Partial pressure and air–sea CO2 flux in the Aegean Sea during February 2006

Data on the distribution of fCO₂ were obtained during a cruise in the Aegean Sea during February 2006. The fCO₂ of surface water (fCO₂^sw) was lower than the atmospheric fCO₂ (fCO₂^atm) throughout the area surveyed and ΔfCO₂ values varied from ?34 to ?61 μatm. The observed under-saturation suggests that surface waters in the Aegean represent a sink for atmospheric CO₂ during the winter of 2006. Higher fCO₂^sw values were recorded in the ‘less warm’ and ‘less saline’ shallow northernmost part of the Aegean Sea implying that the lower seawater temperature and salinity in this area play a crucial role in the spatial distribution of fCO₂^sw.A first estimate of the magnitude of the air–sea CO₂ exchange and the potential role of the Aegean Sea in the transfer of atmospheric CO₂ was also obtained. The air–sea CO₂ fluxes calculated using different gas transfer formulations showed that during February 2006, the Aegean Sea absorbs atmospheric CO₂ at a rate ranging from ?6.2 to ?11.8 mmol m^?2 d^?1 with the shipboard recorded wind speeds and at almost half rate (?3.5 to ?5.5 mmol m^?2 d^?1) with the monthly mean model-derived wind speed. Compared to recent observations from other temperate continental shelves during winter period, the Aegean Sea acts as a moderate to rather strong sink for atmospheric CO₂.Further investigations, including intensive spatial and temporal high-resolution observations, are necessary to elucidate the role of the Aegean Sea in the process of transfer of atmospheric CO₂ into the deep horizons of the Eastern Mediterranean.     

Air-sea CO2 fluxes in the southern Yellow Sea: An examination of the continental shelf pump hypothesis

The southern Yellow Sea (SYS), located to the north of the East China Sea (ECS), was considered part of the ECS when Tsunogai et al. (1999) proposed the “continental shelf pump” (CSP) hypothesis. However, the original CSP carbon dioxide (CO₂) uptake flux (2.9 mol C m⁻² yr⁻¹) appears to have been overestimated, primarily due to the differences between the SYS and the ECS in terms of their CO₂ system. In this paper, we estimated air-sea CO₂ fluxes in the SYS using the surface water partial pressure of CO₂ (pCO₂) measured in winter, spring, and summer, as well as that estimated in fall via the relationship of pCO₂ with salinity, temperature, and chlorophyll a. The results indicate that overall, the entire investigated area was a net source of atmospheric CO₂ during summer, winter, and fall, whereas it was a net sink during spring. Spatially, the nearshore area was almost a permanent CO₂ source, while the central SYS shifted from being a CO₂ sink in spring to a source in the other seasons of the year. Overall, the SYS is a net source of atmospheric CO₂ on an annual scale, releasing ∼7.38 Tg C (1 Tg=10¹² g) to the atmosphere annually. Thus, the updated CO₂ uptake flux in the combined SYS and ECS is reduced to ∼0.86 mol C m⁻² yr⁻¹. If this value is extrapolated globally following Tsunogai et al. (1999), the global continental shelf would be a sink of ∼0.29 Pg C yr⁻¹, instead of 1 Pg C yr⁻¹ (1 Pg=10¹⁵ g).The SYS as a net annual source of atmospheric CO₂ is in sharp contrast to most mid- and high-latitude continental shelves, which are CO₂ sinks. We argue that unlike the ECS and the North Sea where carbon on the shelf could be exported to the open ocean, the SYS lacks the physical conditions required by the CSP to transport carbon off the shelf effectively. The global validity of the CSP theory is thus questionable.     

Spatio-temporal variability of size fractionated phytoplankton on the shelf adjacent to the Ebro river (NW Mediterranean)

