, Italy). A new approach to the study of the post depositional alterations on lithics tools

The site of Pirro Nord 13 (PN 13) is located in a fissure that is a residual component of a wider karst system. Many faunal remains and lithic artefacts were accumulated by natural phenomena in the fissure. The degree of any potential post-depositional reworking of the archaeological and paleontological remains is important to be assessed as to evaluate the stratigraphic integrity of the site. In the proposed research, data about the post-depositional alteration (PdA), collected during the use-wear analysis, helps in the identification of the “taphonomic history” of the lithic assemblage and allow to understand if all the assemblage was exposed to the same environmental processes or if there are some difference among the various lithic elements. To recreate this history, we developed a new research protocol called overlapping method for the taphonomy of the lithic artefacts.

Geochronology and geochemistry of the fossil-flora-bearing Wuda Tuff in North China Craton and its tectonic implications

The Wuda Tuff Flora in the North China Craton (NCC) is a “vegetational Pompeii” and one of the most completely preserved marsh ecosystems of the late Paleozoic Northern Hemisphere. However, the precise age, geochemistry, and petrogenesis of the thick tuff containing this peat-forming flora have not been established. We conducted a comprehensive petrographic, mineralogical, whole-rock geochemical, and zircon UPb and HfO isotopic investigation of five samples from a vertical section through the tuff bed. Our results confirm that the material that buried the flora is a volcanic tuff bed that has undergone intensive post-deposition alteration. Whole-rock Al2O3/TiO2 ratios (65.8–103.5) and negative Eu/Eu* values (0.27–0.38) further suggest a felsic volcanic origin. Secondary ion mass spectrometry zircon UPb dating of the altered tuff bed yielded a weighted mean 206Pb/238U age of 295.9 ± 1.4 Ma (1σ, MSWD = 0.94, n = 39), constraining the age of the Wuda Tuff Flora to the earliest Permian. Depletions of Nb and Ta in both the whole-rock and zircon compositions indicate an arc-related setting for the volcanism. The positive εHf(t) values (0.0 to +5.0) and δ18O values (5.05‰–6.17‰) of zircon grains from the Wuda Tuff suggest a juvenile crust origin of the volcanic magma. Geochemical comparisons with contemporaneous igneous rocks suggest that the volcanic eruption that led to the formation of the Wuda Tuff and associated vegetational Pompeii was most probably caused by arc volcanism along the western margin of the NCC related to subduction of Paleo-Asian Ocean oceanic crust, implying that the Alxa Terrane had not yet amalgamated with the NCC during the early Permian.

The meaning of Pinus sylvestris-type charcoal taphonomic markers in Palaeolithic sites in NE Iberia

The aim of this work is to present the results of the classification of wood anatomy alterations on Pinus sylvestris-type charcoal remains and to analyse the quantitative results from Palaeolithic sites in NE Iberia. The high relative percentages of Pinus sylvestris-type charcoal fragments in these anthracological records has enabled us to systematically observe the taphonomic markers producing alterations in the wood cell structure. The analysis is based on a large collection of Pinus sylvestris-type charcoal remains from eight sites. All the assemblages present values above 70%. The observed alterations have been regrouped into biological alterations related to natural processes, combustion alterations, and post-depositional alterations. The results show differences in the relative presence of the markers, which are difficult to interpret in terms of environmental or cultural processes related to fuel exploitation. However, this study suggests that charcoal taphonomy is a useful tool for supporting previous interpretations of fire-management activities related to firewood gathering.

Clumped isotope geochemistry of carbonatites in the north-western Deccan igneous province: Aspects of evolution, post-depositional alteration and mineralisation

