Outer Slope Research Articles

Microbiome of juvenile corals in the outer reef slope and lagoon of the South China Sea: insight into coral acclimatization to extreme thermal environments.

Environmental conditions between the outer reef slope (ORS) and lagoon in tropical atolls are significantly different, but the variations of juvenile coral-microbiomes in the two environments and their relationship with coral thermal acclimatization are poorly understood. We explored this issue based on local water conditions and the microbiome of juvenile corals in the ORS and lagoon in the central South China Sea. Coral-symbiotic Symbiodiniaceae showed significant differences among coral species; Pocillopora verrucosa and Pachyseris rugosa in the ORS, and Acropora formosa in the lagoon were dominated by Durusdinium, but other corals were dominated by Cladocopium. Although A. formosa in the ORS were dominated by Cladocopium (C3u), they were dominated by Durusdinium (D1/D1a) and Cladocopium (C50) in the lagoon. Other coral species were both dominated by Cladocopium in the lagoon and ORS. The relative abundance of bacteria in the Deinococcus-Thermus was generally higher in the lagoon corals than in the ORS corals. Our study indicates that P. verrucosa, P. rugosa and Porites lutea may have high thermal tolerance based on the relatively high abundance of heat-tolerant Durusdinium and Thermus scotoductus. Likewise, A. formosa in the lagoon may acclimatize to the thermal environment based on a high relative abundance of heat-tolerant Durusdinium.

Mantle-derived helium released through the Japan trench bend-faults

Plate bending-related normal faults (i.e. bend-faults) develop at the outer trench-slope of the oceanic plate incoming into the subduction zone. Numerous geophysical studies and numerical simulations suggest that bend-faults play a key role by providing pathways for seawater to flow into the oceanic crust and the upper mantle, thereby promoting hydration of the oceanic plate. However, deep penetration of seawater along bend-faults remains controversial because fluids that have percolated down into the mantle are difficult to detect. This report presents anomalously high helium isotope (3He/4He) ratios in sediment pore water and seismic reflection data which suggest fluid infiltration into the upper mantle and subsequent outflow through bend-faults across the outer slope of the Japan trench. The 3He/4He and 4He/20Ne ratios at sites near-trench bend-faults, which are close to the isotopic ratios of bottom seawater, are almost constant with depth, supporting local seawater inflow. Our findings provide the first reported evidence for a potentially large-scale active hydrothermal circulation system through bend-faults across the Moho (crust-mantle boundary) in and out of the oceanic lithospheric mantle.

Which Method for Which Purpose? A Comparison of Line Intercept Transect and Underwater Photogrammetry Methods for Coral Reef Surveys

The choice of ecological monitoring methods and descriptors determines the effectiveness of a program designed to assess the state of coral reef ecosystems. Here, we comparer the relative performance of the traditional Line Intercept Transect (LIT) method with three methods derived from underwater photogrammetry: LIT on orthomosaics, photoquadrats from orthomosaics, and surface analyses on orthomosaics. The data were acquired at Reunion Island on five outer reef slopes and two coral communities on underwater lava-flows. Coral cover was estimated in situ using the LIT method and with LITs and photoquadrats digitized on orthomosaic. Surface analyses were done on the same orthomosaics. Structural complexity of the surveyed sites was calculated from digital elevation models using three physical descriptors (fractal dimension, slope, surface complexity), and used to explore their possible influence in coral cover estimates. We also compared the methods in terms of scientific outputs, the human expertise and time required. Coral cover estimates obtained with in situ LITs were higher than those obtained with digitized LITs and photoquadrats. Surfaces analyses on orthomosaics yielded the lowest but most the precise cover estimates (i.e., lowest sample dispersion). Sites with the highest coral cover also had the highest structural complexity. Finally, when we added scientific outputs, and requirements for human expertise and time to our comparisons between methods, we found that surface analysis on the orthomosaics was the most efficient method. Photoquadrats were more time-consuming than both in situ and digitized LITs, even though they provided coral cover estimates similar to those of digitized LITs and yielded more than one descriptor. The LIT in situ method remains the least time-consuming and most effective for species-level taxonomic identifications but is the most limited method in terms of data outputs and representativeness of the ecosystem.

