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Organic carbon and apparent oxygen utilization in the western South Pacific and the central Indian Oceans
Marine Chemistry 68 Ž2000. 249–264 www.elsevier.nlrlocatermarchem
Organic carbon and apparent oxygen utilization in the western South Pacific and the central Indian Oceans M.D. Doval ) , Dennis A. Hansell Bermuda Biological Station for Research, 17 Biological Lane, St. Georges, GE-01, Bermuda Received 11 June 1998; received in revised form 23 July 1999; accepted 23 July 1999
Abstract Samples for total organic carbon ŽTOC. analysis were collected on WOCE Line P15S Ž08 to 678S along 1708W. and from 538 to 678S along 1708E in the western South Pacific, and on Line I8 Ž58N to 438S along 808r908E. in the central Indian Ocean. TOC concentrations in the upper ocean varied greatly between the regions studied. Highest surface TOC concentrations Ž81–85 mM C and 68–73 mM C. were observed in the warmest waters Ž) 278C. of the western South Pacific and central Indian Oceans, respectively. Lowest surface TOC concentrations Ž45–65 mM C. were recorded in the southernmost waters occupied Ž) 508S along 1708W and 1708E.. Deep water Ž) 1000 m. TOC concentrations were uniform across all regions analyzed, averaging between 42.3 and 43 mM C ŽSD: "0.9 mM C.. Mixing between TOC-rich surface waters and TOC-poor deep waters was indicated by the strong correlations between TOC and temperature Ž r 2 ) 0.80, north of 458S. and TOC and density Ž r 2 ) 0.50, southernmost regions.. TOC was inversely correlated with apparent oxygen utilization ŽAOU. along isopycnal surfaces north of the Polar Frontal Zone ŽPFZ. and at depths - 500 m. The TOC:AOU molar ratios at densities of s T 23–27 ranged from y0.15 to y0.34 in the South Pacific and from y0.13 to y0.31 in the Indian Ocean. These ratios indicate that TOC oxidation was responsible for 21%–47% and 18%–43% of oxygen consumption in the upper South Pacific and Indian Oceans, respectively. At greater depths, TOC did not contribute to the development of AOU. There was no evidence for significant export of dissolved and suspended organic carbon along isopycnal surfaces that ventilate near the PFZ. q 2000 Elsevier Science B.V. All rights reserved. Keywords: total organic carbon; dissolved organic carbon; apparent oxygen utilization; Western South Pacific Ocean; Central Indian Ocean
1. Introduction The source of total organic carbon ŽTOC. in the open ocean is in situ production from several biological processes ŽCopin-Montegut and Avril, 1993; ´ ) Corresponding author. Instituto de Investigaciones Marinas, C.S.I.C., Eduardo Cabello, 6, E-36208 Vigo, Spain. Tel.: q3486-231930; Fax: q34-86-292762; E-mail: [email protected]
Kirchman et al., 1993.. DOC production mechanisms are largely influenced by the composition of the plankton community, while concentrations are largely controlled by vertical mixing processes ŽCarlson and Ducklow, 1995; Hansell and Waterhouse, 1997.. The distribution of DOC in the equatorial Pacific Ocean is mostly controlled by the physical system ŽTanoue, 1993; Carlson and Ducklow, 1995; Peltzer and Hayward, 1996; Hansell and Water-
0304-4203r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 0 3 Ž 9 9 . 0 0 0 8 1 - X
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M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
house, 1997., while biological processes may exert greater control in areas such as the Southern Ocean ŽKarl et al., 1996; Carlson et al., 1998.. Surveys of TOC concentrations at the basin scale Žca 10 3 –10 4 km. allow an assessment of controls on distribution by the various biological and physical processes. Apart from the work by Hansell and Waterhouse Ž1997. in the eastern Pacific Ocean, there is a paucity of recent studies covering this spatial scale. Earlier surveys of DOC distribution at the basin scale include those of Duursma Ž1961., Skopintsev and Timofeyeva Ž1962., Menzel Ž1964; 1970., Skopintsev et al. Ž1966., Menzel and Ryther Ž1968., Ogura Ž1970. and Romankevich and Ljutsarev Ž1990.. The semi-labile material that accumulates in the surface ocean Ži.e., with a residence time of months; Kirchman et al., 1993; Carlson and Ducklow, 1995. can be: consumed biologically over time within surface waters; exported vertically to depth, thereby contributing to the vertical gradient