Mixing Of Seawater Research Articles

Mineralogy, geochemistry and microbiology insights into precipitation of stibnite and orpiment at the Daiyon-Yonaguni Knoll (Okinawa Trough) hydrothermal barite deposits

Samples of active chimneys, chimney flanges and massive sulfides from the Daiyon-Yonaguni Knoll hydrothermal field are composed of major barite and minor stibnite and orpiment. Barite is inferred to precipitate from focused-discharge fluids composed of >40% hydrothermal end-member fluid at T = 100-240°C, whereas the stibnite and orpiment are later and lower temperature precipitates. The hydrothermal fluids from this field were subject of sub-seafloor boiling and phase separation and, consequently, are brine-rich depleted in volatile and enriched in non-volatile elements. Boiling and phase separation exerted major control on the rare earth elements (REE) partitioning in the vent fluids: high-chlorinity high-temperature fluids were enriched in light REE and low-chlorinity low-temperature fluids were enriched in heavy REE. Y/Ho molar ratio and Ce anomaly of the vent fluids suggest that the seawater has not completely reacted with the basement rocks and has not equilibrated with them. The trace element concentrations in the hydrothermal deposits suggest a complex interplay among hydrothermal, hydrogenetic and microbial processes. Sulfur isotope composition of the sulfides suggests that the sulfide S is a mixture of both basement rock and seawater S with a higher proportion of the basement rock S. The sulfate dissolved in the fluids was subjected to reduction during a slow mixing of hydrothermal fluid and seawater within the chimney walls of the Tiger and Abyss vents and this resulted in a heavy S-isotope composition of the vent fluid sulfate. Lead isotope composition of the hydrothermal deposits indicates mixing relationships suggesting that Pb and potentially other metals with similar geochemical behavior were derived from two or three sources. The Pb isotopes in the hydrothermal deposits imply that an enriched source, either sediments or extended continental lithosphere, and a depleted source, potentially back-arc mafic volcanics, are present in the area of Daiyon-Yonaguni Knoll. Filamentous orpiment found in the deposits is supposed to be either heavily mineralized fungal hyphae or pure abiogenic biomorphs. Presence of carbonaceous matter on and around the orpiment filaments suggests for microbial activity during filament formation. The filaments experienced temperature of 209.1±37.1°C which falls within the temperature range of the Daiyon-Yonaguni Knoll vent fluids. Stability phase diagrams modeling reveals that the stability of stibnite does not depend on the vent fluid chlorinity, but depends on the vent fluid temperature: the area of stibnite stability increases with decreasing vent fluid temperature and results in stibnite precipitation at low log10a of Sb2S42- and less reduced environment (Eh still <0). Orpiment is stable in a wide range of log10a of H2AsO4-, in reduced conditions and at high S activity. Barite is stable in wide range of log10a of Ba2+ and precipitates in slightly reduced to slightly oxic conditions.

Flexural behavior of GFRP bars reinforced seawater sea sand concrete beams exposed to marine environment: Experimental and numerical study

Experimental and numerical investigations were conducted on the flexural behavior of GFRP bars reinforced seawater sea sand concrete (GFRP-reinforced SWSSC) beams after exposed to marine environment. A total of eight beams were tested by using a four-point static loading facility. Variables included the exposure environment, the exposure duration, the water for mixing and curing, and the sea sand replacement ratio. The failure process and failure modes of the specimens were observed, the load versus deflection curves and the deflection versus maximum width crack curves were obtained. The effects of various parameters on the ultimate bearing capacity, initial stiffness, and ductility coefficient were analyzed. In addition, the finite element analysis (FEA) method was used to simulate the flexural behavior of GFRP-reinforced SWSSC beams, and the parametric analysis was then conducted to investigate the effect of reinforcement ratio and shear-to-span ratio on the flexural behavior of the beam. The test results show that, after 270 days of exposure, compared with the general atmospheric environment, the ultimate bearing capacity of the specimen is increased by 28.0 % after being exposed to salt-spray environment while decreased by 13.0 % after being exposed to tidal environment. During a 270-day exposure duration to tidal environment, with the increase of the exposure duration, the ultimate bearing capacity increases first and then decreases, while both the initial stiffness and ductility coefficient of specimens decrease. The use of seawater for mixing and curing is detrimental to the initial stiffness and ductility coefficient of the specimens. The increase in sea sand replacement ratio is detrimental to specimens' ultimate bearing capacity and initial stiffness. Besides, FEA results indicate that increasing the longitudinal reinforcement ratio can obviously improve the ultimate bearing capacity and the post-cracking stiffness of the specimen. Based on the current China, America, and Canada codes, the calculation method of the flexural capacity of GFRP-reinforced SWSSC beams exposed to marine environment is discussed and the prediction value of long-term ultimate bearing capacity is given.

