Problems of core correlation,sediment source ascription and yield estimation in Ponsonby Tarn,West Cumbria,UK

A suite of 27 short cores, 10 of which have been used for magnetic measurements and four for radiometric dating, provides a framework for reconstructing the processes, patterns and rates of sedimentation in Ponsonby Tarn, a small artificial impoundment created towards the end of the 19th century, close to the Sellafield nuclear reprocessing plant in NW England. Spatial and temporal changes in sedimentation are reconstructed and evidence presented for non-synchroneity in magnetic property changes from core to core in the upper part of the sequence, as a result of sorting and selective deposition at different distances from the inflow to the Tarn. Magnetic measurements alone are therefore not a secure basis upon which to quantify sediment yield for defined time intervals at this site. The chronology, established mainly from ²¹⁰Pb and ¹³⁴Cs analyses, allows estimates of mean sediment yield per annum for four periods: prior to AD 1940, 1940–1964, 1964–1986 and 1986–1991. The rates of sediment accumulation have increased in recent times, especially since 1964, with evidence for input from both magnetically enhanced soils and gleyed alluvial and/or podsolized subsoil sources. Pre-1940 mean annual deposition within the present area of the lake is calculated as 19·5 t a⁻¹ and for the period since 1986 (the period of maximum sedimentation rates), as 111·3 t a⁻¹. These represent yields of 7·0 t km⁻² a⁻¹ and 39·8 t km⁻² a⁻¹, respectively, for the catchment as a whole. Rock magnetic evidence, based on measurements of both bulk samples and the finest particle size separates, suggests that bacterial magnetite, formed within the lake, contributes to the magnetic properties of the sediments, thus modifying the signatures relating to allochthonous sediment input. Copyright © 1999 John Wiley & Sons, Ltd.     

Accretion of a New England (U.S.A.) Salt Marsh in Response to Inlet Migration, Storms, and Sea-level Rise

Sediment accumulation rates were determined at several sites throughout Nauset Marsh (Massachusetts, U.S.A.), a back-barrier lagoonal system, using feldspar marker horizons to evaluate short-term rates (1 to 2 year scales) and radiometric techniques to estimate rates over longer time scales (¹³⁷Cs,²¹⁰Pb,¹⁴C). The barrier spit fronting theSpartina-dominated study site has a complex geomorphic history of inlet migration and overwash events. This study evaluates sediment accumulation rates in relation to inlet migration, storm events and sea-level rise. The marker horizon technique displayed strong temporal and spatial variability in response to storm events and proximity to the inlet. Sediment accumulation rates of up to 24 mm year⁻¹were recorded in the immediate vicinity of the inlet during a period that included several major coastal storms, while feldspar sites remote from the inlet had substantially lower rates (trace accumulation to 2·2 mm year⁻¹). During storm-free periods, accumulation rates did not exceed 6·7 mm year⁻¹, but remained quite variable among sites. Based on¹³⁷Cs (3·8 to 4·5 mm year⁻¹) and²¹⁰Pb (2·6 to 4·2 mm year⁻¹) radiometric techniques, integrating sediment accumulation over decadal time scales, the marsh appeared to be keeping pace with the relative rate of sea-level rise from 1921 to 1993 of 2·4 mm year⁻¹. At one site, the²¹⁰Pb-based sedimentation rate and rate of relative sea-level rise were nearly similar and peat rhizome analysis revealed thatDistichlis spicatarecently replaced this onceS. patenssite, suggesting that this portion of Nauset Marsh may be getting wetter, thus representing an initial response to wetland submergence. Horizon markers are useful in evaluating the role of short-term events, such as storms or inlet migration, influencing marsh sedimentation processes. However, sampling methods that integrate marsh sedimentation over decadal time scales are preferable when evaluating a systems response to sea-level rise.     

Seasonal variability in velocity and ablation of Te Moeka o Tuawe/Fox Glacier, south Westland, New Zealand

Abstract:  Seasonal variations in ablation and surface velocity were investigated on the lower Fox Glacier. Variations occur between summer and winter ablation, with surface velocity also showing marked seasonality. Recent advance has resulted in the glacier gaining around 200 m length since late 2004. Longer term, Fox Glacier appears linked to the Southern Oscillation Index, with positive glacier mass balances associated with negative Southern Oscillation Index (El Niño). An estimated glacier response time of approximately 9–14 years suggests the current terminus advance was linked to mass gains in the mid-1990s. Recent collapses at the terminal face continue to prove a hazard at this busy tourist destination.     

