GGR Biennial Critical Review: Analytical Developments Since 2014

This GGR biennial critical review covers developments and innovations in key analytical methods published since January 2014, relevant to the chemical, isotopic and crystallographic characterisation of geological and environmental materials. In nine selected analytical fields, publications considered to be of wide significance are summarised, background information is provided and their importance evaluated. In addition to instrumental technologies, this review also presents a summary of new developments in the preparation and characterisation of rock, microanalytical and isotopic reference materials, including a précis of recent changes and revisions to ISO guidelines for reference material characterisation and reporting. Selected reports are provided of isotope ratio determinations by both solution nebulisation MC‐ICP‐MS and laser ablation‐ICP‐MS, as well as of radioactive isotope geochronology by LA‐ICP‐MS. Most of the analytical techniques elaborated continue to provide new applications for geochemical analysis; however, it is noted that instrumental neutron activation analysis has become less popular in recent years, mostly due to the reduced availability of nuclear reactors to act as a neutron source. Many of the newer applications reported here provide analysis at increasingly finer resolution. Examples include atom probe tomography, a very sensitive method providing atomic scale information, nanoscale SIMS, for isotopic imaging of geological and biological samples, and micro‐XRF, which has a spatial resolution many orders of magnitude smaller than conventional XRF.     

The constancy of galactic cosmic rays as recorded by cosmogenic nuclides in iron meteorites

We measured the He, Ne, and Ar isotopic concentrations and the ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca concentrations in 56 iron meteorites of groups IIIAB, IIAB, IVA, IC, IIA, IIB, and one ungrouped. From ⁴¹Ca and ³⁶Cl data, we calculated terrestrial ages indistinguishable from zero for six samples, indicating recent falls, up to 562 ± 86 ka. Three of the studied meteorites are falls. The data for the other 47 irons confirm that terrestrial ages for iron meteorites can be as long as a few hundred thousand years even in relatively humid conditions. The ³⁶Cl‐³⁶Ar cosmic ray exposure (CRE) ages range from 4.3 ± 0.4 Ma to 652 ± 99 Ma. By including literature data, we established a consistent and reliable CRE age database for 67 iron meteorites. The high quality of the CRE ages enables us to study structures in the CRE age histogram more reliably. At first sight, the CRE age histogram shows peaks at about 400 and 630 Ma. After correction for pairing, the updated CRE age histogram comprises 41 individual samples and shows no indications of temporal periodicity, especially not if one considers each iron meteorite group separately. Our study contradicts the hypothesis of periodic GCR intensity variations (Shaviv 2002, 2003), confirming other studies indicating that there are no periodic structures in the CRE age histogram (e.g., Rahmstorf et al. 2004; Jahnke 2005). The data contradict the hypothesis that periodic GCR intensity variations might have triggered periodic Earth climate changes. The ³⁶Cl‐³⁶Ar CRE ages are on average 40% lower than the ⁴¹K‐K CRE ages (e.g., Voshage 1967). This offset can either be due to an offset in the ⁴¹K‐K dating system or due to a significantly lower GCR intensity in the time interval 195–656 Ma compared to the recent past. A 40% lower GCR intensity, however, would have increased the Earth temperature by up to 2 °C, which seems unrealistic and leaves an ill‐defined ⁴¹K‐K CRE age system the most likely explanation. Finally, we present new ²⁶Al/²¹Ne and ¹⁰Be/²¹Ne production rate ratios of 0.32 ± 0.01 and 0.44 ± 0.03, respectively.     

MORPHOLOGICAL DEVELOPMENT OF ANATURE-LIKE BANK PROTECTION

1 INTRODUCTIONFor the pmpose of river restoration of the river Isar in the city area of Munich several inveshgationswere cAned out at the InshtUte for Hydrosciences, German Armed Forces University Munich. The calculation ofwater level for the differnt planned variants were realized by a 2d mathematical model. Although thesesimulahons were a very helpfol basis for fuIther planing, a physical model was constrUcted in order toinvestigate detalled problems.The main interest was focussed …     

Rutile occurrence and trace element behavior in medium-grade metasedimentary rocks: example from the Erzgebirge, Germany

