Stable isotopic signature of Southern Ocean deep water CO2 ventilation

Deep-Sea Research II 118 (2015) 177–185

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Deep-Sea Research II journal homepage: www.elsevier.com/locate/dsr2

Stable isotopic signature of Southern Ocean deep water CO2 ventilation K. Prasanna a, Prosenjit Ghosh a,b,n, N. Anil Kumar c a b c

Centre for Earth Sciences (CEaS), Indian Institute of Science, Bangalore 560012, India Divecha Centre for Climate Change, Indian Institute of Science, Bangalore 560012, India National Centre for Antarctic and Ocean Research, Headland Sada, Goa 403804, India

art ic l e i nf o

a b s t r a c t

Available online 25 April 2015

The link between atmospheric CO2 level and ventilation state of the deep ocean is poorly understood due to the lack of coherent observations on the partitioning of carbon between atmosphere and ocean. In this Southern Ocean study, we have classified the Southern Ocean into different zones based on its hydrological features and have binned the variability in latitudinal air-CO2 concentration and its isotopic ratios. Together with air-CO2, we analysed the surface water for the isotopic ratios in dissolved inorganic carbon (DIC). Using the binary mixing approach on the isotopic ratio of atmospheric CO2 and its concentration, we identified the δ13C value of source CO2. The isotopic composition of source CO2 was around
9.22 70.26‰ for the year 2011 and 2012, while a composition of
13.49 7 4.07‰ was registered for the year 2013. We used the δ13C of DIC to predict the CO2 composition in air under equilibrium and compared our estimates with actual observations. We suggest that the degeneration of the DIC in presence of warm water in the region was the factor responsible for adding the CO2 to the atmosphere above. The place of observation coincides with the zone of high wind speed which promotes the process of CO2 exsolution from sea water. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Southern Ocean CO2 Deep water ventilation DIC δ13C

1. Introduction Fossil fuel emissions during the industrial era were found responsible for the increase in air-CO2 concentration from its preindustrial value of  280 PPMV to the present day concentration of  400 PPMV. This led to an overall increase in the global mean temperatures by approximately 0.76 1C 70.19 1C (IPCC, 2007). The global ocean currently absorbs annually about 2 Pg C yr
1 (1 Pg ¼1 petagram¼1015 g¼1 billion tons) of CO2 from the air (Takahashi et al., 2012). The rate of absorption of CO2 generated due to natural and anthropogenic emissions (e.g. fossil fuel burning, cement manufacture, and gas flaring) played a significant role in defining the residence time of CO2 in the atmosphere. The CO2 thus emitted stayed in the atmosphere for a duration of 5–200 years until it was scavenged by either land biosphere or oceanic sinks. However, the rate of exchange of CO2 varies significantly between land and ocean. The Southern Ocean (SO), demarcated by the oceanic region south of 441S is unique as it has been identified as a major sink of global carbon accounting for 1 Pg C yr
1 which is close to 50% of the CO2 emitted into the atmosphere (Caldeira and Duffy, 2000; Fletcher et al., 2006).

n Corresponding author at: Centre for Earth Sciences (CEaS), Indian Institute of Science, Bangalore 560012, India. E-mail address: [email protected] (P. Ghosh).

http://dx.doi.org/10.1016/j.dsr2.2015.04.009 0967-0645/& 2015 Elsevier Ltd. All rights reserved.

Projections suggested that the region will continue to be an important sink of atmospheric CO2, although the rate of sinking might decrease in the future (Roy et al., 2011). The process of CO2 uptake in the SO is poorly understood due to the lack of data; however the role of temperature and wind in the process of CO2 uptake is well documented in the literature (Longinelli et al., 2012). The uptake of CO2 occurs through a combination of biological and physical processes. Even though the SO has a net negative flux of CO2 per year, there are certain zones in the SO where the net flux of CO2 is positive during summer (Metzl, 2009). Wind stress has a positive role in driving the variations in CO2 fluxes (Anderson et al., 2009; Waugh, 2014; Waugh et al., 2013). CO2 efflux in the SO is important because it is regarded as a High Nutrient Low Chlorophyll (HNLC) region where the chlorophyll biomass remains low, despite an abundant supply of the major nutrients (Martin et al., 2013). The poorly utilised nutrients in the region of the SO have important implications for the global carbon cycle. The process of assimilating inorganic carbon (atmospheric CO2) into organic matter during photosynthetic pathways and its subsequent conversion to organic carbon during burial, and inorganic carbon by means of heterotrophic respiration and reequilibration with atmospheric CO2, is collectively referred to as the “biological pump”. This pump is one of the factors responsible for maintaining DIC concentrations and atmospheric CO2 concentration over the SO. However, the HNLC water of the SO represents an “open window” through which a CO2 efflux is possible, thus

K. Prasanna et al. / Deep-Sea Research II 118 (2015) 177–185

70ºS

40ºS

10ºS

Agulhas Retroflection Front (ARF)

35 34.5

Several researchers have demarcated major hydrological fronts and zones of water masses over the SO based on sharp contrasts in the temperature, salinity and productivity (Eynaud et al., 1999; Sparrow et al., 1996). For ease of understanding, we demarcated the entire SO into four fronts which were named: The Agulhas Retroflection Front (ARF) which covers the latitude from 381S to 391S and is characterised by a drop in temperature and salinity from 19.6 1C to 18.8 1C and 35.7 to 35.5; the Sub Tropical Front (STF) which lies between 401S and 421S and isolates the warmer tropical water from the cooler subtropical water where the temperature drops from 18.7 1C to 12 1C, and the salinity drops from 34.11 to 33.8; the Sub Antarctic Front (SAF) which covers latitudes from 451S to 481S and is characterised by a drop in salinity level from 33.9 to 33.8 and the temperature drops from 10.5 1C to 6.3 1C and the Polar Front (PF) which extends from 491S till 561S where the salinity values fall in narrow range of 33.8–33.9 while the temperature varies from 4.8 1C to 3.1 1C (Anilkumar et al., 2005; Srivastava et al., 2007). These zones, marked by the sea surface temperature and salinity changes were easily traceable on the surface (Srivastava et al., 2007) and remained nearly similar during the austral summers of the years 2011, 2012 and 2013 as shown in Fig. 1.

