Pelagic molybdenum concentration anomalies and the impact of sediment resuspension on the molybdenum budget in two tidal systems of the North Sea

The seasonal dynamics of molybdenum (Mo) were studied in the water column of two tidal basins of the German Wadden Sea (Sylt-R��m�� and Spiekeroog) between 2007 and 2011. In contrast to its conservative behaviour in the open ocean, both, losses of more than 50% of the usual concentration level of Mo in seawater and enrichments up to 20% were observed repeatedly in the water column of the study areas. During early summer, Mo removal by adsorption on algae-derived organic matter (e.g. after Phaeocystis blooms) is postulated to be a possible mechanism. Mo bound to organic aggregates is likely transferred to the surface sediment where microbial decomposition enriches Mo in the pore water. First ��98/95Mo data of the study area disclose residual Mo in the open water column being isotopically heavier than MOMo (Mean Ocean Molybdenum) during a negative Mo concentration anomaly, whereas suspended particulate matter shows distinctly lighter values. Based on field observations a Mo isotope enrichment factor of �� = ���0.3��� has been determined which was used to argue against sorption on metal oxide surfaces. It is suggested here that isotope fractionation is caused by biological activity and association to organic matter. Pelagic Mo concentration anomalies exceeding the theoretical salinity-based concentration level, on the other hand, cannot be explained by replenishment via North Sea waters alone and require a supply of excess Mo. Laboratory experiments with natural anoxic tidal flat sediments and modelled sediment displacement during storm events suggest fast and effective Mo release during the resuspension of anoxic sediments in oxic seawater as an important process for a recycling of sedimentary sulphide bound Mo into the water column.

Small aliquot and single grain IRSL and post-IR IRSL dating of fluvial and alluvial sediments from the Pativilca valley, Peru

Infrared stimulated luminescence (IRSL) and post-IR IRSL are applied to small aliquots and single grains to determine the equivalent dose (De) of eleven alluvial and fluvial sediment samples collected in the Pativilca valley, Central Peru at ca. 10��S latitude. Small aliquot De distributions are rather symmetric and display over-dispersion values between 15 and 46%. Small aliquot g-values range between 4 and 8% per decade for the IRSL and 1 and 2% per decade for the post-IR IRSL signal. The single grain De distributions are highly over-dispersed with some of them skewed to higher doses, implying partial bleaching; this is especially true for the post-IR IRSL. Measurements of a modern analog reveal that residuals due to partial bleaching are present in both the IRSL as well as the post-IR IRSL signal. The g-values of individual grains exhibit a wide range with high individual uncertainties and might contribute significantly to the spread of the single grain De values, at least for the IRSL data. Electron Microprobe Analysis performed on single grains reveal that a varying K-content can be excluded as the origin of over-dispersion. Final ages for the different approaches are calculated using the Central Age Model and the Minimum Age Model (MAM). The samples are grouped into well-beached, potentially well-bleached and partially bleached according to the evaluation of the single grain distributions and the agreement of age estimates between methods. The application of the MAM to the single grain data resulted in consistent age estimates for both the fading corrected IRSL and the post-IR IRSL ages, and suggests that both approaches are suitable for dating these samples. Keywords

Mo isotopic composition of the mid-Neoproterozoic ocean: an iron formation perspective

The Neoproterozoic was a major turning point in Earth's surficial history, recording several widespread glaciations, the first appearance of complex metazoan life, and a major increase in atmospheric oxygen. Marine redox proxies have resulted in many different estimates of both the timing and magnitude of the increase in free oxygen, although the consensus has been that it occurred following the Marinoan glaciation, the second globally recorded ���snowball Earth��� event. A critically understudied rock type of the Neoproterozoic is iron formation associated with the Sturtian (first) glaciation. Samples from the <716 Ma Rapitan iron formation were analysed for their Re concentrations and Mo isotopic composition to refine the redox history of its depositional basin. Rhenium concentrations and Re/Mo ratios are consistently low throughout the bottom and middle of the iron formation, reflecting ferruginous to oxic basinal conditions, but samples from the uppermost jasper layers of the iron formation show significantly higher Re concentrations and Re/Mo ratios, indicating that iron formation deposition was terminated by a shift towards a sulfidic water column. Similarly, the ��98Mo values are close to 0.0��� throughout most of the iron formation, but rise to ~+0.7��� near the top of the section. The ��98Mo from samples of ferruginous to oxic basinal conditions are the product of adsorption to hematite, indicating that the Neoproterozoic open ocean may have had a ��98Mo of ~1.8���. Together with the now well-established lack of a positive Eu anomaly in Neoproterozoic iron formations, these results suggest that the ocean was predominantly oxygenated at 700 Ma.

