Rapid increasing trend of CO 2 and ocean acidification in the surface water of the Ulleung Basin , East / Japan Sea inferred from the observations from 1995 to 2004

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Introduction
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Introduction
The IPCC (2007) announced that the anthropogenic CO 2 that has accumulated in the atmosphere since the Industrial Revolution is responsible for the enhanced greenhouse effect.The ocean is the ultimate mobile carbon storage reservoir in the Earth system.The surface ocean absorbs atmospheric CO 2 by means of both physical and biogeochemical processes.In the oceanic carbonate system, the dissolved carbon is transported to the deep ocean by the oceanic carbon pumps and eventually sequestrated in deep-sea sediments.Sabine et al. (2004) reported that about one-third of the CO 2 Figures

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Full huge carbon storage capacity, atmospheric CO 2 is constantly increasing at an unprecedented rate; this disturbs the carbonate system in ways that will make air-sea exchange difficult in the high-CO 2 world of the future.Currently, results from established oceanic time-series stations show that the trend of increasing CO 2 in surface seawater exceeds that of the atmosphere (Bates, 2001;Keeling et al., 2004) and is accelerating ocean acidification (Caldeira and Wickett, 2003;Key et al., 2004;Orr et al., 2005).Ocean carbon uptake is commonly described as a variety of carbon pumps, which operate by thermodynamics (e.g., temperature effects on the solubility of CO 2 ), physical transport (e.g., mixing and advection of water masses carrying various forms of carbon), sinking of soft-tissue (e.g., carbon uptake/release and export by phytoplankton during photosynthesis/respiration), and sinking of carbonate shells (e.g., formation and dissolution of CaCO 3 ) (Volk and Hoffert, 1985).These processes are easily affected, directly or indirectly, by temperature.The world ocean's temperature from the surface to 3000 m increased by 0.06 • C between the mid 1950s and mid 1990s because of an enhanced greenhouse effect (Levitus et al., 2005), and previous works have shown that the trend of increasing global sea surface temperature (SST) is about 0.08-0.14• C decade −1 (Nicholls et al., 1996;Molinari et al., 1997;Casey and Cornillon, 2001).Soaring SST creates stronger stratification between the surface sea (upper mixed layer) and the deep ocean.It might affect the great ocean conveyor system by weakening deep water formation.The warming will also weaken the solubility pump and the biological carbon pump through fortified stratification.Overall, a positive feedback exists between global warming and ocean carbon uptake, thus making the situation worse.Nearly all the ocean's dynamics can be observed and studied in the East/Japan Sea (EJS) despite its relatively small dimensions (Kim and Kim, 1996;Kim et al., 2001;Kang et al., 2003).The Ulleung Basin (UB), located on the western side of the southern EJS, has complex hydrography.The bifurcation of warm current entering through the Korea Strait leaves numerous spin-off mesoscale eddies at the surface, and beneath the surface, a number of cold currents originate from the winter overturning along the Siberian coast of the EJS.Winter mode water formation is occasionally observed (Kim Introduction

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Full  et al., 1991;Seung and Kim, 1993;Talley et al., 2003).The UB supports a high productivity (> 220 g C m −2 yr −1 ) owing to its surface dynamics and coastal upwelling (Yamada et al., 2004;Yoo and Park, 2009).Because the UB has strong solubility pumps and biological pumps, it is an ideal laboratory for ocean carbon uptake studies.For this reason some studies were carried out in the UB (Oh et al., 1999;Choi et al., 2011Choi et al., , 2012)).
In particular, it has been noted that the role of the coastal ocean in the global carbon budget has been underestimated (Wollast, 1998;Borges et al., 2005) and under debate (Cai and Dai, 2004;Thomas et al., 2004).Therefore, the role of the carbon pump in marginal seas should be reconsidered.
To reduce the scientific uncertainty in the prediction of future climate, a better understanding of the carbon flux among various reservoirs, especially the fluxes across the air-sea interface, is required.The Takahashi climatology (Takahashi et al., 2002) utilized all of the available data for modeling but still neglected marginal seas.Here, we like to emphasize the role of the marginal sea, as exemplified by the UB, and compare it to that for the entire ocean.There is no time-series station comparable to BATS or HOT in the EJS.However, we attempted to fill the gap of oceanic CO 2 time series in the UB by interpolation using linear regression and harmonic function analysis of the data from repeated measurements.Although simplified and crude, an attempt to discern the long-term trend of CO 2 uptake and acidification in a marginal sea that operates its own conveyor belt at a much faster time scale than the global ocean is here made for the first time.
2 Experimental methods and data analysis