The transport and mass balance of fallout radionuclides in Blelham Tarn, Cumbria (UK)

Although lake sediment archives are widely used for reconstructing historical records of atmospherically delivered pollutants, the quantitative relationship between fallout levels and their record in the sediments is complex and not well known. The original input signal from the atmosphere can be significantly distorted by mediating transport processes in the catchment, through the water column and within the sediments themselves. Since these processes also affect the fallout radionuclide ²¹⁰Pb commonly used to date sediments, a better understanding of their impact is also important to improving the accuracy and reliability of sediment dating. Blelham Tarn has been the subject of a number of palaeolimnological investigations using radiometric dating techniques since the early 1970s. More recently it was the site of a major study carried out within the EU Transuranics project concerning the long-term fate of fallout radionuclides in catchment/lake systems. This paper reviews the radiometric data from this study and uses the results to determine mass balances for fallout ²¹⁰Pb, ¹³⁷Cs and ^239+240Pu in Blelham Tarn, and their spatial distribution over the bed of the lake. Atmospheric fluxes were determined by measuring concentrations in rainwater and cumulative inventories in soil cores from non-eroding sites. Sediment records in a grid of 16 cores were used to determine the spatial distribution over the bed of the lake, and net inputs from the catchment. Mass balance calculations indicate that c. 47% of ²¹⁰Pb in the sediments derives from erosive inputs from the catchment. For ^239+240Pu the figure rises to 61%. Reduced amounts of ¹³⁷Cs in the sediments are attributed to greater losses of this radionuclide from the water column via the outflow due to its greater solubility. Inputs of radionuclides from the catchment are concentrated near one of the major input streams. Away from this part of the lake the sediment record is dominated by direct atmospheric fallout, though the detailed pattern is influenced by sediment focussing. A one parameter catchment/lake transport model is developed that incorporates the assumption that transport rates will decline with time as fallout on the catchment diffuses into the soil and becomes less available for removal. Values of the transport parameter were calculated for ²¹⁰Pb and ^239+240Pu and found to be comparable. The results suggest that it will take c. 11000 years to remove 50% of ^239+240Pu from the catchment to the lake.     

Lake-Sediment Evidence for Local and Remote Sources of Atmospherically Deposited Pollutants on Svalbard

Twenty-one lakes along the west coast of Svalbard were cored between 28 July and 14 August 1995. Five cores were ²¹⁰Pb dated and analysed for spheroidal carbonaceous particles (SCPs), indicators of deposition from fossil-fuel combustion sources, and trace metals. Surface and pre-industrial levels of five cores were analysed for 10 polychlorinated biphenyl (PCB) isomers and 16 polycyclic aromatic hydrocarbon (PAH) compounds, whilst all 21 surface sediments were analysed for SCPs. Temporal and spatial trends in the atmospheric deposition of pollutants were identified. Whilst temporal patterns of deposited pollutants may have been truncated due to concentrations falling below the limits of detection, spatial patterns showed elevated levels of SCP, PAH, PCB, and possibly Pb within 60–70 km of Isfjord. Tenndammen (U), the closest site to Longyearbyen, showed the highest levels of contamination for all pollutants. Differences in the deposition patterns of PAH and PCB are thought to be due to local sources of PAH from coal combustion whilst PCB sources are remote. It is concluded that the impact from atmospheric deposition on Svalbard is due to a combination of spatially limited local sources superimposed on a broad-scale, long-range pattern.     

Regional Applications of Lake Sediment Dating by Spheroidal Carbonaceous Particle Analysis I: United Kingdom

Spheroidal carbonaceous fly-ash particles (SCPs) are produced by the high temperature combustion of fossil-fuels such as coal and oil. They are not produced by any natural processes and therefore are unambiguous indicators of atmospheric deposition from power generation and other industrial sources. In lake sediments, SCP concentration profiles are robust, replicable and often regionally characteristic such that the main profile features can be used for sediment dating. Previous work in the UK provided dates for the three main SCP profile features for Scotland, North Wales and Northern Ireland and also highlighted the lack of information for the rest of the UK. In this study, archived SCP data and new SCP concentration profiles were compiled from 80 radionuclide-dated sediment cores from across the whole of the UK in order to determine the regional coherence of temporal trends. Using SCP cumulative percentage profiles, with the SCP concentration peak set as 100%, dates for each 10% were produced for each core. Comparing these dates, eight distinct regions were found within the UK and dated cumulative profiles, with errors, were determined for each. Despite this regionality, the whole of the UK showed the same date for the start of the record (1850±25 years) and for the 40- and 50% (1940±15 and 1955±15, respectively). The national coherence of the former is thought to be due to the large confidence limits in the radionuclide dates for this period, whilst the latter is thought to be due to the impact from the major increase in electricity demand following the Second World War and the widespread introduction and use of cheap fuel-oil. Since the 1960s the trend has been one of increasing regionalisation resulting from the introduction of particle arrestor technology, the decline of heavy industry, the move to fewer and larger sources, particularly power stations, and an increase in accuracy for the dates of recent horizons. Regional trends are seen to compare well with industrial development in the UK and in some western areas influenced by emissions from Ireland. Areas of uncertainty remain in South Wales and the northern isles. A cumulative percentile was also calculated for each region for 1963 in order that this can be compared with the ²⁴¹Am and ¹³⁷Cs peaks.     