Iron isotopes constrain sub-seafloor hydrothermal processes at the Trans-Atlantic Geotraverse (TAG) active sulfide mound

Sub-seafloor hydrothermal processes along volcanically active plate boundaries are integral to the formation of seafloor massive sulfide deposits and to oceanic iron cycling, yet the nature of their relationship is poorly understood. Here we apply iron isotope analysis to sulfide minerals from the Trans-Atlantic Geotraverse (TAG) mound and underlying stockwork, 26°N Mid-Atlantic Ridge, to trace hydrothermal processes inside an actively-forming sulfide deposit in a sediment-free mid-ocean ridge setting. We show that data for recently formed chalcopyrite imply hydrothermal fluid–mound interactions cause small negative shifts (<−0.1‰) to the δ56Fe signature of dissolved iron released from TAG into the North Atlantic Ocean. Texturally distinct types of pyrite, in turn, preserve a δ56Fe range from −1.27 to +0.56‰ that reflects contrasting precipitation mechanisms (hydrothermal fluid–seawater mixing vs. conductive cooling) and variable degrees of progressive hydrothermal maturation during the >20 kyr evolution of the TAG complex. The identified processes may explain iron isotope variations found in fossil onshore sulfide deposits.

Application of Water Quality Index as a vulnerability indicator to determine seawater intrusion in unconsolidated sedimentary aquifers in a tropical coastal region of Sri Lanka

Coastal freshwater aquifers are under threat and in many cases gradually decline due to climate change and over-extraction of water with the rapidly expanding human activities including agriculture and tourism. In this study, coastal sedimentary aquifer systems in northern Sri Lanka were characterized by integrated approaches such as mass balance calculations, Seawater Mixing Index (SMI) and Water Quality Index (WQI). A total of 34 groundwater samples were collected and analyzed for field parameters (pH, electrical conductivity (EC), dissolved oxygen and temperature) their major ions (Ca2+, Mg2+, Na+, K+, Cl-, SO42−, NO3−, HCO3−) and selected trace elements (Sr, Ba, B) as well as stable water isotopes (δ18OH2O and δ2HH2O). The EC in investigated groundwater samples ranged from 81 to 5690 μS/cm (mean - 1260 μS/cm) and served as the first indication of seawater intrusion. The study region consists of predominantly Ca2+-HCO3− and Na+-Cl- water types indicating freshening and seawater intrusions as revealed by a Hydrogeochemical Facies Evaluation (HFE) diagram. The WQI and SMI approach indicated that in the study region, 50–58% of samples are affected by seawater intrusion. Up to 28% of seawater admixture was noted based on the mass balance calculation using Cl- (0.55–156 meq/L) as a conservative tracer. This result significantly correlates with the calculated WQI (r = +0.944; p < 0.000). Water isotope values indicated that most of the groundwater in the area is recharged directly from precipitation while some samples showed influences of evaporation and also mixing with seawater. In summary, the investigated coastal sedimentary aquifer system in the northern part of Sri Lanka is already severely affected by seawater intrusion. However, as marked by the dissolution of carbonate minerals under the influence of monsoonal rains leads to the freshening of groundwater. This study showed that WQI combined with mass balance calculations is useful in assessing the vulnerability of coastal aquifers in arid and semi-arid regions.

The deposition and significance of an Ediacaran non-glacial iron formation.

