Anthropogenic versus natural control on trace element and Sr-Nd-Pb isotope stratigraphy in peat sediments of southeast Florida (USA), ∼1500 AD to present

Analysis of a well-dated peat core from Blue Cypress Marsh (BCM) provides a detailed record of natural and anthropogenic factors that controlled the geochemical cycles of a number of trace elements in Florida over the last five centuries. The trace elements were divided into “natural” and “anthropogenic” groups using concentration trends from the bottom to the top of the core. The “natural” group includes Li, Sc, Cr, Co, Ga, Ge, Zr, Nb, Cs, Ba, Hf, Y, Ta, Th, and REE (Rare Earth Elements). These elements show similar concentrations throughout the core, indicating that changes in human activities after European arrival in the “New World” did not affect their geochemical cycles. The “anthropogenic” group includes Pb, Cu, Zn, V, Sb, Sn, Bi, and Cd. Upcore enrichment of these elements indicates enhancement by anthropogenic activities. From the early 1500s to present, fluxes of the “anthropogenic” metals to the marsh increased significantly, with modern accumulation rates several-fold (e.g., V) to hundreds of times (e.g., Zn) greater than pre-colonial rates. The dominant input mechanism for trace elements from both groups to the marsh has been atmospheric deposition. Atmospheric input of a number of the elements, including the anthropogenic metals, was dominated by local sources during the last century. For several elements, long-distant transport may be important. For instance, REE and Nd isotopes provide evidence for long-range atmospheric transport dominated by Saharan dust.The greatest increase in flux of the “anthropogenic” metals occurred during the 20th century and was caused by changes in the chemical composition of atmospheric deposition entering the marsh. Increased atmospheric inputs were a consequence of several anthropogenic activities, including fossil fuel combustion (coal and oil), agricultural activities, and quarrying and mining operations. Pb and V exhibit similar trends, with peak accumulation rates in 1970. The principal anthropogenic source of V is oil combustion. The decline in V accumulation after 1970 in the BCM peat corresponds to the introduction of low-sulfur fuels and the change from heavy to distilled oils since the 1970s. After the 1920s, Pb distribution in the peat follows closely the history of alkyl lead consumption in the US, which peaked in the 1970s. Pb isotopes support this inference and furthermore, record changes in the ore sources used to produce leaded gasoline. Idaho ores dominated the peat Pb isotope record until the 1960s, followed by Pb from Mississippi Valley Type deposits from the 1960s to the 1980s. Enhanced fluxes of Cu, Zn, Cd, Sn, Sb, Bi, and to some extent Ni during the last century are likely also related to fossil fuel combustion. Local agricultural activities may also have influenced the geochemical cycles of Cu and Zn. The peat record shows enhanced U accumulation during the last century, possibly related to phosphate mining in western Florida. Sr isotopes in the peat core also reflect anthropogenic influence. The ⁸⁷Sr/⁸⁶Sr ratio decreases from natural background values in the basal part of the core to lower values in the upper part of the core. The Sr isotope shift is probably related to quarrying operations in Florida, and marks the first time an anthropogenic signal has been detected using the Sr isotope record in a peat core.     

Radio sources in the 2dF Galaxy Redshift Survey – II. Local radio luminosity functions for AGN and star-forming galaxies at 1.4 GHz

We have cross-matched the 1.4-GHz NRAO VLA Sky Survey (NVSS) with the first 210 fields observed in the 2dF Galaxy Redshift Survey (2dFGRS), covering an effective area of 325 deg² (about 20 per cent of the final 2dFGRS area). This yields a set of optical spectra of 912 candidate NVSS counterparts, of which we identify 757 as genuine radio identifications – the largest and most homogeneous set of radio source spectra ever obtained. The 2dFGRS radio sources span the redshift range     to 0.438, and are a mixture of active galaxies (60 per cent) and star-forming galaxies (40 per cent). About 25 per cent of the 2dFGRS radio sources are spatially resolved by NVSS, and the sample includes three giant radio galaxies with projected linear size greater than 1 Mpc. The high quality of the 2dF spectra means we can usually distinguish unambiguously between AGN and star-forming galaxies. We make a new determination of the local radio luminosity function at 1.4 GHz for both active and star-forming galaxies, and derive a local star formation density of         .     

P-to-S conversion for a thin anisotropic zone produced by vertical fracturing

Point-source synthetic seismic responses for a thin, fractured bed are generated, interpreted and processed. The synthesis is carried out for a compressional source and multicomponent surface receivers. The anisotropy considered has hexagonal symmetry, with a horizontal symmetry axis, and represents oil- and gas-filled, aligned vertical fractures for a broad range of fracture densities and aspect ratios. P-to-S reflected conversions recorded on the horizontal geophones show both kinematic and dynamic anomalies that increase with increasing fracture density and are only weakly dependent on aspect ratio. In contrast, the vertical component P-wave reflections provide a much poorer diagnostic of fracturing. Analytic expressions for the eigenvalues and eigenvectors of a vertically fractured system are presented, that have the same simplicity as those for transverse isotropy. New linearized expressions for mode-converted amplitudes are developed for small angles of incidence and are used to interpret the synthetic response.     

