Seasonal evolution of nitrate isotopic composition in the River Scheldt

The Scheldt Estuary is the outlet of one of the most densely populated watersheds of Europe (425 inh./km2). Following the implementation of wastewater management measures the Scheldt River has experienced important biogeochemical changes during the last decade. Among others, this resulted in better water oxygenation, and nitrate has become the dominant form of the dissolved nitrogen pool. We present results for the nitrogen and oxygen isotopic composition of nitrate in the River Scheldt and its main tributaries, as sampled monthly from December 2009 till present in the framework of a regional monitoring programme. The method of analysis was based on the denitrifier methodology developed by Sigman and Casciotti (2001), involving bacterial reduction of nitrate and subsequent analysis of the released N2O by Isotope Ratio Mass Spectrometry (IRMS). To that purpose we used a custom build purification and cryofocusing system connected on-line to a Thermo Delta V IRMS. Along-river sections and tributaries show a clear seasonality with increased d15N and decreased d18O signatures in spring-summer, and the inverse in winter. Likewise, d18O vs d15N plots for waters from the Scheldt and its tributaries clearly differentiate winter from spring-summer conditions, the latter having highest d15N and lowest d18O values. While the isotopic composition for winter is expected to represent the condition prevailing in groundwater feeding the river, during spring-summer, within-river transformation processes impact on the isotopic composition. These processes are mainly nitrification, decreasing nitrate d18O and nitrate uptake, increasing both, d15N and the d18O. The fact that seasons are well resolved imply that nitrification occurs early in the season when ammonium is still abundant and uptake by phytoplankton limited, while later in the season, once ammonium drops to very low concentrations and nitrate is the main inorganic N-species, uptake by phytoplankton becomes the predominant process and is witnessed also by decreasing nitrate concentrations on from June

Stable isotopic composition of bivalve shell organic matrix: <i>Mytilus edulis</i> collected along the Scheldt estuary

Bivalve shells are biostructures composed of a mineral and an organic phase. For paleoclimatology applications, the mineral part (carbonates) is most widely studied. In contrast, understanding of the composition and the proxyfunction of the organic matrix is much less developed. The quantity of organic matrix in shells is relatively small compared to the mineral phase (a few wt %) and the biochemical composition is quite complex, consisting mainly of sugars and proteins. Lipids, which represent a small fraction of the organic matrix, are rather poorly known. We studied the potential of stable isotope composition (C, N, H) of bulk organic matrix and specific lipid compounds of Mytilis edulis shells, as environmental and climatic proxies, with special focus on the effects due to changing salinity. Mytilus specimens were collected along the salinity gradient of the Scheldt estuary (The Netherlands) and we analysed the isotopic composition of the organic matrix and associated specific lipid compounds and related these to averaged physico-chemical characteristics of the water, in particular salinity. We discuss these relationships in the light of their usefulness as proxies for reconstructing past environmental conditions

Identification of the accretion rate for annually resolved archives

International audienceThe past environment is often reconstructed by measuring a given proxy (e.g. ?18O) in an environmental archive, i.e. a species which gradually accumulates mass and records the current environment during this mass formation (e.g. corals, shells, trees, etc...). When such an environmental proxy is measured, its values are known as a function of distance. However, to relate the data to environmental variations, the date associated with each measurement, i.e. the time base, should be known. This is not straightforward solved, since species usually do not grow at constant rates. In this paper, we investigate this problem for annually resolved archives, which exhibit a certain periodicity. Such signals are often found in clams or corals. Due to variations in accretion rate the data along the distance axis have a disturbed periodic profile. A method is developed to extract information about the accretion rate, such that the original (periodic) signal as function of time can be recovered. Simultaneously the exact shape of the periodic signal is estimated. The final methodology is quasi-independent of choices made by the investigator. Every step in the procedure is described in detail and finally, the method is exemplified on a real world example