Terrigenous Research Articles

Rapid Recycling of Subducted Sediments in the Subcontinental Lithospheric Mantle

ABSTRACT Subduction recycling of sediments plays a key role in the geochemical evolution of Earth. The presence of recycled terrigenous sediments in upwelling plumes has been cited to explain the EM2 signature in ocean island volcanics, characterized by particularly high 87Sr/86Sr (>0.706). However, the origin of such isotopic anomalies in continental regions and the role of subducted sediments in the subcontinental lithospheric mantle (SCLM) remain unclear. The Himalaya–Tibet orogen is one of the world’s best places for deciphering continental subduction and the fate of subducted crustal materials in the mantle. Here we present a systematic study of the geochronology, mineral chemistry (especially clinopyroxene), whole-rock chemistry and Sr–Nd–Pb–Hf–O isotopic compositions of Cenozoic potassic–ultrapotassic lavas from the western Kunlun area of northwestern Tibet. New secondary ion mass spectrometry (SIMS) zircon U–Pb dating, coupled with published age results, constrain the timing of volcanism from ~8.3 Ma to the present. These lavas show geochemical characteristics that closely resemble the EM2 mantle end-member represented by the Samoan hotspot. Both whole rocks and individual magmatic clinopyroxenes display arc-like trace-element patterns and remarkably enriched Sr–Nd–Pb–Hf isotope compositions (87Sr/86Sr ≥ 0.7080; εNd ≤ −4.8; 206Pb/204Pb ≥ 18.704; εHf ≤ −2.6). Together with high zircon δ18O values (6.3–10.4‰), the data point to a mantle source enriched by recycled sedimentary materials. Geochemical modeling and geophysical evidence further indicate that the sediments were directly derived from the subducted Indian continental lithosphere during India–Eurasia collision. Partial melting models assuming a hybridized mantle source that contains ~5% Indian continental crust suggest that the primary melts of the potassic–ultrapotassic lavas could be formed by melting of a phlogopite-bearing garnet lherzolite at low melting degrees (1–5%). The magma geochemistry is consistent with the model of mélange melting, implying that the subducted sediments may detach from the downgoing Indian slab and rise up diapirically into the overlying mantle lithosphere. Unlike traditional models of subducted sediments entering the deep mantle, the western Kunlun EM2-like lavas reveal that subducted sediments can be rapidly recycled into the SCLM during continental subduction (probably <50 Myr). We suggest that the SCLM could be an important reservoir for subducted sediments. The findings are important to our understanding of mantle circulation rates and chemical heterogeneities.

Different B[sbnd]Mo isotopic fractionation processes controlled by redox conditions in the subduction zone

The control of redox conditions on the geochemical recycling process in subduction zones is still poorly understood due to large uncertainties in oxygen fugacity (fO2) for subducted slabs. We present the first systematic geochemical and B-Mo-Sr-Nd-Hf-Pb isotopic data for continental arc basalts (CABs) and back-arc basalts (BABs) from Sumatra to investigate the relationships between oxygen fugacity and BMo isotopic fractionation processes during subduction. The Sumatran CABs and BABs yield high FeOT/MgO ratios and low K2O contents (0.48–1.42 wt%) and can be classified as tholeiitic. They have variable Sr-Nd-Hf-Pb isotopic compositions, consistent with the input of different amounts of melt from subducted terrigenous sediments into mantle sources. The BAB samples show much lower δ11B (−9.0‰ to −7.3‰) values than the CABs (δ11B = −7.0‰ to +0.17‰), reflecting significant B isotopic fractionation during multistage melting of subducted sediments. This is distinct from the similar δ98/95Mo values for the Sumatran CABs (−0.21‰ to −0.01‰) and BABs (−0.17‰ to −0.08‰), suggesting limited Mo isotopic fractionation during subduction. The Sumatran CABs show low V/Yb ratios, suggesting that melts from subducted sediments were possibly generated at fO2 values lower than FMQ + 1.5. The limited Mo isotopic fractionation could be induced by the relatively low fO2 during melting of subducted sediments, which would significantly decrease the mobility of Mo but have no influence on B. Alternatively, rocks from highly oxidized arcs, such as the Izu arc (e.g., >FMQ + 3), exhibit across-arc lightening trends for both B and Mo isotopes, which is consistent with enhanced Mo isotopic fractionation due to elevation of fO2 during subduction. Thus, the BMo isotopes of arc rocks could provide diagnostic indicators for distinct geochemical recycling processes controlled by changes in redox conditions in subduction zones.

