Clay mineral distribution characteristics of surface sediments in the South Mid-Atlantic Ridge

Clay minerals are usually considered as important indicators to identify sources in both terrigenous and marine sediments.In particular,clay minerals in metalliferous sediments(MS)have long been studied in global oceans except in South Mid-Atlantic Ridge(SMAR)due to limited explorations.Thus,32 MS and 34 non-MS(NMS)samples were analyzed to clarify the distribution characteristics and mineral compositions of clay minerals.All the sediments were collected along the SMAR between 12°S and 27°S.After removal of organic matter and carbonate,clay fractions(<2μm)were investigated by Xray diffraction(XRD)analysis.Results show that clay mineral assemblages of surface MS consist dominantly of smectite,less abundant illite,chlorite,and kaolinite in average weight percentage of 30%,21%,18%,and 16%,respectively.On the other hand,clay mineral assemblages in the NMS consist mainly of illite,less abundant kaolinite,chlorite,and very scarce smectite in average weight percentage of 47%,29%,24%,and 0.2%,respectively.The clay fractions in MS are mainly composed of amorphous or poorly crystallized Fe/Mn oxyhydroxides,clay mineral,quartz,and plagioclase.However,the counterparts in the NMS are mainly composed of well-crystallized clay minerals,quartz,and plagioclase without the presence of Fe/Mn oxyhydroxides.It is suggested that most of the illite,kaolinite,and chlorite in both MS and NMS are likely aeolian dust in origin from South Africa continent.In addition,the abundance of kaolinite dominates the clay mineral assemblage at low latitudes,where the intensive chemical weathering of continental source rocks facilitating the formation of kaolinite.In terms of smectite,it is indicated of authigenic origin in consideration of only smectite is available in several MS and the absence in NMS.Moreover,the MS samples with only smectite available are always accompanied by goethite.Therefore,it is assumed that most of smectite occurred in studied area is the results of interaction between hydrothermal Fe-oxyhydroxide,silica,and seawater.

Geochemical Features of Sulfides from the Deyin-1 Hydrothermal Field at the Southern Mid-Atlantic Ridge near 15?S

In this study, geochemical compositions of elements in sulfide samples collected from the Deyin-1 hydrothermal field near the 15?S southern Mid-Atlantic Ridge(SMAR) were analyzed by the X-ray fluorescence spectrometry(XRF) and inductively coupled plasma mass spectrometry(ICP-MS) to examine the enrichment regulations of ore-forming elements and hydrothermal mineralization. These sulfide precipitates can be classified macroscopically into three types: Fe-rich sulfide, Fe-Cu-rich sulfide and Fe-Zn-rich sulfide, and are characterized by the enrichment of base metal elements along with a sequence of Fe>Zn>Cu. Compared with sulfides from other hydrothermal fields on MAR, Zn concentrations of sulfides in the research area are significantly high, while Cu concentrations are relatively low. For all major, trace or rare-earth elements(REE), their concentrations and related characteristic parameters exhibit significant variations(up to one or two orders of magnitude), which indicates the sulfides from different hydrothermal vents or even a same station were formed at different stages of hydrothermal mineralization, and suggests the variations of chemical compositions of the hydrothermal fluid with respect to time. The hydrothermal temperatures of sulfides precipitation decreased gradually from station TVG10(st.TVG10) to st.TVG12, and to st.TVG11, indicating that the precipitation of hydrothermal sulfides is subjected to conditions changed from high temperature to low temperature, and that the hydrothermal activity of study area was at the late stage of a general trend of evolution from strong to weak. The abnormally low concentrations of REE in sulfides and their similar chondrite-normalized REE patterns show that REEs in all sulfides were derived from a same source, but underwent different processes of migration or enrichment, or sulfides were formed at different stages of hydrothermal mineralization. The sulfides collected from the active hydrothermal vent were mainly attributed to precipitating directly from the hydrothermal fluid, while those collected from the extinct hydrothermal chimney might have already been altered by the seawater. Generally, ore-forming elements in the sulfides can be divided into three groups: Fe-based element group, Cu-based element group and Zn-based element group. The first group includes Fe, Mn, Cr, Mo, Sn, Rb and bio-enriching elements, such as P and Si, reflecting the similar characteristics to Fe in the study area. And the second group contains Cu, W, Co, Se, Te and Bi, suggesting the similar behavior with Cu. Moreover, the third group includes Zn, Hf, Hg, Cd, Ta, Ga, Pb, As, Ag, Ni and Sb, which indicates the geochemical characteristics of most dispersed trace elements controlled by Zn-bearing minerals to some extent.

