Anthropogenic carbon in the eastern South Pacific Ocean

We present results from the BIOSOPE cruise in the eastern South Pacific Ocean. In particular, we present estimates of the anthropogenic carbon C ant TrOCA distribution in this area using the TrOCA method recently developed by Touratier and Goyet (2004a, b) and Touratier et al. (2007). We study the distribution of this anthropogenic carbon taking into account of the hydrodynamic characteristics of this region. We then compare these results with earlier estimates in nearby areas of the anthropogenic carbon as well as other anthropogenic tracer (CFC-11). The highest concentrations of C ant TrOCA are located around 13° S 132° W and 32° S 91° W, and their concentrations are larger than 80 µmol kg -1 and 70 µmol kg -1 , respectively. The lowest concentrations were observed below 800 m depths (=2 µ mol kg -1 ) and at the Oxygen Minimum Zones (OMZ), mainly around 140° W ( -1 ). The comparison with earlier work in nearby areas provides a general trend and indicates that the results presented here are in general agreement with previous knowledge. This work further improves our understanding on the penetration of anthropogenic carbon in the eastern Pacific Ocean.


Introduction
Studies on the fate of anthropogenic CO 2 in the ocean interior are essential for understanding the global carbon cycle and its consequences on climate change and/or on the ocean itself (mainly due to its pH change induced by CO 2 acidification) (Doney and Ducklow, 2006). 20 In order to understand the role of the oceans as a sink for anthropogenic CO 2 , it is important to determine the distribution of carbon species in the ocean interior as well as the processes affecting the transport and storage of CO 2 taken up from the atmosphere (Chen and Millero, 1979;Peng et al., 2003).
Estimates of anthropogenic CO 2 concentrations in the oceans have been improved belt. Yet the equatorial belt (from 10 • N to 10 • S) plays a significant role in the global carbon cycle (Le Borgne et al., 2002). It annually supplies approximately 0.7-1.5 Pg C as CO 2 gas to the atmosphere, and it is the largest natural source of CO 2 from the ocean (Takahashi et al., 1997;Feely et al., 1999). In particular, as much as 72% of the 10 CO 2 outgassing from the world's oceans can be attributed to the equatorial Pacific.
The South Pacific Ocean (from the equator down to 60 • S) is globally considered as a sink area for anthropogenic carbon (Chen and Chen, 1993), but there is a strong zonal variability. Indeed, the western equatorial zone is the largest oceanic source of CO 2 to the atmosphere, while the eastern equatorial (Tans et al., 1990;Murray et 15 al., 1992;Murray et al., 1995) is considered as CO 2 sink. Here we present results from the BIOSOPE (BIogeochemistry & Optics SOuth Pacific Experiment) cruise in the eastern (East of 142 • W) South Pacific Ocean along a ∼8000 km transect a gradient ( Fig. 1). This transect covered a variety of hydrodynamic and trophic properties from the extremely rich upwelling waters of the Chilean coasts to the extremely oligotrophic 20 waters of the central gyre of the South Pacific Ocean, a priori expected to be the poorest oceanic area of the world (Longhurst, 1998;Claustre et al., 2007).
These specific zones could play a major role on CO 2 sequestration by the way of biological activity, thus taking up a significant amount of anthropogenic carbon from the atmosphere (Archer et al., 1996;Sabine and Key, 1998). Here, we use the data 25 from the BIOSOPE cruise to estimate the anthropogenic carbon. The TrOCA method recently developed by Touratier andGoyet (2004a, b), andTouratier et al. (2007) is then applied. We study the distribution of this anthropogenic carbon taking into account the hydrodynamic characteristics of the area. We then compare these results with those Introduction From the 223 vertical profiles, 23 were sampled for studying the oceanic carbon cycle ( Fig. 1). At each of these stations an average of 22 samples was collected throughout the water column from the near surface (∼3 m depth) to 500 m (excepted for 3 stations to the near bottom depth (>2000 m)). 15 Seawater was sampled into 500 ml borosilicate bottles and then after poisoned with a saturated solution of mercuric chloride before being sealed. They were then stored and shipped back to the laboratory where the measurements have been performed within one month after the end of the cruise 2.2 Parameter measurements 20 Total dissolved inorganic carbon (C T ) and Total Alkalinity (A T ) were determined by the potentiometric acid titration method (DOE, 1994). From replicate analysis of a reference seawater sample of known A T concentration, (CRM from Dr Andrew Dickson of Scripps Institution of Oceanography), the precision and the accuracy of the EGU analyses were determined to be within 1.5 µmol kg −1 for C T and 1.7 µmol kg −1 for A T . In this study, we use also salinity (S), potential temperature (θ; • C), oxygen (O 2 ; µmol kg −1 ), potential density (σ θ ; kg m −3 ), parameters measured on board. Oxygen (O 2 ; µmol kg −1 ) was measured using a photometric endpoint detector and a piston burette (Metrohm, Dosimat 716); dissolved oxygen was determined using a protocol 5 taken from "WHP Operations and methods" (Culberson, 1991).
Next we calculate the anthropogenic CO 2 concentrations (C TrOCA ant ) using the TrOCA (Tracer combining Oxygen, inorganic Carbon and total Alkalinity) approach developed by Touratier and Goyet (2004a, b) and Touratier et al. (2007). We then compare these results with the anthropogenic CO 2 concentrations (C ∆C * ant ) estimated earlier 10 (http://cdiac.ornl.gov/ftp/oceans/Pacific GLODAP ODV) using the ∆C* method (Gruber et al., 1996) and the WOCE and the JGOFS data sets.

