Deep-Sea Research II 54 (2007) 1469–1485
Depth distributions of alkalinity, TCO
2
and d
13
C
TCO
2at SEATS
time-series site in the northern South China Sea
W.C. Chou
a,b, D.D. Sheu
c,, B.S. Lee
d, C.M. Tseng
a,e,
C.T.A. Chen
c, S.L. Wang
f, G.T.F. Wong
ga
National Center for Ocean Research, P.O. Box 23-13, Taipei 106, Taiwan, ROC
bInstitute of Marine Environmental Chemistry and Ecology, National Taiwan Ocean University, 2, Pei-Ning Rd., Keelung, Taiwan, ROC cInstitute of Marine Geology and Chemistry, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, ROC
dDepartment of Biotechnology, ChungChou Institute of Technology, Yuanlin, Changhua 510, Taiwan, ROC eInstitute of Oceanography, National Taiwan University, P.O. Box 23-13, 106 Taipei , Taiwan, ROC fDepartment of Marine Environmental Engineering, National Kaohsiung Marine University, Kaohsiung, Taiwan, ROC gResearch Center for Environmental Changes, Academia Sinica, 128, Academia Road, Section 2, Nankang, 115 Taipei, Taiwan, ROC
Accepted 10 May 2007 Available online 10 July 2007
Abstract
In this study, measurements of titration alkalinity (TA), total dissolved carbon dioxide (TCO2), and d13C of TCO2
(d13CTCO2) throughout the water column at the SouthEast Asian time-series study (SEATS) site were investigated in order
to understand better the fundamental processes controlling their vertical distributions in the South China Sea (SCS). The linear correlations between TA and salinity in the shallow waters, as identified by the mixing line between the surface water and salinity maximum water suggested the predominant control of physical mixing on the variability of TA. In contrast, TCO2and d13CTCO2showed the non-conservative behavior in the respective TCO2and d
13C
TCO2vs. salinity plot due to the
effect of biological production. A stoichiometric model further showed that the depth profile of NTA ( ¼ TA salinity/35) largely reflects the increase of preformed NTA in the shallow waters, whereas carbonate dissolution was responsible for the continuous increase of NTA in the deep waters. A one-dimensional diffusion–advection model further revealed that the carbonate dissolution could account for 28% of NTCO2 ( ¼ TCO2salinity/35) increase in deep waters, and the
remaining 72% of NTCO2was from organic decomposition. Calculation of excess TA further showed that it emerged well
above the aragonite and calcite saturation depths at 600 and 2500 m, respectively, indicating that some biologically, chemically, and physically-mediated processes must be involved to provide excess TA into the shallow waters. The decrease in d13CTCO2 with depth primarily resulted from organic decomposition.
The influence of anthropogenic CO2 throughout the water column was assessed with the carbon chemistry and the
isotope-based approach in this study. Both methods obtained nearly the same results in which the signal of anthropogenic CO2 decreased exponentially with depth, and its penetration depth were found to be at 1000 m. The inventory of
anthropogenic CO2in the water column was estimated to be 16.6 mol C m2, which was less than that reported in the
northwest Pacific at the same latitude, presumably due to the enhanced upwelling in the SCS. Such an anthropogenic CO2
www.elsevier.com/locate/dsr2
0967-0645/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2007.05.002
Corresponding author. Tel.: +886 7 525 5148; fax: +886 7 525 5348. E-mail address:ddsheu@mail.nsysu.edu.tw (D.D. Sheu).
penetration had led to decreases of the saturation levels of aragonite and calcite by 17% and 14%, respectively, in the surface water, and an upward migration of aragonite saturation depth by 100 m since industrial revolution.
r2007 Elsevier Ltd. All rights reserved.
Keywords: Anthropogenic CO2; Carbon chemistry; South China Sea
1. Introduction
Recent concerns about the role of ocean in regulating the atmospheric CO2concentration have prompted oceanographers to reexamine the funda-mental processes controlling the distributions of total dissolved carbon dioxide (TCO2), total alka-linity (TA), and d13C of TCO2 (d13CTCO2) in
seawater, because these parameters not only provide insight into our current understanding of anthro-pogenic CO2in the oceans, e.g., the penetration and storage of anthropogenic CO2, but also can be used as a baseline for future estimates of oceanic CO2 uptake (Wallace, 2001; Quay et al., 2003). For instance, results from the WOCE/JGOFS program and other studies in 1990s have greatly improved our knowledge of the fate of anthropogenic CO2 and its impact on carbonate system in the oceans (Sabine et al., 2004;Feely et al., 2004and references therein). Nonetheless, most of these studies were conducted in open-ocean regimes with very limited data available in the marginal seas, despite the fact that 20% of the global ocean’s net annual uptake of anthropogenic CO2 (0.4 Pg C yr1) may have been sequestrated in the marginal seas and delivered to the open ocean via the ‘‘continental shelf pump’’ (Thomas et al., 2004).
In order to better understand the role of marginal seas in marine carbon cycles, a time-series program (i.e. the SouthEast Asian time-series study, SEATS) was initiated in 1998 in the northern South China Sea (SCS;Fig. 1). The seasonal variability of carbon parameters within the mixed layer at SEATS was reported previously byChou et al. (2005, 2006)and a long-term variation trend can be found inTseng et al. (2007). Here, instead of discussing their temporal variations, we have compiled the high quality data collected from 19 cruises at the SEATS site to examine the general processes controlling the vertical variations of NTA (normalized TA ¼ TA 35/salinity), NTCO2 (normalized TCO2¼TCO2 35/salinity) and d13CTCO2 below the surface mixed
layer, and to evaluate the influences of anthropo-genic CO2 in the water column. The techniques employed in this study to examine the carbon
system and the influence of anthropogenic CO2are similar to those applied in the North Pacific so that results from SEATS can be compared to the results from its major source area, i.e. the North Pacific. Moreover, results from this study also shed light on our current understanding of the role of SCS in the uptake of anthropogenic CO2, and could be used as a baseline for future evaluation of the long-term variability of the carbon system in the SCS. However, it is worth to note that since our data are limited to a single site, potential influences of horizontal processes on the vertical characters delineated in this study are largely ignored.
