Nordic seas transit time distributions and anthropogenic CO 2

Transcription

1 Click Here for Full Article JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2009jc005488, 2010 Nordic seas transit time distributions and anthropogenic CO 2 Are Olsen, 1,2 Abdirahman M. Omar, 1,3 Emil Jeansson, 1 Leif G. Anderson, 2 and Richard G. J. Bellerby 1,3 Received 4 May 2009; revised 27 November 2009; accepted 15 December 2009; published 7 May [1] The distribution and inventory of anthropogenic carbon (DIC ant ) in the Nordic seas are determined using the transit time distribution (TTD) approach. To constrain the shape of the TTDs in the Nordic seas, CO 2 is introduced as an age tracer and used in combination with water age estimates determined from CFC 12 data. CO 2 and CFC 12 tracer ages constitute a very powerful pair for constraining the shape of TTDs. The highest concentrations of DIC ant appear in the warm and well ventilated Atlantic water that flows into the region from the south, and concentrations are typically lower moving west into the colder Arctic surface waters. The depth distribution of DIC ant reflects the extent of ventilation in the different areas. The Nordic seas DIC ant inventory for 2002 was constrained to between 0.9 and 1.4 Gt DIC ant, corresponding to 1% of the global ocean DIC ant inventory. The TTD derived DIC ant estimates were compared with estimates derived using four other approaches, revealing significant differences with respect to the TTD derived estimates, which can be related to issues with some of the underlying assumptions of these other approaches. Specifically, the Tracer combining Oxygen, inorganic Carbon and total Alkalinity (TrOCA) method appears to underestimate DIC ant in the Nordic seas, the DC* shortcut and the approach of Jutterström et al. (2008) appear to overestimate DIC ant at most depths in this area, and finally the approach of Tanhua et al. (2007) appears to underestimate Nordic seas DIC ant below 3000 m and overestimate it above 1000 m. Citation: Olsen, A., A. M. Omar, E. Jeansson, L. G. Anderson, and R. G. J. Bellerby (2010), Nordic seas transit time distributions and anthropogenic CO 2, J. Geophys. Res., 115,, doi: /2009jc Introduction [2] The rising concentration of carbon dioxide (CO 2 )in the atmosphere is currently exercising a very significant influence on the evolution of the global climate system [Forster et al., 2007]. The increase in CO 2 is caused by fossil fuel burning, cement production and land use change, while it is dampened by ocean and terrestrial CO 2 uptake. The current annual ocean uptake of anthropogenic carbon is 2.2 ± 0.3 Gt C [Gruber et al., 2009], corresponding to about 25% of the emissions. The current uptake by the land biosphere is of approximately equal magnitude [Manning and Keeling, 2006]. On longer timescales the oceans appear to be the more important sink. For instance, Sabine et al. [2004] estimated the integrated ocean CO 2 sink between the industrial revolution and 1994 to be 118 Gt C, corresponding to 50% of the emitted CO 2 and after considering the magnitude of the emissions in themselves and the atmospheric CO 2 inventory, Sabine et al. [2004] deduced that the land biosphere must have been a net CO 2 source over 1 Bjerknes Centre for Climate Research, Uni Research, Bergen, Norway. 2 Department of Chemistry, Gothenburg University, Gothenburg, Sweden. 3 Geophysical Institute, University of Bergen, Bergen, Norway. Copyright 2010 by the American Geophysical Union /10/2009JC that period. As regard the future, coupled climate carbon cycle model simulations indicate a sustained or increasing ocean sink, while the terrestrial sink may diminish [Friedlingstein et al., 2006]. [3] A sustained ocean sink for anthropogenic carbon (DIC ant ) relies on vertical mixing since this transports water that has been exposed to the present atmosphere from the upper to the deeper ocean while it brings older unexposed water to the surface. This results in a transfer of DIC ant from the surface to the deep ocean, which has the larger volume thus, storage capacity. The Nordic seas (Figure 1) are potentially important in this respect, as they annually generate 6 Sv of overflow water [Hansen and Østerhus, 2000], which is a main source of North Atlantic deep water [Dickson and Brown, 1994]. The overflow water is primarily formed by modification of North Atlantic Water (NAW) that enters from further south as the northward extension of the Gulf Stream, North Atlantic Current and North Atlantic Drift system [Eldevik et al., 2009] and which have high concentrations of DIC ant [Olsen et al., 2006]. [4] However, despite their potential importance, there have only been two dedicated basin scale studies to determine the DIC ant content of waters in the Nordic seas: Chen et al. [1990] and Jutterström et al. [2008]. Chen et al. [1990] presented sections of DIC ant (actually DTCO 2 0, the difference of preformed normalized total carbon in old and new waters), using the data from the Hudson 1982 cruise 1of14

2 Figure 1. Map of the Nordic seas including the sampling positions of the data used in this analysis. The Nordic seas is the ocean area limited by the Greenland Scotland ridge to the south and the Fram Strait to the north. Bathymetry drawn at 250, 1000, 2000, and 3000 m. The lines along 70 N and from Iceland to the northern Greenland Sea indicate the location of the sections plotted in Figure 4. and the method of Chen and Millero [1979]. However, studies have revealed issues with the data [Olsen, 2009a, 2009b] as well as the method [e.g., Shiller, 1981; Chen et al., 1982; Broecker et al., 1985; Chen and Drake, 1986]. In particular, in the Nordic seas one cannot expect the deep waters to be fully devoid of anthropogenic CO 2. The approach of Jutterström et al. [2008] used observed relationships between nitrate, phosphate, and DIC versus CFCs and assumptions on the DIC ant content of CFC free waters to determine DIC ant concentrations in this area. But their method was only applicable in waters colder than 1 C and with salinity lower than 35, which leaves out the NAW in the Norwegian Atlantic Current (NAC), a key feature of the area and which carries DIC ant from further south