of dissolved inorganic carbon in the ocean ŽCopin-Montegut ´ and Avril, 1993; Carlson et al., 1994.; or exported horizontally from sites of formation ŽPeltzer and Hayward, 1996; Hansell et al., 1997a,b.. Only in oceanic regions where winter overturn ventilates deep isopycnal layers can DOC be exported to the deep ocean. Some early studies have shown a correlation between DOC and apparent oxygen utilization ŽAOU; Ogura, 1970; Craig, 1971. in the ocean, while others have indicated that DOC does not contribute significantly to deep ocean oxygen consumption ŽMenzel and Ryther, 1968; Menzel, 1970; Williams, 1975; Hansell et al., 1993.. The biological refractiveness of DOC ŽBarber, 1968., its great age ŽWilliams and Druffel, 1987; Bauer et al., 1992. and relatively uniform deep ocean concentrations ŽMartin and Fitzwater, 1992. provided further evidence for the conservative nature of DOC. In the past few years, however, a correlation between DOC and AOU has again been demonstrated ŽGuo et al., 1994; Thomas et al., 1995; Peltzer and Hayward, 1996. and the conservative nature of deep ocean DOC has been challenged ŽHansell and Carlson, 1998a.. The discrepancies between these disparate findings have not been adequately evaluated. In this paper, we Ž1. evaluate the basin scale distributions of TOC in the western South Pacific
Ocean and the central Indian Ocean in the context of hydrographic variability in those regions; and Ž2. estimate the contribution of TOC oxidation to oxygen consumption along isopycnal surfaces in the subsurface and deep ocean.
2. Methods The work described here was performed as a component of the NOAA Climate and Global Change program Ocean–Atmosphere Carbon Exchange Study ŽOACES.. Samples for TOC analysis were collected along WOCE Line P15 Ž1708W. and 1708E on the NOAA Ship DiscoÕerer between January–March 1996, and along WOCE Line I8 Ž808E. on the NOAA Ship Malcom Baldrige between September and October of 1995 ŽFig. 1.. Cruise details are listed in Table 1. Data for potential temperature Ž u ., salinity Ž S ., potential density anomaly Ž su ., total inorganic nitroy. gen ŽTIN s NOy and AOU on the I8 line 3 q NO 2 were taken from Peltola et al. Ž1998.. Hydrographic data from the P15S line were taken from McTaggart and Johnson Ž1997. while nutrient data were provided by C. Mordy ŽPMEL.. Water samples taken for organic carbon determination were not filtered; they were collected in precleaned 40 ml glass vials or 60 ml polyethylene bottles. TOC concentrations were determined from up to 24 depths over the entire water column. TOC analysis was by a high temperature combustion method like that described by Hansell and Waterhouse Ž1997.. All measurements were referenced each analytical day against 2600 m Sargasso Sea water. To minimize the system blank in the analytical system, conditioning of the combustion tube was required prior to analysis of samples. Conditioning was performed through repeated injections of Milli-Q w water andror seawater. After conditioning, the system blank was assessed with ampoulated low carbon waters ŽLCW. that had been referenced against blank water provided by Dr. Jonathan Sharp for the 1994 DOC community intercomparison program. Typical relative standard deviations of replicate TOC analyses were ; 2%. The instrument response factor was determined two to four times daily using
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
251
Fig. 1. Map of transects occupied in Region A of the western South Pacific Ocean, Region B of the central Indian Ocean and Region C in the Southern Ocean. The dashed line across 1708W separates Regions A and C.
glucose in Milli-Q w water. Typically, the response factors on an analytical day differed by - 2% and their mean was within ; 5% of the mean of all calibrations. The instrument blank was measured every four to six samples using carbon-free distilled water. All TOC concentrations reported are corrected for the instrument blank. A portion Žf 50%. of the samples collected along the northern part of the I8 line was analyzed at the shore laboratory of Bermuda Biological Station for Research ŽBBSR.. Those samples were stored frozen in 60 ml polyethylene bottles until analysis. All other samples were analyzed at the time of collection. In the data analysis, we have employed the Model II linear regression ŽSokal and Rohlf, 1995..
