Helmuth Thomas

The seasonal cycle of carbon dioxide in Baltic Sea surface waters

A detailed analysis of the seasonal cycle of carbon dioxide (CO2) within the surface waters of the Baltic Proper is presented. Based on the data sets of two cruises during winter and summer, a model has been developed interpolating the annual cycles of dissolved inorganic carbon (DIC) and the partial pressure of CO2 (pCO2). The model balances the CO2 budget of the surface waters referring to the processes determining DIC and pCO2 instead of using non-causal parameterisations. The carbonate system of the Baltic Proper is strongly influenced by the input of low salinity waters during spring and summer originating from the Bothnian Sea, the Gulf of Finland and from the Gulf of Riga as well as by the input of high salinity waters during autumn and winter originating from the North Sea. These hydrographic changes are enforced by the biological processes together causing high annual amplitudes of DIC (up to 250 μmol/kg) and of pCO2 (up to 400 μatm). The mean annual uptake of atmospheric CO2 is calculated to 0.9±0.09 mol CO2/(m2 a) and the mean new production of organic matter amounts to 3±0.3 mol C/(m2 a).

At-sea intercomparison of two newly designed underway pCO2 systems — encouraging results

Two newly designed underway systems for the measurement of CO2 partial pressure (pCO2) in seawater and the atmosphere are described. Results of an intercomparison experiment carried out in the North Sea are presented. A remarkable agreement between the two simultaneously measured (pCO2) data sets was observed even though the spatial variability in surface pCO2 was high. The average difference of all l -min averages of the seawater pCO2 was as low as 0.15 μatm with a standard deviation of 1.2 μatm indicating that no systematic difference is present. A closer examination of the profiles shows that differences tend to be highest during maxima of the pCO2 gradient (up to 14 μatm/min). The time constants of both systems were estimated from laboratory experiments to 45 s, respectively, 75 s thus quantitatively underlining their capability of a fast response to pCO2 changes

Dynamics of pCO2 and related air-ice CO2 fluxes in the Arctic coastal zone (Amundsen Gulf, Beaufort Sea)

We present an Arctic seasonal survey of carbon dioxide partial pressure (pCO 2 ) dynamics within sea ice brine and related air-ice CO 2 fluxes. The survey was carried out from early spring to the beginning of summer in the Arctic coastal waters of the Amundsen Gulf. High concentrations of pCO 2 (up to 1834 matm) were observed in the sea ice in early April as a consequence of concentration of solutes in brines, CaCO 3 precipitation and microbial respiration. CaCO 3 precipitation was detected through anomalies in total alkalinity (TA) and dissolved inorganic carbon (DIC). This precipitation seems to have occurred in highly saline brine in the upper part of the ice cover and in bulk ice. As summer draws near, the ice temperature increases and brine pCO 3 shifts from a large supersaturation (1834 matm) to a marked undersaturation (down to almost 0 matm). This decrease was ascribed to brine dilution by ice meltwater, dissolution of CaCO 3 and photosynthesis during the sympagic algal bloom. The magnitude of the CO 2 fluxes was controlled by ice temperature (through its control on brine volume and brine channels connectivity) and the concentration gradient between brine and the atmosphere. However, the state of the ice-interface clearly affects air-ice CO 2 fluxes.

Determination of anthropogenic CO2 in the North Atlantic Ocean using water mass ages and CO2 equilibrium chemistry

Abstract A new method to calculate the anthropogenic CO 2 (ΔDIC ant ) within the water column of the North Atlantic Ocean is presented. The method exploits the equilibrium chemistry of the carbonate system with reference to temperature, salinity and the partial pressure of atmospheric CO 2 ( p CO 2,atm ). ΔDIC ant is calculated with reference to the ventilation ages of water masses derived from tracer data and to the time history of p CO 2,atm . The method is applied to data recorded during the WOCE program on the WHP A1/E transect in the North Atlantic Ocean, where we characterise six key water masses by their relationships of dissolved inorganic carbon (DIC) and apparent oxygen utilisation (AOU). The error in determining ΔDIC ant is reduced significantly by minimising the number of values referred to, especially by avoiding any use of remineralisation ratios of particulate organic matter. The distribution of ΔDIC ant shows highest values of up to 45 μmol kg −1 in the surface waters falling to 28–33 μmol kg −1 in the Irminger Sea west of the Mid-Atlantic Ridge. The eastern basin is imprinted by older water masses revealing decreasing values down to 10 μmol kg −1 ΔDIC ant in the Antarctic Bottom Water. These findings indicate the penetration of the whole water column of the North Atlantic Ocean by anthropogenic CO 2 .