into the region [Olsen et al., 2006]. This omission implies that their Nordic seas DIC ant inventory estimate of 1.2 Gt C may be biased low. [5] In this paper the transit time distribution (TTD) approach of Hall et al. [2002] is used to determine DIC ant. The method has been employed in several regions, including the North Atlantic [Waugh et al., 2004; Steinfeldt et al., 2009], Arctic Ocean [Tanhua et al., 2009], Indian Ocean [Hall et al., 2004], and Labrador Sea [Terenzi et al., 2007] as well as on the Global Data Analysis Project (GLODAP) [Key et al., 2004] data set [Waugh et al., 2006]. The TTD approach is in principle an extension of the DC* shortcut method of Gruber et al. [1996] which determines DIC ant from water mass ages derived from observations of transient tracers and assuming a single ventilation time for each water parcel. The TTD method takes mixing into account by using the spectrum of waters mass ages found in each water parcel to determine DIC ant. [6] In this contribution we derive the parameters characterizing the TTD in the Nordic seas, determine the distribution and inventory of DIC ant in the area, compare the results of this approach with those of four other widely used approaches, and rationalize the difference that we observe. 2. Data and Methods 2.1. Data [7] The data used in this study were collected at three cruises carried out in the Nordic seas in 2002 and The 2002 cruise of I/B Oden, the 2002 cruise of R/V Knorr, and the 2003 cruise of R/V G.O. Sars. The expocodes for the cruises are 77DN , 316N , and 58GS , respectively. A map of the Nordic seas with the stations occupied on these three cruises is provided in Figure 1. The data have been described by Olsen et al. [2006], Jeansson et al. [2008], and Jutterström et al. [2008] and only a brief summary of their stated precision and accuracy is provided here. The precision of the DIC and alkalinity data (Alk) have been estimated to approximately ±1 mmol kg 1 and accuracy was ensured by analyses of certified reference material (CRM) supplied by A. Dickson, Scripps Institution of Oceanography, USA [Jutterström et al., 2008; Olsen et al., 2006]. Oxygen and CFC data were all obtained with a precision of 1% [Jutterström et al., 2008]. The data from the three cruises are included in the CARINA data synthesis product [Key et al., 2010] and they have been found to be internally consistent [Falck and Olsen, 2010; Jeansson et al., 2010; Olsen et al., 2009; Olsen, 2009a, 2009b], except the CFC 12 data obtained at the G.O. Sars cruise, which should be adjusted by a factor of 0.95 [Jeansson et al., 2010]. This adjustment was applied to the data used in the work presented here. To avoid complications due to the seasonal cycle and the recent decline in the atmospheric CFC concentrations, only data from deeper than 250 m are used for the calculations presented here. 2of14

3 2.2. TTD Method [8] The TTD framework applies to passive tracers with a time dependent surface history. For these, the interior ocean concentrations at location r and time t can be expressed as (assuming steady transport and uniform c 0 over the source region) cðr; tþ ¼ Z 1 0 c 0 ðt ÞGðr;Þd; where c 0 is the time dependent surface history of the tracer in question and G(r, t) is the distribution of transit times (the TTD) at the location. Here we use three passive tracers, CFC 11, CFC 12, and anthropogenic CO 2. For the two former, the surface history was determined using the atmospheric history compiled by Walker et al. [2000], and solubility calculated from temperature and salinity according to Warner and Weiss [1985], assuming a surface saturation of 98% which is consistent with the observations of Jutterström et al. [2008]. The DIC ant history was determined as the difference between the preindustrial equilibrium DIC and that at time t, using updated Law Dome and Mauna Loa atmospheric CO 2 mole fraction records. This requires estimates of preformed alkalinity, Alk 0, and these were obtained from salinity using the relationships specifically determined for the Nordic seas by Nondal et al. [2009]. Preformed silicate and phosphate was set to 8 and 1 mmol kg 1, respectively, typical concentrations of Nordic seas surface water in winter as determined from the CARINA data set [Key et al., 2010; Olafsson and Olsen, 2009]. The CO 2 system calculations were carried out using CO2sys [Lewis and Wallace, 1998] for Matlab [van Heuven et al., 2009] using the constants of Mehrbach et al. [1973] refit by Dickson and Millero [1987]. [9] The TTD is approximated by an inverse Gaussian distribution [Hall et al., 2002; Waugh et al., 2004, 2006] rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! GðÞ ¼ exp ð Þ2 4 2 ; ð2þ where G is the mean age of the water sample, D is the width of the TTD, and t is the age of the water, the transit time. When the ratio between D and G is known the TTD can be determined by using a single transient tracer. For our DIC ant calculation we use observations of CFC 12, combined with an estimate of Nordic seas D/G, derived as described in section Determination of TTD Parameters [10] The ratio between D and G describes the shape of the TTD. Large ratios imply broad TTDs indicating that propagation of surface signals occurs over a wide range of transit times. The smaller the ratio the less mixing and, thus, the narrower range of transit times. The special case D/G =0 indicates that tracer signals are propagated into the ocean interior through pure bulk advection. The ratio, which reflects the degree of ocean mixing, is expected to vary and must be determined on regional scales. [11] To constrain D/G for the Nordic seas we follow the approach of Waugh et al. [2004], i.e., by comparing the ð1þ relationship between tracer ages. Tracer concentrations in themselves could have been compared in order to remove the effect of the nonlinear relationship between concentration and age that is typical for many tracers. However, this approach did not provide any additional information, and we chose to compare the ages to enable direct comparison of our results with those of Waugh et al. [2004]. [12] By the tracer age we mean the age estimate that is obtained by comparing the concentration in