Flood risk reduction by parallel flood defences – Case-study of a coastal multifunctional flood protection zone

In this paper the safety of a double-dike system (or twin dikes) is assessed. Such a system consist of two parallel lines of flood defences. During storms the combined strength of the parallel flood defences must prevent flooding of the hinterland. A culvert can be implemented for the tidal exchange of sea water to enable new land-uses in the area between the dikes such as aquaculture, saline agriculture, salt marsh restoration and clay extraction. We develop a general framework for assessing the safety of such double dike systems and apply a simplified version to the Double Dike between Eemshaven and Delfzijl (The Netherlands) to test this method. In doing so, we aim to quantify the flood protection benefits of parallel flood defenses and enable their use in multifunctional flood protection strategies.Within the framework the transmission of hydraulic loads by the seaward dike to the landward dike in the case-study was described by overtopping, overflow and erosion of the outer slope, alongside discharge through the culvert in the event of a non-closure. For the subset of coastal double dike systems with a tall seaward dike (as in the case-study), the results show only a negligible improvement in flood protection compared to a single dike system. With the addition of a culvert in the first dike, flood risk will only be reduced by the second landward dike if its height is sufficient to retain water in the event of a non-closure during common storm events. These double dike systems are implemented for potential uses of the inter-dike zone, e.g. for nature restoration, rather than as a measure to primarily improve flood protection.

Marine biodiversity of a pristine coral reef in French Polynesia

Understanding the natural state of coral reefs is paramount to evaluate the response of these ecosystems to local and global human impacts as well as management and conservation strategies. In French Polynesia, some islands are still pristine or uninhabited, such as Tupai atoll. Tupai has been uninhabited, with access to the lagoon prohibited since 2010. However, fishers from nearby islands often take from the outer reef slope at Tupai. Our marine biodiversity survey (coral, macro-invertebrates, and fish) conducted in 2019 highlighted a low density of commercial fish species and top-predators on the outer slope in comparison to the lagoon, where the top-predators represented 16% (of the density) of functional trophic groups. Our surveys also showed a high living coral cover (46%) on the outer slope of Tupai, perhaps due to the absence of both touristic sub-aquatic activities and local pollution from private and commercial activities. Overall, this initial scientific assessment of Tupai has granted an understanding of the spatial patterns of coral, macro-invertebrates, and fish assemblages in the absence of human impacts (i.e., in the lagoon), representing an ecological baseline that could inform conservation management strategies to ensure the preservation of coral reef ecosystem.

Free Discharge of Subsurface Drainage Effluent: An Alternate Design of the Surface Drain System in Pakistan

In Pakistan, many subsurface (SS) drainage projects were launched by the Salinity Control and Reclamation Project (SCARP) to deal with twin problems (waterlogging and salinity). In some cases, sump pumps were installed for the disposal of SS effluent into surface drainage channels. Presently, sump pumps have become dysfunctional due to social and financial constraints. This study evaluates the alternate design of the Paharang drainage system that could permit the discharge of the SS drainage system in the response of gravity. The proposed design was completed after many successive trials in terms of lowering the bed level and decreasing the channel bed slope. Interconnected MS-Excel worksheets were developed to design the L-section and X-section. Design continuity of the drainage system was achieved by ensuring the bed and water levels of the receiving drain were lower than the outfalling drain. The drain cross-section was set within the present row with a few changes on the service roadside. The channel side slope was taken as 1:1.5 and the spoil bank inner and outer slopes were kept as 1:2 for the entire design. The earthwork was calculated in terms of excavation for lowering the bed level and increasing the drain section to place the excavated materials in a specific manner. The study showed that modification in the design of the Paharang drainage system is technically admissible and allows for the continuous discharge of SS drainage effluent from the area.

Fast Pliocene integration of the Central Anatolian Plateau drainage: Evidence, processes, and driving forces

