Atmospheric nitrate concentrations and isotope composition for samples collected in the marine boundary layer in 2006 and 2007

The comprehensive isotopic composition of atmospheric nitrate (i.e., the simultaneous measurement of all its stable isotope ratios: 15N/14N, 17O/16O and 18O/16O) has been determined for aerosol samples collected in the marine boundary layer (MBL) over the Atlantic Ocean from 65°S (Weddell Sea) to 79°N (Svalbard), along a ship-borne latitudinal transect. In nonpolar areas, the d15N of nitrate mostly deriving from anthropogenically emitted NOx is found to be significantly different (from 0 to 6 per mil) from nitrate sampled in locations influenced by natural NOx sources (-4 ± 2) per mil. The effects on d15N(NO3-) of different NOx sources and nitrate removal processes associated with its atmospheric transport are discussed. Measurements of the oxygen isotope anomaly (D17O = d17O - 0.52 × d18O) of nitrate suggest that nocturnal processes involving the nitrate radical play a major role in terms of NOx sinks. Different D17O between aerosol size fractions indicate different proportions between nitrate formation pathways as a function of the size and composition of the particles. Extremely low d15N values (down to -40 per mil) are found in air masses exposed to snow-covered areas, showing that snowpack emissions of NOx from upwind regions can have a significant impact on the local surface budget of reactive nitrogen, in conjunction with interactions with active halogen chemistry. The implications of the results are discussed in light of the potential use of the stable isotopic composition of nitrate to infer atmospherically relevant information from nitrate preserved in ice cores.

(Table 1) Conditions of vertical near-bottom profiling and the main parameters of the benthic boundary layer in several regions of the Black Sea shelf

Measurements of 14 vertical profiles of currents and hydrological parameters in the near-bottom layer with depth resolution of 0.1 m were carried out in several regions of the Black Sea shelf, at five points over the continental slope, and in three deep water regions. The upper boundary of the benthic boundary layer (BBL) was reliably determined at a point at distance from 5-7 to 35-40 m from the bottom where the gradients of density and current velocity changed. Experimental data obtained were used to determine the coefficient of bottom friction, friction velocity, coefficients of vertical diffusion of momentum and density, and vertical fluxes of temperature and salinity in the BBL.

Mineralogy and stable oxygen isotope ratios of the Cretaceous-Tertiary boundary

Results of detailed mineralogical, chemical, and oxygen isotope analyses of the clay minerals and zeolites from two Cretaceous-Tertiary (K/T) boundary regions, Stevns Klint, Denmark, and Deep Sea Drilling Project (DSDP) Hole 465A in the north central Pacific Ocean, are presented. In the central part of the Stevns Klint K/T boundary layer, the only clay mineral detected by x-ray diffraction is a pure smectite with > 95 percent expandable layers. No detrital clay minerals or quartz were observed in the clay size fraction in these beds, whereas the clay minerals above and below the boundary layer are illite and mixed-layer smectite-illite of detrital origin as well as quartz. The mineralogical purity of the clay fraction, the presence of smectite only at the boundary, and the d18O value of the smectite (27.2 ± 0.2 per mil) suggest that it formed in situ by alteration of glass. Formation from impact rather than from volcanic glass is supported by its major element chemistry. The high content of iridium and other siderophile elements is not due to the cessation of calcium carbonate deposition and resulting slow sedimentation rates. At DSDP Hole 465A, the principal clay mineral in the boundary zone (80 to 143 centimeters) is a mixed-layer smectite-illite with >=90 percent expandable layers, accompanied by some detrital quartz and small amounts of a euhedral authigenic zeolite (clinoptilolite). The mixed-layer smectite-illite from the interval 118 to 120 centimeters in the zone of high iridium abundance has a very low rare earth element content; the negative cerium anomaly indicates formation in the marine environment. This conclusion is corroborated by the d18O value of this clay mineral (27.1 ± 0.2 per mil). Thus, this mixed-layer smectite-illite formed possibly from the same glass as the K/T boundary smectite at Stevns Klint, Denmark.

