Multiple Sulfur Isotope Determination by SIMS: Evaluation of Reference Sulfides for Δ33S with Observations and a Case Study on the Determination of Δ36S

High spatial resolution multiple sulfur isotope studies undertaken by multi‐collector secondary ion mass spectrometry (SIMS) commonly use well‐characterised sulfide reference materials that do not (or are assumed not to) exhibit mass‐independent fractionation in ³³S and ³⁶S, taking advantage of the three‐isotope plot to evaluate the extent of such fractionation in unknown targets. As a result, few studies to date have used a mass independently fractionated reference sulfide to demonstrate accuracy of measurement and/or data reduction procedures. This article evaluates two mass independently fractionated sulfides, a pyrite from the 3.7 Ga Isua greenstone belt and a pyrrhotite from a 2.7 Ga gold deposit in Minas Gerais, Brazil, which may be used to provide additional confidence in the obtained multiple sulfur isotope data. Additionally, the article presents a method for measuring quadruple sulfur isotopes by SIMS at a comparable spatial and volume resolution to that typically employed for triple sulfur isotopes. This method has been applied to the Isua pyrite as well as to a sample of 2.5 Ga pyrite from the Campbellrand, Transvaal, South Africa, previously investigated using SIMS for triple sulfur isotopes, illustrating its potential for quadruple sulfur investigations.     

Hässeldala – a key site for Last Termination climate events in northern Europe

The Last Termination (19 000–11 000 a BP) with its rapid and distinct climate shifts provides a perfect laboratory to study the nature and regional impact of climate variability. The sedimentary succession from the ancient lake at Hässeldala Port in southern Sweden with its distinct Lateglacial/early Holocene stratigraphy (>14.1–9.5 cal. ka BP) is one of the few chronologically well‐constrained, multi‐proxy sites in Europe that capture a variety of local and regional climatic and environmental signals. Here we present Hässeldala's multi‐proxy records (lithology, geochemistry, pollen, diatoms, chironomids, biomarkers, hydrogen isotopes) in a refined age model and place the observed changes in lake status, catchment vegetation, summer temperatures and hydroclimate in a wider regional context. Reconstructed mean July temperatures increased between c. 14.1 and c. 13.1 cal. ka BP and subsequently declined. This latter cooling coincided with drier hydroclimatic conditions that were probably associated with a freshening of the Nordic Seas and started a few hundred years before the onset of Greenland Stadial 1 (c. 12.9 cal. ka BP). Our proxies suggest a further shift towards colder and drier conditions as late as c. 12.7 cal. ka BP, which was followed by the establishment of a stadial climate regime (c. 12.5–11.8 cal. ka BP). The onset of warmer and wetter conditions preceded the Holocene warming over Greenland by c. 200 years. Hässeldala's proxies thus highlight the complexity of environmental and hydrological responses across abrupt climate transitions in northern Europe.     

Flocculation settling characteristics of mud: sand mixtures

When natural muds become mixed with sandy sediments in estuaries, it has a direct effect on the flocculation process and resultant sediment transport regime. Much research has been completed on the erosion and consolidation of mud/sand mixtures, but very little is known quantitatively about how mixed sediments interact whilst in suspension, particularly in terms of flocculation. This paper presents the settling velocity findings from a recent laboratory study which examined the flocculation dynamics for three different mud/sand mixtures at different concentrations (0.2–5 g.l^?1) and turbulent shear stresses (0.06–0.9 Pa) in a mini-annular flume. The low intrusive video-based Laboratory Spectral Flocculation Characteristics instrument was used to determine floc/aggregate properties (e.g., size, settling velocity, density and mass) for each population. Settling data was assessed in terms of macrofloc (>160 μm) and microfloc (<160 μm) settling parameters: Ws_macro and Ws_micro, respectively. For pure muds, the macroflocs are regarded as the most dominant contributors to the total depositional flux. The parameterised settling data indicates that by adding more sand to a mud/sand mixture, the fall velocity of the macrofloc fraction slows and the settling velocity of microflocs quickens. Generally, a mainly sandy suspension comprising 25% mud and 75% sand (25M:75S), will produce resultant Ws_macro which are slower than Ws_micro. The quickest Ws_micro appears to consistently occur at a higher level of turbulent shear stress (τ?～?0.6 Pa) than both the macrofloc and microfloc fractions from suspensions of pure natural muds. Flocculation within a more cohesively dominant muddy-sand suspension (i.e., 75M:25S) produced macroflocs which fell at similar speeds (±10%) to pure mud suspensions at both low (200 mg l^?1) and intermediate (1 g?l^?1) concentrations at all shear stress increments. Also, low sand content suspensions produced Ws_macro values that were faster than the Ws_micro rates. In summary, the experimental results of the macrofloc and microfloc settling velocities have demonstrated that flocculation is an extremely important factor with regards to the depositional behaviour of mud/sand mixtures, and these factors must be considered when modelling mixed sediment transport in the estuarine or marine environment.     

