Changes In Depositional Environment Research Articles

Cretaceous magnetostratigraphy of the southern Simao Basin, SE Tibetan Plateau, and its paleogeographic implications

Precise stratigraphic chronology is the basis of many studies (e.g., tectonic, paleoclimate, and mineralization) in geoscience. Here, we carried out a detailed youngest detrital zircon U-Pb age-constrained magnetostratigraphic study on the middle-lower parts of a 2309-m-thick Upper Jurassic−Lower Cretaceous sequence from west Jiangcheng in the southern Simao Basin, southeastern Tibetan Plateau. A total of 2262 paleomagnetic samples were collected for magnetostratigraphy, and five siltstone/sandstone samples were collected for detrital zircon U-Pb analyses. Progressive thermal and alternating field demagnetizations isolated 1575 well-defined interpreted primary characteristic remanent magnetization directions, which yielded 25 normal polarity (N7−N31) and 25 reverse polarity (R8−R32) magnetozones. The interpreted maximum depositional ages calculated using the youngest detrital zircon U-Pb dates are 159.0 Ma, 154.0 Ma, 161.0 Ma, 149.3 Ma, and 139.5 Ma for the stratigraphic horizons at 2303 m, 1986 m, 1600 m, 1350 m, and 1110 m, respectively, in the section. Together with the previous magnetostratigraphic results of the upper part of the section, the observed polarity zones are best correlated with chrons C34n−M17r of the geomagnetic polarity timescale (GPTS2012), yielding magnetostratigraphic age estimates of ca. 143.5 Ma to ca. 65.0 Ma for the entire stratigraphic sequence. Paleogeographic analysis shows that the Simao Basin was in a low-latitude offshore environment during the Late Jurassic−earliest Cretaceous (before 142.3 Ma), a dominant fluvial environment during the early−middle Early Cretaceous (142.3−111.5 Ma), with widespread desertification at the end of this period, a saline lake sedimentary environment during the late Early Cretaceous−early Late Cretaceous (111.5−88.7 Ma), and a fluvial environment during the late Late Cretaceous (88.7−65.0 Ma). The changes in depositional environment are consistent with regional/global tectonic and paleoclimate changes, such as the dismission of Pangea, the termination of the megamonsoon, and the closure of the Meso-Tethys Ocean.

Distinguishing climate and tectonic signals in the stratigraphy of the Kura Basin, the southeastern foreland of the Greater Caucasus

Assessing the relative contributions of tectonics and climate in orogenic systems and the stratigraphy preserved within their fringing basins has guided research for decades. Determining the role of these contributions is non-trivial and is difficult due to variations in both magnitude and period over which fluctuations in tectonics and climate occur, typically >105 – 7 years and <105 years, respectively. The Greater Caucasus is a young orogen that offers a unique opportunity to assess these critical roles through analysis of exposures of the foreland stratigraphy. Here, we synthesize available measured stratigraphic sections from within the Kura Fold-Thrust Belt and adjoining regions, creating multiple paleogeographic reconstructions for key regional chronostratigraphic stages, and then assessing the Kura Basin’s response time throughout these stages. We use basin response time as a proxy for whether tectonics or climate fluctuations could be preserved within the Kura Fold-Thrust Belt stratigraphy and, thus, what changes in depositional environments during those periods are more likely to reflect. In general, estimates of basin response times indicate that tectonic signals could be preserved throughout the Kura Basin during the deposition of the Productive Series. Climatic signals would likely be preserved during the deposition of the Akchagyl stage, although tectonics signals cannot be ruled out. During the Apsheronian stage, both tectonic and climate signals can be preserved. These results highlight that a foreland basin system can fluctuate between being able to record mixtures of tectonic and climatic signals during both different geologic stages and within the same stage across a foreland.

