Showing papers by "Hilary Kennedy published in 2005"

The effect of acidification on the determination of organic carbon, total nitrogen and their stable isotopic composition in algae and marine sediment.

Abstract: We investigated the effects of sample acidification on the stable carbon and nitrogen isotopic composition (delta13C and delta15N), as well as the organic carbon (OC) and total nitrogen (TN) composition, of an algal culture and a marine sediment. Replicate measurements of untreated and acid-treated samples were made using 1 M, 2 M and 6 M HCl, 6% H2SO3 and 1 M H3PO4. For all treatments the precision of the analysis for the acid-treated sample was equal to or less than that in the non-acidified sample. For the algae, analysis of variance (ANOVA) indicated no significant differences in the mean OC and TN concentration, or delta13C and delta15N composition, between any acid treatment and non-acidified samples. For the sediment sample a comparison could only be made between the different acid treatments because the untreated contained significant amounts ( approximately 30%) of carbonate carbon. ANOVA indicated that the mean OC determined in sediment samples after the 1 M HCl treatment and the mean delta13C values after the 6% H2SO3 and 1 M H3PO4 treatments were significantly different (p < 0.013 and < .05, respectively) from all other treatments. Mass balance calculations indicate that in some instances delta13C values were biased due to a contribution from unreacted carbonate carbon. There were no significant differences in the mean TN between any acid-treated and non-acidified samples. The mean delta15N values after 6 M HCl, 6% H2SO3 and 1 M H3PO4 treatments were significantly different from the untreated sediment sample (p < 0.044). Based on the significant bias observed for the delta15N and delta13C values, a weak (1-2 M) HCl solution is confirmed as the most appropriate acid for the removal of inorganic carbon from natural materials requiring elemental and isotopic analysis.

Mg/Ca, Sr/Ca, and stable-isotope (δ18O and δ13C) ratio profiles from the fan mussel Pinna nobilis: Seasonal records and temperature relationships

Abstract: [1] We present new annually resolved δ18O, δ13C, Mg/Ca, and Sr/Ca ratio records for two shells of the fast growing Mediterranean fan mussel Pinna nobilis, collected from proximal Spanish coast sea grass meadows. The relationship between the potential geochemical proxies and ontogenetic and environmental controlling factors is investigated. Specifically, the use of shell Mg/Ca and Sr/Ca ratios as potential calcification temperature proxies, the latter calculated from measured shell δ18O values, has been assessed. The δ18O cycles along the growth axis indicate that our P. nobilis specimens are ∼10.5 and ∼4.5 years old. Shell Sr/Ca ratios do not exhibit any consistent interannual cyclicity and are not correlated to temperature. A subtle ontogenetic effect on shell Mg/Ca ratios was observed during the first 4.5 years of recorded growth but was highly evident during the organism's later growth years. In P. nobilis shells, different mechanisms influence ontogenetic variation in shell Mg/Ca and δ18O records. Shell Mg/Ca ratios from the first 4.5 years of growth correlate significantly to temperature, in a best fit relationship described by the equation Mg/Ca = 17.16 ± 1.95 * exp(0.022 ± 0.004 * T). P. nobilis shell Mg/Ca records therefore are a valid temperature proxy only during an early growth phase. For the same range of temperatures, shell Mg/Ca ratios in P. nobilis are approximately 1/3 lower than those reported for inorganic calcite but 3 to 4 times higher than in another bivalve species, Mytilus trossulus, and 4 to 16 times higher than in foraminifera. We suggest these offsets are due to a higher degree of similarity between seawater and calcification-fluid composition in P. nobilis than in other bivalves and foraminifera. The observed shell Mg/Ca ratio change per °C of 2.2% also is lower than that observed for inorganic and other biogenic calcites. Our findings strongly support taxon- and species-specific Mg/Ca–temperature relationships for bivalves and other calcifying organisms. An appreciation of the physiology and calcification mechanisms of any biogenic carbonate archive therefore is paramount prior to the application of stable-isotope and element/Ca ratio proxies for paleotemperature reconstructions.