Experimental methods
Measurements of the partial pressure of CO 2 (pCO 2 ) in the surface water and overlying atmosphere, sea surface temperature (SST), and sea surface salinity (SSS) were Introduction

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Full collected in the UB, the southwestern part of the EJS, were used in this study.The study area was defined as the region from 36-38 • N latitude and 130-133 • E longitude (Fig. 1).
The pCO 2 of surface seawater and the overlying atmosphere was measured by LiCor model 6252 non-dispersive infrared (NDIR) gas analyzer with a two-stage Weiss-type equilibrator.The CO 2 concentration from the NDIR was acquired every 2 s and, after statistical treatment for 1 min, averaged data were obtained and used as the final data.Three different standard gases were used to calibrate the NDIR.Every 12 h, a series of standard gases was analyzed for calibration; one of the standard gases was analyzed every 6 h to check for drift of the machine.Atmospheric and surface seawater pCO 2 were analyzed every 15 min and 45 min on the hour, respectively.Surface seawater pCO 2 was analyzed by equilibrating air with surface seawater fed to the equilibrator.In-situ SST and SSS were measured using thermosalinograph (SeaBird Electronics, Model SBE-21).
Total alkalinity (TA) was determined onboard by a potentiometric titration method using a closed cell (Millero et al., 1993).The TA data were obtained during cruises 9906, 0306, 0405, and 0410 (Table 1).Surface seawater pH values were obtained immediately after sampling by a spectrophotometric method using the indicator dye mcresol purple (Clayton and Byrne, 1993).Determination of surface seawater pH values was carried out during cruises 9906, 0306, 0406, and 0410 (Table 1).

Air-sea flux estimation
The CO 2 flux (mmol C m −2 d −1 ) was estimated from following equation; where k is the gas transfer velocity (cm h −1 ), s is the solubility of CO 2 gas in seawater (mol kg −1 atm −1 ; Weiss, 1974), and ∆f CO 2 is the difference between atmospheric and surface seawater f CO 2 .We choose the formulas for k and the wind speed relationships Introduction

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Full  The secular variation of mean f CO 2 in the surface water and overlying atmosphere of the UB from 1995 to 2004 is shown in Fig. 2. The f CO 2 in the surface seawater ranged from 125 to 472 µatm with a mean value of 348.2 µatm (sd = 41.6), and the f CO 2 from the overlying atmosphere varied from 344 to 393 µatm with a mean value of 369 µatm (sd = 7.8).The f CO 2 in the surface water was lower than the f CO 2 in the atmosphere indicating that the surface seawater was undersaturated to atmospheric CO 2 in this area.
Generally, the secular trend of f CO 2 includes the long-term linear trend as well as the cyclic seasonal variation (Keeling et al., 2004).The long-term trend can be regarded as a linear increase with a constant rate.We determined trends of annual increase for both the f CO sea 2 and the f CO air 2 by applying a linear fit to the time-series data (Fig. 2).The slope of the regression result, which means annual increasing rate, for f CO sea 2 was estimated to be 3.36 µatm yr −1 and that for the f CO air 2 was 1.97 µatm yr −1 .It was assumed that f CO 2 had increased annually in the UB from 1995 through 2004 without any consideration about seasonal variability.To analyze seasonal data, the effect of the annual increase should be excluded.Hence, the data were normalized to the year 1995, when it was the initial time of observation.
The harmonic function, which is the sum of two components defined as the one-year and half-year component, has been used primarily to reconstruct time series data.Since this analysis method was first introduced by Nojiri et al. (1999), harmonic functions have been used extensively for f CO 2 variability studies (Zeng et al., 2002;Lüger Introduction