seawater with the atmospheric history of the tracer in question cðtþ ¼c 0 ðt Þ; ð3þ where c(t) is the interior concentration, c 0 is the surface concentration history, and t is the tracer age. This assumes that tracer signals are propagated into the ocean interior through pure bulk advection so that a single transit time rather than a distribution of transit times describes the timescale of ocean transport from one location to another, i.e., D/G = 0. Any given D/G value will lead to a specific relationship between tracer ages from tracers with different surface histories, and the true D/G value can be identified by comparing observed tracer age relationships with theoretical ones, i.e., those expected for given D/G values [Waugh et al., 2004]. For the North Atlantic, for instance, Waugh et al. [2004] constrained D/G to 0.75 or larger using this approach. Assuming a D/G of unity has since then become more or less a routine [Waugh et al., 2006; Tanhua et al., 2009]. [13] As shown by Waugh et al. [2003] not all tracer pairs are equally suitable for constraining D/G. Pairs with surface histories that differ in shape results in the strongest constraint. For instance, the pair CFC 11 and CFC 12 does not impose strong constraints on D/G. This is also the case for the Nordic seas, as is illustrated in Figure 2 which compares the observed relationships between Nordic seas t CFC 11 and t CFC 12 values with the theoretical relationships between these tracer ages estimated for different D/G values. The pattern is more or less similar to that observed in North Atlantic data by Waugh et al. [2004, Figure 3a], and it imposes little, if any constraint on the D/G values. In fact, the observed relationships do not appear compatible with any of the theoretical ones for ages less than 25 years. The same was observed in the North Atlantic data presented by Waugh et al. [2004], and they proposed that it was caused by different surface saturations of these two tracers. However, assuming different surface saturation affects the observed and theoretical CFC 11 CFC 12 relationships equally. Therefore, invoking different surface saturations for the two tracers did not make the observed relationships fall at the family of theoretical ones. We therefore believe that this feature may indicate that the data for at least one of the two CFC components are slightly biased ( 5%, section A1). This possibility does not have any large influence on our results, as is fully evaluated in Appendix A. [14] Waugh et al. [2003] evaluated the ability of several tracer pairs to constrain D/G, illustrated in their Figure 8. It is evident from their Figure 8 that CFC 12 and a radioactive tracer with a decay rate similar to the atmospheric CO 2 growth rate is one of the more suitable pairs. This implies that the pair t CFC 12 t CO2 would impose strong constraints on D/G. This strategy is followed in the present study. 3of14

4 Figure 2. Relationship between Nordic seas t CFC 12 and t CFC 11 determined by comparing pcfc (from measured CFC and the equation of Warner and Weiss [1985], assuming 98% saturation) with the atmospheric CFC history of Walker et al. [2000] through equation (3). Solid lines show the theoretical relationships determined for TTDs with D/G ranging from 0.25 to 2 by steps of The D/G = 0.25 curve has been labeled. A D/G = 0 corresponds to t CFC 11 t CFC 12 = 0 over the whole range of t CFC 12. The breaks in the relationships and negative t CFC 11 t CFC 12 values at t CFC 12 of approximately 15 years or less are the result of the recent decline of atmospheric CFC 11 and CFC 12 concentrations. This prohibits determination of a unique tracer age for samples within the range of declining values, and t CFC 11 and t CFC 12 were set to zero in these periods. The period is longer for CFC 11 than for CFC 12. [15] To use CO 2 as an age tracer, i.e., to find the carbon dioxide tracer age, we employ the conceptual framework of the DC* approach of Gruber et al. [1996] for estimating anthropogenic carbon concentrations. This framework assumes pure bulk advection which allows for the separation of observed inorganic carbon concentrations into (1) the water sample s equilibrium concentration when at the surface, (2) the degree of disequilibrium the water sample had when at the surface, which is assumed to be constant over time, and (3) the change of dissolved inorganic carbon concentration that has taken place since the water parcel left the surface, associated with remineralization and calcium carbonate dissolution DIC obs ¼ DIC eq@pco2ðt Þ þ DIC diseq þ DIC bio : ð4þ The DIC eq@pco 2(t t) term holds a time stamp, and this allows for the calculation of the CO 2 tracer age. The approach requires an independent estimate of DIC diseq and has not, as far as we are aware, been described in the literature. The lack of this implementation is a result of the far from homogenous surface distribution of CO 2 saturation degree [e.g., Takahashi et al., 2009]. However, for the Nordic seas this can be circumvented by using the relationship between surface pco 2 and SST identified by Olsen et al. [2003], as will be described in the following. [16] The determination of the carbon disequilibrium, DIC diseq, takes advantage of a fundamental assumption of the TTD, as well as of the DC* approach for calculation of DIC ant, namely that the disequilibrium has remained constant with time. Despite recent observations that indicate otherwise for parts of the Nordic seas over the last two decades [Olsen et al., 2006], this appears to be a reasonable assumption for the region as a whole over the time since the industrial revolution. The effect of assuming otherwise is evaluated in section A1. Now, given that the surface disequilibrium is assumed to be constant, there is no need to propagate it using TTDs, because this method must only be employed for propagation of transients. Thus, if it can be parameterized in terms of conservative properties, this parameterization can be applied to every water sample to get an estimate of the original DIC diseq when at the surface. This enables us to utilize the northern North Atlantic wintertime pco 2 SST relationship [Olsen et al., 2003] to determine DIC diseq since