Description: The Southern Ocean is the most important area of anthropogenic carbon (Cant) uptake in the world ocean, only rivalled in importance by the North Atlantic Ocean. Significant variability on decadal time-scales in the uptake of Cant in the Southern Ocean has been observed and modelled, likely with consequences for the interior ocean storage of Cant in the region, and implications for the global carbon budget. Here we use eight cruises between 1973 and 2012 to assess decadal variability in Cant storage rates in the southeast Atlantic sector of the Southern Ocean. For this we employed the extended multiple linear regression (eMLR) method. We relate variability in DIC (dissolved inorganic carbon) storage, which is assumed to equal anthropogenic carbon storage, to changes in ventilation as observed from repeat measurements of transient tracers. Within the Antarctic Intermediate Water (AAIW) layer, which is the dominant transport conduit for Cant into the interior ocean, moderate Cant storage rates were found without any clear temporal trend. In Subantarctic Mode Water (SAMW), a less dense water mass found north of the Subantarctic Front and above AAIW, high storage rates of Cant were observed up to about 2005 but lower rates in more recent times. The transient tracer data suggest a significant speed-up of ventilation in the summer warmed upper part of AAIW between 1998 and 2012, which is consistent with the high storage rate of Cant. A shift of more northern Cant storage to more southern storage in near surface waters was detected in the early 2000s. Beneath the AAIW the eMLR method as applied here did not detect significant storage of Cant. However, the presence of the transient tracer CFC-12 all through the water column suggests that some Cant should be present, but at concentrations not reliably quantifiable. The observed temporal variability in the interior ocean seems at a first glance to be out of phase with observed surface ocean Cant fluxes, but this can be explained by the time delay for the surface ocean signal to manifest itself in the interior of the ocean.

Description: The transit time distribution method was applied to dichlorodifluoromethane and sulfur hexafluoride measurements from four cruises to the tropical North Atlantic between 2006 and 2009 in order to estimate anthropogenic carbon (C-ant) concentrations. By assuming an Inverse Gaussian distribution of the transit time distribution the best fit to the data was achieved with the ratio of mean age to width equals 1. Significant differences in the mean age and C-ant concentrations between the equatorial belt (5 degrees S-5 degrees N) and the Guinea dome area (5 degrees-15 degrees N) was found. Mean ages are higher and C-ant concentrations are lower in the Guinea dome area than at same depths, or densities, in the equatorial belt. The mean column inventories in the upper 1200 m are higher by about 3 mol m(-2) in the equatorial belt compared to the Guinea dome area. The mean column inventory of C-ant, for the whole water column, in the tropical Atlantic is 32.2 mol m(-2) (error range: 30.6-45.2 mol m(-2)), which is significantly lower than the previous estimates. The total C-ant inventory in the eastern tropical Atlantic is 2.5 Pg (error range: 2.3-3.5 Pg) for an area of 6 x 10(6) km(2), comprising the Guinea dome region and the equatorial belt. The equatorial belt has 40% higher storage of C-ant compared to the Guinea dome area which reflects the occurrence of relatively young deep waters at the equator, being high in anthropogenic carbon. Our tracer based C-ant estimates were compared to C-ant concentrations calculated with the TrOCA method applied to measurements conducted in 1999. The TrOCA based estimates are significantly higher than our tracer based C-ant estimates. Comparison between tracer measurements in 1999 and the 2006-2009 time-frame revealed possible speed-up of ventilation in the upper water column, increasing the C-ant concentration in this depth range at a faster rate and a C-ant increase of 12.1 mu mol kg(-1) in the tropical surface water was found

Description: During the last decade, two important collections of carbon relevant hydrochemical data have become available: GLODAP and CARINA. These collections comprise a synthesis of bottle data for all ocean depths from many cruises collected over several decades. For a majority of the cruises at least two carbon parameters were measured. However, for a large number of stations, samples or even cruises, the carbonate system is under-determined (i.e., only one or no carbonate parameterwas measured) resulting in data gaps for the carbonate system in these collections. A method for filling these gaps would be very useful, as it would help with estimations of the anthropogenic carbon (Cant) content or quantification of oceanic acidification. The aim of this work is to apply and describe, a 3D moving window multilinear regression algorithm (MLR) to fill gaps in total alkalinity (AT) of the CARINA and GLODAP data collections for the Atlantic. In addition to filling data gaps, the estimated AT values derived from the MLR are useful in quality control of the measurements of the carbonate system, as they can aid in the identification of outliers. For comparison, a neural network algorithm able to performnon-linear predictionswas also designed. The goal herewas to design an alternative approach to accomplish the sametask of filling AT gaps. Bothmethods return internally consistent results, thereby giving confidence in our approach. Highlights: ► Estimation of alkalinity by multilinear regression (MLR) techniques ► Estimation of alkalinity by neural network techniques ► Intercomparison between alkalinity prediction techniques ► Use of Alkalinity estimation for carbon calculations ► Use of alkalinity estimation for quality control of measurements