Carbonatites crystallise along a wide range of solidus temperatures and are commonly affected by post-magmatic textural re-equilibration and diagenesis. Further insights into the formation and modification of carbonatites are provided using carbon, oxygen and clumped isotope (Δ47) data of rocks from spatially associated Amba Dongar and Siriwasan alkaline complexes in the north-western Deccan igneous province, India. We derive apparent equilibrium blocking temperatures to help constrain the thermal evolution of the different rock types found within the alkaline complexes in a petrographic context. The apparent temperatures for the carbonatites are significantly low but are consistent with reports on other global carbonatites and model predictions. Rapidly cooled Oldoinyo Lengai natrocarbonatite yielded similar low temperatures, even in the absence of bulk isotopic alteration. The isotopic proxies and petrographic observations favour both isotopic exchange reactions and diagenesis in altering Δ47 values in calciocarbonatites. Diagenetic reactions are however strongly favoured, as secondary calcites in nephelinites and ferrocarbonatites record much lower temperatures than in the calciocarbonatites, highlighting the effect of fluids and diagenetic reactions in 13C18O bond ordering in carbonatites. Variations in the CO isotope data reveal the coupling of fractional crystallisation and post-magmatic fluid-rock interactions on bulk rock composition. After emplacement, the resetting of clumped isotope signatures in carbonatites is facilitated by post-magmatic processes in both open and closed systems.

Depositional environment and diagenesis of Late Cretaceous-Early Paleogene carbonates on the Benin flank, southwestern Nigeria

Carbonates of Late Cretaceous-Early Paleogene occur on the Benin Flank, southwestern Nigeria. Depositional environment and diagenetic history of the carbonates have not been extensively described. Outcrop sections and cores on Benin Flank were logged, sampled and analyzed in order to determine the depositional environment and diagenetic history of the carbonate rocks, through field-based studies, petrographic studies (employing both optical and scanning electron microscopy, SEM) and mineralogical studies using XRD (X-ray powder diffraction) analytical technique. Based on field studies, core description and petrographic investigations, two lithofacies have been identified, namely: wackestone (associated with mottled dolomite) and bioclastic wackestone-packstone. The Late Cretaceous-Early Paleogene limestone on the Benin Flank is interpreted to have been formed in low-energy environment along the shelf. Mineralogical composition of the analyzed limestone samples indicate that calcite was the original carbonate mineral in the limestone. The observed diagenetic fabrics of the analyzed carbonate samples suggest that the Late Cretaceous-Early Paleogene carbonates on the Benin Flank have been subjected to some post-depositional alterations, namely micritization, development of microspar, compaction, dissolution, dolomitization and calcite-filled microfractures.

Post-depositional alterations of terrestrial and marine finds of Roman ceramics from Crikvenica production centre (NE Adriatic, Croatia) – A contribution towards chemometric classification

• Different post-depositional alterations of Roman ceramics from Crikvenica's pottery production were identified. • The effects of seawater, anoxic marine sediments and desalination are recognizable by multi-analytical methods. • Multivariate statistics on FT-IR spectra represent a suitable method for a first screening and identification of representative samples of each sub-group. • Ceramics excavated from the site of the workshop (local ceramics) can be used as a reference group for future classification studies of Roman ceramics from the Crikvenica production centre. Different post-depositional conditions and after-recovery treatments may provoke pronounced physicochemical alterations that can make archaeological ceramics’ classification and comparison difficult even when its provenance is known. The present study describes the differences in the chemical composition verified on sherds of Roman ceramics produced in Crikvenica (NE Adriatic, Croatia). Inductive coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), as well as Fourier transform infrared spectroscopy (FT-IR) were applied to characterise the chemical composition and post-depositional alterations in ceramics recovered onshore and underwater. Chemometrical treatment of ICP-MS results of elemental composition and FT-IR absorption bands was performed using univariate and multivariate statistical tools. Principal component analysis (PCA) led to clear distinction of several clusters of examined samples, i.e. samples from the mainland, samples from the sea bottom, and desalinated samples. All sherds that have undergone desalination represent a distinct sub-group by means of changes in the ratio of accumulated chemical elements. It was established that accumulation of Ca, Mg and Na and formation of salts such as FeS, Mg 6 Al 2 CO 3 (OH) 16 ·4(H 2 O), and CaCO 3 reflect post-depositional environments and post-recovery treatments in terms of desalination and specific marine underwater conditions (oxic and anoxic). Our results outlined a pattern of post-depositional alterations that could be applied in upcoming classification studies. Roman ceramics produced in Crikvenica and excavated from the workshop possess attributes of a reference group, revealing that Ho, Ce, La, Lu, Tb, Gd, Eu and Sm are the elements least susceptible to post-depositional alterations.