where and DIC diseq ¼ DIC diseq;95 ¼ DIC 95 DIC eq;95 ; DIC eq;95 ¼ f 2 ; Alk 0 ; Si 0 ; PO 0 4 ; Sal; pco atm;95 ð5þ ð6þ DIC 95 ¼ f pco 95 2 ; Alk0 ; Si 0 ; PO 0 4 ; Sal; ; ð7þ where the function is the thermodynamic equations relating the inorganic carbon species. The pco 95 2 was found using the equation determined by Olsen et al. [2003] pco 95 2 ¼ 391:13 8:71 0:362 þ 0:11 3 e 0:0423ð 5Þ ð8þ and pco 2 atm,95 was determined from the 1995 atmospheric mole fraction according to Dickson et al. [2007] using a pressure of hpa, and in situ and salinity. [17] The impact of remineralization and CaCO 3 dissolution on DIC, DDIC bio, was determined as DIC bio ¼ r C:O2 AOU 1 2 Alk Alk0 þ r N:O2 AOU ; ð9þ where r C:O2 and r N:O2 are the carbon to oxygen and nitrogen to oxygen remineralization ratios, respectively, and AOU is the apparent oxygen utilization. The remineralization ratios derived by Körtzinger et al. [2001] were used, and the sensitivity of the calculations to the choice of r C:O2 and r N:O2 is quantified in section A1. For AOU the following expression was used AOU ¼ O sat 2 O obs 2 O 2 ; ð10þ where O 2 sat is the oxygen saturation concentration and O 2 obs is the observed oxygen concentration. The term DO 2 is the surface disequilibrium at the time of subsurface water mass formation, which is winter. As defined here, positive values mean undersaturation. Wintertime Nordic seas DO 2 is significant and must be accounted for. For instance, the sim- 4of14

5 Figure 3. Relationship between Nordic seas t CFC 12 and t CO2, shown along with the theoretical relationships (solid lines) for TTDs with D/G ranging from 0.25 to 2 by steps of Every other curve has been labeled with its D/G. ulated disequilibrium [Ito et al., 2004] in the Nordic seas is between 0 and 20 mmol l 1 (from their Figure 1), and was attributed to fast heat loss with oxygen uptake lagging. Observations confirm the simulation of Ito et al. [2004]. At OWS M at 66 N and 2 E, Falck and Gade [1999] estimated a mean disequilibrium ranging from approximately 10 mmol l 1 in January to 5 mmol l 1 in March (from their Figure 3) and in the Barents Sea, the study of Olsen et al. [2002] revealed disequilibriums of typically between 10 and 20 mmol l 1 during winter. For a better constraint on Nordic seas DO 2 during winter, the CARINA O 2 values [Falck and Olsen, 2010] were examined. Average DO 2 in wintertime Nordic seas surface waters was determined to 15 ± 7 mmol kg 1.An upper temperature cutoff of 1 C was used here in order to avoid unduly influence of Norwegian Atlantic Current waters. Given these observations a DO 2 value of 15 mmol kg 1 is used in equation (10). The sensitivity of our calculations to the value of DO 2 is evaluated in section A1. [18] With DDIC bio and DIC diseq in place, DIC eq@pco 2(t t) is derived using equation (4). The time stamp, t, is extracted by first finding pco ðt Þ eq@pco2 t 2 ¼ f DIC ð Þ ; Alk 0 ; Si 0 ; PO 0 4 ; Sal; ð11þ (t t) then converting this to the corresponding xco 2 and matching this to the atmospheric CO 2 history through equation (3). 3. Results 3.1. TTD Parameters Derived From CFC 12 and CO 2 Ages [19] Figure 3 compares the CO 2 and CFC 12 tracer ages. For waters with t CFC 12 less than 20 years, t CO2 t CFC 12 does not provide enough resolution to determine D/G, neither did t CFC 11 t CFC 12. However, for waters with t CFC 12 greater than 20 years the theoretical relationships are far better separated than those determined for t CFC 11 t CFC 12 (Figure 2), confirming that t CO2 t CFC 12 is a suitable pair for constraining D/G. For instance, unlike t CFC 11 t CFC 12, as well as several of the other tracer pairs considered by Waugh et al. [2004], the theoretical relationships between t CO2 and t CFC 12 clearly resolves differences of D/G values of The observed relationships take on a range of values. To a large extent we believe this is due to uncertainties in the determination of t CO2, which are addressed in section A1, but it may also reflect true variations of the ratio. Important here is that essentially all observed points fall above the theoretical relationship for D/G of 0.5, and this should be considered the lower limit of possible Nordic seas D/G values. Most of the data fall within the space spanned by the theoretical lines for D/G 0.75 and 1.5, and seem centered around unity. An absolute upper limit cannot be determined with these data. Theoretical relationships up to 2.0 have been drawn in Figure 3, but it requires unreasonably high D/G values, e.g., on the order of , to explain some of the points, and their presence are better explained as being the result of uncertainties in the parameters used for determination of t CO2. The Nordic seas TTD thus appear broad, and similar to those determined for the surrounding ocean regions, i.e., North Atlantic [Waugh et al., 2004] and Arctic Ocean [Tanhua et al., 2009], which is not unreasonable. Given these observations 0.5 is considered as the lower limit for probable Nordic seas D to G ratios, 1 the most probable, and 1.5 as the upper limit. The upper limit of 1.5 was in part motivated by Figure 3, and in part by the observations from further south [Waugh et al., 2004]. This range was robust even after consideration of the uncertainties of our approach (section A1) Anthropogenic CO 2 Distribution [20] The distribution of anthropogenic CO 2 in the year 2002, determined from equation (1) and a D/G of unity is shown in Figure 4, along with the contours of potential density (s ). Figure 4a shows a west east section along 70 N and Figure 4b shows a section that goes approximately south north, from the Iceland shelf edge into the Greenland Sea, both of these have been highlighted in Figure 1. In particular, in the section that goes across the Norwegian and Iceland seas (Figure 4a), the distribution of DIC ant follows the density surfaces. The largest concentrations, between 40 and 45 mmol kg 1, are observed in the lightest waters, of s less than which are found above 1000 m in the Norwegian Sea. This is the well ventilated and warm Norwegian Atlantic Current that transports DIC ant into and through the region. Immediately below this, the Arctic Intermediate Waters (AIW) are found, these have potential densities of up to [Aagaard et al., 1985], and DIC ant concentrations span the range of values from 15 to 35 mmol kg 1. The AIW is found at shallower depths in the Iceland Sea, which is one of their formation areas [Blindheim, 1990]. The isolines of DIC ant and s slopes upward into this area, which has lower DIC ant concentrations in the upper 1000 m than the Norwegian Sea. The DIC ant concentration in the deep waters of the Norwegian Sea are between 5 and 15 mmol kg 1, this is lower than the concentrations in the deep Greenland Sea, which are normally between 10 and 15 mmol kg 1 (Figure 4b). This is because the fraction of relatively old deep waters from the Arctic 5of14

