Post-depositional Processes Research Articles

New insights into shepherds’ activities: Multi-proxy approach applied to fumier deposits from the north of Iberian Peninsula

The use of caves and/or rock shelters as a refugee for herds was a common practice in the Mediterranean area since the early Neolithic. Shepherds move the herds to pastures in higher altitudes, away from the settlements where the animals graze nearby the shelters where livestock is regrouped. Dung and residues produced during the occupation of these shelters were generally heaped and burnt to clean the site and prepare it for new usage, forming a characteristic deposit known as fumier. Fumier deposits are a great source of information to understand past herding practices including the animal's stabled, the management of the residues, the landscape, and the use of plants. In addition, they can provide a better understanding of site formation and postdepositional processes of archaeological sediments in caves and shelters including the impact of fire. Specific microarchaeological analyses are usually utilized to study fumier deposits. Here we present a multi-proxy approach that includes phytoliths, spherulites, ash pseudomorphs, and FTIR analyses, applied to the Neolithic levels of San Cristóbal, Los Husos II (Álava, Spain) and El Mirador (Burgos, Spain), located in the upper Ebro and Duero basins. This area in the north of the Iberian Peninsula constitutes a strategic location for the spread of the productive economy to the north, west, and interior of the Iberian Peninsula. Except for the lower Level XIII of San Cristóbal, where domestic activities were documented, our results showed similar behavior for the three studied sites, with no significant differences neither in terms of plant presence nor in the formation processes of the deposits. Up to seven and two possible complete stabling episodes were identified: three and two possible ones in San Cristóbal, and four in Los Husos II. Among the plants identified grasses of the C3 Pooideae subfamily were dominant, probably representing local grasses. The combining presence of thermally altered phytoliths, spherulites, and ash pseudomorphs points that the stabling episodes burned for several hours with temperatures ranging between 500 °C and 700 °C.

The new frontier of microstructural impurity research in polar ice

AbstractDeciphering the localisation of solid and dissolved impurities on the micron-scale in glacial ice remains a challenge, but is critical to understand the integrity of ice core records and internal deformation. Here we report on the state-of-the-art in microstructural impurity research by highlighting recent progress in bringing together cryo-Raman spectroscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We show the potential of both methods and discuss possibilities to improve inter-method approaches aiming for a more holistic understanding of the evolution of impurity localisation throughout the ice column, including post-depositional processes. In this framework, we elaborate on future research priorities such as LA-ICP-MS imaging on firn samples and integrating a large cryo-cell with imaging capabilities.

Utilizing Geochemical Analysis for Provenance Determination in Sedimentary Systems

Geochemical and isotopic analysis represents a potent approach for investigating sediment provenance, a crucial aspect for paleogeographic reconstructions, understanding the evolution of the Earth's crust and characterizing geology in areas that are no longer exposed at the surface. The sediment composition can be influenced by multiple processes including the weathering and erosion of the source rock, mixing with other sediment sources, grain size sorting and hydrodynamic processes such as water or wind transport. Additionally, climate and topography can impact the sediment composition by affecting the rate and nature of weathering and erosion as well as the transport and deposition of sediments. Moreover, post-depositional processes such as diagenesis and metamorphism have the potential to modify the original isotopic and geochemical composition of sediments. The objective of the present investigation is to conduct an overview of diverse geochemical proxies frequently employed in the evaluation of provenance as well as to assess their capabilities and limitations in estimating the origin of sediment. Additionally, statistical methodologies might be advantageous for sediment source determination, although it is crucial to acknowledge their restrictions and to merge them with traditional geochemical and isotopic techniques. A multidisciplinary strategy that encompasses several techniques and methodologies is probable to yield the most sturdy and dependable outcomes for identifying sediment provenance.

Influences of bulk structure of Calcarenitic rocks on water storage and transfer in order to assess durability and climate change impact.