2.2. Biogeochemical regions. Across the meridional transect, the SO is broadly classified into five biogeochemical zones (Pollard et al., 2002; Racape et al., 2010) based on productivity and temperature. This includes the Tropical Indian Ocean (TIO) which extends from 201N to 201S, the SubTropical Zone (STZ) between 201S and 351S, the Transition Zone (TZ) between 351S and 401S, the Sub-Antarctic Zone (SAFZ) between 401S and 451S, the Polar Frontal Zone (PFZ) between 451S and 501S, and the Antarctic Zone (AAZ) beyond the south of 501S.

3. Material and methods The work presented here is based on the δ13C and concentration measurement of air-CO2 and the δ13C measurement of DIC measured on the air and water samples collected at intervals across the SO on-board ORV Sagar Nidhi on three expeditions coinciding with the austral summers of 2011, 2012 and 2013. The sampling stations are marked in Fig. 2.

Salinity

35.5

34 33.5 30 20

2006

2011

2012

2013

10 0

2. Hydrological and biogeochemical context 2.1. Major hydrological fronts and water masses

20ºN

36

Polar Front (PF)

reducing the efficiency of the biological pump. Hence it is important to study the SO for the zone of CO2 efflux and characterise the source. In this paper, we present observations on atmospheric CO2 concentration, the δ13C in atmospheric CO2 sampled in the glass flask and the δ13C of DIC in sea surface water samples collected across the latitudes over the Indian sector of the SO during the year 2011, 2012 and 2013. The intercept value in the plot of δ13C in atmospheric CO2 and 1/CO2 concentration, popularly known as the Keeling's mixing model (Keeling, 1958), was used to trace the source. We have also discussed the role of wind influencing the CO2 venting from the SO.

Subantarctic Front (SAF) Subtropical Front (STF)

178

70ºS

40ºS

10ºS

20ºN

Fig. 1. Variation in salinity and temperature of sea water (SST till 75 m water depth) across the Southern Ocean is shown in this plot. The data recorded during the Southern Ocean expedition 2011, 2012 and 2013 are compared with observations from the previous study in the same region (Srivastava et al., 2007). Note the large shift in salinity and temperature in the region lying between latitudes 401S and 551S, while the inter-annual variability of the positions of different fronts was negligible.

3.1. δ13C and concentration measurement of air-CO2 Air samples were collected once in a day at different stations (mentioned in Table 1) when the sky was clear. The samples were stored in glass flasks of three litre capacity after ensuring complete removal of moisture using a trap filled with Mg (ClO4)2. The individual flasks were conditioned with sample air using an external pump (NMP 850 KNDC.KNF Neuberger, Freiburg, Germany) operated with a 12 V battery, where 15 min of flushing time was given at a flow rate of 4.5 l/min. The final pressure of  1.2 bar was achieved after the complete filling of air in the flask. CO2 from the air samples were extracted using the cryogenic extraction procedure described in (Guha and Ghosh, 2013) for δ13C analysis in an Isotope ratio mass spectrometer (Thermo Fisher- MAT 253) with dual inlet peripheral. The standard primary carbonate NBS-19 and internal air reference material, ‘OASIS AIRMIX’(Guha and Ghosh, 2013) were analysed intermittently to check the consistency and reproducibility of the analyses. JRAS 06, which is a multi-point scale anchor for isotope measurements of CO2 in air (Wendeberg et al., 2013) was analysed together with air samples to express the delta values of air-CO2 samples in the VPDB scale (shown in Supplementary Fig. 1). An offset correction was incorporated for the presence of N2O in the sample air and the inconsistency detected while analysing the JRAS 06 reference air. The CO2 concentration was measured using two different approaches; during the year 2011, CO2 mixing ratios were measured using a Gas Chromatograph (Thermo Fisher - Chemito GC 8610), whereas for other years 2012 and 2013, the CO2 mixing ratios were determined using a gravimetric technique. Gas Chromatography method involves the separation of CO2 from other components by passing an aliquot of an air sample through a CPPora PLOT Q column and analysing the CO2 by a methanizer with a Flame Ionization Detector (FID). The Gravimetric method involves the cryogenic extraction of CO2 followed by the transfer of CO2 to an ampoule with known volume, where the pressure was accurately measured using a MKS Baratron at a temperature of 25○C. The value was further converted to number of micro moles of CO2 using the pressure volume relationship of the ideal gas law (pV¼nRT). By knowing the number of moles of the air sample, the CO2 mixing ratio was calculated based on the volume of sample air in the flasks and initial and final pressure in the flask

K. Prasanna et al. / Deep-Sea Research II 118 (2015) 177–185

179

2013 2012 2011

Agulhas front Sub tropical front Sub-Antarctic front Polar front SACC front

Fig. 2. Map showing the sampling locations across the meridional transects during three years of air and water observations. (Abbreviated as J¼ January and F¼ February). Also displayed in the figure are the position of the fronts (refer to text).