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Full  et al., 2004;Chierici et al., 2006).We applied the harmonic function analysis to f CO 2 data from the UB to estimate the decadal trend of CO 2 and to separate thermal and non-thermal effects on the f CO 2 .Because harmonic function analysis is a technique for annual data, we used f CO 2 data that had been normalized to the reference year (1995) by assuming a constant annual increase rate (r a ) of 3.36 µatm yr −1 for the surface seawater and 1.97 µatm yr −1 for the atmosphere, respectively.The f CO 2 was normalized to the year 1995 by using the following equation: where f CO in-situ 2 is the measured data, r a is the annual increasing rates in the surface seawater (3.36 µatm yr −1 ) and in the atmosphere (1.97 µatm yr −1 ), and Year is the year during which the data were collected.We subsequently applied the following harmonic function to the normalized f CO 2 : The set of five constant coefficients, C, C 1 , C 2 , C 3 , and C 4 , ensured a satisfactory fit of the harmonic function of Eq. (2) to the observed data (R2 = 0.82 for seawater, R 2 = 0.86 for air), when the constant coefficients were equal to these values: The regression of observed (f CO 2 ) and fitted values (f CO * 2 ) appeared on Fig. 4 and yielded the following: The mean of the differences between f CO 2 and f CO * 2 for the seawater and the atmosphere were 0.63 µatm and −0.15 µatm, respectively.The root mean square deviations were estimated 13.6 µatm (sea) and 2.5 µatm (air), which were smaller than the standard deviation derived from the spatiotemporal averaging data of the UB (41.6 µatm in f CO sea 2 and 7.8 µatm in f CO air 2 ).
The sea-air CO 2 fluxes in 1995 and 2004 were estimated in order to evaluate the effect of the last decadal increasing trend in f CO 2 .The sea-air CO 2 fluxes were evaluated to be −0.95 ± 0.53 mol m −2 yr −1 for 1995 and −0.68 ± 0.49 mol m −2 yr −1 for 2004.This result shows that the UB acts as a carbon sink and its carbon sink efficiency in unit area is almost 1.6 (±0.3) -fold higher than global ocean (−0.51 mol m −2 yr −1 , Takahashi et al., 2002).However the flux had been decreased about 28 % during the last decade.Choi et al. (2012) and Oh et al. (1999) reported that the annual integrated CO 2 flux in this area was −2.47 ± 1.26 mol m −2 yr −1 and −2.2 mol m −2 yr −1 , which were larger than our results.The transiency of their observations could lead overestimation in CO 2 flux because of the complexity of the monthly variability and significant seasonal amplitude in f CO sea 2 in the UB.

Seasonal variability and controlling factors of fCO 2 in the UB
The monthly variation in f CO mer (August and September) and had a mean value of 18.4 • C. The monthly variation in f CO air 2 was opposite in phase to that of SST.The monthly variation pattern of f CO air 2 followed the general seasonal cycle of f CO 2 (or pCO 2 ) monitored in the North Hemisphere; i.e., the f CO air 2 reached its maximum value in spring and minimum value in summer.The f CO sea 2 , meanwhile, showed more complex variation than did the f CO air 2 .
The monthly mean of the f CO sea 2 in February, March, and April was lower than the average value (348.2 µatm) but that in August and December was above the average.The May-September data were close to the average.During the warm season (June and August), relatively high f CO sea 2 was associated with elevated SST.During the fall and winter (October to February), monthly f CO sea 2 values were higher than those in spring (April and May).This difference was a result of vertical entrainment of CO 2 -rich subsurface water despite the decreasing SST.Deepening of MLD (> 50 m) supported high f CO sea 2 phenomenon in winter (Fig. 5b).Comparison of the normalized total CO 2 (NTCO 2 ) in the EJS showed the difference of ∼ 100 µmol kg −1 in surface layer between summer and winter (Park, 1997).It also could be a result of the biological drawdown of CO 2 in March and April, when the f CO sea 2 attained its lowest value.During March and April, satellite-based chlorophyll a concentration reached a peak value (> 1 mg m −3 ) suggesting a possibility of biological CO 2 drawdown (Fig. 5c).Takahashi et al. (2002) proposed a method for estimating the relative importance of the effects of biological activity and seasonal temperature change on the pCO 2 of surface seawater.In order to estimate the relative magnitude of these effects, the ratio of the thermal effect (effect of temperature change) to the non-thermal effect (effect of biological activity and vertical mixing) was adapted after the method proposed by Introduction