Semiarid climate and hyposaline lake on early Mars inferred from reconstructed water chemistry at Gale

Salinity, pH, and redox states are fundamental properties that characterize natural waters. These properties of surface waters on early Mars reflect palaeoenvironments, and thus provide clues on the palaeoclimate and habitability. Here we constrain these properties of pore water within lacustrine sediments of Gale Crater, Mars, using smectite interlayer compositions. Regardless of formation conditions of smectite, the pore water that last interacted with the sediments was of Na-Cl type with mild salinity (~0.1–0.5 mol/kg) and circumneutral pH. To interpret this, multiple scenarios for post-depositional alterations are considered. The estimated Na-Cl concentrations would reflect hyposaline, early lakes developed in 104–106-year-long semiarid climates. Assuming that post-depositional sulfate-rich fluids interacted with the sediments, the redox disequilibria in secondary minerals suggest infiltration of oxidizing fluids into reducing sediments. Assuming no interactions, the redox disequilibria could have been generated by interactions of upwelling groundwater with oxidized sediments in early post-depositional stages.

Coprolites from a brackish ecosystem in the Pennsylvanian Joggins Formation, Nova Scotia, Canada and their palaeoecological implications

The Joggins Fossil Cliffs UNESCO World Heritage Site contains a wealth of terrestrial and aquatic fossils from the late Carboniferous (Pennsylvanian) Period. Fossils from the aquatic realm have historically been under-studied at Joggins, and the brackish ecosystem they represent is poorly understood globally. We used coprolites, which are abundant in the limestones of the Joggins Formation, to broaden our knowledge of the aquatic ecosystem and help reconstruct the food web. Coprolites were studied in hand sample and with standard transmitted-light microscopy to determine morphology and contents. Cathodoluminescence was used to examine coprolite microfabrics to interpret the degree of post-depositional alteration. Specimens range in size from 1 to 10 cm in length and were divided into six broad groups based on size and shape (in order of most to least abundant): small/equant; cylindrical; heteropolar macrospiral; large; irregular; and amphipolar spiral. Where possible, specimens were identified to ichnospecies. The host rock (limestone) and large quantities indicate that at least most, perhaps all, coprolites originated from fish, with some potentially originating from amphibious tetrapods. Bone fragments and/or coated grains were found in the majority of samples, however their relative abundance differs between morphotypes: small/equant coprolites contained the most coated grains and fewest bones and heteropolar macrospiral coprolites contained the most bones and almost no coated grains. Specific species identification of bones was not possible. The mineralogy of the coprolites is phosphatic, specifically carbonate fluorapatite, suggesting that the fish producing these coprolites were carnivorous and that there was a lack of coprolites generated by herbivores. Based on our findings of coprolite morphology and contents and what is currently known about the biotic environment, we introduce a hypothetical trophic pyramid, with rhizodonts at the apex producing the largest coprolites, and smaller fish species responsible for the small/equant-shaped coprolites at the secondary consumer level. This research provides a foundation for further studies on coprolites and other bromalites for a deeper understanding of brackish ecosystems where fish diversified further into fresh water during the Palaeozoic.

Effect of reduced pH on physiology and shell integrity of juvenile Haliotis iris (pāua) from New Zealand.

The New Zealand pāua or black footed abalone, Haliotis iris, is one of many mollusc species at potential risk from ocean acidification and warming. To investigate possible impacts, juvenile pāua (~24 mm shell length) were grown for 4 months in seawater pH/pCO2 conditions projected for 2100. End of century seawater projections (pHT 7.66/pCO2 ~1,000 μatm) were contrasted with local ambient conditions (pHT 8.00/pCO2 ~400 μatm) at two typical temperatures (13 and 15 °C). We used a combination of methods (morphometric, scanning electron microscopy, X-ray diffraction) to investigate effects on juvenile survival and growth, as well as shell mineralogy and integrity. Lowered pH did not affect survival, growth rate or condition, but animals grew significantly faster at the higher temperature. Juvenile pāua were able to biomineralise their inner nacreous aragonite layer and their outer prismatic calcite layer under end-of-century pH conditions, at both temperatures, and carbonate composition was not affected. There was some thickening of the nacre layer in the newly deposited shell with reduced pH and also at the higher temperature. Most obvious was post-depositional alteration of the shell under lowered pH: the prismatic calcite layer was thinner, and there was greater etching of the external shell surface; this dissolution was greater at the higher temperature. These results demonstrate the importance of even a small (2 °C) difference in temperature on growth and shell characteristics, and on modifying the effects at lowered pH. Projected CO2-related changes may affect shell quality of this iconic New Zealand mollusc through etching (dissolution) and thinning, with potential implications for resilience to physical stresses such as predation and wave action.