2. Methods
During the course of this study, the SEATS site was investigated 19 times from September 1999 to October 2003 aboard R/V Ocean Research I or III (September and November 1999, January, March, May, July, and October 2000, February, June, October, and December 2001, March, July, September, and November 2002, as well as January, March, August and October 2003). All raw data of TA and TCO2and other pertinent measurements of SEATS are archived in the Ocean Data Bank (ODB) at the National Center for Ocean Research. Interested readers may contact NCOR-ODB for data requests (NCOR;http://www.ncor.ntu.edu.tw/ ODBS/). Discrete water samples for TCO2, TA and d13CTCO2 analyses were transferred into 250 ml BOD bottles from Go-Flo bottles mounted onto a Rosette sampling assembly. All water samples were poisoned with 200 ml saturated HgCl2 solution immediately after collection and stored at 5 1C in darkness to prevent biological alteration.
TCO2 and TA were determined following the standard operating procedures described in DOE (1994). The coulometric method was used for TCO2 measurements with a precision of 0.1%. The single operator multiparameter metabolic analyzer (SOM-MA) system was used to extract CO2from acidified seawater samples, and the extracted CO2 was quantified by a coulometric detector (UIC, coulo-metric Inc., model 5011). TA was determined by the
potentiometric titration method with a precision of 0.2%. The titration data passing the carbonic acid endpoint (4.5 pH) were calculated to obtain TA using the mass and charge balance method devel-oped byButler (1992). Seawater references prepared and provided by A.G. Dickson were used through-out this study for calibration and accuracy assess-ment. The differences between the certified values and our measurements were less than 2 and 3 mmol kg1for TCO2and TA, respectively.
For d13CTCO2 analysis, 40 ml of sample were
injected into a pre-evacuated vessel and then reacted with 2 ml 85% H3PO4 to liberate TCO2. The evolved CO2 was trapped in a 6-mm glass finger submerged in liquid nitrogen after complete re-moval of water vapor and other condensable gases by a slurry of dry ice and alcohol mixture, and then torch sealed (Sheu et al., 1996). Isotopic analysis was performed with a VG Optima mass
spectro-meter. Results of isotopic measurement were expressed with the conventional d notation and reported as per mil (%) difference relative to the PDB standard (Craig, 1957). The overall procedural error for d13CTCO2 analyses was better than 70.05%. It should be noted that, unlike TCO2 and TA analyses, which began on September 1999, d13C analysis was performed on samples collected after March 2002.
The analytical uncertainties in TCO2, TA and d13CTCO2 were further verified by their variations in
deep waters (42000 m), where they should remain constant through time. The variations (71s) of TCO2, TA and d13CTCO2 at st of 27.6370.01
(corresponding to the depth of 25007200 m) were 1.9, 2.4 mmol kg1, and 0.03%, respectively (Fig. 2). These variations are all less than the reported analytical errors, and thus lend to the credit of the high quality of our data.
Fig. 1. Bathymetric map showing the location of the South East Asian time-series study (SEATS) site (181150N, 1151350E). Contours are
3. Results and discussion
3.1. Water mass structure and its role on the vertical distributions of TA, TCO2, and d13CTCO2
Since the northwestern Pacific (NWP) is the primary source of open-ocean water to the SCS, the SCS shares an analogous water mass structure with NWP, i.e. both are characterized by extremes
in salinity (Gong et al., 1992). The plot of potential temperature vs. salinity (Fig. 3) at SEATS clearly shows linear trends that can be inferred to be the result of mixing of the four distinct water masses, namely the surface water (SW), salinity maximum water (SmaxW), salinity minimum water (SminW), and deep water (DW). The low salinity of SW is due to basin-wide precipitation and river runoff from the surrounding landmass. The SmaxW with a
TC O2 (μ mo l kg -1) 2335 2340 2345 2350 2355 TA (μ mol kg -1) 2410 2415 2420 2425 2430 2435 1σ = 1.9 μmol kg-1 1σ = 2.4 mmol kg-1 Se p1 99 9 No v1 99 9 Ja n2 00 0 Ma r2 00 0 Ma y2 00 0 Ju l2 00 0 Oc t2 00 0 Fe b2 00 1 Ju n2 00 1 Oc t2 00 1 De c2 00 1 Ma r2 00 2 Ju l2 00 2 Se p2 00 2 No v2 00 2 Ja n2 00 3 Ma r2 00 3 Au g2 00 3 Oc t2 00 3 δ 13C TC O 2 (‰ ) -0.4 -0.3 -0.2 -0.1 0.0 1σ = 0.03 ‰ (A) (B) (C)
Fig. 2. Variations of (A) TCO2(B) TA, and (C) d13CTCO2at stof 27.6370.01 (corresponding to the depth of 25007200 m) from the 19
cruises between September 1999 and October 2003 at the SEATS site. Note the measurements of d13C
salinity of 34.65 centered at approximate 150 m is indicative of the North Pacific tropical water (Nitani, 1972). The core of SminW with a salinity of 34.40 at about 500 m fingerprints the North Pacific intermedi-ate wintermedi-ater (Nitani, 1972). Nonetheless, it is worth to note that due to the intensive upwelling and vertical mixing, the salinity extremes in the SCS become less pronounced than those in NWP (Shaw et al., 1996), i.e. salinity maximum becomes smaller (34.6 vs. 34.9) but salinity minimum becomes larger (34.4 vs. 34.2) in the SCS (Fig. 3). The DW bears nearly the same properties as the water from the adjacent NWP through the Luzon Strait at a sill depth of 2200 m, indicating that DW in the SCS originates mainly from the North Pacific deep water (NPDW;
Gong et al., 1992;Chen et al., 2006).
Fig. 4depicts the distributions of TA, TCO2, and d13CTCO2 vs. salinity at the SEATS site. As shown in
Fig. 4A, the relationships between TA and salinity fit well with the linear mixing lines in the shallow waters (waters between SW and SmaxW; line A in
Fig. 4A), evidencing that the physical mixing rates are faster than TA changes caused by biological processes. On the contrary, TCO2and d13CTCO2 vs.
salinity plots show a nonlinear mixing in the shallow
waters, suggesting that biological production of carbonate and organic carbon is predominant in defining the depth distributions of TCO2 and d13CTCO2 in the shallow waters, and consequently
results in the observed lower TCO2 and heavier d13CTCO2 with respect to their hypothetical linear mixing line (line A inFig. 4B and C).