A magnetic and geochemical approach to the changing sedimentation accumulation on the upper slope of the great barrier reef, northeastern Australian margin

The Great Barrier Reef (GBR) located along the northeastern margin of Australia is the largest coral reef system in the world. Modern climatic alterations are quickly changing the GBR ecosystem. To understand the implications of these changes it is important to reconstruct the geological history of GBR. Here we use geochemical and magnetic proxies to evaluate past climatic fluctuations and their consequences on sediment deposition along the GBR margin. IODP Expedition 325 – Hole M0058A, drilled on the uppermost slope at ca. 170 m water depth, reveals the depositional history of the GBR margin during the interval of MIS 7 to 5 and MIS 1. Magnetic and geochemical variations along the core section reveal detailed information on sediment accumulation and on the variations in terrigenous input in relation to sea-level fluctuations and climate change. Sea-level variations influenced margin deposition between MIS 7 and 6 impacting shoreline progradation/retrogradation and siliciclastic redistribution, resulting in a mixture of finer to coarser magnetic assemblages with no significant changes in terrigenous input. At the end of MIS 6 a decline in the deposition of carbonate sediments concomitant with the deposition of fine-grained magnetite-rich terrigenous sediments suggests an intensification of the monsoon in response to global warming trends. Arid periods over NE Australia were established after the glacial/interglacial transition (the MIS 6-5e) and at the middle Holocene (after the MIS 2–1 transition at ca. 7 ka), which favored dust deposition over the region. Enhanced dust fertilization subsequently promoted primary productivity at these intervals resulting in the presence of biogenic magnetite at Hole M0058A produced by magnetotactic bacteria.

The Clay Mineralogy and Geochemistry of Sediments in the Beibu Gulf, South China Sea: A Record of the Holocene Sedimentary Environmental Change

In this study, we analyzed the clay mineralogy and geochemistry of surface and drill core samples from the northeastern Beibu Gulf in order to unravel the sediment provenance of, and factors controlling, the sedimentary environment. The main clay mineral assemblage in the surface sediment samples included kaolinite (27–72%), smectite (4–51%), illite (7–20%), and chlorite (8–17%). The study area comprises three major clay distribution zones (from the northeastern coastal area to central Beibu Gulf basin), i.e., the kaolinite-dominated, kaolinite–smectite, and smectite-dominated zones. The zoning of the clay mineralogy and major and trace elements indicated the mixing of coarse terrigenous sediments with distal fine sediments. Early Holocene sea level rise was documented in core B15-1, which had three sedimentary units (divided into Units 1–3 from top to bottom), as revealed by the changes in the dominant clay minerals and geochemical/oxide ratios (SiO2/Al2O3, Rb/Sr, Sr/Ba, and Ti/Ca) in Unit 2. Unit 1 and Unit 3 were likely deposited in the continental and marine environments, respectively. The low sedimentation rate and hydrodynamic disturbance may have influenced the deposition process more than climatic fluctuations.

Geochemistry and Mineralogy of Rare-Earth Elements in Ferruginous Deposits and Bottom Sediments of the Laptev Sea