The mesoscale distribution and seasonal variation of the size structure of phytoplankton biomass, as measured by chlorophyll a (chl a), was studied in the Ebro shelf area (NW Mediterranean) during three different seasons: autumn, winter and summer. In autumn and summer, when the water column was, respectively, slightly or strongly stratified and nutrient concentrations were low at surface, average total chl a values were 0.31 and 0.29 mg m⁻³, respectively. In winter, the intrusion of nutrients into the photic zone by intense vertical mixing and strong riverine inputs, produced an increase of the total autotrophic biomass (0.76 mg m⁻³). In the three seasons, the main contributor to total chl a was the picoplanktonic (<2 μm) size fraction (42% in winter and around 60% in autumn and summer). The nanophytoplankton (2–20 μm) contribution to total chl a showed the lowest variability amongst seasons (between 29% and 39%). The microplanktonic (>20 μm) chl a size fraction was higher in winter (27%) than in the other seasons (less than 13%). The maximum total chl a concentrations were found at surface in winter, at depths of 40 m in autumn and between 50 and 80 m in summer. The relative contribution of the <2 μm size fraction at these levels of the water column tended to be higher than at other depths in autumn and winter and lower in summer. In autumn and winter, nutrient inputs from Ebro river discharge and mixing processes resulted in an increase on the >2 μm contribution to total chl a in the coastal zone near the Ebro Delta area. In summer, the contribution of the <2 and >2 μm chl a size fractions was homogeneously distributed through the sampling area. In autumn and summer, when deep chl a maxima were observed, the total amount of the autotrophic biomass in the superficial waters (down to 10 m) of most offshore stations was less than 10% of the whole integrated chl a (down to 100 m or to the bottom). In winter, this percentage increased until 20% or 40%. The >2 μm chl a increased linearly with total chl a values. However, the <2 μm chl a showed a similar linear relationship only at total chl a values lower than 1 mg m⁻³ (in autumn and summer) or 2 mg m⁻³ (winter). At higher values of total chl a, the contribution of the <2 μm size fraction remained below an upper limit of roughly 0.5 mg m⁻³. Our results indicate that the picoplankton fraction of phytoplankton may show higher seasonal and mesoscale variability than is usually acknowledged.     

Seasonal variations in pCO2 and its controlling factors in surface seawater of the northern East China Sea

Surface partial pressure of CO₂ (pCO₂), temperature, salinity, nutrients, and chlorophyll a were measured in the East China Sea (ECS; 31°30′–34°00′N to 124°00′–127°30′E) in August 2003 (summer), May 2004 (spring), October 2004 (early fall), and November 2005 (fall). The warm and saline Tsushima Warm Current was observed in the eastern part of the survey area during four cruises, and relatively low salinity waters due to outflow from the Changjiang (Yangtze River) were observed over the western part of the survey area. Surface pCO₂ ranged from 236 to 445 μatm in spring and summer, and from 326 to 517 μatm in fall. Large pCO₂ (values >400 μatm) occurred in the western part of the study area in spring and fall, and in the eastern part in summer. A positive linear correlation existed between surface pCO₂ and temperature in the eastern part of the study area, where the Tsushima Warm Current dominates; this correlation suggests that temperature is the major factor controlling surface pCO₂ distribution in that area. In the western part of the study area, however, the main controlling factor is different and seasonally complex. There is large transport in this region of Changjiang Diluted Water in summer, causing low salinity and low pCO₂ values. The relationship between surface pCO₂ and water stability suggests that the amount of mixing and/or upwelling of CO₂-rich water might be the important process controlling surface pCO₂ levels during spring and fall in this shallow region. Sea–air CO₂ flux, based on the application of a Wanninkhof [1992. Relationship between wind speed and gas exchange over the ocean. Journal of Geophysical Research 97, 7373–7382] formula for gas transfer velocity and a set of monthly averaged satellite wind data, were −5.04±1.59, −2.52±1.81, 1.71±2.87, and 0.39±0.18 mmol m⁻² d⁻¹ in spring, summer, early fall, and fall, respectively, in the northern ECS. The ocean in this study area is therefore a carbon sink in spring and summer, but a weak source or in equilibrium with the atmosphere in fall. If the winter flux value is assumed to have been the mean of autumnal and vernal values, then the northern ECS absorbs about 0.013 Pg C annually. That result suggests that the northern ECS is a net sink for atmospheric CO₂, a result consistent with previous studies.     

Nutrients,irradiance, and mixing as factors regulating primary production in coastal waters impacted by the Mississippi River plume

Relationships among primary production, chlorophyll, nutrients, irradiance and mixing processes were examined along the salinity gradient in the Mississippi River outflow region. A series of six cruises were conducted during 1988–1992 at various times of year and stages of river discharge. Maximum values of biomass and primary production were typically observed at intermediate salinities and coincided with non-conservative decreases in nutrients along the salinity gradient. Highest values of productivity (>10 gC m⁻² d⁻¹) and biomass (>30 mg chlorophyll a m⁻³) were observed in April 1988, July–August 1990 and April–May 1992; values were lower in March and September 1991. Rates of primary production were apparently constrained by low irradiance and mixing in the more turbid, low salinity regions of the plume, and by nutrient limitation outside the plume. Highest values of primary production occurred at stations where surface nutrient concentrations exhibited large deviations from conservative mixing relationships, indicating that depletion of nutrients was related to phytoplankton uptake. Mixing and advection were important in determining the location and magnitude of primary production maxima and nutrient depletion. In addition to growth within plume surface waters, enhanced growth and/or retention of biomass may have occurred in longer residence time waters at the plume edge and/or beneath the surface plume. Vertical structure of some plume stations revealed the presence of subsurface biomass maxima in intermediate salinity water that was depleted in nutrients presumably by uptake processes. Exchange between subsurface water and the surface plume apparently contributed to the reduction in nutrients at intermediate salinities in the surface layer. DIN (=nitrate+nitrite+ammonium) : PO₄ (=phosphate) ratios in river water varied seasonally, with high values in winter and spring and low values in late summer and fall. Periods of high DIN : PO₄ ratios in river nutrients coincided with cruises when surface nutrient concentrations and their ratios indicated a high probability for P limitation. N limitation was more likely to occur at high salinities and during late summer and fall. Evidence for Si limitation was also found, particularly in spring.     