Most Neoproterozoic iron formations (NIF) are closely associated with global or near-global "Snowball Earth" glaciations. Increasingly, however, studies indicate that some NIFs show no robust evidence of glacial association. Many aspects of non-glacial NIF genesis, including the paleo-environmental setting, Fe(II) source, and oxidation mechanisms, are poorly understood. Here, we present a detailed case study of the Jiapigou NIF, a major non-glacial NIF within a Neoproterozoic volcano-sedimentary sequence in North Qilian, northwestern China. New U-Pb geochronological data place the depositional age of the Jiapigou NIF at ~600 Ma. Petrographic and geochemical evidence supports its identification as a primary chemical sediment with significant detrital input. Major and trace element concentrations, REE + Y systematics, and εNd (t) values indicate that iron was sourced from mixed seawater and hydrothermal fluids. Iron isotopic values (δ56 Fe=-0.04‰-1.43‰) are indicative of partial oxidation of an Fe(II) reservoir. We infer that the Jiapigou NIF was deposited in a redox stratified water column, where hydrothermally sourced Fe(II)-rich fluids underwent oxidation under suboxic conditions. Lastly, the Jiapigou NIF has strong phosphorous enrichments, which in other iron formations are typically interpreted as signals for high marine phosphate concentrations. This suggests that oceanic phosphorus concentrations could have been enriched throughout the Neoproterozoic, as opposed to simply during glacial intervals.

Characteristics of the flow field inside and around a square fish cage considering the circular swimming pattern of a farmed fish school: Laboratory experiments and field observations

The management of marine aquaculture requires an accurate assessment of environmental impacts to achieve an environmentally sustainable system. To study the influence of farmed fish schools on the flow field around a fish cage, a pseudo-fish-school structure model was developed to simulate the circular swimming patterns of farmed fish in laboratory experiments. The drag force acting on the fish cage and current velocities inside and around a modeled square fish cage were investigated. A field survey was also conducted. As a result, the non-uniformity and complexity of the flow field of a stocked cage were observed in laboratory experiments. Circular swimming patterns were found to have obvious influences on the flow field distribution. The drag force of the square fish cage decreased by 11.8% because of the presence of the fish-school structure. Based on the field measurements, the averaged velocity inside and downstream of the full-scale fish cage was reduced by 46.2% and 57.8%, respectively. An acceleration was also observed in the current velocity below the fish cage. The vertical mixing of seawater may be because of the water blockage resulting from the cage and the circular swimming pattern of farmed fish schools. The findings presented in this study can be used as benchmarks for numerical models in assessing the potential environmental impact of fish farm sites.

Characteristics of alkali-activated slag powder mixing with seawater: Workability, hydration reaction kinetics and mechanism

Marine engineering is faced with the problem of shortage of engineering materials. The local use of marine raw materials, such as seawater and sea sand, can greatly promote the development of marine engineering construction. In order to explore the application potential of seawater in marine engineering materials, in this paper, the influences of seawater on the physical & mechanical properties, hydration kinetics and microstructure of alkali-activated slag powder (AAS) were investigated compared with deionized water. The results show that seawater can reduce the hydration exothermic rate and cumulative heat of AAS. From analysis of hydration kinetics, seawater delays AAS from crystal nucleation and growth (NG) stage into the phase boundary reaction (I) stage and diffusion (D) stage. And the reaction rate of NG and I decrease by seawater, which is manifested by the increase in setting times and fluidity of AAS. Compared with the deionized water mixing, the compressive strength at curing age 3 d to 28 d of the alkali-activated slag mortar mixed with seawater decreases by about 6%. The volume of harmless or less-harmful pores decreases and the porosity of AAS paste increases after seawater mixing, which explains the decrease of strength for AAS with seawater. In additional, AAS can bound chloride ions in seawater.