6 Figure 4. Sections of Nordic seas DIC ant (mmol kg 1 ) along (a) the section at 70 N and (b) the section from Iceland (at approximately 67 N, 15 W) northeastward to Greenland Sea (to approximately 75 N, 0 E), where it turns northward and ends at almost 79 N (as indicated by the lines in Figure 1). The dots show the sampling locations. Ocean is higher in the Norwegian than in the Greenland Sea. The Greenland Sea is otherwise more vertically homogenous than both the Iceland and Norwegian Sea, reflecting the more extensive convective activity in this area, and in the central Greenland Sea the upper 1000 m have DIC ant concentrations between 30 and 35 mmol kg 1. [21] The distribution of DIC ant in the upper 1000 m of the Nordic seas is governed partly by ventilation time and partly by temperature. The waters in the upper 1000 m in the Greenland Sea are around 5 years older than the North Atlantic Waters (NAW) of the Norwegian Atlantic Current. This age difference explains almost half of the difference in DIC ant concentrations of approximately 10 mmol kg 1. The rest of the difference is explained by the temperature difference with the NAW being 5 10 C warmer than the waters of the Greenland Sea. The Revelle factor is thus lower for the NAW, i.e., the capacity for DIC ant uptake is larger. In addition the alkalinity of NAW is greater, but this effect explains only about 1 mmol kg 1 of the difference in DIC ant. [22] The AIW of the Nordic seas flows over the Greenland Scotland ridge into the deep North Atlantic and is a main source of North Atlantic deep water [Swift et al., 1980; Dickson and Brown, 1994]. To obtain an estimate of the surface to deep water export of DIC ant associated with this process, we combine our DIC ant estimate with a volume flux estimate from the literature. The total flux of cold overflow water across the ridge has been estimated to almost 6Sv[Hansen and Østerhus, 2000]. The mean properties of the water were summarized by Eldevik et al. [2009]. The Denmark Strait overflow water spans the theta and salinity ranges of approximately C and , respectively, while the respective ranges for Faeroe Shetland Channel overflow water are approximately C, and The waters with these properties were identified in our data, and although not marked in Figure 4, they correspond to waters found in the s range of between and and which have DIC ant of typically between 25 and 30 mmol kg 1. Combining these concentrations with the volume flux, gives us an annual mean export of DIC ant from the Nordic seas into the deep North Atlantic of between 0.06 and 0.07 Gt C, which corresponds to 3% of the annual global ocean uptake of DIC ant of 2.2 Gt C [Gruber et al., 2009] Anthropogenic CO 2 Inventory [23] The Nordic seas DIC ant inventory was determined by using the approach described for the Arctic Ocean by Tanhua et al. [2009] which interpolates each DIC ant profile onto 50 m depth intervals using a piecewise cubic Hermite, and then use the topography following mapping scheme described by Davis [1998] and Rhein et al. [2002] to map the interpolated data onto a regular grid. The upper 250 m were not included in our calculations (section 2.1) so we assumed that these were saturated with DIC ant. The mapped column inventories are shown in Figure 5. They range from less than 10 to more than 70 mol m 2 and show a clear dependence on bottom depth (Figure 1). The largest column inventories are found over the deep Greenland, Lofoten and Norwegian Basins, while the smaller are found over the Iceland Plateau, and the continental shelves. However, the distribution over the three deep basins is also modulated by the distribution of water masses and their DIC ant content. The depth of the layer of NAW is deeper in the Lofoten Basin than in the Norwegian Basin [Orvik, 2004], and hence the column inventory is greater over the Lofoten Basin than over the Norwegian Basin. The column inventory is even greater over the Greenland Basin, this is in part because the Greenland Basin is the deepest, but it is also because relatively recently ventilated water masses has penetrated deeper here than in the other two deep basins, as can be appreciated from Figure 4. [24] The total inventory of the Nordic seas and its subregions are provided in Table 1. The limits between the subregions were set as by Jakobsson [2002] who follows International Hydrographic Organization [2001]. The ocean volumes estimated through our mapping routine (Table 1) do not match the volume estimates determined from the International Bathymetric Chart of the Arctic Ocean (IBCAO) exactly [Jakobsson, 2002]. This is caused by the lower resolution of the bathymetry (the 5 min TerrainBase of NOAA/National Geophysical Center) used for the mapping as well as the 50 m layer thickness. 6of14