Cave air ventilation and CO2 outgassing by radon-222 modeling: How fast do caves breathe?

In general, the rate and timing of calcite precipitation is in part affected by variations in cave air CO₂ concentrations. Knowledge of cave ventilation processes is required to quantify the effect variations in CO₂ concentrations have on speleothem deposition rates and thus paleoclimate records. In this study we use radon-222 (²²²Rn) as a proxy of ventilation to estimate CO₂ outgassing from the cave to the atmosphere, which can be used to infer relative speleothem deposition rates. Hollow Ridge Cave, a wild cave preserve in Marianna, Florida, is instrumented inside and out with multiple micro-meteorological sensor stations that record continuous physical and air chemistry time-series data. Our time series datasets indicate diurnal and seasonal variations in cave air ²²²Rn and CO₂ concentrations, punctuated by events that provide clues to ventilation and drip water degassing mechanisms. Average cave air ²²²Rn and CO₂ concentrations vary seasonally between winter (²²²Rn = 50 dpm L^? 1, where 1 dpm L^? 1 = 60 Bq m^? 3; CO₂ = 360 ppmv) and summer (²²²Rn = 1400 dpm L^? 1; CO₂ = 3900 ppmv). Large amplitude diurnal variations are observed during late summer and autumn (²²²Rn = 6 to 581 dpm L^? 1; CO₂ = 360 to 2500 ppmv).We employ a simple first-order ²²²Rn mass balance model to estimate cave air exchange rates with the outside atmosphere. Ventilation occurs via density driven flow and by winds across the entrances which create a ‘venturi’ effect. The most rapid ventilation occurs 25 m inside the cave near the entrance: 45 h^? 1 (1.33 min turnover time). Farther inside (175 m) exchange is slower and maximum ventilation rates are 3 h^? 1 (22 min turnover time). We estimate net CO₂ flux from the epikarst to the cave atmosphere using a CO₂ mass balance model tuned with the ²²²Rn model. Net CO₂ flux from the epikarst is highest in summer (72 mmol m^? 2 day^? 1) and lowest in late autumn and winter (12 mmol m^? 2 day^? 1). Modeled ventilation and net CO₂ fluxes are used to estimate net CO₂ outgassing from the cave to the atmosphere. Average net CO₂ outgassing is positive (net loss from the cave) and is highest in late summer and early autumn (about 4 mol h^? 1) and lowest in winter (about 0.5 mol h^? 1). Modeling of ventilation, net CO₂ flux from the epikarst, and CO₂ outgassing to the atmosphere from cave monitoring time-series can help better constrain paleoclimatic interpretations of speleothem geochemical records.     

Himalayan metamorphic CO2 fluxes: Quantitative constraints from hydrothermal springs

Hot springs in the Marsyandi Valley, Nepal, vent CO₂ sourced from metamorphic fluids that mix with shallow groundwaters before degassing near the Earth's surface. The δ¹³C of spring waters ranges up to + 13‰, while that of the coexisting free gas phase is close to ? 4‰. Empirical and thermodynamic modelling of this isotopic fractionation suggests > 97 ± 1% CO₂ degassing. The calculated minimum total CO₂ degassing in the Marsyandi catchment is 5.4 × 10⁹ mol/yr from a Cl-based estimate of the spring water discharge to the Marsyandi River and the fraction of CO₂ degassed. Extrapolated to the whole of the Himalayas, this implies a probable minimum metamorphic CO₂ flux of 0.9 × 10¹² mol/yr, or ～ 13% of solid Earth CO₂ degassing. The calculated flux is a factor of three greater than the estimated CO₂ drawdown by silicate weathering in the Himalayas. Himalayan metamorphic degassing contributes a significant fraction of the global solid Earth CO₂ flux and implies that metamorphism may cause changes in long-term climate that oppose those resulting from the orogenic forcing of chemical weatherability.     