(Affek and Eiler, 2006; Guha and Ghosh, 2013). The average value obtained from 19 replicates of CO2 concentration in the JRAS 06 air standard cylinder was 401 PPMV (Standard error¼ 0.77 PPMV), while the true value was 402 PPMV (shown in Supplementary Fig. 1). 3.2. δ13C measurement in DIC (δ13CDIC) Dissolved Inorganic Carbon (DIC) is designated as the cumulative of all species of carbon, namely carbonic acid (H2CO3), bicarbonate ðHCO3 Þ
and carbonate ions ðCO3 Þ2
. To determine the carbon isotopic ratio of inorganic carbon, surface sea water was collected (  1–5 m depth) in a clean bucket at different stations. The water samples were collected in a glass amber bottle capped with butyl rubber septa and crimped with aluminium caps. 1 ml of saturated HgCl2 solution was added to the water samples to arrest any post sampling biological activities and for long storage. The δ13C of DIC was measured by acidifying 2 ml of water with 0.5 ml of 100% ortho-phosphoric acid (Assayag et al., 2006). An Isotope ratio mass spectrometer (Thermo Fisher- MAT 253)

coupled with a Gas bench II peripheral was used in continuous flow mode for the analysis. While performing the analyses, both international carbonate standards and in-house standards (NBS19 and MARJ1) were analysed along with the Na2CO3 standard solution (DIC standard) prepared for DIC measurement. The DIC standard solutions with variable concentrations of DIC (1500– 2000 μmol mL
1) were generated for the isotopic analyses. These solutions were prepared by dissolving 7.94 mg to 13.24 mg of analytical grade Na2CO3 in 50 ml of double distilled water. While analysing the samples, the standards mentioned above were analysed intermittently for estimating the analytical reproducibility of δ13C. The standard deviation recorded in multiple analyses of the internal lab standard was 0.09‰ for δ13C. The δ13C of atmospheric CO2 and the DIC of surface ocean water in the majority of cases is out of equilibrium (Tans et al., 1993). However, in the window of our study area between 40 and 531S, the disequilibrium is near zero (Quay et al., 2007). In this study, we estimated the δ13C of CO2 in equilibrium with DIC at the observed Sea Surface Temperature (SST) (5–25 1C). The empirical relationship used to derive the δ13C of air-CO2 on top after taking into account

180

K. Prasanna et al. / Deep-Sea Research II 118 (2015) 177–185

Table 1 Location, date and time of samples collected for δ13C of air-CO2 and air-CO2 concentration across latitudes for the years 2011, 2012 and 2013. Longitude

Date

Time

Concentration PPMV

δ13C

δ13C and CO2 5th Southern Ocean Expedition, 2011 19M11 10.60 18M11
0.75 17M11
11.08 16M11
18.45 1/J11
38.13 2F11
40.12 14F11
41.98 13F11
43.98 3F11
44.98 12F11
46.38 5F11
52.00 6F11
54.02 11F11
54.00 10F11
58.10 7F11
60.03 8F11
60.05 9F11
60.07

89.10 80.05 72.77 59.98 57.50 57.48 47.00 46.98 57.50 47.00 57.50 57.50 47.00 46.88 51.98 48.55 48.55

29/03/2011 23/03/2011 19/03/2011 14/03/2011 31/01/2011 02/01/2011 26/02/2011 25/02/2011 02/04/2011 24/02/2011 02/11/2011 02/12/2011 20/02/2011 18/02/2011 16/02/2011 17/02/2011 17/02/2011

14:30 14:40 18:40 17:30 14:55 14:45 17:00 16:00 16:55 17:30 15:30 17:00 17:30 0:00 16:00 16:00 16:00

367.06 366.09 342.92 329.65 378.88 367.77 331.09 414.31 370.77 379.45 230.84 336.28 298.75 357.66 414.79 356.01 368.11


8.75
8.42
8.40
8.37
8.26
8.37
8.25
8.44
8.86
8.30
8.32
8.22
8.28
8.47
8.28
8.45
8.24

δ13C and CO2 6th Southern Ocean Expedition, 2012 1/D11 14.38 2/D11 10.67 3/D11 2.78 4/D11 0.98 5/J12
5.15 6/J12
11.03 7/J12
16.47 8/J12
19.93 9/J12
25.10 20/F12
27.33 10/J12
31.27 11/J12
35.05 13/J12
39.98 12/J12
40.00 19/J12
41.75 13a/J12
42.22 14/J12
43.18 18/J12
46.18 17/J12
48.58 15/J12
50.58 16/J12
51.27

74.08 75.17 78.77 77.55 75.30 72.85 70.27 68.67 66.23 57.28 63.15 61.22 54.10 56.50 57.65 53.50 58.58 57.47 57.57 57.50 57.67

25/12/2011 26/12/2011 29/12/2011 30/12/2011 01/01/2012 03/01/2012 05/01/2012 06/01/2012 08/01/2012 03/02/2012 10/01/2012 11/01/2012 16/01/2012 15/01/2012 28/01/2012 17/01/2012 19/01/2012 26/01/2012 25/01/2012 22/01/2012 24/01/2012

17:00 14:42 14:30 11:23 12:00 9:30 10:50 17:22 13:55 16:25 15:53 17:53 17:38 17:13 19:19 19:36 16:04 16:44 18:30 15:53 18:55

