A review of present techniques and methodological advances in analyzing Th in aquatic systems

The short-lived thorium isotope ²³⁴Th (half-life 24.1 days) has been used as a tracer for a variety of transport processes in aquatic systems. Its use as a tracer of oceanic export via sinking particles has stimulated a rapidly increasing number of studies that require analyses of ²³⁴Th in both marine and freshwater systems. The original ²³⁴Th method is labor intensive. Thus, there has been a quest for simpler techniques that require smaller sample volumes. Here, we review current methodologies in the collection and analysis of ²³⁴Th from the water column, discuss their individual strengths and weaknesses, and provide an outlook on possible further improvements and future challenges. Also included in this review are recommendations on calibration procedures and the production of standard reference materials as well as a flow chart designed to help researchers find the most appropriate ²³⁴Th analytical technique for a specific aquatic regime and known sampling constraints.     

Crustal Provenance and Cooling of the Basement Complexes of the Sierra de San Luis: An Insight Into the Tectonic History of the Pro to-Andean Margin of Gondwana

The Sierra de San Luis constitutes the southernmost tip of the Eastern Sierras Pampeanas. Its Palaeozoic metamorphic basement units define a key location for the understanding of the accretional history along the proto-Andean margin of Gondwana. Although, it is largely accepted that the polyphase accretional history of the Sierras Pampeanas is preluded by the docking of the Pampean Terrane followed by the Famatinian Orogenic Cycle that involves subduction along the margin of Gondwana and the accretion of the Precordillera (Cuyania) Terrane and finally ceased with the collision of the Chilenia terrane, a vast amount of controversial information concerning the timing and mode of collisions as well as the origin of the different involved crustal fragments within the Eastern Sierras Pampeanas is published. In this paper, those different hypothesis are presented and evaluated under the light of new isotopic data of the Sierra de San Luis. Nd-systematics of the metasedimentary sequences of the Sierra de San Luis indicate that the studied sequences were developed on the Pampean Terrane. An Amazonian origin of the Pampean Terrane that was probably detached from the Arequipa Antofalla Craton is proposed. Furthermore, the correlation of two low-grade phyllitic belts (San Luis Formation) with the widespread Puncoviscana Formation is not supported by Sm-Nd data. It is suggested that the sedimentary precursors of the Pringles Metamorphic Complex and the topping phyllites were sourced on the Pampean Orogen and accommodated in a newly formed back arc basin during the early Famatinian.

The cooling history of the basement complex is recorded by an extensive amount of K-Ar muscovite and biotite ages. A high variability in muscovite ages is only partly related to different intrusion times of two pegmatoid generations. Post Famatinian to Achalian crustal scale mylonite formation (-359 Ma) and a rotational exhumation of the central basement unit are causal for the observed K-Ar muscovite age pattern in the range from 395 Ma to 447 Ma. Therefore, the decrease in metamorphic degree from west to east is the result of the erosion level of a crustal profile from the mid lower crust to the upper crust. An even higher variability in K-Ar biotite cooling ages covering the range from 315 Ma to 418 Ma is related to the slow cooling after the Famatinian Orogenic Cycle or reheating during the Achalian Orogenic Cycle and consequent variable reset of the isotopic system. However, ages recorded by biotite booklets substantiate the hypothesis of a differential exhumation of the basement of the Sierra de San Luis.     

Utilisation de dépôts d'argile sur lame de verre pour leur étude en spectroscopie infrarouge

IR spectra of clay samples can be collected in transmission through oriented deposits onto glass slide and are similar to the IR spectra of the same samples obtained by traditional way (KBr pellets). As examples, it is shown that it is possible to differentiate dickite from kaolinite and smectites of various chemistries, as easily by both ways. It is thus possible to use oriented clay deposits onto glass slide for both XRD and IR studies and to easily take benefit of the complementarity of both techniques. To cite this article: S. Petit et al., C. R. Geoscience 335 (2003).     

Magmatic processes and mantle heterogeneity beneath the slow-spreading northern Kolbeinsey Ridge segment,North Atlantic

We present new data on mineralogical, major and trace element compositions of lavas from the northernmost segment of the Kolbeinsey Ridge (North Kolbeinsey Ridge, NKR). The incompatible element enriched North Kolbeinsey basalts lie on a crystal fractionation trend which differs from that of the other Kolbeinsey segments, most likely due to higher water contents (~0.2%) in the NKR basalts. The most evolved NKR magmas erupt close to the Jan Mayen Fracture Zone, implying increased cooling and fractionation of the ascending magmas. Mainly incompatible element-enriched basalts, as well as some slightly depleted lavas, erupt on the NKR. They show evidence for mixing between different mantle sources and magma mixing. North Kolbeinsey Ridge magmas probably formed by similar degrees of melting to other Kolbeinsey basalts, implying that no lateral variation in mantle potential temperature occurs on the spreading axis north of the Iceland plume and that the Jan Mayen Fracture Zone does not have a cooling effect on the mantle. Residual garnet from deep melting in garnet peridotite or from enriched garnet pyroxenite veins does not play a role. The incompatible element-enriched source has high Ba/La and Nb/Zr, but must be depleted in iron. The iron-depleted mantle is less dense than surrounding mantle and leads to the formation of the North Kolbeinsey segment and its shallow bathymetry. The enriched NKR source formed from a relatively refractory mantle, enriched by a small degree melt rather than by recycling of enriched basaltic crust. The depleted mantle source resembles the mantle of the Middle Kolbeinsey segment with a depletion in incompatible elements, but a fertile major element composition.     