Seasonal and vertical distributional patterns of siphonophores and medusae in the Chiloé Interior Sea,Chile

The horizontal and vertical distribution of jellyfish was assessed in the Chiloé Inland sea, in the northern area of the Chilean Patagonia. A total of 41 species of cnidarians (8 siphonophores, 31 hydromedusae, 2 scyphomedusae) were collected. Eleven jellyfish species were recorded for the first time in the area. Species richness was higher in spring than in winter (37 vs. 25 species, respectively). Species such as Muggiaea atlantica, Solmundella bitentaculata, and Clytia simplex were extremely abundant in spring. The total abundance (408,157 ind 1000 m^?3) was 18 times higher in spring than in winter (22,406 ind 1000 m^?3).The horizontal distribution of the most abundant species (four in winter, five in spring) showed decreasing abundances in the north–south direction in winter and spring. Peak abundances occurred in the northern microbasins (Reloncaví Fjord, Reloncaví and Ancud gulfs), where the water column stability, phytoplankton and zooplankton abundance were higher, compared with the southern microbasins (Corcovado Gulf, Boca del Guafo). During the spring higher jellyfish abundance season, the vertical distribution of the dominant species (except M. atlantica) showed peak values at mid-depth (30–50 m) and in the deepest sampled layer (50–200 m). This vertical distribution pattern reduced seaward transport in the shallowest layer through estuarine circulation and also limited mortality by predation in the more illuminated shallow layers. Thus, jellyfish were able to remain in the interior waters during the season of maximum biological production.     

A nitrogen fixation estimate for the Baltic Sea based on continuous pCO2 measurements on a cargo ship and total nitrogen data

Organic matter production and nitrogen fixation in the central Baltic Sea were studied on the basis of high-resolution CO₂ partial pressure data that were obtained from an automated measurement system deployed on a cargo ship. The net organic carbon (OC) production was calculated from a surface water CO₂ mass balance and used to estimate the nitrogen uptake by organic matter during the period March to August 2005. It was shown that the net OC production continued despite the exhaustion of dissolved inorganic nitrogen (DIN) after the spring bloom in April. The nitrogen demand for this production was calculated on the basis of the C/N ratio of organic matter. It was of the same order of magnitude than the winter DIN concentration that fuelled the spring bloom. Since the atmospheric DIN deposition was negligible and no indications of alternative DIN sources were found, it was assumed that N₂ fixation had taken place despite the low temperatures (4–8 °C) in April/May. This “cold fixation” amounted to 74 mmol m^?2 whereas a value of 99 mmol m^?2 was obtained for the summer N₂ fixation during June/July. Due to the contribution of the April/May N₂ fixation, a total annual rate (173±35 mmol m^?2) was obtained for 2005 which is considerably higher than presently accepted estimates. These findings were confirmed by a nitrogen budget based on long-term data (1993–2006) for total nitrogen and total phosphorus concentrations. Furthermore, these data revealed a 30% increase in N₂ fixation during the years 1994–2006.     

Eutrophication and warming-driven green tides (Ulva rigida) are predicted to increase under future climate change scenarios

The incidence and severity of extraordinary macroalgae blooms (green tides) are increasing. Here, climate change (ocean warming and acidification) impacts on life history and biochemical responses of a causative green tide species, Ulva rigida, were investigated under combinations of pH (7.95, 7.55, corresponding to lower and higher pCO₂), temperature (14, 18 °C) and nitrate availability (6 and 150 μmol L^? 1). The higher temperature accelerated the onset and magnitude of gamete settlement. Any two factor combination promoted germination and accelerated growth in young plants. The higher temperature increased reproduction, which increased further in combination with elevated pCO₂ or nitrate. Reproductive success was highest (64.4 ± 5.1%) when the upper limits of all three variables were combined. Biochemically, more protein and lipid but less carbohydrate were synthesized under higher temperature and nitrate conditions. These results suggest that climate change may cause more severe green tides, particularly when eutrophication cannot be effectively controlled.     

