Relationship between geoacoustic properties and chemical content of submarine polymetallic crusts from offshore Brazil

Abstract

Marine mining is the ocean’s new exploration frontier, and polymetallic crusts (PMCs) and ferromanganese nodules are considered a strategic resource for the future. Acoustic geophysical methods are a valuable tool for oceanic research and have been employed for several decades in the exploration of marine resources and environmental evaluations. The main objective of this work was to investigate the correlation between the chemical composition of PMCs from three different areas along the Brazilian continental margin and their physico-acoustic attributes (P-wave velocity, density and acoustic impedance). The results show that the geoacoustic properties of PMCs are correlated to their chemical composition. Measurements showed positive and negative trends between acoustic impedance and studied elements (Fe, Mn, Co, Cu, Zn, W and Ni) according to the direction of measurement, indicting high anisotropy levels. Our study sheds initial light on the correlation between acoustic properties and metal content of PMCs. The study facilitates assessments of the acoustic responses of PMCs allowing more efficient prospection and exploitation compared to ship-board geophysical techniques that are too qualitative to identify PMCs. The results can contribute to determining the best procedures and techniques for more efficient future exploration of this resource.     

Data-Assimilative Hindcast and Climate Forecast of Storm Surges with an ASGF Regression Model

ABSTRACT

This study demonstrates that long-term climate model solutions can be efficiently converted to storm surge time series at points of interest (POIs) for the future. The all-source Green's function (ASGF) regression model is used for this conversion. In addition to being data assimilative, the ASGF regression model can also simulate storm surges at a POI faster than the traditional modelling approach by orders of magnitude. This is demonstrated using the tidal gauge at Sept-Îles (Quebec, Canada) in the Gulf of St. Lawrence as the POI. First the ASGF regression model is used to assimilate 32 years of tidal gauge data, producing a continuous hindcast of storm surges and a set of best-estimate regression parameters. Second, the ASGF regression model with the best-estimate parameters is used to convert a Canadian Regional Climate Model solution (CRCM/AHJ) to an hourly time series of storm surges from 1961 to 2100. Gumbel's extreme value analysis (EVA) is then applied to the time series as a whole and also to tri-decadal segments. The tri-decadal approach is used to investigate whether there is any progressive shortening or lengthening of storm surge return periods as a result of future climate change. A method for correcting for bias due to the forcing field at the EVA level is also demonstrated.     

Petrogenesis of base metal sulphide assemblages of some peridotites from the Kaapvaal craton (South Africa)

Thirty-two peridotite xenoliths from kimberlitic pipes of the Kaapvaal craton were analysed for S and studied in reflected light microscopy and electron microprobe. Correlation between whole-rock S concentrations and sulphide modal abundances has been obscured by kimberlite-related sulphur within the mantle and by low-temperature contamination processes during emplacement. Mantle-derived base metal sulphides (BMS) occur as solitary inclusions (SI) and intergranular blebs. Unfractured SI encloses intergrowths of Ni-poor and Ni-rich monosulphide solid solution (Mss) phases, coexisting with pentlandite (Pn) and Cu-rich sulphides. Textural relationships between Mss phases and Cu-sulphides are consistent with fractional crystallization of Mss from a Cu–Fe–Ni sulphide melt. Pn-rich euhedral SI may have crystallized from a more metal-rich sulphide melt. However, the opaque mineral assemblages of both fractured sulphide inclusions and intergranular BMS point to a progressive desulphurization of Mss, yielding Pn-rich grains, often replaced by Fe-poor heazlewoodite and abundant magnetite, while Cu-sulphides are replaced by native copper. This trend is consistent with reducing conditions generated by low-temperature serpentinization. A residual origin cannot be ruled out for the Mss enclosed in the most refractory peridotites, although their Ni/Fe ratios are too low to be consistent with an equilibration with olivine at magmatic temperatures. Modal abundances of mantle-derived BMS increase in the Fe-enriched metasomatized peridotites. At least two BMS precipitation processes can be recognized: (1) precipitation of Fe–Cu-rich immiscible sulphide melts in phlogopite-(ilmenite–rutile) peridotites and (2) sulphidation reactions from an H₂S-rich fluid phase in phlogopite-K richterite peridotites.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Pyrite tracks assimilation of crustal sulfur in Pyrenean peridotites

