Potential implications of differential preservation of testate amoeba shells for paleoenvironmental reconstruction in peatlands

Testate amoebae are now commonly used in paleoenvironmental studies but little is known of their taphonomy. There is some experimental evidence for differential preservation of some testate amoeba shell types over others, but it is unclear what, if any impact this has on palaeoenvironmental reconstruction. To investigate this issue we looked at palaeoecological evidence for the preservation of different shell types. We then investigated the possible impact of selective preservation on quantitative palaeoenvironmental inference. We first used existing palaeoecological data sets to assess the vertical patterns of relative abundance in four testate amoeba shell types: (1) shells made of secreted biosilica plates (idiosomes, e.g. Euglypha), (2) idiosomes with thick organic coating (Assulina), (3) proteinaceous shells (e.g. Hyalosphenia), (4) shells built from recycled organic or mineral particles (xenosomes) (e.g. Difflugia, Centropyxis). In three diagrams a clear pattern of decay was only observed for the idiosome type. In order to assess the implications of differential preservation of testate amoeba taxa for paleoenvironmental reconstruction we then carried out simulations using three existing transfer functions and a wide range of scenarios, downweighting different test categories to represent the impact of selective test decomposition. Simulation results showed that downweighting generally reduced overall model performance. However downweighting a shell type only produced a consistent directional bias in inferred water table depth where that shell type is both dominant and shows a clear preference along the ecological gradient. Applying a scenario derived from previous experimental work did not lead to significant difference in inferred water table. Our results show that differential shell preservation has little impact on paleohydrological reconstruction from Sphagnum-dominated peatlands. By contrast, for the minerotrophic peatlands data-set loss of idiosome tests leads to consistent underestimation of water table depth. However there are few studies from fens and it is possible that idiosome tests are not always dominant, and/or that differential decomposition is less marked than in Sphagnum peatlands. Further work is clearly needed to assess the potential of testate amoebae for paleoecological studies of minerotrophic peatlands.     

Landscape-scale patterns of sediment sulfur accumulation in Swedish lakes

Sulfur has played a central role in the acidification of many lakes in Scandinavia and other regions. As part of the research into sulfur cycling, numerous studies have analyzed the sediment record in order to develop insights into past in-lake cycling of sulfur, particularly in the context of reconstructing past deposition rates. Although many of these studies have shown that it is not easy to interpret the sediment record in terms of past sulfur deposition rates, analyses of sulfur in sediment still provide valuable information on the response of lakes to anthropogenic sulfur deposition. Here, we have analyzed sulfur in top and bottom samples from short surface cores (25–35 cm, representing ≥250 years) as well as bulk cores from ∼110 lakes located throughout Sweden, which were collected during 1986, as well as in more-detailed profiles from six lakes. The lakes with the highest surface sediment concentrations (9–24 mg S g⁻¹ dry mass) and the highest calculated inventories of ‘excess’ sulfur (20–180 g S m⁻²) are found in southern Sweden and around one industrial area along the northeastern coast where sulfur deposition rates and lake-water concentrations have been highest. For many lakes in the central and northern inland region it is common that the sediment cores exhibit either no enrichment or even a decline in sulfur concentrations in near-surface sediments, which we suggest was the pre-pollution norm for lakes. Although interpreting sulfur sediment profiles is problematic for reconstructing deposition, a more-comprehensive spatial sampling approach shows that there is a good geographic agreement between sulfur deposition, lake-water chemistry and sediment sulfur accumulation.     

Instantaneous deformation from continuous GPS: contributions from quasi-periodic loads

Continuous GPS (CGPS) coordinate time-series are known to experience repeating deformation signals with seasonal and other periods. It is unlikely that these signals represent perfect sinusoids with temporally constant amplitude. We develop an analysis method that accommodates temporal variations in the amplitudes of sinusoidal signals. We apply the method to simulated coordinate time-series to numerically explore the potential consequences of neglecting decadal variation in amplitude of annual motions on the residual-error spectra of CGPS measurements, as well as potential bias in estimates for secular site velocity. We find that secular velocity bias can be appreciable for shorter time-series, and that residual-error time-series of longer duration may contain significant power in a broad band centred on semi-annual frequency if temporal variation in the amplitude of annual motions is not accounted for in the model used to reduce the observations to residuals. It may be difficult to differentiate the bandpass filtered signature of mismodelled loading signals from power-law noise, using residual-error spectra for shorter time-series. We provide an example application to a ∼9-yr coordinate time-series for a CGPS station located in southern California at Carbon Creek Control Structure (CCCS), which is known to experience large amplitude seasonal motions associated with the Santa Ana aquifer system.     