394.74 392.86 400.00 395.28 395.12 383.95 390.05 392.02 386.18 433.37 378.26 389.51 391.83 374.77 377.39 377.62 348.91 371.97 376.22 382.30 371.83


8.41
8.58
8.37
8.41
8.42
8.33
8.32
8.32
8.42
8.40
8.41
8.41
8.41
8.36
8.39
8.37
8.32
8.33
8.45
8.47
8.37

δ13C and CO2 7th Southern Ocean Expedition, 2013 2/J13 5.72 3/J13
1.42 4/J13
5.05 5/J13
11.46 6/J13
18.17 7/J13
24.84 23/F13
25.83 8/J13
28.40 9/J13
31.78 22/F13
35.71 10/J13
37.73 11/J13
40.44 21/F13
41.54 12/J13
43.44 13/F13
48.85 20/F13
50.50 19/F13
51.01 18/F13
53.13 14/F13
55.69 16/F13
56.50 15/F13
56.52 17/F13
56.60

80.09 78.01 79.28 76.35 72.29 68.10 56.96 65.75 63.40 57.27 57.84 55.07 57.56 58.16 57.48 54.71 51.51 47.87 57.59 54.69 56.49 49.20

14/01/2013 16/01/2013 17/01/2013 19/01/2013 21/01/2013 23/01/2013 25/02/2013 24/01/2013 25/01/2013 22/02/2013 27/01/2013 28/01/2013 20/02/2013 30/01/2013 01/02/2013 17/02/2013 15/02/2013 13/02/2013 04/02/2013 08/02/2013 05/02/2013 11/02/2013

9:01 16:58 17:21 16:15 16:25 16:30 16:34 15:41 16:16 18:40 15:41 19:17 18:00 16:20 17:27 18:15 17:14 18:30 17:16 16:40 18:00 16:55

400.68 389.88 392.68 402.16 394.96 384.80 396.66 388.61 404.27 389.88 379.30 417.40 394.54 390.31 391.15 405.97 389.46 380.99 430.94 387.77 395.39 394.96


8.78
8.51
8.34
8.93
8.22
8.32
8.36
8.26
8.29
8.23
8.20
8.80
8.14
8.34
8.15
8.08
8.41
8.22
8.31
8.31
8.36
8.31

Sample number

Latitude


the fractionation of the δ13C between HCO and 3  the gaseous  CO2 HCO3
3 at known temperatures is: 10 ln αCO2 ¼ 9:36 103 =T Kelvin
23:5 (Zhang et al., 1995). The biogeochemical zones described earlier are mapped with mean sea surface δ13CDIC, δ13C air-CO2 and CO2 concentrations along with their standard deviations. The mean and standard deviations were calculated based on the number of samples in

each zone. The TIO had 8 samples for the year 2012 and 7 samples for the year 2013 for δ13CDIC, whereas the data for δ13C in air-CO2 and CO2 concentration are based on 4, 8 and 5 samples for the years 2011, 2012 and 2013, respectively. The STZ had 4 samples for the year 2012 and 5 samples for the year 2013 for δ13CDIC, whereas for δ13C in air-CO2 and CO2 concentrations the number of samples were 4 and 5 for the years 2012 and 2013, respectively. The TZ had

4. Results 4.1. δ13C of DIC Prior to our work, Racape et al. (2010) reported the summer and winter time distributions of sea surface δ13CDIC across the SO. In their study, the amplitude of change in δ13CDIC between summer and winter is attributed to biological activity during summer and to deep vertical mixing during winter. In more recent times, a compilation of isotopic signature in DIC for the world ocean was presented in (Schmittner et al., 2013). In this study, we have documented the variability of δ13C in DIC across the Indian sector of the SO. The meridional average δ13CDIC for the years 2012 and 2013, corresponding to the regions of biogeochemical distinctions, are shown in Fig. 3. The mean δ13CDIC values for the years 2012 and 2013 in TIO are 0.78 70.08 and 0.66 70.14‰, respectively. Similarly, the observed δ13CDIC values in the STZ are 0.83 70.09‰ for the year 2012 and 0.91 7 0.28‰ for the year 2013. At the TZ, the mean δ13CDIC values for the years 2012 and 2013 are 1.07 70.13 and 0.81‰, respectively. The SAFZ showed an enriching trend in δ13CDIC with average δ13CDIC values approaching 1.01 70.16‰ and 1.54 70.28‰ for the years 2012 and 2013, respectively. The δ13CDIC values recorded for the PFZ are 1.48 70.12‰ and 1.53 70.09‰ for the years 2012 and 2013, respectively. The AAZ registered a consistent depletion compared to that of the SAFZ and PFZ with average values for the years 2012 and 2013 being 1.4‰ and 1.28 70.11‰, respectively. 4.2. Concentration in air-CO2 The inter-annual variability of CO2 concentration in air over the SO is presented in Table 1 and displayed in Fig. 4(a). Here we presented the mean value for air-CO2 concentration and the standard deviation for each of the biogeochemical zones previously described. The CO2 concentration over the TIO for the years 2012 and 2013 showed a concentration of 393 7 4.6 PPMV and 396 7 5.2 PPMV, whereas for the year 2011, the mean value recorded was 3517 18.31 PPMV. The mean concentration recorded in the air samples collected during the years 2012 and 2013 from the STZ are 396 724.8 PPMV and 392 7 7.6 PPMV respectively. Similarly, the mean value of air CO2 recorded at the TZ for the years 2012 and 2013 were 383 712 PPMV and 398 7 26.9 PPMV, respectively, while for 2011, the average value was 373 77.8 PPMV. Likewise, the average value recorded in the SAFZ