Cobalt-bearing pyrite (0.5?C2.0?wt.% Co) is abnormally abundant (up to 35?vol.% of the total volume of the sulfide phase) in some eastern Pyrenean peridotite massifs, compared to pieces of subcontinental mantle studied so far for sulfides. Pyrite occurs as vermicular intergrowths inside pentlandite and/or chalcopyrite or as coarser, blocky grains in the intergranular pores of host peridotites. Those different pyrites are characterized by different platinum-group element systematics (measured by laser ablation microprobe and ICP-MS). Vermicular pyrite intergrown with pentlandite displays Os-, Ir-, Ru- and Rh-enriched chondrite normalized PGE patterns of Monosulfide solid solution (Mss). In contrast, coarse-grained intergranular (??blocky??) pyrites, are PGE-poor. Chalcophile trace elements (i.e. Zn, Pb, Ag, Au) that are not usually concentrated in mantle-derived sulfides were commonly detected. By contrast, selenium contents are generally low, yielding thus pyrite with high S/Se ratio (>10⁵), consistent with a sedimentary sulfur source. Pyrite microtextures and chalcophile trace element contents support a process of assimilation of crustal sulfur from the metamorphosed sedimentary country rocks. These latter generated highly reactive CO₂-S fluids, which were injected into structural discontinuities of the lherzolitic bodies. Sulfur has reacted at T?=?300?C550°C with pre-existing, mantle-derived, metal-rich sulfide assemblages (pentlandite-chalcopyrite). Addition of crustal sulfur did produce Mss which, on cooling, exsolved the Os-rich pyrite in addition to pentlandite. The coarse-grained pyrite types have crystallized directly from S-rich fluids.     

Kinematics of the Southern Rhodope Core Complex (North Greece)

The Southern Rhodope Core Complex is a wide metamorphic dome exhumed in the northern Aegean as a result of large-scale extension from mid-Eocene to mid-Miocene times. Its roughly triangular shape is bordered on the SW by the Jurassic and Cretaceous metamorphic units of the Serbo-Macedonian in the Chalkidiki peninsula and on the N by the eclogite bearing gneisses of the Sideroneron massif. The main foliation of metamorphic rocks is flat lying up to 100 km core complex width. Most rocks display a stretching lineation trending NE–SW. The Kerdylion detachment zone located at the SW controlled the exhumation of the core complex from middle Eocene to mid-Oligocene. From late Oligocene to mid-Miocene exhumation is located inside the dome and is accompanied by the emplacement of the synkinematic plutons of Vrondou and Symvolon. Since late Miocene times, extensional basin sediments are deposited on top of the exhumed metamorphic and plutonic rocks and controlled by steep normal faults and flat-ramp-type structures. Evidence from Thassos Island is used to illustrate the sequence of deformation from stacking by thrusting of the metamorphic pile to ductile extension and finally to development of extensional Plio-Pleistocene sedimentary basin. Paleomagnetic data indicate that the core complex exhumation is controlled by a 30° dextral rotation of the Chalkidiki block. Extensional displacements are restored using a pole of rotation deduced from the curvature of stretching lineation trends at core complex scale. It is argued that the Rhodope Core Complex has recorded at least 120 km of extension in the North Aegean, since the last 40 My.     

喜马拉雅的两个造山体系和一个转换-转移断层:证据及推论

位于中喜马拉雅和巴基斯坦境内西喜马拉雅的两个相互结合的剖面在一级单元、断层中展现出不同的构造形式;并在不同时期,以不同速率发育了二级构造。沿两剖面岩性单元的显著差异显示通常指的圆柱状喜马拉雅带并没有越过喀喇昆仑山断层。与此同时,在近来许多区域研究中显示出来的构造轮廓强调主中央逆冲断层是一个貌似与中喜马拉雅断层带和越过西部山脉的西喜马拉雅断层带有联系的独立部分。上述两个地区展现出不同的碰撞历史。这些不同之处揭示喀喇昆仑山断层是西部岛弧保留造山带与东部岛弧俯冲造山带之间转移/转换断层的再活动或衍变。     

Crystal chemistry of suspended matter in a tropical hydrosystem, Nyong basin (Cameroon, Africa)