Intercomparison of Boron Isotope and Concentration Measurements. Part II: Evaluation of Results

The Istituto di Geoscienze e Georisorse (IGG), on behalf and with the support of the International Atomic Energy Agency (IAEA), prepared eight geological materials (three natural waters and five rocks and minerals), intended for a blind interlaboratory comparison of measurements of boron isotopic composition and concentration. The materials were distributed to twenty seven laboratories - virtually all those performing geochemical boron isotope analyses in the world -which agreed to participate in the intercomparison exercise. Only fifteen laboratories, however, ultimately submitted the isotopic and/or concentration results they obtained on the intercomparison materials. The results demonstrate that interlaboratory reproducibility is not well reflected by the precision values reported by the individual laboratories and this observation holds true for both boron concentration and isotopic composition. The reasons for the discrepancies include fractionations due to the chemical matrix of materials, relative shift of the zero position on the δ¹¹B scale and a lack of well characterized materials for calibrating absolute boron content measurements. The intercomparison materials are now available at the IAEA (solid materials) and IGG (waters) for future distribution.     

The petrogenesis of Tertiary cone-sheets in Ardnamurchan, NW Scotland: petrological and geochemical constraints on crustal contamination and partial melting

Abundant cone sheets form one of the last magmatic stages in the Tertiary central complex on the Scottish peninsula of Ardnamurchan and can be grouped into a younger inner and an older outer suite relative to a gabbro intrusion. Most of the cone-sheets consist of tholeiitic to transitional basalt with MgO contents between 7.5% and 4%, although more evolved rocks also occur (to 0.5% MgO). The mafic samples are slightly enriched in the light rare earth elements (Chondrite-normalized La/Sm ∼1.1), the enrichment increases in the more evolved rocks. The compositional variation of the basaltic rocks is mainly due to crystal fractionation of olivine and clinopyroxene at depths of ∼10 km but trace elements show simultaneous assimilation of Archean Lewisian granulite crust. The andesitic to rhyolitic lavas formed by fractional crystallization from the contaminated basaltic magma coupled with assimilation of Proterozoic Moine metasediments at uppermost crustal levels. The occurrence of composite cone-sheets with basaltic and rhyolitic parts and mixtures between these magmas implies that the melts ascended successively but within a short period of time. The parental magmas of the Ardnamurchan cone-sheets must have formed at relatively shallow depths in the mantle and are comparable to the youngest tholeiitic lavas from the neighbouring island of Mull. Received: 5 June 1997 / Accepted: 12 November 1997     

Human impact on Holocene sediment dynamics in the Eastern Mediterranean – the example of the Roman harbour of Ephesus

During the past millennia, many erosion and accumulation processes have been modified by anthropogenic impact. This holds especially true for the environs of ancient settlements and their harbours along the Mediterranean coasts. Our multi‐proxy investigations in the Roman harbour and the harbour canal of Ephesus (western Turkey) reveals that humans have significantly triggered soil erosion during the last three millennia. Since the eighth century bc , and especially since the Hellenistic period, a high sedimentation rate indicates fast alluviation and delta progradation of the Küçük Menderes. Deforestation, agriculture (especially ploughing) and grazing (especially goats) were the main reasons for erosion of the river catchment area. One consequence was significant siltation of the Hellenistic/Roman harbour basin. This sediment trap archives the human impact, which was strongly enhanced from Hellenistic/Roman to Byzantine times (second/first centuries bc to the sixth/seventh centuries ad ), evidenced by high sedimentation rates, raised values of heavy metal contaminations [lead (Pb), copper (Cu)], the occurrence of fruit tree pollen and of intestinal parasites. From the middle to the end of the first millennium ad , the influence of Ephesus declined, which resulted in a decrease of human impact. Studies of several ancient settlements around the Mediterranean Sea tell a comparable story. They also confirm that during their most flourishing periods the human impact totally overprinted the climatic one. To detect the latter, geo‐bio‐archives of relatively pristine areas have to be investigated in detail. Copyright © 2016 John Wiley & Sons, Ltd.     

Effects of sampling microhabitats with low coverage within the STAR/AQEM macroinvertebrate sampling protocol on stream assessment

In the STAR/AQEM protocol microhabitats covering less than 5% of the sampling area were neglected. Driven by an ongoing discussion on the importance of these underrepresented microhabitats we tested the influence of sampling them. We investigated 48 streams representing 14 different stream types from all over Germany. Macroinvertebrates of underrepresented microhabitats were sampled in addition to the STAR/AQEM protocol. To ensure the method remains feasible in routine monitoring programmes the total sampling and sorting effort of additional sampling was limited to 20 min. Particularly those taxa were picked, which were not recognised during the routine STAR/AQEM sorting.To identify the effect of additional sampling on stream assessment results, we calculated the stream type-specific Multimetric Index (MMI) with the “main” and the “main+additional” data for each sample. The mean and median difference in MMI values between “main” and “main+additional” samples was 0.02 and 0.01, respectively. In seven of 48 samples (14.6%) a different ecological quality class was calculated with the “main+additional” dataset. Regarding common metrics within the MMI as well as intercalibration metrics differences between “main” and “main+additional” samples were analysed. The values differed most in richness metrics (e.g., number of EPTCBO Taxa, number of Trichoptera Taxa). The results of the present study show that additional sampling of underrepresented microhabitats could alter multimetric assessment results.