70ºS

60ºS

50ºS

40ºS

30ºS

20ºS

10ºS

0

1

‰ DIC

1.5

VPDB

2

13

2013

2.5 Individual data

δ C

2012

Tropical Indian Ocean (TIO)

Polar frontal Zone(PFZ) Sub antarctic frontal Zone (SAFZ)

Meridonial average

Transition Zone (TZ)

2012 2013

Antarctic Zone(AAZ)

3 samples for the year 2012 and 1 sample for the year 2013 for δ13CDIC whereas for δ13C in air-CO2 and CO2 concentrations the number of samples were only 2 for the years 2011, 2012 and 2013. The SAFZ had 8 samples for the year 2012 and 5 samples for the year 2013 for δ13CDIC whereas for δ13C air-CO2 and CO2 concentrations 3 samples were considered for the year 2011 and 2012 while 2 samples for the year 2013 were considered. In the PFZ, we had 3 samples for the year 2012 and 5 samples for the year 2013 for δ13CDIC whereas for δ13C air-CO2 and CO2 concentrations the number of samples were 1, 3 and 2 samples for the years 2011, 2012 and 2013, respectively. And, finally, in the AAZ we had 1 sample for the year 2012 and 21 samples for the year 2013 for δ13CDIC whereas for δ13C air-CO2 and CO2 concentration the number of samples were 5, 1 and 4 for the years 2011, 2012 and 2013, respectively. The meridional distribution of wind speed was calculated using available long term data on zonal-mean wind speed in the southern hemisphere between 471E to 1001E, for the austral summer months of January, February, and March. (Data from National Centers for Environmental Prediction reanalyses averaging 1971–2000).

181

Subtropical Zone (STZ)

K. Prasanna et al. / Deep-Sea Research II 118 (2015) 177–185

0.5 0

10ºN 20ºN

Fig. 3. Variation of δ13C of DIC w.r.t. VPDB across the meridional transects. Hollow squares and circles represent the individual data points while filled squares and circles are estimates of the zonal average with standard deviation (as error bar) for the specific biogeochemical zones described in the text.

for the years 2012 and 2013 were 367716.5 PPMV and 392 72.9 PPMV, respectively and for the year 2011, the average value was 372 741 PPMV. The PFZ, for the years 2012 and 2013, showed mean values of 376 75 PPMV and 398 7 10 PPMV, respectively, and for the year 2011, the mean value recorded was 379 PPMV. At the AAZ for the year 2011, the value recorded was 355738 PPMV. The years 2012 and 2013 registered concentration values of 371 PPMV and 396 717 PPMV, respectively, in the air samples collected at the same zone. A notable feature documented in our observation is a large scatter in the concentration values as one moves to progressively higher latitudes. The drop in CO2 concentration in the air samples is suggestive of either enhanced productivity or a lowering of sea water temperature enabling a large suction of CO2 from the air to the water. 4.3. δ13C of Air CO2 The data on the inter-annual variability of δ13C of air-CO2 across the SO are shown in Table 1 and Fig. 4(b). The δ13C of air-CO2 over the TIO showed a consistent interannual pattern where the lighter carbon isotope composition of air-CO2 was recorded in the sample air collected from the northern hemisphere. The mean values recorded for individual years were
8.4870.18,
8.4070.09 and
8.567 0.3‰ for the years 2011, 2012 and 2013, respectively. The location at 11127.530 S 76121.260 E during 2013 captured an extremely light δ13C value of
8.93‰ whereas for the years 2011 and 2012, the δ13C values were
8.4 and
8.33‰, respectively. On reaching the STZ the δ13C of air-CO2 varied within
8.42 and
8.23‰. There were no samples collected in this region in the year 2011, whereas in the year 2012, 4 samples had a nearly identical composition (
8.4170.01‰). 5 samples collected from this region in the year 2013 showed a composition of
8.2970.05‰. The mean δ13C values measured in the air-CO2 samples collected during the years 2011, 2012 and 2013 from TZ were
8.3270.08,
8.3870.04 and -8.5070.42‰, respectively. It was observed that the analytical results of the air-CO2 sample collected at 40 126.120 S55104.140 E during the 2013 expedition showed an extremely light δ13C value of
8.79‰, which closely matched the observations documented in another independent experimental study conducted on-board the M/V Italica during the year 2013 (Longinelli et al., 2013). The SAFZ experienced a large variation in air-CO2 δ13C values. The δ13C values recorded in our observation ranged between
8.86 to
8.14‰. A lighter value of
8.86‰ was recorded at the station 431590 S 461590 E. The mean values estimated were
8.5270.3,
8.3670.04 and
8.2470.14‰ for the years 2011, 2012 and 2013, respectively. The air-CO2 samples collected from PFZ presented in this study measured δ13C values of
8.30,
8.4270.07 and
8.127 0.05‰ for the year 2011, 2012 and 2013, respectively. The AAZ air-CO2 δ13C value laid between
8.47 to
8.22‰, with the mean δ13C values

K. Prasanna et al. / Deep-Sea Research II 118 (2015) 177–185

5. Discussion 5.1. Zonal variability of wind strength and CO2 composition The oceans are the main store house of CO2, and they continuously exchange carbon dioxide during photosynthesis, upwelling, degassing and hydration. Limited information is available on the role of wind as one of the factors driving CO2 concentration over the oceanic atmosphere. We investigated the role of wind on the SO air-CO2 concentration, more specifically at the SAFZ. The meridional distribution of wind speed is shown in Fig. 6(a) top panel. Since the SO has no landward boundary, the atmospheric westerlies drive a strong eastward flow of Antarctic circumpolar current (ACC). Together with the Ekman drift, the process facilitates the overturning of meridional circulation. This upwelling process in the Antarctic region causes divergence, which exposes deep waters with elevated DIC concentrations to the surface. The CO2 produced from the dissociation of DIC mixes 430