Abstract Continental sedimentation was widespread across the Central Anatolian Plateau in Miocene–Pliocene time, during the early stages of plateau uplift. Today, however, most sediment produced on the plateau is dispersed by a well-integrated drainage and released into surrounding marine depocenters. Residual long-term (106–107 yr) sediment storage on the plateau is now restricted to a few closed catchments. Lacustrine sedimentation was widespread in the Miocene–Pliocene depocenters. Today, it is also restricted to the residual closed catchments. The present-day association of closed catchments, long-term sediment storage, and lacustrine sedimentation suggests that the Miocene–Pliocene sedimentation also occurred in closed catchments. The termination of sedimentation across the plateau would therefore mark the opening of these closed catchments, their integration, and the formation of the present-day drainage. By combining newly dated volcanic markers with previously dated sedimentary sequences, we show that this drainage integration occurred remarkably rapidly, within 1.5 m.y., at the turn of the Pliocene. The evolution of stream incision documented by these markers and newly obtained 10Be erosion rates allow us to discriminate the respective contributions of three potential processes to drainage integration, namely, the capture of closed catchments by rivers draining the outer slopes of the plateau, the overflow of closed lakes, and the avulsion of closed catchments. Along the southern plateau margin, rivers draining the southern slope of the Central Anatolian Plateau expanded into the plateau interior; however, only a small amount of drainage integration was achieved by this process. Instead, avulsion and/or overflow between closed catchments achieved most of the integration, and these top-down processes left a distinctive sedimentary signal in the form of terminal lacustrine limestone sequences. In the absence of substantial regional climate wetting during the early Pliocene, we propose that two major tectonic events triggered drainage integration, separately or in tandem: the uplift of the Central Anatolian Plateau and the tectonic completion of the Anatolian microplate. Higher surface uplift of the eastern Central Anatolian Plateau relative to the western Central Anatolian Plateau promoted more positive water balances in the eastern catchments, higher water discharge, and larger sediment fluxes. Overflow/avulsion in some of the eastern catchments triggered a chain of avulsions and/or overflows, sparking sweeping integration across the plateau. Around 5 Ma, the inception of the full escape of the Anatolian microplate led to the disruption of the plateau surface by normal and strike-slip faults. Fault scarps partitioned large catchments fed by widely averaged sediment and water influxes into smaller catchments with more contrasted water balances and sediment fluxes. The evolution of the Central Anatolian Plateau shows that top-down processes of integration can outcompete erosion of outer plateau slopes to reintegrate plateau interior drainages, and this is overlooked in current models, in which drainage evolution is dominated by bottom-up integration. Top-down integration has the advantage that it can be driven by more subtle changes in climatic and tectonic boundary conditions than bottom-up integration.

High connectivity within restricted distribution range in Pocillopora corals

AbstractAimConvergence, stasis and plasticity can frequently confound our understanding of species distributions in the seas. Yet delimiting species and understanding population connectivity across marine environments is mandatory for establishing appropriate management of coral reefs, which are experiencing critical declines. We test whether morphospecies from Pocillopora corals found on outer reef slopes are unique species or species complexes, in order to correctly define their respective distributions and consequently to accurately assess their population connectivity.LocationArchipelagos and islands of the Western Indian Ocean and the Southern Pacific (New Caledonia, Tonga, French Polynesia).TaxonPocillopora eydouxi/meandrina and Pocillopora verrucosa morphospecies (Scleractinia).MethodsWe analysed the 13‐microsatellite genotypes of 4837 colonies from six understudied ecoregions in the southern part of the genus distribution, to first explore the genetic partitioning within morphospecies. We then characterized the spatial distribution of each delimited species and analysed patterns of genetic diversity and connectivity for each species separately.ResultsBoth morphospecies are complexes of species, each found almost exclusively in the Indian or the Pacific Oceans. Moreover, some of these cryptic species are found in sympatry over their whole distribution, which sometimes was very restricted. However, within each species, genetic diversity and connectivity were relatively high, although some populations were found differentiated for some species, while not for others.Main conclusionsA weak connectivity was found between the Indian and Pacific Oceans, but high connectivity within both oceans, supporting the existence of a barrier impeding gene flow between both ocean basins in Pocillopora. Although constrained by the same geography and current patterns, some sympatric species present different connectivity patterns, demonstrating the importance of multi‐species connectivity models to set up appropriate management plans.