High‐frequency water quality monitoring in an urban catchment: hydrochemical dynamics,primary production and implications for the Water Framework Directive

This paper describes the hydrochemistry of a lowland, urbanised river‐system, The Cut in England, using in situ sub‐daily sampling. The Cut receives effluent discharges from four major sewage treatment works serving around 190 000 people. These discharges consist largely of treated water, originally abstracted from the River Thames and returned via the water supply network, substantially increasing the natural flow. The hourly water quality data were supplemented by weekly manual sampling with laboratory analysis to check the hourly data and measure further determinands. Mean phosphorus and nitrate concentrations were very high, breaching standards set by EU legislation. Although 56% of the catchment area is agricultural, the hydrochemical dynamics were significantly impacted by effluent discharges which accounted for approximately 50% of the annual P catchment input loads and, on average, 59% of river flow at the monitoring point. Diurnal dissolved oxygen data demonstrated high in‐stream productivity. From a comparison of high frequency and conventional monitoring data, it is inferred that much of the primary production was dominated by benthic algae, largely diatoms. Despite the high productivity and nutrient concentrations, the river water did not become anoxic, and major phytoplankton blooms were not observed. The strong diurnal and annual variation observed showed that assessments of water quality made under the Water Framework Directive (WFD) are sensitive to the time and season of sampling. It is recommended that specific sampling time windows be specified for each determinand, and that WFD targets should be applied in combination to help identify periods of greatest ecological risk. © 2015 The Authors. Hydrological Processes published by John Wiley & Sons Ltd.     

High temperature gas–solid reactions in calc–silicate Cu–Au skarn formation; Ertsberg,Papua Province,Indonesia

The 2.7–3 Ma Ertsberg East Skarn System (Indonesia), adjacent to the giant Grasberg Porphyry Copper deposit, is part of the world’s largest system of Cu–Au skarn deposits. Published fluid inclusion and stable isotope data show that it formed through the flux of magma-derived fluid through contact metamorphosed carbonate rock sequences at temperatures well above 600° C and pressures of less than 50 MPa. Under these conditions, the fluid has very low density and the properties of a gas. Combining a range of micro-analytical techniques, high-resolution QEMSCAN mineral mapping and computer-assisted X-ray micro-tomography, an array of coupled gas–solid reactions may be identified that controlled reactive mass transfer through the ~ 1 km³ hydrothermal skarn system. Vacancy-driven mineral chemisorption reactions are identified as a new type of reactive transport process for high-temperature skarn alteration. These gas–solid reactions are maintained by the interaction of unsatisfied bonds on mineral surfaces and dipolar gas-phase reactants such as SO₂ and HCl that are continuously supplied through open fractures and intergranular diffusion. Principal reactions are (a) incongruent dissolution of almandine-grossular to andradite and anorthite (an alteration mineral not previously recognized at Ertsberg), and (b) sulfation of anorthite to anhydrite. These sulfation reactions also generate reduced sulfur with consequent co-deposition of metal sulfides. Diopside undergoes similar reactions with deposition of Fe-enriched pyroxene in crypto-veins and vein selvedges. The loss of calcium from contact metamorphic garnet to form vein anhydrite necessarily results in Fe-enrichment of wallrock, and does not require Fe-addition from a vein fluid as is commonly assumed.     

Petrological and experimental evidence for differentiation of water-rich magmas beneath St. Kitts,Lesser Antilles

St. Kitts lies in the northern Lesser Antilles, a subduction-related intraoceanic volcanic arc known for its magmatic diversity and unusually abundant cognate xenoliths. We combine the geochemistry of xenoliths, melt inclusions and lavas with high pressure–temperature experiments to explore magma differentiation processes beneath St. Kitts. Lavas range from basalt to rhyolite, with predominant andesites and basaltic andesites. Xenoliths, dominated by calcic plagioclase and amphibole, typically in reaction relationship with pyroxenes and olivine, can be divided into plutonic and cumulate varieties based on mineral textures and compositions. Cumulate varieties, formed primarily by the accumulation of liquidus phases, comprise ensembles that represent instantaneous solid compositions from one or more magma batches; plutonic varieties have mineralogy and textures consistent with protracted solidification of magmatic mush. Mineral chemistry in lavas and xenoliths is subtly different. For example, plagioclase with unusually high anorthite content (An_≤100) occurs in some plutonic xenoliths, whereas the most calcic plagioclase in cumulate xenoliths and lavas are An₉₇ and An₉₅, respectively. Fluid-saturated, equilibrium crystallisation experiments were performed on a St. Kitts basaltic andesite, with three different fluid compositions (XH₂O = 1.0, 0.66 and 0.33) at 2.4 kbar, 950–1025 °C, and fO₂ = NNO ? 0.6 to NNO + 1.2 log units. Experiments reproduce lava liquid lines of descent and many xenolith assemblages, but fail to match xenolith and lava phenocryst mineral compositions, notably the very An-rich plagioclase. The strong positive correlation between experimentally determined plagioclase-melt Kd_Ca–Na and dissolved H₂O in the melt, together with the occurrence of Al-rich mafic lavas, suggests that parental magmas were water-rich (> 9 wt% H₂O) basaltic andesites that crystallised over a wide pressure range (1.5–6 kbar). Comparison of experimental and natural (lava, xenolith) mafic mineral composition reveals that whereas olivine in lavas is predominantly primocrysts precipitated at low-pressure, pyroxenes and spinel are predominantly xenocrysts formed by disaggregation of plutonic mushes. Overall, St. Kitts xenoliths and lavas testify to mid-crustal differentiation of low-MgO basalt and basaltic andesite magmas within a trans-crustal, magmatic mush system. Lower crustal ultramafic cumulates that relate parental low-MgO basalts to primary, mantle -derived melts are absent on St. Kitts.