Description: The accumulation of anthropogenic CO2 in the oceans is altering seawater carbonate chemistry. Investigation and monitoring of the carbonate parameters is therefore necessary to understand potential impacts on ocean ecosystems. Total alkalinity (AT) and dissolved inorganic carbon (CT) were sampled across the Rockall Trough in Feb 2009 (CE0903) and Feb 2010 (CE10002) as part of a baseline study of inorganic carbon chemistry in Irish shelf waters. The results have been compared with data from WOCE surveys A01E (Sept 1991), A01 (Dec 1994), AR24 (Nov 1996) and A24 (June 1997). The 2009 and 2010 datasets provide a snapshot of the biogeochemical parameters which can act as a baseline of inorganic carbon and acidity levels in surface waters of the Rockall Trough in late winter for future comparison since previous surveys in the area have been affected by biological activity. The dataset also offers the possibility to compare decadal changes in subsurface waters. The temporal evolution of anthropogenic carbon (Delta C-ant) between the 1990s and 2010 was evaluated using two separate methods; (i) a comparison of the concentrations of C-T between surveys, after correcting it for remineralisation of organic material and formation and dissolution of calcium carbonate (Delta CT-abio) and (ii) an extended Multiple Linear Regression was used to calculate the Delta C-ant (Delta C-ant(eMLR)). There was an increase in Delta CT-abio and Delta C-ant(eMLR) of 18 +/- 4 mu mol kg(-1) and 19 +/- 4 mu mol kg(-1), respectively, in the subsurface waters between 1991 and 2010, equivalent to a decrease of 0.040 +/- 0.003 pH units over the 19 year period. There was an increase in both Delta CT-abio and Delta C(ant)e(mLR) of 8 +/- 4 mu mol kg(-1) in Labrador Sea Water (LSW) in the Trough between 1991 and 2010, and LSW has acidified by 0.029 +/- 0.002 pH units over the same time period. A reduction in calcite and aragonite saturation states was observed, which may have implications for calcifying organisms in the region

Description: Noble gas radionuclides, including 81Kr (t1/2 = 229,000 years), 85Kr (t1/2 = 10.8 years), and 39Ar (t1/2 = 269 years), possess nearly ideal chemical and physical properties for studies of earth and environmental processes. Recent advances in Atom Trap Trace Analysis (ATTA), a laser-based atom counting method, have enabled routine measurements of the radiokrypton isotopes, as well as the demonstration of the ability to measure 39Ar in environmental samples. Here we provide an overview of the ATTA technique, and a survey of recent progress made in several laboratories worldwide. We review the application of noble gas radionuclides in the geosciences and discuss how ATTA can help advance these fields, specifically: determination of groundwater residence times using 81Kr, 85Kr, and 39Ar; dating old glacial ice using 81Kr; and an 39Ar survey of the main water masses of the oceans, to study circulation pathways and estimate mean residence times. Other scientific questions involving a deeper circulation of fluids in the Earth's crust and mantle are also within the scope of future applications. We conclude that the geoscience community would greatly benefit from an ATTA facility dedicated to this field, with instrumentation for routine measurements, as well as for research on further development of ATTA methods.

Description: Highlights: • D_Ba is investigated along a high resolution and quasi-zonal transect in the MedSea. • The D_Ba content ranges from 38 to 85 nmol kg− 1 with local deep D_Ba maxima reaching up to 172 nmol kg− 1. • The water column is largely undersaturated with respect to barite (0.2 〈 SI 〈 0.6). • The D_Ba distribution is impacted by the large-scale Mediterranean circulation and biogeochemical processes. • Local changes in the D_Ba patterns may be the key to better constrain the C dynamics in the MedSea. Abstract: The dissolved barium (D_Ba) data set for the Mediterranean Sea is here expanded with data from a large-scale transect sampled in April 2011 (M84/3 cruise) at high resolution. A total of 833 seawater samples have been analyzed for D_Ba. Over the basin the D_Ba content ranges from 38 to 85 nmol kg− 1 with local deep D_Ba maxima reaching up to 172 nmol kg− 1. Deep D_Ba maxima are associated with near bottom waters influenced by benthic processes and brine waters. The water column is largely undersaturated with respect to barite (BaSO4, the main phase of particulate biogenic barium P_Ba), with water column barite saturation state ranging between 0.2 and 0.6 over the basin. This new D_Ba dataset shows that the general zonal distribution of D_Ba is impacted by the large-scale Mediterranean circulation, as evidenced by the Levantine Intermediate Water zonal and meridional progression as well as by the eastward flow of surface Atlantic Water. However biogeochemical processes are also at play, as suggested by an elevated D_Ba content of deep waters and by local lower D_Ba contents in intermediate waters. These features could be attributed to active cycling between the particulate and dissolved Ba phases. Since P_Ba barite has been recognized in previous studies as a proxy for particulate organic carbon remineralization at intermediate depths, the significance of local changes in the water column D_Ba patterns may be the key to better constrain the Ba and carbon dynamics in the Mediterranean Sea.