Identifying RR Lyrae Variable Stars in Six Years of the Dark Energy Survey

Abstract We present a search for RR Lyrae stars using the full six-year data set from the Dark Energy Survey covering ∼5000 deg2 of the southern sky. Using a multistage multivariate classification and light-curve template-fitting scheme, we identify RR Lyrae candidates with a median of 35 observations per candidate. We detect 6971 RR Lyrae candidates out to ∼335 kpc, and we estimate that our sample is >70% complete at ∼150 kpc. We find excellent agreement with other wide-area RR Lyrae catalogs and RR Lyrae studies targeting the Magellanic Clouds and other Milky Way satellite galaxies. We fit the smooth stellar halo density profile using a broken-power-law model with fixed halo flattening (q = 0.7), and we find strong evidence for a break at R 0 = 32.1 − 0.9 + 1.1 kpc with an inner slope of n 1 = − 2.54 − 0.09 + 0.09 and an outer slope of n 2 = − 5.42 − 0.14 + 0.13 . We use our catalog to perform a search for Milky Way satellite galaxies with large sizes and low luminosities. Using a set of simulated satellite galaxies, we find that our RR Lyrae-based search is more sensitive than those using resolved stellar populations in the regime of large (r h ≳ 500 pc), low-surface-brightness dwarf galaxies. A blind search for large, diffuse satellites yields three candidate substructures. The first can be confidently associated with the dwarf galaxy Eridanus II. The second has a distance and proper motion similar to the ultrafaint dwarf galaxy Tucana II but is separated by ∼5 deg. The third is close in projection to the globular cluster NGC 1851 but is ∼10 kpc more distant and appears to differ in proper motion.

Evaluating residual dyke resistance using the Random Material Point Method

Due to a lack of large deformation dyke assessment models, primary failure mechanisms, such as inner slope failure, are often used as a proxy to assess the probability of failure of a dyke. However, a dyke continues to fulfil its main function unless, or until, flooding occurs. The Random Material Point Method (RMPM) is used here to investigate residual dyke resistance, which is the resistance against flooding after initial failure. RMPM combines random fields with MPM in a Monte Carlo simulation and has been extended here to include the effects of an external hydrostatic pressure on a dyke’s outer slope. The residual resistance of an idealised dyke (computed using RMPM) is shown to reduce the probability of flooding by 25% with respect to the initial failure. A lower degree of anisotropy of the spatial variability increases the residual dyke resistance. RMPM simulates, as expected, a lower residual dyke resistance for larger initial failures and/or a higher water level. A ‘safe’ remaining geometry has not been found, since even small initial failures can result in an unacceptable probability of flooding, highlighting the importance of modelling the entire failure process.

Persistent, Depth‐Intensified Mixing During The Western Mediterranean Transition's Initial Stages

AbstractMajor deep‐convection activity in the northwestern Mediterranean during winter 2005 triggered the formation of a complex anomalous deep‐water structure that substantially modified the properties of the Western Mediterranean deep layers. Since then, evolution of this thermohaline structure, the so‐called Western Mediterranean Transition (WMT), has been traced through a regularly sampled hydrographic deep station located on the outer continental slope of Minorca Island. A rapid erosion of the WMT's near‐bottom thermohaline signal was observed during 2005–2007. The plausible interpretation of this as local bottom‐intensified mixing motivates this study. Here, the evolution of the WMT structure through 2005–2007 is reproduced by means of a one‐dimensional diffusion model including double‐diffusive mixing that allows vertical variation of the background mixing coefficient and includes a source term to represent the lateral advection of deep‐water injections from the convection area. Using an optimization algorithm, a best guess for the depth‐dependent background mixing coefficient is obtained for the study period. WMT evolution during its initial stages is satisfactorily reproduced using this simple conceptual model, indicating that strong depth‐intensified mixing (K ∞ (z) ≈ 22 × 10−4 m2 s−1; z ⪆ 1,400 dbar) is a valid explanation for the observations. Extensive hydrographic and current observations gathered over the continental slope of Minorca during winter 2018, the first deep‐convective winter intensively sampled in the region, provide evidence of topographically localized enhanced mixing concurrent with newly formed dense waters flowing along‐slope toward the Algerian sub‐basin. This transport‐related boundary mixing mechanism is suggested to be a plausible source of the water‐mass transformations observed during the initial stages of the WMT off Minorca.

Miocene Seep-Carbonates of the Northern Apennines (Emilia to Umbria, Italy): An Overview

The natural emission of methane-rich fluids from the seafloor, known as cold seepage, is a widespread process at modern continental margins. The studies on present-day cold seepages provide high-resolution datasets regarding the fluid plumbing system, biogeochemical processes in the sediment, seafloor seepage distribution and ecosystems. However, the long-term (hundreds of thousands to millions of years) evolution of cold seepage remains elusive. The identification and study of outcrop analogous now exposed on land represent a valuable method for better understanding the effects of geological processes and climate forcing on the development of cold seepage systems. Here, we provide an overview on Miocene seep-carbonate deposits of the northern Apennines (from Emilia to the Umbria-Marchean sector, Italy), based on decades of field research integrated with detailed sedimentological and geochemical investigations. We report a total of 13 seep-carbonate outcrops, which formed in three different structural settings of the paleo-accretionary wedge corresponding to wedge-top basins, outer slope and intrabasinal highs at the deformational front. We discuss the recurring lithostratigraphic occurrence of seep deposits and the main compositional features (carbonate facies, carbon and oxygen stable isotopes) in order to interpret the seepage dynamics, duration and infer the contribution of methane-rich fluids released by paleo-gas hydrates. The datasets presented in this study represent a valuable complete record of cold seepage spanning ~12 Myr, that can be used to better understand factors controlling the regional-scale spatial and temporal evolution of cold seepage systems at modern active continental margins.