Tracing post-depositional processes and preservation of architectural materials and deposits in the semi-arid environment of southern Cyprus: A micromorphological approach

Post-depositional processes have a significant role in the analysis of archaeological deposits and materials as, through the synergic combination of their action, depositional contexts may be disturbed and modified, thus affecting archaeological analysis and interpretations. In this paper, the effects of post depositional alterations in semi-arid environment of southern Cyprus are presented using the example of Middle Bronze Age Erimi-Laonin tou Porakou. Combined stratigraphic and micromorphological analysis have indicated that a variety of natural and anthropogenic agents impacted on archaeological deposits after their accumulation and burial. The climatic fluctuations between more humid and dry periods, which occurred in the Eastern Mediterranean during the Holocene, have strongly impacted on processes of calcium carbonate dissolution, mobilisation and re-precipitation, contributing to development of calcitic pedofeatures, which are ubiquitous in all the depositional sequences at Erimi. Oscillation in water saturation as consequence of climatic variations also promoted processes of oxidation and organic decay in sediments analysed. Biogenic activity, including roots infiltration and microorganism actions, has produced diffused reworking and homogenisation of deposits, and – in combination with processes of compaction by trampling – has modified the horizontal distribution of artefacts and micro-materials embedded in sediments by introducing new patterns in their spatial distribution. On the contrary, post-depositional fire recorded in few buildings of the settlement contributed to the preservation of organic materials and deposits, especially mudbricks, through the backing and strengthening of their internal structure. The evaluation of agents of transformations in depositional sequences analysed was key for enhancing analysis and reconstruction of the macro and micro-history of the study-context. The present study, despite preliminary, provides a documented example of post-depositional features in support of further integrative geoarchaeological analysis in the southern region of Cyprus.

Archaeo-ceramic 2.0: investigating ancient ceramics using modern technological approaches

Since the first attempts to the petrographic analysis on pottery, a multitude of archaeometric studies have been carried out to solve important issues on provenance, production technology, dating, functionality and conservation state of ancient ceramic materials. According to the scientists’ background, the period when the investigations were done, destructive versus non-destructive analyses (also in relation to the unicity of the studied objects), as well as the purpose of the research, numerous analytical methods, typically used in various disciplines such as geology, physics and chemistry, have been applied. This contribution focuses on the use of innovative approaches to solve archaeological issues on pottery provenance, production technology and post-depositional alteration, reporting some examples of new methods applied to perform mineralogical, petrographic and chemical analyses. The traditional microscopic characterisation of ceramic pastes, for instance, can now be easily coupled with quantitative analysis of abundance and size distribution of their textural features, through the application of digital image analysis. This method, applied to images acquired by polarised transmitted light microscope, scanning electron microscope in back-scattered mode and microchemical mapping, can quantitatively and quickly describe production recipes, contributing to the definition of the pottery production choices and changes over time. As for the mineralogical analysis, the possibility of simultaneously processing numerous X-ray diffraction patterns by statistical tools, such as cluster analysis on raw data, is another example of a modern way to analyse ancient pottery and interpreting, with an objective comparison method, possible differences or analogies in the production recipes and/or firing technology of groups of ceramics. Moreover, modern microdiffraction techniques, both using conventional (X-ray tubes) and advanced sources (synchrotron radiation), also coupled with micro-Raman spectroscopy, allow the determination of mineralogical composition of small-sized phases both in ceramic bodies and slips/glazes, supplying important information on the production technology and possible post-deposition processes. As for the provenance issue, especially on fine ware, constraining relationships between far away communities/societies can now be evaluated not only on the basis of conventional bulk chemistry but also by isotope analysis. These new methodological approaches give rise to the new concept of archaeometric analysis on ancient ceramic materials that can be defined as “archaeo-ceramic 2.0”.