3. Results 3.1. Hydrographic features of the ocean regions This study covered several oceanic areas: the equatorial regions in the Pacific and Indian Oceans, the oligotrophic Pacific and Indian subtropical gyres, and the Subantarctic and Antarctic PacificrSouthern Ocean. Here, in order to simplify the evaluation, these areas are grouped into three regions ŽFig. 1.: Region A — the southwest Pacific Ž08–428S, 1708W.; Region B — the central Indian Ocean Ž58N–438S, 808–958E.; and Region C — the Southern Ocean Ž428–678S, 1708W and 538–678S, 1708E.. The equatorial systems in the Pacific ŽRegion A. and Indian ŽRegion B. Oceans differed greatly dur-
Table 1 Dates of the OACES cruises ŽWOCE sections P15S in the western South Pacific Ocean including 1708E, and I8 in the central Indian Ocean; Fig. 1., stations sampled and regions considered in this study. The Stations column indicates the range of stations occupied, the number of stations occupied for TOC analysis within the range, and the number of samples analyzed Dates
Ocean section
Stations
Region
February 1–March 6, 1996 September 28–October 24, 1995 January 18–31, 1996 January 9–17, 1996
P15S, 1708W I8, 808–958E P15S, 1708W 1708E
a90–174 Ž83 stations, 705 samples. a2–101 Ž51 stations, 657 samples. a33–87 Ž51 stations, 365 samples. a5–32 Ž21 stations, 213 samples.
A Ž08–428S. B Ž58N–438S. C Ž428–678S. C Ž538–678S.
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M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
ing the periods of sampling. Surface layer TIN concentrations were ; 5 mmolrkg in the equatorial Pacific but below limits of detection in the equatorial Indian Ocean ŽFig. 2a,b.. Warm waters Ž) 278C. characterized the upper mixed layer within the equa-
torial central Indian Ocean ŽFig. 2b., as reported previously by Jochem et al. Ž1993.. The oligotrophic gyre waters found in the western South Pacific and in the central Indian Oceans Žsouth of 108S in Regions A and B, respectively. were
Fig. 2. Distribution of mean TOC ŽmM C., temperature Ž u , 8C. and TIN Žmmolrkg. in the upper layer within Ža. Region A Župper 100 m., Žb. Region B Župper 50 m., and Žc. on line 1708W in Region C Župper 50 m. and Žd. line 1708E in Region C Župper 50 m..
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
depleted in nutrients and showed a decrease in surface temperature with increasing latitude ŽFig. 2a,b.. A limb of the low salinity, western Equatorial Pacific warm pool Ž) 278C; Radenac and Rodier, 1996. was located between 58 and 238S within region A ŽFig. 2a and Fig. 3b.. Region C Ž) 428S, 1708W and ) 508S, 1708E. covers Subantarctic Žbetween 428 and 608S. and Antarctic Žsouth of 608S. waters, with the two separated by the Polar Frontal Zone ŽPFZ.. These cold, saline waters showed relatively high nutrient concentrations ŽFig. 2c.. 3.2. Distributions of TOC The latitudinal distributions of TOC ŽFigs. 3a, 4a, 5a. showed a wide range of surface TOC concentrations between the different regions considered and within the same regions: 65–90 mM C ŽRegion A.,
253
55–80 mM C ŽRegion B. and 45–75 mM C ŽRegion C.. The variation in deep water TOC Ždepth ) 1000 m. was - 1 mM C, with mean concentrations Ž"SD. of 42.6 " 0.7 mM C in Region A, 42.2 " 0.6 mM C in Region B, and 43.0 " 0.9 mM C in Region C. All these means were statistically different Ž p - 0.05.. The distributions of TOC along 1708W and 808E ŽFig. 3a and Fig. 4a. resembled those of temperature ŽFig. 3b and Fig. 4b., broadly indicating the mixing of TOC-rich, warm surface water with TOC poor, cold deep water. TOC in the upper layer Žmean concentrations in the upper 100 m within Region A and upper 50 m within Region B. paralleled the distribution of mean temperature and inversely correlated with mean TIN concentrations ŽFig. 2a,b.. The warmest waters Ž) 278C. were those of the warm pools located at 58S–238S ŽRegion A. and 58N–108S ŽRegion B.. These waters showed the maximum TOC values in the upper layer for the entire lines
Fig. 3. Distribution of TOC ŽmM C. and temperature Ž8C. in the upper 500 m of the South Pacific Ocean.