Metamorphic textures in medium-grade (~500–550°C) metasedimentary rocks from the Erzgebirge give evidence of prograde rutile crystallization from ilmenite. Newly-crystallized grains occur as rutile-rich polycrystalline aggregates that pseudomorph the shape of the ilmenites. In-situ trace element data (EMP and SIMS) show that rutiles from the higher-grade samples record large scatter in Nb content and have Nb/Ti ratios higher than coexisting ilmenite. This behavior can be predicted using prograde rutile crystallization from ilmenite and indicates that rutiles are reequilibrating their chemistry with remaining ilmenites. On the contrary, rutiles from the lowest grade samples (~480°C) have Nb/Ti ratios that are similar to the ones in ilmenite. Hence, rutiles from these samples did not equilibrate their chemistry with remaining ilmenites. Our data suggest that temperature may be one of the main factors determining whether or not the elements are able to diffuse between the phases and, therefore, reequilibrate. Newly-crystallized rutiles yield temperatures (from ~500 to 630°C, Zr-in-rutile thermometry) that are in agreement with the metamorphic conditions previously determined for the studied rocks. In quartzites from the medium-grade domain (~530°C), inherited detrital rutile grains are detected. They are identified by their distinct chemical composition (high Zr and Nb contents) and textures (single grains surrounded by fine grained ilmenites). Preliminary calculation, based on grain size distribution of rutile in medium-grade metapelites and quartzites that occur in the studied area, show that rutiles derived from quartzites can be anticipated to dominate the detrital rutile population, even if quartzites are a minor component of the exposure.     

Application of the sine-Poisson equation in solar magnetostatics

Solutions of the sine-Poisson equation are used to construct a class of isothermal magnetostatic atmospheres, with one ignorable coordinate corresponding to a uniform gravitational field in a plane geometry. The distributed current in the model j is directed along the x-axis, where x is the horizontal ignorable coordinate. The current j varies as the sine of the magnetostatic potential and falls off exponentially with distance vertical to the base with an e-folding distance equal to the gravitational scale height. We investigate in detail solutions for the magnetostatic potential A corresponding to the one-soliton, two soliton, and breather solutions of the sine-Gordon equation. Depending on the values of the free parameters in the soliton solutions, horizontally, periodic magnetostatic structures are obtained possessing either (a) a single X-type neutral point, (b) multiple neutral X-points, or (c) solutions without X-points. The solution cases (b) and (c) contain two families of intersecting current sheets, in which the line of intersection forms flux concentration points (or singularities) for the magnetic field. The solutions illustrate the contribution of the anisotropic J × B force (B, magnetic field induction), the gravitational force, and the gas pressure gradient to the force balance.     

Calcium isotope record of Phanerozoic oceans: Implications for chemical evolution of seawater and its causative mechanisms

A total of 280 brachiopods of Ordovician to Cretaceous age, complemented by published data from belemnites and planktonic foraminifera, are used to reconstruct the evolution of calcium isotope composition of seawater (δ^44/40Ca_SW) over the Phanerozoic. The compiled δ^44/40Ca_SW record shows a general increase from ∼1.3‰ (NIST SRM 915a) at the beginning of the Ordovician to ∼2‰ at present. Superimposed on this trend is a major long-term positive excursion from the Early Carboniferous to Early Permian as well as several short-term, mostly negative, oscillations.A numerical model of the global cycles of calcium, carbon, magnesium and strontium was used to estimate whether the recorded δ^44/40Ca_SW variations can be explained by varying magnitudes of input and output fluxes of calcium to the oceans. The model uses the record of marine ⁸⁷Sr/⁸⁶Sr ratios as proxy for seafloor spreading rates, a record of oceanic Mg/Ca ratios to estimate rates of dolomite formation, and reconstructed atmospheric CO₂, discharge and erosion rates to estimate continental weathering fluxes.The model results indicate that varying magnitudes of the calcium input and output fluxes cannot explain the observed δ^44/40Ca_SW trends, suggesting that the isotope signatures of these fluxes must also have changed. As a possible mechanism we suggest variable isotope fractionation in the sedimentary output flux controlled by the dominant mineralogy in marine carbonate deposits, i.e. the oscillating ‘calcite-aragonite seas’. The ultimate control of the calcium isotope budget of the Phanerozoic oceans appears to have been tectonic processes, specifically variable rates of oceanic crust production that modulated the hydrothermal calcium flux and the oceanic Mg/Ca ratio, which in turn controlled the dominant mineralogy of marine carbonates, hence the δ^44/40Ca_SW. As to the causes of the short-term oscillations recorded in the secular δ^44/40Ca_SW trend, we tentatively propose that these are related to variable rates of dolomite formation and/or to changing chemical composition of the riverine flux, in particular and ratios, induced by variable proportions of silicate vs. carbonate weathering rates on the continents.     