A geochemical evaluation of the genetic relationship between Ouachita Mountains Paleozoic rocks and the Mississippi Valley-type mineralization in the southern Ozark Region, USA

The Pb and/or Zn deposits of the Tri-State and the Northern Arkansas Mississippi Valley-type (MVT) districts in the southern Ozark Region, located north of the Arkoma Basin and the Ouachita fold-thrust belt in North America, are genetically connected to the Pennsylvanian-Permian Ouachita orogeny. This orogenic event triggered a south-north topographic gradient flow of basinal brines, leaching metal-rich sediments en route. To establish whether the organic-rich shales of the Ouachita Mountains and the Ozark Region provided metals during the mineralization event, the Pb isotope compositions of the ores (sphalerite) have been compared to those of the sedimentary rocks (whole-rocks and leachates) of the Ouachita Mountains (Cambrian Collier Shale, Early Ordovician Mazarn Shale, Middle Ordovician Womble Shale, Late Ordovician Polk Creek Shale, Mississippian Stanley Shale, and Pennsylvanian Jackfork Sandstone) and the Ozark Plateau (Devonian/Mississippian Chattanooga Shale and Mississippian Fayetteville Shale). The Pb isotope compositions of ores in both districts show a broad and a linear range of isotopic values. This trend suggests mixing of Pb from two distinct end-member components. One end-member must be highly radiogenic, with Pb isotope ratios equal to or higher than the highest noticed value for ores. The other end-member must be less radiogenic, with Pb isotope ratios equal to or lower than the lowest value recorded for ores. The Pb isotope compositions of the analyzed whole rocks and leachates indicate that the best candidates for contributing the less radiogenic Pb to the ores are the sandstone members of the upper and middle Jackfork Sandstone and the lowermost section of the Chattanooga Shale.The depositional environment of the potential source rocks and their ability to sequester metals has been appraised using redox sensitive trace elements (Cd, Co, Cr, Cu, Mo, Ni, Pb, Th, U, V, Zn). Paleoredox proxies (e.g., enrichment factors of redox sensitive trace elements and V/V + Ni ratios) indicate deposition of the analyzed rocks mainly under anoxic and euxinic conditions, which are favorable for metal enrichment. However, oxic-suboxic conditions are also indicated by other geochemical proxies, suggesting a possible first order control of primary production that has resulted in an increased flux of organic matter. A disagreement in the redox conditions yielded by the paleoredox proxies has been noticed in several cases. This inconsistent behavior may be associated with several factors, including redox potential, pH, sulfur abundance, chemical availability, TOC values, and sediment provenance. The authigenic flux is the only fraction needed to unravel past redox conditions; however, the analyzed bulk sediments consist of authigenic, biogenic, and siliciclastic components, which may also have led to the observed disagreement in the redox conditions. Additional factors such as the basin geometry, the rates of sediment accumulation, the post depositional alteration processes associated with diagenesis and low-grade metamorphism during the Ouachita orogeny might have overprinted the depositional signatures.

Late Pleistocene Chronology of Sediments From the Yermak Plateau and Uncertainty in Dating Based on Geomagnetic Excursions

AbstractThe Yermak Plateau is one of several regions in the Arctic Ocean where paleomagnetism yields controversial results. Despite low sedimentation rates, late Pleistocene paleomagnetic excursions have been reconstructed from many cores in the region, but they are characterized by considerably longer durations when compared to established ones. Self‐reversal during maghemitization of (titano)magnetite has been proposed as one explanation. Rock magnetic, 14C dating, sedimentological and stable isotope (δ18O) methods were employed to three new sediment cores to put paleomagnetic results in the context of the regional stratigraphy and chronology. Coherence of lithological parameters and δ18O variations validated the ratio of anhysteretic remanent susceptibility to bulk magnetic susceptibility (κARM/κ) as a parameter for cross‐core correlation. As established by earlier studies, we use the link between glacial/interglacial cycles and κARM/κ to tune our records to a global δ18O stack, which provides age models that are independent of radicarbon ages and paleomagnetic data. Our results show that zones of negative magnetic inclination are asynchronous across the plateau. Alternating field demagnetization data revealed that negative inclinations are contained in a medium‐high‐coercivity (>25–35 mT) magnetic phase that may be the result of postdepositional alteration of (titano)magnetite. We note a positive relationship between water depth and excursion duration, which may be driven by changes in water mass circulation on glacial/interglacial timescales.