Abstract —The geochemical and mineralogical aspects of the distribution of rare-earth elements (REE) in ferruginous deposits (FD) and bottom sediments of the southeastern Laptev Sea are analyzed. The FD are dominated by biomorphs developed after polychaete tubes. The REE patterns depend on the proportion of ore and non-ore substances. The ore substance is represented mainly by iron hydroxides (limonite). It is a product of suboxic diagenesis enhanced by bioturbation and determines the slight enrichment of FD in MREE and cerium deficiency (Cean = 0.94). The non-ore substance comes from terrigenous sediments and has an (alumino)silicate composition. It controls the scandium content and is the source of REE mineral grains, among which monazite-(Ce) prevails. The sediments demonstrate a common similarity in REE patterns to shales and suspended material transported to the Laptev Sea by the Lena River, with elevated LREE and MREE contents and the value of Ce anomaly almost equal to unity (Cean = 1.06). In the sediments, REE minerals occur mostly in the silt fraction (<63 μm in size). The high content of organic matter (Corg of up to 2.15%) of predominantly terrigenous origin (OMter of up to 85%) in the sediments explains the oxygen deficiency and weak diagenetic mineral formation with low accumulation of trace elements, including REE, in the FD. The total REE content in the FD is lower than that in the sediments (on average, 173 ppm against 206 ppm).

Jurassic terrigenous deposits of the Buchara-Khiva region – a reserve of exploration in oil and gas

Jurassic terrigenous deposits of the Buchara-Khiva region – a reserve of exploration in oil and gas

Morphology, seismic stratigraphy, and tectonic control of the Yitong submarine canyons – fan apron system in the northern South China Sea

Submarine fans are ubiquitous in modern deep-water environments. Such systems reported in the literature are largely fed by shelf-indented canyons receiving ample fluvial or longshore drift sediment inputs, whereas those nourished by slope-confined canyons far away from terrigenous sediment sources remain poorly understood. Herein, we report such a system, the Yitong submarine canyons – fan apron system (YCFS), from the northern South China Sea. This system was studied using multibeam bathymetric and two-dimensional multichannel seismic data, combined with geological constraints from nearby International Ocean Discovery Program drilling sites. YCFS consists of approximately 12 submarine canyons confined in the lower continental slope of >1200 m water depth, which feed a fan apron in the connecting abyssal plain. The canyons are erosion-dominated, as evidenced by their V-shaped cross-sections with up to 708 m deep incisions. Canyon infillings comprise mass transport deposits and turbidites, and interfluves exhibit features such as landslides, creep-typed sediment waves, and gullies, indicating the canyons and interfluves are dominated by mass transport and turbidity-current processes. The fan apron is generally depositional, in which channels and channel-levee complexes, turbidite lobes, and mass transport deposits comprise the main depositional elements. In the canyon-to-fan transitions, supercritical turbidity current features and bedforms, including plunge pools and connected sediment mounds, cyclic steps, and antidunes, are developed, which are linked to the deceleration and hydraulic-jump effects of the high-velocity turbidity currents shedding off the canyons. Seismic stratigraphic analysis revealed that YCFS was initiated in Late Miocene. The formation and maintenance of YCFS are linked to the long-term existence of an upslope margin-parallel basement high. The latter could have maintained a steep slope condition (up to 6.5°) that favors the development of canyons and supercritical turbidity currents. Because of the blocking effect of the basement high, YCFS receives limited supply of the shallow-water sediments.

Paleoclimate characteristics of source area weathering and metallogenic implication of cretaceous black shales in the Mamfe basin, (SW Cameroon): Evidence from lithogeochemistry

Black shales are highly enriched in essential elements containing critical information on metallic richness and paleoclimatic imprints during deposition. The Cretaceous period marks the age of Basin formation from which the Mamfe Basin was formed from the Albian to Cenomanian. The purpose of this study is to investigate the geochemical behaviour of the shales in the Mamfe Basin and their paleoclimatic and metallogenic significance. Twenty-five representative shale samples were collected from seven sites outcrops in the Mamfe Basin. The major and trace elements composition in the shale samples were analysed using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). In comparison with standard values for metalliferous shales, the shales from the Mamfe Basin are enriched in Boron (B), with only few samples enriched in Li, Rb, Cu, Pb and Zn. The Aluminium module (Alm) and ternary diagram plotted from Al–Fe–Mn shows that all the shales are considered as terrigenous sediments with only 02 samples (08%) falling within the zone of metalliferous sediments. The paleoclimatic characteristics that were inferred from the C-values (0.3–3.1), Sr/Cu (0.6–8.9), Sr/Ba (0.1–2.8), Rb/Sr (0.3–0.9), Fe/Mn (1.8–180), Al/Mg (0.31–31) and Mg/Ca (0.05–16.4) data of the shales in the Mamfe Basin indicate that the climate that existed during source area weathering ranged from a sub humid to a more humid climate with few samples ≤5% displaying arid climate.