The distribution and composition of hydrocarbons in sediments of the South Mid-Atlantic Ridge

Sediment samples obtained from the South Mid-Atlantic Ridge are analyzed by a gas chromatography-mass spectrometer（GC-MS） for the abundances and distributions of hydrocarbons. The hydrocarbons in the samples exhibit a bimodal distribution of n-alkanes and are rich in 3-methylalkanes, 8-methylalkanes and 2, 4,（n-1）-trimethylalkanes, which may be the result of metabolic activity of benthic microorganism. Terpanes, hopanes and steranes are all enriched in the samples, which also support the microbial origin of hydrocarbons in samples.Bitumen and hydrocarbons in the samples show a trend that the contents are the highest in the Samples 22V-TVG10 and 26V-TVG05 collected near hydrothermal areas, and the lowest in samples 22IV-TVG01, 22V-TVG11,and 22V-TVG14 collected far from the hydrothermal areas, which suggest the possible influence on the samples by hydrothermal activity.

Mineralogical characteristics of polymetallic sulfides from the Deyin-1 hydrothermal field near 15°S, southern Mid-Atlantic Ridge

A seafloor hydrothermal field, named Deyin-1 later, near 15°S southern Mid-Atlantic Ridge（SMAR） was newly found during the 22 nd cruise carried out by the China Ocean Mineral Resources Research ＆ Development Association（COMRA）. Sulfide samples were collected at three stations from the hydrothermal field during the26 th cruise in 2012. In this paper, mineralogical characteristics of the sulfides were analyzed with optical microscope, X-ray diffractometer, scanning electron microscope and electron microprobe to study the crystallization sequence of minerals and the process of hydrothermal mineralization. According to the difference of the ore-forming metal elements, the sulfide samples can be divided into three types：（1） the Ferich sulfide, which contains mainly pyrite and chalcopyrite;（2） the Fe-Cu-rich sulfide consisting predominantly of pyrite, chalcopyrite and isocubanite, with lesser amount of sphalerite, marmatite and pyrrhotine; and（3） the Fe-Zn-rich sulfide dominated by pyrite, sphalerite and marmatite, with variable amounts of chalcopyrite, isocubanite, pyrrhotine, marcasite, galena and gratonite. Mineral precipitations in these sulfides are in the sequence of chalcopyrite（isocubanite and possible coarse pyrite）, fine pyrite,sphalerite（marmatite）, galena, gratonite and then the minerals out of the dissolution. Two morphologically distinct generations（Py-I and Py-II） of pyrite are identified in each of the samples; inclusions of marmatite tend to exist in the coarse pyrite crystals（Py-I）. Sphalerite in the Fe-Zn-rich sulfide is characterized by a＂chalcopyrite disease＂ phenomenon. Mineral paragenetic relationships and a wide range of chemical compositions suggest that the environment of hydrothermal mineralization was largely changing. By comparison, the Fe-rich sulfide was formed in a relatively stable environment with a high temperature, but the conditions for the formation of the Fe-Cu-rich sulfide were variable. The Fe-Zn-rich sulfide was precipitated during the hydrothermal venting at relatively low temperature.