Seismic-aseismic impact on marine depositional dynamics and sedimentary architecture: Study on multilayer-multiscale SSDS in Proterozoic Rohtas Limestone, India

The present study explores a diverse suite of soft-sediment deformation structures (SSDSs) preserved at multiple levels within the Mesoproterozoic carbonate succession of Rohtas Limestone in the Vindhyan Supergroup of India. The carbonate succession shows lithological heterogeneity, characterized by a sharp change of facies association and depositional environment. Three layers of SSDS, traced over 57 km, demarcate the lower lagoonal from upper open shelf deposits. Contorted laminae, load casts and flame structures, convolute laminae, ball and pillow structures, accordion folds and pillar structures within the limestone beds infer liquefaction-fluidization and plastic deformation. The lateral continuity, vertical repetition and confinement of SSDS beds at the interface of lagoon and open shelf deposits indicate the role of seismicity on facies succession. The SSDS beds show lateral variation in thickness and structures, depending on the distance from epicenter. The seismic trigger was accompanied by sudden deepening of the depositional environment and related marine transgression. The coarsening up packages within the lagoonal succession comprise load cast and flame structures, syn-sedimentary faults and intraclastic conglomerates. The open shelf succession shows several fining up cycles, with multiple load and flame structures with intraclastic conglomerate at the top. Origin of SSDS in association of the coarsening up sequence, and fining up cycle infers that intrabasinal tectonics and storms were the triggering agents for lagoon and open shelf successions respectively. The seismite in the studied succession not only reveals the seismicity during the Proterozoic, but also changed the sedimentary facies association, which may use as a diagnostic criterion for the identification of seismite. This study also highlights that seismic and aseismic impacts on sedimentary architecture and paleoceanography evolution.

Repeated Coseismic Uplift of Coastal Lagoons Above the Patton Bay Splay Fault System, Montague Island, Alaska, USA

AbstractCoseismic slip on the Patton Bay splay fault system during the 1964 Mw 9.2 Great Alaska Earthquake contributed to local tsunami generation and vertically uplifted shorelines as much as 11 m on Montague Island in Prince William Sound (PWS). Sudden uplift of 3.7–4.3 m caused coastal lagoons along the island's northwestern coast to gradually drain. The resulting change in depositional environment from marine lagoon to freshwater muskeg created a sharp, laterally continuous stratigraphic contact between silt and overlying peat. Here, we characterize the geomorphology, sedimentology, and diatom ecology across the 1964 earthquake contact and three similar prehistoric contacts within the stratigraphy of the Hidden Lagoons locality. We find that the contacts signal instances of abrupt coastal uplift that, within error, overlap the timing of independently constrained megathrust earthquakes in PWS—1964 Common Era, 760–870 yr BP, 2500–2700 yr BP, and 4120–4500 yr BP. Changes in fossil diatom assemblages across the inferred prehistoric earthquake contacts reflect ecological shifts consistent with repeated draining of a lagoon system caused by >3 m of coseismic uplift. Our observations provide evidence for four instances of combined megathrust‐splay fault ruptures that have occurred in the past ∼4,200 years in PWS. The possibility that 1964‐style combined megathrust‐splay fault ruptures may have repeated in the past warrants their consideration in future seismic and tsunami hazards assessments.

Deep learning‐based grain‐size decomposition model: A feasible solution for dealing with methodological uncertainty

ABSTRACTTerrigenous clastic sediments cover a large area of the Earth's surface and provide valuable insights into the Earth's evolution and environmental change. Sediment grain‐size decomposition has been widely used as an effective approach to inferring changes in sediment sources, transport processes and depositional environments. Several algorithms, such as single sample unmixing, end‐member modelling analysis and the universal decomposition model, have been developed for grain‐size decomposition. The performance of these algorithms is highly dependent on parameter selections, introducing subjective uncertainty. This uncertainty could undermine the reliability of decomposition results, limit the application of grain‐size decomposition techniques and reduce comparability across different studies. To mitigate the methodological uncertainty, a novel deep learning‐based framework for grain‐size decomposition of terrigenous clastic sediments is proposed. First, an improved universal decomposition model is used to analyse the collected grain‐size data, in order to provide training sets for the end‐to‐end decomposers. To meet the data size requirements of supervised learning, generative adversarial networks are also trained for data augmentation. The performance of the new framework is then evaluated using a small‐scale dataset (73 393 samples from 18 sites) of three sedimentary types (loess, fluvial and lake delta deposits). The decomposed grain‐size results demonstrate high feasibility and great potential of the framework in constructing a robust grain‐size decomposition model. Finally, it is proposed that future grain‐size research should aim to establish guidelines for grain‐size data sharing and produce a big grain‐size database for deep learning.