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Full where SST is the surface seawater temperature in ) and nonthermal (f CO non-therm 2 ) effects was separated by the equations above (Eqs.6 and 7).
The results are shown in Fig. 5d and e.The f CO therm 2 followed the same pattern as the seasonal SST variation (Fig. 5d and a).The peak-to-peak amplitudes of the seasonal cycles of SST and f CO therm 2 (∆f CO therm 2 , Eq. 8) were ∼ 17 • C and 236 µatm, respectively, which means that an 1 • C temperature change makes a 14 µatm change in f CO 2 .Takahashi et al. (1993) suggested the thermodynamic relationship between pCO 2 and temperature ((∂pCO 2 /∂T )/pCO 2 = 0.0423 • C −1 ), our data showed almost close to it (0.0376 • C −1 ).However, the ∆f CO therm 2 was larger than that observed at Station "P" (100 µatm, Wong and Chan, 1991) and at BATS (150 µatm, Bates, 2001), which located at the comparable latitude as the UB.Larger seasonal variation of SST (> 17 • C) may have caused such a difference.
On the other hand, f CO non-therm 2 reached its highest value during the cold season and its lowest value during the warm season.The peak-to-peak amplitude was 193 µatm, which was smaller than the value from the Ross Sea (260 µatm, Sweeney, 2000) but much larger than the value from BATS (115 µatm, Bates, 2001).This high f CO non-therm 2 value in winter was due to (1) relatively low biological carbon uptake during the cold season, and (2) stronger vertical mixing that pumps up high-CO 2 subsurface water.The decrease in the f CO to carbon fixation by the spring bloom (Fig. 5e and c).In summer, the value decreased because of weakening of vertical mixing caused by stratification (Fig. 5b and e).The seasonal variation in f CO 2 can be explained by the sum of the thermal and nonthermal effects.The ratio between the thermal effect and the non-thermal effect was estimated to be almost equal to one (∆f CO therm 2 /∆f CO non-therm 2 = 1.2) in the UB, which suggested that the thermal effect was almost balanced with the non-thermal effect.A similar result (0.9) was obtained at Station "P" in the eastern subarctic Pacific Ocean (50 • N, 145 • W) during the period 1973-1978 (Wong and Chan, 1991).On the other hand, the ratio differed significantly from that found for the BATS (2.7) (Bates, 2001) or the Ross Sea (0.02) (Sweeney, 2000) values (Table 2).
In summary, the contribution of temperature variation to the seasonality of f CO 2 was almost equivalent to the non-thermal effect.However, the relative contribution varied with the season (Fig. 5f).Non-thermal effect contributed to the surface f CO 2 drawdown in summer, while the surface f CO 2 elevation in winter.According to Sarmiento and Gruber (2006), pCO 2 in the North Pacific is not affected by one dominant factor among SST, biological activity, and vertical mixing but is affected by their combined effect.Their explanation about the controlling factors of pCO 2 in the North Pacific could be applied to similar features of the f CO 2 in the UB.

Decadal trend of fCO 2 in the UB
Decadal trend of the modeled f CO 2 (i.e., f CO * 2 ) based on the combination of a linear increasing trend and a harmonic function analysis using observed data from the UB is shown in Fig. 6.The long-term atmospheric pCO 2 variation observed at Gosan (33 • 17.4 N, 126 • 9.9 E), Jeju Island, Korea (Cho et al., 2005)  (3.2 µatm yr −1 ; Keeling et al., 2004).However, when we compared the peak-to-peak amplitudes of the f CO sea 2 in the UB with those at station ALOHA, we found that the UB exhibits a stronger seasonality (99.5 µatm) than did station ALOHA (40 µatm).
Since the work of Inoue et al. (1995), numerous studies have focused on the rate of CO 2 increase based on monitoring and survey data.For the sake of comparing the decadal trend of the f CO 2 in the UB with other regions, previously published data covering approximately 30 different oceanic areas are listed in Table 3.Rates of increase of f CO 2 are plotted as a function of the latitude in Fig. 7. Generally, the increasing rate in most areas is close to the global average (1.5 µatm yr −1 ; IPCC 2007), but the rate in the middle latitudes of the Northern Hemisphere is higher.We could infer from this result that human activities might influence the rate of increase of CO 2 in surface seawater.

