Barium in twilight zone suspended matter as a potential proxy for particulate organic carbon remineralization : results for the North Pacific

Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 55 (2008): 1673-1683, doi:10.1016/j.dsr2.2008.04.020.This study focuses on the fate of exported organic carbon in the twilight zone at two contrasting environments in the North Pacific: the oligotrophic ALOHA site (22°45' N 158°W; Hawaii; studied during June–July 2004) and the mesotrophic Subarctic Pacific K2 site (47°N, 161°W; studied during July-August 2005). Earlier work has shown that non-lithogenic, excess particulate Ba (Baxs) in the mesopelagic water column is a potential proxy of organic carbon remineralization. In general Baxs contents were significantly larger at K2 than at ALOHA. At ALOHA the Baxs profiles from repeated sampling (5 casts) showed remarkable consistency over a period of three weeks, suggesting that the system was close to being at steady state. In contrast, more variability was observed at K2 (6 casts sampled) reflecting the more dynamic physical and biological conditions prevailing in this environment. While for both sites Baxs concentrations increased with depth, at K2 a clear maximum was present between the base of the mixed layer at around 50m and 500m, reflecting production and release of Baxs. Larger mesopelagic Baxs contents and larger bacterial production in the twilight zone at the K2 site indicate that more material was exported from the upper mixed layer for bacterial degradation deeper, compared to the ALOHA site. Furthermore, application of a published transfer function (Dehairs et al., 1997) relating oxygen consumption to the observed Baxs data indicated that the latter were in good agreement with bacterial respiration, calculated from bacterial production. These results corroborate earlier findings highlighting the potential of Baxs as a proxy for organic carbon remineralization. The range of POC remineralization rates calculated from twilight zone excess particulate Ba contents did also compare well with the depth dependent POC flux decrease as recorded by neutrally buoyant sediment traps, except in 1 case (out of 4). This discrepancy could indicate that differences in sinking velocities cause an 3 uncoupling of the processes occurring in the fine suspended particle pool from those affecting the larger particle pool which sustains the vertical flux, thus rendering comparison between both approaches risky.This research was supported by Federal Science Policy Office, Brussels through contracts EV/03/7A, SD/CA/03A, the Research Foundation Flanders through grant G.0021.04 and Vrije Universiteit Brussel via grant GOA 22, as well as the US National Science Foundation programs in Chemical and Biological Oceanography

Effect of Chelate Ring Expansion on Jahn–Teller Distortion and Jahn–Teller Dynamics in Copper(II) Complexes

The expanded ligand <i>N</i>,<i>N</i>′-dimethyl-<i>N</i>,<i>N</i>′-dipyridin-2-yl-pyridin-2,6-diamine (ddpd) coordinates to copper­(II) ions in a meridional fashion giving the dicationic complex <i>mer</i>-[Cu­(ddpd)₂]­(BF₄)₂ (<b>1</b>). In the solid state at temperatures below 100 K the cations of <b>1</b> localize in Jahn–Teller elongated CuN₆ polyhedra with the longest Cu–N bond pointing in the molecular <i>x</i> or <i>y</i> directions while the <i>z</i> axis is constrained by the tridentate ddpd ligand. The elongated polyhedra are ordered in an antiferrodistortive way giving an idealized zincblende structure. At higher temperature dynamically averaged (fluxional) polyhedra in the molecular <i>x</i>/<i>y</i> directions are observed by multifrequency variable temperature electron paramagnetic resonance (EPR) and by variable temperature X-ray diffraction studies. Compared to [Cu­(tpy)₂]²⁺ (tpy = 2,2′;6′,2″-terpyridine) the Jahn–Teller splitting 4δ₁ of <b>1</b> is larger. This is very probably caused by the much more favorable orbital overlap in the Cu–N bonds in <b>1</b> which results from the larger bite angle of ddpd as compared to tpy. The “freezing-in” of the Jahn–Teller dynamics of <b>1</b> (<i>T</i> ≈ 100 K) occurs at higher temperature than observed for [Cu­(tpy)₂]²⁺ (<i>T</i> < 77 K) which is also probably due to the larger Jahn–Teller distortion of <b>1</b> resulting in a larger activation barrier

Coinage Metal Complexes of Tris(pyrazolyl)methanide-Based Redox-Active Metalloligands