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Fig. 4. Distribution of TOC ŽmM C. and temperature Ž8C. in the upper 500 m of the central Indian Ocean.
ŽFig. 2a,b.. This trend can also be inferred from the linear regression between TOC and temperature in both areas ŽTable 2.. The average TOC concentrations within the Equatorial Pacific region were lower than those recorded within the Pacific warm pool. TOC concentrations in the surface layer of Region C were only slightly higher than values found at greater depths. The Subantarctic and Antarctic waters within the southernmost part of 1708W showed an average TOC concentration of 60.4 " 9.2 mM C in the upper 50 m, decreasing from 74.2 to 56.5 mM C from 428 to 608S and from 56.5 to 45 mM C from 608 to 678S ŽFig. 2c.. The average TOC concentration between 538 and 678S was 55.0 " 4.6 mM C along 1708W and 53.1 " 3.6 mM C along 1708E ŽFig. 2d.. Linear regression between TOC and temperature Ž p - 0.001. from all depths in Regions A and B
showed r 2 of 0.81 and 0.87, respectively ŽTable 2.. The slopes obtained were slightly higher Ž1.85 and 1.36 for Regions A and B, respectively. when only samples with temperature ) 78C were considered, but the correlation coefficients were lower Ž0.73 and 0.79 for Regions A and B, respectively; data not showed.. The narrower ranges of both TOC and temperature in the Southern Ocean ŽRegion C; 1708E; Fig. 5a and b. resulted in a low correlation Ž r 2 s 0.62 from 1708W, and 0.22 from 1708E; Table 2.. These southern waters showed a better correlation when sigma theta was considered as the hydrographic index Ž r 2 : 0.78 and 0.54, respectively, data not shown.. Linear regression showed that temperature explained 50% and 79% of the variance observed in TOC in the upper mixed layers Ždepths - 40 m, data not shown. in Regions A and B, respectively. However, the variance explained was as low as 28% and
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
255
Fig. 5. Distribution of TOC ŽmM C. and temperature Ž8C. in the upper 500 m on the 1708E line in the Southern Ocean.
32% when all thermocline waters Žtemperature) 208C, su - 25. were considered Ždata not shown.. The lack of correlation with temperature or su in the upper thermocline waters was particularly evident in the warm pools. The correlation of TOC with temperature in the upper mixed and upper thermocline waters within Region C was especially low and insignificant on 1708E Ž r 2 - 0.01, data not shown..
3.2.1. TOC r AOU relationship TOCrAOU relationships ŽModel II. are first considered for the entire water column Žwhere AOU ) 0, i.e., subsurface waters. in different areas of the western Pacific and central Indian Oceans ŽTable 3.. This analysis is similar to those performed in several earlier efforts Žsee references above., although some of the molar ratio determined using Model I regres-
Table 2 Selected linear regressions between TOC ŽmM C. and temperature Ž u , 8C. for the entire water column from the various regions studied Ž p - 0.001.. Numbers in parentheses are the standard error of the coefficients Region
Equation
r2
n
A Ž08–428S, 1708W. B Ž58N–438S, 808–958E. C Ž428–678S, 1708W. C Ž538–678S, 1708E.