The main purpose of this study is to understand influences of bulk structure of geomaterials on water storage and transfer, in order to assess durability and climate change impact on the UNESCO world heritage sites in Alexandria, Egypt. This study deals with the responses of water towards the physiochemical and physicomechanical behaviours of Calcarenitic rocks, that are utilized in Greek and Roman monuments at Alexandria. Many vulnerable archaeological sites [2.3–2.5 m above mean sea level (M.S.L)] are identified at Alexandria, specifically at the Eastern Harbor, El-Shatby the Greek necropolis and Moustafa Kamil Roman tombs and addition to the Roman Catacombs of Kom El-Shoqafa which excavated with deepth − 18 m from the land surface. These UNESCO heritage sites suffer climate change impact (heavy rains and sea water rising) as well as multiple geoenvironmental and geophysical hazards. In this study a general outline of the various tests, surveys and analyses is presented, highlighting the most important issues related to the durability and climate change impact. This paper represents the comprehensive in-situ, field and laboratory surveys and tests undertaken in these outstanding world heritage sites. The field testing program comprises various geotechnical and geophysical field and laboratory tests aiming to define the physical, mechanical and dynamic properties of the hard soils/soft rock materials of the archaeological sites where these outstanding monuments are excavated and constracted. By analysis of linear correlations, some essential mechanisms should be underlined, which may connect the macrostructure to the microstructure of the geomaterial. A systematic method of analysis clearly appeared and emphasized the role of the bulk structure (i.e. grain size, grain contact, specific area, pore shape and microporosity) on petrophysical and petromechanical behavior of rock materials. The study revealed that the petrophysical and geomechanical properties of Calcarenitic rocks are influenced by size, shape, packing of grains, porosity, cement and matrix content, all controlled strongly by depositional fabric and postdepositional processes. The accurate analysis of the physiochemical and physicomechanical behaviours of Calcarenitic rocks that are utilized in Greek and Roman monuments at Alexandria allowed us to define the pathology of these monuments and to estimate the durability, climate change impact and ultimate geostatic loads that they can survive under their present geoenvironmental conditions.

Rare-Earth Minerals in Kaolin Ore, Mine Tailings, and Sands – Central Georgia, Upper Coastal Plain

AbstractThe total concentrations of rare-earth elements (REE) in the mined kaolin (0.02–0.06 wt.%), kaolin mine tailings (0.03–1.9 wt.%), and the kaolin-associated Marion Member sand lithology (0.03–4.6 wt.%) opened questions regarding the modes of occurrence of the REE and the role(s) of chemical weathering and secondary processes to explain the presence of REE in these materials. The REE were hosted primarily by phosphate minerals (monazite, xenotime) based on mineralogic analyses (scanning electron microscopy, X-ray diffraction). Enrichments in the light rare-earth elements (LREE: La–Gd) and the high correlation coefficient values were noted between P and the total REE concentrations (r2 = 0.99) for the sands and the mine tailings. Lower correlation coefficient values were noted between total REE concentrations and Zr (r2 = 0.31). The coarse fractions of the mined kaolins were enriched in the heavy rare-earth elements (HREE: Y, Tb–Lu) relative to the kaolin-associated sand lithologies. The REE inventory cannot be explained solely by mineral inheritance within the mined kaolins. Lower correlation coefficient values between P and total REE, positive Eu/Eu* anomalies, and the presence of xenotime overgrowths on zircon showed the importance of the role of chemical weathering of the detrital minerals during post-depositional processes (such as diagenesis) leading to redistributed and fractionated REE within the mined kaolin. The possibility of adsorption of the REE to kaolin mineral surfaces in the fine fraction of the mined kaolins remains open and permits further study to characterize fully the multi-modal fractionation of REE possible in the Georgia kaolin deposits.

