Gas Exchange Parameterization Research Articles

Dissolved gases in the deep North Atlantic track ocean ventilation processes

Gas exchange between the atmosphere and ocean interior profoundly impacts global climate and biogeochemistry. However, our understanding of the relevant physical processes remains limited by a scarcity of direct observations. Dissolved noble gases in the deep ocean are powerful tracers of physical air-sea interaction due to their chemical and biological inertness, yet their isotope ratios have remained underexplored. Here, we present high-precision noble gas isotope and elemental ratios from the deep North Atlantic (~32°N, 64°W) to evaluate gas exchange parameterizations using an ocean circulation model. The unprecedented precision of these data reveal deep-ocean undersaturation of heavy noble gases and isotopes resulting from cooling-driven air-to-sea gas transport associated with deep convection in the northern high latitudes. Our data also imply an underappreciated and large role for bubble-mediated gas exchange in the global air-sea transfer of sparingly soluble gases, including O2, N2, and SF6. Using noble gases to validate the physical representation of air-sea gas exchange in a model also provides a unique opportunity to distinguish physical from biogeochemical signals. As a case study, we compare dissolved N2/Ar measurements in the deep North Atlantic to physics-only model predictions, revealing excess N2 from benthic denitrification in older deep waters (below 2.9 km). These data indicate that the rate of fixed N removal in the deep Northeastern Atlantic is at least three times higher than the global deep-ocean mean, suggesting tight coupling with organic carbon export and raising potential future implications for the marine N cycle.

Gas Fluxes and Steady State Saturation Anomalies at Very High Wind Speeds

AbstractGas exchange at high wind speeds is not well understood—few studies have been conducted at wind speeds above 20 ms−1 and significant disagreement exists between gas exchange models at high wind speeds. In this study, noble gases (He, Ne, Ar, Kr, and Xe) were measured in 35 experiments in the SUSTAIN wind‐wave tank where the wind speeds ranged from 20 to 50 m s−1 and mechanical waves were generated as monochromatic or with a short‐crested JONSWAP frequency spectrum. Bubble size spectra were determined using shadowgraph imagery and wave statistics were measured using a wave wire array. The steady state saturation anomalies and gas fluxes initially increased as wind speeds increased but then leveled off, similar to prior studies of heat and momentum flux coefficients. Noble gas fluxes and steady state saturation anomalies are correlated most strongly with bubble volumes for the less soluble noble gases and with wind speed and wave Reynolds number for the more soluble noble gases. In the JONSWAP experiments, significant wave height was the most important predictor for gas steady state saturation anomalies with correlation coefficients of greater than 0.92 for He, Ne, and Ar (P < 0.05). Furthermore, invasion fluxes were larger than evasion fluxes when other conditions were similar. Taken together, these lab‐based experiments suggest more attention should be paid to parameterizations based on wave characteristics and bubbles and that current wind‐speed based gas exchange parameterizations should not be applied to conditions with very high wind speeds.

A Next Generation Ocean Carbon Isotope Model for Climate Studies I: Steady State Controls on Ocean 13C

AbstractThe 13C/12C of dissolved inorganic carbon (δ13CDIC) carries valuable information on ocean biological C‐cycling, air‐sea CO2 exchange, and circulation. Paleo‐reconstructions of oceanic 13C from sediment cores provide key insights into past as changes in these three drivers. As a step toward full inclusion of 13C in the next generation of Earth system models, we implemented 13C‐cycling in a 1° lateral resolution ocean‐ice‐biogeochemistry Geophysical Fluid Dynamics Laboratory (GFDL) model driven by Common Ocean Reference Experiment perpetual year forcing. The model improved the mean of modern δ13CDIC over coarser resolution GFDL‐model implementations, capturing the Southern Ocean decline in surface δ13CDIC that propagates to the deep sea via deep water formation. Controls on δ13CDIC of the deep‐sea are quantified using both observations and model output. The biological control is estimated from the relationship between deep‐sea Pacific δ13CDIC and phosphate (PO4). The δ13CDIC:PO4 slope from observations is revised to a value of 1.01 ± 0.02‰ (μmol kg−1)−1, consistent with a carbon to phosphate ratio of organic matter (C:Porg) of 124 ± 10. Model output yields a lower δ13CDIC:PO4 than observed due to too low C:Porg. The ocean circulation impacts deep modern δ13CDIC in two ways, via the relative proportion of Southern Ocean and North Atlantic deep water masses, and via the preindustrial δ13CDIC of these water mass endmembers. The δ13CDIC of the endmembers ventilating the deep sea are shown to be highly sensitive to the wind speed dependence of air‐sea CO2 gas exchange. Reducing the coefficient for air‐sea gas exchange following OMIP‐CMIP6 protocols improves significantly surface δ13CDIC relative to previous gas exchange parameterizations.