7 Figure 5. Nordic seas anthropogenic CO 2 column inventory (mol m 2 ). Therefore all inventories have been scaled to the ocean volumes published by Jakobsson [2002] using the simple normalization equation Inv scaled ¼ Invorg V J V org ; ð12þ where Inv org and V org are the inventory and volume estimates determined through the mapping routine and V J is the volume estimates of Jakobsson [2002]. This scaling changed the Greenland Sea inventory estimate by 2%, the Norwegian Sea estimate by 0.8%, and the estimates for the Iceland Sea and Denmark Strait by 0.3% and 10%, respectively. The large relative change of the Denmark Strait estimate is due to its small size and corresponds to only Gt C. These estimates indicate that the Nordic seas contain 1.24 Gt DIC ant. This is approximately 1% of the global ocean DIC ant inventory estimate of Sabine et al. [2004]. Given the estimates of the carbon transport of the overflow waters of 3% of the annual global ocean DIC ant uptake, determined above, the Nordic seas appear more important for mediating surface to deep ocean DIC ant transport than for storage. The small inventory of this area is a consequence of the small volume, corresponding to 0.3% of the global ocean volume. [25] Our estimate of the Nordic seas DIC ant inventory can be combined with the inventory estimate for the Arctic Ocean of 3 Gt DIC ant, determined through a similar approach by Tanhua et al. [2009]. This estimate was for the year 2005, and assuming transient steady state our Nordic seas estimate scaled to 2005 is 1.3 Gt. Adding these numbers we get a year 2005 Arctic Ocean and Nordic seas DIC ant inventory of 4.3 Gt C. This is 2.1 Gt C smaller than the estimate of Sabine et al. [2004] scaled to 2005, of 6.4 Gt C[Tanhua et al., 2009] for these areas. [26] There are four significant sources of uncertainty that affects our DIC ant estimates, as identified in section A2, and thus the Nordic seas inventory estimate: (1) lowering the D/G ratio increases the DIC ant estimates and vice versa, (2) the CFC 12 data we have used are possibly biased high by 5%, which translates into a potential DIC ant bias of between and +1.6 mmol kg 1, depending on depth, (3) a time variant CFC 12 surface saturation will increase the DIC ant estimates, and (4) an increasing air sea CO 2 disequilibrium will reduce the DIC ant estimates. These uncertainties will also affect our Nordic seas anthropogenic carbon inventory estimate. An absolute upper limit of the Nordic seas DIC ant inventory was determined by assuming a D/G of 0.5 and a time variant CFC 12 surface saturation, this gave an inventory of 1.42 Gt C. An absolute lower limit was determined by adjusting all the CFC 12 data down by 5%, applying a D/G of 1.5, and assuming that the air sea CO 2 disequilibrium in the Nordic seas has increased over time. In addition we assumed that the DIC ant concentration in the upper 250 m are the same as the concentrations in the m depth layer, rather than assuming that this layer is saturated with DIC ant as was originally done. This gave an inventory of 0.86 Gt C. Thus our Nordic seas DIC ant inventory estimate of 1.24 Gt C may possibly deviate by 0.38 Gt C and Gt C Comparison With Other Methods [27] Figure 6 compares the TTD derived DIC ant estimates with the estimates derived using the DC* shortcut approach 7of14

8 Table 1. Nordic Seas DIC ant Inventory Estimates a Region V mapped (10 3 km 3 ) V Jakobsson (10 3 km 3 ) DIC ant Inventory (Gt C) Greenland Sea Norwegian Sea Iceland Sea Denmark Strait Total a Determined using a D/G value of 1, a time invariant air sea CO 2 disequilibrium, a surface CFC 12 saturation of 98%, and assuming that the upper 250 m of the water column is saturated with anthropogenic CO 2. See text for a discussion of the uncertainties. V mapped is the volume mapped by our interpolation routines, and V Jakobsson is the volume estimates published by Jakobsson [2002]. [Gruber et al., 1996], the approach of Jutterström et al. [2008], the revised Tracer combining Oxygen, inorganic Carbon and total Alkalinity (TrOCA) approach [Touratier et al., 2007], and the approach of Tanhua et al. [2007] which we have labeled emlre (extended Multi Linear Regression extension). This latter approach requires an estimate of the DIC ant increase over decadal timescales; these were obtained from Olsen et al. [2006]. Unlike another comparison [Vázques Rodríguez et al., 2009], negative DIC ant estimates have not been set to zero. The uncertainty of the TTD derived estimates is given by the gray area, with the upper and lower bounds determined in the same way as for the inventory, i.e., by assuming D/G of 0.5 and timedependent CFC 12 saturation for the upper bound, and by adjusting the CFC 12 data down by 5%, and assuming D/G of 1.5 and time dependent CO 2 surface saturation for the lower. [28] The DIC ant estimates derived using the DC* shortcut approach [Gruber et al., 1996] are larger than the upper bound of the TTD derived estimates, except in the upper 1000 m. This is as expected since this approach is essentially an implementation of the TTD approach with D/G set to zero, i.e., one assumes that the tracer age represents the true and unique age of the water parcel, and the lower the D/G the higher the DIC ant. [29] The DIC ant estimates derived using the approach of Jutterström et al. [2008] are also larger than the ones derived using the TTD approach, except for above 750 m, where the estimates derived with the approach of Jutterström et al. [2008] are lower than the estimate derived using D/G = 1. The lower values in the upper 750 m results from the fact that the method of Jutterström et al. [2008] is not applicable in the Norwegian Atlantic Current, which has large concentrations of DIC ant. The larger values below this, is the result of assumptions employed when a key number for this approach was determined, the DIC ant at the time of zero CFC 11, which is used to determine the intercept of the theoretical CFC 11 DIC regression line. This line is the expected CFC 11 DIC relationship in the absence of DIC ant, and DIC ant is estimated as the difference between observed DIC and values estimated from this theoretical line. The intercept was determined by Jutterström et al. [2008] through evaluating the relationship between calculated CFC 11 and DIC ant at a temperature, salinity, alkalinity, and saturation level typical for the Greenland Sea. For CFC 11 = 0 the DIC ant was found to be 17.6 mmol kg 1. This estimate was subtracted from the observed intercept of mmol kg 1, to give the intercept of the theoretical relationship of mmol kg 1. This approach assumes bulk advective transport, i.e., D/G = 0. Thus Jutterström et al. [2008] overestimates DIC ant at CFC = 0 which leads to a too small intercept of the theoretical CFC 11 DIC relationship. Using TTDs with D/G of 1.0 we get a DIC ant of around 5 mmol kg 1 at the time of zero CFC 12, using this would lower the DIC ant