Burial Of Sediments Research Articles

“Excess Ar” by laboratory alteration of biotite

AbstractMany biotite phenocrysts from marine tephra layers have substoichiometric potassium concentrations and alkali occupation <<2.0 atoms per formula unit. Diagenetic alteration is an expected effect of exposure of fresh magmatic minerals to interstitial water and brine intrusions after the deposition and burial of sediments. To test the effect of diagenetic alteration on potassium-argon ages, we irradiated and step heated untreated Fish Canyon biotite (t = 28.2 Ma) and several aliquots leached to various extents in strong and weak acids. Laboratory alteration caused loss of K, age spectrum discordance, high step ages and total gas ages, Ar release at lower furnace temperature, higher Cl/K and Ca/K, and a slight decrease in 36Ar concentration. Potassium loss was always higher than 40Ar* loss. Electron microprobe element maps document that acids preferentially penetrated in phyllosilicate interlayers, removing K (and Na). Because Ar* is removed to a lesser extent than K, we propose that natural 40K decay partly implants radiogenic Ar* into the tetrahedral-octahedral-tetrahedral (T-O-T) phyllosilicate layer, where Ar is shielded from interlayer leaching. The recoiled 39Ar, which was produced by irradiation after the leaching, also partitioned between T-O-T and the interlayer; age spectrum discordance was probably enhanced by the heterogeneous partition of 39Ar and 40Ar* in leached samples.

Hafnium isotope evidence for enhanced weatherability at high southern latitudes during Oceanic Anoxic Event 2

The Oceanic Anoxic Event 2 (OAE 2; ca. ∼94 Ma) represents one of the most extreme carbon cycle perturbations of the Phanerozoic, which coincided with major environmental and climate reorganization in both terrestrial and marine realms. Chemical weathering of continental silicate rocks is thought to have played a crucial role during OAE 2, through enhanced release of bio-essential nutrients to the ocean, promoting high rates of marine primary production and organic carbon burial, but also due to its effect on atmospheric CO2 drawdown, which altogether possibly drove the OAE 2 termination. Yet, the evolution of continental chemical weathering during OAE 2 remains poorly defined, especially in high-latitude regions. In this study, we present a combined hafnium-neodymium isotope investigation of the clay-size detrital fraction (△εHfclay) of late Cenomanian to early Turonian sediments from the southwest Australian margin, at a site (International Ocean Discovery Program U1516) located in the southern high latitudes (∼62°S) during the late Cretaceous. The reliability of △εHfclay as a proxy for continental chemical weathering in ancient anoxic marine sediments was assessed by analyzing a suite of samples retrieved from methanogenic sediments experiencing marine silicate weathering at ocean margins, suggesting negligible effect of reverse weathering on hafnium-neodymium isotope compositions. At Site U1516, the early stage of OAE 2 was characterized by relatively low △εHfclay values (−5.9 ± 2), typical of reduced chemical weathering in nearby continental regions. At the onset of the most prominent carbon isotope excursion, an abrupt decrease in △εHfclay points towards accelerated export of poorly weathered sediments resulting from the abrupt reactivation of river systems in southwest Australia. This period was followed by a pronounced △εHfclay shift towards positive values, indicative of intensifying chemical weathering conditions during the OAE 2 interval showing the highest δ13C anomaly. Based on these results, we posit that enhanced hydrological cycle, most likely caused by a southward shift of the westerlies, led to a large increase in weatherability at southern high latitudes during peak OAE 2 warmth. This finding provides empirical support for the potential role played by high-latitude weathering systems in driving the termination of OAE 2, via weathering-driven consumption of atmospheric CO2 and accelerated riverine fluxes of nutrients leading to enhanced organic carbon burial in marine sediments.

