Silicon in iron meteorite metal

Abstract– We report Si concentrations in the metal phases of iron meteorites. Analyses were performed by secondary ion mass spectrometry using a CAMECA 1270 ion probe. The Si concentrations are low (0.09–0.46 μg g^?1), with no apparent difference in concentration between magmatic and nonmagmatic iron meteorites. Coexisting kamacite and Ni‐rich metal phases have similar Si contents. Thermodynamic calculations show that Fe,Ni‐metal in equilibrium with silicate melts at temperatures where metal crystallizes should contain approximately 100 times more Si than found in iron meteorites in this work. The missing Si may either occur as tiny silicate inclusions in metal or it may have diffused as Si‐metal into surrounding silicates at low temperatures. In both cases, extensive low‐temperature diffusion of Si in metal is required. It is therefore concluded that low Si in iron meteorites is a result of subsolidus reactions during slow cooling.     

Early Holocene environment on Bjørnøya (Svalbard) inferred from multidisciplinary lake sediment studies

Bjørnøya, a small (178 km²) island situated between the mainland of Norway and southern Spitsbergen, provides the opportunity for the reconstruction of early Holocene terrestrial and limnic palaeoenvironments in the southwestern Barents Sea. The AMS ¹⁴C dating technique, geochemical, mineral magnetic, micro and macrofossil analyses were applied to sediments recovered from lake Stevatnet and the results are interpreted in terms of palaeoenvironmental conditions between 9800 and 8300 ¹⁴C bp. After the disappearance of local glaciers before ca 9800¹⁴C BP, the lake productivity increased rapidly at the same time as pioneer plant communities developed on soils which gradually became more stable. Insect data indicates that strong seasonal contrasts with mean July temperatures around 9°C and mean January temperatures around −12°C prevailed between 9500 and 8300 ¹⁴C BP. These high summer temperatures, possibly as much as 4-5°C higher than the present, favoured the development of a flora including Dryas and Angelica cf. archangelica . The enhanced freeze/thaw processes led to an increased erosion of minerogenic and organic material. After 8000 ¹⁴C BP the temperatures may have gradually declined. The environmental reconstruction derived from our data set supports the conceptual insolation model which proposes maximum Holocene seasonality for the Northern Hemisphere at ca 9000 ¹⁴C BP.     

Geochemical Evolution of the Soufriere Hills Volcano, Montserrat, Lesser Antilles Volcanic Arc

The geochemical evolution of Montserrat provides an importantbackground to understanding the current activity of this islandarc volcano. Here we present major and trace element, and U-,Th- and O-isotope data for rocks generated in the last 300 kyrthat provide constraints on the magmatic processes occurringbeneath the volcano. Samples range from low- to medium-K calc-alkalinebasalts to dacites. Three suites can be distinguished on thebasis of major and trace element compositions: the South SoufrièreHills suite; the Soufrière Hills suite, including thelava from the current eruption; and the mafic inclusions. Magmaticdifferentiation of the magma that crystallized to form the maficinclusions appears to have been governed by closed-system processes,modelled by fractional crystallization (F     

Monitoring baseline suspended sediment in forested basins: the effects of sampling on suspended sediment rating curves

Abstract

Rating curves are widely used for directly assessing changes in the suspended sediment delivery process and indirectly for estimating total yields. Four sampling methods were simulated over a 31-day record of suspended sediment from the North Fork of the Mad River near Korbel, California. The position and size of the four groups of plotted slope/intercept pairs indicated differences in bias and variance among the methods. Estimates of total yield for the 31-day period and for storms of three sizes were also biased according to sampling method. A standard bias-correcting technique improved yield estimates, but did not remove sampling bias uniformly. Methods of data collection have a large and systematic effect on the estimation of rating-curve parameters and on estimates of suspended sediment yield. Differences attributed to land management may, in fact, result from changes in sampling methods.     

Paper II – Dirt,dates and DNA: OSL and radiocarbon chronologies of perennially frozen sediments in Siberia,and their implications for sedimentary ancient DNA studies

Arnold, L. J., Roberts, R. G., MacPhee, R. D. E., Haile, J. S., Brock, F., Möller, P., Froese, D. G., Tikhonov, A. N., Chivas, A. R., Gilbert, M. T. P. & Willerslev, E. 2010: Paper II – Dirt, dates and DNA: OSL and radiocarbon chronologies of perennially frozen sediments in Siberia, and their implications for sedimentary ancient DNA studies. Boreas, Vol. 40, pp. 417–445. 10.1111/j.1502‐3885.2010.00181.x. ISSN 0300‐9483 The sedimentary ancient DNA (sedaDNA) technique offers a potentially invaluable means of investigating species evolution and extinction dynamics in high‐latitude environments. An implicit assumption of the sedaDNA approach is that the extracted DNA is autochthonous with the host deposit and that it has not been physically transported from older source deposits or reworked within the sedimentary profile by postdepositional mixing. In this paper we investigate whether these fundamental conditions are upheld at seven perennially frozen wetland sites across the Taimyr Peninsula and coastal lowlands of north‐central Siberia. Optically stimulated luminescence (OSL) and radiocarbon (¹⁴C) dating are used to constrain the ages of both the inorganic and organic fractions of perennially frozen deposits from which sedaDNA of extinct and extant species have been recovered. OSL and ¹⁴C age/depth profiles, as well as single‐grain equivalent dose (D_e) distribution characteristics, are used to assess the stratigraphic integrity of these sedaDNA sequences by (i) identifying the presence of primary or reworked organic and inorganic material, and (ii) examining the types of depositional and postdepositional processes that have affected specific sedimentary facies. The results of this study demonstrate that even though DNA preservation and stratigraphic integrity are commonly superior in perennially frozen settings, this does not, in itself, guarantee the suitability of the sedaDNA approach. The combined OSL and ¹⁴C chronologies reveal that certain perennially frozen sites may be poorly suited for sedaDNA analysis, and that careful site selection is paramount to ensuring the accuracy of any sedaDNA study – particularly for ‘latest appearance date’ estimates of extinct taxa.     

Seasonal and interannual variability of Siberian river discharge in the Laptev Sea inferred from stable isotopes in modern bivalves

Stable oxygen and carbon isotope profiles from modern bivalve shells were investigated in order to reconstruct short-term hydrographical changes in the river-shelf system of the Laptev Sea. Oxygen isotopic profiles obtained from the aragonitic species Astarte borealis exhibit amplitude cycles interpreted as annual hydrographical cycles. These records reflect the strong contrast between summer and winter bottom water conditions in the Laptev Sea. The seasonal variations in δ¹⁸O are mainly controlled by the riverine freshwater discharge during summer with 0.5‰ per salinity unit. Corrected for a defined species-dependent fractionation offset of -0.37‰, time-dependent salinity records were reconstructed from these δ¹⁸O profiles. They indicate a good correspondence to seasonal hydrographic changes and synoptical data. Persistent trends with shell growth towards more negative δ¹³C values are observed in all specimens and appear to be related to metabolic changes of the bivalves during ontogeny. In contrast, short-term fluctuations are likely linked to seasonal variabilities of the river water outflow patterns and enhanced phytoplankton productivity during summer. This is corroborated by a clear watermass-related distinction of the various δ¹³C records made on the basis of water depth and distance from the riverine source.