Arsenic geochemistry in Chesapeake Bay: Dependence upon anthropogenic inputs and phytoplankton species composition

Arsenic is not conservative in Chesapeake Bay. Inputs of man-derived arsenic, of the order of 100 kg d⁻¹, cause substantial positive deviation from theoretical dilution. The chemical form of the arsenic varies both seasonally and along the axis of the bay. During winter, arsenic is present only as arsenate. During summer, substantial quantities of reduced and methylated forms are present in different areas, indicative of separate formation processes. Arsenite, present in low-salinity regions, may have been formed by chemical reduction in anoxic, subsurface waters and then mixed into the surface layer. Methylated arsenicals correlate highly with algal standing stocks. One particular form, methylarsonate, is highly correlated with the dominant algal genus, Chroomonas. As both arsenic reactivity and toxicity are altered by transformation of chemical form, the observed variations in arsenic speciation have considerable geochemical and ecological significance.     

Recent increase in atmospheric deposition of mercury to California aquatic systems inferred from a 300-year geochronological assessment of lake sediments

Age-dated sediment cores from 4 remote lakes across California were analyzed for total Hg (Hg_T) concentration as a function of pre- and post-industrialization. Particle size, magnetic susceptibility and organic C and N, were measured to determine if the Hg concentration in sediment cores could be related to atmospheric deposition and/or watershed processes. Results indicate that (a) for each lake modern (1970–2004) Hg_T lake sediment concentrations have increased by an average factor of 5 times more than historic (pre-1850) Hg_T concentrations; (b) the ratio of modern to pre-industrial lake sediment Hg_T for these lakes are higher than estimated for other locations where atmospheric deposition is presumed to be the main source of Hg; (c) 2 of the 4 studied lakes demonstrated significant relationships between Hg_T concentrations and percentage organic material (r² = 0.68 and p < 0.01; r² = 0.67 and p < 0.01) whereas the other two indicated no significant relationship (r² = 0.05 and p = 0.51; r² = 0.12 and p = 0.36).     

Modeling IR spectral energy distributions: a?pilot study of?starburst parameters and silicate absorption curves for some GOALS galaxies

This paper describes a pilot study into the spectral energy distribution (SED) fitting and the derivation of physical parameters for 19 galaxies observed as part of the Great Observatories All-sky LIRG Survey (GOALS) survey as observed with the Spitzer Space Telescope. For this we have used the pan-spectral fitting tools developed in a series of papers by Dopita and his co-workers. We show that the standard Lee and Draine ??astronomical silicate?? model cannot provide a good fit to the silicate absorption features as observed in the heavily dust-extinguished (A _V??50 mag.) starbursts. We have derived an empirical fit to the ??starburst silicate?? absorption in these objects. This absorption curve is consistent with the silicate grains being systematically larger in starburst environments than in the local Galactic interstellar medium. We demonstrate the sensitivity of the SED fitting to each of the fitted parameters, and derive these parameters for those galaxies which do not have an embedded AGN. This technique is simple and provides reasonably robust and uniform parameters for the starburst, especially as far as the star formation rate, population of old stars, compactness of the starburst region and total foreground extinction are concerned. However, the chemical abundances and the optical extinction cannot be reliably determined by this analysis, and optical SEDs will also be required to provide a complete characterization of the starburst region and of the surrounding galaxy.     

Hydrologic fragmentation-induced eutrophication in Dove Sound,Upper Florida Keys,USA

Anthropogenic impacts to island systems can have deleterious effects on coastal aquatic ecosystems. These effects can alter water quality, primary production as well as habitat. Land development often fragments hydrologic connectivity within aquatic ecosystems forcing alterations in nutrient transport and increases the potential for eutrophication. Dove Sound, a tidal lagoon located in the Upper Florida Keys on Key Largo, has been subjected to anthropogenic influences of land development during the last century. To investigate these influences a short sediment core was collected from within Dove Sound and investigated using ²¹⁰Pb dating, stable isotopes of carbon and nitrogen, and sedimentary pigments. Results indicated that Dove Sound has undergone eutrophication and the primary producer community structure has shifted from dominantly macrophytic to a system that supports substantial algal production. While septic waste was a possible source for eutrophication, low δ¹⁵N did not support this conclusion. However, the timing of the shifts in Dove Sound along with indicators of anoxia leads to the conclusion that fragmentation caused by the construction of a railroad was the root cause. The hydrologic fragmentation reduced the flushing rates, thereby enhancing anoxic conditions in the system and increasing the internal nutrient loading.     

Meteoric lithification of catastrophic rockslide deposits: Diagenesis and significance

Deposits of catastrophic rockslides composed of lithologies rich in carbonate minerals may undergo precipitation of cements that can be used to proxy-date the rockslide event and/or subsequent geomorphic changes of the rockslide mass.In the Alps, localized to widespread lithification of post-Glacial rockslide deposits is observed in lithologies ranging from limestones and dolostones to metacarbonates to calcphyllites. Lithification of rockslide deposits to breccias may be localized to meteoric ‘runoff-shadows’ below larger boulders, or may comprise a layer of breccia or may affect a rockslide mass down its base. In addition, precipitation of cements and small stalactites may take place in megapores on boulder undersides. Cements found in rockslide deposits comprise skalenohedral calcite, prismatic calcite, blocky calcite, calcitic micrite and micropeloidal calcitic cement and, rarely, botryoidal aragonite. Initial cement formation probably is driven by meteoric dissolution–re-precipitation of (mini-) micritic abrasive rock powder generated by dynamic disintegration during the rockslide event. Preliminary ²³⁴U/²³⁰Th ages of rockslide cements support a concept that cementation starts immediately or early after a rockslide event. In rockslide deposits of calcphyllite with accessory pyrite, oxidation of pyrite probably also propels the process of carbonate dissolution–re-precipitation. Limestone-precipitating springs emerging from rockslide masses, and well-cemented talus slopes and fluvial conglomerates percolated by rockslide-derived groundwaters, indicate that rockslide deposits remain diagenetically active long after emplacement.     

Mercury flux to estuarine sediments, derived from Pb-210 and Cs-137 geochronologies (Guaratuba Bay, Brazil)

A sediment core from Guaratuba Bay was used to indicate possible Hg modifications to this coastal environment brought about by growing agricultural activity. Sedimentation rates were estimated to be 6.1 mm/year and 5.2 mm/year through 210Pb and 137Cs geochronologies, respectively. Mercury concentrations and organic matter ratios in the surface layers are greater than in the older sediments, supporting the hypothesis of anthropogenic enrichment. Results show that the Hg flux has raised more than twofold during the second half of the 20th century. These results point to the need for further studies to substantiate the hypothesis of anthropogenic enrichment and to quantify point sources of Hg to this estuary.