By comparison, TA, TCO2 and d13CTCO2 vs.
salinity plots all show nonlinear mixing in the deep waters (waters between SminW and DW), where TA, TCO2are higher and d13CTCO2 is lighter than their
theoretical values expected from the purely linear mixing lines (line C inFig. 4A–C). This non-linearity indicates that organic matter decomposition and carbonate dissolution are the responsible processes for the vertical distributions of TA, TCO2 and d13CTCO2 observed in the deep waters. Moreover,
TA, TCO2 and d13CTCO2 vs. salinity plots also
demonstrate a nonlinear mixing trend in the inter-mediate waters (waters between SmaxW and SminW). Nonetheless, this non-linearity hardly results from the non-conservative biological production, because the depth range of intermediate waters (150–500 m) is far below the euphotic zone (80–90 m) where major biological production is taking place. Therefore, the physical mixing among three end members, i.e. SW, SmaxW, SminW, is more likely responsible for the observed non-linearity with lower TA and TCO2and heavier d13CTCO2.
3.2. Analyses of controlling factors on the depth variations of NTA, NTCO2and d13CTCO2 in the
water column at the SEATS site
It has been well recognized that TA, TCO2, and d13CTCO2 measured in the water column consist of
three major components: (1) the preformed values, i.e. values of a water mass at the time of its formation at surface outcrops; (2) changes resulting from organic decomposition (the organic carbon pump); (3) changes caused by the carbonate dissolution (the carbonate pump). Accordingly, their measured values at any given depth in the water column can be represented respectively by the following equations (Chen and Millero, 1979;
Kroopnick, 1985;Feely et al., 2002):
NTAmeas¼NTApreþTAorgþTAcarb, (1) NTCOmeas2 ¼NTCOpre2 þTCOorg2 þTCOcarb2 , (2) d13Cmeas ¼d13Cpreþd13Corgþd13Ccarb, (3)
Pot. Temp. ( Ο C ) Salinity 25 24 22 σθ= 21 26 23 27 33.0 33.5 34.0 34.5 35.0 0 5 10 15 20 25 30 SW SmaxW SminW DW NPIW NPDW NPTW
Fig. 3. Potential temperature vs. salinity relationships at the SEATS site (open circles) and adjacent northwestern Pacific (solid squares). SW, SmaxW, SminW, and DW indicate the surface
water, salinity maximum water, salinity minimum water, and deep water, respectively, in the South China Sea (SCS). NPTW, NPIW, and NPDW denote North Pacific tropical water, North Pacific intermediate water, and North Pacific deep water, respectively. Solid lines represent the hypothetically linear mixing lines between the four water masses in the SCS.
where the superscripts ‘‘meas’’, ‘‘pre’’, ‘‘org’’ and ‘‘carb’’ stand for measured, preformed, organic decomposition and carbonate dissolution, respec-tively. In Eqs. (1) and (2), TA and TCO2 were normalized to a constant salinity of 35 (NTA and NTCO2) to remove the effect of evaporation/ precipitation on their variations. In the following discussion, we will apply these equations to assess their relative contribution to the observed depth variations of NTA, NTCO2, and d13CTCO2 at the
SEATS site. Data from all 19 cruises were averaged to carry out these assessments.
3.2.1. NTA
The preformed TA equation of Sabine et al. (2002a) was used to calculate the TApre values, TApre¼148:7 þ ð61:36 SÞ
þ ð0:0941 POÞ ð0:582 yÞ, ð4Þ where S is salinity, PO is a quasi-conservative tracer (PO ¼ dissolved oxygen+170 phosphate), and y is the potential temperature. The reason for choos-ing this equation lies in the fact that it was derived from the WOCE/JGOFS data in the surface water (0–60 m) of entire Pacific and, as mentioned earlier, that the SCS water originates mainly from the NWP. Results (Fig. 5, open squares) show that NTApre increases gradually from surface to ap-proximately 1500 m, and then remains essentially constant below 1500 m. The increase in NTAprewith depth therefore accounts for, in part, the observed vertical gradient of NTAmeas in the water column, especially in the shallow waters.
The component ‘‘TAorg’’ in Eq. (1) represents the change of TA due to production and/or reminer-alization of organic matters. Conventionally, the net effect between organic production and decomposi-tion on TA changes is estimated by apparent oxygen utilization (AOU). In this study, we adopted a coefficient of 0.119 ofFeely et al. (2002)to calculate
Fig. 4. (A) TA, (B) TCO2, and (C) d13CTCO2 vs. salinity
relationships at the SEATS site. SW, SmaxW, SminW, and DW
denote the surface water, salinity maximum water, salinity minimum water, and deep water, respectively, in the South China Sea. Dashed lines in (A) and (B) represent the hypothe-tically linear mixing lines between the four water masses. Solid lines in (C) represent the hypothetically non-linear mixing lines for d13CTCO2between the four water masses. The end members of
SW, SmaxW, SminW, and DW were determined as the averages of
all data collected in top 10 m, at the depth of 150, 500 m, and greater than 2500 m, respectively, from the 19 cruises between September 1999 and October 2003.
33.0 33.5 34.0 34.5 35.0 2150 2200 2250 2300 2350 2400 2450 33.0 33.5 34.0 34.5 35.0 800 900 2000 2100 2200 2300 2400 Salinity 33.0 33.5 34.0 34.5 35.0 -0.5 0.0 0.5 1.0 S W SmaxW SminW D W S W S W SmaxW SmaxW SminW SminW D W D W A B C A C B A B C TCO 2 (μ mol kg -1) TA ( μ mol kg -1) δ 13C TCO2 (‰ ) (A) (B) (C)
the TAorg,
TAorg¼ 0:119 AOU: (5)
The above conversion includes the contributions from nitrate, organic phosphorus and sulfur, and assumes a nitrate/AOU ratio of 16/170 (Anderson and Sarmiento, 1994). The magnitude of TAorg is graphically represented by the difference between NTApre(open squares) and (NTApre+TAorg) (open triangles) inFig. 5.