Acidification rate
As surface water CO 2 has been increasing, the pH of the surface seawater has been decreasing, an effect called ocean acidification.The decadal trend of pH in the UB was estimated based on total alkalinity (TA) data from this study and f CO * 2 .The pH values were calculated from TA and simulated f CO * 2 by CO2SYS (Lewis and Wallace, 1998).The carbonate dissociation constants (K 1 and K 2 ) used in these calculations was those of Mehrbach et al. (1973) as refit by Dickson and Millero (1987).We assumed the TA of surface seawater to be constant at 2266 ± 17 µeq kg −1 , the average value of 60 measurements during the study period, because the seasonality and secular trends were not significant.These calculated pH values were in good agreement with the measured ones (R 2 = 0.8).As the surface f CO 2 increased, the pH value in the surface seawater of the UB, while fluctuating seasonally, decreased at the rate of 0.04 pH units decade −1 since 1995 (Fig. 8).The pH in the surface ocean decreased by 0.1 pH units between 1750 and 1994, which was noted as an unprecedented decline by Sabine et al. (2004).To compare f CO 2 increasing trend and pH decreasing trend in the UB with global trend, we also plotted time series of f CO 2 and pH from Station ALOHA data.Since 1988, the pCO 2 at Station ALOHA has increased at a rate of ∼ 2 µatm yr −1 and mixed layer pH has declined by 0.02 pH units per decade (Fig. 8, Doney et al., 2012).IPCC (2007) reported that the rate of decrease of pH was estimated at about 0.02 pH units decade −1 based on station data at HOT, BATS, and ESTOC since 1980.Compared with the global trend, surface seawater of the UB was being acidified almost twice as rapidly.
Under the assumption of constant TA and surface seawater temperature, 10 % increase of f CO 2 (i.e.33.6 µatm increase in a decade) may reduce pH by 0.036 pH units.A seawater temperature rise of 1 • C may also decrease pH by 0.01 pH units at a pressure of 1 atm (Gieskes, 1969).The sea surface temperature in the EJS increased by 0.2-0.7 • C during the last decade (Kim et al., 2007;Yeh et al., 2010), which is a much faster warming than the global average (0.13 Rayner et al., 2006).This warming contributed to a pH decrease of 0.002-0.007pH units which was equivalent to 5-20 % of the pH decline in the UB during the last decade.Therfore, the ocean acidification in the UB was mainly driven by the f CO 2 increase (∼ 90 %), while the warming effect was relatively small (about 5-20 %).Introduction

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Full  Full  Full  Full Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | .15, C 2 = 13.88,C 3 = 8.19, C 4 = 23.94(for seawater), C = 357.12,C 1 = 4.4, C 2 = 5.74, C 3 = −2.33,C 4 = −0.13(for air).Monthly variations in f CO 1995 2 in the atmosphere and in the surface seawater which subtract the annual increaing trend by normalizing to the year of 1995 and the harmonic function analysis results are shown in Fig. 3. Lastly, using Eq.(1), we recalculated f CO Discussion Paper | Discussion Paper | Discussion Paper | in Fig. 3 and the f CO sea 2 controlling parameters such as sea surface temperature (SST), mixed layer depth (MLD) based on the climatology data (de Boyer-Montégut et al., 2004), and chlorophyll a concentration inferred from the SeaWiFS data between 1998 and 2007 are represented in Fig. 5a-c.The SST reached a minimum in winter (February) and a maximum in sum-Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Takahashi et al. (2002) as the following equations: Discussion Paper | Discussion Paper | Discussion Paper | is also shown in the figure for comparison with f CO air 2 measured in the UB (Fig. 6a).The f CO air 2 in the UB coincided with pCO Discussion Paper | Discussion Paper | Discussion Paper | values were slightly larger than the global mean 1.5 ppm ν yr −1 (IPCC, 2007).Still larger was the decadal increase trend of f CO 2 observed in the UB.The f CO sea 2 in the UB had been increasing gradually at a rate of 3.36 µatm yr −1 , which was almost similar to result measured from Station ALOHA (22 • 45 N, 158 • W) of the HOT between 1997 and 2004 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Figure 1 .Fig. 1 .
Figure 1.Maps showing the tracks of the cruises conducted between 1995 and 2004 for this 3 study.4 5

Fig. 6 .
Fig. 6.Decadal trend of f CO 2 at the Ulleung Basin from 1995 to 2004.(a) f CO air 2 (filled diamonds) of overlying atmosphere of the Ulleung Basin.Atmospheric pCO 2 data, measured at Gosan, Jeju Island, Korea (black dots), are also shown for comparison with f CO air 2 , (b) f CO sea 2(filled circles) of surface seawater of the Ulleung Basin.The gray curves represent f CO * 2 modeled from a linear increasing trend and a harmonic function analysis (after Eq. 3).

Fig. 7 .
Fig. 7.A plot of the rate of increase of surface seawater CO 2 versus latitude.Dashed line represents the global average of surface seawater increasing rate (1.5 µatm yr −1 ).

3 Results and discussion 3.1 General characteristics of fCO 2 and CO 2 flux in the UB
• C, and the f CO 2 and SST refer to the annual average of observed f CO 2 and SST values, respectively.The dependence of f CO 2 variability in the UB on the thermal (f CO

Table 1 .
Summary of cruise information and data for this study.SST, f CO air 2 , f CO sea 2 , ∆f CO 2 , total alkalinity (TA), and pH are reported as averages with one standard deviations.The number of data is given in parentheses.

Table 2 .
Comparison of the contribution of thermal and non-thermal effects on the f CO 2 of surface seawater in various regions.

Table 3 .
Rate of increase of surface seawater CO 2 at various regions of the world ocean.