Depth-integrated Flux Research Articles

Impact of submarine groundwater discharge on biogeochemistry and microbial communities in pockmarks

The impact of submarine groundwater discharge (SGD) on coastal sea biogeochemistry has been demonstrated in many recent studies. However, only a few studies have integrated biogeochemical and microbiological analyses, especially at sites with pockmarks of different degrees of groundwater influence. This study investigated biogeochemical processes and microbial community structure in sediment cores from three pockmarks in Hanko, Finland, in the northern Baltic Sea. Pockmark data were supplemented by groundwater and seawater measurements. Two active pockmarks showed SGD rates of 0.02 cm d−1 and 0.31 cm d−1, respectively, based on porewater Cl− profiles, while a third pockmark had no SGD influence. Reactive transport modelling (RTM) established that the porewater systems of these active pockmarks are dominated by advection, resulting in the focusing of biogeochemical reactions and the microbial community into a thin zone at the sediment surface. The advection further reduces the accumulation of organic matter in the surface sediments, resulting in the absence of a sulfate-methane transition zone (SMTZ) at these pockmarks. Furthermore, the RTM estimated low rates of consumption of SO42−, and low rates of production of CH4, NH4+, DIC at the active pockmarks. Archaeal communities in the active pockmarks were dominated by ammonia-oxidizing archaea of predominantly groundwater origin. In contrast, at the inactive pockmark, the lack of SGD has permitted rapid deposition of organic-rich mud. The porewater system in the inactive pockmark is dominated by diffusion, leading to orders of magnitude higher metabolite concentrations at depth compared to the active pockmarks. The biogeochemical environment in the inactive pockmark resembles typical organic-rich mud seafloor in the area, with sulphate reduction and methanogenesis dominating organic matter remineralization. Accordingly, methanogens dominate the archaeal community, whereas sulfate reducers dominate the bacterial community. RTM results suggest that sulfate-mediated anaerobic oxidation of methane (S-AOM) also occurs at this site. Although depth-integrated fluxes of SO42−, CH4, NH4, DIC at the inactive pockmark are orders of magnitude higher compared to the active pockmarks, processes at the inactive pockmark represent internal recycling in the coastal sea. Fluxes observed at the active pockmarks, although comparatively small in magnitude, are partly influenced by external inputs to the sea through SGD. Hence, effluxes across the sediment–water interface at these sites partly represent direct external fluxes to the marine environment, in addition to diagenetic recycling at the benthic interface. The study highlights that SGD can result in significant spatial heterogeneity of biogeochemical processes and microbial community structure in the coastal zone, and that the overall effects of SGD and associated solute fluxes at an SGD site are a function of the number of pockmarks, the rate of SGD, and the ratio of active to inactive pockmarks.

How important is it to integrate riverine suspended sediment chemical composition with depth? Clues from Amazon River depth-profiles

The vertical variability in mineralogical, chemical and isotopic compositions observed in large river suspended sediments calls for a depth-integration of this variability to accurately determine riverine geochemical fluxes. In this paper, we present a method to determine depth-integrated chemical particulate fluxes of large rivers, based on river sampling along depth-profiles, and applied to the Amazon Basin lowland tributaries. The suspended particulate matter (SPM) concentration data from depth-profiles is modeled for a number of individual grain size fractions using the Rouse model, which allows to predict the grain size distribution of suspended sediment throughout the whole river cross-section. Then, using (1) the relationship between grain size distribution and the Al/Si ratio (2) relationships between the Al/Si ratio and the chemical concentrations, the chemical composition of river sediment is predicted throughout the river cross-section, and integrated to yield the depth-integrated chemical particulate flux for a number of chemical elements (e.g. Si, Al, Fe, Na, REEs, …). For elements such as Al, Fe, REEs, Th, the depth-integrated flux is around twice as high as the one calculated from river surface sample characteristics. For Na and Si, the depth-integrated flux is three times higher than the “surface” estimate, due to the enrichment of albite and quartz at the bottom of the river. Depth-integrated 87Sr/ 86Sr composition of suspended sediment, also predictable using this method, differs by more than 10 −3 from the surface sample composition. Finally, potential implications of depth-integrated estimates of Amazon sediment chemistry are explored. Depth-integration of particulate 87Sr/ 86Sr isotopic ratios is necessary for a reliable use of Sr isotopes as a provenance tracer. The concept of steady-state weathering of a large river basin is revisited using depth-integrated sediment composition. This analysis shows that, in the Amazon Basin river, the previously observed discrepancy between (1) weathering intensities of channel surface sediment and (2) silicate-derived dissolved fluxes is only slightly accounted for by the vertical variability of suspended sediment weathering intensities. This observation confirms that most large rivers basins are not eroding at steady-state.