An off-line 3D model of anthropogenic CO2 uptake by the oceans

A key unanswered question in global climate research is the extent to which the oceans absorb and redistribute anthropogenic CO2 (ΔDICant). We apply an offline ocean carbon cycle model, one that uses atmospheric CO2 concentrations and simplified ocean ventilation estimates. We estimate ocean ventilation by introducing the full age distributions of water masses obtained from a global ocean circulation model. The two model cases we use are known to respectively underestimate and overestimate observed ventilation rates, thereby providing upper and lower bounds on CO2 uptake. The error in determining ΔDICant is reduced significantly by minimising the number of tunable parameters. The development of the oceanic ΔDICant inventory is resolved from the beginning of industrialisation to the present time. According to the model, 177Gt anthropogenic carbon have been locked up in the oceans by 1999, corresponding to an annual uptake rate of 3.9GtC*a−1 during 1999. Uncertainties in our estimated oceanic CO2 uptake are discussed.

Vulnerability of Polar Oceans to Anthropogenic Acidification: Comparison of Arctic and Antarctic Seasonal Cycles

Polar oceans are chemically sensitive to anthropogenic acidification due to their relatively low alkalinity and correspondingly weak carbonate buffering capacity. Here, we compare unique CO2 system observations covering complete annual cycles at an Arctic (Amundsen Gulf) and Antarctic site (Prydz Bay). The Arctic site experiences greater seasonal warming (10 vs 3°C), and freshening (3 vs 2), has lower alkalinity (2220 vs 2320 μmol/kg), and lower summer pH (8.15 vs 8.5), than the Antarctic site. Despite a larger uptake of inorganic carbon by summer photosynthesis, the Arctic carbon system exhibits smaller seasonal changes than the more alkaline Antarctic system. In addition, the excess surface nutrients in the Antarctic may allow mitigation of acidification, via CO2 removal by enhanced summer production driven by iron inputs from glacial and sea-ice melting. These differences suggest that the Arctic system is more vulnerable to anthropogenic change due to lower alkalinity, enhanced warming, and nutrient limitation.

Impact of sea-ice processes on the carbonate system and ocean acidification at the ice-water interface of the Amundsen Gulf, Arctic Ocean

[1] From sea-ice formation in November 2007 to onset of ice melt in May 2008, we studied the carbonate system in first-year Arctic sea ice, focusing on the impact of calciumcarbonate (CaCO3) saturation states of aragonite (XAr) and calcite (XCa) at the ice-water interface (UIW). Based on total inorganic carbon (CT) and total alkalinity (AT), and derived pH, CO2, carbonate ion ([CO3 22 ]) concentrations and X, we investigated the major drivers such as brine rejection, CaCO3 precipitation, bacterial respiration, primary production and CO2-gas flux in sea ice, brine, frost flowers and UIW. We estimated large variability in seaice CT at the top, mid, and bottom ice. Changes due to CaCO3 and CO2-gas flux had large impact on CT in the whole ice core from March to May, bacterial respiration was important at the bottom ice during all months, and primary production in May. It was evident that the sea-ice processes had large impact on UIW, resulting in a five times larger seasonal amplitude of the carbonate system, relative to the upper 20 m. During ice formation, [CO2] increased by 30 mmol kg 21 ,[ CO 3 22 ] decreased by 50 mmol kg 21 , and the XAr decreased by 0.8 in the UIW due to CO2-enriched brine from solid CaCO3. Conversely, during ice melt, [CO3 22 ] increased by 90 mmol kg 21 in the UIW, and X increased by 1.4 between March and May, likely due to CaCO3 dissolution and primary production. We estimated that increased ice melt would lead to enhanced oceanic uptake of inorganic carbon to the surface layer.