Component characteristics of polycyclic aromatic compounds in sediments of the South Mid-Atlantic Ridge

10 samples of sediments obtained from the South Mid-Atlantic Ridge were measured for the abundances and distributions of polycyclic aromatic compounds（PAHs）. The total concentrations of PAHs（∑PAHs） ranged from 2.768 to 9.826 μg/g dry sediment. The ∑PAHs was higher in sample 22V-TVG10 and sample 26V-TVG05 which were close to hydrothermal fields, with the lowest value in sample 22V-TVG14 which was farthest from hydrothermal fields, suggesting a probable hydrothermal origin of ∑PAHs of samples. Approximately nine kinds of PAHs were identified, and low molecular mass tricyclic and tetracyclic aromatic compounds were predominant in the samples. The concentrations of fluoranthene which were typical as hydrothermal alteration compounds were the highest among PAHs with dry weight between 0.913–3.157 μg/g. The phenanthrene homologue was most abundant in the samples, and the ratios between parent phenanthrene and methylphenanthrene which probably reflected the degree of hydrothermal alteration ranged from 0.097 to 1.602. The sample 22V-TVG10 possessing a maximum ratio value showed the intense influence of the hydrothermal alteration on this sample, which might further imply that PAHs in sediments were mainly derived from the hydrothermal alteration.

Petrology and geochemistry of South Mid-Atlantic Ridge(19°S) lava flows:Implications for magmatic processes and possible plume-ridge interactions

The South Mid-Atlantic Ridge(SMAR)19°S segment,approximately located along the line of Saint Helena volcanic chain(created by Saint Helena mantle plume),is an ideal place to investigate the issue whether the ridgehotpot interaction process affected the whole MAR.In this study,we present major and trace elemental compositions and Sr-Nd-Pb isotopic ratios of twenty fresh lava samples concentrated in a relatively small area in the SMAR 19°S segment.Major oxides compositions show that all samples are tholeiite.Low contents of compatible trace elements(e.g.,Ni=239-594 ppm and Cr=456-1010 ppm)and low Fe/Mn(54-67)and Ce/Yb(0.65-1.5)ratios of these lavas show that their parental magmas are partially melted by a spinel lherzolite mantle source.Using software PRIMELT3,this study obtained mantle potential temperatures(Tp)beneath the segment of1321-1348℃,which is lower relative to those ridges influenced by mantle plumes.The asthenospheric mantle beneath the SMAR 19°S segment starts melting at a depth of^63 km and ceases melting at^43 km with a final melting temperature of^1265℃.The extent of partial melting is up to 16%-17.6%with an average adiabatic decompression value of 2.6%/kbar.The correlations of major oxides(CaO/Al2 O3)and trace elements(Cr,Co,V)with MgO and Zr show that the parental magma experienced olivine and plagioclase fractional crystallization during its ascent to the surface.87Sr/86Sr(0.702398-0.702996),143 Nd/144 Nd(0.513017-0.513177)and 206Pb/204Pb(18.444-19.477)ratios of these lavas indicate the mantle source beneath the SMAR 19°S segment is composed of a three-component mixture of depleted MORB mantle,PREMA mantle,and HIMU mantle materials.The simple,binary mixing results among components from plume-free SMAR MORB,Saint Helena plume and Tristan plume show that asthenospheric mantle beneath the SMAR 19°S segment may be polluted by both Saint Helena and Tristan plume enriched materials.The abovementioned mantle potential temperatures,together with the low Saint Helena(<10%)and Tristan(<5%)components remaining in the asthenospheric mantle at present,show that the physically ridge-hotspot interactions at SMAR 19°S segment may have ceased.However,the trace element and SrNd-Pb isotopically binary mixing calculation results imply that these lavas tapped some enriched pockets left when Saint Helena and/or Tristan plume were once on the SMAR during earlier Atlantic rifted history.

Spreading-rate dependence of hydroacoustic and teleseismic seismicity of ridge-transform systems:East Pacific Rise,Galapagos Ridge,and Mid-Atlantic Ridge