Two‐dimensional SH‐wave and acoustic P‐wave full waveform inversion: A Midland Basin case study

AbstractBuilding a reliable S‐wave velocity model remains challenging from P‐wave land seismic data as multi‐parameter elastic full waveform inversion which updates P‐ and S‐wave velocities simultaneously is not yet widely adopted due to its computational cost, highly nonlinear nature and noise contamination from land seismic data. To overcome this challenge, we propose implementing the SH‐wave full waveform inversion to obtain the S‐wave velocity model. The value of this approach has been examined by applying two‐dimensional time‐domain SH‐ and P‐wave acoustic full waveform inversion to a nine‐component three‐dimensional seismic survey acquired in the Midland Basin. The nine‐component seismic survey was acquired by using both vertical and horizontal component vibrators and receivers, which enables us to apply both SH‐ and P‐wave full waveform inversion to the raw and preprocessed shot gathers along a two‐dimensional line. The SH‐ and P‐wave full waveform inversion applied to the raw shot gathers provide more detailed P‐ and S‐wave velocities compared to the velocities from the stacking velocity analysis. Additionally, there are no problematic artefacts in the inverted S‐wave velocity model, which shows the stability of the SH‐wave inversion. Through the SH‐wave full waveform inversion from the preprocessed shot gathers, we reveal the lateral velocity variations in the Grayburg–San Andres interval, which coincides with the depositional environment changes suggested by the previous regional study mainly based on well log and cores. These variations demonstrate that SH‐wave full waveform inversion can identify additional geological features that are not observed in the inverted P‐wave velocity model. The inverted P‐ and SH‐wave velocities are validated by the flattening of the events observed in the common image gathers after Kirchhoff depth migration. The SH‐wave migration image further reveals the irregular geometries of carbonates in the Spraberry formation. Vp/Vs ratios calculated from the independently inverted P‐ and S‐wave velocity models show strong lateral variations in the Wolfcamp interval (key‐producing interval), which could be caused by the organic content variations between the reservoir and non‐reservoir rocks. The inversion results demonstrate that the SH‐wave full waveform inversion can be used to provide an S‐wave velocity model that is comparable to the P‐wave velocity model derived from acoustic full waveform inversion, which is widely used for P‐wave velocity model building from P‐wave land seismic data.

A facies‐constrained geostatistical seismic inversion method based on multi‐scale sparse representation

AbstractGeostatistical seismic inversion is an important method for establishing high‐resolution reservoir parameter models. There is no accurate representation method for reservoir structural features, and prior information about structural features cannot be incorporated into geostatistical inversion. Based on the assumption of the sparsity of stratigraphic sedimentary features, the same type of structural feature is used to represent the sedimentary pattern of reservoirs within the same facies. Different sparse representation patterns are used to represent the differences in sedimentary patterns between facies. Although changes in depositional environment might result in the multi‐scale characteristics of geological structures for varying sedimentary rhythms, this paper proposes a facies‐constrained geostatistical inversion method based on multi‐scale sparse representation to better accommodate such situation. Using the method of sparse representation combined with wavelet transform, the multi‐scale sedimentary structural features of reservoirs are learned from well‐logging data. Seismic facies and multi‐scale features are used as prior information for geostatistical inversion. Further, the likelihood function is constructed using seismic data to obtain the posterior probability distribution of reservoir parameters. Finally, the accurate inversion result is obtained by using multi‐scale sparse representation as a constraint in the posterior probability distribution of reservoir parameters. Compared with conventional geostatistical methods, this algorithm can better match the structural features of reservoir parameters with varying geological conditions. Field data tests have shown the effectiveness of this method in improving the accuracy and resolution of reservoir parameter structural features.