Proposal for an International Molybdenum Isotope Measurement Standard and Data Representation

Molybdenum isotopes are increasingly widely applied in Earth Sciences. They are primarily used to investigate the oxygenation of Earth's ocean and atmosphere. However, more and more fields of application are being developed, such as magmatic and hydrothermal processes, planetary sciences or the tracking of environmental pollution. Here, we present a proposal for a unifying presentation of Mo isotope ratios in the studies of mass-dependent isotope fractionation. We suggest that the ��98/95Mo of the NIST SRM 3134 be defined as +0.25���. The rationale is that the vast majority of published data are presented relative to reference materials that are similar, but not identical, and that are all slightly lighter than NIST SRM 3134. Our proposed data presentation allows a direct first-order comparison of almost all old data with future work while referring to an international measurement standard. In particular, canonical ��98/95Mo values such as +2.3��� for seawater and ���0.7��� for marine Fe���Mn precipitates can be kept for discussion. As recent publications show that the ocean molybdenum isotope signature is homogeneous, the IAPSO ocean water standard or any other open ocean water sample is suggested as a secondary measurement standard, with a defined ��98/95Mo value of +2.34 �� 0.10��� (2s). Les isotopes du molybd��ne (Mo) sont de plus en plus largement utilis��s dans les sciences de la Terre. Ils sont principalement utilis��s pour ��tudier l'oxyg��nation de l'oc��an et de l'atmosph��re de la Terre. Cependant, de plus en plus de domaines d'application sont en cours de d��veloppement, tels que ceux concernant les processus magmatiques et hydrothermaux, les sciences plan��taires ou encore le suivi de la pollution environnementale. Ici, nous pr��sentons une proposition de pr��sentation unifi��e des rapports isotopiques du Mo dans les ��tudes du fractionnement isotopique d��pendant de la masse. Nous sugg��rons que le ��98/95Mo du NIST SRM 3134 soit d��finit comme ��tant ��gal �� +0.25 ���. La raison est que la grande majorit�� des donn��es publi��es sont pr��sent��s par rapport �� des mat��riaux de r��f��rence qui sont similaires, mais pas identiques, et qui sont tous l��g��rement plus l��ger que le NIST SRM 3134. Notre proposition de pr��sentation des donn��es permet une comparaison directe au premier ordre de presque toutes les anciennes donn��es avec les travaux futurs en se r��f��rant �� un standard international. En particulier, les valeurs canoniques du ��98/95Mo comme celle de +2,3 ��� pour l'eau de mer et de -0,7 ��� pour les pr��cipit��s de Fe-Mn marins peuvent ��tre conserv��s pour la discussion. Comme les publications r��centes montrent que la signature isotopique moyenne du molybd��ne de l'oc��an est homog��ne, le standard de l'eau oc��anique IAPSO ou tout autre ��chantillon d'eau provenant de l'oc��an ouvert sont propos�� comme standards secondaires, avec une valeur d��finie du ��98/95 Mo de 2.34 �� 0.10 ��� (2s).

Denudation rates of small transient catchments controlled by former glaciation: The H��rnli nunatak in the northeastern Swiss Alpine Foreland

Extensive glaciers repeatedly occupied the northern Alpine Foreland during the Pleistocene and left a strongly glacially overprinted low slope landscape. Only few islands appeared as nunataks standing above the surface of the large piedmont glacier lobes. These nunatak areas kept their original shape, manifested in steep catchments with mean slopes up to 33 . Even though not glaciated, these catchments where significantly affected by base-level changes occurring as a consequence of phases of glacier advances and retreats. Both domains, the glacially eroded and non-eroded, are therefore prone to different mechanisms and time-scales of fluvial and colluvial re-adjustment. In this study we investigate these effects by exploring the spatial distribution and magnitude of denudation in the H��rnli region of the eastern Swiss Alpine Foreland in the present Interglacial. The area represents both domains in a relatively small area with largely uniform tectonic, lithologic and climatic conditions. The differences in Holocene andscape evolution are investigated using topographic analyses and catchment-averaged denudation rates derived from 10Be concentrations in fluvial quartz sand. We find that in formerly non-glaciated, fluvially dominated catchments close hillslope-channel coupling prevails and that these catchments yield high average denudation rates of 350 mm/ka. Glacially overprinted catchments yielded catchment-wide denudation rates an order of magnitude lower. These low denudation rates are hypothesized to be the consequence of both (i) a dominance of slow hillslope processes and (ii) admixture of high concentration, pre-LGM glacial sediment. This suggests that a) a careful field investigation must accompany the denudation rate studies and b) that the concept of area-weighted cosmogenic nuclide denudation rates must be considered in light of the predominant catchment processes.

Kyzylkumite, Ti2V3+O5(OH): new structure type, modularity and revised formula

The crystal structure of kyzylkumite, ideally Ti2V3+O5(OH), from the Sludyanka complex in South Baikal, Russia was solved and refined (including the hydrogen atom position) to an agreement index, R1, of 2.34 using X-ray diffraction data collected on a twinned crystal. Kyzylkumite crystallizes in space group P21/c, with a = 8.4787(1), b = 4.5624(1), c = 10.0330(1) ��, �� = 93.174(1)��, V = 387.51(1) ��3 and Z = 4. Tivanite, TiV3+O3OH, and kyzylkumite have modular structures based on hexagonal close packing of oxygen, which are made up of rutile TiO2 and montroseite V3+O(OH) slices. In tivanite the rutile:montroseite ratio is 1:1, in kyzylkumite the ratio is 2:1. The montroseite module may be replaced by the isotypic paramontroseite V4+O2 module, which produces a phase with the formula Ti2V4+O6. In the metamorphic rocks of the Sludyanka complex, vanadium can be present as V4+ and V3+ within the same mineral (e.g. in batisivite, schreyerite and berdesinskiite). Kyzylkumite has a flexible composition with respect to the M4+/M3+ ratio. The relationship between kyzylkumite and a closely related Be-bearing kyzylkumite-like mineral with an orthorhombic norbergite-type structure from Byrud mine, Norway is discussed. Both minerals have similar X-ray powder diffraction patterns.