Hydrography and circulation
This research concerns seawater dominated by both the South Equatorial Current and The hydrography of the region is summarized by the θ-S diagram shown in the Fig. 2. Both the easterly winds which drive away the superficial waters and the prevailing southerly winds off the Peruvian coast provoke an upwelling along the equator and the Peruvian coast. The cold and relatively low-salinity waters of the Humboldt Current are Introduction EGU advected northward from Chile to offshore of Peru (Strub et al., 1998;Kessler, 2006). These eastern boundary waters merge to supply the westward-flowing South Equatorial Current (SEC), which is subject to the divergence, north and south of the equator, and generates upwelling of waters which brings waters with higher salinity, C T , and nutrient concentrations to the surface (Kessler, 2006). Since chlorophyll concentrations remain low and the macronutrients are not depleted, this region is called the HNLC (high-nutrient/low-chlorophyll) area (Minas et al., 1986).
In the SEC (South Equatorial Current) the surface seawater is characterized by the warm and high-salinity SubTropical Surface Water (STSW, S>35). Along the coast of South America, Peru Current is characterized by cold, low-salinity water (Fiedler and 10 Talley, 2006). Closer to the coast, the Gunther Undercurrent is located between 100 and 400 m depth, and is characterized by the Equatorial SubSurface Water (ESSW) with a relatively high salinity (34.7-34.9) and nutrients concentrations, low temperatures (∼12.5 • C) and dissolved oxygen (Shaffer et al., 1995;Blanco et al., 2002). Underneath the SEC, the Subtropical Underwater (ESPCW: Emery and Meincke, 1986;15 Tomczak and Gogfrey, 2001) is located between 110 • W and 150 • W, and 10 • S to 20 • S around 150 m depth. At a few hundred meters water depth (around 200-400 m), there are two oxygen minimum zones (OMZ) which are driven by the degradation of organic matter sinking out of the euphotic zone and modified by ocean circulation (Wyrtki, 1962). The oxygen minimum zone is strongly linked with one of the most productive 20 marine ecosystem in the world, so that the oxygen deficiency is attributable to the high biological productivity at the surface seawater. This largest area of low oxygen in the world lies under the thermocline in the Eastern Tropical South Pacific Ocean. The area of low oxygen extends as tongues to either side of the equator from central and northern South  EGU ESSW closer to the coast (Blanco et al., 2001). The influence of Antarctic Intermediate Water (AAIW), can be seen at depths around 500 to 700 m, typically south of 26 • S and with salinities <34.5 and temperatures <7 • C (Blanco et al., 2001;Zenk et al., 2005;Fiedler and Talley, 2006). The bottom water (not shown in Fig. 3) originates from the Lower Circumpolar Water (LCPW). 5 The South Pacific subtropical gyre (SPSG) region (13-23 • S, 80/87-140 • W) has the highest surface (0-100 m) salinities (average value = 36.1) of the Eastern Tropical South Pacific Ocean regions and is defined by the "STSW" of Fiedler and Talley (2006).