Lithology, facies and dynamics of formation of the albian-cenomanian reservoir rocks of the Pokuttya-Bukovyna part of the Carpathian autochthon

The purpose of this work is to study the Upper Albian-Lower Cenomanian psammitic complex of the platform basement of the Pokuttya-Bukovyna part of the Carpathian Foredeep, which is one of the main reservoir strata of the Lopushna oil field. Methodology. The complex of investigations includes correlation of the well-log data, lithological studies of core material, mineralogical and petrographic analysis of rocks, and paleogeographic studies of the sedimentary basin. Results. On the basis of the analysis of all existing wells, it was established that the deposits of the Late Albian-Early Cenomanian age within the entire territory of the autochthon of the Pokuttya-Bukovyna Carpathians are mostly represented by sandstones. They are composed of quartz (50-80%), feldspar (1–5%), muscovite (0.1-3%), pyrite (0.1-3%) and accessory minerals – zircon and epidote. A significant part of sandstone is composed of the authigenic minerals – glauconite (10-15%, sometimes up to 45%) and phosphate (1–3 up to 10%). The matrix in the rocks is mainly porous and contact-porous (10-15% of the rock volume), which provides these psammites with good reservoir properties. On the basis of paleogeographic reconstructions, it was established that in the Late Albian-Early Cenomanian, terrigenous sedimentation prevailed within the entire shelf basin of the Carpathian branch of the Meso-Tethys, which caused the accumulation of the thick sandstones’ strata. The burial history and post-sedimentary evolution of rocks took place under the influence of the thrusting of the Carpathian Fold Belt. This resulted in the formation of reservoir rocks and traps in the Upper Albian-Lower Cenomanian sequence. The peculiarities of hydrocarbon migration in the studied region indicate a high probability of discovering new deposits in the autochthon of the Pokuttya-Bukovyna Carpathians. Originality. The applied complex of lithological, geological-geophysical and paleogeographical studies made it possible to investigate the features of distribution of the Albian-Cenomanian psammite complex of rocks, both laterally and in section, their mineralogical and petrographic characteristics, sedimentary environments. The post-sedimentary evolution of the study strata and features of hydrocarbon migration were caused by the dynamics of the Carpathian Flysch Belt thrusting over the margin of the East-European platform. Practical significance. The obtained results show that the Upper Albian-Lower Cenomanian sandstones, which are oil and gas-bearing in the Lopushna field, represent the potential reservoirs in terms of both thickness and mineral composition within the entire autochthon of the Pokuttya-Bukovyna Carpathians. Since the Pokuttya fault could serve as a migration path for the hydrocarbons from the Carpathian flysch sequence into the platform basement strata, there is a high probability of the discovery of new accumulations in the structures similar to the Lopushna, revealed by seismic studies in the autochthon of the Pokuttya-Bukovyna Carpathians.

Devonian continental arc magmatism in the southern Central Asian Orogenic Belt: Evidence from the Dunhuang Block, NW China