Influence of coastal alluvial aquifer on groundwater chemistry in Wates, Yogyakarta, Indonesia

Saline groundwater is found far from the coastline, whereas closer groundwater has low salinity in the Wates area, Indonesia. This situation which has led to problems linked to the coastal aquifer, shows different groundwater chemistries. This connection needs to highlight the geochemical evolution of aquifers. This research aims to evaluate the hydrochemical characteristics in Wates Sub-District, Kulon Progo District, Indonesia. The study area is located in the southern part of the Kulon Progo with an aquifer of marine and fluvial deposits. The boundary of the south is the Indian Ocean. The approach is based on analyzing the major ions of 27 groundwater samples, and the XRD results include the Stiff diagram and the Stuyfzand classification. The Stiff diagram showed in agreement with that of the Stuyfzand classification. Results showed that the F2-(CaHCO3)2Ø and F1-(NaMix)Ø types reflect the recharge process and dominate the dissolution of silicate minerals. Halite dissolution originates in topographically flat areas of the Bs1-NaCl (-) water. Total hardness is softer than others as a result of cation exchange. The groundwater chemistry’s geochemical processes are the dissolution of silicate and halite minerals, ion exchange, and fresh and seawater mixing. This research indicates to focus on pumping problems in the aquifer.

Depth Profiles of Re-Os Geochemistry in Drill Cores from Hole U1530A in Brothers Volcano Hydrothermal Field, Kermadec Arc: Mobilization and Extreme Enrichment of Os by Volcanic Gas

AbstractBetter understanding metallogenesis in oceanic crust depends on costly sea-floor drilling projects in areas where metal-bearing deposits, such as sea-floor massive sulfide deposits, are currently forming. In 2018, International Ocean Discovery Program (IODP) Expedition 376 recovered drill cores from an active hydrothermal field at Brothers volcano, in the Kermadec arc. These provide insight into the formation of mineral deposits along arcs, the structure and permeability of hydrothermal sites, and the relationship between the discharge of magmatic fluids and the deep biosphere. We report whole-rock major and trace element compositions and the Re-Os isotope geochemistry of hydrothermally altered volcanic rocks in a core from Hole U1530A, extending 453 m beneath the sea floor, and unaltered volcanic rocks in cores from four other drilling sites and interpret these data to better understand subseafloor mixing of hydrothermal fluids and ambient seawater. The core exhibits more radiogenic 187Os/188Os values than typical basal values in four intervals. We propose two causal mechanisms of these radiogenic values: (1) mixing between seawater and hydrothermal fluid, associated with abundant deposition of sulfide or sulfate minerals; and (2) ingress of seawater with radiogenic 187Os/188Os values, associated with abundant chlorite and high porosity. Extreme Os enrichments up to 61.5 ppb are interpreted as the result of mobilization of Os as OsO4 or OsF6 and transport by volcanic gas, which also affected the Re-Os geochemistry of the rocks from the other Expedition 376 holes. Mobilization and transport of Os by volcanic gas may be an appreciable factor in the influx of unradiogenic Os into the ocean.

Linkages Between Optical and Molecular Signatures of Dissolved Organic Matter Along the Yangtze River Estuary-to-East China Sea Continuum

Changes in the molecular composition of dissolved organic matter (DOM) and its light-absorbing component (CDOM) along the river–coastal ocean continuum are crucial for better understanding the source and fate of DOM in coastal oceans. Both optical (absorbance and fluorescence) techniques and ultrahigh-resolution mass spectrometry have been widely used to trace DOM cycling. However, to the best of our knowledge, the linkage between these two techniques is rarely investigated along the river–coastal ocean continuum. In this regard, bulk characterization, optical techniques, and ultrahigh-resolution mass spectrometry [Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS)] were applied to determine the DOM chemistry along a spatial transect from the Yangtze River Estuary (YRE) to the East China Sea. The results showed that DOM in the YRE was mainly controlled by the mixing of freshwater and seawater. Besides terrestrial input, multiple sources (tributary input from the Huangpu River, wastewater input, and sediment resuspension and subsequent release at the turbidity maximum zone) have been identified. In addition, the linkages between CDOM and thousands of formulas were developed based on Spearman’s rank correlations between optical parameters and FT-ICR MS peaks. The linkages showed that the molecular groups associated with the optical parameters generally agreed with conventional biogeochemical interpretations. Nevertheless, each technique has its unique advantage and weakness in interpreting DOM composition. Therefore, the combination of the untargeted FT-ICR MS approach and optical techniques could be valuable for studying the DOM sources and transformation in large river estuarine systems.