Late Quaternary eolian dune-field mobilization and stabilization near the Laurentide Ice Sheet limit, New Jersey Pine Barrens, eastern USA

Well-preserved stabilized dune fields are widespread in the New Jersey Pine Barrens, northern Atlantic Coastal Plain, USA. In this area, which was unglaciated throughout the Quaternary, quartz-rich Miocene–Pleistocene age fluvial and marginal marine sands provided source sediments for eolian mobilization. Parabolic and transverse dunes within fluvial source-bordering dune fields in small-river watersheds migrated to the east-southeast (110–125°) over unconsolidated sands and gravels. The short eolian transport distance of most dune-field sand in the presence of moderately to sub-rounded quartz grains with low sphericity indicates eolian abrasion and dune-sand fashioning occurred within a short duration of transport. Although the absolute duration of eolian transport remains unknown, dune stabilization occurred about 23–17.5 ka, with a weighted mean of 19.5 ± 0.5 ka from six dated dunes. Dune stabilization coincided with northward retreat of the Laurentide Ice Sheet from its maximum position at ∼41.500° N (∼100 km north of the study area), to ∼41.375°N (∼200 km north). The well-preserved dune morphology and narrowly constrained ages suggest rapid dune stabilization. Dune-forming katabatic winds from the WNW declined abruptly with northward migration of the ice sheet, accompanied by climatic amelioration and stabilization by vegetation. A short-lived period of eolian mobilization may have been associated with a temporary increase in sand availability from adjacent fluvially derived sediments. Post-depositional processes included soil eluviation, with dissolution features and breakage blocks on quartz grains signifying long-term in-situ soil weathering.

Cross-mended ceramic sherds as a proxy for depositional processes at two Late Archaic shell rings in coastal Georgia

ABSTRACT Characterizing the depositional and temporal nature of sediments lends insight into the construction of monuments and midden accumulation. Identifying discrete deposits at Late Archaic shell rings can be challenging due to the seemingly homogenous nature of shell deposits. Data from cross-mended artifacts can help identify surfaces and determine whether deposits are contemporaneous. We present cross-mend results from the St. Catherines Shell Ring and the Sapelo Island Shell Ring complex. In both cases, we observed vertical distance between cross-mends suggesting that detailed spatial control of artifacts and cross-mend analysis can be used to understand the nature of anthropogenic deposition and postdepositional processes.

Antimony isotope fractionation and the key controls in the soil profiles of an antimony smelting area

The controlling factors of antimony migration and transformation in soil profiles are still unclear. Antimony isotopes might be a useful tool to trace it. In this paper, antimony isotopic compositions of plant and smelter-derived samples, and two soil profiles were measured for the first time. The δ123Sb values of the surface and bottom layers of the two soil profiles varied in 0.23‰−1.19‰ and 0.58‰−0.66‰, respectively, while δ123Sb of the smelter-derived samples varied in 0.29‰−0.38‰. The results show that the antimony isotopic compositions in the soil profiles are affected by post-depositional biogeochemical processes. The enrichment and loss of light isotopes at 0–10 cm and 10–40 cm layers of the contrasted soil profile may be controlled by plant uptake process. The loss and enrichment of heavy isotopes in the 0–10 cm and 10–25 cm layers of the antimony from smelting source in the polluted soil profile may be controlled by the adsorption process, while the enrichment of light isotopes in the 25–80 cm layer may be related to the reductive dissolution process. The conclusion emphasizes that the promotion of the Sb isotope fractionation mechanism will play a crucial role in understanding the migration and transformation behaviors of Sb in soil systems.

Pleniglacial dynamics in an oceanic central European loess landscape

Abstract. Loess–palaeosol sequences (LPSs) of the oceanic-influenced European loess belt underwent frequent post-depositional processes induced by surface runoff or periglacial processes. The interpretation of such atypical LPSs is not straightforward, and they cannot be easily used for regional to continental correlations. Within the last few years, however, such sequences gained increased attention, as they are valuable archives for regional landscape dynamics. In this study, the Siersdorf LPS was analysed using a multi-proxy approach using sedimentological, geochemical, and spectrophotometric methods combined with luminescence dating and tentative malacological tests to unravel Pleniglacial dynamics of the Lower Rhine Embayment. A marshy wetland environment for the late Middle Pleniglacial to the early Upper Pleniglacial was shown by colour reflectance and grain size distribution. Age inversions from luminescence dating paired with geochemical and sedimentological data reveal long-lasting erosional processes during the early Upper Pleniglacial, which were constrained to a relatively small catchment with short transport ranges. The upper sequence shows typical marker horizons for the study area and indicate harsh, cold-arid conditions for the late Upper Pleniglacial. In comparison with other terrestrial archives, the Siersdorf LPS shows that the Lower Rhine Embayment was more diverse than previously assumed, regarding not only its geomorphological settings and related processes but also its ecosystems and environments.