Seasonal plankton variability in Chilean Patagonia fjords: Carbon flow through the pelagic food web of Aysen Fjord and plankton dynamics in the Moraleda Channel basin

Two research cruises (CIMAR 13 Fiordos) were conducted in the N–S oriented macrobasin of the Moraleda Channel (42–47°S), which includes the E–W oriented Puyuhuapi Channel and Aysen Fjord, during two contrasting productive seasons: austral winter (27 July–7 August 2007) and spring (2–12 November 2007). These campaigns set out to assess the spatio-temporal variability, defined by the local topography along Moraleda Channel, in the biological, physical, and chemical oceanographic characteristics of different microbasins and to quantify the carbon budget of the pelagic trophic webs of Aysen Fjord.Seasonal carbon fluxes and fjord-system functioning vary widely in our study area. In terms of spatial topography, two constriction sills (Meninea and Elefantes) define three microbasins along Moraleda Channel, herein the (1) north (Guafo-Meninea), (2) central (Meninea-Elefantes), and (3) south (Elefantes-San Rafael Lagoon) microbasins. In winter, nutrient concentrations were high (i.e. nitrate range: 21–14 μM) and primary production was low (153–310 mgC m^?2 d^?1), suggesting that reduced light radiation depressed the plankton dynamics throughout Moraleda Channel. In spring, primary production followed a conspicuous N–S gradient, which was the highest (5167 mgC m^?2 d^?1) in the north microbasin and the lowest (742 mgC m^?2 d^?1) in the south microbasin. The seasonal pattern of the semi-enclosed Puyuhuapi Channel and Aysen Fjord, however, revealed no significant differences in primary production (～800 mgC m^?2 d^?1), and vertical fluxes of particulate organic carbon were nearly twice as high in spring as in winter (266 vs. 168 mgC m^?2 d^?1).At the time-series station (St. 79), the lithogenic fraction dominated the total sedimented matter (seston). The role of euphausiids in the biological carbon pump of the Patagonian fjords was evident, given the predominance of zooplankton fecal material, mostly euphausiid fecal strings (46% of all fecal material), among the recognizable particles contributing to the particulate organic carbon flux.The topographic constriction sills partially modulated the exchange of oceanic waters (Subantarctic Surface Water) with freshwater river discharges along the Moraleda Channel. This exchange affects salinity and nutrient availability and, thus, the plankton structure. The north microbasin was dominated by a seasonal alternation of the classical (spring) and microbial (winter) food webs. However, in the south microbasin, productivity was low and the system was dominated year-round by large inputs of glacier-derived, silt-rich freshwater carrying predominantly small-sized diatoms (Skeletonema spp) and bacteria. When superimposed upon this scenario, highly variable (seasonal) solar radiation and photoperiods could exacerbate north–south differences along Moraleda Channel.     

Anomalous CO2 content in the Gallicano thermo-mineral spring (Serchio Valley,Italy) before the 21 June 2013, Alpi Apuane earthquake (M = 5.2)

Since late 2002, a continuous automatic monitoring network is operating in Tuscany, Central Italy, to investigate the geochemical response of selected aquifers to local seismic activity. The monitoring is aimed at identifying possible earthquake geochemical precursors. The network is currently constituted by six stations, all equipped with sensors for the measurement of temperature, pH, redox potential, electrical conductivity, CO₂ and CH₄ dissolved concentration, that have been installed in the areas of highest seismic risk of the region. By combining geochemical data gathered from the automatic station of Gallicano (Garfagnana, Northern Tuscany), and obtained via chemical analyses of spring water samples collected during periodic field surveys in the area surrounding this station, the most significant aspects of the deep fluid circulation paths feeding the Gallicano thermo-mineral system have been investigated, and the geochemical baseline of the Gallicano spring defined. The CO₂ continuous signal recorded by the Gallicano automatic station has been then processed over the period 2003–2013 in the search for anomalies possibly related to local seismic activity. A substantial anomaly in CO₂ content has been observed at Gallicano in conjunction with the Alpi Apuane earthquake (M = 5.2) of 21 June 2013.     

Impact of climate warming on plant growth varied according to the season

Climate changes will promote the growth of invasive aquatic macrophytes. The aim of our study was to analyse the responses of three invasive plants (Elodea canadensis, Egeria densa and Lagarosiphon major) to a temperature increase of 3 °C in spring, summer and winter. At each season we measured four functional traits: the Relative Growth Rate ‘RGR’, Relative Leaf surface area Growth Rate, number of lateral branches and roots. E. canadensis showed the highest growth and vigour in spring and in summer. The Relative Growth Rate was enhanced by an elevation of temperature for E. canadensis and for E. densa. A 3 °C temperature increase stimulated root production for all species in summer, whereas the number of lateral branches was favoured in spring for E. canadensis. Climate warming had a greater impact on plant growth in spring than in summer and late winter. The response of species to climate warming was species-specific and so an increase in temperature should favour the spread of E. canadensis.     