TOC s 34.5Ž"0.3. q 1.51Ž"0.03.u TOC s 38.1Ž"0.2. q 1.2Ž"0.02.u TOC s 39.9Ž"0.3. q 1.9Ž"0.1.u TOC s 41.4Ž"0.4. q 1.8Ž"0.1.u
0.81 0.87 0.62 0.22
701 645 384 213
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
256
Table 3 Slope Žmolar ratio. of the linear regression between TOC Žmmolrkg. and AOU Žmmolrkg. and the multiple regression between TOC, temperature Ž u , 8C. and AOU when AOU) 0 ŽModel II.. Numbers in brackets are the standard error, ns number of data pairs included Region
Variables
TOCrAOU
r2
South Pacific 08–678S Ž ns 735. 08–428S Ž ns 235. 428–678S Ž ns 274.
TOCrAOU TOCru ,AOU TOCrAOU TOCru ,AOU TOCrAOU TOCru ,AOU
y0.121 Ž"0.00. y0.074 Ž"0.002. y0.13 Ž"0.005. y0.10 Ž"0.008. y0.099 Ž"0.004. y0.071 Ž"0.004.
0.44 0.75 0.68 0.81 0.44 0.69
Central Indian Ž ns 511. TOCrAOU TOCru ,AOU
y0.085 Ž"0.003. y0.030 Ž"0.001.
0.17 0.86
1708E (538– 678S) Ž ns165. TOCrAOU TOCru ,AOU
y0.074 Ž"0.003. y0.087 Ž"0.004.
0.54 0.54
sions can be underestimated. Second, an evaluation of the relationship between TOC and AOU is performed along isopycnal surfaces.
The correlation between TOC and AOU is partially due to the dependence of both variables on water mass mixing ŽMenzel and Ryther, 1968.. Therefore, when both variables show a strong correlation with temperature, the multiple correlation between TOC, temperature and AOU must be considered ŽAlvarez-Salgado X.A., personal communication.. Regressions with and without temperature considered indicate that water mass mixing tends to overestimate the contribution of TOC to AOU in all regions, except along 1708E in Region C where the correlation of both TOC and AOU with temperature is low ŽTable 3.. This overestimation is evident by the reduction in slope when temperature is included in the multiple regression. The TOCrAOU relationship for the entire water column is of limited value because it combines depths in the ocean where the relationship is strong with depths where the relationship is weak. To identify where TOC oxidation makes a substantive contribution to the development of AOU an evaluation of the two properties along isopycnal surfaces is required. The distribution of sigma theta is shown in Figs. 6 and 7 for the western South Pacific and central Indian Oceans, respectively. In Fig. 8, we
Fig. 6. Distribution of sigma theta in the upper 750 m of the South Pacific Ocean. The upper and lower dashed lines approximately indicate the shallowest and greatest depths, and the spatial extent, to which net TOC oxidation contributed to AOU development.
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
257
Fig. 7. Distribution of sigma theta in the upper 750 m of the central Indian Ocean. The dashed line approximately indicates the depth and spatial extent over which net TOC oxidation contributed to AOU development.
