ALBERT

Description: Measurements 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.

Description: Oceanic bromoform (CHBr3) is the major source of organic Br to the atmosphere and may be significant for ozone depletion through the contribution of reactive bromine to the upper troposphere and lower stratosphere of the midlatitudes and tropics. We report the first analyses of boundary layer air, surface and deep ocean waters from the tropical Atlantic. The data provide evidence of a source of CHBr3 throughout the tropical open ocean associated with the deep chlorophyll maximum within the tropical thermocline. Equatorial upwelling carries the CHBr3 to the surface, adding to increased concentrations in the equatorial mixed layer and driving oceanic emissions that support locally elevated atmospheric concentrations. In air masses that had crossed the coastal upwelling region off NW Africa even higher atmospheric mixing ratios were measured. The observations suggest a link between climate, wind-driven upwelling, and the supply of Br to the upper atmosphere of the tropics.

Description: Total alkalinity (AT) was measured during the Meteor 51/2 cruise, crossing the Mediterranean Sea from west to east. AT concentrations were high (∼2600 μmol kg−1) and alkalinity-salinity-correlations had negative intercepts. These results are explained by evaporation coupled with high freshwater AT inputs into coastal areas. Salinity adjustment of AT revealed excess alkalinity throughout the water column compared to mid-basin surface waters. Since Mediterranean waters are supersaturated with respect to calcite and aragonite, the excess alkalinity likely reflects alkalinity inputs to coastal areas close to regions of deep and intermediate water formation. An alkalinity budget shows that main alkalinity inputs come from the Black Sea and from rivers, whereas the Strait of Gibraltar is a net sink. The major sink appears to be carbonate sedimentation. The basin-average net calcification rate and CaCO3 sedimentation was estimated to be 0.38 mol m−2 yr−1. The estimated residence time of AT is ∼160 yr.

Description: Time series of hydrographic and transient tracer measurements were used to study the variability of Greenland Sea water mass transformation between 1991 and 2000. Increases in tracer inventories indicate active renewal of Greenland Sea Intermediate Water (GSIW) at a rate of 0.1 to 0.2 Sv (1 Sv = 1 × 106 m3 s−1) (10-year average). A temperature maximum (Tmax) was established at the base of the upper layer (500 m) as a consequence of anomalously strong freshwater input into the near-surface layer at the beginning of the 1990s. Tmax rapidly descended to 1500 m by 1995 followed by a much slower rate of descent. GSIW became warmer and less saline compared to the 1980s. During the deepening phase of Tmax, atmospheric data revealed above-average wind stress curl and oceanic heat loss. In addition, high Arctic Ocean sea-ice export and lack of local sea-ice formation have been documented for that period. A combination of all these factors may have evoked the renewal of GSIW with anomalously freshwater from the upper layers. The Tmax layer established a stability maximum that inhibits vertical exchange between intermediate and deeper waters. Temperature and salinity of deep waters continued to increase at rates of 0.01°C yr−1 and 0.001 yr−1, respectively. However, since 1993, decrease in and homogenization of deep water transient tracer concentrations indicate that renewal occurred predominantly by addition of Arctic Ocean waters. In 2000 the water column (500 m to 3400 m) required an additional 60 W m−2 (110 W m−2) over the annual mean heat loss to restore its heat content to 1989 (1971) values.

Description: Surface seawater pCO2 and related parameters were measured at high frequency onboard the volunteer observing ship M/V Falstaff in the North Atlantic Ocean between 36° and 52°N. Over 90,000 data points were used to produce monthly CO2 fluxes for 2002/2003. The air-sea CO2 fluxes calculated by two different averaging schemes were compared. The first approach used gas transfer velocity determined from wind speed retrieved at the location of the ship and called colocated winds, while for the second approach a monthly averaged gas transfer velocity was calculated from the wind for each grid pixel including the variability in wind. The colocated wind speeds determined during the time of passage do not capture the monthly wind speed variability of the grid resulting in fluxes that were 47% lower than fluxes using the monthly averaged wind products. The Falstaff CO2 fluxes were in good agreement with a climatology using averaged winds. Over the entire region they differed by 2–5%, depending on the time-dependent correction scheme to account for the atmospheric in increase in pCO2. However, locally the flux differences between the ship measurements and the climatology were greater, especially in regions north of 45°N, like the eastern sector. A comparison of two wind speed products showed that the annual CO2 sink is 4% less when using 6 hourly NCEP/NCAR wind speeds compared to the QuikSCAT wind speed data.