Contrasting nutrients input along with groundwater discharge to east Dongting Lake, central China: A geological perspective

The spatial patterns of lacustrine groundwater discharge (LGD) and associated nutrient inputs are crucial for effective management and protection of lakes. Multiple factors have been found to influence the spatial differences in LGD rates and associated nutrients loads, but the influence of geological conditions on the differences have not been well understood. In this study, we quantified LGD rates and associated nutrients loads on the west and east of the lake with contrasting geological conditions of East Dongting Lake (EDL) within central Yangtze catchment and discuss the influence of geology on the spatial differences, through 222Rn mass-balance model, water chemistry coupled with existing geological data. The results showed that LGD rates were 34.76 ± 23.36 mm/d in the east EDL which is characterized by hilly geomorphy, deep/fast/narrow flowing, coarse-grained lakebed and large hydraulic gradients (0.004–0.006). Surprisingly, LGD rates were higher (71.47 ± 52.16 mm/d) in the west EDL which is characterized by alluvial-lacustrine plain geomorphology, shallow/sluggish flowing, clayey or silty lakebed and low hydraulic gradients (0.0002–0.0015). The remaining factor determining the higher LGD rates in the west EDL is the permeability of the porous aquifer connected with the lake. The groundwater around the east EDL existed in a less confined environment, and frequent flushing led to low concentrations of nutrients. By contrast, rapid burial of sediments and deposition of paleo-lake sediments since Last Deglaciation formed an organic-rich and reducing environment, which facilitated the enrichment of geogenic nutrients. As a result, the loads of LGD-derived nutrients in the west generally exceeded that in the east by one order of magnitude. The study provides new understanding of the spatial differences in LGD and associated nutrients input to the large lakes resulting from variations in geological conditions, thus serving as a reference for ecological protection of lakes.

Organic Carbon Burial With Reactive Iron Across Global Environments

AbstractPreservation of organic carbon (OC) in marine and terrestrial deposits is enhanced by bonding with reactive iron (FeR). Association of OC with FeR (OC‐FeR) provides physical protection and hinders microbiological degradation. Roughly 20% of all OC stored in unconsolidated marine sediments and 40% of all OC present in Quaternary terrestrial deposits is preserved as OC‐FeR, but this value varies from 10% to 80% across global depositional environments. Here, we provide a new assessment of global OC‐FeR burial rates in both marine and terrestrial environments, using published estimates of OC associated with FeR, carbon burial, and probabilistic modeling. We estimate the marine OC‐FeR sink between 31 and 70 Mt C yr−1 (average 52 Mt C yr−1), and the terrestrial OC‐FeR sink at between 146 and 917 Mt C yr−1 (average 446 Mt C yr−1). In marine environments, continental shelves (average 17 Mt C yr−1) and deltaic/estuarine environments (average 11 Mg C yr−1) are the primary settings of OC‐FeR burial. On land, croplands (279 Mt C yr−1) and grasslands (121 Mt C yr−1) dominate the OC‐FeR burial budget. Changes in the Earth system through geological time impact the OC‐FeR pools, particularly in marine settings. For example, periods of intense explosive volcanism may lead to increased net OC‐FeR burial in marine sediments. Our work highlights the importance of OC‐FeR in marine carbon burial and demonstrates how OC‐FeR burial rates may be an order of magnitude greater in terrestrial environments, but here OC‐FeR stocks are most sensitive to the anthropogenic impacts of climatic change.

Sand burial has more negative impacts than interspecific competition on a riparian pioneer plant species: The Dwarf bulrush (Typha minima Hoppe)

Both biotic interactions and abiotic processes are important drivers of changes in riparian communities. However, little is known about how their effects could lead to the riparian pioneer species exclusion over time. We therefore conducted an ex-situ experiment in the Isère valley (France) to disentangle the effects of interspecific competition and sediment burial on the survival, growth and reproduction strategies of an endangered pioneer herbaceous plant species: Typha minima Hoppe. We crossed two factors with two modalities each: with the presence or absence of Salix alba L., (a common pioneer riparian tree) and in buried (with 10 cm of silty and -sandy alluvial sediments) or unburied conditions. The plants were placed in individual containers with 12 replicates for each combination of modalities. During the second vegetation season, we monitored mortality rates and traits, related to competitive vigor, energy storage ability and spatial exploration. Our analysis showed that interspecific competition had little effect on T. minima: it only reduced inflorescence and stem number. On the contrary, sediment burial induced a high mortality rate and strongly decreased all the monitored traits related to growth and reproduction strategies. When applied together, competition and sediment burial induced reduced leaf length and rhizome biomass. Our results highlight that T. minima is more resilient to interspecific competition than to sediment burial. To better understand alpine population trends, future research should be focused on other abiotic factors such as water resource availability, disturbance regimes and habitat disconnection induced by bar aggradation processes.

