珍珠层有机质及其矿化响应研究

珍珠层中的有机质极其重要,它不仅是生物矿物与纯无机矿物的根本区别,而且具有重要的生物矿化响应效应.但珍珠层中有机质含量极低,约为珍珠层体积的5％,致使研究手段受到了很大限制.选择广西、广东、浙江、江苏等地的海水养殖珍珠、淡水养殖珍珠、育珠贝贝壳中的珍珠层以及现代珊瑚、珊瑚化石作为研究对象,通过日立835-50型高速氨基酸分析仪获得了各样品氨基酸的种类和含量,继而进行相关性分析和聚类分析,得到一些具有特点的氨基酸组合或类,结合对珍珠层进行的TEM、SEM等电镜观察结果,发现这些组合和类很可能分别对应于珍珠层中不同空间位置、不同结构、不同种类和不同形态大小的有机大分子.由此认为有机质的矿化响应十分复杂、多样,并可以通过诱导和控制作用形成多样化、相互演化、复杂的无机矿物及珍珠层独特的微结构.这一结论对于阐释珍珠层有机质复杂多样的矿化响应具有重要意义.     

Electron Microprobe/SIMS Determinations of Al in Olivine: Applications to Solar Wind,Pallasites and Trace Element Measurements

Electron probe microanalyser measurements of trace elements with high accuracy are challenging. Accurate Al measurements in olivine are required to calibrate SIMS implant reference materials for measurement of Al in the solar wind. We adopt a combined EPMA/SIMS approach that is useful for producing SIMS reference materials as well as for EPMA at the ~ 100 µg g^?1 level. Even for mounts not polished with alumina photoelectron spectroscopy shows high levels of Al surface contamination. In order to minimise electron beam current density, a rastered 50 × 100 µm electron beam was adequate and minimised sensitivity to small Al‐rich contaminants. Reproducible analyses of eleven SIMS‐cleaned spots on San Carlos olivine agreed at 69.3 ± 1.0 µg g^?1. The known Al mass fraction was used to calibrate an Al implant into San Carlos. Accurate measurements of Al were made for olivines in the pallasites: Imilac, Eagle Station and Springwater. Our focus was on Al in olivine, but our technique could be refined to give accurate electron probe measurements for other contamination‐sensitive trace elements. For solar wind, it is projected that the Al/Mg abundance ratio can be determined to 6%, a factor of 2 more precise than the solar spectroscopic ratio.     

Chemical Separation of Tungsten and Other Trace Elements for TIMS Isotope Ratio Measurements Using Organic Acids

One requirement for isotope ratio measurement results with small measurement uncertainties is that the element of interest is effectively separated from the sample matrix. Efficient chemical separation of W from matrix components, especially Ti, can be challenging, particularly for large test portion masses (> 1 g). We present a new W separation procedure that takes advantage of the distinct complexation behaviour of Ti and W with citrate ligand in a moderately low pH, oxidising solution. This preparation procedure can reduce the Ti/W ratio of large (4–10 g) basaltic (i.e., high‐matrix) test portions by a factor of 10⁵, relative to their original compositions, in a two‐step separation procedure. The procedure additionally provides a separate, well‐purified Mo fraction. We show that optimal separation requires precise selection of reagent concentrations and sample load. The procedure was employed to determine the μ¹⁸²W composition of BHVO‐2 as ?6.7 ± 4.2 (2 standard deviation, 2s). The principles derived from this method may prove useful for chemical separation of other elements used for geochemical and cosmochemical applications given an appropriate selection of organic acid. Future successful applications of this method may reveal that the use of organic acids as procedural reagents is a currently under‐utilised tool for efficient chemical separation protocols.     

