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In this study we report full‐depth water column profiles for nitrogen and oxygen isotopic composition (δ15N and δ18O) of nitrate (NO3−) during the GEOTRACES GA01 cruise (2014). This transect intersects the double gyre system of the subtropical and subpolar regions of the North Atlantic separated by a strong transition zone, the North Atlantic Current. The distribution of NO3− δ15N and δ18O shows that assimilation by phytoplankton is the main process controlling the NO3− isotopic composition in the upper 150 m, with values increasing in a NO3− δ18O versus δ15N space along a line with a slope of one toward the surface. In the subpolar gyre, a single relationship between the degree of NO3− consumption and residual NO3− δ15N supports the view that NO3− is supplied via Ekman upwelling and deep winter convection, and progressively consumed during the Ekman transport of surface water southward. The co‐occurrence of partial NO3− assimilation and nitrification in the deep mixed layer of the subpolar gyre elevates subsurface NO3− δ18O in comparison to deep oceanic values. This signal propagates through isopycnal exchanges to greater depths at lower latitudes. With recirculation in the subtropical gyre, cycles of quantitative consumption‐nitrification progressively decrease subsurface NO3− δ18O toward the δ18O of regenerated NO3−. The low NO3− δ15N observed south of the Subarctic Front is mostly explained by N2 fixation, although a contribution from the Mediterranean outflow is required to explain the lower NO3− δ15N signal observed between 600 and 1500 m depth close to the Iberian margin. Key Points: Nitrate assimilation controls the nitrate isotopic composition in surface waters of the North Atlantic subpolar gyreNitrate isotopic composition reveals that nitrate is supplied from subpolar to subtropical gyre by Ekman transport and isopycnal mixingN2 fixation and Mediterranean outflow have an impact on nitrate isotopic composition in the North Atlantic Ocean [ABSTRACT FROM AUTHOR]

Winter to summer CO2 dynamics within landfast sea ice in McMurdo Sound (Antarctica) were investigated using bulk ice pCO2 measurements, air-snow-ice CO2 fluxes, dissolved inorganic carbon (DIC), total alkalinity (TA), and ikaite saturation state. Our results suggest depth-dependent biotic and abiotic controls that led us to discriminate the ice column in three layers. At the surface, winter pCO2 supersaturation drove CO2 release to the atmosphere while spring-summer pCO2 undersaturation led to CO2 uptake most of the time. CO2 fluxes showed a diel pattern superimposed upon this seasonal pattern which was potentially assigned to either ice skin freeze-thaw cycles or diel changes in net community production. In the ice interior, the pCO2 decrease across the season was driven by physical processes, mainly independent of the autotrophic and heterotrophic phases. Bottom sea ice was characterized by a massive biomass build-up counterintuitively associated with transient heterotrophic activity and nitrate plus nitrite accumulation. This inconsistency is likely related to the formation of a biofilm. This biofilm hosts both autotrophic and heterotrophic activities at the bottom of the ice during spring and may promote calcium carbonate precipitation. [ABSTRACT FROM AUTHOR]

The East Siberian Sea and contiguous western Arctic Ocean basin are characterized by a subsurface nutrient maximum in the halocline, generally attributed to both Pacific inflow and intensive remineralization in shelf bottom waters that are advected into the central basin. We report nitrogen and oxygen isotopic measurement of nitrate from the East Siberian Sea and western Eurasian Basin, in order to gain insight into how nitrate is processed by the microbial community and redistributed in the Arctic Ocean. A large decoupling between nitrate δ15N and δ18O is reported, increasing and decreasing upward from the Atlantic temperature maximum layer toward the surface, respectively. A correlation between water and nitrate δ18O indicates that most of the nitrate (> 60%) at the halocline has been regenerated within the Arctic Ocean. The increase in nitrate δ15N correlates with the fixed N deficit, indicating a causal link between the loss of fixed N and the 15N enrichment. This suggests that a significant share of benthic denitrification is driven by nitrate supplied by remineralization and partial nitrification, allowing residual 15N‐enriched ammonium to diffuse out of the sediments. By increasing nutrient concentrations and fixed N deficit in shelf bottom waters, this imprint is attenuated offshore following advection into the halocline by nitrate regeneration and mixing. Estimation of the sedimentary isotope effect related to benthic denitrification yields values in the range of 2.4–3.8‰, with its magnitude driven by both the degree of coupling between remineralization and nitrification, and fixed N concentrations in shelf bottom waters. [ABSTRACT FROM AUTHOR]