Sea-level change, carbon cycling and palaeoclimate during the Late Cenomanian of northwest Europe; an integrated palaeoenvironmental analysis

A new high resolution sea-level curve for the Late Cenomanian M. geslinianum Zone has been generated using sequence stratigraphic analysis on transects through the margins of the Anglo-Paris Basin in the UK and Saxony Basin in Germany. Transgressive sediments that bury a rocky shoreline in the Dresden area have proved particularly useful in determining both the absolute amount of sea-level change and the rate of rise. After a brief fall at the base of the M. geslinianum Zone, sea level rose rapidly through the higher part of the zone, resulting in an overall short term eustatic rise of 22–28 m. Biostratigraphy and carbon isotope stratigraphy have enabled detailed correlations to be made between marginal locations and thick, relatively complete, basinal successions. The basinal successions at Eastbourne, UK, and Gröbern, Germany, provide both geochemical proxies for palaeoenvironmental change, including oxygen and carbon isotope records, and an orbital timescale graduated in precession and eccentricity cycles. Integration of the sea-level history with palaeoclimate evolution, palaeoceanography and changes in carbon cycling allows a detailed reconstruction of events during the Late Cenomanian. Orbital forcing on long eccentricity maxima provides the underlying drive for these changes, but amplification by tectonic events and feedback mechanisms augmented the orbital effects and made the Cenomanian/Turonian Boundary Event distinctive. In particular, variations in atmospheric CO₂ caused by oceanic drawdown and a brief period of intense volcanic outgassing resulted respectively in short term cooling and warming events. The magnitude and high rates (up to 1 m/1 kyr) of sea-level rise are diagnostic of glacioeustasy, however improbable this may appear at the height of the Cretaceous greenhouse.     

<Superscript>10</Superscript>Be surface exposure dating of the last deglaciation in the Aare Valley,Switzerland

The combined Rhone and Aare Glaciers presumably reached their last glacial maximum (LGM) extent on the Swiss Plateau prior to 24 ka. Two well-preserved, less extensive moraine stades, the Gurten and Bern Stade, document the last deglaciation of the Aare Valley, yet age constraints are very scarce. In order to establish a more robust chronology for the glacial/deglacial history of the Aare Valley, we applied ¹⁰Be surface exposure dating on eleven boulders from the Gurten and Bern Stade. Several exposure ages are of Holocene age and likely document post-depositional processes, including boulder toppling and quarrying. The remaining exposure ages, however yield oldest ages of 20.7 ± 2.2 ka for the Gurten Stade and 19.0 ± 2.0 ka for the Bern Stade. Our results are in good agreement with published chronologies from other sites in the Alps.     

Oscillatory cosmic-ray shock structures

A multiple scales analysis is used to derive a mixed Burgers-Korteweg-de Vries (BKdV) equation in the long wavelength regime for a two-fluid MHD model used to describe cosmic-ray acceleration by the first-order Fermi process in astrophysical shocks. The BKdV equation describes the time evolution of weak shocks in the theory of diffusive shock acceleration for all possible cosmic-ray pressures. Previous work on weak shocks in the cosmic-ray MHD model has assumed that dissipation alone is sufficient to balance nonlinearity, but, as cosmic-ray pressures become small, the weak shock becomes discontinous. By including Hall current effects into the MHD model, the low cosmic-ray pressure limit leads smoothly into solitary wave behaviour. For low cosmic-ray pressures, the shock has a downstream oscillatory precursor which is smoothed into the standard Taylor shock profile with increasing cosmic-ray pressure. As a by-product of the perturbation analysis, a dissipative KdV equation is derived. In conclusion, dispersive effects on Alfvén waves are discussed and a modulational stability analysis is presented.