Nanostructure, composition and mechanisms of bivalve shell growth

Freshwater and marine cultured pearls form via identical processes to the shells of bivalves and can therefore serve as models for the biomineralization of bivalve shells in general. Their nanostructure consists of membrane-coated granules (vesicles) which contain amorphous calcium carbonate (ACC) at the beginning of the biomineralization sequence, preceding the crystallization of aragonite and vaterite. In contrast to the commonly accepted view, crystallization of ACC occurs rapidly and within the granular nano-compartments mediated by organic molecules much earlier than platelet formation. The interlamellar organic sheets in nacre that form the platelet structure of nacre themselves form by self-organization after the crystallization process of CaCO₃ is completed and, thus, cannot serve as a nucleation template for aragonite. Pores in the organic sheets are postulated to be a result of this process rather than to represent the pathways for CaCO₃ through pre-existing interlamellar sheets. The amorphous phase has the highest concentrations of Mg (5.8 mol%), Mn (6.6 mol%), S (4.7 mol%) and P (1 mol%) of the three CaCO₃-polymorphs. Mg/Ca and Mn/Ca ratios are found to decrease in the order ACC > vaterite > aragonite, corresponding to decreasing organic content in the different phases. This, as well as an observed enrichment of Mg in the organic-rich growth-banding of the pearls, suggests an at least partially organic speciation of Mg and Mn in bivalves and may be responsible for the observed physiological influence on Mg/Ca and Mn/Ca ratios in bivalves as a proxy for environmental parameters.     

贝壳珍珠层微结构及成因理论

珍珠层是由多边形文石板片和少量有机质（〈５％）组成的具微层理的结构单元,可分为砖型和堆垛型两种类型珍珠层中文石板片晶体的结晶学方向、粒径及形貌等严格受到控制,呈现出高度有序的结构。珍珠层的形成与软体动物的生理活动有关,目前珍珠层的成因理论主要有：与外套的外上皮细胞年龄有关;细胞内结晶细胞外组装说;隔室说;矿物桥说及模板说等。     

Hydrogeological investigation in Santiago Island (Cabo Verde) using magnetotellurics and VLF methods

A geoelectromagnetic research was carried out in the Santa Cruz region (Santiago Island, Cabo Verde) during June 2004. The survey comprised MT soundings and VLF profiles. The main purpose of the MT profile, carried out across three important valleys associated with freshwater aquifers, was to study the tectonic structures correlated to seawater infiltration. The VLF method was used inside of the valleys for investigating shallow structures related to the aquifer contamination by seawater.Numerical modelling shows that the ocean effect is not important for MT data collected at periods shorter than 1 s. The MT data were inverted using a two-dimensional approach, to obtain the sub-superficial electrical conductivity distribution. The VLF data were processed applying the Karous–Hjelt filters to obtain the equivalent current distribution and inverted using 2-D approach. The results obtained in one of the most important valleys show anomalous current concentration/low resistivity (<20 Ω m) areas at depths greater than 40 m that may correspond to an increase in seawater content.The MT data modelling show that the deep zones beneath the valley are strongly fractured representing good pathways for seawater circulation. The depth of the conductive zones increases from south to north, suggesting a northward decreasing of the seawater infiltration effect. This observation correlates very well with in situ geochemical observations.     

Apatite Chlorine Concentration Measurements by LA‐ICP‐MS

Apatite incorporates variable and significant amounts of halogens (mainly F and Cl) in its crystal structure, which can be used to determine the initial F and Cl concentrations of magmas. The amount of chlorine in the apatite lattice also exerts an important compositional control on the degree of fission‐track annealing. Chlorine measurements in apatite have conventionally required electron probe microanalysis (EPMA). Laser ablation inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) is increasingly used in apatite fission‐track dating to determine U concentrations and also in simultaneous U‐Pb dating and trace element measurements of apatite. Apatite Cl measurements by ICP‐MS would remove the need for EPMA but the high (12.97 eV) first ionisation potential makes analysis challenging. Apatite Cl data were acquired using two analytical set‐ups: a Resonetics M‐50 193 nm ArF Excimer laser coupled to an Agilent 7700× quadrupole ICP‐MS (using a 26 μm spot with an 8 Hz repetition rate) and a Photon Machines Analyte Excite 193 nm ArF Excimer laser coupled to a Thermo Scientific iCAP Qc (using a 30 μm spot with a 4 Hz repetition rate). Chlorine concentrations were determined by LA‐ICP‐MS (1140 analyses in total) for nineteen apatite occurrences, and there is a comprehensive EPMA Cl and F data set for 13 of the apatite samples. The apatite sample suite includes different compositions representative of the range likely to be encountered in natural apatites, along with extreme variants including two end‐member chlorapatites. Between twenty‐six and thirty‐nine isotopes were determined in each apatite sample corresponding to a typical analytical protocol for integrated apatite fission track (U and Cl contents) and U‐Pb dating, along with REE and trace element measurements. ³⁵Cl backgrounds (present mainly in the argon gas) were ~ 45–65 kcps in the first set‐up and ~ 4 kcps in the second set‐up. ³⁵Cl background‐corrected signals ranged from ~ 0 cps in end‐member fluorapatite to up to ~ 90 kcps in end‐member chlorapatite. Use of a collision cell in both analytical set‐ups decreased the low mass sensitivity by approximately an order of magnitude without improving the ³⁵Cl signal‐to‐background ratio. A minor Ca isotope was used as the internal standard to correct for drift in instrument sensitivity and variations in ablation volume during sessions. The ³⁵Cl/⁴³Ca values for each apatite (10–20 analyses each) when plotted against the EPMA Cl concentrations yield excellently constrained calibration relationships, demonstrating the suitability of the analytical protocol and that routine apatite Cl measurements by ICP‐MS are achievable.     