show TOCrAOU relationships from select density surfaces in the South Pacific. TOC and AOU concentrations from water forming the su s 25–25.5 density layer are plotted against latitude in Fig. 8a. The two variables varied little at locations ) 308S. At these latitudes, the su s 25–25.5 density layer was above the seasonal thermocline ŽFig. 6.. As the layer deepened to ) 200 m from 308S to near 128S ŽFig. 6. TOC decreased and AOU increased ŽFig. 8a.. AOU and TOC remained at fairly constant values at latitudes - 128S ŽFig. 8a.. A contribution of TOC oxidation to AOU development was largely limited to the depth and latitudinal ranges of 50–200 m and 128S–308S, respectively, in this density layer. As another example, TOC and AOU concentrations in the su s 26.5–26.75 density layer of the western South Pacific, which ventilates at a higher latitude and reaches greater depths than the su s 25– 25.5 layer ŽFig. 6., are shown in Fig. 8b. TOC concentrations decreased and AOU concentrations increased from the site of ventilation Žnear 558S. to near 208S Ž450–500 m.. At lower latitudes TOC was largely invariant but AOU continued to increase. Several density layers were evaluated in this way to determine where TOC oxidation showed a contri-
bution to AOU. The start and end points ŽTOC concentration and latitudinal ranges along each isopycnal surface. are shown in Fig. 8c for the western South Pacific Ocean. The corresponding depths for these points are approximated by the dashed lines in Figs. 6 and 7, with the upper dashed line Žabsent in Fig. 7. indicating where TOC begins contributing to AOU and the lower dashed line indicating the end of a contribution. The concentrations of TOC decreased with increasing sigma theta, both near the surface where TOC begins to contribute to AOU and at depth where the contribution ends ŽFig. 8c.. The same behavior was found in central Indian Ocean waters. The TOCrAOU molar ratios along isopycnal surfaces in the South Pacific Ocean Ž08–678S. and in the central Indian Ocean Ž58N–438S., assessed where TOC contributes to AOU as described above, are reported in Table 4a and b, respectively. In the lowest density waters Ž su - 23. TOC and AOU did not correlate because these waters reside in the surface layers where AOU is variable and near zero due to air–sea exchange processes. At intermediate densities Ž su s 23–27., the TOCrAOU molar ratio ranged from y0.15 to y0.34 Žmean of y0.26 " 0.6.
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M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
Fig. 8. Distribution of TOC ŽmM C. and AOU Žmmolrkg. along selected density surfaces in the South Pacific Ocean. Ža. su s 25– 25.5, and Žb. su s 26.5–26.75; and Žc. maxima and minima of TOC concentrations ŽmM C. on each density surface.
and y0.13 to y0.31 Žmean of y0.23 " 0.05. along 1708W and 808E, respectively. At potential densities anomalies of su ) 27 the ratio was very low, ranging from y0.03 to q0.08, and with low correlation coefficients ŽTable 4a,b.. The contribution of TOC to oxygen consumption along isopycnal surfaces can be determined after converting AOU to carbon equivalents. The ratio expected from the Redfield stoichiometry ŽC:N:O:P s 106:42:16:1, Anderson, 1995. would predict a yDCrDO 2 ratio of 0.72. The contribution of TOC to oxygen consumption at intermediate densities, in equivalent carbon units, ranged from 20.8–47.2%
with a mean of 40.4 " 8.8% in the South Pacific and from 18.0–43.0% with a mean of 32.7 " 7.8% in the central Indian Ocean ŽTable 4a and b.. These contributions change somewhat when we consider a nonRedfield CrN ratio of 12 for DOM. The average contribution of TOC to AOU development becomes 36% in the South Pacific and 29% in the central Indian Ocean. The contributions of isopycnal mixing to the horizontal gradients in AOU and TOC are difficult to ascertain. Model results from the North Pacific ŽSonnerup et al., 1999. suggest that advection dominates transport in the main thermocline of the subtropics so the contribution of diffusion is small. Taking a diffusion coefficient of 1000 m2 sy1 suggests that - 30% of the change in AOU is due to diffusive mixing. Indeed, by taking the ratio of TOC to AOU as was done here, the contributions of diffusion are to some extent negated since diffusion should affect the two terms similarly. Another uncertainty is the effect of diapycnal mixing on the ratio. This uncertainty can be minimized by grouping the data into broadly delimited density surfaces, so that most mixing is within the chosen surface and not across it. Using only the data demonstrating TOC:AOU covariance as described above, the data have been grouped as su s 23–25 Župper thermocline waters. and su ) 25.5. In the upper thermocline the molar ratios Žand the contributions of TOC to AOU in equivalent carbon units. were y0.35 Ž48.6%. and y0.21 Ž29.1%. along 1708W and 808E, respectively, while in the deeper water the slopes were y0.26 Ž36.1%. and y0.22 Ž30.5%., respectively Ždata not shown.. The molar ratio from the 808E shallow water regression has greater uncertainly since there were only 11 data points, but the ratios are in general agreement with those reported in Table 4a and b for narrower density intervals. To summarize the findings for the various density ranges, we have the following. ŽI. su - 23 Župper mixed layer from 208S to 08 within the South Pacific and surface water between 108S and 58N within the central Indian Ocean.: TOCrAOU was highly variable and the linear regression not significant. ŽII. su s 23–27, at depths - 500 m Žsubsurface waters of the western Pacific Ocean and the central
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
259
Table 4 Slope ŽModel II. between TOC Žmmolrkg. and AOU Žmmolrkg. TOCrAOU-C Ž%.