2

Sub antarctic frontal Zone (SAFZ)

-8.0

2012

230

-8.1 Tropical Indian Ocean (TIO)

-8.8 -8.9 -9.0 70°S

60°S

Subtropical Zone (STZ)

-8.7

Transition Zone (TZ)

-8.6

Polar frontal Zone(PFZ)

-8.5

Antarctic Zone(AAZ)

-8.4

VPDB

280

2013

-8.3

δ

330

2011

-8.2

13

C

of atmospheric CO (‰)

380

2

being
8.3270.1,
8.37 and
8.3270.06‰ for air-CO2 analysed during the years 2011, 2012 and 2013, respectively. To check the variability of air-CO2 concentration over the SO and identify the isotopic composition of dominant source CO2, a two component mixing model approach was adopted where the intercept value was used to characterise the source CO2 composition. A representative binary mixing model plot with dataset collectively from the SAFZ upto the AAZ are plotted in Fig. 5(a) and (b). The end member signature of the source air-CO2 contributed from degassing or ventilation varies annually. The collective record showed the δ13C of the end member as
9.2270.24‰ based on several data points for the years 2011 and 2012, whereas, for 2013, the end member is
13.4974.07. The error envelope with a 99% and 95% confidence limit showed the variability of the slope and the intercept. We identified outliers in 2011 data, where an air packet originating from coastal South America carrying an anomalous signature of a coastal upwelling was documented. This was further verified using HYSPLIT air trajectories (Supplementary Fig. 2).

CO Concentration (PPMV)

182

50°S

40°S

30°S

20°S

10°S

0

10°N

20°N

13

Fig. 4. (a) CO2 concentration across the meridional transects also demarcated is the biogeochemical zones. (b) δ C of atmospheric air-CO2 measured in air collected onboard the ORV Sagar Nidhi for three years (5th Southern Ocean Expedition, 2011 designated as filled triangles, 6th Southern Ocean Expedition, 2012 designated as filled squares and 7th Southern Ocean Expedition, 2013 designated as filled diamonds).

CO2 Concentration PPMV 434.8

400.0

416.7

384.6

370.4

357.1

CO2 Concentration PPMV 344.8

425.5

322.6

333.3

400.0

392.2

fide

nc

l

0. 22 ± t -9. ) cep 2=0.51 r e t (r In

-8.3

26 ‰

-8.4

95

%C

-8.6 0.0023

384.6

fide

0.0024

val

0.0025

0.0026

0.0027

0.0028

0.0029 -1

1/CO mixing ratio (µmol mol ) 2

0.0030

0.0031

Con

fiden

ce in

377.4

l terva

-8.0

-8.5

3.49 ± ept -12 .28) (r =0

4.07 ‰

Interc

-9.0

13

on

ter

δ C

-8.5

in nce

99% 2

on

rva

of atmospheric CO (‰)

9

C 9%

te e in

VPDB

2

-8.2

VPDB

of atmospheric CO (‰) 13

408.2

-7.5

-8.1

δ C

416.7

95

%C

-9.5 0.00235

on

f

n ide

ce

int

0.00240

erv

al

0.00245

0.00250

0.00255

0.00260

0.00265

-1

1/CO mixing ratio (µmol mol ) 2

Fig. 5. (a) Plot suggesting an inverse relationship between air-CO2 concentration and δ13C value measured across the sampling stations occupied between 401S and 531S latitudes for the years 2011 and 2012. The δ13C value of carbon source predicted based on the data is
9.22 7 0.26‰. The observed value closely matches with the δ13C value estimated from the DIC of surface water under equilibrium conditions in this region. (b) Inverse relationship for the year 2013 suggests the source composition to be
13.49 74.07‰.

K. Prasanna et al. / Deep-Sea Research II 118 (2015) 177–185

Polar frontal Zone (PFZ)

12

Transition Subtropical Zone Zone (STZ) (TZ)

Tropical Indian Ocean (TIO) Jan

10

Feb

Mar

8

Increase in wind speed Sub antarctic frontal Zone 2 Antarctic Zone(AAZ) (SAFZ) 6 4 0

2012

0 75

2050

50

510

2100 21

2125

22

2275

00

50oS

22

2200

1020 60oS

2150

2125

2200

2250

21

Depth (m)

2250

Contours represent DIC in μmol kg-1

2013

40oS

30oS

25

20oS

2250 2300

10oS

0o

(Calculated)

-6 -6.5 -7 -7.5 -8 -8.5 -9 -9.5 -10

δ13C

Wind Speed (ms-1)

14

183

10oN

0M 8 200 M

7.95

400 M

7.85

600 M

7.8

800 M

7.75 7.7 16

1000 M 0M

10 8

600 M

6 800 M

o

12 400 M

Temperature ( C)

