High‐resolution diatom δ18O records,from the last 150 years,reflecting changes in amount of winter precipitation in two sub‐Arctic high‐altitude lakes in the Swedish Scandes

Waters from high‐altitude alpine lakes are mainly recharged by meteoric water. Because of seasonal variations in precipitation and temperature and relatively short hydraulic residence times, most high‐altitude lakes have lake water isotopic compositions (δ¹⁸O_lake) that fluctuate due to seasonality in water balance processes. Input from snowmelt, in particular, has a significant role in determining lake water δ¹⁸O. Here we compare two high‐resolution δ¹⁸O_diatom records from lake sediments in the Swedish Scandes with instrumental data from the last century obtained from nearby meteorological stations. The time period AD 1900–1990 is characterised by an increase in winter precipitation and high winter/summer precipitation ratios and this is recorded in δ¹⁸O_diatom as decreasing trends. Lowest δ¹⁸O_diatom values and highest amount of winter precipitation are found around AD 1990 when the winter North Atlantic Oscillation index was above +2. We conclude that for the last 150 a the main factor affecting the δ¹⁸O_diatom signal in these sub‐Arctic high‐altitude lakes with short residence times has been changes in amount of winter precipitation and that δ¹⁸O_diatom derived from high‐altitude lakes in the Swedish Scandes can be used as a winter precipitation proxy. Copyright © 2010 John Wiley & Sons, Ltd.     

Subsurface structure of Planum Boreum from Mars Reconnaissance Orbiter Shallow Radar soundings

We map the subsurface structure of Planum Boreum using sounding data from the Shallow Radar (SHARAD) instrument onboard the Mars Reconnaissance Orbiter. Radar coverage throughout the 1,000,000-km² area reveals widespread reflections from basal and internal interfaces of the north polar layered deposits (NPLD). A dome-shaped zone of diffuse reflectivity up to 12 μs (∼1-km thick) underlies two-thirds of the NPLD, predominantly in the main lobe but also extending into the Gemina Lingula lobe across Chasma Boreale. We equate this zone with a basal unit identified in image data as Amazonian sand-rich layered deposits [Byrne, S., Murray, B.C., 2002. J. Geophys. Res. 107, 5044, 12 pp. doi:10.1029/2001JE001615; Fishbaugh, K.E., Head, J.W., 2005. Icarus 174, 444-474; Tanaka, K.L., Rodriguez, J.A.P., Skinner, J.A., Bourke, M.C., Fortezzo, C.M., Herkenhoff, K.E., Kolb, E.J., Okubo, C.H., 2008. Icarus 196, 318-358]. Elsewhere, the NPLD base is remarkably flat-lying and co-planar with the exposed surface of the surrounding Vastitas Borealis materials. Within the NPLD, we delineate and map four units based on the radar-layer packets of Phillips et al. [Phillips, R.J., and 26 colleagues, 2008. Science 320, 1182-1185] that extend throughout the deposits and a fifth unit confined to eastern Gemina Lingula. We estimate the volume of each internal unit and of the entire NPLD stack (821,000 km³), exclusive of the basal unit. Correlation of these units to models of insolation cycles and polar deposition [Laskar, J., Levrard, B., Mustard, J.F., 2002. Nature 419, 375-377; Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772] is consistent with the 4.2-Ma age of the oldest preserved NPLD obtained by Levrard et al. [Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772]. We suggest a dominant layering mechanism of dust-content variation during accumulation rather than one of lag production during periods of sublimation.     

The Effects of Varying Salinity on Ammonium Exchange in Estuarine Sediments of the Parker River,Massachusetts

We examined the effects of seasonal salinity changes on sediment ammonium (NH₄ ⁺) adsorption and exchange across the sediment–water interface in the Parker River Estuary, by means of seasonal field sampling, laboratory adsorption experiments, and modeling. The fraction of dissolved NH₄ ⁺ relative to adsorbed NH₄ ⁺ in oligohaline sediments rose significantly with increased pore water salinity over the season. Laboratory experiments demonstrated that small (∼3) increases in salinity from freshwater conditions had the greatest effect on NH₄ ⁺ adsorption by reducing the exchangeable pool from 69% to 14% of the total NH₄ ⁺ in the upper estuary sediments that experience large (0–20) seasonal salinity shifts. NH₄ ⁺ dynamics did not appear to be significantly affected by salinity in sediments of the lower estuary where salinities under 10 were not measured. We further assessed the importance of salinity-mediated desorption by constructing a simple mechanistic numerical model for pore water chloride and NH₄ ⁺ diffusion for sediments of the upper estuary. The model predicted pore water salinity and NH₄ ⁺ profiles that fit measured profiles very well and described a seasonal pattern of NH₄ ⁺ flux from the sediment that was significantly affected by salinity. The model demonstrated that changes in salinity on several timescales (tidally, seasonally, and annually) can significantly alter the magnitude and timing of NH₄ ⁺ release from the sediments. Salinity-mediated desorption and fluxes of NH₄ ⁺ from sediments in the upper estuary can be of similar magnitude to rates of organic nitrogen mineralization and may therefore be important in supporting estuarine productivity when watershed inputs of N are low.     