Reconstruction of periods and sources of continental arc magmatism provides critical insights for deciphering the thermal evolution and geodynamics of the continental lithosphere and unveiling subduction-related episodes of accretion and orogeny. The Dunhuang Block, located in the southern Central Asian Orogenic Belt, is built upon a Precambrian metamorphic crystalline basement that is intruded by numerous Paleozoic granitoids. In this paper, we present new zircon U-Pb ages, whole-rock geochemistry and Sr-Nd-Pb-Hf isotopes from the Hanxiakouzi and Shuangta granodiorite plutons outcropped in the central and northern Dunhuang Block, respectively. The U-Pb zircon dating indicate that the Hanxiakouzi and Shuangta granodiorites were emplaced at ca. 366–363 Ma, i.e., in the late Devonian. Geochemically, the Hanxiakouzi granodiorites are medium- to high-K, I-type granites possessing high Sr and Sr/Y, i.e., similar to adakites. The Hanxiakouzi granodiorites show negative εNd(t) values (−8.2 to −6.6), significant positive εHf(t) values (+0.7 to +8.1), and evolved Pb isotope features suggesting their derivation by melting of subducted terrigenous sediments, interaction of melts with mantle peridotite, and assimilation of ancient crust during magma ascent. The Shuangta granodiorites belong to high-K calc-alkaline I-type granites with medium Sr and low Sr/Y. The Shuangta granodiorites yield negative εNd(t) values (−2.9 to −3.1) and negative to positive zircon εHf(t) values (−1.8 to +3.6), suggesting their formation by partial melting of ancient mafic crust with addition of minor mantle-derived melt and subducted terrigenous sediments. In general, Hf-in-zircon isotopes characteristics of the late Paleozoic granitic rocks in the Dunhuang Block indicate that the southern Dunhuang area carries more ancient crust materials than the central and northern Dunhuang areas, indicating a lateral heterogeneity of the middle Paleozoic crust. Conclusively, the middle Paleozoic granitoids of the Dunhuang Block were emplaced at an active continental margin of the southern Paleo-Asian Ocean.

Microfossils from the Bangkok Clay Formation (Upper Holocene) of central Thailand: Paleoenvironmental insight

Palynomorphs and benthic foraminifers are reported from the Upper Holocene Bangkok Clay Formation at a whale-fall excavation site in Samut Sakhon Province, located 15 kilometers from the shoreline of the Gulf of Thailand. The aim of this research is to interpret the paleoenvironment of the whale-trapped layer using bioindicator assemblages. Twenty-two palynomorphs are identified. The palynomorphs are composed of tropical plants and categorized based on their ecological communities into five groups: mangrove, back mangrove, floodplain, terrestrial, and ferns. Of these, mangrove and floodplain pollen assemblages are dominant. The pollen was transported via aeolian and fluvial systems to deposit in the delta system, then wave and tide processes dispersed them to the deeper part of the sea floor. The foraminiferal assemblage is dominated by marine ammonioids and miliolids without agglutinated forms, representing a shallow marine setting with low freshwater influence. The assemblages of foraminifers together with ostracods, gastropods and diatoms suggest sedimentary deposition in shallow subtidal marine environments, where salinity varied from slightly brackish to normal. The terrigenous sediment influx was high; the depositional environment is likely to be the distal part of a prodelta.

Titanium transport and isotopic fractionation in the Critical Zone

Stable Ti isotopes have been applied in the detrital sediment record to reconstruct the bulk composition of Earth’s continental crust due to the relationship between magmatic differentiation and Ti isotopic compositions. However, no study has systematically evaluated the influence of provenance, physical, and chemical weathering on the composition of sediments relative to the protolith they originated from. To test the influence of these processes on Ti isotopic compositions we investigate the Ti isotope composition of 82 surface samples including loess, volcaniclastic rocks, river sediment, and two separate weathering profiles through igneous rocks, collected from a broad geographical area and a range of environmental conditions. Limited but significant Ti isotope fractionation exists in samples subjected to extreme chemical weathering processes, potentially as a result of elemental mobilization. For example, the δ49Ti isotopic composition of bauxites developed on Columbia River basalt varies by up to 0.1‰, becoming isotopically heavier with increasing weathering intensity. However, negligible variation in δ49Ti was found in a second profile of saprolites developed on weathered diabase. Titanium isotope variations in loess do not correlate with chemical weathering intensity or size sorting, but may instead be related to the provenance of the sediment. We find that the δ49Ti of Amazon River sediments is correlated with the Al/Zr ratio, indicating that δ49Ti is impacted by sediment sorting. At our study sites, the river averaged offset between the isotopic composition of the bedload and the suspended sediment fraction is 0.051‰, with the largest offset being + 0.116‰. Our data suggest that during chemical weathering, heavy Ti isotopes are preferentially incorporated into secondary minerals producing higher δ49Ti in intensely weathered soils. During fluvial transport, the Ti isotopic composition of fine-grained sediment is heavier than that of its coarser counterpart. Crustal protolith composition and sorting during transport and sedimentation have a stronger effect on the Ti isotopic composition than chemical weathering. Our results have implications for studies that utilize the Ti elemental concentration to calculate relative enrichment or depletion during chemical weathering and physical transport processes in the Critical Zone and for studies using Ti isotopes in terrigenous sediments to infer the composition of their provenance.