Effects of Seawater Recharge on the Formation of Geothermal Resources in Coastal Areas and Their Mechanisms: A Case Study of Xiamen City, Fujian Province, China

The southeast coastal areas of China have abundant geothermal resources. Most especially, seawater-recharged geothermal systems in the coastal areas have large quantiles of recharge but suffer water salinization and low water temperature. Moreover, the geothermal water development in these areas may induce seawater intrusion. Understanding the genetic patterns of geothermal resources is significant for rational exploration and protection. This study analyzed the hydrochemical and environmental isotopic characteristics of geothermal water, groundwater, and surface water samples collected in the area with geothermal resources in Xiamen Province in the southeast coastal areas of China. Based on this, the recharge of geothermal water circulation and the genetic patterns of geothermal resources were revealed. The results of this study indicate that the geothermal water in mountainous areas and piedmonts in Xiamen is mainly recharged by rainfall infiltration. In contrast, the geothermal water in coastal areas in Xiamen is recharged by seawater mixing to different extents, as indicated by hydrochemical types, isotopic characteristics, and the C1-/Br- ratio of geothermal water. As revealed by the calculation results using the Cl−mixing model, 10 of 13 geothermal fields in Xiamen are recharged by seawater mixing, with a mixing ratio of up to 73.20% in the Pubian geothermal field. After being recharged by rainfall in the low mountainous areas, geothermal water migrates toward deep parts along NW-trending faults. Then, it converges with regional NE-trending deep faults to absorb heat conducted from deep parts to form deep geothermal reservoirs. The deep geothermal reservoirs were estimated to be 185–225°C using the silica-enthalpy mixing model. The geothermal water is mixed with cold water or seawater while rising along faults. The temperature of shallow geothermal reservoirs was estimated to be 71–145°C using SiO2geothermometers.

Stable potassium (K) isotope characteristics at mid-ocean ridge hydrothermal vents and its implications for the global K cycle

Recent discoveries of significant variations in stable K isotope ratios (41K/39K or δ41K) among various terrestrial samples indicate that K isotopes can be a novel tracer for the global K cycle, but a key observation that seawater δ41K is ∼0.6‰ higher than the bulk silicate Earth remains unexplained. An unconstrained component critical to this puzzle is hydrothermal systems that represent both a major K source and sink in the ocean. Here we report δ41K results on mid-ocean ridge (MOR) hydrothermal fluids from the Gorda Ridge and ∼9°N East Pacific Rise (EPR), including time-series samples that recorded major perturbations in fluid chemistry induced by a local volcanic eruption. Fluid δ41K values range from -0.46‰ to -0.15‰, falling between those of fresh basalts and seawater. δ41K values of “time-zero” fluids collected shortly after the volcanic eruption are shifted towards the seawater value, followed by a return to pre-eruption values within ∼2 years. Fluid δ41K variations are largely influenced by water–rock interactions, but they cannot be solely explained by simple mixing of seawater and K leached from basalts at high temperatures. Instead, these data imply small but significant isotope fractionation that enriches heavy K isotopes in basalts, likely caused by low-temperature alteration during the recharge stage of hydrothermal circulation. Our results preclude MOR hydrothermal systems as the cause for the heavy δ41K value of seawater. Using fluid δ41K data and K isotope fractionation constrained here for hydrothermal systems, a K mass-balance model implies a critical role for a marine sedimentary sink, possibly authigenic clay formation, in the global K cycle. Also, applying the K isotope fractionation constrained here to the published δ41K data from ophiolites shows the possibility for significantly lower seawater δ41K during the Ordovician, which can be explained by enhanced reverse weathering in response to distinct climate and tectonics at that time.