Seismicity in ocean ridge-transform systems reveals fundamental processes of mid-ocean ridges,while comparisons of seismicity in different oceans remain rare due to a lack of detection of small events.From 1996 to 2003,the Pacific Marine Environmental Laboratory of the National Oceanic and Atmospheric Administration(NOAA/PMEL)deployed several hydrophones in the eastern Pacific Ocean and the northern Atlantic Ocean.These hydrophones recorded earthquakes with small magnitudes,providing us with opportunities to study the seismic characteristics of ridge-transform systems at different spreading rates and make further comparisons of their differences.This study comparatively analyzed hydroacoustic and teleseismic data recorded on the fast-spreading East Pacific Rise(EPR,10°S to 12°N),intermediate-spreading Galapagos Ridge(103°W to 80°W),and slow-spreading Mid-Atlantic Ridge(MAR,15°N to 37°N).We present a systematic study of the spatial and temporal distribution of events,aftershock seismicity,and possible triggering mechanisms of aftershock sequences.Our analysis yields the following conclusions.(1)From the hydroacoustic data,the EPR transform faults had the highest average seismicity rate among the three regions.(2)Along-ridge event distributions show that a high number of earthquakes were concentrated on the EPR,while they became dispersed on the GR and fewer and more scattered on the MAR,reflecting that the different tectonic origins were closely correlated with the spreading rate.(3)Analysis from mainshock-aftershock sequences shows no significant differences in the aftershock decay rate among the three regions.(4)Multiple types of aftershock triggering models were inferred from Coulomb stress changes:strike-slip mainshocks triggered strike-slip aftershocks and normal faulting aftershocks,and normal faulting mainshocks triggered normal faulting aftershocks.Although these results are case studies,they may be applicable to other ocean ridge-transform systems in future investigations.Our results provide important new insights into the seismicity of global ocean ridge-transform systems.

Radio Direction Finding Method to Mitigate Tsunami Risk in Sierra Leone

In this study,the Radio Direction Finding method is proposed for the detection of electromagnetic signals,in the VLF band,to try to anticipate the occurrence of potentially destructive geophysical events.The experimentation concerns the interception of electromagnetic anomalies in Sierra Leone,in the five-day time window,associated with seismic events that could potentially generate tsunamis.The area of investigation is Sierra Leone,whose coastline is subjected to tidal wave hazards triggered by earthquakes generated in the Mid-Atlantic Ridge.Although Sierra Leone is not affected by recurrent earthquakes,there is nevertheless a low probability,estimated at 2 percent,of the occurrence of destructive earthquakes in the next 50 years.Also in estimates,the risk of rogue and potentially damaging waves is estimated to strike the Sierra Leone coast at least once in the next 10 years.The Radio Direction Finding experiment carried out continuously 24/7,has shown a close relationship between increased radio-anomalies,in the frequencies of 6,000 Hz,a time window between electromagnetic anomaly detection and the imminence of an earthquake,and higher frequency times for the risk of earthquake occurrence in the Mid-Atlantic Ridge.

Conspecific negative density dependence in American beech

Background： One of the most important drivers of forest biodiversity is conspecific negative density dependence（CNDD）, a reduction in performance when conspecific densities are high. While the majority of CNDD research has focused on tropical forests, evidence is mounting that CNDD may also play an important role in temperate forests.To further explore the potential reach of this phenomenon, we investigated CNDD in American beech（Fagus grandifolia）in a mature mid-Atlantic forest.Methods： We used bivariate point pattern analyses to examine spatial relationships between large beech trees and conspecific saplings, and we also contrasted these patterns with comparable patterns for heterospecifics. In addition, to address the possibility of dispersal limitation and the associated effects on spatial patterns, we analyzed seedling density as a function of adult conspecific abundance.Results： We found that beech saplings were more repel ed from large conspecifics than large heterospecifics, despite the fact that beech seedling density was positively correlated with beech basal area. However, saplings of other canopy tree species were also repel ed from adult beech trees, suggesting a general suppressive effect. Nonetheless, the discrepancy between beech seedling and sapling densities beneath adult conspecifics suggests that beech seedling survival rates were reduced in vicinity of conspecific adults.Conclusions： Regardless of the extent to which beech inhibits heterospecific trees, a negative effect on conspecific recruits may be critical for biodiversity maintenance. Without this conspecific suppression, a dense layer of shade-tolerant beech saplings could form beneath adult beech trees. If this were to occur, beech would have a substantial head-start fol owing canopy disturbance, and this late-successional species could potential y dominate a stand in perpetuity,through repeated disturbance cycles.