A series of coinage metal complexes containing the redox-active metalloligands [RuCp^X(κ³<i>N</i>-Tpmd)] {κ³<i>N</i>-Tpmd = κ³<i>N</i>-[C­(pz)₃] with pz = pyrazolyl; [RuCp­(Tpmd)] (<b>2a</b>) and [RuCp*­(Tpmd)] (<b>2b</b>)} are presented. <b>2a</b> and <b>2b</b> are isolable, relatively stable compounds, despite the fact that they feature a “naked” carbanion at the bridgehead position of the κ³<i>N</i>-coordinated tris­(pyrazolyl)­methanide ligand scaffold. As expected, both complexes act as κ¹<i>C</i> ligands toward coinage metal fragments to yield dinuclear complexes of the general formula [RuCp^X(μ-Tpmd)­{MX}] (μ-Tpmd = μ-κ¹<i>C</i>:κ³<i>N</i>-[C­(pz)₃]; M = Au, X = Cl, Cp^X = C₅H₅ (<b>3a</b>) or C₅Me₅ (<b>3b</b>); M = Au, X = CN, Cp^X = C₅H₅ (<b>4a</b>) or C₅Me₅ (<b>4b</b>); M = Cu, X = OC­(O)­Me, Cp^X = C₅H₅ (<b>5a</b>); M = Cu, X = Si­(SiMe₃)₃, Cp^X = C₅H₅ (<b>6a</b>) or C₅Me₅ (<b>6b</b>); M = Ag, X = SC­(S)­NEt₂, Cp^X = C₅H₅ (<b>7a</b>), M = Au, X = CC–Ar, Cp^X = C₅H₅ {Ar = C₆H₅ (<b>8a</b>), 4-NH₂-C₆H₄ (<b>9a</b>), 3,5-(CF₃)₂-C₆H₃ (<b>10a</b>)}). All complexes under study were fully characterized by common spectroscopic techniques; the structural parameters of <b>2a</b>, <b>3a</b>, <b>5a</b>, <b>6a</b>, <b>7a</b>, and <b>10a</b> were determined by X-ray diffraction. Coordination of the {MX} fragment leads to electronic effects on the metalloligand unit, as reflected by the corresponding ¹H and ¹³C NMR spectra. Density functional theory calculations were performed in order to elucidate a conceivable interplay between the metal atoms. The bonding characteristics within the {MX} fragment are only marginally affected upon electronic excitation of the ruthenium-based metalloligand. However, some effect of the influence of {MX} on the <i>E</i>⁰_1/2(Ru^II/Ru^III) value was detected with the aid of cyclic voltammetry measurements. A strong Lewis-acidic metal fragment such as GaCl₃ (<b>11a</b>) leads to an <i>E</i>⁰_1/2 value of 0.37 V, while electron-richer coinage metal fragments facilitate the oxidation of the ruthenium center significantly (<i>E</i>⁰_1/2 = 0.14–0.23 V). This dependence suggests an interaction between both metals due to their close spatial proximity

Methane sources, distributions, and fluxes from cold vent sites at Hydrate Ridge, Cascadia Margin

To constrain the fluxes of methane (CH4) in the water column above the accretionary wedge along the Cascadia continental margin, we measured methane and its stable carbon isotope signature (?13C-CH4). The studies focused on Hydrate Ridge (HR), where venting occurs in the presence of gas-hydrate-bearing sediments. The vent CH4 has a light ?13C-CH4 biogenic signature (?63 to ?66‰ PDB) and forms thin zones of elevated methane concentrations several tens of meters above the ocean floor in the overlying water column. These concentrations, ranging up to 4400 nmol L?1, vary by 3 orders of magnitude over periods of only a few hours. The poleward undercurrent of the California Current system rapidly dilutes the vent methane and distributes it widely within the gas hydrate stability zone (GHSZ). Above 480 m water depth, the methane budget is dominated by isotopically heavier CH4 from the shelf and upper slope, where mixtures of various local biogenic and thermogenic methane sources were detected (?56 to ?28‰ PDB). The distribution of dissolved methane in the working area can be represented by mixtures of methane from the two primary source regions with an isotopically heavy background component (?25 to ?6‰ PDB). Methane oxidation rates of 0.09 to 4.1% per day are small in comparison to the timescales of advection. This highly variable physical regime precludes a simple characterization and tracing of “downcurrent” plumes. However, methane inventories and current measurements suggest a methane flux of approximately 3 × 104 mol h?1 for the working area (1230 km2), and this is dominated by the shallower sources. We estimate that the combined vent sites on HR produce 0.6 × 104 mol h?1, and this is primarily released in the gas phase rather than dissolved within fluid seeps. There is no evidence that significant amounts of this methane are released to the atmosphere locally

Enhanced marine CH4 emissions to the atmosphere off Oregon caused by coastal upwelling

Methane in surface waters and marine air off Oregon (44°24´N–44°54´N, 124°36´W–125°24´W) was continuously surveyed in July 1999. During a high-resolution survey after a period of steady winds from the north, CH4 concentrations were high in the northeastern region, near the shelf edge. The highest CH4 concentrations were 2.5 times higher than equilibrium with the atmospheric partial pressure. In contrast, concentrations were near equilibrium in the western part of the survey area, the Hydrate Ridge. The increase in CH4 from southwest to northeast correlates with a drop in sea surface temperature (SST), from 16.5°C to &lt;13.5°C, toward the shelf edge. The observed SST pattern was caused by summer upwelling off Oregon. The results suggest that CH4 derived from bottom sources near the shelf/slope break and methane found in connection with shallow (100–300 m) turbidity layers is transported to the surface by coastal upwelling, which causes an enhanced net flux of CH4 to the atmosphere. Vertical profiles of the methane distribution on the shelf in October demonstrate the accumulation of methane introduced by shelf sources. Surface concentrations at these stations in October (during nonupwelling conditions) were lower than in July (during upwelling) and were only slightly oversaturated with respect to the atmosphere. An acoustic Doppler current profiler survey indicates that the observed trend cannot be attributed to a surface flow reversal in the area. The low-salinity waters in the core of the Columbia River plume (S &lt; 31) showed no enhanced CH4 concentrations. The trend of higher CH4 concentrations at lower temperatures existed over the whole 17-day survey, but large spatial and temporal variations existed. The presence of methane sources in regions of coastal upwelling worldwide, such as shallow seeps, gas hydrates, and intermediate nepheloid layers, suggests that the enhancement of CH4 fluxes to the atmosphere by coastal upwelling occurs on a global scale