Factors influencing the CO 2 distribution
The distributions of C T and pCO 2 are regulated by physical mixing and upwelling pro-10 cesses, biological uptake, and air-sea gas exchange (Watson et al., 1991;Broecker and Peng, 1992;Keeling, 1993;Takahashi et al., 1997).
In the Eastern Tropical South Pacific Ocean the flux of carbon dioxide gas (CO 2 ) into and out of the oceans is controlled by: (1) upwelling of C T rich water; (2) warming of cooler waters either recently upwelled or advected from higher latitudes; (3) wind 15 speed; and (4) the biological uptake of CO 2 by phytoplankton (Takahashi et al., 1997). Upwelling brings new C T rich water to the surface which CO 2 then diffuses into the atmosphere. Surface warming decreases the solubility of gasses in water and acts to increase venting to the atmosphere. The wind then facilitates the rate of transfer across the air-water interface. This venting to the atmosphere is reduced by biological uptake 20 of CO 2 by phytoplankton. However, widespread iron limitation in the open-ocean eastern tropical Pacific causes excess C T (in addition to nitrate) to remain unused in the region of HNLC waters. Conversely, where CO 2 is used by phytoplankton, the euphotic zone partial pressure of CO 2 can be drawn below atmospheric levels, and CO 2 will diffuse into the ocean. As with phytoplankton production there is a north-south 25 asymmetry in the eastern margin CO 2 fluxes, with high fluxes out of the ocean in the SEC and neutral conditions north of the equator. Similarly, upwelling of nitrate-deficient waters (Pennington et al., 2006) in the PCU region may also restrict biological produc-1821 Introduction EGU tion, leaving an excess of CO 2 which vents to the atmosphere.

General pattern
All physicochemical parameters show a large spatial variability in their distribution along the longitudinal section (Fig. 3). This variability is very large whatever the depth for most 5 of the parameters. However, the distributions of AOU and C T which exhibit a similar pattern, are less heterogeneous in the upper layer (between 0 and 200 m) than in the deeper layers. In general, two patterns appear: the values of θ, S, O 2 and A T decrease with depth while the other parameters (C T , AOU and σ θ ) increase.

Relationships with water masses and currents 10
The localization of the highest θ and S suggests that it corresponds to the water masses STSW at the surface and ESPCW, the Eastern South Pacific Central Waters at ∼100 m (Fiedler, 2006); see the θ-S diagram; Fig. 2). Low C T and AOU values (<2100 and 50 µmol kg −1 respectively) also correspond to ESPCW and STSW (located between 100 • W and 140 • W longitude and between the surface and 100 m depth). 15 In the eastern South Pacific Ocean the distributions of S, θ, σ θ , C T , A T and O 2 (Fig. 3) reflect the influence of the Peru Current (Pennington et al., 2006). Large concentrations of C T at the surface waters indicate that CO 2 flux increased from Peruvian Current Upwelling region surface waters to the atmosphere. This is indirectly caused by the upwelling of nitrate-deficient water (Pennington et al., 2006). 20 Not far from the Chilean coast (88 • W) and around 500 m depth, elevated concentrations of C T (≥2200µmol kg −1 ) and low values of A T (≤ 2300 µmol kg −1 ) (not shown) are located in the dense, cold and low salinity AAIW waters. In this area, C T distribution follows a general pattern of a consistent increase with increasing depth down to 500 m. This corresponds to what was observed by Fiedler and Talley (2006)  EGU nutrients and suggests the effects of biological uptake in surface waters and remineralization in deeper waters as well as to upwelling of thermocline waters into the surface layer evidenced by cold waters. The zone of low oxygen concentrations (Fig. 3) corresponds to the OMZ (between the coasts and the South Equatorial Current) described above. ) was computed below 150 m (the maximum depth of the wintertime mixed layer) (Fig. 3) and was found more or less all over the studied region, independently of water masses; however its distribution is not homogeneous. 10 The anthropogenic CO 2 (C TrOCA ant ) variations in subsurface waters are related to the duration of waters exposition to the air-sea interface. As shown in Fig. 3, the largest concentrations are located in areas where the densities are equal to or smaller than 26 kg m −3 . This is explained by the fact that C TrOCA ant invades the ocean by the exchanges at the air-sea interface; this corresponds to the subsurface water masses (between 150 15 and 350 m depth). A close correspondence exists between C TrOCA ant distribution and the distribution of θ, S at the western part of the studied area. However, in the Eastern part, C TrOCA ant doesn't seem to be linked to these parameters. This Eastern part corresponds to the upwelling zone described above.
The EGU lower penetration in the region at the Chilean coast and around the longitudes between 100 • W and 110 • W. This place represents the eastern part of the subduction of the Subtropical Underwater, and exhibits the deepest mixed layers. Surrounding South America (east of 90 • W) the anthropogenic CO 2 concentrations are controlled by the origin of the upwelling (especially its depth) and by the thermocline position. According 5 to Carr and Kearns (2003), water constituting the upwelling comes from σ θ surfaces ranging from 25.6 to 26.2 kg m −3 ; moreover, the vertical displacement of the upwelling at the ocean surface doesn't exceed 50 m between 15 • S and 34 • S. Therefore, these values are representative of this area.
The Using data from the three deep stations, we interpolated the estimated C TrOCA ant to the 20 lowest boundary of anthropogenic CO 2 penetration. This lowest boundary was defined by the depth at which the anthropogenic CO 2 was less than 6.25 µmol kg −1 that is the uncertainty of the involved parameters and variables in the TrOCA approach Touratier et al., 2007, Figs. 5 and 6 EGU anthropogenic CO 2 follow tightly those of AOU (Fig. 4).