Barotropic and Baroclinic M2 Tides in the Monterey Bay Region

Abstract A high-resolution (250 m) primitive equation model is used to simulate the depth-averaged and baroclinic M2 tides in the Monterey Bay region. The model shows a high level of skill in comparisons with sea level observations. ADCP current observations within the submarine canyon are reasonably well reproduced. The modeled depth-averaged currents have a complex spatial pattern with magnitudes in excess of 0.1 m s−1 near Moss Landing. During the second half of flood (ebb) tide, the flow is into (out of) the bay through the canyon but out of (into) the bay over the shelf regions. This depth-averaged flow pattern is not evident in a diagnostic run, implying a feedback between the internal tides and the pressure gradient in the bay. The depth-integrated baroclinic (total minus depth-averaged) energy fluxes show a complex beamlike pattern due to the interaction of energy from multiple sources. The beams have magnitudes up to 1 kW m−1 interspersed with regions of near-zero depth-integrated flux. Energy entering the bay propagates northward from generation regions along the continental margins to the south of the bay (notably on the flanks of Sur Platform) and is steered into the canyon. A localized increase in depth-integrated baroclinic flux magnitude (up to 5 times the values elsewhere in the domain) closely follows the canyon axis near the mouth of the bay, presumably due to topographic focusing. Upper ocean velocities from a long-term surface mooring (MBARI M1) are used to assess the temporal variability of the M2 tide outside the canyon.

Role of deep-sea bacteria in organic matter mineralization and adaptation to hydrostatic pressure conditions in the NW Mediterranean Sea

Ectoaminopeptidase activity (EAA), 14 C-glutamic acid assimilation (GA) and respiration (GR) rates were measured over spring and fall, whilst maintaining in situ hydrostatic pressure condi- tion through a 2000 m water column in the NW Mediterranean. Depth-integrated EAA was maximal in the deep-sea layer due to the lack of ready-to-use compounds. Conversely, depth-integrated fluxes for GA and GR in the bathypelagic zone, though far from negligible, were much lower than in the photic layer, the exception being in the 1000 to 2000 m layer during fall when respiration fluxes and particle fluxes were optimal. Metabolic rates obtained from samples recovered and incubated under in situ pressure conditions were 4.53 ± 4.45 (mean ± SD; n = 19) times higher than their decompressed counterparts, proving that deep-sea bacteria were adapted to high pressure. 14 C-glutamic acid uptake rates (GU = GA + GR) measured under in situ pressure conditions were used to calculate the bacterial 14 C-glutamic acid assimilation yield (GYG = GA/GU). In decompressed samples this yield was always lower by around 20% than in those measured under in situ pressure conditions. Because experiments currently used to estimate bacterial growth efficiency (BGE) are not carried out under in situ pressure conditions, we hypothesize that pressure affects BGE in the same way that it affects GYG. Thus, the stress caused by decompression induces an increase in energy cost, and so under- estimates BGE. Growth of deep-sea bacteria is highly dependent on the quantity and quality of sink- ing particles reaching 1000 m. During mesotrophic conditions (spring), high fluxes of relatively fresh particulate organic carbon generated relatively small depth-integrated EAA rates (20.6 mgC m -2 h -1 ) and high GYG (65%). Meanwhile, during oligotrophic conditions (fall), a minimal flux of old organic matter generated maximal depth-integrated EAA rates (62.2 mgC m -2 h -1 ) and lower GYG (12%). From these observations and from the literature, we explore the relationship between the nutritive sources available to deep-sea bacteria, their role in the mineralization of peptide compounds and consequently the implications for the global carbon cycle.