Preliminary analysis on roles of metal–organic compounds in the formation of invisible gold

The paper comprises new analytical data on the nature and occurrence of gold in solid pyrobitumen, closely associated with the main gold-bearing sulfide arsenic ores of the Bakyrchik gold deposit (Kazakhstan), related to post-collisional magmatic-hydrothermal origin. Gold mineralization of the deposit occurs mainly in the form of an “invisible” type of gold in the structures of arsenian pyrite and arsenopyrite, and the form of gold-organic compounds of pyrobitumen in carbonaceous-terrigenous sequences of Carboniferous formation. Microscopic and electron microscopic analysis, Raman and FT-Infrared analysis, mineralogical and three-step sequential extraction analysis (NH₂OH·HCl, H₂O_2, HNO₃ + HCl) has been carried out using 9 ore samples (from 3 different types of ores) for a comprehensive study of pyrobitumen and sulfide arsenic ores focusing mainly on organic matter. The sequentially extracted precious metal content of pyrobitumen reaches up to 7 ppm gold and other metals like Ag 4 ppm, Pt 31 ppb, and Pd 26 ppb, forming metal–organic compounds, while arsenic sulfide minerals incorporate 11 ppm gold, 39 ppm Ag, 0.49 ppm Pt. The enrichment of gold associating with organic matter and sulfide ore minerals was confirmed in this study. Organic matter was active in the migration of gold and the capture of gold by pyrobitumen. Moreover, the reductive organic matter agent released gold, most likely for the sulfide arsenic ore minerals. Pyrobitumen was a decisive factor in the concentration, transportation, and preservation of gold in the deposit.

     

A detailed pyrolysis-GC/MS analysis of a black carbon-rich acidic colluvial soil (Atlantic ranker) from NW Spain

Despite the potentially large contribution of black carbon (BC) to the recalcitrant soil organic matter pool, the molecular-level composition of aged BC has hardly been investigated. Pyrolysis-GC/MS, which provides structural information on complex mixtures of organic matter, was applied to the NaOH-extractable organic matter of an acidic colluvial soil (Atlantic ranker) sampled with high resolution (5 cm) that harbours a fire record of at least 8.5 ka. Additionally, 5 charcoal samples from selected soil layers were characterised using pyrolysis-GC/MS for comparison. Pyrolysis-GC/MS allowed distinguishing between BC and non-charred organic matter. It is argued that a large proportion of the polycyclic aromatic hydrocarbons (PAHs), benzenes and benzonitrile in the pyrolysates of the extractable organic matter, together accounting for 21–54% of total identified peak area, derived from BC. In charcoal samples, these compounds accounted for 60–98% of the pyrolysis products. The large quantity of BC in almost all samples suggested a key role of fire in Holocene soil evolution. The high C content of the soil (up to 136 g C kg⁻¹ soil) may be attributed to the presence of recalcitrant organic C as BC, in addition to the sorptive preservation processes traditionally held responsible for long-term C storage in acid soils. Interactions between reactive Al hydroxides and BC could explain the longevity of BC in the soil. This work is the first thorough pyrolysis-GC/MS based study on ancient fire-affected organic matter.     