Remineralization ratios of carbon, nutrients, and oxygen in the North Atlantic Ocean: A field databased assessment

[1] Remineralization ratios of carbon, nutrients, and oxygen have been assessed in the North Atlantic Ocean along the WOCE 1A/E section. The study is based on an extensive field data set comprising dissolved inorganic carbon (DIC), nitrate and nitrite (NO 3/2 ), phosphate (PO 4 ), and oxygen (O 2 ) data as well as hydrographic data. A procedure has been introduced which normalizes DIC data to constant salinity and temperature and corrects for the contamination from anthropogenic CO 2 . The remaining variability on the normalized DIC values (DIC bio ) can be attributed to the remineralization of organic matter. DIC bio can thus be seen as carbon-analogy to the apparent oxygen utilization (AOU). The consecutive evaluation of the remineralization ratios obtains two different regimes separated at the density level p = 1027.7 kg m -3 . In the shallower level the ratios (C/N = 4.5; C/P = 67, AOU/C = 2.0; N:P = 15; AOU:P = 134; AOU/N = 9.0) are shifted toward relatively higher nutrient release and higher oxygen consumption with respect to the Redfield ratios of particulate organic matter (POM). In contrast, in the deeper levels the ratios are shifted toward relatively higher carbon release and lower oxygen demand (C/N = 11; C/P = 152, AOU/C = 0.86; N:P = 13.9; AOU:P = 130; AOU/N = 9.4). The depth integrated inventories of the remineralization products (DIC, NO 3/2 , PO 4 , and AOU) provide water column averaged ratios for the investigation area (C/N = 8.8; C/P = 124, AOU/C = 1.1; N:P = 14.2: AOU:P = 131; AOU/N = 9.3) which imply a higher efficiency of the biological carbon pump in the North Atlantic Ocean than predicted with respect to the elemental composition of POM.

New production enhanced by nutrient supply from non-Redfield remineralisation of freshly produced organic material

Abstract Budgets of dissolved inorganic carbon (DIC), nitrate and phosphate for the euphotic surface layer of the Baltic Sea and the Baltic Intermediate Water (BIW) are estimated for the growing season 1994/1995. These budgets are the bases for the calculation of new production in the euphotic layer based on DIC consumption (net community production) or nitrate and phosphate consumption, respectively. The net community production is on average 1.5 times the new production based on nitrate consumption. It is assumed that the freshly produced particulate organic matter (POM) adheres to the Redfield ratios, but that nitrogen and phosphorus of the POM are preferentially remineralised, enabling primary production to continue when the nutrients are exhausted and thus use DIC to a greater extent than would be estimated by the nitrate consumption alone. Since the BIW is enclosed at the top by a strong thermocline during the summer and at the bottom by the permanent halocline, all changes during the growing season are related to the biology within this layer. The carbon-enriched particulate material of the surface layer is exported to the BIW where it is remineralised resulting in a high DIC surplus during the growing season. In the BIW, the apparent oxygen utilisation (AOU) corresponds to the production of excess DIC, while no corresponding excess in nitrogen or phosphorus is observed. This confirms the export of POM to the BIW, and the preferential remineralisation of nitrogen and phosphorus in the euphotic surface layer. The role of the so-called biological carbon dioxide pump is discussed.

Barium and carbon fluxes in the Canadian Arctic Archipelago

The seasonal and spatial variability of dissolved Barium (Ba) in the Amundsen Gulf,southeastern Beaufort Sea, was monitored over a full year from September 2007 to September 2008. Dissolved Ba displays a nutrient・type behavior: the maximum water column concentration is located below the surface layer. The highest Ba concentrations are typically observed at river mouths, the lowest concentrations are found in water masses of Atlantic origin. Barium concentrations decrease eastward through the Canadian Arctic Archipelago. Barite (BaSO 4 ) saturation is reached at the maximum dissolved Ba concentrations in the subsurface layer, whereas the rest of the water column is undersaturated. A three end‐member mixing model comprising freshwater from sea・ice melt and rivers, as well as upper halocline water, is used to establish their relative contributions to the Ba concentrations in the upper water column of the Amundsen Gulf. Based on water column and riverine Ba contributions, we assess the depletion of dissolved Ba by formation and sinking of biologically bound Ba (bio・Ba), from which we derive an estimate of the carbon export production. In the upper 50 m of the water column of the Amundsen Gulf, riverine Ba accounts for up to 15% of the available dissolved Ba inventory, of which up to 20% is depleted by bio-Ba formation and export. Since riverine inputs and Ba export occur concurrently, the seasonal variability of dissolved Ba in the upper water column is moderate. Assuming a fixed organic carbon to bio・Ba flux ratio, carbon export out of the surface layer is estimated at 1.8 ・ 0.45 mol C m −2 yr −1 . Finally, we propose a climatological carbon budget for the Amundsen Gulf based on recent literature data and our findings, the latter bridging the surface and subsurface water carbon cycles.