14

200 M

1000 M

pH

7.9

4 o

52 S

o

50 S

o

48 S

o

46 S

o

44 S

o

42 S

o

40 S

Fig. 6. (a) Zonal mean wind speed between 471E and 1001E for the austral summer months (Jan, Feb, and March) based on data from the National Centres for Environmental Prediction reanalyses (1971–2000 averages) are plotted. (b) δ13C calculated from the δ13CDIC and temperature of water measured in our study. (c) The available data on the depth profile of DIC concentration upto the oxygen minima zone (OMZ) for the year 2007 adapted from WOCE I8s-I9n cruise (www.pmel.noaa.gov/co2/story/I9N) was used for comparison. Our measurements of (d) pH variation (e) and temperature change (interpolated based on discrete measurements) in the water column across the latitudes 401S to 531S coincide with the defined zone of DIC excursion.

with the atmosphere on top. The δ13C of air-CO2 in equilibrium with the δ13C of DIC is calculated (Table 2) at known temperatures (SST). As shown in the Fig. 6(b), the δ13C values of 
8.5‰ estimated using the equilibrium model matches with the proposed offset of 0.6‰ suggested as the factor of air-sea δ13C disequilibrium for the Indian Ocean (Quay et al., 2003). This further validated the δ13C values predicted for the source CO2 using the binary mixing approach shown in Fig. 5(a). This is also supported by the observation of elevated DIC concentration measured in our study along with the data available across the cross section of the SO based on the WOCE expedition (Fig. 6c). We captured the variability of pH as well as temperature in this region reflecting DIC concentration (Fig. 6(d), (e)). Observations suggest that during the last few decades, a strengthening of winds (Lenton and Matear, 2007) has caused a rise in CO2 concentrations and induced significant warming in the southern hemisphere

(Thompson and Solomon, 2002). Direct observations demonstrating the role of wind in modulating the atmospheric CO2 concentration and its composition are limited (Metzl, 2009) and require extended observations. The zone demarcated as the AAZ experienced a favourable wind condition supporting a higher level of CO2 in air with unique carbon isotope ratios. 5.2. Carbon isotope ratios in CO2 from deep ocean Upwelling in the SO at coastal Antarctica and wind induced divergence expose the deep waters, with elevated concentrations of DIC, to the surface, causing exsolution of CO2 to the atmosphere. The zone (SAFZ and PFZ) lying between 401S and 531S latitude is ideal zone to capture the signature of CO2 venting from the ocean water. The end member composition shown in Fig. 5(a) suggests the average composition of source CO2 as
9.22 70.26‰ from

184

K. Prasanna et al. / Deep-Sea Research II 118 (2015) 177–185

Table 2 Location, SST, δ13CDIC of surface water along with calculated δ13C of air CO2 (refer text) for the years 2012 and 2013. Latitude

Longitude

SST (1C)

δ13C(DIC) VPDB ‰

103ln α*

δ13 CðCO2 Þ VPDB ‰

2012 14.38 10.97 2.78 0.98
10.97
15.53
19.93
24.90
30.73
34.95
38.02
40.00
41.75
43.18
48.58
50.58
51.27

74.08 75.00 78.77 77.55 72.85 70.27 68.67 66.23 63.15 61.22 54.10 56.50 57.65 58.58 57.57 57.50 57.67

28.60 27.20 28.10 28.30 29.00 26.30 25.90 24.10 20.90 21.00 18.50 16.00 16.10 14.50 7.00 3.80 4.50

0.77 0.67 0.80 0.73 0.79 0.76 0.71 0.74 0.75 0.92 0.98 1.54
0.88 1.36 1.60 1.48 1.40

10.77 7.52 7.66 7.57 7.55 7.48 7.76 7.80 7.99 8.33 8.32 8.59 8.87 8.86 9.04 9.91 10.30


9.95
6.82
6.84
6.81
6.73
6.70
7.02
7.03
7.21
7.38
7.31
7.03
9.71
7.47
7.41
8.39
8.86

2013 5.72
1.42
5.05
11.46
18.17
24.84
28.40
31.78
37.73
40.44
41.54
43.44
48.85
50.50
51.01
53.13
55.69
56.50
56.52
56.60

80.09 78.01 79.28 76.35 72.29 68.10 65.75 63.40 57.84 55.07 57.56 58.16 57.48 54.71 51.51 47.87 57.59 54.69 56.49 49.20

26.70 28.60 27.50 27.70 26.70 25.80 25.60 24.00 19.60 14.20 17.40 10.70 4.30 4.90 5.30 3.40 2.70 1.90 2.60 3.70

0.60 0.75 0.55 0.70 0.77 0.72 0.81 0.80 0.81 0.82 1.31 1.52 1.50 1.42 1.43 1.22 1.15 1.14 1.17 1.28

7.52 7.63 7.61 7.72 7.81 7.83 8.00 8.47 9.07 8.71 9.48 10.24 10.16 10.11 10.35 10.43 10.53 10.44 10.31 10.55


6.89
6.86
7.04
6.99
7.02
7.09
7.16
7.64
8.23
7.87
8.13
8.68
8.63
8.66
8.87
9.17
9.33
9.26
9.09
9.23