Distinguishing geobiological signatures from organic matter in the Ediacaran chert nodules

The Doushantuo Formation (DST) in South China preserved significant bio- and chemostratigraphic records that are critical to the study of Ediacaran life and environment. Abundant microfossils and organic matter (Corg) were preserved in the DST nodules. We investigated the mineralogy, petrography and the structural characteristics of Corg in three different types (Type I: silica-rich; Type II: pyrite-rich; Type III: barite-rich) of DST nodules, in order to better understand the geobiological processes during the formation of nodules and the taphonomic details of Corg preserved therein. According to Raman spectral parameters, the Corg experienced low-grade metamorphism, and represents obvious heterogeneities of structural order and H:C in the center, rim and the host rock, respectively. Furthermore, the structural characteristics of Corg show distinct bias to various mineral matrices (silica, pyrite, goethite and barite) where they were preserved, probably due to differential mineral-organic interactions during the formation of different nodules. These mm- and μm-scale structural heterogeneities of Corg reflect the nature of precursor organism, which were subsequently affected by various post-depositional processes. Our study proposes the possible formation processes of three types of DST nodule, suggests the geobiological influences on the Corg preserved therein, and sheds light on the geobiological signatures from the DST using Raman-based approaches.

Pests or prey? Micromammal species within an ancient anthropic environment at the Norse settlement site of Tuquoy (Westray, Orkney).

Micromammals, like rodents and shrews, adapt rapidly to take advantage of new food sources, habitats and ecological niches, frequently thriving in anthropogenic environments. Their remains, often retrieved during archaeological investigations, can be a valuable source of information about the past environmental conditions as well as interspecies interactions and human activity. However, the research on such finds rarely covers multiple approaches, often relying on single species or data type (e.g. identification/information for proxy studies). Here we investigate micromammal remains from the Norse and medieval (AD tenth-fourteenth centuries) archaeological site at Tuquoy, Orkney, to elucidate the relationships between micromammals, humans and other species present using a variety of data. Four micromammal species were identified, and their species dynamics as well as relationships with humans could be inferred by tracking changes in spatial and temporal location of remains, from their taphonomic history and by age estimation for individual animals. A larger, predatory assemblage was also identified, with species composition differing from that in the rest of the archaeological assemblage, and possibly therefore representing small mammal species composition in the wild. The assemblage was probably deposited by a diurnal raptor, though identification to species is not certain due to post-depositional processes.