Chronostratigraphic framework and depositional environments in the organic‐rich,mudstone‐dominated Eagle Ford Group,Texas, USA

Since the beginning of the century, several authors have hypothesized and documented the presence of bottom currents during the deposition of mudstones, including mudstones rich in organic matter, challenging the assumption that persistent low‐energy conditions are necessary prerequisites for deposition of such sediments. More processes responsible for transport and deposition of mudstones mean also more processes acting contemporaneously in different parts of a basin. Without a precise and robust chronostratigraphic framework, however, it is not possible to characterize these differences. The new data reported here provide a profoundly different understanding of the controls on sedimentation in distal continental shelf platforms. To enhance the understanding of the different coeval environments of deposition coexisting in a muddy system, the Upper Cretaceous Eagle Ford Group, deposited on the Comanche carbonate platform, has been investigated by integrating sedimentology, mineralogy, geochemistry and palaeoecology, and creating age models in different physiographic sectors using biostratigraphy and geochronology. Data from two cores and 41 outcrops were analysed with a telescopic approach, from grain scale to basin scale. Nine temporal stages over a ca 8 Myr interval (ca 98 to 90 Ma) were defined in an area that spans 75 000 km². Finally, the different environments of deposition recorded within each of the nine stages were interpreted. The construction of the chronostratigraphic framework also allowed: measuring the duration of a basin‐wide gradational increase in energy in the water column (ca 1 Myr) and a hiatus confined into the shallower water sector (ca 2 Myr); determining the mean eruption frequency of volcanoes (ca 9 kyr); and the time of inundation of the Western Interior Seaway (97·5 to 97·1 Ma). The context, the outcrops–cores–logs correlations, the large data set (Appendix  S1 ), the high‐precision and well‐calibrated constraints represent an unprecedented contribution for future regional facies models of organic‐rich units and for improvements of key aspects in the industry of unconventional resources.     

Biomarker transformations as constraints for the depositional environment and for maximum temperatures during burial of Opalinus Clay and Posidonia Shale in northern Switzerland

Samples from two argillaceous formations (Opalinus Clay and Posidonia Shale) of near-identical maturity from northern Switzerland were subjected to a geochemical characterisation of organic matter and to confined-system pyrolysis experiments. Throughout the study area, the characteristics of organic matter are similar, indicating a spatially homogeneous sedimentary facies. Posidonia Shale contains marine organic matter deposited in a reducing environment, while a predominantly terrigenous source and a more oxidising environment of deposition was identified for Opalinus Clay. In the western and central parts of the study area, organic maturity is close to the onset of oil generation. In the easternmost part, a higher maturity has been reached due to a deeper burial below thick Tertiary Molasse deposits.Isothermal pyrolysis experiments were conducted at temperatures between 250 and 390 °C over 24 h. Bitumen yields increase along similar pathways for both Opalinus Clay and Posidonia Shale, but the maximum values are displaced by 10–20 °C. Data pertaining to maturity were determined from GC–MS analyses of saturated hydrocarbons, and specific attention was given to C₂₉-sterane and C₃₂-hopane isomerisation ratios. The evolution of these parameters with rising temperature is slightly different in the two formations, which is attributed to the contrasting organic facies. The pyrolysis data, together with literature data from natural basins, were used to calculate kinetic parameters for C₂₉-sterane and C₃₂-hopane, assuming a single-step isomerisation scheme according to the Arrhenius law. The resulting values based on pyrolysis data alone are very similar to those based on the combination of pyrolysis and natural data. Activation energies are similar in both formations, while the frequency factors are up to one order of magnitude higher for Posidonia Shale when compared to Opalinus Clay. For the Benken site, maximum temperature during Cretaceous burial was calculated on the basis of the kinetic data, using the TTI approach. The resulting temperatures of 75–80 °C are 5–10 °C below those derived in the literature from apatite fission-track analysis, vitrinite reflectance and basin modelling.     

Organic phosphorus fractionation in wetland soil profiles by chemical extraction and phosphorus-31 nuclear magnetic resonance spectroscopy