Evaluating Oceanic Uptake of Atmospheric CCl4: A Combined Analysis of Model Simulations and Observations

AbstractWe provide new estimates of the air‐sea flux of CCl4using simulations from a global ocean biogeochemistry model (NEMO‐PlankTOM) in combination with depth‐resolved CCl4observations from global oceanic databases. Estimates of global oceanic CCl4uptake are derived from a range of model analyses, including prescribed parameterizations using reported values on hydrolysis and degradation, and analyses optimized using the global observational databases. We evaluate the sensitivity of our results to uncertainties in air‐sea gas exchange parameterization, estimation period, and circulation processes. Our best constrained estimate of ocean CCl4uptake for the period 1996–2000 is 20.1 Gg/year (range 16.6–22.7), corresponding to estimates of the partial atmospheric lifetime with respect to ocean uptake of 124 (110–150) years. This new oceanic lifetime implies higher emissions of CCl4than currently estimated and therefore a larger missing atmospheric source of CCl4.

Using Noble Gases to Assess the Ocean's Carbon Pumps.

Natural mechanisms in the ocean, both physical and biological, concentrate carbon in the deep ocean, resulting in lower atmospheric carbon dioxide. The signals of these carbon pumps overlap to create the observed carbon distribution in the ocean, making the individual impact of each pump difficult to disentangle. Noble gases have the potential to directly quantify the physical carbon solubility pump and to indirectly improve estimates of the biological organic carbon pump. Noble gases are biologically inert, can be precisely measured, and span a range of physical properties. We present dissolved neon, argon, and krypton data spanning the Atlantic, Southern, Pacific, and Arctic Oceans. Comparisons between deep-ocean observations and models of varying complexity enable the rates of processes that control the carbon solubility pump to be quantified and thus provide an important metric for ocean model skill. Noble gases also provide a powerful means of assessing air-sea gas exchange parameterizations.

Using Noble Gases to Compare Parameterizations of Air‐Water Gas Exchange and to Constrain Oxygen Losses by Ebullition in a Shallow Aquatic Environment

AbstractAccurate determination of air‐water gas exchange fluxes is critically important for calculating ecosystem metabolism rates from dissolved oxygen in shallow aquatic environments. We present a unique data set of the noble gases neon, argon, krypton, and xenon in a salt marsh pond to demonstrate how the dissolved noble gases can be used to quantify gas transfer processes and evaluate gas exchange parameterizations in shallow, near‐shore environments. These noble gases are sensitive to a variety of physical processes, including bubbling. We thus additionally use this data set to demonstrate how dissolved noble gases can be used to assess the contribution of bubbling from the sediments (ebullition) to gas fluxes. We find that while literature gas exchange parameterizations do well in modeling more soluble gases, ebullition must be accounted for in order to correctly calculate fluxes of the lighter noble gases. In particular, for neon and argon, the ebullition flux is larger than the differences in the diffusive gas exchange flux estimated by four different wind speed‐based parameterizations for gas exchange. We present an application of noble gas derived ebullition rates to improve estimates of oxygen metabolic fluxes in this shallow pond environment. Up to 21% of daily net oxygen production by photosynthesis may be lost from the pond via ebullition during some periods of biologically and physically produced supersaturation. Ebullition could be an important flux of oxygen and other gases that is measurable with noble gases in other shallow aquatic environments.