Chronology of detrital carbonate events in Baffin Bay reveals different timing but similar average recurrence time of North American-Arctic and Laurentide ice sheet collapse events during MIS 3

Baffin Bay Detrital Carbonate (BBDC) layers represent periods of elevated discharge of terrigenous sediments, icebergs and freshwater originating from the North American Arctic-ice sheet complex. Distinct from Heinrich event layers in their dolomitic composition, these sedimentary deposits are found throughout Baffin Bay. They are considered a signature of large-scale instability of the ice sheets discharging into the Baffin Bay, akin to the Heinrich events of the North Atlantic. However, the precise timing of and potential forcing mechanisms for the North American Arctic-ice sheet instabilities remains elusive. Previous work suggests that BBDC events were not in phase with Heinrich events originating from the Hudson Strait, but this hypothesis could not be tested without rigorous chronological constraints. By combining radiocarbon ages and relative paleointensity (RPI) dating in a central western Baffin Bay marine sedimentary sequence, we were able to derive a new chronology for BBDC events covering a large part of MIS3. Sedimentological and elemental data indicate the occurrence of seven distinct BBDC events over the last 52 ka. These events occur during both glacials (stadials) and interglacials (interstadials) and have no consistent phase relationship with Heinrich events. However, the mean duration and mean recurrence time of BBDC events appears similar to Heinrich events. We conclude that BBDC events reflect repeated ice sheet instability periods, with similar statistical properties but different timing compared to the Heinrich events. This implies that episodic instability was an intrinsic property of the North American Arctic-ice sheet complex as well, but over the studied period of MIS3, instabilities of the Arctic and the Laurentide Ice sheets did not appear to have been linked.

Impacts of Particle Shape, Skeletal Porosity and Density on the Settling Velocity of Gravel‐Size Coral Debris

AbstractKnowledge of the hydrodynamic behavior of carbonate bioclasts is essential for solving coastal morphodynamic problems in marine carbonate depositional environments. Marine carbonate grains are produced within the sedimentary system itself and are deformed by subsequent hydrodynamics, resulting in particle densities, sizes and shapes that are distinct from those of terrigenous sediments. In this study, we investigated the shapes and settling characteristics of gravel‐size natural coral debris collected from the coasts of Okinawa Prefecture, Japan. Based on the shape parameters and settling velocities obtained using image analysis techniques and settling tube experiments, respectively, a correlation model for predicting the settling velocity of coral gravel particles was developed by extending the previous formula of Dietrich (1982, https://doi.org/10.1029/wr018i006p01615) over particle Reynolds number values ranging from 384.23 to 4439.76. The model determines the settling velocity as a function of particle nominal diameter, form, roundness, and surface texture parameters that were selected after examining several other shape descriptors. Compared with other models, the proposed model showed better settling velocity predictive skills for coral gravel particles classified into four shape types. The proposed model also determines how the drag coefficient of coral gravel particles should be modified from that for spherical particles, by accounting for the abovementioned coral gravel particle shape parameters. The obtained data set of settling velocity and modification of the drag coefficient can be used for quantitative interpretation of suspension, transport, and sedimentation processes in marine carbonate depositional environments.