Prevalence of Autotrophy in Non-humic African Lakes

Heterotrophic respiration of organic matter (OM) is thought to dominate over aquatic primary production (PP) in most freshwater lake ecosystems. This paradigm implies that lateral transport of OM from the terrestrial biosphere subsidize the major fraction of aquatic respiration and that many lakes are a net source of carbon dioxide (CO2) to atmosphere. Nevertheless, African lakes were absent of the datasets upon which this paradigm was built. Here, we report a comprehensive and methodologically consistent data set of pelagic PP and community respiration (CR) obtained over the last decade in contrasting non-humic African lakes including 5 of the East African Great lakes (Tanganyika, Kivu, Edward, Albert, Victoria) and smaller shallow lakes located in Eastern Africa. Also, we determined the partial pressure of CO2 in surface waters and examined the sources and dynamics of organic and inorganic carbon by means of stable isotope tools across a wide range of physical and chemical conditions and productivity status. Our observations revealed that the threshold value at which the equivalence between PP and CR is met is substantially lower in Africa (10 mmol C m−3 d−1) than at higher latitude (25 mmol C m−3 d−1), suggesting that non-humic African lakes tend to be more autotrophic than expected from empirical relationships derived from data collected in boreal and temperate regions. Integrated at the regional scale, we estimate that PP is about 20 times higher than the organic carbon burial in sediments. It implies that a large fraction (< 90%) of PP is effectively recycled in the warm water column of non-humic African lakes.

Major sulfur cycle perturbations in the Panthalassic Ocean across the Pliensbachian-Toarcian boundary and the Toarcian Oceanic Anoxic Event

The early Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) was characterized by marine deoxygenation and the burial of organic-rich sediments at numerous localities worldwide. However, the extent of marine anoxia and its impact on the sulfur cycle during the T-OAE is currently poorly understood. Here, stable sulfur isotopes of reduced metal-bound sulfur (δ 34 S pyrite ) and pyrite sulfur concentrations (S PY ) have been analyzed across the Pliensbachian-Toarcian boundary (Pl-To) and the T-OAE from the Sakahogi and Sakuraguchi-dani sections (Japan), which were deposited in the deep and shallow Panthalassic Ocean, respectively. Our data reveal marked positive δ 34 S pyrite excursions of >10‰ across both the Pl-To and the T-OAE at Sakahogi, coincident with increases in S PY , and a positive excursion of >20‰ at the onset of the T-OAE at Sakuraguchi-dani. Whilst the development of deep-water anoxic/euxinic conditions could have resulted in an enhanced burial of pyrite, and also partly contributed to the positive excursion of δ 34 S pyrite , variations in δ 34 S pyrite at Sakahogi were most likely controlled by elevated export production and/or preservation. On the shallow shelf generally low and highly variable S PY and the positive shift in δ 34 S pyrite were likely attributable mainly to elevated sedimentation rates, with redox playing only a minor role in controlling pyrite abundance. Our discovery of a positive δ 34 S pyrite excursion across the Pl-To at Sakahogi indicates a hitherto unrecognized perturbation to the deep-water sulfur cycle, potentially associated with increased seafloor organic matter flux and pyrite burial at this time, consistent with a transient interval of anoxia. • First pyrite S-isotope (δ 34 S pyrite ) data across the Toarcian OAE (T-OAE) • Positive pyrite S-isotope excursions across T-OAE from deep and shallow Panthalassa • Excursions mainly attributable to local depositional conditions. • Deep-water sulfur isotope excursion across Pliensbachian-Toarcian boundary