Using Noble Gas Measurements to Derive Air‐Sea Process Information and Predict Physical Gas Saturations

AbstractDissolved gas distributions are important because they influence oceanic habitats and Earth's climate, yet competing controls by biology and physics make gas distributions challenging to predict. Bubble‐mediated gas exchange, temperature change, and varying atmospheric pressure all push gases away from equilibrium. Here we use new noble gas measurements from the Labrador Sea to demonstrate a technique to quantify physical processes. Our analysis shows that water‐mass formation can be represented by a quasi steady state in which bubble fluxes and cooling push gases away from equilibrium balanced by diffusive gas exchange forcing gases toward equilibrium. We quantify the rates of these physical processes from our measurements, allowing direct comparison to gas exchange parameterizations, and predict the physically driven saturation of other gases. This technique produces predictions that reasonably match N2/Ar observations and demonstrates that physical processes should force SF6 to be ∼6% more supersaturated than CFC‐11 and CFC‐12, impacting ventilation age calculations.

Intense oceanic uptake of oxygen during 2014–2015 winter convection in the Labrador Sea

AbstractMeasurements of near‐surface oxygen (O2) concentrations and mixed layer depth from the K1 mooring in the central Labrador Sea are used to calculate the change in column‐integrated (0–1700 m) O2 content over the deep convection winter 2014/2015. During the mixed layer deepening period, November 2014 to April 2015, the oxygen content increased by 24.3 ± 3.4 mol m−2, 40% higher than previous results from winters with weaker convection. By estimating the contribution of respiration and lateral transport on the oxygen budget, the cumulative air‐sea gas exchange is derived. The O2 uptake of 29.1 ± 3.8 mol m−2, driven by persistent undersaturation (≥5%) and strong atmospheric forcing, is substantially higher than predicted by standard (nonbubble) gas exchange parameterizations, whereas most bubble‐resolving parameterizations predict higher uptake, comparable to our results. Generally large but varying mixed layer depths and strong heat and momentum fluxes make the Labrador Sea an ideal test bed for process studies aimed at improving gas exchange parameterizations.

Numerical investigation of the Arctic ice–ocean boundary layer and implications for air–sea gas fluxes

Abstract. In ice-covered regions it is challenging to determine constituent budgets – for heat and momentum, but also for biologically and climatically active gases like carbon dioxide and methane. The harsh environment and relative data scarcity make it difficult to characterize even the physical properties of the ocean surface. Here, we sought to evaluate if numerical model output helps us to better estimate the physical forcing that drives the air–sea gas exchange rate (k) in sea ice zones. We used the budget of radioactive 222Rn in the mixed layer to illustrate the effect that sea ice forcing has on gas budgets and air–sea gas exchange. Appropriate constraint of the 222Rn budget requires estimates of sea ice velocity, concentration, mixed-layer depth, and water velocities, as well as their evolution in time and space along the Lagrangian drift track of a mixed-layer water parcel. We used 36, 9 and 2 km horizontal resolution of regional Massachusetts Institute of Technology general circulation model (MITgcm) configuration with fine vertical spacing to evaluate the capability of the model to reproduce these parameters. We then compared the model results to existing field data including satellite, moorings and ice-tethered profilers. We found that mode sea ice coverage agrees with satellite-derived observation 88 to 98 % of the time when averaged over the Beaufort Gyre, and model sea ice speeds have 82 % correlation with observations. The model demonstrated the capacity to capture the broad trends in the mixed layer, although with a significant bias. Model water velocities showed only 29 % correlation with point-wise in situ data. This correlation remained low in all three model resolution simulations and we argued that is largely due to the quality of the input atmospheric forcing. Overall, we found that even the coarse-resolution model can make a modest contribution to gas exchange parameterization, by resolving the time variation of parameters that drive the 222Rn budget, including rate of mixed-layer change and sea ice forcings.