Seawater carbonate chemistry and concentration boundary layers around complex assemblages of macroalgae in a laboratory experiment

Metabolic processes have the potential to modulate the effects of ocean acidification (OA) in nearshore macroalgal beds. We investigated whether natural mixed assemblages of the articulate coralline macroalgae Arthrocardia corymbosa and understory crustose coralline algae (CCA) altered pH and O2 concentrations within and immediately above their canopies. In a unidirectional flume, we tested the effect of water velocity (0-0.1 m/s), bulk seawater pH (ambient pH 8.05, and pH 7.65), and irradiance (photosynthetically saturating light and darkness) on pH and O2 concentration gradients, and the derived concentration boundary layer (CBL) thickness. At bulk seawater pH 7.65 and slow velocities (0 and 0.015 m/s), pH at the CCA surface increased to 7.90-8.00 in the light. Although these manipulations were short term, this indicates a potential daytime buffering capacity that could alleviate the effects of OA. Photosynthetic activity also increased O2 concentrations at the surface of the CCA. However, this moderating capacity was flow dependent; the CBL thickness decreased from an average of 26.8 mm from the CCA surface at 0.015 m/s to 4.1 mm at 0.04 m/s. The reverse trends occurred in the dark, with respiration causing pH and O2 concentrations to decrease at the CCA surface. At all flow velocities the CBL thicknesses (up to 68 mm) were much greater than those previously published, indicating that the presence of canopies can alter the CBL substantially. In situ, the height of macroalgal canopies can be an order of magnitude larger than those used here, indicating that the degree of buffering to OA will be context dependent.

Diffusion boundary layers ameliorate the negative effects of ocean acidification on the temperate coralline macroalga Arthrocardia corymbosa

Anthropogenically-modulated reductions in pH, termed ocean acidification, could pose a major threat to the physiological performance, stocks, and biodiversity of calcifiers and may devalue their ecosystem services. Recent debate has focussed on the need to develop approaches to arrest the potential negative impacts of ocean acidification on ecosystems dominated by calcareous organisms. In this study, we demonstrate the role of a discrete (i.e. diffusion) boundary layer (DBL), formed at the surface of some calcifying species under slow flows, in buffering them from the corrosive effects of low pH seawater. The coralline macroalga Arthrocardia corymbosa was grown in a multifactorial experiment with two mean pH levels (8.05 'ambient' and 7.65 a worst case 'ocean acidification' scenario projected for 2100), each with two levels of seawater flow (fast and slow, i.e. DBL thin or thick). Coralline algae grown under slow flows with thick DBLs (i.e., unstirred with regular replenishment of seawater to their surface) maintained net growth and calcification at pH 7.65 whereas those in higher flows with thin DBLs had net dissolution. Growth under ambient seawater pH (8.05) was not significantly different in thin and thick DBL treatments. No other measured diagnostic (recruit sizes and numbers, photosynthetic metrics, %C, %N, %MgCO3) responded to the effects of reduced seawater pH. Thus, flow conditions that promote the formation of thick DBLs, may enhance the subsistence of calcifiers by creating localised hydrodynamic conditions where metabolic activity ameliorates the negative impacts of ocean acidification.

Concentration and composition of aerosols in the near-water atmosphere layer above the Laptev Sea in July-September 1995

During the ARK-XI/1 expedition of R/V Polarstern in July-September 1995 12 samples of aerosols were collected in lower atmosphere layer over the Laptev Sea by filtration of air through AFA-HA filters. Element composition of the samples was determined by instrumental neutron activation analysis. Average atmospheric concentrations of Cr, Mn, Fe, Co, Zn and As are higher than in other regions of the Arctic. This can be explained by natural reasons: (1) by input of particles from the surface microlayer of sea water enriched by many chemical elements, (2) by atmospheric transfer of organic matter and lithogenic material from the land, and (3) by resuspension of particles from ice-rafted sediments. In some samples anthropogenic pollution was registered.