The carbon budget of crustal reworking during continental collision: Clues from nanorocks and fluid inclusions

The source of volatiles in the continental crust is a long-standing issue. In addition to controlling the amount of melt generated during anatexis, H2O and CO2 budgets of the middle and lower siliciclastic crust are also of great importance for carbonate precipitation, ore concentration, orogenic degassing and carbon storage. Here we focus on two case studies of partially melted metamorphic rocks of crustal affinity– the Ivrea Zone in the Western Alps (0.8 GPa) and the Central Maine Terrane (1.8 GPa), USA. These terrains contain fluid inclusions and carbon-bearing nanogranitoids (former melt inclusions) from which the H2O and CO2 content has been estimated via in-situ analyses. Thermodynamic modelling is used to quantify the amount of internally derived, mineral-bound bulk rock CO2 necessary to reproduce the volatile contents of these melt inclusions. The minimum amount of bulk rock CO2 present at peak metamorphic conditions is estimated at 400 ppm for the Ivrea Zone and 3000 ppm for the Central Maine Terrane. This suggests that the flux of carbon associated with the burial of siliciclastic sediments in the lower crust during the Phanerozoic is 0.2–4.4 Mt. C/yr. These values, as well as the nature of the source of the deep crustal carbon might have changed with time, with periods dominated by internal reworking rather than external inputs. The protracted growth and differentiation of the continental crust through the reworking of supracrustal materials in continental collision settings is a key element of carbon storage processes. The stability of the continental crust through time provides an ultimate, long-lasting reservoir of carbon.

Hybrid machine learning models for estimating total organic carbon from mineral constituents in core samples of shale gas fields

The analysis of total organic carbon (TOC) contents is an important activity in exploring potentially hydrocarbon-generating intervals. Petroleum source rocks have, by definition, high TOC values due to the accumulation of organic matter in anoxic and low-energy deposition environments over geological time. Such petroleum generation is called conventional because it results from the increasing temperature and pressure conditions during continuous sedimentary burial. On the other hand, unconventional generation refers to reserves formed due to another heat source, for example, associated with intrusive igneous emplacement. In the last decade, there has been a growing economic interest in exploring unconventional hydrocarbon reserves because of the abundance of its occurrences. However, finding ways to assess the total organic carbon (TOC) content at lower cost and time-consuming laboratory experiments is becoming fundamental due to the importance of TOC in the analysis of the composition of these resources. This study predicts TOC using a hybrid approach, integrating machine learning models into a Grey Wolf Optimization Algorithm to adjust their parameters. The performance of four methods was compared: Elastic Net Linear Model, Extreme Learning Machine, Multivariate Adaptive Regression Splines, and Linear Support Vector Regression. The methodology was evaluated by applying core samples of the shale gas field YuDong-Nan belonging to the Sichuan Basin. The results show that the machine learning methods assisted by the evolutionary algorithm could accurately estimate TOC and be used to further exploratory geological analysis, especially those related to the prospects of unconventional oil-gas resources.

Characteristics, formation mechanism and influence on physical properties of carbonate minerals in shale reservoirs of Wufeng-Longmaxi formations, Sichuan Basin, China

The characteristics, formation mechanisms, and influences on physical properties of carbonate minerals in shale reservoirs of Wufeng-Longmaxi formations in Sichuan Basin are systematically investigated by utilizing electron probe microscope with spectrometer and energy spectrometer, combined with physical properties and whole rock X-diffraction and organic carbon data. The research yielded the findings that follow: First, the main carbonate minerals are calcite, dolomite, and ferriferous dolomite. Calcite is a single mineral that fills the siliceous shell cavity of radiolarians and exists between mineral particles. Ferriferous dolomite always rings dolomite, which is a single mineral that is present among mineral particles and aggregates. Second, calcite is produced by microorganisms that secrete calcium carbonate in the surface of seawater. The siliceous skeleton cavity of radiolarian and seawater both precipitate calcite, which partially dissolves while settling in seawater before depositing on the seabed and being preserved by burial. Thirdly, the dolomite is a diagenetic mineral formed on the water–sediment interface with physiological activities of sulfate bacteria, and the ferriferous dolomite is produced by methanogenic metabolism during the initial burial of muddy sediments. Fourthly, organic carbon, pyrite, quartz, and clay minerals are closely related to reservoir physical properties, while carbonate has no effect on porosity and permeability as a whole. Future research on shale reservoir diagenesis should make use of the in-situ detection and element area scanning, in particular with the spectrogram from electron probe microscope technology, which provides typical petrological evidences for the study of characteristics, formation mechanism, and influence on physical properties of carbonate minerals in shale reservoirs.