Circulation and volume flux of the North Atlantic using synoptic hydrographic data in a Bernoulli inverse

A new formulation of the Bernoulli inverse is used to determine the circulation and flux of the North Atlantic from synoptic CTD observations. The inverse assumes that the compressible Bernoulli function, modified potential temperature and salinity are conserved on streamlines in steady, geostrophic, hydrostatic, mass and density conserving flow. Crossings between CTD stations in the distribution of modified potential temperature versus salinity define a set of streamlines. The Bernoulli function is conserved along streamlines and the difference in the Bernoulli function at each crossing is related to the contribution of the unknown sea-surface height (SSH) to the Bernoulli function. These crossings form a set of overdetermined simultaneous linear equations which we solve using a singular value decomposition. A covariance matrix of the SSH solution gives a good estimate of the SSH solution error. From the SSH we calculate a barotropic reference velocity which is added to the baroclinic velocity from the observed density distribution giving the total geostrophic velocity. The inverse is tested using output from the Ocean Circulation and Climate Advanced Model (OCCAM) where the SSH is known a priori: the mean SSH error is 2.2 cm, corresponding to a velocity error of ~1 cm/s for stations separated by 300 km. Inverses of the CTD data have a significantly smaller mean SSH error of 1.2 cm which corresponds to a velocity error of ~0.5 cm/s. Solutions are sensitive to the inclusion of deep crossings which result from observational error as a consequence of small meridional salinity gradients in North Atlantic Deep Water. The inverse is better than a classical dynamic height analysis of the data, by which we mean that the variance of the inverse circulation at depth is greater than the error variance. In the upper ocean (shallower than ?2 = 36.873 which is the top of the Labrador Sea Water) the North Atlantic Current and west wind drift transport 35 ± 4 Sv eastward between 39N and 54N. In the eastern North Atlantic the North Atlantic Current turns northward, west of 20W, with a flux of 14 ± 3 Sv into the Iceland Basin west of the Rockall-Hatton Plateau. The depth-integrated flux of the subpolar gyre in the Irminger Basin is 16 ± 6 Sv fed equally from sources in the western North Atlantic and from flow which crosses the Reykjanes Ridge east to west from the eastern North Atlantic. The circulation at the depth of the Labrador Sea Water is a mid-depth minimum and is generally dominated by error estimates. However, the flux west to east across the Mid-Atlantic Ridge is 3 ± 2 Sv. A simple estimate of the mean flushing time of the Labrador Sea Water layer in the eastern North Atlantic is ~16 years. At depth, over the Porcupine Abyssal Plain the inverse puts in place a basin scale cyclonic circulation and we note agreement with mean circulation rates of 1 to 2 cm/s derived from current meters.

A SEMI-ANALYTICAL SOLUTION FOR ONE-DIMENSIONAL SOLUTE TRANSPORT IN SOILS

Analytical solutions to the convection-dispersion model (CDM) of solute transport require linear reaction terms, strict initial and boundary concentration conditions, and are often complex to evaluate because of the inherent mathematical functions. We present a flexible and mathematically very simple solution to the CDM at steady water flow, labeled a semi-analytical (SA) solution. The SA solution allows for nonlinear reaction terms, variable initial and boundary conditions, and is based on the recently presented moving concentration slope (MCS) model for solute transport. To derive the SA solution, a solute flux approximation at the upper boundary and a small, constant depth increment of 0.5 cm are used, and two features of the MCS model are exploited, i.e., an explicit, depth-integrated flux equation is already inherent in the model and all numerical error and stability equations are unique functions of the solute unit mean travel distance (SUMTD). The SA solution contains seven constants; one is the solute dis-persivity, and the remaining six are functions only of the SUMTD. Excellent agreement between the SA solution and ordinary analytical solutions to the CDM was obtained. For variable boundary conditions, the SA solution was also tested against data for chloride transport in sandy soil columns. Measured and calculated outlet concentrations compared well. The SA model allows for linear or nonlinear reaction terms without increasing the complexity of the solution. In the case of nonlinear reactions, the SA model offers a simple solution in situations where conventional analytical solutions are not available. This was illustrated by successfully comparing the SA solution, including a Michaelis-Menten reaction term, with measured data for simultaneous transport and reduction of nitrate in porous media columns.