Organic P (OP) plays an important role in soil P cycling and is a potential P source for wetland plants. In this study, a modified chemical sequential fractionation method and ³¹P nuclear magnetic resonance spectroscopy (³¹P NMR) of NaOH–EDTA extracts were used to examine the distribution of organic P fractions and compounds in soil profiles of the Beijing Yeyahu Wetland, China. The influence of acid treatment prior to NaOH–EDTA extraction on ³¹P NMR spectra was also investigated. Results show that highly resistant OP was the major class of organic P. The rank order of organic P fractions was highly resistant OP (on average accounting for 68.5% of total OP) > moderately resistant OP (15.8%m of total OP) > moderately labile OP (11.4% of total OP) > labile OP (4.3% of total OP). Most of the organic P fractions decreased with soil depth due to the accumulation of plant residues in surface soils and the deposition and diagenesis of soils. Moderately (r = 0.586, p < 0.01) and highly (r = 0.741, p < 0.01) resistant OP fractions were positively correlated with soil organic matter. Phosphorus compounds including orthophosphate (23–74.6% of total P in spectra), monoester phosphate (18.6–76%), diester phosphate (nil-7.8%) and pyrophosphate (nil-6.7%) were characterized using ³¹P NMR. Monoester-P was the dominant soil organic P compound identified. The proportion of monoester-P increased significantly in NaOH–EDTA extracts with HCl pretreatment and it was confirmed by chemical analysis. Therefore, it can be concluded that HCl pretreatment can remove more than half of the inorganic P and increase the overall recovery rate of organic P during subsequent NaOH–EDTA extraction, which might be a new approach for organic P detection. Furthermore, the OP chemical sequential fractionation method presented in this study is an integrated and comprehensive approach which can be used for further verification.     

Iron speciation in interaction with organic matter: Modelling and experimental approach

The present study aims to model iron speciation when interacting with natural organic matter. Experimental data for iron speciation were achieved with insolubilized humic acid as an organic matter analogue for 1.8 × 10^− 3 M and 1.8 × 10^− 4 M iron concentrations and 2–5 pH range. Combining EPR spectroscopy and chemical analysis allowed us to fit NICA-Donnan model parameters for both organic complexation of iron and oxides precipitation.     

CHIME dating of monazite, xenotime, zircon and polycrase: Protocol, pitfalls and chemical criterion of possibly discordant age data

This paper outlines the CHIME (chemical Th–U-total Pb isochron method) dating method, which is based on precise electron microprobe analyses of Th, U and Pb in Th- and U-bearing accessory minerals such as monazite, xenotime, zircon and polycrase. The age-mapping technique that is applicable to young monazite and zircon is also described. CHIME dating consists of analyzing multiple spots within homogeneous age domains that show sufficient compositional variation, and then these data are used to construct a “pseudo-isochron” from which an age can be obtained via regression. This method, when coupled with discrimination of possibly concordant age data by chemical criteria such as the (Ca + Si)/(Th + U + Pb + S) ratio for monazite and Ca and S contents for zircon, has the potential advantage of significant precision, and the ability to work with minerals that have a significant initial common Pb component. This technique can identify two or more homogeneous domains that are separated by age gaps smaller than the error on individual spot age analysis. Many features that are insignificant in major element analysis can have major impact in the acquisition of trace element data. Critical factors include the roles of collimator slit, detector gas, background estimation, accelerating voltage, probe current, X-ray interferences and count rate in affecting the accuracy, and a way to apply the Th and U interference correction without pure Th- and U-oxides or synthesized pure ThSiO₄. The age-mapping procedure for young monazite and zircon includes acquiring PbMα (or PbMβ) intensity of individual pixels with multiple spectrometers, correcting background with background maps computed from a measured background intensity by the intensity relationships determined in advance of the measurement, calibrating of intensity with standards and calculating of ages from the Th, U and Pb concentrations. This technique provides age maps that show differences in age domains on the order of 20 Ma with in monazite as young as 100 Ma. The effect of sample damage by irradiation of intense and prolonged probe measurement is also described.     

Turbidite stratigraphy in proglacial lakes: Deciphering trigger mechanisms using a statistical approach

Turbidites embedded in lacustrine sediment sequences are commonly used to reconstruct regional flood or earthquake histories. A critical step for this method to be successful is that turbidites and their trigger mechanisms are determined unambiguously. The latter is particularly challenging for prehistoric proglacial lake records in high-seismicity settings where both earthquake-generated and flood-generated turbidites interrupt the background varved sedimentation. This calls for a new method to allow efficient and objective identification and classification of turbidites. This study examined turbidites in five long (9 to 17 m) sediment cores from Eklutna Lake, a proglacial lake in south-central Alaska, using standard core logging and grain-size data. A novel statistical approach is presented, in which varve-thickness distributions were first analyzed to objectively identify the thickest turbidites and distinguish them from background sedimentation. For each turbidite, a selection of variables were then measured, including: basal grain-size, thickness, magnetic susceptibility and spectrophotometric variables. Triggering mechanisms were discriminated by a combination of principal component analysis and clustering, and by calibration with historical events. Using this approach, a 2250 year long lake-wide event stratigraphy was constructed, with 94 prehistoric events, including 24 earthquake and 70 flood events. Basal grain-size and thickness variables turn out to be the most effective proxies for discrimination. This statistical approach is a powerful and new method to identify turbidites and their triggering mechanisms in long prehistoric sediment records. It opens up new prospects for palaeoseismological, palaeohydrological and palaeoclimate studies in proglacial lakes worldwide.     