Changes in Organic Carbon Delivery to the Yangtze River Delta Over the Last 2000 Years

Natural processes and anthropogenic activities are vital in dictating the amount and character of organic carbon (OC) input into large river deltas and adjacent shelves. Previous studies have indicated that sediment from the Huanghe River (HR) has significantly affected the formation of the northern Yangtze River subaqueous delta (YRD) over the past several hundred years. However, whether this process has changed sedimentary OC burial in the YRD remains unclear. A sediment core was collected from the YRD in 2018 CE for optically stimulated luminescence and 210Pb dating as well as grain size, total OC, total nitrogen, and stable-isotope analyses to investigate temporal changes in sedimentary OC over the past 2000 years. The results indicate that changes in terrestrial OC inputs to the YRD have been controlled mainly by the East Asian summer monsoon and anthropogenic influences in the past 2000 years. However, the decreased terrestrial OC inputs after 1385 CE, have been significantly affected by increased contribution of HR sediment to the YRD when the HR lower courses shifted to enter the southern Yellow Sea. This study demonstrates that sediment source changes should not be neglected in analyses of mechanisms and variations in OC burial in estuarine and coastal areas.

Sedimentary Hydrodynamic Processes Under Low-Oxygen Conditions: Implications for Past, Present, and Future Oceans

Continental margin sediments represent a major global sink of organic carbon (OC), and as such exert a key control on Earth’s climate. Today, OC burial in marine sediments mainly takes place under oxygen-rich water columns, where most OC is stabilized through intimate association with sediment grains and biogenic minerals. In prior episodes of Earth’s past, when large parts of the oceans were anoxic, the mode of sedimentary OC burial must have been very different, however. Present-day analogues indicate that surface sediments accumulating under low-oxygen water columns are often “soupy” in texture. Moreover, most OC occurs in large (100–2,000 μm diameter) organic and organo-mineral aggregates which, due to their low density, are prone to wave- and current-induced resuspension. Upon mobilization, these aggregates can undergo lateral transport within so-called nepheloid layers, and may be translocated hundreds of kilometres, and on timescales of thousands of years. Little is known about processes of formation, resuspension and hydrodynamic properties of these aggregates in oxygen-poor waters, or which factors control their eventual breakdown or burial. The goal of this study is to examine the drivers and biogeochemical consequences of this resuspension on OC cycling in modern, oxygen-depleted, “Semi-Liquid Ocean Bottom” (SLOB) regions. We argue that models of sediment and OM hydrodynamics and redistribution that describe sedimentation processes in oxygenated ocean waters of the modern ocean are a poor analogue for equivalent processes occurring under oxygen-deficient conditions. In the latter, we hypothesize that 1) the abundance of low-density organic-rich particles and aggregates leads to a greater propensity for sediment remobilization at low(er) shear stress, and 2) upon resuspension into low-oxygen bottom waters, remobilized OM may be subject to less degradation (less attenuation) during lateral transport, leading to efficient and widespread translocation to distal centres of deposition. We address specific aspects of the SLOB hypothesis utilizing a combination of literature and new data, focussing on the Benguela Upwelling Region as a model system.