RETRACTED: Monitoring of chloride and Friedel’s salt, hydration components, and porosity in high-performance concrete

The innovative paper evaluated the water of the sea as a curing component and mixing constituent in high-performance concrete (HPC). This motivation would provide an understanding for the reader of how the water from the sea affects the HPC. This paper would provide information about the mechanism and the influence of prepared high-performance concrete mixed with either freshwater or seawater and made to cure in freshwater, seawater, or air. The mechanism and the influence were evaluated by XRD tests, SEM observations, and petrography analysis of HPCs at 3 days, 7 days, 28 days, and 90 days. The structure of the Friedel's salt increases tobermorite and ettringite (3CaO.Al 2 O 3 .3CaSO 4 32H 2 O) and retards portlandite (Ca(OH) 2 ) and calcium silicates (2CaO.SiO 2 and 3CaO.SiO 2 ) in HPC mixed with seawater for 3–90 days. Air curing reduces the formation of portlandite (Ca(OH) 2 ), tobermorite (3CaO.2SiO 2 .3H 2 O), ettringite (3CaO.Al 2 O 3 .3CaSO 4 32H 2 O), and calcium silicate (3CaO.SiO 2 ) in seawater HPC while increasing the formation of calcium silicate (2CaO.SiO 2 ). SEM analysis supports the mechanism and influence of mixing of seawater, curing in the water of the sea, and curing treatment in the air. According to data from petrographic analysis and porosity testing, the porosity was decreased by the seawater and curing treatment in the seawater in HPC mixes. In terms of hydration components, SEM observation, porosity, and petrographic analysis, the HPC-3 shows the best performance in the work.

Organic matter and sedimentary accumulation rates in a transect of cores in the Bay of Bengal offshore Bangladesh and Andaman Sea offshore Myanmar

Eight sediment multi-cores samples collected from the Bay of Bengal and the Andaman Sea were examined to evaluate organic matter (OM) sources to the marine setting for a large sediment and carbon sink. Total organic carbon (TOC) for sediments from the northern Bay of Bengal (NBB), eastern Bay of Bengal (EBB) and the northern Andaman Sea (NAS) range from 0.34 to 0.79 wt%, 0.84 to 1.06 wt%, and 0.20 to 1.18 wt%, respectively, with higher concentrations in fine silts than in the sandy fractions suggesting hydrodynamic sorting and surface area control TOC. The TOC to total nitrogen (TN) ratio (C/N) (NBB: 5.31–8.52; EBB: 5.51–6.97; NAS: 6.44–11.92) indicates OM is a mixture of dominant contributions from marine plankton and lesser contributions from land plants. The organic carbon (δ13Corg) isotopic values display a similar range of values in each of the regions of the NBB (−12.0 to −19.9‰), EBB (−10.1 to −20.2‰) and NAS (−8.3 to −20.3‰). δ13Corg values may derive from marine phytoplankton, terrestrial C4 plants and/or aquatic macrophytes. The total nitrogen (δ15N) isotopic compositions (NBB: 3.7–4.1; EBB: 4.1–5.9; NAS: 4.2–6.5‰) diagnose input from planktonic sources or aquatic macrophytes. Hydrocarbons from the sediments have a bimodal distribution with high relative abundance of mid-chain n-alkanes (C23 and C25 peaks) and long-chain n-alkanes (C29 and C31). Average chain length (ACL25-35), carbon preference index (CPI25-35), and grassland (Pgrass) ratios suggest contributions from terrestrial vegetation including grasses. High TOC to total sulphur (TS) ratios (C/S) observed suggest well-oxygenated interval associated with vertical mixing of seawater about 1000 years ago. The average sedimentation rates of Bay of Bengal and Andaman Sea are about 0.4 mm yr−1 and 0.07 mm yr−1, respectively. The organic carbon accumulation rate of the Bay of Bengal is found to be several times higher than that of Andaman Sea. Rapid deposition of both marine and terrestrial OM in the Bay of Bengal could be a favourable sedimentary system for methane fermentation to form methane hydrate in this regime.

Hydrochemistry and water isotopes of a deep sinkhole in north Quintana Roo, Mexico

Sinkholes are common karst forms were we can access the aquifer. This study describes an 85 m deep sinkhole in north Quintana Roo (Mexico), located at 45 km from the coast, allowing us to explore an open section of the aquifer. The profile of the water column was realized to evaluate the physical chemistry, hydrochemistry and water isotopes (δ2H, δ 18O and 3H) in order to distinguish the stratification. It was identified water-rock interaction (limestone) in the upper section (0–40 m), typical from karst groundwater; a mixing zone approximately 10 m thick, and saline intrusion in the deep section (50–80 m). Salinization of the freshwater was observed, likely due to diffusion of sodium and chloride from the deep water. The hydrochemistry suggests seawater intrusion contributing as much a 60% of the water in the deep section. With the stable isotopes, meteoric water, groundwater and seawater mixing was attested. The tritium measurements suggest that the aquifer has short residence time and fast recharge. Few deep sinkholes in Mexico have been studied, the information obtained in Yumkin contributes to the knowledge and understanding of the Yucatan aquifer system.