In situ dehydration behavior of zeolite-like pentagonite: A single-crystal X-ray study

The structural modifications upon heating of pentagonite, Ca(VO)(Si4O10)��4H2O (space group Ccm21, a=10.3708(2), b=14.0643(2), c=8.97810(10) ��, V=1309.53(3) ��3) were investigated by in situ temperature dependent single-crystal X-ray structure refinements. Diffraction data of a sample from Poona district (India) have been measured in steps of 25 up to 250 ��C and in steps of 50 ��C between 250 and 400 ��C. Pentagonite has a porous framework structure made up by layers of silicate tetrahedra connected by V4+O5 square pyramids. Ca and H2O molecules are extraframework occupants. Room temperature diffraction data allowed refinement of H positions. The hydrogen-bond system links the extraframework occupants to the silicate layers and also interconnects the H2O molecules located inside the channels. Ca is seven-fold coordinated forming four bonds to O of the tetrahedral framework and three bonds to extraframework H2O. The H2O molecule at O9 showing a high displacement parameter is not bonded to Ca. The dehydration in pentagonite proceeds in three steps. At 100 ��C the H2O molecule at O8 was released while O9 moved towards Ca. As a consequence the displacement parameter of H2O at O9 halved compared to that at room temperature. The unit-cell volume decreased to 1287.33(3) ��3 leading to a formula with 3H2O per formula unit (pfu). Ca remained seven-fold coordinated. At 175 ��C Ca(VO)(Si4O10)��3H2O transformed into a new phase with 1H2O molecule pfu characterized by doubling of the c axis and the monoclinic space group Pn. Severe bending of specific T���O���T angles led to contraction of the porous three-dimensional framework. In addition, H2O at O9 was expelled while H2O at O7 approached a position in the center of the channel. The normalized volume decreased to 1069.44(9) ��3. The Ca coordination reduced from seven- to six-fold. At 225 ��C a new anhydrous phase with space group Pna21 but without doubling of c had formed. Release of H2O at O7 caused additional contraction of T���O���T angles and volume reduction (V=1036.31(9) ��3). Ca adopted five-fold coordination. During heating excursion up to 400 ��C this anhydrous phase remained preserved. Between room temperature and 225 ��C the unit-cell volume decreased by 21% due to dehydration. The dehydration steps compare well with the thermo-gravimetric data reported in the literature.

Crystal chemistry and hydrogen bonding of rustumite Ca10(Si2O7)2(SiO4)(OH)2Cl2 with variable OH, Cl, F

Three samples of the skarn mineral rustumite Ca10(Si2O7)2(SiO4)(OH)2Cl2, space group C2/c, a ���7.6, b ��� 18.5, c ��� 15.5 ��, �� ��� 104��, with variable OH, Cl, F content were investigated by electron microprobe, single-crystal X-ray structure refinements, and Raman spectroscopy. ���Rust1LCl��� is a low chlorine rustumite Ca10(Si2O7)2(SiO4)(OH1.88F0.12)(Cl1.28,OH0.72) from skarns associated with the Rize batholith near Ikizedere, Turkey. ���Rust2F��� is a F-bearing rustumite Ca10(Si2O7)2(SiO4)(OH1.13F0.87) (Cl1 96OH0.04) from xenoliths in ignimbrites of the Upper Chegem Caldera, Northern Caucasus, Russia. ���Rust3LClF��� represents a low-Cl, F-bearing rustumite Ca10(Si2O7)2(SiO4)0.87(H4O4)0.13(OH1.01F0.99) (Cl1.00 OH1.00) from altered merwinite skarns of the Birkhin massif, Baikal Lake area, Eastern Siberia, Russia. Rustumite from Birkhin massif is characterized by a significant hydrogarnet-like or fluorine substitution at the apices of the orthosilicate group, leading to specific atomic displacements. The crystal structures including hydrogen positions have been refined from single-crystal X-ray data to R1 = 0.0205 (Rust1_LCl), R1 = 0.0295 (Rust2_F), and R1 = 0.0243 (Rust3_LCl_F), respectively. Depletion in Cl and replacement by OH is associated with smaller unit-cell dimensions. The substitution of OH by F leads to shorter hydrogen bonds O-H���F instead of O-H���OH. Raman spectra for all samples have been measured and confirm slight strengthening of the hydrogen bonds with uptake of F.This study discusses the complex crystal chemistry of the skarn mineral rustumite and may provide a wider understanding of the chemical reactions related to contact metamorphism of limestones.