Nitrification, the microbially mediated oxidation of ammonium into nitrate, is generally expected to be low in the Southern Ocean mixed layer. This paradigm assumes that nitrate is mainly provided through vertical mixing and assimilated during the vegetative season, supporting the concept that nitrate uptake is equivalent to the new primary production (i.e., primary production which is potentially available for export). Here we show that nitrification is significant (~40-80% of the seasonal nitrate uptake) in the naturally iron-fertilized bloom over the southeast Kerguelen Plateau. Hence, a large fraction of the nitrate-based primary production is regenerated, instead of being exported. It appears that nitrate assimilation (light dependent) and nitrification (partly light inhibited) are spatially separated between the upper and lower parts, respectively, of the deep surface mixed layers. These deep mixed layers, extending well below the euphotic layer, allow nitrifiers to compete with phytoplankton for the assimilation of ammonium. The high contributions of nitrification to nitrate uptake are in agreement with both low export efficiency (i.e., the percentage of primary production that is exported) and low seasonal nitrate drawdown despite high nitrate assimilation. [ABSTRACT FROM AUTHOR]

The impact of iron fertilization on mesopelagic carbon (C) remineralization was assessed during the European Iron Fertilization Experiment (EIFEX) in the Southern Ocean by following the temporal change of excess particulate barium (also called biogenic or Baxs) in the mesopelagic waters of a mesoscale eddy. Before the iron infusion the site was already sustaining a significant vertical flux of particles leading to organic remineralization in the mesopelagic. Approximately 35 d after the fertilization, mesopelagic Baxs contents provided evidence of changes in surface particulate export and of enhanced and particularly fast remineralization extending down to 1000 m. At the end of the experiment (day 36) both export and remineralization decreased. Organic carbon remineralization took place mainly in the lower part of the mesopelagic zone (500-1000 m) and increased more than 5-fold during EIFEX, reaching values up to 92 ± 15 mg C m-2 d-1. However, such remineralization rates are not particularly high when compared to other values reported for the natural Southern Ocean during summer. Though export and remineralization reached peak values around day 34, remineralization integrated over the 150 to 1000 m layer accounted only for 13 ± 1.4% of the export at 150 m, suggesting that a substantial amount of exported carbon reached deeper in the water column. Compared to natural blooms in high-nutrient low-chlorophyll (HNLC) waters of the Southern Ocean, the Fe-induced bloom during EIFEX resulted in a lower ratio of remineralized organic carbon over exported carbon, similar to what we recently observed for natural Fe-replete conditions prevailing above the Kerguelen Plateau. Unexpected similarities and phase lags of profiles in the mesopelagic between casts sampled inside and outside the fertilized patch during EIFEX indicated that eddy dynamics were a determinant in setting particle patterns on a broad spatial scale exceeding the extent of the fertilization, thereby homogenizing the signals of mesopelagic remineralization. [ABSTRACT FROM AUTHOR]

Gut contents of sand goby Pomatoschistus minutus showed higher C and N isotope values than the food before consumption. This enrichment was more pronounced in the hindgut than in the foregut, probably because of preferential assimilation of 12C and 14N along the gastro-intestinal tract. The results indicated that the shift towards higher values in the alimentary canal occurs in the first 2 h after feeding. [ABSTRACT FROM AUTHOR]

We analyzed δ29Si of dissolved silicate for eight water column profiles across the Southern Ocean (south of Australia in spring 2001) from the Seasonal Ice Zone (SIZ) north to the Subantarctic Zone (S AZ), including the first isotopic compositions measured for Si-depleted seawaters. All profiles display mixed layer enrichments in heavy Si isotopes relative to deep water in accordance with preferential uptake of the light isotope by diatoms. As silicate levels decrease from the SIZ northward across the Polar Front Zone (PFZ) to the SAZ, surface and mesopelagic δ29 Si signatures generally become progressively heavier, but the most Si-depleted SAZ waters do not exhibit δ29Si values heavier than in the PFZ. This intricacy appears to derive from variations in the vertical and horizontal supply of silicate to surface waters, and by applying a steady state open system model, we estimate a fractionation factor, 29ϵ between diatoms and seawater of -0.45 ± 0.1 7‰, independently of zones and phytoplankton community. Though encouraging, these results are related to latitudinal changes in mesopelagic δ29Si values, complexity in surface silicate-δ29Si correlations, and differences from previous studies, which underline the need for caution in the use of silicon isotopes in paleoceanographic studies until systematic efforts have been undertaken to better understand modem variations. [ABSTRACT FROM AUTHOR]

In the present study, the high-resolution stable carbon (13C/12C) and oxygen (18O/16O) isotope ratio profiles in the wood of the mangroveRhizophora mucronataLam., a tropical tree species lacking distinct growth rings, were investigated. Variations of both isotope ratios revealed a remarkable annual cyclicity with lowest values occurring at the latewood/earlywood boundary (April–May) and highest values during the transition from earlywood to latewood (October–November). Based on the current knowledge of the physiology of this mangrove species, as well as on the current literature available on high-resolution profiles of stable isotope ratios in tree rings, possible driving forces responsible for this seasonal pattern are discussed. The annual cyclicity, together with a conspicuous isotope pattern appearing in the El-Niño year 1997, promises great potential for tropical dendrochronology. [ABSTRACT FROM AUTHOR]