Samples
r2
(a) South Pacific Ocean (08–678S) 27.9–27.5 q0.062 Ž"0.007. 27.5–27 y0.032 Ž"0.004. 27–26.75 y0.23 Ž"0.01. 26.75–26.5 y0.15 Ž"0.011. 26.5–26 y0.30 Ž"0.01. 26–25.5 y0.28 Ž"0.01. 25.5–25 y0.33 Ž"0.03. 25–24.5 y0.34 Ž"0.05. 24.5–24 y0.32Ž"0.04. 24–23.5 y0.34 Ž"0.04. 23.5–23 y0.33 Ž"0.06. 23–22.0 insignificant
– 4.4 31.9 20.8 41.7 38.9 45.8 47.2 44.4 47.2 45.8 –
08S–408, 1400–3000 m 08S–308, 520–1070 m 448–638S, 20–570 m 208S–568S, 20–470 m 208S–408S, 70–400 m 138S–408S, 50–300 m 128S–308S, 80–260 m 128S–308S, 46–210 m 98S–268S, 50–162 m 98S–218S, 50–164 m 88S–208S, 70–165 m 58S–238S, - 100 m
0.003 0.025 0.45 0.49 0.76 0.89 0.73 0.58 0.72 0.83 0.92 –
64 52 84 98 54 35 20 20 17 13 8 –
(b) Central Indian Ocean (58N–438S) 27.8–27.5 q0.08 Ž"0.004. 27.5–27 q0.039 Ž"0.004. 27–26.75 y0.13 Ž"0.01. 26.75–26.5 y0.26 Ž"0.02. 26.5–26 y0.25 Ž"0.04. 26–25.5 y0.21Ž"0.01. 25.5–25 y0.31 Ž"0.05. 25–24 y0.26 Ž"0.03. 24–23 y0.23 Ž"0.06. 23–21 insignificant
– – 18.0 36.1 34.7 29.2 43.0 36.1 31.9 –
58S–308N, 1100–2500 m 58S–458N, 500–1400 m 158S–438S, s–600 m 158S–438S, s–500 m 158S–358S, s–300 m 158S–308S, 20–270 m 158S–308S, s–200 m 178S–258S, s–120 m 58S–128N, 50–100 m 58S–108N, - 90 m
0.08 0.14 0.44 0.52 0.47 0.46 0.81 0.91 0.58 –
40 64 68 128 28 5 10 7 8 –
su
TOCrAOU Žmolar ratio.
n
Numbers in parentheses are the standard error. TOCrAOU is the molar ratio; TOCrAOU-C is calculated with AOU in carbon equivalents, Žusing a Redfield ratio yDCrDO 2 s 0.72; Anderson, 1995.. TOCrAOU-C thus represents the contribution of TOC to AOU along specific isopycnal surfaces. s s surface.
Indian Ocean.: The TOCrAOU molar ratio ranged between y0.15 and y0.34 in the South Pacific Ocean and between y0.18 and y0.31 in the central Indian Ocean. The coefficient of regression, r 2 was ) 0.4. A total of 30% to 50% of the AOU could be assigned to oxidation of suspendedrdissolved organic carbon, with the remainder due to oxidation of sinking POM. ŽIII. su ) 27 and depths ) 500 m Ždeep waters of the subtropical Pacific and central Indian Ocean.: The TOCrAOU molar ratios and the correlations were insignificant. AOU at these depths are controlled by mixing and oxidation of sinking POM. TOC was not exported to a significant extend along these density surfaces. The relative importance of TOC mineralization varied with depth and latitude. Variability in the
South Pacific region is showed on Fig. 9: the shadowed area indicates where TOC contributes significantly to oxygen consumption.