Sources and Fate of Organic Carbon in West Spitsbergen Fjord Systems: A Review

Abstract Fjord margins in the high latitude regions are the hotspots for long term carbon burial. Studying the source and fate of organic carbon in high latitude fjords would help assess the sedimentary processes and burial. With the unprecedented warming, the marine productivity patterns are changing, leading to changes in the sources of organic carbon and its eventual burial in marine sediments. In the present study, high latitude fjords in the West Spitsbergen region has been reviewed, subjected to competing effects of large freshwater influx from the glacial melts, enhanced West Spitsbergen Current and its control on marine productivity. The West Spitsbergen Current advects warm North Atlantic waters enriched in nutrients to the fjords in the West Spitsbergen region. The increased freshwater from the glacial melts also dominates the fjords. Thus, the composition of organic matter in the fjord is mainly controlled by the inflow of freshwater from the glaciers and intrusion of oceanic waters to the fjords influencing the organic matter stability and carbon burial efficiency in the fjord sediments. The ongoing rapid warming in the Arctic changes the hydrographic settings, and changes in the strength of West Spitsbergen Current may also influence the long-term organic carbon burial in the West Spitsbergen fjords.

The Biotic‐Abiotic Control of Si Burial in Marine Carbonate Systems of the Pre‐Eocene Si Cycle

AbstractThe silicon (Si) cycle in the modern ocean might still be representative of some of the processes that occurred in the Si‐depleted post‐Eocene oceans resulting after the expansion of diatoms. However, silicon‐rich pre‐Eocene seas, where sponges and radiolarians were major Si users before the emergence of diatoms, were radically different from modern ocean scenarios. The spatial and temporal evolution of Si cycling in Earth history is recorded in geological deposits and could be reconstructed by petrographic and mineralogical analyses. The thick successions of carbonate siliceous rocks deposited in marine environments during the Paleozoic and Mesozoic indicate the significant role of Si outflow from the Si cycle via burial in sediments. The aim of this study is to fill an important gap in knowledge concerning the functioning of the main silicon sink in the oceans: burial in sediments. Si outflow from the marine biogeochemical Si cycle occurs via early diagenetic silica crystallization within seabed mud. The mechanisms leading to Si binding in silica minerals are a complex process controlled by abiotic global events, biological activity of silicifiers and geochemical conditions mediated by microbes. This study concentrates on reconstructions of the mechanisms of Si outflow from the Si cycle by silica polymorph crystallization below the seabed surface and was realized through mineralogical and microtextural analyses of the main components of Upper Cretaceous carbonate–siliceous rocks.

First evidence for Neoproterozoic rocks offshore South-East Greenland

AbstractMeta-sedimentary rocks recovered beneath Palaeogene basalts near the base of Ocean Drilling Program (ODP) Leg 152-917A offshore South-East Greenland were thought to be of Late Cretaceous age. This interpretation, however, has several inconsistencies as it requires a tectono-metamorphic event during the Cretaceous not recognized in the North Atlantic region, and the presence of a wide Mesozoic sedimentary basin that extended from SE-Greenland to the Rockall Plateau, for which there is currently no evidence. Here, we report a Neoproterozoic U/Pb apatite age of 905 ± 21 Ma and a younger 40Ar/39Ar isochron whole-rock age of 820 ± 40 Ma for an altered tuff layer that occurs in the upper part of the meta-volcaniclastic sequence recovered from hole 917A. The 40Ar/39Ar step-heating ages on biotite and whole-rock mini-cores from deeper in hole 917A yielded Palaeoproterozic dates that cluster around 1950 to 1850 Ma, pointing toward a Palaeoproterozoic source. The U/Pb apatite date is interpreted as the eruption age of the tuff layer, whereas the younger whole-rock 40Ar/39Ar age is consistent with low-temperature greenschist alteration of volcanic glass and secondary mineral growth during sedimentary burial in an extensional regime. The c. 905 Ma age for the tuff provides the first evidence for Neoproterozoic rocks offshore South-East Greenland and suggests a correlation between this sequence and the Torridon Group in the Hebridean Foreland of the Scottish Caledonides. The calc-alkaline nature of the volcaniclastic rocks and the age of the tuff layer point toward a source area with arc-magmatism related to the Renlandian event of the Valhalla Orogeny.