Seasonal and spatial variation in the organic carbon and nitrogen concentration and their stable isotopic composition in Zostera marina (Denmark)

Abstract: The spatial and seasonal variation in the concentration and stable isotopic composition of carbon and nitrogen was studied in Zostera marina (eelgrass) meadows in Roskilde Fjord and Oresund, Denmark. Organic carbon (C org) and nitrogen (Norg) concentrations in new tissues (leaf, rhizome, roots) ranged from 11 to 40 mmol g 21 C and 0.4 to 3.6 mmol g 21 N, whereas their stable isotopic composition in new leaf ranged from 222‰ to 28‰ ( d 13 Cleaf) and 17‰ to 120‰ ( d 15 Nleaf). The d 13 Cleaf correlated weakly (r 2 ; 0.3) with the d 13 C of total dissolved inorganic carbon (DIC) in the surface waters (d 13 CDIC range: 25.4‰ to 20.3‰). The temporal change was more pronounced than the spatial change for Norg, the atomic C : N ratio, and d 13 Cleaf but was much smaller for Corg and d 15 Nleaf. Eelgrass plants had higher Norg concentrations, lower C : N ratios, and more depleted d 13 Cleaf in winter than summer, reflecting the temporal imbalance between inorganic nutrient supply and plant demand imposed by seasonality in the growth rate. The apparent carbon isotope enrichment factor of new leaf relative to external DIC («eelgrass-DIC range: 212.1‰ to 25.3‰) indicated that eelgrass must be reliant on direct HCO uptake, especially during the period of high 2 3 growth rate (May to September). The steep spatial gradient of d 15

Nutrient dynamics and ecosystem metabolism in the Bay of Blanes (NW Mediterranean)

Abstract: The dynamics of the nutrient pools and their stoichiometry as well as their control by ecosystem metabolism (benthic and planktonic) and benthic–pelagic exchanges (sedimentation rates and sediment waterfluxes) were examined in the Mediterranean littoral (Blanes Bay, NE Spain). Dissolved organic nitrogen comprised about half of the nitrogen present in the water column and the carbon pool was dominated by the inorganic pool (95% of the carbon present in the water column). The dissolved and particulate organic pools were deficient in P relative to C and N, indicating a rapid recycling of P from organic matter. The pelagic compartment was heterotrophic, supported by significant allochthonous inputs of land material, which also contributed greatly to the sedimentary inputs (37% of total sedimenting carbon). In contrast, the benthic compartment was autotrophic, with the excess net benthic community production balancing the deficit in pelagic community production, leading to metabolic equilibrium at the station studied. Sedimentary inputs of nitrogen, phosphorus and silicon exceeded the benthic release, indicating that the benthic compartment acted as a sink for nutrients, consistent with its autotrophic nature. Carbon inputs to the benthic compartment also exceeded requirements, due to the allochthonous subsidies to the system, so that the benthic compartment stored or exported organic carbon.

The effects of megafaunal burrows on radiotracer profiles and organic composition in deep-sea sediments: preliminary results from two sites in the bathyal north-east Atlantic

Abstract: Megafaunal burrows were detected in boxcores from two sites in the bathyal north-east Atlantic. Burrow contents were analysed to assess their significance to sediment radiotracer profiles and organic composition. At 1100 m depth, burrow openings up to 3 cm diameter occurred at a density of approximately 5 m−2. Burrows at 12–18 cm sediment depth extending horizontally for up to 35 cm and linked to the surface by vertical shafts were provisionally attributed to echiuran worms, although no occupants were found in situ. In one example the horizontal burrow section was filled with green slurry, for which scanning electron microscopy, 210Pbexcess and organic content all indicated a phytodetrital origin. At 1920 m depth no large burrow openings were found in five boxcores examined, but large subsurface biogenic structures were present. Galleries at 15–26 cm depth were traced horizontally for up to 30 cm, but contained no occupants or filling. Extended linear bands of faecal pellets were found in three boxcores at 13–17 cm depth. Excess 210Pb content indicated that most of these structures resulted from surface deposit feeding. Faecal pellet bands may partially explain the occurrence of subsurface peaks detected in profiles of 210Pbexcess at this site. Results suggest that ‘caching’ of phytodetritus and subsurface deposition of faeces are two mechanisms for the rapid, deep burial of relatively fresh organic matter, but the significance of these processes to sediment geochemistry cannot be quantified without much information on the distribution, identity and abundance of burrowing megafauna in the deep sea.