The effect of ammonia ice on the outgoing thermal radiance from the atmosphere of Jupiter

We examine the effects of NH₃ ice particle clouds in the atmosphere of Jupiter on outgoing thermal radiances. The cloud models are characterized by a number density at the cloud base, by the ratio of the scale height of the vertical distribution of particles (H_p) to the gas scale height (H_g), and by an effective particle radius. NH₃ ice particle-scattering properties are scaled from laboratory measurements. The number density for the various particle radius and scale height models is inferred from the observed disk average radiance at 246 cm^?1, and preliminary lower limits on particle sizes are inferred from the lack of apparent NH₃ absorption features in the observed spectral radiances as well as the observed minimum flux near 2100 cm^?1. We find lower limits on the particle size of 3 μm if H_p/H_g = 0.15, or 10μmif H_p/H_g = 0.50 or 0.05. NH₃ ice particles are relatively dark near the far-infrared and 8.5-μm atmospheric windows, and the outgoing thermal radiances are not very sensitive to various assumptions about the particle-scattering function as opposed to radiances at 5 μm, where particles are relatively brighter. We examined observations in these three different spectral window regions which provide, in principle, complementary constraints on cloud parameters. Characterization of the cloud scale height is difficult, but a promising approach is the examination of radiances and their center-to-limb variation in spectral regions where there is significant opacity provided by gases of known vertical distribution. A blackbody cloud top model can reduce systematic errors due to clouds in temperature sounding to the level of 1K or less. The NH₃ clouds provide a substantial influence on the internal infrared flux field near the 600-mbar level.     

Absorption line studies of reflection from horizontally inhomogeneous layers

A discussion of literature relevant to horizontal inhomogeneities in planetary atmospheres shows this to be an increasingly important yet largely unexplored topic. Section II details an inhomogeneous reflecting layer (IRFL) model designed to survey absorption line behavior from a Squires-like cloud cover (which is characterized by convection cell structure). Computational problems and procedures are discussed in detail, with results presented for center to limb equivalent width variations at phase angles α = 0° and 90°, followed by equivalent width variations as a function of phase angle for specific points across the planetary equator. We examine a range of cloud and gas configurations, of line and continuum opacities, and we compare phase variations of bright versus dark limbs.The results in general show trends quite dissimilar to (usually opposite) those predicted by a simple reflecting layer model. Percent equivalent width variations for the tower model are usually somewhat greater for weak than for relatively strong absorption lines, with differences of a factor of about two or three. Also, IRFL equivalent width variations do not differ drastically as a function of geometry when the total volume of absorbing gas is held constant. The IRFL results are in many instances consistent with observed equivalent width variations of Jupiter, Saturn, and Venus. Thus, consideration of horizontal inhomogeneities evidently worsens current uniqueness problems. Future more detailed observations will ameliorate this impass if, it is argued, interpretive studies encompass the complexities of realistic horizontal cloud structure.     

Glonass Laser Ranging Accuracy With Satellite Signature Effect

GLONASS satellites have been tracked by the worldwide laser ranging networkas well as by the GLONASS-borne microwave-based technique. Owing to thelarge size of their corner cube reflector arrays, the amount of ranging data is enough to determine their orbits from laser ranging data alone. We found, however, that the large size of the array affected the accuracy of measurement to an extent that is dependent on the characteristics of the ranging systems. An azimuthal variation of the reflector array response was also detected in observations from single-photon laser ranging. Orbital analysis reveals that the effect makes the measured range on average 22 mm shorter than expected in the absence of the large array, which explains more than half of the offset of 39 mm previously discovered between microwave and laser orbits.     

Sediment heavy metal record in Lake Baikal: natural and antrhopogenic sources

Geochemical analysis of six radiometrically dated short cores of recent sediment from Lake Baikal shows clear evidence of enhanced Pb supply. However, the sediment concentration increases are very small; the average Pb concentration rises from a baseline value of 10.9 g g^-1 to a peak value of only 14.8 g g^-1. In contrast to the more polluted lakes commonly studied in Europe and North America, variation in Pb concentration is far more strongly influenced by natural variation than by pollution. In sediment deposited over the last 150–200 years 73% of the variance in the sediment Pb concentration can be accounted for by variation in bulk composition of the sediment, and by atmospheric pollution. Factors influencing Pb concentrations over this time period are, in order of decreasing average importance (fraction of total variance explained), catchment supply (indicated by ²²⁶Ra activity variation) (43%), anthropogenic Pb emissions (24%), and dilution by ferromanganese hydroxides (6%). On longer (1000s of years) time scales dilution by biogenic silica is probably more important.The recent enhanced supply of catchment Pb correlates with accelerating accumulation rates, indicating a link with enhanced erosion. Anthropogenic sources dominate only in the southern basin, where local fossil-fuel burning industry is situated. The evidence for a local industrial source for the Pb pollution is strengthened by the high correlation between the inventories for Pb and for spheroidal carbonaceous particles. The absence of detectable anthropogenic Pb enrichment in the northern part of the lake suggests that long-distance Pb pollution is small compared with the local natural supply.