The amount of TA produced by carbonate dissolution (TAcarb) can be readily computed by subtracting (NTApre+TAorg) from NTAmeas, as formulated in Eq. (1). As shown, TAcarb (the difference between open circles and triangles) appears below 200 m and increases sharply to 1500 m, then remains constant to the bottom. Together, these profiles demonstrate the importance of TAcarbin controlling the observed large increase of NTAmeas in the deeper waters, whereas increase of NTAmeas in the shallower waters results mainly
from the increase of NTApre, and TAorgplays a role in modulating the vertical gradient of NTAmeas. 3.2.2. NTCO2
Referring to Eq. (2), ‘‘NTCO2meas’’ is the sum of ‘‘NTCO2pre’’, ‘‘TCO2org’’, and ‘‘TCO2carb’’, in which ‘‘TCO2org’’ can be evaluated from the AOU on the basis of the stoichiometric ratio (C/AOU) of 117/170 (Anderson and Sarmiento, 1994):
TCOorg2 ¼117=170 AOU: (6)
For ‘‘TCO2 carb
’’, it is calculated from TAcarbbecause of the fact that dissolution of CaCO3can result in an increase of TA and TCO2 at a ratio of 2/1 (i.e. TCO2carb¼0.5 TAcarb). Substituting Eq. (1) and (5) into this relationship, and rearranging the equation, the term ‘‘TCO2carb’’ was computed as follows: TCOcarb2 ¼0:5 TAcarb¼0:5
ðNTAmeasNTApreTAorgÞ ¼0:5 ðNTAmeasNTApreÞ
þ0:0593 AOU: ð7Þ
Finally, NTCO2pre was calculated by subtracting (TCO2org+TCO2carb) from NTCO2meas (ref. Eq. (2)). Results from these calculations were depicted graphi-cally in Fig. 6. As seen, NTCO2pre, TCO2org (the difference between open triangles and squares), and TCO2carb (the difference between open circles and triangles) all increase gradually with depth. In terms of their relative contributions, the vertical increment of NTCO2measin the upper 400 m comes equally from the increase of NTCO2pre and TCO2org. Below 400 m, although TCO2carb, NTCO2preand TCO2orgall account for the continuous increase of NTCO2measwith depth, TCO2carbis much less important than the other two.
The relative contribution of carbonate and organic pumps to TCO2 increase with depth was further examined with TCO2carb/TCO2org (IC/OC) ratios. As shown in Fig. 7, IC/OC ratio increases from 0.04 at 300 m to 0.36 at 2000 m, and then remains fairly constant below 2000 m. The value of 0.36 indicates that approximately 26% of TCO2 production in waters deeper than 2000 m at the SEATS site is from carbonate dissolution. This percentage of carbonate dissolution is consistent with that reported previously byChen (1990)in the Pacific and the SCS (Table 1). The high IC/OC ratio of 0.36 found in the deep waters of SCS also reflects its origin from the oldest and hence most corrosive water mass, i.e. NPDW. The increasing IC/OC with depth further confirms the increasing
NTA (μmol kg-1) 2250 2300 2350 2400 2450 2500 Depth (m) 0 500 1000 1500 2000 2500 3000 NTAmeas NTApre
NTApre+TAorg
TAcarb
TAorg
Fig. 5. Depth profiles of measured NTA (NTAmeas), preformed NTA (NTApre), and NTApre+TAorg at the SEATS site. See details in text for the definitions and calculations of TAorgand
TAcarb. Open circle and horizontal bar represent the average and
variation of NTAmeas at a given depth from the 19 cruises
between September 1999 and October 2003. Horizontal bars on NTApreand NTApre+TAorgprofiles represent the uncertainties
importance of carbonate dissolution with depth in water column.
Since the y vs. salinity plot is relatively linear between the SminW and DW at the SEATS site, a one-dimensional advection–diffusion model can be applied to cross-check the IC/OC ratios derived from the above stoichiometric model. By taking the same technique of Craig (1969), the resultant Z*( ¼ K/w), J/w for TA, and TCO2 are 0.44 km, 3.60 and 6.58 mmol kg1km1(Fig. 8), respectively, where Z*, K, w, and J represent the mixing parameter, eddy-diffusion coefficient, upwelling rate, and production rates, respectively. The calcu-lated Z* at SEATS is apparently lower than those reported previously in the North Pacific (0.44 vs. 0.86–1.20 km; Craig and Weiss, 1970; Chung and Craig, 1973;Table 1). Since K is relatively invariable in the ocean, the smaller Z* at SEATS suggests a higher upwelling rate (larger w) in the SCS, which agrees well with the consensus that the rates of
vertical mixing and upwelling in the SCS are high (Chao et al., 1996). Substituting the calculated values of J/w for TA ((J/w)Alk) and TCO2 (ðJ=wÞTCO
2) into the following equation:
IC=OC ¼ 0:5ðJ=wÞAlk=½ðJ=wÞTCO20:5ðJ=wÞAlk,
(8) where an IC/OC ratio of 0.38 was obtained, which agrees well with that (0.36) estimated from the stoichiometric model discussed above. This agree-ment further supports the validity of preformed equation used in the stoichiometric model for IC/OC estimates.
3.2.3. d13CTCO2
To evaluate the respective contribution of d13Cpre, d13Corg and d13Ccarb to the observed vertical gradient of d13Cmeas, we rewrote the Eq. (3) in a mass-balance form:
d13CmeasTCOmeas2 ¼d13CpreTCOpre2 þd13Corg0TCOorg2
þd13Ccarb0 TCOcarb2 , ð9Þ
NTCO2 (μmol kg-1) 2000 2100 2200 2300 2400 Depth (m) 0 500 1000 1500 2000 2500 3000 NTCO2 meas NTCO2 pre NTCO2 pre +TCO2 org TCO2 org TCO 2carb
Fig. 6. Depth profiles of measured NTCO2 (NTCO2meas),
preformed NTCO2 (NTCO2pre), and NTCO2pre+TCO2org at the
SEATS site. See text for the definitions and calculations of TCO2organd TCO2carbin details. Open circle and horizontal bar
represent the average and variation of NTCO2measat a given depth
from the 19 cruises between September 1999 and October 2003. Horizontal bars on NTCO2pre and NTCO2pre+TCO2org profiles
represent the uncertainties in their calculations.