Geochemical insights into secular changes in the depositional environment of ferromanganese nodules in the western North Pacific

Understanding Earth's surface dynamics and biological evolution requires deciphering past marine environmental changes. Hydrogenetic ferromanganese nodules formed by the direct precipitation of Fe–Mn oxide/hydroxide from seawater could record the marine environment at the time of deposition, as in the case of ferromanganese crusts. In this study, petrological and geochemical analyses were conducted on ferromanganese nodules collected from the Japanese Exclusive Economic Zone around Minamitorishima Island in the western North Pacific Ocean. Additionally, the results were compared with those of ferromanganese crusts distributed in the same area to elucidate the secular changes in the depositional environment. The mineralogical features (consisting solely of Fe-vernadite with an absence of 10 Å Mn oxide) and the geochemical characteristics (including positive Ce-anomaly and negative Y-anomaly in rare-earth element patterns, high Co contents, and low Cu and Ni contents) of the nodules suggest that the studied nodules are of a hydrogenetic origin. From the inside to the outside of the nodule, the Mn/Fe and Y/Ho ratios decrease while the Th/U ratio increases, indicating a changing depositional environment over time. A similar trend was observed in ferromanganese crusts from the nearby Takuyo-Daigo Seamount. Furthermore, similar compositional changes were observed on the surfaces of the crust samples (uppermost 1–3 mm), which correlated with depth-related changes in seawater oxygen concentrations. Therefore, the observed cross-sectional changes in the chemical compositions of the nodules and crusts may reflect changes in seawater-dissolved oxygen concentrations over time. The Mn/Fe and Y/Ho trends indicate a shift towards a less oxic environment with increasing age while maintaining the current depth profile, implying an expansion of the oxygen minimum zone (OMZ). Moreover, the observed Ni and Cu enrichment, together with the changes in the Mn/Fe and Y/Ho ratios, suggest that the expansion of the OMZ was caused by increased surface ocean productivity.

Sedimentary porewaters record regional tectonic and climate events that perturbed a deep-sea brine pool in the Gulf of Aqaba, Red Sea

Brine pools in deep-sea environments provide unique perspectives into planetary and geological processes, extremophile microbial communities, and sedimentary records. The NEOM Brine Pool Complex was the first deep-sea brine pool system found in the Gulf of Aqaba, representing a significant extension of the geographical range and depositional setting of Red Sea brine pools. Here, we use a combination of brine pool samples collected via cast using a conductivity, temperature, depth instrument (CTD), as well as interstitial porewaters extracted from a sediment core collected in the NEOM Brine Pool to characterize the chemical composition and subsurface evolution of the brine. New results indicate that the NEOM brines and the subsurface porewaters may originate from different sources. Elemental concentrations suggest the brines in the NEOM pool are likely derived from dissolution of sub-seabed evaporites. In contrast, the sedimentary porewaters appear to have been influenced by periodic turbidite flows, generated either by earthquakes, submarine landslides, or flash floods, in which normal marine waters from the overlying Red Sea became entrained, periodically disturbing the chemistry of the brine pool. Thus, sediment porewaters beneath brine pools may record transient and dynamic changes in these deep marine depositional environments, reflecting the interplay between climate, tectonics, and sedimentation patterns along a rapidly urbanizing coastline. In concert, new results from NEOM extend the range and chemical constraints on Red Sea Brine Pools and highlight the dynamic interplay between Red Sea Deep water, dissolving evaporites, turbidites, and subsurface fluids that produce these unique depositional environments which host microbial life at the edge of habitability. In concert with sedimentological indicators, the chemistry of porewaters beneath deep-sea brine pools may present detailed records of natural hazards arising from interactions between the atmosphere, lithosphere, hydrosphere, and anthroposphere.

Is there a link between carbon isotopes and sea level in epicontinental carbonate settings?