Implications of the melting depth and temperature of the Atlantic mid-ocean ridge basalts

Mid-ocean ridge basalts(MORBs) are characterized by large variations in trace element compositions and isotopic ratios, which are difficult to be interpreted solely by using magmatic process such as partial melting of a peridotitic mantle and subsequently fractional crystallization. Geochemical diversity of MORBs have been attributed to large-scale heterogeneity within the underlying mantle, and the heterogeneity might have been caused by addition of recycled crustal component, subcontinental lithosphere, metasomatized lithosphere and outer core contribution. In this study, we investigated the MORBs along the Mid-Atlantic Ridge(MAR) by estimating the temperature and pressure of partial melting, and comprehensively comparing trace element and isotope ratios. The data for MORBs from areas close to mantle plumes show large variations. Mantle plumes can affect mid-oceanic ridges 1 400 km away, but plume effects did not cover all of the ridge segments, and those segments without plume effects did not have any abnormalities in temperature, trace element or isotope ratios.We ascribed the above phenomena to result from the shapes of the plume flow, which we categorized as "pipelike channels" and "pancake-like channels". The "pancake-like channels" plumes affected the ambient mantle nondirectionally, but the range of the mantle affected by the "pipe-like channels" plumes were selective. Element ratios of MORBs reveal that the mantle source of the MORBs along the MAR is highly heterogeneous. We suggest that most of source heterogeneities of the MORBs may be due to the presence of subducted slab and delaminated lower crust in the source. In addition, the plume that carried materials from the core-mantle boundary may affect some of the segments.

A Quantitative Method for Active Fault Migration Distance Assessment on both Sides of Mid-Ocean Ridges——Based on Multi-Beam Data

Fracture-fissure systems found at mid-ocean ridges are dominating conduits for the circulation of metallogenic fluid.Ascertaining the distribution area of active faults on both sides of mid-ocean ridges will provide a useful tool in the search for potential hydrothermal vents,thus guiding the exploration of modern seafloor sulfides.Considering the MidAtlantic Ridge 20°N–24°N(NMAR)and North Chile Rise(NCR)as examples,fault elements such as Fault Spacing(?S)and Fault Heave(?X)can be identified and quantitatively measured.The methods used include Fourier filtering of the multi-beam bathymetry data,in combination with measurements of the topographic slope,curvature,and slope aspect patterns.According to the Sequential Faulting Model of mid-ocean ridges,the maximal migration distance of an active fault on either side of mid-ocean ridges—that is,the distribution range of active faults—can be measured.Results show that the maximal migration distance of active faults at the NMAR is 0.76–1.01 km(the distance is larger at the center than at the ends of this segment),and at the NCR,the distribution range of active faults is 0.38–1.6 km.The migration distance of active faults on the two study areas is positively related to the axial variation of magma supply.In the NCR study area,where there is an abundant magma input,the number of faults within a certain distance is mainly affected by the variation of lithospheric thickness.Here a large range of faulting clearly corresponds to a high proportion of magmatism to seafloor spreading near mid-ocean ridges(M)value,and in the study area of the NMAR,there is insufficient magmatism,and the number of faults may be controlled by both lithospheric thickness and magma supply,leading to a less obvious positive correlation between the distribution range of active faults and M.

Temperature-Dependent Newtonian Rheology in Advection-Convection Geodynamical Model for Plate Spreading in Eastern Volcanic Zone, Iceland

Geodynamic process as advection-convection of the Mid-Atlantic Ocean Ridge (MAR), that is exposed on land in Iceland is investigated. Advection is considered for the plate spreading velocity. Geodetic GPS data during 2000-2010 is used to estimate plate spreading velocity along a profile in the Eastern Volcanic Zone (EVZ), Iceland striking N102。E, approximately parallel to the NUVEL-1A spreading direction between the Eurasian and North American plates. To predict subsurface mass flow patterns, temperature-dependent Newtonian rheology is considered in the finite-element models (FEM). All models are considered 2-D with steady-state, incompressible rheology whose viscosity depends on the subsurface temperature distribution. The thickness of lithosphere along the profile in the EVZ is identified by 700。C isotherm and 1022 Pa s iso-viscosity, those reach 50 ± 3 km depth at distance of 100 km from rift axis. Geodetic observation and model prediction results show the ~90% of spreading is accommodated within ~45 km of the rift axis in each direction. Model predicts ~8.5 mm.yr-1 subsidence at the surface of rift center when magmatic plumbing is inactive. The rift center (the highest magmatic influx is ~11 mm.yr-1) in model shifts ~10 - 20 km west to generate observed style surface deformation. The spreading velocity, isotherm and depth of isotherm are the driving forces resulting in the surface deformation. These three parameters have more or less equal weight. However, as the center of deformation in the EVZ shifts and most of the subsidence takes place in the volcanic system that is currently the active which is the located of plate axis.