Simulated diagenesis of the iron-silica precipitates in banded iron formations

Abstract Banded iron formations (BIF) are chemically precipitated sediments that can record Archean ocean geochemistry. BIFs are laminated silica- and iron-rich deposits that host a range of iron(II, III) minerals, including hematite, magnetite, siderite, greenalite, minnesotaite, and stilpnomelane. This diverse mineralogical assemblage reflects secondary mineralization reactions due to diagenesis and/or post-depositional alteration. While petrographic observations of BIFs sparingly contain the iron silicate greenalite, recent evidence of greenalite nanoparticles preserved in early-mineralizing BIF chert suggest this mineral was a primary phase in BIF progenitor sediments. Therefore, it is critical to investigate the formation and alteration of greenalite to constrain the Archean ocean environment and help unravel post-depositional processes. To examine how iron silicates precipitate and then crystallize and/or transform during diagenesis, we simulated these two processes under Archean ocean conditions. We first precipitated a poorly ordered Fe-rich serpentine with subsidiary ferrihydrite at neutral pH by performing in situ Fe(II) oxidation experiments at 25 °C in the presence of silica. Subjected to simulated diagenesis at 80 °C, the rudimentary Fe-phyllosilicate transformed into a crystalline phyllosilicate characterized as 30% cronstedtite and 70% greenalite accompanied by magnetite and persistent ferrihydrite. At temperatures ≤150 °C, we continued to observe ferrihydrite, increased magnetite formation, and elevated incorporation of Mg into the phyllosilicate as it further recrystallized into Mg-greenalite. Our findings demonstrate a possible formation mechanism of early silicates through partial Fe(II) oxidation and support petrographic observations that magnetite likely mineralizes during diagenesis. Additionally, we suggest that Mg contents in BIF iron phyllosilicates could serve as a tracer for diagenesis, with Mg signaling phyllosilicate-fluid interactions at elevated temperatures. Ultimately, our experiments help reveal how initial iron-silica coprecipitates are altered during diagenesis, providing novel insights into the interpretation of greenalite and magnetite in ancient BIF assemblages.

Exploring the role of snow metamorphism on the isotopic composition of the surface snow at EastGRIP

Abstract. Stable water isotopes from polar ice cores are invaluable high-resolution climate proxy records. Recent studies have aimed to improve our understanding of how the climate signal is stored in the stable water isotope record by addressing the influence of post-depositional processes on the isotopic composition of surface snow. In this study, the relationship between surface snow metamorphism and water isotopes during precipitation-free periods is explored using measurements of snow-specific surface area (SSA). Continuous daily SSA measurements from the East Greenland Ice Core Project site (EastGRIP) during the summer seasons of 2017, 2018 and 2019 are used to develop an empirical decay model to describe events of rapid decrease in SSA linked to snow metamorphism. We find that SSA decay during precipitation-free periods at the EastGRIP site is best described by the exponential equation SSA(t)=(SSA0-22)⋅e-αt+22, and has a dependency on wind speed. The relationship between surface snow SSA and snow isotopic composition is primarily explored using empirical orthogonal function analysis. A coherence between SSA and deuterium excess is apparent during 2017 and 2019, suggesting that processes driving change in SSA also influence snow deuterium excess. By contrast, 2018 was characterised by a covariance between SSA and δ18O highlighting the inter-annual variability in surface regimes. Moreover, we observed changes in isotopic composition consistent with fractionation effects associated with sublimation and vapour diffusion during periods of rapid decrease in SSA. Our findings support recent studies which provide evidence of isotopic fractionation during sublimation, and show that snow deuterium excess is modified during snow metamorphism.

Local variability of a taiga snow cover due to vegetation and microtopography

ABSTRACT The taiga snow cover accumulates in relatively stable and windless weather. This should produce a uniform snow cover with continuous, laterally homogeneous stratigraphy and snow properties when the snow is deposited on a level, smooth substrate. However, such substrates are rare, and local variations in vegetation and ground surface topography alter the structure of the snow cover and produce irregular snow layers. In this study, we investigated the effects of vegetation, microtopography, and microclimatic variability on the taiga snow near Fairbanks, Alaska. Through the winter of 2019–2020, in situ measurements were made at three locations with distinctly different local microtopographic features and radically different (but typical) vegetation. One site was an open grassy field, the second a mature spruce forest, and the third a birch forest located on thermokarst terrain with steep microrelief where ice wedges had degraded. Widely different canopy interception processes proved to have the strongest impact on the resulting snow cover heterogeneity by altering the initial deposition, with surface microtopography having the second strongest influence through postdepositional processes. In this article, we suggest a conceptual framework for understanding and modeling taiga snow variability in terms of the vegetation and microtopography that created it.