Evaluating the effects of diagenesis on the δ13C and δ18O compositions of carbonates in a mud-rich depositional environment: A case study from the Midland Basin, USA

Marine carbonate δ13C and δ18O values are commonly utilized as indicators of past climate and are thought to record primary seawater conditions. However, these minerals can be altered following deposition and burial, overprinting the primary signal to reflect subsurface conditions. Whereas the influence of post-depositional alteration is widely understood in carbonate-dominated sediments (Swart, 2015), the influence has not been as widely studied in mud-rich settings. This study analyzed the carbonate δ13C and δ18O values from carbonate intervals in the Late Pennsylvanian Wolfcamp D shale unit in the Midland Basin to evaluate the impact of post-depositional alteration on the interpretation of stable isotope compositions of carbonates in a mud-rich depositional environment. Here we show the importance of using multiple indicators to fully evaluate the impact of post-depositional alteration on the interpretation of the marine carbonate δ13C and δ18O values in mud-rich environments. The extent of diagenesis in the Wolfcamp D was primarily controlled by fluctuations in benthic redox chemistry, which controlled sulfate reduction and the preservation of organic matter. These processes influenced the extent of carbonate cementation in the bacterial sulfate reduction zone and during deep-burial, making the carbonates unreliable for intra-basinal correlations. The results of this study were incorporated into a conceptual model that can likely be applied as a framework for other studies in mud-rich depositional environments.

Organic signatures in Pleistocene cherts from Lake Magadi (Kenya) – implications for early Earth hydrothermal deposits

Abstract. Organic matter in Archean hydrothermal cherts may provide an important archive for molecular traces of the earliest life on Earth. The geobiological interpretation of this archive, however, requires a sound understanding of organic matter preservation and alteration in hydrothermal systems. Here we report on organic matter (including molecular biosignatures) enclosed in hydrothermally influenced cherts of the Pleistocene Lake Magadi (Kenya; High Magadi Beds and Green Beds). The Magadi cherts contain low organic carbon (< 0.4 wt %) that occurs in the form of finely dispersed clots, layers, or encapsulated within microscopic carbonate rhombs. Both extractable (bitumen) and non-extractable organic matter (kerogen) were analyzed. The bitumens contain immature “biolipids” like glycerol mono- and diethers (e.g., archaeol and extended archaeol), fatty acids, and alcohols indicative for, inter alia, thermophilic cyanobacteria, sulfate reducers, and haloarchaea. However, co-occurring “geolipids” such as n-alkanes, hopanes, and polycyclic aromatic hydrocarbons (PAHs) indicate that a fraction of the bitumen has been thermally altered to early or peak oil window maturity. This more mature fraction likely originated from defunctionalization of dissolved organic matter and/or hydrothermal petroleum formation at places of higher thermal flux. Like the bitumens, the kerogens also show variations in thermal maturities, which can partly be explained by admixture of thermally pre-altered macromolecules. However, findings of archaea-derived isoprenoid moieties (C20 and C25 chains) in kerogen pyrolysates indicate rapid sequestration of some archaeal lipids into kerogen while hydrothermal alteration was active. We posit that such early sequestration may enhance the resistance of molecular biosignatures against in situ hydrothermal and post-depositional alteration. Furthermore, the co-occurrence of organic matter with different thermal maturities in the Lake Magadi cherts suggests that similar findings in Archean hydrothermal deposits could partly reflect original environmental conditions and not exclusively post-depositional overprint or contamination. Our results support the view that kerogen in Archean hydrothermal cherts may contain important information on early life. Our study also highlights the suitability of Lake Magadi as an analog system for hydrothermal chert environments on the Archean Earth.