Investing in Strategies to Accelerate Conservation and Measure Impact in the Delaware River Watershed

The Delaware River watershed provides drinking water to over 15 million people, critical habitat for plants and animals, including many threatened and endangered species, and recreational and economic enterprise valued at $10 billion per year in direct wages. Water quality and associated economic, environmental and social values have improved dramatically since the 1950s when the lower portion of the river was declared a dead zone during parts of the summer due to excessive inputs of domestic and industrial waste. The question today is how to ensure that progress continues in the face of persistent and growing threats to water quality. Recognizing the challenges facing the watershed, over 40 of the leading conservation groups in this 13,000 square mile region are pursuing a 10-year strategic initiative focused on water quality through the Delaware River Watershed Initiative, a conservation program advancing a combination of place-based work in watershed protection, restoration, education, collaboration and innovation through collective impact. This paper serves as an invitation for broader strategic involvement to accelerate watershed protection and restoration;it also is a springboard for stakeholders to set an agenda for ensuring that the Delaware River watershed delivers clean water for humans, plants and animals. The paper identifies eight “clusters” of sub-watersheds, constituting approximately 25 percent of the total Delaware Basin, where analysis has shown that investment in water quality could deliver significant returns. Diverse geology, land use, development patterns, population density and en-vironmental stressors are present throughout these sub-watershed clusters. Focusing conservation actions in these places contributes directly to local water quality, and by fostering experimentation and innovation, it also cultivates a wide range of effective approaches for scaling up investment across the Delaware River watershed and beyond. This paper emphasizes five strategies for investing in protection of high quality waters and restoration of impaired waters: 1. protect forested headwaters to maintain high water quality;2. manage agricultural lands to reduce polluted runoff and increase groundwater infiltration;3. implement best practices and new financial incentives to reduce urban stormwater pollution through natural processes;4. increase the evidence base for watershed protection by monitoring trends in water quality and assessing project impacts;5. improve policy and practice through applied research focused on water quality outcomes. These strategies demand place-based work, and the Delawre River Watershed Initiative will focus on advancing these efforts through the cooperation of organizations located in the eight distinct watershed clusters. Proceeding downstream from the headwaters, the eight landscapes are: Pocono Mountains and Kittatinny Ridge;New Jersey Highlands;Upper Lehigh River;Middle Schuylkill River;Schuylkill Highlands;Brandywine and Christina Rivers Upstream;Suburban Philadelphia;and Kirkwood-Cohansey Aquifer (comprising New Jersey’s Bayshore;and Pine Barrens). These clusters bring together many of the most ecologically valuable and significantly impaired areas of the watershed. They are strategically located where strong organizations and critical natural vatues provide measurable opportunities for advancing local water quality while having regional impact. The selection of areas and strategies was based on research and planning undertaken by the Open Space Institute (OSI) and the Academy of Natural Sciences of Drexel University (ANSDU} with support from the William Penn Foundation. Researchers at OSI and ANSDU were joined by the National Fish and Wildlfe Foundation (NFWF) in engaging over 40 organizations working across the eight sub-watershed clusters to develop collaborative plans for implementing and measuring local conservation strategies essential to the long-term health and vibrancy of the region. These implementation plans tackle major threats to water quality and include strategies to track progress and share lessons learned. The plans provide a framework for public agencies and philanthropic funders seeking to pursue targeted watershed protection outcomes supported by monitoring, technical assistance and ongoing communications. Organizations large and small, public and private, are invited to read this paper and consider this program as an opportunity to align investment for greater impact and help ensure a bright future for the Delaware River watershed.