Growth and collapse of the 2018–2019 lava dome of Merapi volcano

Lava dome collapses are a major threat to the population living near such volcanoes. However, it is not possible to forecast collapses reliably because the mechanisms are not clearly understood, due partly to the lack of continuous observations of such events. To address this need for field data, we have developed new monitoring stations, which are adapted to the volcanic environment. The stations tracked the complete evolution of the 2018–2019 lava dome of Merapi volcano (Indonesia) and the associated pyroclastic density currents. During the 14 months of activity, the stations acquired thermal, high-resolution visual images and movies in stereoscopic configurations. The dome developed on a plateau flanked by steep sides (~ 40°–50°) inside the crater, which was open to the SE. We observed that the dome behaved in a viscous manner (with a viscosity of 109 Pa s for the interior to 1013 Pa s for external parts of the dome) on gentle slopes, and in a brittle way (friction angle ~ 35°, cohesion < 100 kPa) on slopes steeper than 35°. Thus, the lava dome was unable to grow on the outer slopes of the plateau and a significant volume of lava (350–750 × 103 m3) accumulated and collapsed daily to the SE in relatively small volumes (< 10,000 m3), preventing the lava dome from reaching the critical volume necessary for pyroclastic density currents to form and threaten the surrounding population. The cause of the small and frequent collapses was purely gravitational during the dome activity. This suggests that relatively small differences in the summit morphology can control dome evolution, favouring either a lava dome restricted to a small volume and leading to only a minor crisis, or more voluminous dome growth and a catastrophic collapse.

Application of hydrodynamic models for designing structural measures for river flood mitigation: the case of Kulfo River in southern Ethiopia

Flood is one of life-threatening events in different parts of Ethiopia. The causes of flood might vary from place to place, for instance, rates of deforestation, agricultural expansion, urbanization, wetland drainage, climate change, siltation of river bed, and several other types of land use change might be the cause for flood in different rivers banks. This study aims to assess, investigate, and design suitable river training works on lower Kulfo river reach. The study reach has been seriously affected by extreme floods due to the above reasons. For minimizing the loss due to flood, and to use the advantage of flood for different developmental activities various flood control measures should be adopted. The flood control measures which should more correctly be termed as “flood management” can be planed either through structural engineering measures or non-structural measures. Structural measures comprise retarding structures which store flood water, channel improvements which increase flood-carrying capacity of the river, embankments, and levees which keep the waterway from flood-prone area, detention basin which retards and absorbs most of the floodwater. Within the identified 6 km reach, field investigation including secondary data collection has been done to predict the flood extent using 1D hydrodynamic model, HECRAS and HEC Geo RAS. Estimated flood depth and extent helped us in fixing the dimension of different river training structures selected. The modelling result indicates a maximum channel bed flood depth of 4.3 m and flood plain flood depth 2.3 m obtained using a 100-year return flow. Analysis of soil samples indicates that the lower reach of Kulfo river is gravel and sand dominated meandering river, with estimated scour depth up to 3.41 m along the river course. Levees have been designed in conjunction with Groynes to protect the Upstream farm located at the prison and upper part of the Limat households. Frequent floods happening near both bridges shall be reduced by using Guide banks without influencing the bridges and diversion structures. Consideration is given for the ecosystem to stay in equilibrium, by providing suitable outer slopes so that plantation is possible on the top and side slope of the levees and guide banks.