Site formation process of the Dadong Paleolithic site in Jilin province, China: A geoarchaeological approach

The Dadong site, located in the Changbaishan region of Jilin province, China, is an important Upper Paleolithic site characterized by its large distribution area and abundant stone artifacts. This study presents a geoarchaeological study of a newly excavated area of this site. Soil micromorphology, particle size analysis, and pH measurements were used to reconstruct the site formation process from around 60 ka to the present. Additionally, this study examines the impact of volcanic eruptions and local geomorphology on ancient human adaptation in this area. This paper also investigates the effects of post-depositional processes, particularly freeze-thaw, on the distribution of lithics. This study is the first soil micromorphological study of a Paleolithic site in Northeast China, and is important for studying the changes of the Pleistocene environment in this region as well as the development of microblade technology in Northeast Asia.

The (in)visibility of lost wax casting moulds in the archaeological record: observations from an archaeological experiment

Apart from mould and core remnants attached to metal objects from the Chalcolithic Southern Levant (ca. 4500–3800 BCE), production remains of early lost wax casting are seemingly invisible in the archaeological record. An experiment using reproduced casting moulds was performed to simulate the Chalcolithic processes to investigate whether the moulds might have deteriorated to an unrecognisable state after the casting process. Results from previous studies on mould remains attached to metal objects were complemented with ethnographical accounts for the experimental set-up. The moulds were prepared from local clays already in use during the Chalcolithic. Petrography, simple alteration tests, and general technological considerations reveal that preservation may be poor but is still visible to the naked eye. The preservation of multi-layered fragments is crucial, despite their friable or brittle state after casting. A further, previously unmentioned, clear indication of lost wax casting moulds may be the combination of vegetal temper with mineral/rock temper in their clay paste. This composition derives purely from technological considerations and is independent of depositional and post-depositional processes.

Geophysical Survey and Changes in the Use of the Cultural Landscape

The results of detailed and large-scale geophysical measurements in archaeology have been steadily increasing for years. The growth in measured data has also increased the need for processing and interpretation; in archaeology, this primarily means the archaeological interpretation of the measured data. However, the information contained in geophysical data includes a substantial volume or area of data of varying size or thickness of some different natural or modern anthropogenic origin (beyond archaeological interest). Like the archaeological situations themselves, these must also be identified and demarcated. The presented article consists of a wide range of case studies in which the result of a specific applied geophysical method includes both the desired interpretations of archaeological features and the differentiation and warning of other anomalies, the origin of which may or may not be unambiguous or related to the post-deposition processes of archaeological features. The purpose of selecting several different examples of results in our paper is to point out that there are many more consequences of anthropogenic activity hidden beneath the surface of the terrain of the contemporary cultural landscape than just those that archaeologists have in their viewfinder. Other anomalies in specific environments may be of natural origin or related to various geological, pedological or hydrological changes in a site’s natural environment. This should be dealt with by the alternative differentiation of anomalies of various probable origins; the interpretive descriptions, diagrams or maps should not just focus strictly on the anticipated subsurface relics of the archaeological features and situations, as these are not there alone.

Carbonate texture identification using multi-layer perceptron neural network

Abstract This study proposes an approach to identifying carbonate rocks depositional texture utilizing more than 1250 thin-section photos as a data set from shallow-water carbonates of the Kruja zone (Albania). The proposed technique uses a numerical methodology based on digitized images of thin sections. According to the Dunham classification, it permits the user to recognize the carbonate texture automatically. The carbonate rock should not be affected by post-depositional processes. This technique uses as input gray-scale digital images taken from thin sections. Image processing can extract (as output) a set of 22 numerical parameters measured in the entire image, including white areas representing calcite cement. A multi-layer perceptron (MLP) neural network inputs these numerical parameters and produces, as output, the estimated depositional texture. We utilized 654 images of thin sections to evaluate this technique to train the neural network. We used 348 pictures taken from the same data set and 250 from the Burizana section (Kruja zone, Albania) to test the technique. The proposed method has shown 90.5 and 91.3% accuracy in identifying the automatically depositional texture using digitized images on the 348 and 250 test sets. Based on the good results obtained, this technique can be extending not only in identifying carbonate rocks texture but in every type of rock, getting fast and correct results. Python was used as a computer programming language for image processing and displaying images.