estimates of Jutterström et al. [2008]. Despite of the differences in the DIC ant distribution, the inventory determined by Jutterström et al. [2008] is similar to our TTD derived inventory estimate of 1.24 Gt. This is partially because Jutterström s method overestimates DIC ant at depth while at the same time missing the high concentrations in the NAW, and partly because their inventory estimate does not include the upper 250 m, which holds large amounts of DIC ant. For instance, of our TTDderived inventory estimate of 1.24 Gt DIC ant, 0.35 Gt is found in the upper 250 m. Adding this to the estimate by Jutterström et al. [2008] would increase their inventory to 1.55 Gt DIC ant. [30] The DIC ant estimates derived through the revised TrOCA approach [Touratier et al., 2007] are mainly negative at depth, and this was also the case for estimates (not shown) derived using the original TrOCA approach [Touratier and Goyet, 2004a]. This is unrealistic, and in conflict with the use of anthropogenic CO 2 as an explanation for the high TrOCA values in the Nordic seas observed by Touratier and Goyet [2004b]. The very low DIC ant values derived through the TrOCA approach are in agreement with the TrOCA based estimates of Vázques Rodríguez et al. [2009], which were derived using a subset of the data used in this study. They are also in agreement with the 1990s (their Figure 5), but not 1980s (their Figure 4) estimates of Touratier and Goyet [2004a]. The difference between these Figure 6. Mean profiles of Nordic seas DIC ant determined using the TTD approach (solid lines) with upper and lower bounds determined as described in the text, the emlre, the revised TrOCA, the DC* shortcut approach, and the approach of Jutterström et al. [2008]. The mean profiles were determined by arithmetic bin averaging of the individual profiles into 250 m intervals. 8of14

9 two latter estimates may be due to lower accuracy of the older data. Regardless, the TrOCA approach appears to underestimate DIC ant in the Nordic seas. We believe this happens because the empirical fit to determine TrOCA 0 of Touratier and Goyet [2004a], as well as that of Touratier et al. [2007], overestimates Nordic seas TrOCA 0 values. TrOCA is essentially the carbon version of the tracer NO introduced by Broecker [1974], and besides air sea gas exchange and nitrification/denitrification, their distributions are governed by the same processes in the ocean. Hence, these are quite similar as can be appreciated from Figures 2c and 2f of Touratier and Goyet [2004b], and both tracers also tend to decrease with increasing potential temperature [Touratier and Goyet, 2004a, Figure 2; Broecker, 1974, Figure 2]. This temperature dependency was utilized by Touratier and Goyet [2004a] and in large part by Touratier et al. [2007] for parameterizing the preindustrial distribution of TrOCA, TrOCA 0. However, and analogously to NO, Nordic seas TrOCA 0 are likely to fall below the values expected from the TrOCA 0 mixing line that applies elsewhere. For NO this is in large part due to the lower preformed nitrate concentrations in the Nordic seas [Broecker, 1974, Figure 2] caused by the export production that occurs as the waters travels northward as part of the thermohaline circulation. Export production would also affect DIC, but would be compensated by uptake of atmospheric CO 2. The difference in preformed nitrate that explains the difference in NO between Weddell Sea water and Nordic seas water is 11 mmol kg 1 (from Figure 2 of Broecker [1974]). Using the classical carbon to nitrogen remineralization ratio of 106:16 [Redfield et al., 1963] this translates to a DIC difference of 72 mmol kg 1. Using the set of equations of Touratier and Goyet [2004a] this would translate to a difference in TrOCA 0 values of 86 mmol kg 1 with the Nordic seas values being lower. The difference between our TTD and TrOCA based estimates of DIC ant implies that Nordic seas TrOCA 0 is overestimated by between approximately 12 and 18 mmol kg 1 (1.2*(DIC ant, TTD DIC ant, TrOCA )), much less than the 86 mmol kg 1 expected from export production alone. We believe this reflects the compensating effect of air sea CO 2 exchange. [31] The final approach we have employed here is the emlre [Tanhua et al., 2007]. The DIC ant estimates derived through this approach are at the lower boundary of our TTD derived estimates in deep waters and higher than the upper boundary in surface waters. The emlre method is essentially a scaling of recent changes in DIC ant with the full growth history since preindustrial times. A fundamental assumption here is that the relative DIC ant response to the atmospheric CO 2 perturbation over the last few decades is the same as the relative response to the full atmospheric perturbation. This assumption may be violated in the Nordic seas. At depth the fraction of deep waters from the Arctic Ocean has increased over the last two decades [Blindheim and Rey, 2004; Skjelvan et al., 2008]. It is not unlikely that these relatively older waters carry less DIC ant than locally formed relatively younger Greenland Sea Deep Water. Thus, the deep water DIC ant response to the atmospheric CO 2 growth over the last two decades may be atypically low compared to the full increase. This would cause the emlre method to underestimate DIC ant at depth. As regard the surface waters, the response observed by Olsen et al. [2006] in particular to the south, may be atypically high and in part related to shifts in the ocean circulation associated with changes in the atmospheric circulation pattern [Thomas et al., 2008]. This would cause emlre to overestimate DIC ant in surface waters. 