Sea water intrusion in the Yellow River Delta of Shandong Province based on hydrogeochemical characteristics Analysis

To study the source of sea water intrusion in the Yellow River Delta, groundwater sampling was conducted in typical sections of the Yellow River Delta. Through the analysis of major ions and isotopes of water samples, the study showed that the groundwater in the Yellow River Delta originated from ancient seawater and atmospheric precipitation deposited by sea intrusion during the geological-historical period, and there was basically no mixing of groundwater and modern seawater.

Distribution and Environmental Impact Factors of Phytoplankton in the Bay of Bengal during Autumn

In order to better understand the effects of environmental factors and water mixing on the phytoplankton community structure in the Bay of Bengal, a field investigation was conducted from October to December 2016. A total of 276 species from 68 genera were identified, including Bacillariophyta (81 species), Miozoa (188 species), Cyanobacteria (four species), and Ochrophyta (three species). The abundance and distribution of dominant cyanobacteria gradually decreased along the latitude. This is evidenced in the vertical direction, which shows that cyanobacteria were affected by changes in environmental factors caused by the vertical mixing of seawater. The relationship between stratified N:P and phytoplankton also revealed that the changes in the vertical direction of the water deeply affected the phytoplankton community structure in the Bay of Bengal. The regions with strong vertical stratification were more favorable for the growth of cyanobacteria, while the regions with weak vertical stratification were more favorable for the growth of diatoms and dinoflagellates. According to the canonical correspondence analysis, nitrogen, silicates, phosphates, vertical stratification, and temperature were key control factors for phytoplankton communities. However, changes in these chemical parameters in the study area were also caused by the seawater cycle process.

Multi-isotopes revealing the coastal river anthropogenic pollutants and natural material flux to ocean: Sr, C, N, S, and O isotope study.

Coastal river exports massive terrestrial materials to the adjacent marine environment with information about chemical weathering, providing critical insights on riverine flux and the potential impact on marine ecosystem. In this study, the preliminary data of dissolved strontium (Sr) and 87Sr/86Sr in a typical coastal river in southeastern China were collected along with hydrochemistry and C, N, S, and O isotopes to discriminate the source of terrestrial weathering and the riverine flux. Sr concentrations exhibited a range of 0.084 ~ 1.307μmol L-1, and 87Sr/86Sr values ranged 0.7089 ~ 0.7164. The total cationic charge (TZ+) ranged 0.2 ~ 11.7meq L-1 with the predominant Ca2+ which accounted for > 50% of TZ+, while the anions were dominated by HCO3-. The extremely high Na+ and Cl- near the estuary indicated seawater mixing in such a coastal river. δ13C-DIC, δ15N-NO3-, δ18O-NO3-, and δ34S-SO42- of river water ranged - 24.1‰ ~ - 9.2‰, 0.3‰ ~ 22.7‰, - 2.1‰ ~ 21.4‰, and - 9.3‰ ~ 18.0‰, respectively. δ13C enhanced correspondingly to decreased δ34S, confirming the attendance of H2SO4 in carbonate weathering. Most δ18O values exhibited within ± 10‰, indicating the dominant nitrification process. δ15N presented slightly negative relationship with δ13C and no obvious correlation with δ34S, indicating relatively limited impact of denitrification. The depleted δ13C and δ15N may be attributed to carbonate dissolution with nitric acids and the oxidation of organic matters into C and N pools. Quantitative analysis revealed that silicate weathering accounts for 79% of total dissolved Sr, indicating the dominant weathering process. The estimated monthly flux of dissolved Sr to the East China Sea was 138.1 tons, demonstrating an potential impact on seawater Sr isotope evolution. Overall, the investigations of multi-isotopes revealed the enhancement of weathering rates and the consequently depleted CO2 consumption, which further proved the involvement of strong acids (H2SO4 and HNO3). This study provides scientific insight in terrestrial weathering and anthropogenic impact of a typical coastal watershed and may orient the management of environmental issues related to coastal ecosystems.