Description: Observations and model results both indicate increasing oxygen minimum zones (OMZ) in the tropical oceans. Here we report on record low dissolved oxygen minimum concentrations in the eastern tropical North Atlantic in fall of 2008, with less than 40 mu mol kg(-1) in the core of the OMZ. There we find a deoxygenation rate of similar to 0.5 mu mol kg(-1) a(-1) during the last decades on two repeat sections at 7.5 and 11 degrees N. The potential temperature and salinity in the surface and central water layers increased on both sections compared to previous observations. However, in contrast to the oxygen decrease in the core of the OMZ, increasing oxygen concentrations were observed in the central water layer above the OMZ. The observed deoxygenation was thus restricted to the core of the oxygen minimum layer. It remains unclear whether the vertical expansion of the oxygen minimum represents a long time trend or decadal variations

Description: Biological dinitrogen fixation provides the largest input of nitrogen to the oceans, therefore exerting important control on the ocean’s nitrogen inventory and primary productivity. Nitrogen-isotope data fromocean sediments suggest that the marine-nitrogen inventory has been balanced for the past 3,000 years (ref. 4). Producing a balanced marine-nitrogenbudget based on direct measurements has proved difficult, however, with nitrogen loss exceeding the gain from dinitrogen fixation by approximately 200 TgNyr-1 (refs 5, 6). Here we present data from the Atlantic Ocean and show that the most widely used method of measuring oceanic N2-fixation rates underestimates the contribution of N2-fixing microorganisms (diazotrophs) relative to a newly developed method. Using molecular techniques to quantify the abundance of specific clades of diazotrophs in parallel with rates of 15N2 incorporation into particulate organic matter, we suggest that the difference between N2-fixation rates measured with the established method and those measured with the new method8 can be related to the composition of the diazotrophic community. Our data show that in areas dominated by Trichodesmium, the established method underestimatesN2-fixation rates by an averageof 62%. We also find that the newly developed method yields N2-fixation rates more than six times higher than those from the established method when unicellular, symbiotic cyanobacteria and c-proteobacteria dominate the diazotrophic community. On the basis of average areal rates measured over the Atlantic Ocean, we calculated basin-wide N2-fixation rates of 14+/-1TgNyr-1 and 24+/-1TgNyr-1 for the established and new methods, respectively. If our findings can be extrapolated to other ocean basins, this suggests that the global marine N2-fixation rate derived from direct measurements may increase from 103+/-8TgNyr-1 to 177+/-8TgNyr-1, and that the contribution of N2 fixers other than Trichodesmium is much more significant than was previously thought.

Description: We present full 2004–2005 seasonal cycles of CO2 partial pressure (pCO2) and dissolved oxygen (O2) in surface waters at a time series site in the central Labrador Sea (56.5°N, 52.6°W) and use these data to calculate annual net air-sea fluxes of CO2 and O2 as well as atmospheric potential oxygen (APO). The region is characterized by a net CO2 sink (2.7 ± 0.8 mol CO2 m−2 yr−1) that is mediated to a major extent by biological carbon drawdown during spring/summer. During wintertime, surface waters approach equilibrium with atmospheric CO2. Oxygen changes from marked undersaturation of about 6% during wintertime to strong supersaturation by up to 10% during the spring/summer bloom. Overall, the central Labrador Sea acts as an O2 sink of 10.0 ± 3.1 mol m−2 yr−1. The combined CO2 and O2 sink functions give rise to a sizable APO flux of 13.0 ± 4.0 mol m−2 yr−1 into surface waters of the central Labrador Sea. A mixed layer carbon budget yields a net community production of 4.0 ± 0.8 mol C m−2 during the 2005 productive season about one third of which appears to undergo subsurface respiration in a depth range that is reventilated during the following winter. The timing of the spring bloom is discussed and eddies from the West Greenland Current are thought to be associated with the triggering of the bloom. Finally, we use CO2 and O2 mixed layer dynamics during the 2005 spring bloom to evaluate a suite of prominent wind speed-dependent parameterizations for the gas transfer coefficient. We find very good agreement with those parameterizations which yield higher transfer coefficients at wind speeds above 10 m s−1.