A modified EK method with an I<Superscript>−</Superscript>/I<Subscript>2</Subscript> lixiviant assisted and approaching cathodes to remedy mercury contaminated field soils

Wanshan mercury mine is the largest cinnabar deposit in Guizhou, China. Few effective methods had been achieved to remedy Hg heavily contaminated field soils. In this paper, a modified EK method with approaching cathodes (AC-EK) and an I⁻/I₂ lixiviant was described to remedy mercury-contaminated field soils near Wanshan mercury mine. Paddy Soil I and Paddy Soil II were sampled and contained 576.73 ± 45.50 and 491.35 ± 4.73 mg/kg Hg, respectively. Although they contained 6.9 and 9.4% organic matter respectively, more than 92 and 89% Hg were removed by AC-EK within 5 days. Removal ratio increased by 0.21 and 0.68 times using EK process with ACs over that with one single cathode (SC-EK). AC-EK method saved nearly 26.4–28.1% electric power as compared to SC-EK method. As an I⁻/I₂ lixiviant solution was used to solubilize HgS(HgO) during EK process, the bonding of Hg to organic functional S groups should be less important than the binding to inner sites of organic matter in soil. The relationship between EK remediation effect and soil organic matter content was fitted to a linear model. It turned out that when soil OM increased by 1.0%, EK removal ratio (%) of Hg would decrease by 2.63%.     

Peat Formation Processes Through the Millennia in Tidal Marshes of the Sacramento–San Joaquin Delta,California, USA

The purpose of this study was to determine peat formation processes throughout the millennia in four tidal marshes in the Sacramento–San Joaquin Delta. Peat cores collected at each site were analyzed for bulk density, loss on ignition, and percent organic carbon. Core data and spline fit age–depth models were used to estimate inorganic sedimentation, organic accumulation, and carbon sequestration rates in the marshes. Bulk density and percent organic matter content of peat fluctuated through time at all sites, suggesting that peat formation processes are dynamic and responsive to watershed conditions. The balance between inorganic sedimentation and organic accumulation at the sites also varied through time, indicating that marshes may rely more strongly on either inorganic or organic matter for peat formation at particular times in their existence. Mean carbon sequestration rates found in this study (0.38–0.79 Mg C ha⁻¹ year⁻¹) were similar to other long-term estimates for temperate peatlands.     

Exploring former subglacial Hodgson Lake,Antarctica. Paper II: palaeolimnology

Direct exploration of subglacial lakes buried deep under the Antarctic Ice Sheet has yet to be achieved. However, at retreating margins of the ice sheet, there are a number of locations where former subglacial lakes are emerging from under the ice but remain perennially ice covered. One of these lakes, Hodgson Lake (72°00.549′S, 068°27.708′W) has emerged from under more than 297–465 m of glacial ice during the last few thousand years. This paper presents data from a multidisciplinary investigation of the palaeolimnology of this lake through a study of a 3.8 m sediment core extracted at a depth of 93.4 m below the ice surface. The core was dated using a combination of radiocarbon, optically stimulated luminescence, and relative palaeomagnetic intensity dating incorporated into a chronological model. Stratigraphic analyses included magnetic susceptibility, clast provenance, organic content, carbonate composition, siliceous microfossils, isotope and biogeochemical markers. Based on the chronological model we provisionally assign a well-defined magnetic polarity reversal event at ca 165 cm in the lake sediments to the Mono Lake excursion (ca 30–34 ka), whilst OSL measurements suggest that material incorporated into the basal sediments might date to 93 ± 9 ka. Four stratigraphic zones (A–D) were identified in the sedimentological data. The chronological model suggests that zones A–C were deposited between Marine Isotope Stages 5–2 and zone A during Stage 1, the Holocene. The palaeolimnological record tracks changes in the subglacial depositional environment linked principally to changing glacier dynamics and mass transport and indirectly to climate change. The sediment composition in zones A–C consists of fine-grained sediments together with sands, gravels and small clasts. There is no evidence of overriding glaciers being in contact with the bed reworking the stratigraphy or removing this sediment. This suggests that the lake existed in a subglacial cavity beneath overriding LGM ice. In zone D there is a transition to finer grained sediments characteristic of lower energy delivery coupled with a minor increase in the organic content attributed either to increases in allochthonous organic material being delivered from the deglaciating catchment, a minor increase in within-lake production or to an analytical artefact associated with an increase in the clay fraction. Evidence of biological activity is sparse. Total organic carbon varies from 0.2 to 0.6%, and cannot be unequivocally linked to in situ biological activity as comparisons of δ¹³C and C/N values with local reference data suggest that much of it is derived from the incorporation of carbon in catchment soils and gravels and possibly old CO₂ in meteoric ice. We use the data from this study to provide guidelines for the study of deep continental subglacial lakes including establishing sediment geochronologies, determining the extent to which subglacial sediments might provide a record of glaciological and environmental change and a brief review of methods to use in the search for life.     