Chemical Weathering Indices and their Use for Paleoclimatic Reconstructions (on the Example of the Vendian‒Lower Cambrian Section of Podolsk Transnistria)

The article provides an overview of various indices/indicators (Vogt, Parker, CIA, CIW, PIA, MIA, etc.) used in the study of weathering profiles/crusts and reconstruction of paleoclimatic environments of sedimentary sequence accumulation. Their possibilities are demonstrated by the example of Vendian-Lower Cambrian terrigenous deposits of Podolsk Transnistria (southwestern slope of the Ukrainian Shield). The distribution of the ba1 index values in this section indicates the presence of the material most transformed by the processes of chemical weathering in the mudstones of the Nagoryany Formation, the lower part of the Danylivka and the middle part of the Studenytsya formations. For mudstones of the Danylivka–Zbruch interval, the HM values are close to the HMPAAS. For rocks of the Yaryshiv–Nagoryany interval, the HM values are slightly higher, and for mudstones in the lower part of the Yaryshiv Formation, they are comparable to those inherent in continental clays of a hot tropical climate. The average SA index for mudstones of the entire section is 5.6 ± ± 0.7. Mudstones of the Hrushka–Nagoryany interval, where SA SAPAAS, are composed of more weathered material. The WIP values in the mudstones of the Mohyliv and Yaryshiv formations, as well as in the upper part of the Zbruch Formation, correspond to the interval of their values between PAAS and the average Archean granite. Clay rocks of other formations have WIP ≤ WIPPAAS values. The average CIA value for mudstones is 71 ± 4, which practically corresponds to the CIA value (70), which separates the sediments of cold/arid and warm/humid climates. Variations in the CIW index along the section are symbate with variations in the CIA. The vast majority of mudstones are characterized by PIA PIAPAAS values. The average CPA value is 91 ± 4, which is also typical for PAAS. The above and other data show that on the basis of a “direct” interpretation of the values of various indices of chemical weathering inherent in fine-grained clastic rocks, the paleoclimate that existed in the Vendian‒Early Cambrian on the territory of Podolsk Transnistria was rather moderate or warm humid. Comparison of the CIA values of mudstones with the values of this indicator for the particulate suspended matter of modern rivers suggests that the climate in the Vendian‒Early Cambrian resembled dry and humid subtropical or dry tropical with elements of humid.

The sources and transport pathways of sediment in the northern Ninety-east Ridge of the India Ocean over the last 35000 years

The Ninety-east Ridge (NER) is located in the southern Bay of Bengal in the northeast Indian Ocean and is composed of pelagic and hemipelagic sediments. In addition to contributions from marine biomass, the ridge also contains terrestrially sourced sedimentary material. However, considerable disagreement remains regarding the origin of these terrestrial materials and transport pathways. This paper discusses the collection of seafloor surface sediments and three sediment cores recovered from the northern region of the NER, as well as the analysis of clay minerals, Sr-Nd isotopes, and sediment grain size. The ages of the three core sediments are constrained by AMS 14C dating to better establish the source and transport pathways of the terrestrial materials within NER sediments over the past 35000 years. The research results show that the Qinghai-Tibet Plateau is the predominate source of terrigenous sedimentary material in the NER. In the plateau, the crustal materials were weathered and stripped and then transported to the Andaman Sea via the Irrawaddy River. From there, the material was transported westward by monsoon-driven circulation to the northernmost part of the NER before being transported to the south for final deposition. This transport mode has changed little over the past 35000 years. However, during the rapidly changing climate of the Younger Dryas (12.9~11.5 ka BP), there were some variations in the input amount, grain size, and Sr-Nd isotope value of the source material. The above conclusions are significant for re-evaluating the source of terrigenous sediments, the temporal and spatial changes in transport modes, and the sensitivity of the NER to climatic shifts.

Carbonate diagenesis: A celebration of the work of John Anthony Dawson (Tony) Dickson

Carbonate diagenesis: A celebration of the work of John Anthony Dawson (Tony) Dickson

Provenance and transport mechanism of gravity core sediments in the deep-water area of the Qiongdongnan Basin, northern South China Sea