Intergeneric and geomorphological variations in Symbiodiniaceae densities of reef-building corals in an isolated atoll, central South China Sea

The healthy status of corals in the isolated atolls of the central South China Sea (SCS) remains unclear. Symbiodiniaceae density (SD) can effectively reflect the thermal tolerance and health of hard corals. Here, the SDs of 238 samples from the Huangyan Atoll (HA) were analyzed. The results revealed significantly intergeneric and geomorphological differences in SD. Intergeneric variation may reflect that corals with high SD have stronger thermal tolerance. Geomorphic analysis showed that the SDs at the outer reef slope were higher than in the lagoon. Hydrodynamics and sea surface temperature were likely the main influencing factors. Most notably, corals in SCS HA had higher SDs than those at neighboring reefs, indicating that their thermal tolerance were strong, which may be related to HA's local upwelling. These results suggest that the HA has the potential to serve as a refuge for corals, but increasing human disturbance limit its function.

Skin δ13C and δ15N reveal spatial and temporal patterns of habitat and resource use by free-ranging odontocetes from the southwestern Atlantic Ocean

Large diversity and abundance of cetacean species occur along the Brazilian outer continental shelf and slope waters. In the present study, we analyzed carbon (δ13C) and nitrogen (δ15N) stable isotopes in skin biopsies of ten odontocete species (Delphinidae and Physeteridae) to assess the intra- and inter-specific patterns in the use of resources along the region. Our isotopic analysis allowed us to identify two potential subpopulations of Stenella frontalis. High core isotopic niche overlap between S. frontalis from the southern range of the study area and Delphinus delphis, especially in spring, suggested that they share similar resources and rely on spatiotemporal segregation to achieve niche differentiation and to minimize competition. Our isotopic data also pointed to high similarity among Tursiops truncatus, S. frontalis, and Globicephala melas in the use of resources. Steno bredanensis had the highest δ15N and δ13C, which agrees with its occurrence in neritic waters. S. longirostris showed consistently low δ15N values, indicating that they feed at relatively lower trophic levels, and lack of niche overlap with most delphinid species, except S. attenuata. Orcinus orca and G. melas had very similar δ13C and δ15N values, that were consistent with feeding in offshore waters. δ13C and δ15N in Physeter macrocephalus suggested that individuals feed on similar trophic levels, but over a wide spatial range. The analyses of stable isotopes in skin biopsies helped us to refine our knowledge about the intra-specific resource and habitat use, ecological niches, and the trophic interactions amongst co-occurring cetaceans from the oceanic waters of the subtropical western South Atlantic.

Redox fluctuations and organic accumulation on the outer shelf of the Early Cambrian (Ages 2–3) Yangtze sea: Geochemical records on a seaward submarine sill

The Early Cambrian was a critical interval marked by dramatic oceanic, geochemical, and biological changes in Earth's history. Although numerous studies have documented oceanic redox changes and revealed a heterogeneous pattern from the inner shelf to outer shelf margin in the Yangtze Block, less attention has been paid to their palaeogeographic‐specific occurrences, particularly in the deeper basinal regime during that time. To better understand marine redox variations and organic accumulation across the entire outer shelf‐basin transect, we selected a coeval organic‐rich section (i.e., Guantang section, western Hunan) deposited on a farther seaward submarine sill for a comparison. Integrated geochemical data of iron speciation, redox‐sensitive trace element [i.e., molybdenum (Mo) and uranium (U)] concentrations and total organic carbon (TOC) contents as well as P/Al and Ba/Al ratios, in combination with previously published data from the outer shelf slope‐basin, reveal an overall high TOC content on the sill and further attest to the occurrence of a fluctuating euxinic water wedge overlying the deep ferruginous water on the outer shelf slope to basin transect in the presence of oceanic upwelling system. The fluctuating euxinic watermass could be dynamically advanced upslope and retreated downslope in the pace of sea‐level rise and fall. All these signify the critical roles of palaeogeographic setting and eustatic changes in controlling the spatiotemporal productivity and redox fluctuations in the Yangtze sea during the Early Cambrian.

Sedimentary facies characteristics and organic matter enrichment mechanism of lower Cambrian Niutitang Formation in South China