2011 and 2012 measurements, whereas for 2013 the composition of source CO2 was
13.49 74.07‰. The possible precursor responsible for CO2 production is the degradation of organic matter or ex-solution of DIC. We measured the δ13C of organic matter present in the phytoplankton bloom found in this region and it showed a δ13C value of
29‰ while previous researchers recorded similar δ13C values of
30‰ (Goericke and Fry, 1994; Yang et al., 2014). The end member composition deduced from the linear model for 2011 and 2012 is
9.22 70.26‰ which is in disagreement with the value expected from the degradation of phytoplankton. It reflects, rather that the expelled CO2 is from the dissociation of bicarbonate present in the water column. This is further verified with the depth bound pH and temperature data during the time of the expedition as presented in Fig. 6(d) and (e). The set of observations shown in the plot demonstrate that the DIC and pH followed complementary patterns between the 401S and 531S latitudes, i.e., where an increase in pH lead to an increase in DIC concentration. At 401S, the surface water pH recorded is 8.06 whereas on a progressive southward direction i.e. 531S, the pH value drops to 7.88 in the surface water. At 1000 m depth near 451S, the pH value dropped to 7.75. At 501S, the pH value of 7.68 was recorded at 750 m depth, whereas at 531S, the pH value recorded was 7.68 at 200 m (Shown in Fig. 6(d)). Involvement of factors like temperature and upwelling were important in driving the DIC concentration and isotopes beyond 531S. However, with

limited data it is difficult to comment on their individual contributions. The observation documented for the year 2013 was anomalous as the isotopic composition source CO2 was lighter i.e.
13.4974.07‰. Such a depleted composition indicates the presence of CO2 due to the process of organic decomposition along with the mixing of DIC present in the sea water. Such variability of the δ13C in air-CO2 and its concentration for the nearby region i.e. between 50.171S and 70.081S from New Zealand to Antarctica (Longinelli et al., 2012) for the period covering December 2009, also registered a lower δ13C in air-CO2. The authors proposed reasons such as extremely depleted CO2 from bio-degradation and low productivity. An analysis of the same data for an inverse correlation between the δ13C and CO2 (Keeling's two component mixing model) allowed identification of end member composition as
36.7‰ (Supplementary Fig. 3). The presence of such a member, even at a low concentration, will disturb the binary mixing model and would hinder the definition of the end member values. To document the degradation process, we used the indirect approach of capturing the productivity of the ocean during the period of sampling using satellite data. We are aware of the uncertainty in the satellite based data; however, in absence of direct measurement this provided us an alternative means to assess the biological activity of the ocean. A drop in productivity was captured in the chlorophyll-a concentration data (Supplementary Fig. 4) (MODIS Aqua, chlorophyll at 4 km) coinciding with the period (Jan–March) of our sampling (http://

K. Prasanna et al. / Deep-Sea Research II 118 (2015) 177–185

gdata1.sci.gsfc.nasa.gov/daac-bin/G3/gui.cgi?instance_id=ocean_ month). 6. Conclusions In this paper, we have identified the region of CO2 venting over the SO and provided new data on the source of CO2. The isotopic composition of ventilated CO2 closely matches with the estimated δ13C of CO2 found ex-solving from DIC present in the surface water at those locations. This study has demonstrated that the major source of CO2 degassed from the ocean is from the dissociation of DIC, which is mainly caused by ocean acidification. In future, the effect of warming is expected to intensify this process of CO2 removal mainly due to dissociation of DIC. However, in exceptional situations, (i.e. for the year 2013) evidence of CO2 from biological degradation has been captured in the SAFZ and the PFZ. The study provides sufficient motivation to further expand the task of rigorous monitoring of the region of CO2 efflux into the atmosphere over the SO. This will enable us to address the rate of CO2 build-up and its seasonal pattern. The δ13C of a foraminifera shell from paleo-records in this region such as sediments cores (ODP leg 183, Site 1135-1140) reflects the carbon isotopic composition of the DIC in seawater in which the shell calcified, but not in isotopic equilibrium with seawater. The abiotic kinetic fractionation effect causes foraminiferal calcite δ13C to be 1.07 0.2‰ enriched relative to δ13CDIC (Romanek et al., 1992). The signature of carbon isotopes recorded in the shell can act as a proxy for CO2 efflux, which is seen in our monitoring effort. Therefore, this further add values to our effort where understanding the palaeo-record from the SO regions is attempted. Acknowledgement We thank all the anonymous reviewers for their valuable inputs. We thank Dr. Sagarika Roy and Mrs. Meera Rao for editorial inputs to the manuscript. We thank the Ministry of Earth Science, Government of India for providing financial support for the Indian Scientific Expedition to the Indian Ocean sector of the Southern Ocean, under which the present work was carried out. We also thank the Director, NCAOR, Goa, and Divecha Centre for Climate Change, Bangalore, for providing the necessary facilities and support. We also thank all the scientific team, Captain, officers and crew on-board ORV Sagar Nidhi for all their support. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.dsr2.2015.04.009. References Affek, H.P., Eiler, J.M., 2006. Abundance of mass 47 CO2 in urban air, car exhaust, and human breath. Geochim. Cosmochim. Acta 70 (1), 1–12. Anderson, R.F., Ali, S., Bradtmiller, L.I., Nielsen, S.H.H., Fleisher, M.Q., Anderson, B.E., Burckle, L.H., 2009. Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO2. Science 323 (5920), 1443–1448. Anilkumar, N., Dash, M.K., Luis, A.J., Babu, V.R., Somayajulu, Y.K., Sudhakar, M., Pandey, P.C., 2005. Oceanic fronts along 45 degrees E across Antarctic Circumpolar Current during Austral summer 2004. Curr. Sci. 88 (10), 1669–1673. Assayag, N., Rive, K., Ader, M., Jezequel, D., Agrinier, P., 2006. Improved method for isotopic and quantitative analysis of dissolved inorganic carbon in natural water samples. Rapid Commun. Mass Spectrom. 20 (15), 2243–2251. Caldeira, K., Duffy, P.B., 2000. The role of the Southern Ocean in uptake and storage of anthropogenic carbon dioxide. Science 287 (5453), 620–622.

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