Suspended matter (SM) from the Nyong basin (Cameroon, Africa), a tropical watershed, was collected by tangential flow ultrafiltration to separate particulate (>0.45 μm) and colloidal (<0.45 μm; >20 kDa) fractions. In this basin, two distinctive systems in a selected small catchment (Nsimi–Zoétélé) of the Nyong river basin have been considered: (i) colourless water (groundwater and spring) with a low suspended load (<3 mg/l) and a low total organic carbon content (TOC<1 mg/l) and (ii) coloured water (Mengong brook and Nyong river), which is organic rich (TOC>10 mg/l) and contains higher amounts of SM (10–20 mg/l) than the colourless water. Freeze-dried samples of SM have been analysed by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), electron paramagnetic resonance spectroscopy (EPR), and visible diffuse reflectance spectroscopy (DRS).

Colourless water mainly contains mineral phases, such as poorly ordered kaolinite, plus quartz and goethite in the particulate fraction, and euhedral kaolinite plus amorphous iron oxyhydroxides in the colloidal fraction. In contrast, the SM in coloured water is mainly organic in nature. The mineral phases in the particulate fraction are similar to those from clear water, but with additional phytoliths and diatom frustules composed of biogenic opal. In the colloidal fraction, complexation of Fe³⁺ and Mn²⁺ with organic matter is evidenced by EPR, together with significant occurrence of Fe oxyhydroxides associated with organic matter.

The sites of Al, Si, Fe, Mn in colloidal fractions derived from spectroscopic analyses are discussed with reference to chemical analyses performed by inductively coupled plasma mass spectrometry. Most of the observed solid phases or species correspond to those expected from published thermodynamic calculations for the same hydrosystem, except the colloidal iron oxyhydroxides in the coloured water. The presence of such iron phases is emphasised since they are expected to have large sorption capacities for numerous trace elements.

The crystal chemistry of SM is used to discuss the origin of the mineral particles transported from the soil to the main rivers in terms of mechanical and chemical erosion processes.     

Thérèse Mound: a case study of coral bank development in the Belgica Mound Province,Porcupine Seabight

High-resolution seismic profiles, swath bathymetry, side-scan sonar data and video imageries are analysed in this detailed study of five carbonate mounds from the Belgica mound province with special emphasis on the well-surveyed Thérèse Mound. The selected mounds are located in the deepest part of the Belgica mound province at water depths of 950 m. Seismic data illustrate that the underlying geology is characterised by drift sedimentation in a general northerly flowing current regime. Sigmoidal sediment bodies create local slope breaks on the most recent local erosional surface, which act as the mound base. No preferential mound substratum is observed, neither is there any indication for deep geological controls on coral bank development. Seismic evidence suggests that the start-up of the coral bank development was shortly after a major erosional event of Late Pliocene–Quaternary age. The coral bank geometry has been clearly affected by the local topography of this erosional base and the prevailing current regime. The summits of the coral banks are relatively flat and the flanks are steepest on their upper slopes. Deposition of the encased drift sequence has been influenced by the coral bank topography. Sediment waves are formed besides the coral banks and are the most pronounced bedforms. These seabed structures are probably induced by bottom current up to 1 m/s. Large sediment waves are colonised by living corals and might represent the initial phase of coral bank development. The biological facies distribution of the coral banks illustrate a living coral cap on the summit and upper slope and a decline of living coral populations toward the lower flanks. The data suggest that the development of the coral banks in this area is clearly an interaction between biological growth processes and drift deposition both influenced by the local topography and current regime.     

粤北晚泥盆世天子岭组遗迹组构及其环境解释

遗迹组构是由生物成因改造的沉积岩组构,它是物理过程和生物过程相互作用的产物。本文在广东韶关晚泥盆世天子岭组碳酸盐岩沉积中建立了4个遗迹组构,即反映局限台地或封闭海湾环境的Planolies montanus遗迹组构,反映台地潮下低能环境的Thalassinoides—Planolites遗迹组构,反映潮上-潮间环境的．Skolithos遗迹组构和受固底底质控制的Thalassinoides遗迹组构。对遗迹化石和生物扰动构造进行阶层分析,从中识别出受固底底质控制的遗迹化石Thalassinoides.sp,这类遗迹化石常常代表层序地层学中的重要界面。