A presumed link between carbon isotopic trends and sea level change features prominently in many studies of epicontinental carbonates. In these shallow marine environments, a combination of basin restriction, burial/oxidation of organic carbon, proximity to terrestrial carbon sources, carbonate mineralogy, and/or meteoric influence can result in δ13Ccarb records that are distinct from that of the open ocean. Because many of these processes are linked to sea level change, it has been argued that sea level might exert a significant and systematic control on the δ13Ccarb records from epicontinental settings. Multiple studies have attempted to document sea level's influence on carbon isotopic trends, but they do so with only limited constraints on sea level change and without objective evaluations of interpreted trends and relationships. We argue that the complex and complicated set of processes influencing carbon isotopic values in epicontinental settings requires a systematic approach to truly address the question of sea level's influence on δ13Ccarb. Only by integrating carbon isotopic records with a detailed sedimentological and sequence stratigraphic framework can we properly track changes in depositional environments and reconstruct the transgressive-regressive history of the rocks. Trends and relationships in these robust datasets can be evaluated with rank correlation tests specifically designed and empirically tested to deal with noisy datasets. In short, we map a possible path forward for systematic testing of the relationship between sea level and δ13Ccarb.

Provenance and depositional environment of Middle Miocene silicic volcaniclastic deposits from Mt. Medvednica (North Croatian Basin, Carpathian-Pannonian Region)

Extensive and protracted volcanism in the Carpathian-Pannonian Region climaxed during the Early to Middle Miocene with series of major ignimbrite-forming eruptions with their products dispersed across the Alpine-Mediterranean Region. The spatial and temporal dispersion of these volcanic horizons make them potentially important stratigraphic markers throughout the region, allowing better understanding of temporal and spatial changes in depositional environments and paleoclimate. Several of these Middle Miocene volcaniclastic layers are preserved in stratigraphically and environmentally variable sedimentary facies on Mt. Medvednica, located in the North Croatian Basin. In order to decipher the age, depositional environment and provenance of two volcaniclastic horizons intercalated within Central Paratethys marine sediments on Mt. Medvednica, we applied an integrated approach of volcanological, geochronological, and paleontological analyses. New high-precision zircon geochronology and volcanic glass geochemistry data allow to distinguish two primary rhyolitic volcaniclastic horizons derived from distinct eruptions, “Plaz“, and the “Bidrovec“, dated at 14.937 ± 0.012 Ma and 14.835 ± 0.012 Ma. Distinguished mineralogical and geochemical data enabled the correlation of the older (“Plaz”) horizon on Mt. Medvednica with the Demjén eruption, one of the six major Early–Middle Miocene ignimbrite-forming eruptions of the Carpathian-Pannonian Region. However, a correlation of the younger (“Bidrovec“) horizon and assignment to a specific eruption could not be established due to a lack of compositional data from coeval eruption products throughout the region. The newly gathered data establishes both “Plaz” and “Bidrovec” pyroclastic deposits as valuable marker horizons for regional reconstructions, and enable a better understanding of the eruption chronology and tephrostratigraphy of the Carpathian-Pannonian Region.

Insights into Late Quaternary Rock Shelter Sedimentation at Santuario Della Madonna Cave (Northern Calabria, Italy)

Shelter caves serve as crucial repositories that provide valuable insights into Late Pleistocene–Holocene depositional mechanisms and environmental changes. In this study, we conducted a stratigraphic analysis of the easternmost cave-fill succession within the Santuario della Madonna cave, located along the Tyrrhenian coasts of southern Italy. By examining the cave-fill deposits and their relationship with archaeological successions from previous excavation campaigns, we aimed to enhance our understanding of sedimentary evolution, specifically, the interplay between local and global environmental factors. A comprehensive sedimentological analysis was conducted, enabling the identification of three distinct sedimentary facies within the clastic succession that overlays the calcareous cave roof. Our findings reveal a noticeable shift in cave sedimentation processes, characterized by a gap in detritus accumulation followed by the deposition of flowstones. Furthermore, the presence of oversized boulders within the succession was attributed to failures from the cliffs that overhang the cave opening. We interpret these boulders as possible evidence of cave collapses induced by earthquakes, similarly to rockfall phenomena observed in the archaeological test pits within the shelter, which show a strong correlation with paleoseismic events.