4. Summary and Conclusions [32] We have demonstrated that it is possible to calculate a CO 2 age, which together with CFC 12 ages does indeed constitute a powerful pair for constraining the D/G of transit time distributions, as originally suggested by Waugh et al. [2003]. The number of uncertainties involved in the calculation of the CO 2 age, as treated in section A1, prohibits full exploitation of the potential of this tracer pair, and calls for better parameterizations of wintertime sea surface DO 2 and pco 2 using conservative parameters. Accurate determination of the long term time variation of the air sea CO 2 disequilibrium would also be worthwhile in this context. We were able to constrain Nordic seas D/G to between 0.5 and 1.5, with 1 being the most probable value. [33] A very important, but hitherto not explicitly stated, corollary of the calculations is that the assumptions that the air sea CO 2 disequilibrium is time invariant and that tracer age represents the true water mass age, i.e., fundamental assumptions of the DC* shortcut approach, are mutually exclusive in the Nordic seas. Had these assumptions been consistent with each other, then the t CO2 t CFC 12 estimates of Figure 3 should all fall on the zero line. This is not the case, and the observations can only be explained by invoking transit time distributions, or an overall significantly decreasing air sea CO 2 disequilibrium in the subducting waters of the Nordic seas, the latter is inconsistent with observations over the last twenty years (section A1). [34] The calculations enabled us to determine a Nordic seas anthropogenic CO 2 inventory of 1.24 Gt C for the year 2002, with 0.86 and 1.42 Gt C as the lower and upper bounds, corresponding to 1% of the global ocean inventory [Sabine et al., 2004]. While this number is not large in absolute terms, it reflects the overall high DIC ant concentrations of an area which comprises 0.3% of the global ocean volume. Combined with the estimate of Tanhua et al. [2009] we get a combined Nordic seas and Arctic Ocean anthropogenic CO 2 inventory for the year of 2005 of 4.3 Gt C, 3 4% of the global ocean inventory. [35] The column inventory of the Nordic seas is basically a function of water depth, modulated by the DIC ant concentration and water mass distribution. The concentrations are largest in the NAW in the Norwegian Atlantic Current, which has been exposed to the atmosphere on its way northward from the North Atlantic, and which is relatively warm and thus has the greatest buffer capacity. The transport of DIC ant with this inflow is a key source of DIC ant to the Nordic seas and the present results allows for a rudimentary assessment of the fate of this DIC ant. Olsen et al. [2006] estimated an annual influx of 0.12 Gt DIC ant by combining a DIC ant estimate of 53 mmol kg 1 with a volume flux estimate of 6 Sv of NAW flowing into the Nordic seas. Our DIC ant concentration estimates in the NAW are slightly smaller (Figure 4); 45 mmol kg 1 does not appear unreasonable. This implies an inflow of 0.1 Gt DIC ant y 1, this is 5% of the annual global ocean accumulation. The NAW 9of14

10 circulates and overturn in the Nordic seas and Arctic Ocean, and constitutes the primary source of the overflow waters crossing the Greenland Scotland ridge [Eldevik et al., 2009]. As estimated in section 3.2, these waters transport 0.06 Gt DIC ant y 1 (3% of annual global ocean accumulation) into the deep North Atlantic. Thus, by difference and neglecting any surface outflows, 0.04 Gt ( 2% of the annual global ocean accumulation) of the DIC ant carried by the NAW accumulates in the Nordic seas and Arctic Ocean each year. Appendix A: Uncertainties A1. Uncertainties of the TTD Parameters [36] The calculation of t CO2 (section 2.3) involves a number of assumptions and approximations that introduce uncertainties in the results, the most important are identified as (in random order) (1) the DO 2 value, (2) the wintertime sea surface pco 95 2 estimates, (3) the equation for determination of Alk 0, (4) the carbon nitrogen and carbon oxygen remineralization ratios, and (5) the assumed CO 2 surface saturations. The calculation of t CFC 12 is sensitive to the accuracy of the CFC 12 data and the assumed CFC 12 surface saturation. The sensitivity of our results on probable Nordic seas D/G values to each of these factors is evaluated in the following. [37] The DO 2 value directly influences the AOU estimates, which are used for determination of DDIC bio and thus t CO2. Changing the DO 2 value changes the t CO2 Figure A2. Relationship between Nordic seas t CFC 12 and t CO2 when 10 matm is (a) subtracted or (c) added to the pco 2 estimates from equation (8). (b) The unperturbed values are shown. The lines show theoretical relationships for TTDs with D/G ranging from 0.25 to 2 by steps of The curves for D/G of 0.5 and 1.0 have been labeled. Figure A1. Relationship between Nordic seas t CFC 12 and t CO2 for DO 2 set to (a) 8, (b) 15, and (c) 22 mmol kg 1 shown along with the theoretical relationships for TTDs with D/G ranging from 0.25 to 2 by steps of The curves for D/G of 0.5 and 1.0 have been labeled. estimates and thus the range of probable D/G values. The standard deviation of the Nordic seas winter time DO 2 estimate was 7 mmol kg 1. The effect of this uncertainty on the t CO2 t CFC relationships is illustrated in Figure A1. As the value of DO 2 is increased, the range of probable D/G spans smaller values, and with a DO 2 of 22 mmol kg 1, D/G values of between 0.25 and 1 seems most probable. Based on the available data, the DO 2 value of 15 mmol kg 1 that has been used throughout this work appears to be the best estimate for typical Nordic seas surface winter water and we do not expect that the range of D/G values that has been assumed, i.e., 0.5 to 1.5, is biased. However, it is quite likely that variability of the DO 2 around its mean of 15 mmol kg 1 contributes to the spread of the observed t CO2 t CFC 12 relationships. Ideally the distribution of Nordic seas DO 2 should be better understood and preferably parameterized in terms of conservative parameters. [38] The equation of Olsen et al. [2003] was used for determining pco 2 95 that goes into the DIC diseq term of equation (4). These estimates carry an uncertainty of ±10 matm [Olsen et al., 2003]. The effect on the t CO2 t CFC 12 relationships is illustrated in Figure A2. As the pco 2 95 is lowered by 10 matm, the t CO2 t CFC 12 relationships span lower values of probable D/G, and most values fall within ratios of between 0.25 and 1. The opposite effect is seen when 10 matm is added to the pco 2 95 values computed by equation (8). However, we do not believe that uncertainties of the pco 2 95 estimates has introduced biases in the range of 10 of 14