Reconstructing the role of provenance and transport mechanism of sediments deposited in deep-water environments is often complex because of myriad of factors such as variability in sediment sources, tectonics, bottom morphology, climatic changes and sea level fluctuations as well as current regime. The South China Sea (SCS) is situated at the tectonic intersection of the Pacific, Eurasian, and Indo-Australian plates, and provides an excellent natural laboratory for understanding source-to-sink transport processes of fluvial sediments. The Qiongdongnan Basin (QDNB) is located in the northern part of the SCS, and the deep-water area (>300 m) of the QDNB has a good hydrocarbon source and hydrocarbon accumulation conditions with a tremendous thickness since the late Miocene-Quaternary. However, the complex sources and transport mechanisms of sediments in the QDNB are undefined. In previous study QDNB may have a major provenance or a mixture of sources. In this paper, the sediments of two cores from the deep-water area of QDNB have been analyzed using several sedimentological and geochemical approaches, including measurements of grain size distribution(433 samples), the occurrence of minerals, and trace element geochemistry(37 samples)as well as strontium and neodymium isotopes(20 samples).The granularity analysis results first show that the 19S37 core can be divided into three stages while the 19S40 core is divided into two stages by clear vertical variations in grain size characteristics. Chlorite (62.0%) and Muscovite (33.6%) appear to be the predominant minerals in the 19S37 core, while the mineral assemblage of sediments from 19S40 mainly consists of kaolinite (37.4%), muscovite (29.2%) and chlorite (27.1%). The primitive mantle standardization curves and chondrite standardized distribution patterns of 19S37 and 19S40 are consistent and display enrichments in Th, U, Pb, and Nd and depletions in Ba, Nb, Sr, and Eu. In addition, the content of Sr fluctuates greatly with depth, which should reflect the influences of sea level changes and the relative proportion of various provenance components. The 143Nd/144Nd ratios and εNd values of the sediments from 19S37 are higher than those of the sediments from 19S40, and all the εNd values show narrow variations and range from −10.085 to −11.080. We conclude that the sediments of the deep-water area in the upper Pleistocene-Holocene were mainly from the sources of the Red River, Taiwan, Pearl River, Hainan Island, and Vietnam. The transport mechanism of the sediments from Hainan Island can be explained by the short distance between the source region and the sedimentary site, while the large amounts of terrigenous sediment inputs derived from the Red River and advantageous morphology make the Red River the largest sediment source to cores 19S37 and 19S40. The transport distance and amount of Deep Water Current could be even more powerful to make Taiwan be the second largest source of modern sediments in QDNB.

Tracing Sediment Melt Activity in the Sub‐Continental Lithosphere: Insights From Zn‐Fe Stable Isotopes

AbstractRecycling of upper crustal sediments through slab subduction contributes to sub‐continental lithospheric refertilization and heterogeneity. However, the nature of recycled upper crustal components is unclear and direct evidence for sediment melt activity in the sub‐continental lithosphere is lacking. Here, we integrate major and trace elements, zircon U‐Pb dating, Sr‐Nd‐Zn‐Fe isotopic compositions of clinopyroxenites (crust‐mantle boundary) and a “glassy” xenolith from the North China Craton to relate their petrogenesis to the potential recycling of upper continental crust and provide direct insight into the sediment melt‐rock interaction. The clinopyroxenites have relatively uniform δ56Fe values (the permil deviation of the 56Fe/54Fe ratio from the IRMM014; −0.05‰–0.07‰, except for one outlier) and are not affected by melt metasomatism. The clinopyroxenites have highly variable whole‐rock δ66Zn values (the permil deviation of the 66Zn/64Zn ratio from the JMC‐Lyon standard) between 0.04‰ and 0.46‰, that closely correlate with Rb/La, K/U, Ba/Th, and Th/Nb ratios, and generate arrays that trend toward a composition similar to the “glassy” xenolith. The “glassy” xenolith has a high δ66Zn value (0.43‰ ± 0.05‰, 2SD) and a significantly low 143Nd/144Nd ratio (0.510991). This evidence implies that the “glassy” xenolith may represent a quenched sediment melt formed by the melting of carbonate‐bearing terrigenous sediments that may also be responsible for the metasomatism of clinopyroxenite xenoliths. The geochemical evidence from the “glassy” and clinopyroxenite xenoliths provides a direct evidence for the activity of sediment melt with upper continental crust components in the sub‐continental lithosphere.

Permian Terrigenous Deposits of the Omolon Massif: Formation Settings and Source Area (According to Geochemical Data)

Permian Terrigenous Deposits of the Omolon Massif: Formation Settings and Source Area (According to Geochemical Data)