Effect of salinity on the preservation of plant-derived n-alkyl compounds in the terrestrial-aquatic interface

In sediments, plant biomolecules (n-alkanes and fatty acids) serve as robust proxies in paleoenvironmental studies. Although the factors controlling preservation and isotopic composition of plant biomolecules are understood in soil systems, the effect of changes in depositional environments in land-sea interfaces, especially in hypersaline environments, is not so well constrained. In this study, we observed that in bed sediments, the plant biomolecules are preserved with carbon preference index (CPI) values > 1 in high molecular weight (HMW) n-alkanes in the hypersaline, metahaline, and euhaline regions of the Pulicat lagoon, India. However, in sub-surface sediments (up to 70 cm depth), the CPI values are consistently < 1 in the hypersaline region, but > 1 in the euhaline and metahaline regions. In the hypersaline region, the alteration of plant biomolecules possibly results from halophilic bacterial degradation and fatty acid to n-alkane reduction in anaerobic conditions (pristane/phytane values < 1), during the weaker northeast monsoon (44–60 cm sub-surface). Prolonged hypersaline conditions and associated anoxia resulted in a higher isotopic difference between fatty acids and n-alkanes (δ13CFA-ALK value) in the hypersaline region (3.1 ± 1.5‰) compared to metahaline (2.2 ± 1.0‰) and euhaline (1.5 ± 1.0‰) regions. Although in the hypersaline region the HMW fatty acids have CPI values > 1, the higher δ13CFA-ALK value also suggests the alteration of fatty acids. This study suggests that if plant-derived n-alkanes are altered, the δ13C values and distribution patterns of plant-derived fatty acids should be critically evaluated before using it as an alternative proxy in coastal environments.

Investigation of Organic Matter Sources and Depositional Environment Changes for Terrestrial Shale Succession from the Yuka Depression: Implications from Organic Geochemistry and Petrological Analyses

Investigation of Organic Matter Sources and Depositional Environment Changes for Terrestrial Shale Succession from the Yuka Depression: Implications from Organic Geochemistry and Petrological Analyses

Inconsistent mercury records from terrestrial upland to coastal lowland across the Permian-Triassic transition

The enrichment of mercury (Hg) in sediments has been increasingly utilized as a proxy for large-scale volcanism. However, a causal link between these two remains ambiguous, as Hg enrichments can be caused by many non-volcanic factors, e.g., changes in local depositional environments or terrestrial runoffs. In this study, we present high-resolution Hg concentration and isotope records of two drill cores, one located in a coastal lowland (HK-1) and the other in a terrestrial upland (TK-1), spanning the Permian–Triassic boundary (PTB). Interestingly, the coastal lowland core exhibits significant Hg enrichments over a stratigraphic horizon of approximately 200 meters (corresponding to a duration of ∼2 Myrs), with dominating negative Δ199Hg values. In contrast, the terrestrial upland core primarily displays background Hg levels with near-zero Δ199Hg values. Given the spatial proximity of the two cores, separated by only about 70 km, we interpret these discrepant Hg signatures to indicate that terrestrial upland underwent minimal influence from terrestrial and/or atmospheric depositions rather than volcanic input, while the coastal lowland was primarily sourced from enhanced terrestrial influxes, depositing under oxygen-deficient conditions. This interpretation is supported by the high concentrations of organic carbon and sulfur, substantial pyrite layers, and positive correlations between Hg and chalcophile elements, such as Mo, Se, and Co, in the HK-1 core. Our study highlights the need to examine multiple Hg enrichment factors in sedimentary records before comprehensively establishing a robust causal link with volcanic activities. A thorough evaluation of multiple lines of evidence, including Hg isotopes, local environmental proxies, and sedimentology investigations, should be conducted first, particularly necessary for the non-marine sediments.