Relationship between C TrOCA ant and circulation
According to Chen and Chen, (1993), in the Pacific Ocean, excess CO 2 (anthropogenic carbon concentration estimated by back-calculation from the carbonate data) doesn't penetrate below the thermocline because there is no bottom water formation in the 5 North Pacific Ocean while the shallowest penetration outside the Southern Ocean occurs in the eastern equatorial region where the excess CO 2 , penetrates down to 400 m. This shallow penetration in the equatorial Pacific, possibly reflects the high rate equatorial upwelling (Wyrtki, 1981;Broecker and Peng, 1998).

EGU
The two studies point out that the distribution of anthropogenic CO 2 in the Pacific Ocean is mainly concentrated in the isopycnal surface (σ θ ≤ 26.0 kg m 3 ). This reveals the intensive exchanges between these waters and the atmosphere.
Estimation differences between methods can be large Touratier et al., in press. Nevertheless, these early studies provide researchers with new insights into the distribution 5 of anthropogenic CO 2 in the Pacific Ocean.
When we compare the C TrOCA ant concentrations with the anthropogenic CO 2 concentrations (C ∆C * ant ) estimated earlier (http://cdiac.ornl.gov/ftp/oceans/Pacific GLODAP ODV) using the ∆C* method (Gruber et al., 1996) and the WOCE and the JGOFS data sets in the Eastern Pacific Ocean, along the 18 • S and 32 • S latitudes and along the longitudes 10 from 80 • W to 140 • W, C ∆C * ant ranged from 40 µmol kg −1 at the surface to 5 µmol kg −1 at 500 m depth (Figs. 5 and 6). The isolines at approximately 18 • S get progressively deeper westward over approximately 100 • W of longitude (Fig. 5). West of ∼140 • W the isolines are relatively flat. The broad scale of this eastern feature is related to the broad, slow nature of the currents within the gyre of the South Equatorial Current. In 15 the Eastern part of the covered area (from 100 • W towards the coastal area) and at a given depth, concentrations are variable while they don't vary much Westward. Moreover, the areas with maximum anthropogenic carbon (∼35 µmol kg −1 ) move upward from 200 m depth in the Western part to the surface in the Eastern part of the section 32 • S (Fig. 6). 20 The C ∆C * ant results (Sabine et al., 2002), from an area situated along 150 • W, between 10 • S and 35 • S at the proximity of our studied area, could give some indications about comparability of anthropogenic carbon in the Eastern Pacific. The C ∆C * ant ranged from 40 µmol kg −1 at the surface to 20 µmol kg −1 at 500 m depth. EGU cases, the inventory of anthropogenic carbon in the water column, reaches a maximum values around 40 • S and decreases towards the equatorial zone in the south Pacific.

Comparison with the CFCs
As expected, the distributions of CFC-11 and C TrOCA ant are similar. In particular, the 5 distribution on the WOCE and the JGOFS data sets show that the zero isosurface exhibits a behavior that is consistent with the C TrOCA ant results (Figs. 5 and 6). In summary, a detailed comparison among the various estimates of C ant distributions would point out significant differences. However, since today there is no absolute reference for C ant , it would be very difficult to determine the accuracy of any estimated 10 C ant . In spite of the high variability in the anthropogenic carbon distribution, due to the complex interactions between physico-chemical and hydrographic processes in the Eastern Pacific Ocean, this work provides more than a general trend. It specifies that anthropogenic carbon is always present even in oligotrophic areas. It improves our knowledge on the penetration of anthropogenic carbon in this part of the Ocean. EGU Emery, W. J. and Meincke, J.: Global water masses: summary and review, Oceanologica Acta, 9, 383-391, 1986. Feely, R. A., Sabine, C. L., Key, R. M., and Peng, T. H.: CO2 survey synthesis results: Estimating the anthropogenic carbon dioxide sink in the Pacific Ocean, U.S. JGOFS News, 9, 1-16, 1999.