Quantifying air-sea gas exchange using noble gases in a coastal upwelling zone

The diffusive and bubble-mediated components of air-sea gas exchange can be quantified separately using time-series measurements of a suite of dissolved inert gases. We have evaluated the performance of four published air-sea gas exchange parameterizations using a five-day time-series of dissolved He, Ne, Ar, Kr, and Xe concentration in Monterey Bay, CA. We constructed a vertical model including surface air-sea gas exchange and vertical diffusion. Diffusivity was measured throughout the cruise from profiles of turbulent microstructure. We corrected the mixed layer gas concentrations for an upwelling event that occurred partway through the cruise. All tested parameterizations gave similar results for Ar, Kr, and Xe; their air-sea fluxes were dominated by diffusive gas exchange during our study. For He and Ne, which are less soluble, and therefore more sensitive to differences in the treatment of bubble-mediated exchange, the parameterizations gave widely different results with respect to the net gas exchange flux and the bubble flux. This study demonstrates the value of using a suite of inert gases, especially the lower solubility ones, to parameterize air-sea gas exchange.

Influence of current velocity and wind speed on air‐water gas exchange in a mangrove estuary

AbstractKnowledge of air‐water gas transfer velocities and water residence times is necessary to study the fate of mangrove derived carbon exported into surrounding estuaries and ultimately to determine carbon balances in mangrove ecosystems. For the first time, the 3He/SF6 dual tracer technique, which has been proven to be a powerful tool to determine gas transfer velocities in the ocean, is applied to Shark River, an estuary situated in the largest contiguous mangrove forest in North America. The mean gas transfer velocity was 3.3 ± 0.2 cm h−1 during the experiment, with a water residence time of 16.5 ± 2.0 days. We propose a gas exchange parameterization that takes into account the major sources of turbulence in the estuary (i.e., bottom generated shear and wind stress).

Upper-ocean gas dynamics from radon profiles in the Eastern Tropical South Pacific

Uncertainties in the dynamics of dissolved gases limit the accuracy of geochemical primary-productivity estimates. Mixed-layer ventilation, entrainment, and upwelling all affect the interpretation of geochemical data, yet they are rarely measured together. We report upper-ocean dissolved-gas dynamics in the Eastern Tropical South Pacific (10–20°S, 80–100°W) obtained from 222Rn distributions from a research cruise in February 2010. Radon-222-based surface ventilation measurements at seven stations were compared with gas transfer velocities calculated using empirical wind-speed parameterizations. Good agreement between the two methods was observed across different wind data products (ASCAT scatterometer and NCEP/NCAR meterological reanalysis) and grid scales (0.5°×0.5°, 1°×1°, and 2°×2°). Average wind speeds in the region were 5–8ms−1, corresponding to in situ gas transfer velocities of 1–3md−1. Averaged over the region, most recent gas exchange parameterizations performed similarly, and the results are robust with respect to both wind data products and grid scales. Non-steady-state conditions were observed at one station, possibly caused by a recent internal wave. These results suggest that 222Rn could be used to support measurements of biogeochemical O2 and CO2 fluxes in the upper ocean.

A neural network-based estimate of the seasonal to inter-annual variability of the Atlantic Ocean carbon sink