Assessing the Quantity and Quality Controls of the Freshwater Lens on a Semi-Arid Coral-Limestone Island in Sri Lanka

Uncertainties in the contamination and salinization mechanisms of the freshwater lens (FWL) in the semi-arid coral-limestone aquifer of Delft Island, Sri Lanka threatens its water security. The processes governing the quality and distribution of the FWL were therefore investigated through recharge assessment and hydrochemical analysis. Potential groundwater recharge zones based on land classification and geology were first identified. A rootzone water balance model was then built, which revealed the spatiotemporal variability of potential groundwater recharge occurring rapidly during the wet season (October to January) and most abundantly on pasture land underlain by yellow and brown sand. Recharge also varied largely between dry and wet years. Where the water table was shallow, intense rainfall in wet years was seen to result in surface flooding. Geochemical modeling using PHREEQC combined with diagrams (Piper and Stiff) and scatter plots, including stable water isotopes, revealed the meteoric origin of groundwater with salinization mainly caused by seawater mixing and slight evaporation. Findings also suggest that salinization is driven by the island's low-lying nature (maximum elevation of 6 m above sea level), the low hydraulic heads (maximum of 3.7 m above sea level), the shallow depth of the marine water, the presence of lagoons in the center which are inferred to be in hydraulic continuity with the ocean, and to some extent by unregulated abstraction of groundwater through shallow hand-dug wells. We hypothesize about infiltration and percolation of saline water through the root zone during storm inundations near the coast, supported by the combined occurrence of high values of partial CO2 pressure, alkalinity and salinity in groundwater samples. Cation exchange showed indications of salinization of wells mostly in low lying areas (minimum Na/Cl value of 0.66), and freshening in areas near the coast with high potential groundwater recharge (maximum Na/Cl value of 1.04). Elevated nitrate concentrations (maximum of 2.55 mmol/L NO3-) in groundwater samples were observed. This suggests that anthropogenic contamination is further threatening the already scarce resource as well as coastal ecosystems that may be groundwater dependent.

Quaternary Evolutionary Stages of Selinitsa Cave (SW Peloponnese, Greece) Reveal Sea-Level Changes Based on 3D Scanning, Geomorphological, Biological, and Sedimentological Indicators

Significant evolutionary stages of Selinitsa Cave (SW Peloponnese, Greece) were revealed by 3D mapping, as well as geomorphological study of the cave and the nearby landscape. Four marine terraces were identified in the area of the coastal cave at 6, 10.7, 16.6, and 30–32 m above sea level (asl), with the terrace at 16.6 m representing Marine Isotope Stage (MIS) 5. The widest karstified space of Selinitsa Cave clusters between 15.73 and 18.05 m above sea level (asl), with the peak lying at 16.4 m asl, corresponding to the level where the phreatic/epiphreatic zone was stable for a sufficient period of time. A tidal notch at 16.4 m asl at the cave entrance is correlated to the marine terrace at 16.6 m. Both features correspond to the sea-level stand at which intense karstification occurred. The tidal notch bears a horizontal arrangement of Lithophaga borings at the vertex. Sedimentological investigation of the Selinitsa fine-grained deposit revealed the paleohydrologic regime of the cave. It is characterized by “slack-water” facies, indicating very low water flow speeds, whereas the thickness of the deposit points to stable hydrological conditions for prolonged periods. The cave sediment height of 18.8 m asl indicates a flooding level higher than sea level. The overlying Plattenkalk flysch is most probably the major source of detritus, and the predominance of authigenic dolomite (&gt;98% modal in the carbonate fraction) indicates a hyposaline environment related to mixing of sea water with percolating fresh water. The approach of this study shows the significance of 3D mapping, bio-geo-Relative Sea Level (RSL) indicators, and sedimentology in deciphering the paleogeographic evolution of coastal karstic systems and subsequently defining the paleoclimate regime of coastal areas in Greece and the eastern Mediterranean during the Late Quaternary.