Multi-Collector SIMS Determination of Trace Lanthanides in Zircon

A method is presented for the determination of rare earth elements (REE) in zircon (ZrSiO₄) using a multi-collector equipped ion-microprobe operating at moderately high mass resolution (M/ΔM ˜ 3900). The low abundance light REE (La-Eu) were measured simultaneously in six ion counting electron multipliers, reducing analytical time to less than half of that in a monocollection routine. Effective mass filtering, moderate energy filtering and careful set up of detector parameters in order to minimise background counts, yielded a highly coherent set of analyses from the 91500 zircon reference sample.     

Detrital zircon ages from southern Norway – implications for the Proterozoic evolution of the southwestern Baltic Shield

An ion-microprobe (SIMS) U-Pb zircon dating study on four samples of Precambrian metasediments from the high-grade Bamble Sector, southern Norway, gives the first information on the timing of discrete crust-forming events in the SW part of the Baltic Shield. Recent Nd and Pb studies have indicated that the sources of the clastic metasediments in this area have crustal histories extending back to 1.7 to 2.1 Ga, although there is no record of rocks older than 1.6 Ga in southern Norway. The analysed metasediments are from a sequence of intercalated, centimetre to 10-metre wide units of quartzites, semi-metapelites, metapelites and mafic granulites. The zircons can be grouped in two morphological populations: (1) long prismatic; (2) rounded, often flattened. The BSE images reveal that both populations consist of oscillatory zoned, rounded and corroded cores (detrital grains of magmatic origin), surrounded by homogeneous rims (metamorphic overgrowths). The detrital zircons have ²⁰⁷Pb/²⁰⁶Pb ages between 1367 and 1939 Ma, with frequency maxima in the range 1.85 to 1.70 Ga and 1.60 to 1.50 Ga. There is no correlation between crystal habit and age of the zircon. One resorbed, inner zircon core in a detrital grain is strongly discordant and gives a composite inner core-magmatic outer core ²⁰⁷Pb/²⁰⁶Pb age of 2383 Ma. Two discrete, unzoned zircons have ²⁰⁷Pb/²⁰⁶Pb ages of 1122 and 1133 Ma, representing zircon growth during the Sveconorwegian high-grade metamorphism. Also the μm wide overgrowths, embayments in the detrital cores and apparent “inner cores” which represent secondary metamorphic zircon growth in deep embayments in detrital grains, are of Sveconorwegian age. The composite-detrital-metamorphic zircon analyses give generally discordant ²⁰⁶Pb/²³⁸U versus ²⁰⁷Pb/²³⁵U ratios and maximum ²⁰⁷Pb/²⁰⁶Pb ages of 1438 Ma. These data demonstrate the existence of a protocrust of 1.7 to 2.0 Ga in the southwestern part of the Baltic Shield, implying a break in the overall westward younging trend of the Precambrian crust, inferred from the southeastern part of the Baltic Shield. Received: 8 April 1997 / Accepted: 14 July 1997     

4D analogue modelling of transtensional pull-apart basins

Scaled sandbox models were used to investigate the 4D evolution of pull-apart basins formed above underlapping releasing stepovers in both pure strike-slip and transtensional basement fault systems. Serial sectioning and 3D volume reconstruction permitted analysis of the full 3D fault geometries. Results show that very different pull-apart basins are developed in transtension compared to pure strike-slip. Both types of models produced elongate, sigmoidal to rhomboidal pull-apart systems, but the transtensional pull-apart basins were significantly wider and uniquely developed a basin margin of en-echelon oblique-extensional faults. Dual, opposing depocentres formed in the transtensional model whereas a single, central depocentre formed in pure strike-slip. In transtension, a distinct narrow graben system formed above the principal displacement zones (PDZs). Cross-basin fault systems that linked the offset PDZs formed earlier in the transtensional models.Sequential model runs to higher PDZ displacements allowed the progressive evolution of the fault systems to be evaluated. In cross-section, transtensional pull-aparts initiated as asymmetric grabens bounded by planar oblique-extensional faults. With increasing displacement on the PDZs, basin subsidence caused these faults to become concave-upwards and lower in dip angle due to fault block collapse towards the interior of the basin. In addition, strain partitioning caused fault slip to become either predominantly extensional or strike-slip. The models compare closely with the geometries of natural pull-apart basins including the southern Dead Sea fault system and the Vienna Basin, Austria.     