Abstract. The Atlantic Ocean is one of the most important sinks for atmospheric carbon dioxide (CO2), but this sink has been shown to vary substantially in time. Here we use surface ocean CO2 observations to estimate this sink and the temporal variability from 1998 through 2007 in the Atlantic Ocean. We benefit from (i) a continuous improvement of the observations, i.e. the Surface Ocean CO2 Atlas (SOCAT) v1.5 database and (ii) a newly developed technique to interpolate the observations in space and time. In particular, we use a two-step neural network approach to reconstruct basin-wide monthly maps of the sea surface partial pressure of CO2 (pCO2) at a resolution of 1° × 1°. From those, we compute the air–sea CO2 flux maps using a standard gas exchange parameterization and high-resolution wind speeds. The neural networks fit the observed pCO2 data with a root mean square error (RMSE) of about 10 μatm and with almost no bias. A check against independent time-series data and new data from SOCAT v2 reveals a larger RMSE of 22.8 μatm for the entire Atlantic Ocean, which decreases to 16.3 μatm for data south of 40° N. We estimate a decadal mean uptake flux of −0.45 ± 0.15 Pg C yr−1 for the Atlantic between 44° S and 79° N, representing the sum of a strong uptake north of 18° N (−0.39 ± 0.10 Pg C yr−1), outgassing in the tropics (18° S–18° N, 0.11 ± 0.07 Pg C yr−1), and uptake in the subtropical/temperate South Atlantic south of 18° S (−0.16 ± 0.06 Pg C yr−1), consistent with recent studies. The strongest seasonal variability of the CO2 flux occurs in the temperature-driven subtropical North Atlantic, with uptake in winter and outgassing in summer. The seasonal cycle is antiphased in the subpolar latitudes relative to the subtropics largely as a result of the biologically driven winter-to-summer drawdown of CO2. Over the 10 yr analysis period (1998 through 2007), sea surface pCO2 increased faster than that of the atmosphere in large areas poleward of 40° N, while in other regions of the North Atlantic the sea surface pCO2 increased at a slower rate, resulting in a barely changing Atlantic carbon sink north of the Equator (−0.01 ± 0.02 Pg C yr−1 decade−1). Surface ocean pCO2 increased at a slower rate relative to atmospheric CO2 over most of the Atlantic south of the Equator, leading to a substantial trend toward a stronger CO2 sink for the entire South Atlantic (−0.14 ± 0.02 Pg C yr−1 decade−1). In contrast to the 10 yr trends, the Atlantic Ocean carbon sink varies relatively little on inter-annual timescales (±0.04 Pg C yr−1; 1 σ).

Surface ocean p CO2 seasonality and sea-air CO2 flux estimates for the North American east coast

[1] Underway and in situ observations of surface ocean pCO2, combined with satellite data, were used to develop pCO2 regional algorithms to analyze the seasonal and interannual variability of surface ocean pCO2 and sea-air CO2 flux for five physically and biologically distinct regions of the eastern North American continental shelf: the South Atlantic Bight (SAB), the Mid-Atlantic Bight (MAB), the Gulf of Maine (GoM), Nantucket Shoals and Georges Bank (NS+GB), and the Scotian Shelf (SS). Temperature and dissolved inorganic carbon variability are the most influential factors driving the seasonality of pCO2. Estimates of the sea-air CO2 flux were derived from the available pCO2 data, as well as from the pCO2 reconstructed by the algorithm. Two different gas exchange parameterizations were used. The SS, GB+NS, MAB, and SAB regions are net sinks of atmospheric CO2 while the GoM is a weak source. The estimates vary depending on the use of surface ocean pCO2 from the data or algorithm, as well as with the use of the two different gas exchange parameterizations. Most of the regional estimates are in general agreement with previous studies when the range of uncertainty and interannual variability are taken into account. According to the algorithm, the average annual uptake of atmospheric CO2 by eastern North American continental shelf waters is found to be between −3.4 and −5.4 Tg C yr−1 (areal average of −0.7 to −1.0 mol CO2 m−2 yr−1) over the period 2003–2010.

Net sea–air CO<sub>2</sub> flux uncertainties in the Bay of Biscay based on the choice of wind speed products and gas transfer parameterizations

Abstract. The estimation of sea–air CO2 fluxes is largely dependent on wind speed through the gas transfer velocity parameterization. In this paper, we quantify uncertainties in the estimation of the CO2 uptake in the Bay of Biscay resulting from the use of different sources of wind speed such as three different global reanalysis meteorological models (NCEP/NCAR 1, NCEP/DOE 2 and ERA-Interim), one high-resolution regional forecast model (HIRLAM-AEMet), winds derived under the Cross-Calibrated Multi-Platform (CCMP) project, and QuikSCAT winds in combination with some of the most widely used gas transfer velocity parameterizations. Results show that net CO2 flux estimations during an entire seasonal cycle (September 2002–September 2003) may vary by a factor of ~ 3 depending on the selected wind speed product and the gas exchange parameterization, with the highest impact due to the last one. The comparison of satellite- and model-derived winds with observations at buoys advises against the systematic overestimation of NCEP-2 and the underestimation of NCEP-1. In the coastal region, the presence of land and the time resolution are the main constraints of QuikSCAT, which turns CCMP and ERA-Interim in the preferred options.