Sources and distribution of organic matter in northern Patagonia fjords,Chile (∼44–47°S): A multi-tracer approach for carbon cycling assessment

We investigated the provenance of organic matter in the inner fjord area of northern Patagonia, Chile (～44–47°S), by studying the elemental (organic carbon, total nitrogen), isotopic (δ¹³C, δ¹⁵N), and biomarker (n-alkanoic acids from vascular plant waxes) composition of surface sediments as well as local marine and terrestrial organic matter. Average end-member values of N/C, δ¹³C, and δ¹⁵N from organic matter were 0.127±0.010, ?19.8±0.3‰, and 9.9±0.5‰ for autochthonous (marine) sources and 0.040±0.018, ?29.3±2.1‰, and 0.2±3.0‰ for allochthonous (terrestrial) sources. Using a mixing equation based on these two end-members, we calculated the relative contribution of marine and terrestrial organic carbon from the open ocean to the heads of fjords close to river outlets. The input of marine-derived organic carbon varied widely and accounted for 13–96% (average 61%) of the organic carbon pool of surface sediments. Integrated regional calculations for the inner fjord system of northern Patagonia covered in this study, which encompasses an area of ～4280 km², suggest that carbon accumulation may account for between 2.3 and 7.8×10⁴ ton C yr^?1. This represents a storage capacity of marine-derived carbon between 1.8 and 6.2×10⁴ ton yr^?1, which corresponds to an assimilation rate of CO₂ by marine photosynthesis between 0.06 and 0.23×10⁶ ton yr^?1. This rate suggests that the entire fjord system of Patagonia, which covers an area of ～240,000 km², may represent a potentially important region for the global burial of marine organic matter and the sequestration of atmospheric CO₂.     

CO2-induced dissolution of low permeability carbonates. Part II: Numerical modeling of experiments

We used the 3D continuum-scale reactive transport models to simulate eight core flood experiments for two different carbonate rocks. In these experiments the core samples were reacted with brines equilibrated with pCO₂ = 3, 2, 1, 0.5 MPa (Smith et al., 2013 [27]). The carbonate rocks were from specific Marly dolostone and Vuggy limestone flow units at the IEAGHG Weyburn-Midale CO₂ Monitoring and Storage Project in south-eastern Saskatchewan, Canada. Initial model porosity, permeability, mineral, and surface area distributions were constructed from micro tomography and microscopy characterization data. We constrained model reaction kinetics and porosity–permeability equations with the experimental data. The experimental data included time-dependent solution chemistry and differential pressure measured across the core, and the initial and final pore space and mineral distribution. Calibration of the model with the experimental data allowed investigation of effects of carbonate reactivity, flow velocity, effective permeability, and time on the development and consequences of stable and unstable dissolution fronts.The continuum scale model captured the evolution of distinct dissolution fronts that developed as a consequence of carbonate mineral dissolution and pore scale transport properties. The results show that initial heterogeneity and porosity contrast control the development of the dissolution fronts in these highly reactive systems. This finding is consistent with linear stability analysis and the known positive feedback between mineral dissolution and fluid flow in carbonate formations. Differences in the carbonate kinetic drivers resulting from the range of pCO₂ used in the experiments and the different proportions of more reactive calcite and less reactive dolomite contributed to the development of new pore space, but not to the type of dissolution fronts observed for the two different rock types. The development of the dissolution front was much more dependent on the physical heterogeneity of the carbonate rock. The observed stable dissolution fronts with small but visible dissolution fingers were a consequence of the clustering of a small percentage of larger pores in an otherwise homogeneous Marly dolostone. The observed wormholes in the heterogeneous Vuggy limestone initiated and developed in areas of greater porosity and permeability contrast, following pre-existing preferential flow paths.Model calibration of core flood experiments is one way to specifically constrain parameter input used for specific sites for larger scale simulations. Calibration of the governing rate equations and constants for Vuggy limestones showed that dissolution rate constants reasonably agree with published values. However the calcite dissolution rate constants fitted to the Marly dolostone experiments are much lower than those suggested by literature. The differences in fitted calcite rate constants between the two rock types reflect uncertainty associated with measured reactive surface area and appropriately scaling heterogeneous distribution of less abundant reactive minerals. Calibration of the power-law based porosity–permeability equations was sensitive to the overall heterogeneity of the cores. Stable dissolution fronts of the more homogeneous Marly dolostone could be fit with the exponent n = 3 consistent with the traditional Kozeny–Carman equation developed for porous sandstones. More impermeable and heterogeneous cores required larger n values (n = 6–8).     