Describing gonad development and gametogenesis in southern Africa’s endemic box jellyfish Carybdea branchi (Cubozoa,Carybdeidae)

Information on the reproductive development of species of box jellyfish (class Cubozoa) is poor globally, despite their significance as potent stingers and worrying projections about range expansions. While most species are confined to tropical and subtropical waters, the South African box jellyfish Carybdea branchi is commonly found in the cold-water (especially southern) Benguela ecosystem. Its biology is unknown. Here, we examine reproductive development and describe gametogenesis in this dioecious species. Four and five maturity classes were determined for males and females, respectively, as well as five oocyte developmental stages. Oocyte sizes differed significantly between developmental stages. Mature and immature medusae differed significantly in size, but males and females did not. Individuals matured at approximately 30 mm average bell dimensions. Carybdea branchi displayed gonadal development characteristic of a semelparous organism.     

Highly localised distribution patterns of juvenile sea cucumber Australostichopus mollis

Information on the environmental characteristics of the juvenile habitat of many deposit-feeding sea cucumber species is limited, despite most fished species exhibiting rapid localised depletion. The current study combined large and small scale surveying techniques within a New Zealand harbour to identify areas with high densities of juvenile Australostichopus mollis, a commercially valuable aspidochirote holothurian. Data from detailed surveys were used to relate densities of juveniles and adults with measures of physical habitat characteristics including depth, sediment facies type, grain size range, as well as measures of chlorophyll-a, phaeopigment, carbon and nitrogen content of surface sediment. Results revealed a highly localised distribution of juvenile A. mollis focused on one site associated with an area of high adult density. Sites of high juvenile A. mollis density were characterised by sediment qualities favouring epibenthic detritivorous deposit feeding, including high nitrogen content, high phaeopigment:chlorophyll-a ratio and small grain size. The high-density juvenile site had facies that were further characterised by the presence of large shell fragments (>10 cm length) of the horse mussel (Atrina zelandica), which may provide a unique settlement microhabitat for early juveniles. Unlike some other sea cucumber species, juvenile A. mollis shows no distinct spatial separation from adult sea cucumbers, no association with dense macroalgae and no clear preference for shallower depths than adults. Overall, the results illustrate the highly localised pattern of recruitment of this species to a widely distributed adult population, which may help to explain the lack of previous observations of juveniles in this species. These results indicate the importance of identifying and protecting what appear to be very specific juvenile habitats in deposit-feeding sea cucumbers to ensure continuing recruitment to exploited populations.     

An integrated zircon geochronological and geochemical investigation into the Miocene plutonic evolution of the Cyclades, Aegean Sea, Greece: part 2—geochemistry

Zircons from 14 compositionally variable granitic rocks were examined in detail using CL image-guided micro-analysis to unravel the complex magmatic history above the southward retreating Hellenic subduction zone system in the Aegean Sea. Previously published U–Pb ages document an episodic crystallisation history from 17 to 11?Ma, with peraluminous (S-type) granitic rocks systematically older than closely associated metaluminous (I-type) granitic rocks. Zircon O- and Hf isotopic data, combined with trace element compositions, are highly variable within and between individual samples, indicative of open-system behaviour involving mantle-derived melts and evolved supracrustal sources. Pronounced compositional and thermal fluctuations highlight the role of magma mixing and mingling, in accord with field observations, and incremental emplacement of distinct melt batches coupled with variable degrees of crustal assimilation. In the course of partial fusion, more fertile supracrustal sources dominated in the earlier stages of Aegean Miocene magmatism, consistent with systematically older crystallisation ages of peraluminous granitic rocks. Differences between zircon saturation and crystallisation temperatures (deduced from zircon Ti concentrations), along with multimodal crystallisation age spectra for individual plutons, highlight the complex and highly variable physico-compositional and thermal evolution of silicic magma systems. The transfer of heat and juvenile melts from the mantle varied probably in response to episodic rollback of the subducting lithospheric slab, as suggested by punctuated crystallisation age spectra within and among individual granitic plutons.