Uncertainties in gas exchange parameterization during the SAGE dual-tracer experiment

A dual tracer experiment was carried out during the SAGE experiment using the inert tracers SF 6 and 3He, in order to determine the gas transfer velocity, k, at high wind speeds in the Southern Ocean. Wind speed/gas exchange parameterization is characterised by significant variability and we examine the major measurement uncertainties that contribute to that scatter. Correction for the airflow distortion over the research vessel, as determined by computational fluid dynamics (CFD) modelling, had the effect of increasing the calculated value of k by 30%. On the short time scales of such experiments, the spatial variability of the wind field resulted in differences between ship and satellite QuikSCAT winds, which produced significant differences in transfer velocity. With such variability between wind estimates, the comparison between gas exchange parameterizations from diverse experiments should clearly be made on the basis of the same wind product. Uncertainty in mixed layer depth of ∼10% arose from mixed layer deepening at high wind speed and limited resolution of vertical sampling. However the assumption of equal mixing of the two tracers is borne out by the experiment. Two dual tracer releases were carried out during SAGE, and showed no significant difference in transfer velocities using QuikSCAT winds, despite the differences in wind history. In the SAGE experiment, duration limitation on the development of waves was shown to be an important factor for Southern Ocean waves, despite the presence of long fetches.

Linearity of DMS transfer coefficient with both friction velocity and wind speed in the moderate wind speed range

In many air‐sea gas exchange parameterizations, wind speed is the sole environmental controlling factor for exchange velocity, k. Here we examine also the role of friction velocity (u*) in the mid‐range of wind speeds, using data from the 2007 Deep Ocean Gas Exchange Experiment. We measured dimethyl sulfide (DMS) fluxes and divided these by the interfacial concentration difference, ΔCDMS, to derive k. Computed ks are sensitive to spatial gradients in seawater DMS concentration (DMSw). A linear function with either u* or U10N (wind speed) explains 87% of the variance in k. Lower and higher winds will have different physics and curvature in transitions between ranges. This linearity demonstrates that a simple physical model of wind‐driven stirring is sufficient for DMS in the wind speed range U10N = 4–12 m/s. A non‐linear relationship to wind speed would be superfluous and obscures the physics.

Noble gas constraints on air‐sea gas exchange and bubble fluxes

Air‐sea gas exchange is an important part of the biogeochemical cycles of many climatically and biologically relevant gases including CO2, O2, dimethyl sulfide and CH4. Here we use a three year observational time series of five noble gases (He, Ne, Ar, Kr, and Xe) at the Bermuda Atlantic Time series Study (BATS) site in tandem with a one‐dimensional upper ocean model to develop an improved parameterization for air‐sea gas exchange that explicitly includes separate components for diffusive gas exchange and bubble processes. Based on seasonal timescale noble gas data, this parameterization, which has a 1σ uncertainty of ±14% for diffusive gas exchange and ±29% for bubble fluxes, is more tightly constrained than previous parameterizations. Although the magnitude of diffusive gas exchange is within errors of that of Wanninkhof (1992), a commonly used parameterization, we find that bubble‐mediated exchange, which is not explicitly included by Wanninkhof (1992) or many other formulations, is significant even for soluble gases. If one uses observed saturation anomalies of Ar (a gas with similar characteristics to O2) and a parameterization of gas exchange to calculate gas exchange fluxes, then the calculated fluxes differ by ∼240% if the parameterization presented here is used compared to using the Wanninkhof (1992) parameterization. If instead one includes the gas exchange parameterization in a model, then the calculated fluxes differ by ∼35% between using this parameterization and that of Wanninkhof (1992). These differences suggest that the bubble component should be explicitly included in a range of marine biogeochemical calculations that incorporate air‐sea gas fluxes.