Sedimentology and stratigraphic architecture of Barremian synrift barrier island–estuarine depositional systems from blended field and drone‐derived data

ABSTRACTThe Lower Cretaceous (Barremian) Camarillas Formation in the Galve Sub‐basin of eastern Spain is an exceptionally muddy, synrift, aggradational then retrogradational paralic succession. Deposition within these arid, equatorial paralic systems was strongly controlled by crustal rifting of the Iberia plate linked to the geodynamic evolution of the Atlantic Ocean, the Bay of Biscay and the Tethys Ocean. Although synsedimentary extensional tectonics controlled thickness and facies distributions, the parasequence stacking patterns point to a superimposed high‐order allogenic control on the paralic succession. Field data and drone imagery are combined to document changes in sedimentology and three‐dimensional stratigraphic architecture of these deposits to interpret changes in depositional environments as this basin filled. Three evolutionary stages are identified: (i) tide‐dominated estuary; (ii) mixed‐energy estuary, with a well‐developed wave‐dominated barrier island system; and (iii) barrier island–tidal inlet suite. An exceptional record of back‐barrier‐island depositional interactions is preserved in this high‐subsidence, extensional‐basin setting, including deposition of washover fans, flood‐tidal deltas and ebb‐tidal deltas. Drone‐derived imagery facilitates three‐dimensional architectural characterization of these complex paralic deposits, including multi‐episodic tidal inlets, and correlation of basin scale stratigraphic markers. Spatio‐temporal interactions between climate change, sea‐level variations and rift‐related subsidence generated complex estuarine and barrier island geobodies. The general transgressive trend recorded in the Camarillas Formation correlates well with global eustatic sea‐level rise during the Barremian.

Shifts in sediment bacterial communities reflect changes in depositional environments in a fluviatile context

Sediments are complex heterogeneous matrices allowing to some extent the recording of past environmental conditions by integrating sediment characteristics, contamination and the microbial community assembly. In aquatic environments, abiotic environmental filtering is considered the primary deterministic mechanism shaping microbial communities in sediments. However, the number and relative contributions of geochemical and physical factors associated with biotic parameters (reservoir of microorganisms) complicate our understanding of community assembly dynamics. In this study, the sampling of a sedimentary archive in a site alternately subjected to contrasting inputs from the Eure and the Seine Rivers allowed us to study the response of microbial communities to changes in depositional environment over time. The coupling of the quantification and sequencing of the gene encoding the 16S rRNA with analyses of grain size, organic matter and major and trace metal contents demonstrated that microbial communities reflected contrasting sedimentary inputs over time. Total organic carbon (TOC) was the main factor influencing microbial biomass, while the quantity and quality of organic matter (R400, RC/TOC), major elements (i.e. Al, Fe, Ti) and trace metals (i.e. Zn, Pb, Cu, Cr, Ni, As, Co, Ag, Sb) shaped the structure of the microbial community. Besides the effect of geochemical factors, a specific microbial signature was associated with the contrasting sedimentary sources, highlighting the importance of the microbial reservoir in the assembly of microbial communities. Indeed, the main genera identified in the facies influenced by the Eure River were affiliated with the phyla Desulfobacterota (Syntrophus, Syntrophorhabdus, Smithella, Desulfatiglans), Firmicutes (Clostridium_sensu_stricto_1), Proteobacteria (Crenothrix), Verrucomicrobiota (Luteolibacter), while the contributions of the Seine River were characterised by some halophilic genera Salirhabdus (Firmicutes), Haliangium (Myxococcota) and SCGC-AB-539-J10 (Chloroflexi). This study sheds light on the overall processes determining the assembly of microbial communities in sediments and the importance of associating geochemical factors with reservoirs of microorganisms inherited from sediment sources.

