Marked isotopic variability within and between the Amazon River and marine dissolved black carbon pools

Riverine dissolved organic carbon (DOC) contains charcoal byproducts, termed black carbon (BC). To determine the significance of BC as a sink of atmospheric CO2 and reconcile budgets, the sources and fate of this large, slow-cycling and elusive carbon pool must be constrained. The Amazon River is a significant part of global BC cycling because it exports an order of magnitude more DOC, and thus dissolved BC (DBC), than any other river. We report spatially resolved DBC quantity and radiocarbon (Δ14C) measurements, paired with molecular-level characterization of dissolved organic matter from the Amazon River and tributaries during low discharge. The proportion of BC-like polycyclic aromatic structures decreases downstream, but marked spatial variability in abundance and Δ14C values of DBC molecular markers imply dynamic sources and cycling in a manner that is incongruent with bulk DOC. We estimate a flux from the Amazon River of 1.9–2.7 Tg DBC yr−1 that is composed of predominately young DBC, suggesting that loss processes of modern DBC are important

Determination of dissolved organic nitrogen in seawater using Kjeldahl digestion after inorganic nitrogen removal

8 páginas, 6 tablas, 1 figura[EN] An update of the Kjeldahl method is presented for the direct determination of dissolved organic nitrogen (DON) in seawater. Dissolved inorganic nitrogen is previously removed: ammonium as NH3 with NaOH at pH 9.4; and, subsequently, nitrate and nitrite as nitric oxide with FeSO4 in acid medium. The sample is then mineralized to ammonium, which is measured with a Technicon autoanalyzer by the indophenol blue method. The range of recovery for tested standard compounds is similar to those obtained by high temperature oxidation (HTO) techniques. Direct determination of DON by the method described in this work marks an improvement in precision, in comparison with other methods; the standard deviation obtained for samples of seawater is +/-0.2 mu mol.l(-1). The precision of DON measurements is not dependent on dissolved inorganic nitrogen analysis. For several stations in the Northeast Atlantic Ocean, DON values ranged between 3 and 10 mu mol.l(-1).[FR] La méthode de Kjeldahl a été modifiée pour réaliser la mesure directe de l'azote organique dissous (NOD) dans l'eau de mer. Les composés inorganiques dissous dans l'échantillon sont préalablement éliminés comme suit: l'ammonium, en rendant le milieu basique avec de la soude; le nitrate et le nitrite, en les transformant en oxyde nitrique par le FeS04 en milieu acide. L'échantillon est ensuite minéralisé pour transformer le NOD en ammonium dont la concentration est déterminée sur un AutoAnalyseur Technicon. La récupération des produits standard est identique à celle obtenue par des techniques d'oxydation à haute température. La détermination du NOD par cette méthode est améliorée car elle ne dépend plus de la mesure des concentrations en azote minéral comme dans les autres méthodes. L'écart-type est d'environ ±0,2 J.Lmol.l-1 pour des échantillons d'eau de mer. Les concentrations, mesurées par cette technique à plusieurs stations de l'océan Atlantique NE, varient de 3 à 10 J.Lmol.l- 1•Support for this work came from the EEC project MAST2- CT93-0065.Peer reviewe

Fires prime terrestrial organic carbon for riverine export to the global oceans

Black carbon (BC) is a recalcitrant form of organic carbon (OC) produced by landscape fires. BC is an important component of the global carbon cycle because, compared to unburned biogenic OC, it is selectively conserved in terrestrial and oceanic pools. Here we show that the dissolved BC (DBC) content of dissolved OC (DOC) is twice greater in major (sub)tropical and high-latitude rivers than in major temperate rivers, with further significant differences between biomes. We estimate that rivers export 18 ± 4 Tg DBC year−1 globally and that, including particulate BC fluxes, total riverine export amounts to 43 ± 15 Tg BC year−1 (12 ± 5% of the OC flux). While rivers export ~1% of the OC sequestered by terrestrial vegetation, our estimates suggest that 34 ± 26% of the BC produced by landscape fires has an oceanic fate. Biogeochemical models require modification to account for the unique dynamics of BC and to predict the response of recalcitrant OC export to changing environmental conditions