Effect of uncertainties in climatologic wind, ocean pCO2, and gas transfer algorithms on the estimate of global sea‐air CO2 flux

This paper addresses the uncertainties of global sea‐air CO2 flux estimated on the basis of the sea‐air pCO2 differences and sea‐air CO2 gas transfer rate. Uncertainties in the global sea‐air flux estimates are identified from different gas transfer algorithms and monthly wind speeds (NCEP‐2 and SSM/I). The net global sea‐air flux estimates for the contemporary oceans range from 0.9 ± 0.5 Pg‐C a−1 to 1.3 ± 0.8 Pg‐C a−1 for reference year 2000. Including the carbon transferred from rivers in the preindustrial steady oceans, the uptake rates for anthropogenic CO2 range from 1.3 ± 0.6 Pg‐C a−1 to 1.7 ± 0.8 Pg‐C a−1. For all three gas exchange parameterizations used, the difference between global fluxes using SSM/I and NCEP‐2 winds ranged between 3% and 37%. The subtropical regions are the largest sinks of CO2 (–0.8 Pg‐C a−1), and the equatorial Pacific is the largest source (0.4 Pg‐C a−1). The North Pacific (–0.4 Pg‐C a−1) and the North Atlantic (–0.3 Pg‐C a−1) are also significant sinks of CO2. All the remaining regions have relatively weak net yearly exchanges of CO2. Recent estimates (Takahashi et al., 2009) of sea‐air CO2 flux in the ice‐free zone of the Southern Ocean (50°S−62°S) reveal a small mean annual flux (–0.06 Pg‐C a−1) resulting from the cancellation of the summer uptake CO2 flux with the winter release of CO2 caused by deepwater upwelling. Our study results for the same region range from –0.03 to –0.06 Pg‐C a−1, which are within the uncertainties of the estimates.

Reprint of “Seasonal and daily variation of mercury evasion at coastal and off shore sites from the Mediterranean Sea”

Dissolved gaseous mercury (DGM) was measured continuously using two newly developed techniques and a manual technique. The continuous techniques were based on the equilibrium between the aqueous and gaseous phase (DGM = Hg extr / H', Hg extr is the measured mercury concentration in the gas phase, H' is the Henry's Law coefficient at the desired temperature). In order to calculate the annual mercury evasion from the Mediterranean Sea, diurnal and seasonal measurements of DGM, total gaseous mercury in air (TGM), water temperature and wind speed were performed. During August 2003, March–April 2004 and October–November 2004 measurements of these parameters were conducted on board the RV Urania. The continuous measurements of DGM showed a diurnal variation in concentration, at both coastal and off shore sites, with higher concentrations during daytime than nighttime. The concentration difference could be as large as 130 fM between day and night. The degree of saturation was calculated directly from the measurements, S = Hg extr / TGM and was found to vary between the different seasons. The highest average degree of saturation (850%) and the largest variation in saturation (600–1150%) was observed during the summer. The spring showed the lowest variation (260–360%) and the lowest average degree of saturation (320%). The autumn also showed a large variation in saturation (500–1070%) but a lower average (740%) compared to the summer cruise. This might be explained by the temperature difference between the different seasons, since that parameter varied the most. The flux from the sea surface was calculated using the gas exchange model developed by Nightingale et al. [Nightingale, P.D., Malin, G., Law, C.S., Watson, A.J., Liss, P.S., Liddicoat, M.I., Boutin, J., Upstill-Goddard, R. C., 2000. In situ evaluation of air–sea gas exchange parameterization using novel conservative and volatile tracers. Global Biogeochemical Cycles, 14(1):373–387]. The evasion varied between the different seasons with the highest evasion during the autumn, 24.6 pmol m − 2 h − 1 . The summer value was estimated to 22.3 pmol m − 2 h − 1 and the spring to 7.6 pmol m − 2 h − 1 . Using this data the yearly evasion from the Mediterranean Sea surface was estimated to 77 tons.