Underestimated PAH accumulation potential of blue carbon vegetation: Evidence from sedimentary records of saltmarsh and mangrove in Yueqing Bay, China

Sediments of blue carbon vegetation are becoming a sink of natural and anthropogenic pollutants, such as polycyclic aromatic hydrocarbons (PAHs). However, the extent to which PAHs are accumulated and varied in blue carbon sediments, and the impact of blue carbon vegetation on the accumulation and retention capacity of PAHs, have been poorly explored. This study examines the sedimentary records of PAHs in profiles from mangrove plantation, saltmarsh, and mudflat in Ximen Island and Maoyan Island of Yueqing Bay, China. The existence of blue carbon vegetation provides a sheltered environment for the accelerated burial of sediment and OC. Decadal PAH sedimentation records show staged changes characterized by the emission of PAHs and colonization of blue carbon vegetation, reflecting the accelerated burial of PAHs in sediments by blue carbon vegetation colonization. In addition, the colonization of blue carbon vegetation contributes to the shift of PAH compositions in sediments. This study provides new insights into the underestimated PAH accumulation potential and retention capacity of blue carbon vegetation and the corresponding underlying sediments, supporting the environmental benefits of blue carbon vegetation.

Fate of Soil Carbon Transported by Erosional Processes

The accelerated process of soil erosion by water and wind, responsible for transport and redistribution of a large amount of carbon-enriched sediments, has a strong impact on the global carbon budget. The breakdown of aggregates by erosivity of water (raindrop, runoff) and wind weakens the stability of soil C (organic and inorganic) and aggravates its vulnerability to degradation processes, which lead to the emission of greenhouse gases (GHGs) including CO2, CH4, and N2O, depending on the hydrothermal regimes. Nonetheless, a part of the eroded soil C may be buried, reaggregated and protected against decomposition. In coastal steep lands, (e.g., Taiwan, New Zealand) with a short distance to burial of sediments in the ocean, erosion may be a sink of C. In large watersheds (i.e., Amazon, Mississippi, Nile, Ganges, Indus, etc.) with a long distance to the ocean, however, most of the C being transported is prone to mineralization/decomposition during the transit period and is a source of GHGs (CO2, CH4, N2O). Land use, soil management and cropping systems must be prudently chosen to prevent erosion by both hydric and aeolian processes. The so-called plague of the soil, accelerated erosion by water and wind, must be effectively curtailed.

Contrasting P-T-t paths of basement and cover within the Búzios Orogen, SE Brazil – Tracking Ediacaran-Cambrian subduction zones

Deeply eroded collisional orogens show complex structural and inverted stratigraphic relations with juxtaposition of rock stacks from distinct crustal levels, origins and ages, hence with contrasting P-T-t paths during convergent tectonics. This paper presents petrochronological data on Paleoproterozoic (basement) and Ediacaran (cover) gneisses tectonically interleaved during the Ediacaran-Cambrian Búzios Orogeny, in southeastern Brazil. U-Pb in zircon and EPMA U-Th-Pb in monazite data, coupled with geothermobarometric data, plus Zr-in-rutile, reveal that at a first orogenic stage (ca. 530–520 Ma), these units were at distinct crustal levels. Samples within the Paleoproterozoic basement show metamorphic near-peak conditions of ∼800 °C and 10 kbar, at a depth of c.37 km. Contrastingly, Ediacaran kyanite-orthoclase-garnet-biotite granulite with retrometamorphic sillimanite (cover) reached near-peak conditions of 15 kbar and 818 °C–785 °C at depths of c. 55 km, in high-pressure granulite facies. This deep burial of Ediacaran sediments in less than 20 m.y. would be compatible with a low angle subduction zone active from ca. 550 to 530 Ma. Intrusion of ca. 550 Ma tholeiitic dykes in the Paleoproterozoic gneiss indicates a high geothermal gradient for this subduction setting, which is consistent with a low subduction rate. In a second orogenic stage (ca. 520–500 Ma), these distinct stratigraphic units were placed tectonically side by side during a fast exhumation, preserving an inverted metamorphic stack. The cover underwent retrometamorphic conditions of 800 °C and 10 kbar on a clockwise return path due to decompression. It is proposed here that the contact between reworked units within a Paleoproterozoic continental crust and Ediacaran magmatic and sedimentary units represent the suture of an Ediacaran NW-subduction of the Angola continental paleomargin below the Oriental Terrane of the Ribeira belt. This paper reports the highest pressure recorded in Ediacaran-Cambrian metamorphic rocks from the Brasiliano belts along the actual South Atlantic continental margins. The suture we propose here is aligned along strike, with a medium to high-pressure Ediacaran metamorphic occurrence 700 km to the SW, in the Curitiba Terrane.