Age offsets between radiocarbon samples and changes in Holocene depositional ocean environments captured in sediment cores near Ube City, Yamaguchi Prefecture, Japan

Radiocarbon ages from contemporaneous samples of varying types can show large age differences (Nakamura et al., 2016; Raymond and Bauer, 2001), and changes in offsets between these ages (δage) within a core have been scarcely examined. Changes in δage have previously suggested disturbances or shifts to the depositional setting (Nakamura et al., 2016; Ishiwa et al., 2021; Yokoyama et al., 2019). Radiocarbon dating of ocean sediment cores from near Ube City, Yamaguchi prefecture, Japan revealed periods of slower and rapid sedimentation which aligned with, respectively, post-Holocene High Stand (HHS, ∼7–4 cal ka BP (Yokoyama, 2021)) conditions and sea level fall, and pre-HHS conditions with rising sea levels.δage patterns implied that the reliability of sample types as depositional age indicators may depend on the depositional environment. Radiocarbon ages and relative sea level (RSL) curves suggested that local inundation may have occurred up to ∼ 200 years earlier than previously reported. In addition to constraining the relative timing of sea transgression and regression during the HHS, this study highlights the need to examine the depositional setting to accurately represent depositional ages.

The large decline in carbonate δ238U from a PETM section at Tingri (South Tibet) was driven by local sea-level changes, not global oceanic anoxia

Uranium isotope compositions (δ238U) and Th/U in carbonates are being explored as paleoredox proxies to study global oceanic anoxia. However, the impact of changes to local depositional environments that might cause a decline in δ238U and an increase in Th/U has not been thoroughly investigated. Here we report a decline in δ238U (from +0.1 to −0.3‰) and an increase in Th/U from shallow-marine carbonates during the Paleocene-Eocene thermal maximum at Tingri (south Tibet). Changes of δ238U and Th/U are not fully coupled. The decline in δ238U coincided with a local sea-level fall, whereas the increase in Th/U occurred mainly during a sea-level rise. The decline in δ238U likely results from changes in redox conditions of pore waters and in primary carbonate mineralogy. The increase in Th/U is ascribed to reduced authigenic U(IV) accumulation. Our results suggest that changes in local depositional environments can cause behavior of δ238U and Th/U resembling that induced by expanded global oceanic anoxia. This finding calls for caution to consider local factors before applying δ238U and Th/U as global paleoredox proxies.

Storm-driven variations in depositional environments modify pyrite sulfur isotope records

The sulfur isotopic composition of marine pyrite (δ34Spyr) is one of the major geochemical tools to reconstruct global changes in Earth's surface environment. Storm-driven variations in depositional environments, however, can play key roles in modifying the δ34Spyr signal. Here we present δ34Spyr values in a complete Holocene muddy storm deposit on the East China coastal plain, which is identified by comprehensive analyses of sediment components, organic matter composition, bulk organic carbon isotopic ratios, carbon and oxygen isotopic ratios of carbonates and trace element concentrations. We find that positive (+5.8‰ to +15.8‰) and negative (−9.0‰ to −19.7‰) δ34Spyr values were preserved in two successive intervals of the muddy storm deposit (ca. 62 cm long), corresponding to the high-energy storm peak phase (HESPP) and the waning-energy late storm phase (WELSP), respectively. We propose that the 34S enrichment in pyrite from the HESPP is most likely due to a combination of storm reworking of sediments and high sedimentation rates, which involves various physical and chemical processes leading to the accumulation of 34S-enriched pyrite, such as oxidation of near-surface sulfides, reduced exchange of sulfate between sediment pore fluids and the overlying water column, and/or input of excess reactive iron minerals. In contrast, lower sedimentation rates and limited sedimentary remobilization during the WELSP allow the isotopic signal of early-formed 32S-enriched pyrite to be preserved. The striking shift in δ34Spyr values, therefore, reflects a dramatic change in the local depositional environment between the HESPP and WELSP, suggesting that non-steady-state deposition induced by storm activity facilitates the formation of isotopically “heavy” pyrite. This sharp shift can be further generalized as a characteristic feature of δ34Spyr values in storm deposits. Our work highlights the critical impact of weather- and climate-event-driven depositional variations on δ34Spyr values, adding to the growing evidence emphasizing the local environmental and diagenetic controls on these records.