IC/OC 0.0 0.1 0.2 0.3 0.4 0.5 Depth (m) 0 500 1000 1500 2000 2500 3000
Fig. 7. Depth profiles of IC/OC ratio at the SEATS site, which implies the relative contribution of carbonate dissolution and organic decomposition to the increase of TCO2in water samples.
where d13Cmeas, d13Corg0, and d13Ccarb0 were d13C in TCO2 meas , TCO2 org and TCO2 carb , respectively, and values of 20% for d13Corg0 (Goericke and Fry, 1994) and +2% for d13Ccarb0(Bonneau et al., 1980) were adopted. Rearranging Eq. (9), d13Cpre was computed with the following equation:
d13Cpreðin%Þ ¼ ðd13CmeasTCOmeas2 þ20 TCOorg2
2 TCOcarb2 Þ=ðTCOpre2 Þ. ð10Þ
Results show that d13Cpreincreases progressively from 0.7% at the surface to 1.2% at 500 m, and then remains in a narrow range (1.270.05%) throughout the water column to 3000 m (open squares in Fig. 9). The light d13Cpre found in the shallow waters echoes the greater influences of anthropogenic CO2, which has d13C value much lighter than that of seawater (22 to 32%;Andres et al., 2000). A further examination of effects of anthropogenic CO2on the observed d13C trend will be presented in later section.
TCO2 (μmol kg-1) 1800 1900 2000 2100 2200 2300 2400 TA (μmol kg-1) 2150 2200 2250 2300 2350 2400 2450 Depth (m) 0 500 1000 1500 2000 2500 3000 Z* = 0.44km J/w = 6.58 J/w = 3.60
Fig. 8. Measured (A) TCO2and (B) TA data with calculated profiles from the diffusion–advection-production model (dashed line) at the
SEATS site. The values of Z* and J/w are the best fit values calculated for the deep water between the boundary depths of 700 and 3000 m. Table 1
Comparisons of IC/OC, Z*, saturation depth of aragonite, saturation depth of calcite, penetration depth of anthropogenic CO2, total
column inventory of anthropogenic CO2, Dd13C/DTCO2at the SEATS site with those in the North Pacific
SEATS North Pacific References
IC/OC 0.36–0.38 0.36 Chen (1990)
Z* 0.44 km 0.86–1.20 km Craig and Weiss (1970)
Chung and Craig (1973)
Saturation depth of aragonite 600m 600 m Feely et al. (2002)
Saturation depth of calcite 2500m 2500 m Feely et al. (2002)
Penetration depth of anthropogenic CO2 1000 m 1000 m Sabine et al. (2002a)
Chen (1987)
Total column inventory of anthropogenic CO2 16.6 mol m2 20 mol m2 Sabine et al. (2002a)
Dd13C/DTCO
2 0.026% (mmol kg1)1 0.024% (mmol kg1)1 Winn et al. (1998)
To further assess the net change in d13CTCO2 due
to addition of TCO2org, we have assumed a negligible effect of carbonate dissolution. Thus, d13Corgcan be readily calculated by the following mass balance equation:
d13Corg¼ ½ðd13CpreTCOpre2 Þ þ ð20 TCOorg2 Þ= ðTCOpre2 þTCOorg2 Þ d13Cpre. ð11Þ
Fig. 9 shows the depth profile of the calculated d13Corg plus d13Cpre (open triangles) and d13Cmeas (open circles). As seen, they are nearly equal. In other words, the observed decrease of d13Cmeaswith depth results mainly from organic decomposition, while dissolution of carbonates has little effect on the change of d13CTCO2.
3.3. The dissolution and saturation state of carbonates at the SEATS site
The degree of carbonate saturation with respect to aragonite and calcite, i.e. Oaragonite and Ocalcite
(O ¼ [Ca2+] [CO32]/Ksp), was estimated using the program developed by Lewis and Wallace (1998). As shown in Fig. 10, both Oaragonite (open triangles) and Ocalcite(open circles) decrease rapidly with depth in the upper 1000 m due to the decrease of [CO32] (open diamonds). As indicated by open diamonds inFig. 10, [CO32] decreases dramatically from a surface value of 220 to 70 mmol kg1 at 1000 m, and this drop can account for 80% of the decrease of saturation in the upper 1000 m. By contrast, [CO32] stays relatively constant below 1000 m. As a consequence, the continuous decrease in saturation must have resulted from the increase of Ksp due to the increase of pressure and decrease of temperature with depth. As revealed in Fig. 10, the saturation depths of aragonite and calcite could be placed at about 600 and 2500 m, respec-tively, at the SEATS site. The calculated depths derived from this study are consistent with those reported in the same latitude of NWP byFeely et al. (2002)(Table 1).