Benthic foraminiferal assemblages and environmental drivers along the Kveithola Trough (NW Barents Sea)

We describe the population density, biodiversity and vertical distribution in the sediment of benthic foraminifera, and their relationship to environmental parameters, in the Kveithola Trough (NW Barents Sea). Two staining methods, Cell Tracker Green (CTG) and Rose Bengal (RB), were used to distinguish between living and dead tests. CTG proved to be more effective than RB, and we therefore used this approach to document faunal assemblage variability along a transect of the Trough. The outer shelf shows a diverse benthic foraminiferal assemblage suggesting an oxygenated and oligotrophic environment. The central part appears to be a disturbed area due to rapid circulation changes and organic matter burial in sediments where opportunistic foraminifera colonize only the first centimetres. In contrast, the inner part of the Trough seems to be a stressed environment where species associated with organic-rich sediment and oxygen-depleted environments dominate the living assemblage. At all sites, delicate monothalamids species form part of the assemblage. The peculiar geomorphological and environmental conditions of this area, and the high regional primary and secondary production, are key drivers of foraminiferal assemblage distribution. • This work analyses the living benthic foraminifera of the Kveithola Trough. • The peculiar conditions shape the living benthic foraminiferal structure. • Faunal distribution varies along the longitudinal transect of the area. • The main determining factor in the diversity is the organic matter concentration. • CTG method is appropriate to document the vitality of foraminifera in this area.

Thermal maturity of the Hawasina units and origin of the Batinah Mélange (Oman Mountains): Insights from clay minerals

The Oman Mountains developed during Cretaceous to Cenozoic time by obduction of the Semail Ophiolite on top of the Arabian rifted margin. The tectonic pile of the orogenic system is composed of three major domains, which from bottom to top are: (i) the proximal domain of the Arabian rifted margin; (ii) the Hawasina Nappe, including rocks pertaining to the distal portions of the Arabian rifted margin; (iii) the Upper Cretaceous Semail Ophiolite. However, in NE Oman (Batinah Coastal Plain), exposures of Hawasina rocks are resting above the ophiolite.What is the origin of the uncommon structural position of Hawasina rocks in the Batinah Coastal Plain? To address this question, an extensive dataset of X-ray diffraction analyses from rocks of the Hawasina units has been used to reconstruct their thermal and burial history since the Late Cretaceous. Temperature-dependent clay minerals indicate that the Hawasina units experienced different levels of thermal maturity from early diagenetic to anchizone conditions depending on their structural position during orogenic build-up. Rocks from the Hawasina units resting above the ophiolite contain random-ordered mixed layer illite-smectite (I–S) with an illite content between 30 and 45% displaying low levels of thermal maturity reflecting early diagenetic conditions that were acquired because of limited sedimentary burial (600 m) and minor heating during the early Pliocene. Deep-water rocks of the Hawasina units below the ophiolite are characterized by long-range ordered mixed layer I–S with an illite content between 80 and 90% and by a mineralogical assemblage of rectorite and pyrophyllite displaying more evolved levels of thermal maturity in deep diagenetic/anchizone conditions that were acquired during the obduction of the 4100 to 5500 m thick Semail Ophiolite. We finally propose a new evolutionary scheme for the genesis of the Batinah Mélange, that was not buried by the ophiolite but was transported by gravity-driven mass transport on top of the ophiolite. Our results may further improve the assessment of thermal maturity of Arabian passive margin deposits for hydrocarbon exploration purposes.