Chemistry of the organic-rich hot core G327.3-0.6

We present gas-phase abundances of species found in the organic-rich hot core G327.3-0.6. The data were taken with the Swedish-ESO Submillimetre Telescope (SEST). The 1-3 mm spectrum of this source is dominated by emission features of nitrile species and saturated organics, with abundances greater than those found in many other hot cores, including Sgr B2 and OMC-1. Population diagram analysis indicates that many species (CH3CN, C2H3CN, C2H5CN, CH3OH, etc.) have hot components that originate in a compact (~2") region. Gas-phase chemical models cannot reproduce the high abundances of these molecules found in hot cores, and we suggest that they originate from processing and evaporation of icy grain mantle material. In addition, we report the first detection of vibrationally excited ethyl cyanide and the first detection of methyl mercaptan (CH3SH) outside the Galactic center.     

Geophysical monitoring and evaluation of coastal plain aquifers

We use time domain electromagnetic (TDEM) soundings to monitor ground water conditions beneath the coastal plain in eastern North Carolina. The TDEM method measures the earth's response to an induced electromagnetic field. The resulting signal is converted, through a complex inversion process, to apparent resistivity values, which can be directly correlated to borehole resistivity logs. TDEM soundings are used to map the interface between fresh and salt water within coastal aquifers, and estimate depth to basement when siting new monitoring wells. Focused TDEM surveys have identified areas of salt water encroachment caused by high volumes of discharge from local supply wells. Electromagnetic sounding, when used in tandem with the state's network of monitoring wells, is an accurate and inexpensive tool for evaluating fresh water/salt water relationships on both local and regional scales within coastal plain aquifers.     

Influence of algal farming on fish assemblages

We examined the influence of algal farming on fish assemblages in two shallow coastal lagoons in Zanzibar, Tanzania. Fish assemblages were visually investigated using a belt transect method and the line-intercept technique was used to examine the substrate composition. 101 species of fish belonging to 31 families were recorded. Algal farming affected the associated fish fauna in terms of abundance, species richness, trophic identity and fish community composition. However, the impact differed between the lagoons. Algal farms in one lagoon hosted a more abundant and diversified fish fauna than controls, whereas farms in the other lagoon exhibited lower fish densities and similar species diversity compared to controls. The discrepancies between lagoons may be an effect of differences in farming intensity and character of the substratum.     

Global thermal inertia and surface properties of Mars from the MGS mapping mission

We present a new high-resolution map of thermal inertia derived from observations of planetary brightness temperature by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) obtained during the entire MGS primary mapping mission. Complete seasonal coverage provides a nearly global view of Mars, including the polar regions, at a spatial resolution of approximately 3 km. Our map of nighttime thermal-bolometer-based thermal inertia covers approximately 60% of the surface between 80° S and 80° N latitudes. We confirm the global pattern of high and low thermal inertia seen in lower resolution mapping efforts and provide greater detail concerning a third surface unit with intermediate values of both thermal inertia and albedo first identified by Mellon et al. 2000, Icarus 148, 437-455. Several smaller regional units with distinct characteristics are observed. Most notably, a unit of low thermal inertia () and low-to-intermediate albedo (0.09-0.22) dominates the region polewards of 65° S. We consider possible causes for these characteristics and conclude that a low-density mantle formed by desiccation of a previously ice-rich near-surface layer is the most likely explanation for the observed thermophysical properties. Global comparison of thermal inertia and elevation shows that high and low thermal inertia values can be found over a broad range of elevation, with only low values (30-) occurring at the highest elevations and the highest values occurring only at lower elevations. However, the lowest values () are found only at lower elevations, implying that the distribution of low thermal inertia material is not solely controlled by atmospheric pressure and the trapping of fines at high elevations. A new estimate of thermal inertia for the Viking and Pathfinder landing sites helps establish an important link between surface characteristics observed in situ and those derived from remote-sensing data.     

High-resolution reflection seismic imaging of the upper crust at Laxemar, southeastern Sweden

A major cost in exploring the upper 1–2 km of crystalline crust with reflection seismics is the drilling required for explosive sources. By reducing the charge size to a minimum, shallow inexpensive shotholes can be drilled with handheld equipment. Here, we present results from a full-scale test using small charges for high-resolution seismic surveying over a nuclear waste disposal study site (not an actual site). Two 2–2.5-km-long crossing profiles were acquired in December 1999 with 10-m shot and geophone spacing in the Laxemar area, near Oskarshamn in southeastern Sweden. After standard processing, including dip moveout (DMO), several subhorizontal to moderately dipping reflections are imaged. Many of the dipping ones can be correlated to fracture zones observed in a ca. 1700-m-deep borehole where the profiles cross and/or to fracture zones mapped on the surface. The imaged fracture zones form a complex 3D pattern illustrating the necessity of having 3D control before interpreting seismic reflection data. Analyses of sonic and density logs from the borehole show that greenstones have significantly higher impedances than the more dominant granite found in the borehole (granite/greenstone reflection coefficient is +0.065). These greenstones may contribute to the reflectivity when associated with fracture zones. In some cases, where they are present as larger subhorizontal lenses, they may be the dominant source of reflectivity. A set of north-dipping (10°) reflectors at 3–3.5-km depth can be correlated to a similar set observed below the island of Ävrö about 3 km to the east.