The purpose of this study was to examine the sedimentary facies characteristics of lower Cambrian Niutitang Formation (∈1n) in South China, to reveal the mechanism of organic matter enrichment, and to guide exploration of shale gas. Macro investigation and experimental analyses were used to assess the lithology in detail, total organic matter mass fraction w(TOC), mineral composition, and trace element characteristics of ∈1n. The influencing factors of organic matter enrichment were discussed extensively, and a sedimentary facies mode was suggested. In the early stage of ∈1n, the locations of Well E’yangye 1, Well Ciye 1, Well Changye 1, and Well Anye 1 respectively develop, platform inner sag, outer shelf, Jiangnan slope belt, and South China detention basin. In the late stage of ∈1n, the sedimentary facies evolve with decreasing sea level. The study area presents a complete three-step basin in the Early Cambrian. In the early stage of ∈1n, the first step is the Yangtze carbonate platform, the second step is the outer shelf and slope, and the third step is the deep-water basin. From the Yangtze carbonate platform to the deep-water basin, w(TOC) and the mass fraction of quartz gradually increase, the mass fraction of carbonate mineral decreases, and the mass fraction of clay mineral is higher in the second step. The sea level fluctuation results in a higher w(TOC) vertically in the lower ∈1n shale, and the paleogeographic (provenance) conditions lead to better horizontal development of organic matter in the outer shelf, slope and detention basin. Trace elements are abundant in the lower ∈1n, and w(TOC) is correlated positively with many trace elements. In the outer shelf, slope, and adjacent areas, hydrothermal activity and upwelling current bring nutrient-rich material and promote organic matter enrichment under a strong reducing condition. Deep-shelf, slope and deep-water basin are the best facies for the formation and preservation of organic matter, especially deep-water basin facies. It remains necessary to strengthen the exploration of shale gas in the deep-water basin of ∈1n in central Hunan, China.

Macrofracturing of Oceanic Lithosphere in Complex Large Earthquake Sequences

AbstractMajor earthquakes in oceanic lithosphere seaward of the subduction zone outer trench slope are relatively uncommon, but several recent occurrences have involved very complex sequences rupturing multiple nonaligned faults and/or having high aftershock productivity with diffuse distribution. This includes the 21 December 2010 MW 7.4 Ogasawara (Bonin), 11 April 2012 MW 8.6 Indo‐Australia, 23 January 2018 MW 7.9 Off‐Kodiak Island, and 20 December 2018 MW 7.3 Nikol'skoye (northwest Pacific) earthquakes. Major oceanic intraplate event sequences farther from plate boundaries do not tend to be as complex in faulting or aftershocks. Outer trench slope extensional faulting can involve complex distributed sequences, particularly when activated by great megathrust ruptures such as 11 March 2011 MW 9.1 Tohoku and 15 November 2006 MW 8.3 Kuril Islands. Intense faulting sequences also occur near subduction zone corners, with many fault geometries being activated, including some in nearby oceanic lithosphere, as for the 29 September 2009 MW 8.1 Samoa, 6 February 2013 MW 8.0 Santa Cruz Islands, and 16 November 2000 MW 8.0 New Ireland earthquakes. The laterally varying plate boundary stresses from heterogeneous locking, recent earthquakes, or boundary geometry influence the specific faulting geometries activated in nearby major intraplate ruptures in oceanic lithosphere. Preexisting lithospheric structures and fabrics exert secondary influences on the faulting. Intraplate stress release in oceanic lithosphere near subduction zones favors distributed macrofracturing of near‐critical fault systems rather than localized, single‐fault failures, both under transient loading induced by plate boundary ruptures and under slow loading by tectonic motions and slab pull.

Spatiotemporal variations and controlling factors of the surface pCO2 in the northern South China Sea

The focal area of this study was the northern South China Sea (NSCS; 18.5°-22.5°N, 1.0°-8.0°E), located on the northwestern shelf of the SCS. The surface pCO2 (pCO2w), sea surface temperature (SST) and sea surface salinity (SSS) was measured by continuous measurement system during four field surveys to show their spatiotemporal variations in the NSCS. Low pCO2w in the southwestern region of PRE were observed in four seasons, particularly in spring and summer. The Pearl River diluted water (PRDW) discharged a large amount of nutrients into nearshore waters, which promoted phytoplankton propagation and CO2 uptake in spring and summer. On the contrary, primary productivity was low in winter and late autumn, runoff input of dissolved inorganic carbon (DIC) played important role in increasing the pCO2 in nearshore waters, so the PRDW controlled the spatiotemporal variations of pCO2w in the coastal and inner shelf. The pCO2w in the outer shelf and slope were relatively high in four seasons, and SST was the critical controlling factor of pCO2w, sea-air CO2 exchange also played role in the seasonal scales of pCO2w, the impact of weather and climate events on the variations of pCO2w and sea-air CO2 flux in the short term also were remarkable. Generally, the NSCS acted as sink of atmospheric CO2 in spring, late autumn and winter, particularly in latter two seasons, in contrast, it was weak CO21 source in summer.