To assess further the depth levels where carbonate dissolution actually took place in water column at the SEATS site, we superimposed the depth distribution of TCO2carb(TCO2produced by carbo-nate dissolution; termed as TA* by Feely et al. (2002) andSabine et al. (2002b), and as DTACaCO3
byChung et al. (2003)on Oaragoniteand Ocalcitedepth profiles in Fig. 10 (open squares). It is found that appreciable TCO2carbis present in waters well above the calcite saturation depth (2500 m), even at depths shallower than the aragonite saturation depth (600 m). Thus, TCO2carb data indicate that carbonate dissolution might take place at depths shallower than those assessed by saturation data. Although the consensus has been that shallow waters are supersaturated with respect to both aragonite and calcite, and appreciable carbonate dissolution only occurs at greater depths in the ocean, recent studies have suggested that dissolution of carbonates might occur in shallow waters. For instance, Milliman et al. (1999) reported that as much as 60–80% of carbonates exported from the surface layer have been remineralized in the upper 500–1000 m of the ocean.Feely et al. (2002),Sabine et al. (2002b), and Chung et al. (2003) found that excess TA did exist in the shallow water at depths well above the calcite lysocline in the Pacific, the Indian, and the Atlantic oceans. Our results thus are in consistent with their conclusion. Nevertheless, it should be pointed out that our estimates from excess TA data represent the uppermost limit of
δ13C TCO2 (‰) -0.5 0.0 0.5 1.0 1.5 Depth (m) 0 500 1000 1500 2000 2500 3000 δ13Cmeas δ13Cpre δ13 Cpre+δ13Corg
Fig. 9. Depth profiles of measured d13C (d13Cmeas), preformed d13C (d13Cpre), and d13Cpre+d13Corgat the SEATS site. See details
in text for the definition and calculation of d13Corg. Open circle
and horizontal bar represent the average and variation of d13Cmeas
at a given depth from the eight cruises between March 2002 and October 2003. Horizontal bars on d13Cpreand d13Cpre+d13Corg
dissolution because the upwards advection of TA-enriched deep water, the sedimentary calcite dis-solution in the sediment–water interface (de Villiers, 2005) and the denitrification and reduction of organic matter by manganese, iron, and sulfate in the anaerobic environment on the shelves (Chen, 2002) also could contribute to the excess TA found in shallow waters. In fact, there have been various possible mechanisms proposed to explain the carbonate dissolution at the shallower depths, e.g. (1) dissolution within the guts and feces of zooplankton (Jansen and Ahrens, 2004; and refer-ences therein); (2) the microbial oxidation of organic matter may produce a microenvironment conducive to carbonate dissolution (Milliman et al.,
1999); (3) dissolution of more soluble carbonate phases, such as pteropods and high-magnesium calcite (Feely et al., 2002). Consequently, the appearance of excess TA in shallow waters is suggested to multiple interpretations and required further investigation.
3.4. The penetration of anthropogenic CO2and its influence on the saturation state of carbonate
Observation-based methods for estimating the penetration and storage of anthropogenic CO2 in the ocean are derived either from the carbon chemistry (mainly TA and TCO2) or the d13C data (Wallace, 2001;Quay et al., 2003). In this study, we utilized these two data sets separately, which were measured concurrently, to assess the magnitude of anthropogenic CO2 (excess CO2) influence in the water column at the SEATS site.
The back calculation method ofChen and Mill-ero (1979)was adopted to estimate the influence of anthropogenic CO2on TCO2in the water column. The underlying principle of this method and some alterations have been described in great detail by
Chen and Millero (1979), Chen and Pytkowicz (1979), Gruber et al. (1996), Sabine and Feely (2001), andWallace (2001). In this study, changes in NTCO2 resulted from remineralization of organic carbon and carbonate dissolution were estimated by Eqs. (6) and (7), respectively. The preformed TA at the present time was calculated by Eq. (4). The preformed NTCO2 at the present time ðNTCOpre2 presentÞ was calculated separately using the following equations (Chen et al., 1986; Chen and Huang, 1995):
For the DW:
NTCOpre2 presentðmmol kg1Þ ¼2219 11 yð16Þ.
(12) For waters above and the SminW:
NTCOpre2 presentðmmol kg1Þ ¼2242 12:08 yð18Þ.
(13) At depth where the SminW mixed with DW, the y/S plot was further used to estimate their respective contribution. For surface waters, the ‘‘anthropo-genic CO2’’ was estimated from the difference between measured NTCO2 and the ‘‘preindustrial NTCO2’’, which was computed from the measured TA and hydrographical data assuming that pCO2of seawater was at equilibrium with atmospheric pCO2 Ω (Carbonate saturation level)
0 1 3 4 6 7 Depth (m) 0 500 1000 1500 2000 2500 3000
TCO2carb (μmol kg-1)
0 10 20 30 40 50 60
[CO32-] (μmol kg-1)
0 50 100 150 200 250
Saturation depth of aragonite at preindustrial era Saturation depth of aragonite
at present day
Saturation depth of calcite
Ωcalcite-present Ωaragonite-present Ωcalcite-preindustrial Ωaragonite-preindustrail [CO3 2-] TCO2 carb 5 2
Fig. 10. Depth profiles of aragonite (Oaragonite) and calcite
(Ocalcite) saturation levels at the present time (open triangles
and circles) and the pre-industrial era (solid triangles and circles) at the SEATS site. Superimposed are the depth profiles of concentration of carbonate ion ([CO3
2
], open diamonds) and TCO2
carb
(TCO2 produced from carbonate dissolution, open
of 280 ppmv. Although there were considerable uncertainties (720 mmol kg1) associated with this method (Shiller, 1981; Chen et al., 1982), it is thus far the only approach applicable for a single site’s assessment such as the SEATS.
Results from these calculations are depicted in
Fig. 11. As shown, concentrations of anthro-pogenic CO2 decrease exponentially with depth. Below 1000 m, they fluctuate slightly around 075 mmol kg1
, implying the maximal depth they could reach. The penetration depth at 1000 m is in good agreement with other studies reported pre-viously in the North Pacific (Chen, 1987; Sabine et al., 2002a;Table 1). Nonetheless, the concentra-tion of anthropogenic CO2 in the surface water (between 55 and 60 mmol kg1) is appreciably higher than that reported by Sabine et al. (2002a)
in subtropical surface water of the North Pacific Ocean (between 40 and 50 mmol kg1). The discre-pancy is attributed to the different sampling time between that of Sabine et al. (2002a) and ours (1991–1994 vs. 1999–2003) because more anthro-pogenic CO2 must have accumulated in surface waters at the SEATS site. The total column inventory of anthropogenic CO2 was estimated to be 16.6 mol m2 at the SEATS site, which was slightly less than 20 mol m2 reported by Sabine
et al. (2002a) for the same latitude in the NWP (Table 1). The difference can be best explained by the greater upward advection (upwelling) in the SCS that tends to reduce its capacity in storing both natural and anthropogenic CO2. By extra-polating this value to the entire SCS (3.5 106km2), the total anthropogenic CO2 in the SCS is about 0.570.2 PgC (1 PgC ¼ 1015
g carbon), i.e. 0.42% of total anthropogenic CO2 storage in the global oceans (118719 PgC; Sabine et al., 2004). The value is rather small, considering the SCS’s share in terms of size that occupies 0.97% of the total ocean. It thus appears that, unlike other marginal seas in high latitude such as the Bering Sea and the Sea of Okhotsk, SCS plays only a minor role in transporting anthropogenic CO2 to the interior of the North Pacific.