Quantification of seep-related methane gas emissions at Tommeliten,North Sea

Tommeliten is a prominent methane seep area in the Central North Sea. Previous surveys revealed shallow gas-bearing sediments and methane gas ebullition into the water column. In this study, the in situ methane flux at Tommeliten is re-assessed and the potential methane transport to the atmosphere is discussed, with regards to the hydrographic setting and gas bubble modeling. We have compiled previous data, acquired new video and acoustic evidence of gas bubble release, and have measured the methane concentration, and its C-isotopic composition in the water column. Parametric subbottom sonar data reveal the three-dimensional extent of shallow gas and morphologic features relevant for gas migration. Five methane ebullition areas are identified and the main seepage area appears to be 21 times larger than previously estimated. Our video, hydroacoustic, subbottom, and chemical data suggest that ～1.5×10⁶ mol CH₄/yr (～26 tons CH₄/yr) of methane gas is being released from the seepage area of Tommeliten. Methane concentration profiles in the vicinity of the gas seeps show values of up to 268 nM (～100 times background) close to the seafloor. A decrease in δ¹³C-CH₄ values at 40 m water depth indicates an unknown additional biogenic methane source within the well oxygenated thermocline between 30 and 40 m water depth. Numerical modeling of the methane bubbles due to their migration and dissolution was performed to estimate the bubble-derived vertical methane transport, the fate of this methane in the water column, and finally the flux to the atmosphere. Modeling indicates that less than ～4% of the gas initially released at the seafloor is transported via bubbles into the mixed layer and, ultimately, to the atmosphere. However, because of the strong seasonality of mixing in the North Sea, this flux is expected to increase as mixing increases, and almost all of the methane released at the seafloor could be transferred into the atmosphere in the stormy fall and winter time.     

Crustal velocity structure in the southern edge of the Central Alborz (Iran)

Inversion of local earthquake travel times and joint inversion of receiver functions and Rayleigh wave group velocity measurements were used to derive a simple model for the velocity crustal structure beneath the southern edge of the Central Alborz (Iran), including the seismically active area around the megacity of Tehran. The P and S travel times from 115 well-located earthquakes recorded by a dense local seismic network, operated from June to November 2006, were inverted to determine a 1D velocity model of the upper crust. The limited range of earthquake depths (between 2 km and 26 km) prevents us determining any velocity interfaces deeper than 25 km. The velocity of the lower crust and the depth of the Moho were found by joint inversion of receiver functions and Rayleigh wave group velocity data. The resulting P-wave velocity model comprises an upper crust with 3 km and 4 km thick sedimentary layers with P wave velocities (V_p) of ～5.4 and ～5.8 km s^?1, respectively, above 9 km and 8 km thick layers of upper crystalline crust (V_p ～6.1 and ～6.25 km s^?1 respectively). The lower crystalline crust is ～34 km thick (V_p ～ 6.40 km s^?1). The total crustal thickness beneath this part of the Central Alborz is 58 ± 2 km.     

Seasonal variability of primary production in a fjord ecosystem of the Chilean Patagonia: Implications for the transfer of carbon within pelagic food webs

We characterized the seasonal cycle of productivity in Reloncaví Fjord (41°30′S), Chilean Patagonia. Seasonal surveys that included measurements of gross primary production, community respiration, bacterioplankton secondary production, and sedimentation rates along the fjord were combined with continuous records of water-column temperature variability and wind forcing, as well as satellite-derived data on regional patterns of wind stress, sea surface temperatures, and surface chlorophyll concentrations. The hydrography and perhaps fjord productivity respond to the timing and intensity of wind forcing over a larger region. Seasonal changes in the direction and intensity of winds, along with a late-winter improvement in light conditions, may determine the timing of phytoplankton blooms and potentially modulate productivity cycles in the region.Depth-integrated gross primary production estimates were higher (0.4–3.8 g C m^?2 d^?1) in the productive season (October, February, and May), and lower (0.1–0.2 g C m^?2 d^?1) in the non-productive season (August). These seasonal changes were also reflected in community respiration and bacterioplankton production rates, which ranged, respectively, from 0.3 to 4.8 g C m^?2 d^?1 and 0.05 to 0.4 g C m^?2 d^?1 during the productive and non-productive seasons and from 0.05 to 0.6 g C m^?2 d^?1 and 0.05 to 0.2 g C m^?2 d^?1 during the same two periods. We found a strong, significant correlation between gross primary production and community respiration (Spearman, r=0.95; p<0.001; n=12), which suggests a high degree of coupling between the synthesis of organic matter and its usage by the planktonic community. Similarly, strong correlations were found between bacterioplankton secondary production and both gross primary production (Spearman, r=0.7, p<0.05, n=9) and community respiration (Spearman, r=0.8, p<0.05, n=9), indicating that bacterioplankton may be processing an important fraction (8–59%) of the organic matter produced by phytoplankton in Reloncaví Fjord. In winter, bacterial carbon utilization as a percentage of gross primary production was >100%, suggesting the use of allochthonous carbon sources by bacterioplankton when the levels of gross primary production are low. Low primary production rates were associated with a greater contribution of small cells to autotrophic biomass, highlighting the importance of small-sized plankton and bacteria for carbon cycling and fluxes during the less productive winter months. Fecal pellet sedimentation was minimal during this period, also suggesting that most of the locally produced organic carbon is recycled within the microbial loop. During the productive season, on the other hand, the area exhibited a great potential to export organic matter, be it to higher trophic levels or vertically towards the bottom.     