Isotopic and spectral signatures unravel the sources, preservation and degradation of sedimentary organic matter in the Dongzhai Harbor mangrove estuary, southern China

Sedimentary organic matter (SOM) production and transformation processes have a prominent role in regulating biogenic element cycling in mangrove ecosystems. However, aquaculture biodeposits can substantially affect source apportionments and biogeochemical processes of SOM within mangroves. In this study, natural stable isotopes (13C and 15N), spectral techniques, and hydrogeochemical analysis were used to elucidate the sources, preservation, and degradation of SOM across the Dongzhai Harbor (DZH) estuary, southern China, where aquaculture systems are extensive. Along the estuarine salinity gradient, sediment and bottom water samples were collected from the upper fluvial mangrove zones to the lower marine endmember. Results indicated that source contributions to SOM along the estuarine gradient were spatially heterogeneous. Aquaculture-derived organic matter was the largest contributor to the Sanjiang River (44.29 %) in the upper estuary, and terrestrial sources from C3 mangrove forests were highest in the Yanfeng East River (62.87 %) near the middle estuary. Farther seaward, marine plankton sources were elevated in the Dongzhai Harbor channel (64.15 %) and Yanfeng West River (48.83 %) of the lower estuary. The terrestrial/microbial humic-like component (C1), tryptophan-like component (C2), and soil-derived fulvic acid (C3) were identified from the samples using fluorescence excitation emission matrices and parallel factor analysis (EEM-PARAFAC). Results show that in the upper and middle estuary, SOM was preserved by sedimentation due to the abundance of fine-grained aquaculture-derived biodeposits, weak hydrodynamic conditions, and the formation of Fe/Mn-humus complexes. In contrast, in the lower estuary, SOM originated from marine plankton and was rapidly degraded via aerobic respiration. Moreover, correlations between δ13C and δ15N values vs C/N ratios further suggest progressive SOM degradation from initial diagenesis to post-depositional geochemical processes. This study provides new insights into aquacultural and environmental factors controlling SOM preservation and degradation within estuaries. The findings have implications for shoreline stability and the protection and restoration of estuarine mangrove wetlands.

Vanadium Speciation in Ancient Shales Revealed through Synchrotron-Based X-ray Spectroscopy

Redox-sensitive trace elements, such as vanadium, are frequently used as paleoredox proxies to infer the redox status of ancient marine depositional environments. When applied to vanadium, this approach relies on an accurate understanding of vanadium geochemistry in marine sediments. This, in turn, requires the application of analytical techniques capable of providing reliable chemical information on vanadium host phases of vanadium and thus the ability to distinguish authigenic from detrital phases. The increasing availability of synchrotron-based X-ray spectroscopic techniques capable of providing chemical speciation information at both the bulk and micro-scale presents an opportunity to refine our understanding of vanadium geochemistry in ancient marine sediments. Here, we show the utility of synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy and scanning X-ray fluorescence microscopy (SXFM) for investigating vanadium host phases in ancient marine sediments. The studied samples from the 1400 Ma Xiamaling Formation came from strongly contrasting depositional settings (ranging from oxic to euxinic) with a wide range of vanadium enrichments (enrichment factors from 0.7 to 9.6), yet vanadium speciation determined by XANES was consistently found to be phyllosilicate-hosted vanadium (III). High-resolution elemental imaging by SXFM coupled with global statistical colocalization analyses revealed strong associations between vanadium and potassium, consistent with vanadium hosted by potassium-rich clay minerals. These findings, while demonstrating the power of synchrotron-based X-ray methods, imply that differentiating authigenic and detrital vanadium in ancient marine sediments is unlikely to be possible given that post-depositional processes result in similar chemical speciation for both fractions. This will also limit the reliability of vanadium isotope system studies that depend on differentiating authigenic and detrital fractions using selective extraction procedures.