To assess further aragonite and calcite saturation states of seawaters in the preindustrial era, we assumed that TA did not change during industrial era, and the preindustrial levels of TCO2could be calculated by subtracting the anthropogenic CO2 concentration from the TCO2 measured. Results show that the depth of calcite saturation during preindustrial era is same as that of the present day, while the depth of aragonite saturation has migrated upward approximately 100 m. The upward migra-tion of aragonite saturamigra-tion depth implies that carbonate particles settling through the water column may dissolve at shallower depths and decrease the supply of sinking particles to bottom sediments in the SCS during industrial era. The saturation levels of aragonite and calcite in surface waters have decreased considerably from the pre-industrial values of 4.2 and 6.3 to 3.5 and 5.4 at present, respectively, due to the uptake of anthropogenic CO2. Recently Feely et al.
(2004) reported that the calcification rate of all calcifying organisms had decreased in response to a lowered carbonate saturation state, even when the carbonate saturation level was 41. The decrease of carbonate saturation found in the SCS is in line with their observations and deserved to be further investigated.
Conventionally, estimates of anthropogenic CO2 penetration from d13C data could be furnished directly by temporal changes in d13C (Quay et al., 2003, and references therein) or indirectly by the concurrent d13C-nutrient method (Keir et al., 1998;
Ortiz et al., 2000). We had adopted the latter to assess the influence of anthropogenic CO2on d13C in this study because of the lack of historical
Anthropogenic CO2 (μmol kg-1) -10 0 10 20 30 40 50 60 70 Depth (m) 0 500 1000 1500 2000 2500 3000
measurements of d13C at SEATS site. As discussed in Section 3.2.3, vertical distribution of d13C is primarily controlled by the biological cycling of 13
C-depleted organic matter in the water column, and thus a negative correlation between d13C and nutrients should exist. According to Broecker and Maier-Reimer (1992), the slope in the plot of d13C vs. phosphate should be about 1.1, providing that there was no isotopic exchange between seawaters and the overlying atmosphere. The correlation between d13C and phosphate at the SEATS site is plotted inFig. 12. As seen, the slopes of correlation are much less negative (0.71 and 0.23 for waters above and below 100 m, respectively) than the expected biological trend (1.1). Lin et al. (1999)
reported a similar deviation in the northeastern SCS. These deviations imply that processes other than biological cycling, such as isotopic fractiona-tions and uptake of anthropogenic CO2 during air–sea CO2exchange, must have been involved in decoupling the expected correlation between d13C and phosphate (Lynch-Stieglitz et al., 1995).
The influence of the air–sea exchange on oceanic d13C consists of three components, i.e. thermody-namic, kinetic, and anthropogenic CO2effects (Keir
et al., 1998;Ortiz et al., 2000). According toZhang et al. (1995), the temperature-dependent isotopic fractionation of d13C associated with the air–sea
equilibration is about 0.1% 1K1. As a result, a thermodynamic effect tends to make d13C in cold surface water higher than that in warm surface water. The kinetic effects cause surface waters to be enriched in13C in source areas, and depleted in sink areas (Lynch-Stieglitz et al., 1995). According to
Broecker and Maier-Reimer (1992), the net effect of the air–sea exchange on the surface ocean d13C can be derived from the following relationship:
d13Cas ¼d13Cmeas ð2:7 1:1 ½PO34 Þ, (14) where d13Casstands for d13C that is affected by the air–sea exchange. The term in parenthesis on the right-hand side of Eq. (14) is the mean ocean biological trend.Fig. 12shows the d13Cas(the offset between d13Cmeasand the hypothetical mean ocean biological trend) is negative for waters shallower than 800 m (phosphateo2.6 mmol kg1), while it is slightly positive for waters below. The negative d13Cas in shallow waters, however, cannot be attributed to thermodynamic and kinetic effects alone, i.e. deriving from a high temperature and/or a sink area. Instead, portions of the negative d13Cas signal must have come from the anthropogenic CO2, which is depleted in 13C and penetrates to a depth of 1000 m as discussed previously. For deep waters, positive d13Cas indicates that the waters originate from a low temperature and/or source area.
According to Keir et al. (1998) and Ortiz et al. (2000), the combined thermodynamic and kinetic effect, and anthropogenic CO2effect on d13Cascan be resolved from establishing a pre-anthropogenic relation model between d13C and phosphate in the study area. The offset between pre-anthropogenic model and d13Cmeas, denoted as Dd13Ca–p, repre-sented the magnitude of the anthropogenic CO2on d13Cas, while the offset between mean ocean biological trend and pre-anthropogenic model, denoted as Dd13Cther, signified the combined ther-modynamic and kinetic effects on d13Cas (Fig. 12). Here, we employed the similar technique of Keir et al. (1998) and Ortiz et al. (2000) with minor modifications to establish the pre-anthropogenic model for the SEATS site. We first assumed that waters below 2000 m contained no anthropogenic CO2, because deep waters in the SCS originate from the NPDW, the oldest water mass in the global oceans, so that they had exchanged with the atmosphere before the presence of anthropo-genic CO2. We further assumed that the sur-face water d13C has decreased by 1.0% since the
PO43- (μmol kg-1) 0 1 2 3 -1 0 1 2 3
Mean ocean biological trend
Pre-anthropogenic model δ13C as=Δδ 13C ther+Δδ 13C a-p Δδ13 Cther Δδ13C a-p δ 13 CTCO2 (‰) Depth<100 m Depth>100 m
Fig. 12. The correlation between d13CTCO2and phosphate at the
SEATS site. The dashed and solid lines denote respectively the hypothetical mean ocean biological trend and that derived from the pre-anthropogenic model. In addition, superimposed are two regression lines (dotted) calculated separately from water samples above (open circles) and below (solid circles) 100 m. See details in text for the definitions and calculations of the mean ocean biological trend, the pre-anthropogenic model, d13C
as, Dd13Cther,
and Dd13C a–p.
pre-anthropogenic era. This assumption was made because d13C in the surface ocean waters had decreased by about 0.8% from the last 200 years to the end of 1992 (Bo¨hm et al., 1996) and a rate of d13C decrease during the past decade in the surface water of the North Pacific was approximately 0.2% (Sonnerup et al., 1999). By connecting the surface and deep end-points, the pre-anthropogenic model at SEATS was established (solid line inFig. 12), and then the magnitude of anthropogenic CO2influence on d13C (Dd13Ca–p) can be estimated by the offset between the established pre-anthropogenic model and d13Cmeas.