Latitudinal patterns of export production recorded in surface sediments of the Chilean Patagonian fjords (41–55°S) as a response to water column productivity

The Chilean Patagonian fjords region (41–56°S) is characterized by highly complex geomorphology and hydrographic conditions, and strong seasonal and latitudinal patterns in precipitation, freshwater discharge, glacier coverage, and light regime; all of these directly affect biological production in the water column. In this study, we compiled published and new information on water column properties (primary production, nutrients) and surface sediment characteristics (biogenic opal, organic carbon, molar C/N, bulk sedimentary δ¹³C_org) from the Chilean Patagonian fjords between 41°S and 55°S, describing herein the latitudinal pattern of water column productivity and its imprint in the underlying sediments. Based on information collected at 188 water column and 118 sediment sampling sites, we grouped the Chilean fjords into four main zones: Inner Sea of Chiloé (41° to ～44°S), Northern Patagonia (44° to ～47°S), Central Patagonia (48–51°S), and Southern Patagonia (Magellan Strait region between 52° and 55°S). Primary production in the Chilean Patagonian fjords was the highest in spring–summer, reflecting the seasonal pattern of water column productivity. A clear north–south latitudinal pattern in primary production was observed, with the highest average spring and summer estimates in the Inner Sea of Chiloé (2427 and 5860 mg C m^?2 d^?1) and Northern Patagonia (1667 and 2616 mg C m^?2 d^?1). This pattern was closely related to the higher availability of nutrients, greater solar radiation, and extended photoperiod during the productive season in these two zones. The lowest spring value was found in Caleta Tortel, Central Patagonia (91 mg C m^?2 d^?1), a site heavily influenced by glacier meltwater and river discharge loaded with glacial sediments. Biogenic opal, an important constituent of the Chilean fjord surface sediments (Si_OPAL ～1–13%), reproduced the general north–south pattern of primary production and was directly related to water column silicic acid concentrations. Surface sediments were also rich in organic carbon content and the highest values corresponded to locations far away from glacier influence, sites within fjords, and/or semi-enclosed and protected basins, reflecting both autochthonous (water column productivity) and allochthonous sources (contribution of terrestrial organic matter from fluvial input to the fjords). A gradient was observed from the more oceanic sites to the fjord heads (west–east) in terms of bulk sedimentary δ¹³C_org and C/N ratios; the more depleted (δ¹³C_org ?26‰) and higher C/N (23) values corresponded to areas close to rivers and glaciers. A comparison of the Chilean Patagonian fjords with other fjord systems in the world revealed high variability in primary production for all fjord systems as well as similar surface sediment geochemistry due to the mixing of marine and terrestrial organic carbon.     

Mantle-derived carbon in Hercynian granites. Stable isotopes signatures and C/He associations in the thermomineral waters,N-Portugal

Na–HCO₃–CO₂-rich thermomineral waters issue in the N of Portugal, within the Galicia-Trás-os-Montes region, linked to a major NNE-trending fault, the so-called Penacova-Régua-Verin megalineament. Along this tectonic structure different occurrences of CO₂-rich thermomineral waters are found: Chaves hot waters (67 °C) and also several cold (16.1 °C) CO₂-rich waters. The δ²H and δ¹⁸O values of the thermomineral waters are similar to those of the local meteoric waters. The chemical composition of both hot and cold mineral waters suggests that water–rock reactions are mainly controlled by the amount of dissolved CO₂ (g) rather than by the water temperature. Stable carbon isotope data indicate an external CO₂ inorganic origin for the gas. δ¹³C_CO2 values ranging between ? 7.2‰ and ? 5.1‰ are consistent with a two-component mixture between crustal and mantle-derived CO₂. Such an assumption is supported by the ³He/⁴He ratios measured in the gas phase, are between 0.89 and 2.68 times the atmospheric ratio (Ra). These ratios which are higher than that those expected for a pure crustal origin (≈ 0.02 Ra), indicating that 10 to 30% of the He has originated from the upper mantle. Release of deep-seated fluids having a mantle-derived component in a region without recent volcanic activity indicates that extensive neo-tectonic structures originating during the Alpine Orogeny are still active (i.e., the Chaves Depression).