Historical rates of sediment and nutrient accumulation in marshes of the Upper St. Johns River Basin, Florida, U.S.A.

We used 210Pb-dated sediment cores from wetlands and Blue Cypress Lake, in the Upper St. Johns River Basin (USJRB), Florida, USA, to measure historical accumulation rates of bulk sediment, total carbon (C), total nitrogen (N), and total phosphorus (P). Marsh cores displayed similar stratigraphies with respect to physical properties and nutrient content. Wetland sediments typically contained > 900 mg organic matter (OM) g^–1 dry mass, > 500 mg C g^–1, and 30–40 mg N g^–1. OM, C, and N concentrations were slightly lower in uppermost sediments of most cores, but displayed no strong stratigraphic trends. Total P concentrations were relatively low in bottommost deposits (0.01–0.11 mg g^–1), but ranged from 0.38–2.67 mg g^–1 in surface sediments. The mean sediment accretion rate in the marsh since ~ 1900, 0.33 ± 0.05 cm yr^–1, was calculated from ten ²¹⁰Pb-dated cores. All sites displayed increases in accumulation rates of bulk sediment, C, N, and P since the early part of the 20th century. These trends are attributed to recent hydrologic modifications in the basin combined with high nutrient loading from agricultural, residential, and urban sources.     

Remote sensing of sea surface temperature and chlorophyll during Lagrangian experiments at the Iberian margin

Satellite derived sea surface temperature (SST) and chlorophyll data are used to characterise the period of upwelling during a cruise on which two Lagrangian drift experiments were conducted off the Iberian Atlantic coast in August 1998. During the cruise there was a prolonged period of equator-ward winds which favour upwelling; three distinct maxima were observed in the meteorological data interspersed with periods of relaxation. The SST and chlorophyll imagery show upwelling to be active with distinct offshore filaments that are cooler and of higher chlorophyll concentration than the surrounding oceanic water; these filaments represent an important cross-shelf transport mechanism. A front detection methodology has been applied to satellite images and suggests that these filaments are distinct, long-lived features, characterised by enhanced primary production.     

Use of Cell-Specific PAM-Fluorometry to Characterize Host Shading in the Epiphytic Dinoflagellate Gambierdiscus toxicus

Abstract. Cell‐specific fluorescence characteristics were used to characterize the light tolerance of the toxic benthic dinoflagellate Gambierdiscus toxicus. The fluorescence parameter F_v : F_m was measured using pulse amplitude modulation (PAM) fluorometry on individual cells collected from foliose red algae growing in the sub‐tidal margin of South Water Cay, Belize. Samples were collected over several days during sunny and cloudy conditions and compared to samples incubated in situ. The data from individual cells were used to generate both F_v : F_m frequency histograms and averages. Maximum individual cell values of F_v : F_m reached 0.81 in pre‐dawn samples, a value near the theoretical maximum for PAM fluorometry. In field samples from macroalgal hosts, average F_v : F_m values declined only slightly during the day, but cells incubated in bottles under 47 % incident sunlight showed a significant mid‐day depression. In freshly collected samples, near‐maximum F_v : F_m values could be found in individual cells during the entire day; however, the frequency histograms indicated a greater range in F_v : F_m values during the afternoon than in the morning. In contrast, cultures of G. toxicus showed a tight distribution around a mean. Field samples showed a rapid recovery to near‐maximum F_v : F_m within 2 min when assayed using a standardized actinic light series. Similar results were obtained in laboratory cultures of G. toxicus grown at 73 µmol photons · m^‐2 · s^‐1, but not at 383 µmol photons · m^‐2 · s^‐1. These data provide empirical support for suggestions that G. toxicus exploits the three‐dimensional structure of the algal host thallus to minimize light exposure. This strategy permits G. toxicus, a high‐light intolerant species in culture, to thrive in shallow, well‐lit tropical seas. It may also partially explain the observed preference of G. toxicus for complex, foliose macroalgae as hosts.     

Stabilisation of the Stanton Lees Landslip Using an Embedded Pile Retaining Wall

A landslip occurred near the village of Stanton Lees, Derbyshire, UK, in November 2000 following a prolonged period of heavy rainfall. The slip resulted in the gradual, progressive down-slope movement of a steep embankment that was supporting an existing gabion retaining wall and a minor road which was subsequently closed to vehicular traffic. This paper describes the site; the sequence of events leading up to the start of the landslide; an interpretation of the ground conditions and onsite monitoring data; and the engineering works carried out in order to stabilise the slip and return the road to serviceability. The landslip occurred due to the superficial deposits moving over the weathered bedrock after the groundwater levels had been elevated following a prolonged period of heavy rainfall. A stability analysis indicated that the embankment was at a state of limiting equilibrium and its factor of safety on slope instability was found to be particularly sensitive to fairly minor changes in the groundwater levels. The remedial works replaced the existing gabion wall with a new cantilevered, bored pile retaining wall that comprised two rows of piles (600-mm in diameter), which were staggered in plan arrangement.