Description: Bromoform (CHBr3) is the largest single source of atmospheric organic bromine and therefore of importance as a source of reactive halogens to the troposphere and lower stratosphere. The sea-to-air flux, originating with macroalgal and planktonic sources, is the main source for atmospheric bromoform. We review bromoform's contribution to atmospheric chemistry, its atmospheric and oceanic distributions and its oceanic sources and sinks. We have reassessed oceanic emissions, based on published aqueous and airborne concentration data, global climatological parameters, and information concerning coastal and biogenic sources. The goals are to attempt an estimate of the global source strength and partly to identify key regions that require further investigation. The sea-to-air flux is spatially and temporally variable with tropical, subtropical and shelf waters identified as potentially important source regions. We obtain an annual global flux of bromoform of ∼10 Gmol Br yr−1 (3–22 Gmol Br yr−1). This estimate is associated with significant uncertainty, arising from data precision and coverage, choice of air-sea exchange parameterizations and model assumptions. Anthropogenic sources of ∼0.3 (to 1.1) Gmol Br yr−1 (as CHBr3) can be locally significant, but are globally negligible. Our estimate of the global oceanic source is three to four times higher than recent estimates based on the modeling of atmospheric sinks. The reasons for this discrepancy could lie with the limited regional and temporal data available and the broad assumptions that underlie our flux calculations. Alternatively, atmospheric sink calculations, often made on the basis of background CHBr3 levels, may neglect the influence of strong but highly localized sources (e.g., from some coastal and shelf regions). The strongly variable and poorly characterized source of CHBr3, together with its short atmospheric lifetime, complicates model-based estimation of the distribution of reactive Br resulting from its atmospheric degradation. An integrated program of marine and atmospheric observations, atmospheric modeling and mechanistic studies of oceanic bromoform production is required to better constrain present and future Br delivery to the atmosphere.

Description: The meridional oceanic transports of dissolved inorganic carbon and oxygen were calculated using six transoceanic sections occupied in the South Atlantic between 11 degrees S and 30 degrees S. The total dissolved inorganic carbon (TCO2) data were interpolated onto conductivity-temperature-depth data to obtain a high-resolution data set, and Ekman, depth-dependent and depth-independent components of the transport were estimated. Uncertainties in the depth-independent velocity distribution were reduced using an inverse model. The inorganic carbon transport between 11 degrees S and 30 degrees S was southward, decreased slightly toward the south, and was -2150 +/- 200 kmol s(-1) (-0.81 +/- 0.08 Gt C yr(-1)) at 20 degrees S. This estimate includes the contribution of net mass transport required to balance the salt transport through Bering Strait. Anthropogenic CO2 concentrations were estimated for the sections. The meridional transport of anthropogenic CO2 was northward, increased toward the north, and was 430 kmol s(-1) (0.16 Gt C yr(-1)) at 20 degrees S. The calculations imply net southward inorganic carbon transport of 2580 kmol s(-1) (1 Gt C yr(-1)) during preindustrial times. The slight contemporary convergence of inorganic carbon between 10 degrees S and 30 degrees S is balanced by storage of anthropogenic CO2 and a sea-to-air flux implying little local divergence of the organic carbon transport. During the preindustrial era, there was significant regional convergence of both inorganic carbon and oxygen, consistent with a sea-to-air gas flux driven by warming. The northward transport of anthropogenic CO2 carried by the meridional overturning circulation represents an important source for anthropogenic CO2 currently being stored within the North Atlantic Ocean.