The effect of Si on metal-silicate partitioning of siderophile elements and implications for the conditions of core formation

We have determined the liquid metal-liquid silicate partitioning of Ni, Co, Mo, W, V, Cr and Nb at 1.5 GPa/1923 K and 6 GPa/2123 K under conditions of constant silicate melt composition with variable amounts of Si in the Fe-rich metallic liquid. Partitioning of Ni, Co, Mo, W and V is sensitive to the Si content of the metal with, in all five cases, increasing Si tending to make the element more lithophile than for conditions where the metal is Si-free. In contrast, metal-silicate partitioning of Cr and Nb is, at constant silicate melt composition, insensitive to the Si content of the metal.The implications of our data are that if, as indicated by the Si isotopic composition of the silicate Earth ( [Georg et al., 2007] and [Fitoussi et al., 2009]), the core contains significant amounts of Si, the important siderophile elements Ni, Co, W and Mo were more lithophile during accretion and core formation than previously believed.We use our new data in conjunction with published metal-silicate partitioning results to develop a model of continuous accretion and core segregation taking explicit account of the partitioning of Si (this study) and O (from Ozawa et al., 2008) between metal and silicate and their effects on metal-silicate partitioning of siderophile elements. We find that the effect of Si on the siderophile characteristics of Ni, Co and W means that the pressures of core segregation estimated from these elements are ∼5 GPa lower than those derived from experiments in which the metal contained negligible Si (e.g., Wade and Wood, 2005). The core-mantle partitioning of Cr and Nb requires that most of Earth accretion took place under conditions which were much more reducing than those implied by the current FeO content of the mantle and that the oxidation took place late in the accretionary process. Paths of terrestrial accretion, oxidation state and partitioning which are consistent with the current mantle contents of Ni, Co, W, V, Cr and Nb lead to Si and O contents of the core of ∼4.3 wt.% and 0.15%, respectively.     

Extending the Place Glacier mass-balance record to AD 1585, using tree rings and wood density

Recognizing that climate influences both annual tree-ring growth and glacier mass balance, changes in the mass balance of Place Glacier, British Columbia, were documented from increment core records. Annually resolved ring-width (RW), maximum (MXD), and mean density (MD) chronologies were developed from Engelmann spruce and Douglas-fir trees sampled at sites within the surrounding region. A snowpack record dating to AD 1730 was reconstructed using a multivariate regression of spruce MD and fir RW chronologies. Spruce MXD and RW chronologies were used to reconstruct winter mass balance (Bw) for Place Glacier to AD 1585. Summer mass balance (Bs) was reconstructed using the RW chronology from spruce, and net balance was calculated from Bw and Bs. The reconstructions provide insight into the changes that snowpack and mass balance have undergone in the last 400 years, as well as identifying relationships to air temperature and circulation indices in southern British Columbia. These changes are consistent with other regional mass-balance reconstructions and indicate that the persistent weather systems characterizing large scale climate-forcing mechanisms play a significant glaciological role in this region. A comparison to dated moraine surfaces in the surrounding region substantiates that the mass-balance shifts recorded in the proxy data are evident in the response of glaciers throughout the region.     

Source of paleo-groundwater in the Emirate of Abu Dhabi, United Arab Emirates: evidence from unusual oxygen and deuterium isotope data

Paleo-groundwaters of 6000?years BP from the Liwa water-table sand dune aquifer in the Rub al Khali and the Gachsaran artesian carbonate aquifer, on the coast of the Emirate of Abu Dhabi (UAE), exhibit normal ??²H/??¹⁸O slopes, modest ??¹⁸O increases, and large negative deuterium excess ??d?? (Liwa aquifer: 2.19?? VSMOW and d?=??C15; Gachsaran aquifer: 3.16?? VSMOW and d?=??C28) compared to local Shamal precipitation (0.05?? VSMOW and d?=?7). This unusual isotopic signature is hypothesized to result from re-evaporation of continental runoff to the Red Sea catchment basin. It is postulated that this continental water flowed onto the surface of the Indian Ocean providing a moisture source for the monsoon that dominated precipitation during this, the last wet period in the area from 5000 to 9000 BP. Carbonate precipitation, forming speleothems, travertines, tufas, lacustrine, and capillary-zone deposits, subsequently record this ??¹⁸O isotopic signature. This rock record is thus dominated by the water source, rather than environmental conditions of deposition normally assumed to control the rock ??¹⁸O isotopic signature. As a consequence, re-evaluation of paleo-climate??¹⁸O data from the rock record may be necessary for this time period in southern Arabia.     

The stellar mass ratio of GK Persei

We study the absorption lines present in the spectra of the long-period cataclysmic variable GK Per during its quiescent state, which are associated with the secondary star. By comparing quiescent data with outburst spectra we infer that the donor star appears identical during the two states and the inner face of the secondary star is not noticeably irradiated by flux from the accreting regions. We obtain new values for the radial velocity semi-amplitude of the secondary star,     , a projected rotational velocity,     and consequently a measurement of the stellar mass ratio of GK Per,     . The inferred white dwarf radial velocities are greater than those measured traditionally using the wings of Doppler-broadened emission lines suspected to originate in an accretion disc, highlighting the unsuitability of emission lines for mass determinations in cataclysmic variables. We determine mass limits for both components in the binary,     and     .     

AGB stars in the Magellanic Clouds

The stars that will be detectable in the Magellanic Clouds by the DENIS and 2MASS near infrared surveys are enumerated. All thermally-pulsing AGB stars will be observable in I, J, H and K, along with the top two magnitudes of both the early-AGB and the first giant branch. All carbon stars will be visible, and normal (N type) C stars will be easily distinguished by their large J-K colours. However, it will not be possible to separate faint, warm C stars from K and M stars using the photometry alone. Photometry of AGB stars in clusters will allow an accurate evaluation of the AGB tip luminosities as a function of initial mass. Random phase K magnitudes of LPVs and Cepheids should provide a better measure of the LMC tilt and distortions in the SMC. The K survey should turn up 100 to 150 objects undergoing superwind mass loss, these objects being OH/IR stars and the dust-enshrouded C star equivalents of OH/IR stars. It is shown that crowding should not be a problem even in the LMC bar.     

The use of caddisfly fauna (Insecta: Trichoptera) to characterise the Late‐glacial River Trent,England

Trichoptera have been utilised infrequently in palaeoecological studies despite their value as environmental indicators of freshwater habitat structure and quality, via their aquatic larvae, and catchment macroclimate conditions, via the aerial adults. Two sites, dated ca. 11 600 ¹⁴C yr BP, in the middle reaches of the River Trent (England) supported caddisfly (Trichoptera) assemblages indicative of a dynamic braided river with extensive floodplain development. At the first, Barrow‐upon‐Trent, the caddisfly assemblage was dominated by taxa indicative of a medium to large gravel‐bed river (Lepidostoma hirtum, Micrasema setiferum and taxa from the family Hydropsychidae). At the second site, Hemington, the caddisfly fauna was dominated by taxa from the families Limnephilidae and Phryganeidae. These usually occur in slow flowing and standing waters and are indicative of cut‐off channels on the floodplain. Micrasema setiferum does not occur in the contemporary UK fauna and has a modern distribution across central Europe and Fennoscandia. A gridded climatology extracted for those geographical areas where M. setiferum occurs today indicated a greater degree of continentality than currently prevailing in the Trent catchment. The value of Trichoptera in palaeoecological studies is explored and a model of channel evolution for the River Trent is presented based on caddisfly data. Copyright © 2003 John Wiley & Sons, Ltd.