Three‐dimensional seismic characterization of a complex sediment drift in the South China Sea: Evidence for unsteady flow regime

This study describes a previously unobserved reflection seismic configuration comprising a honeycomb planform and a repeated erosion/infill cross‐section, based on high‐resolution three‐dimensional/two‐dimensional seismic data and bathymetric data. The honeycomb structures cover an area of more than 5000 km² and are developed within the Late Miocene to recent deep‐water sediments of the north‐western South China Sea. Linear erosional troughs up to 10 km long and 1 km wide are widely developed in this area, are intimately related to the particular seismic configuration and interpreted to represent a new type of sediment drift that is caused by unsteady bottom current regimes operating since the Late Miocene. The unsteady bottom current regimes are suggested to be triggered by irregular seabed morphologies. Considerable sea‐floor topography was generated as a direct result of tectonic movements in the area since the Late Miocene, and this topography then influenced the pathways of strong bottom currents. This study highlights that: (i) an unsteady bottom current regime can be laterally extensive and persist for millions of years; (ii) structurally controlled sea‐floor relief plays an important role in controlling the depositional pattern; and (iii) the bottom currents were active since the Late Miocene, flowing from the south‐east through the Xisha–Guangle Gateway and crossing the honeycomb structure zone. This study documents a new style of drift and will help to improve current knowledge of palaeoceanography and understanding of the South China Sea deep‐water circulation which is at present still poorly understood.     

The use of vertical electrical sounding resistivity method for the location of low salinity groundwater for irrigation in Chaj and Rachna Doabs

A geoelectrical resistivity survey using vertical electrical sounding (VES) was conducted at Chaj Doab (land between rivers Jhelum and Chenab, Pakistan) and Rachna Doab (land between rivers Chenab and Ravi, Pakistan), with the objective of investigating groundwater conditions. A total of 90 sites were selected with 43 sites in Chaj and 47 sites in Rachna Doabs. The resistivity meter (ABEM Terrameter SAS 4000, Sweden) was used to collect the VES data by employing a Schlumberger electrode configuration, with half current electrode spacings (AB/2) ranging from 2 to 180 m and the potential electrode (MN) from 1 to 40 m. The field data were interpreted using the Interpex IX1D computer software and the resistivity versus depth models for each location was estimated. The outputs of subsurface layers with resistivities and thickness presented in contour maps and 3-D views by using SURFER software were created. A total of 102 groundwater samples from nearby hydrowells at different depths were collected to develop a correlation between the aquifer resistivity of VES and the electrical conductivity (EC) of the groundwater and to confirm the resulted geophysical resistivity models. From the correlation developed, it was observed that the groundwater salinity in the aquifer may be considered low and so safe for irrigation if resistivity >45 Ω m, and marginally fit for irrigation having resistivity between 25 and 45 Ω m. The study area has resistivities from 3.9 to 2,222 Ω m at the top of the unsaturated layer, between 1.21 and 171 Ω m, in the shallow aquifers, and 0.14–152 Ω m in the deep aquifers of the study area. The results indicate that the quality of groundwater is better near the rivers and in the shallow layers compared to the deep layers.