Description: Halogenated transient tracers such as the chlorofluorocarbons CFC-11, CFC-12 and CFC-113 are commonly used in oceanographic studies. These compounds enter the ocean via the atmosphere and their transient atmospheric concentrations make them valuable as oceanic tracers. The trends of rapidly rising atmospheric concentrations of these tracers are however broken, and the oceanic signal becomes increasingly more difficult to decipher. There is a need for a new transient tracer to complement the existing suit of tracers, especially for recently ventilated water masses. One compound that looks promising in this respect is sulphur hexafluoride (SF6), an inert gas whose atmospheric concentration is rising rapidly. In this paper, the use of SF6 as a transient tracer in recently ventilated waters is discussed and the method for determination of SF6 in seawater is described. Tracer data from a section in the Atlantic sector of the Southern Ocean along 6°E, from 60°S to 40°S, occupied during January 1998, are presented. The Antarctic Polar Front, found close to 50°S, was studied with a densely sampled section down to 400 m depth and SF6/CFC-12 ratios are used to deduce ventilation ages and dilution factors. A comparison of apparent ages derived from a variety of tracers is presented together with the uncertainties in these estimates. This work demonstrates that SF6 is a useful and valuable transient tracer for waters ventilated during the last 20 years

Description: The dense overflow across the Denmark Strait is investigated with hydrographic and hydro-chemical data and the water mass composition of the Denmark Strait Overflow Water (DSOW) is determined by multivariate analysis. Hydrographical properties, the transient tracers CFC-11 and CFC-12, oxygen and nutrients are utilized for the water mass definitions. Distribution and characteristics of water masses north of Denmark Strait are described, the important water masses at the sill and the variability on weekly time-scales are discussed, and the entrainment and mixing of water into the overflow plume in the northern Irminger Basin is calculated. The analysis indicates that water masses both from the Nordic Seas and the Arctic Ocean are important for the formation of DSOW. It is found that water masses transported with the East Greenland Current make up about 75% of the overflow at the sill. The overflow at, and shortly south of, the sill is inhomogeneous with a low-salinity component dominated by Polar Intermediate Water. The high-salinity component of the overflow is mainly of Arctic origin. The water mass composition, and the short-term variability for 7 repeats of sections close to the sill are described, and these illustrate that the overflow is in fact a composite of a number of water masses with different formation and transport histories. This indicate that the overflow is a robust feature, but that it responds to variations in the circulation or atmospheric forcing that influences the formation of intermediate and deep water masses within the Arctic Mediterranean and the North Atlantic. At a section about 400 km south of the sill the overflow is well mixed and modified by entrainment of, mainly, Iceland–Scotland Overflow Water and Labrador Sea Water, together constituting 30% of the overflow plume. The entrainment of Middle Irminger Water dominates shortly downstream of the sill, before the overflow plume reaches too deep but the entrainment seems to be intermittent in time.

Description: The Arctic Ocean has wide shelf areas with extensive biological activity including a high primary productivity and an active microbial loop within the surface sediment. This in combination with brine production during sea ice formation result in the decay products exiting from the shelf into the deep basin typically at a depth of about 150 m and over a wide salinity range centered around S not, vert, similar33. We present data from the Beringia cruise in 2005 along a section in the Canada Basin from the continental margin north of Alaska towards the north and from the International Siberian Shelf Study in 2008 (ISSS-08) to illustrate the impact of these processes. The water rich in decay products, nutrients and dissolved inorganic carbon (DIC), exits the shelf not only from the Chukchi Sea, as has been shown earlier, but also from the East Siberian Sea. The excess of DIC found in the Canada Basin in a depth range of about 50–250 m amounts to 90±40 g C m−2. If this excess is integrated over the whole Canadian Basin the excess equals 320±140×1012 g C. The high DIC concentration layer also has low pH and consequently a low degree of calcium carbonate saturation, with minimum aragonite values of 60% saturation and calcite values just below saturation. The mean age of the waters in the top 300 m was calculated using the transit time distribution method. By applying a future exponential increase of atmospheric CO2 the invasion of anthropogenic carbon into these waters will result in an under-saturated surface water with respect to aragonite by the year 2050, even without any freshening caused by melting sea ice or increased river discharge.

Description: The Sulu Sea, a semi-enclosed deep marginal basin, has limited exchange of deep water and poorly constrained ventilation time-scales. Here we use observations of the transient tracer CFC-12 to calculate the ventilation time-scale and the concentration of anthropogenic CO2 (Cant) in the Sulu Sea based on the transient time distribution (TTD) method. We found that CFC-12 has penetrated the entire water column in the Sulu Sea and the mean ages increase with depth monotonically and exceed 400 years below the depth of about 2500 m. The mean ages derived from CFC data in the Sulu Sea below about 500 m are lower than those in the South China Sea (SCS) at the same depth, indicating that the ventilation time of deep water is shorter in the Sulu Sea than the SCS. The mean column inventory of Cant in the Sulu Sea is 39 mol m- 2 (error range (ER): 30-49 mol m- 2) for the deeper part and the estimated total Cant inventory in 2011 is 81 Tg (ER: 66-98 Tg). By applying the Transient Steady State approximation on the Cant data we calculated the lower limit of the diapycnal diffusivity, Kz, to 1.1 × 10- 4 m2 s- 1 (ER: 0.7-1.8 × 10- 4 m2 s- 1) for the Sulu Sea below the surface mixed layer.