The depth distribution of Dd13Ca–pis depicted on
Fig. 13. As seen inFig. 11, the vertical variations of Dd13Ca–p(derived from isotopic data) are very similar to those of ‘‘anthropogenic CO2’’ (derived from TA and TCO2data). Both approaches indicate that the influence of anthropogenic CO2 has penetrated to approximately 1000 m in the water column. Further-more, an averaged Dd13C/DTCO2ratio of 0.026% (mmol kg1)1throughout the penetration depth was obtained by dividing the depth-integrated Dd13Ca–p (426% m) with the depth-integrated ‘‘anthropo-genic CO2’’ (16,600 mmol kg1m). This value is almost identical to the Dd13C/DTCO2ratio (0.024%
(mmol kg1)1) estimated from the temporal changes of TCO2 and d13C at HOT (Winn et al.,
1998; Gruber et al., 1999; Table 1) and the North Atlantic Ocean (Ko¨rtzinger et al., 2003). It differs, however, from ratios between 0.007 and 0.015% (mmol kg1)1 in the Southern Ocean (McNeill et al., 2001), 0.013% (mmol kg1)1 in the south-ern Indian Ocean (Chen and Chen, 1989), and 0.016% (mmol kg1)1in the northeastern Atlan-tic (Keir et al., 1998).
4. Conclusions
In this study, we thoroughly investigated the vertical distributions of TA, TCO2and d13CTCO2 in
the water column at SEATS site, northern SCS, which were collected from 19 different cruises spanning from September 1999 to October 2003. The depth profiles were then compiled to document their general distributions in the SCS. Furthermore, in order to understand better processes controlling their variations and to better quantify the influence of the anthropogenic CO2upon their distributions, we have deconvoluted these profiles into different components using various techniques that were widely used in marine biogeochemical community. The effort thus ensured our results to be comparable with data from other studies, and to be used in the future for a better coverage of carbon synthesis in the world oceans.
The property vs. salinity plots revealed that depth distributions of TA, TCO2 and d13CTCO2 in the
water column at SEATS were affected chiefly by the water mass structure and the rapid vertical advec-tion in the SCS. The vertical gradients of these parameters were all controlled, to different extent, by production/decomposition of organic matter, formation/dissolution of carbonates, and differ-ences in their respective preformed values. For NTA, due to the compensation of increment in preformed values and consumption of TA during organic decomposition, it remained nearly constant in the top 150 m of water column. From 150 to 350 m, the increase of NTA results principally from the increase in preformed values, whereas carbonate dissolution accounted for the continuous rise in NTA below 350 m. For NTCO2, increases in preformed values and organic oxidation with depth were equally important in controlling the observed increasing trend in the upper 400 m. Below 400 m, although carbonate dissolution together with or-ganic decomposition and increase of preformed
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Depth (m) 0 500 1000 1500 2000 2500 3000 Δδ13C a-p (‰)
Fig. 13. Depth profile of Dd13C
a–pat the SEATS site. See text for
the definition and calculation of ‘‘Dd13C
values led to the continuous increase of NTCO2 with depth, carbonate dissolution had a minor effect. As shown by results from stoichiometric model and the one dimensional diffusion-advection model, carbonate dissolution contributed to 25% of added TCO2. The decrease of d13CTCO2 with
depth, however, was attributed mainly to organic oxidation throughout the water column. Moreover, calculations of saturation level of carbonates showed that the saturation level of aragonite and calcite decreased 17% and 14%, respectively, in the surface water, and the aragonite saturation depth had migrated upward 100 m since the industrial revolution. Nonetheless, the observed TAcarb emerged at depths shallower than those hypothetically estimated from calcite and aragonite saturations, may also result from the biologically mediated dissolution of carbonate, upwards advection of TA-enriched deep water, and TA generated by anaerobic reduction reactions in shelves’ sediments.
The depth of anthropogenic CO2penetration in the water column at the SEATS site was estimated using two independent indicators, namely ‘‘anthro-pogenic CO2’’ and ‘‘Dd13Ca–p’’, which were derived from the ‘‘back calculation’’ and ‘‘d13C vs. phos-phate’’ methods, respectively. Results show that anthropogenic CO2 has penetrated to a depth of 1000 m. The calculated column inventory of anthropogenic CO2is slightly less than that in the same latitude of the NWP (16.6 vs. 20 mol m1), suggesting a greater upward advection in the SCS. Since more anthropogenic CO2 invasion is antici-pated, results from this study could provide a baseline for future studies of the impacts of anthropogenic CO2 on the carbon system in the SCS.
Acknowledgments
We are grateful to the Captains, crew and technicians of R/V Ocean Research I and III for assistance with deck operations and shipboard sampling, and to C.N. Sun, S.L. Lee, and Y.Y. Shih for laboratory assistance. We appreciate K.K. Liu, L.S. Wen, F.K. Shiah, and staff of the National Center for Ocean Research (NCOR) for cruise participation and logistic supports during the course of this research. Constructive comments and sug-gestions from two anonymous reviewers have greatly improved the manuscript. This work was supported by the National Science Council grants
(NSC90-2611-M-110-025-OP1, NSC91-2611-M-110-003 and NSC92-2119-M-110-001) to D.D. Sheu. The study is a contribution (#107) to the SEATS program sponsored by NCOR, National Science Council, Republic of China.
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