4. Discussion 4.1. TOC distribution The maximum TOC concentrations Ž) 80 mM C. were found in the western Pacific warm pool, a region characterized by high temperature Ž) 278C. and depleted nutrients. The maximum TOC concentrations in the Indian Ocean, also located in the warmest surface waters, were significantly lower at
260
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
Fig. 9. Schematic diagram depicting the locations, depths and patterns of TOC mineralization as it is exported from the upper ocean. The shadowed area indicates where TOC contributes significantly to oxygen consumption, based on observations in the western South Pacific Ocean during austral summer. The sigma theta contours 25 and 27 are showed in the diagram.
) 70 mM C ŽFigs. 3 and 4.. The Indian Ocean maximum values were similar to the surface layer concentrations reported by Hansell and Peltzer Ž1998. for the Arabian Sea during the same seasonal period Žthe Fall Intermonsoon.. They reported that the lowest concentrations on an annual basis occurred during the Fall Intermonsoon, perhaps due to a change in the composition of the microbial food web. Lower variability of TOC in Region C, especially on the most westerly line Ž1708E., agrees with the results reported for the Southern Ocean by several authors ŽCarlson et al., 1998; Wedborg et al., 1998; Weibinga and De Baar, 1998.. The uniformity of deep water TOC values Ž) 1000 m. reflects the relative homogeneity of bottom water masses in the regions studied. Deep water in Regions A and B have as their source a mixture of Antarctic Bottom Water and North Atlantic Deep Water ŽReid, 1986.. In the Southern Ocean ŽRegion C., the average deep TOC concentration was indistinguishable from the Regions A and B. These bottom TOC concentrations were very similar to those reported by Hansell and Carlson Ž1998a. for these areas. Higher surface water TOC values in the warm subtropical Pacific than in the cool Equatorial Pacific have been reported previously ŽTanoue, 1993; Peltzer and Hayward, 1996; Hansell and Waterhouse, 1997..
These authors suggested physical control as the main determinant for DOC concentrations. The highest TOC concentrations are within the tropical region where high vertical stability allows its accumulation. Net biological consumption of TOC and mixing within the subtropical gyre was indicated by the gradual decrease of the TOC in the upper 100 m from north to south. Within the Equatorial Pacific, TOC accumulation was partially prevented by the low residence time of the surface water and therefore, the high advective export of carbon ŽMurray et al., 1994.. In Region C, lower net production of DOC ŽCarlson et al., 1998; Hansell and Carlson, 1998b. and lower vertical stability resulted in low TOC concentrations. Surface TOC concentrations are controlled both by physical and biological processes ŽDoval et al., 1997; Hansell and Waterhouse, 1997; Carlson et al., 1998; Alvarez-Salgado et al., 1999.. A role for mixing between surface water rich in semi-labile DOC and deep water low in semi-labile DOC was indicated by the good correlation between TOC and temperature when the entire water column was considered in Regions A and B. However, although the maximum accumulation of TOC coincides with the warm pools, the lower correlation with temperature in the upper thermocline waters and the unequal
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
TOC concentrations in the Pacific and Indian warm pools suggest a greater biological control in this layer. The barrier layer associated with the warm pool Ži.e., the water column located between the top of thermocline and the base of the surface mixed layer; Lukas and Lindstrom, 1991., because of its inherent stabilizing effect, should restrict the vertical export of TOC from the upper mixed layer by mixing. Therefore, the high TOC concentrations present in this layer must be due mainly to in situ production and solubilization of POC. The Southern Ocean data set showed little correlation between TOC and temperature, especially along 1708E. This behavior is due in part to the reduced ranges of both TOC and temperature and the higher control by salinity on density at high latitudes. A higher biological control for the distribution of DOC in the Southern Ocean has been suggested in other studies, where DOC production, biogeochemical composition, and utilization by bacteria all strongly affect the accumulation of DOC ŽKarl et al., 1996; Carlson et al., 1998.. 4.2. Subsurface mineralization of dissolÕed organic carbon Several data sets have revealed some degree of covariance between DOC and AOU ŽOgura, 1970; Craig, 1971; Kumar et al., 1990; Druffel et al., 1992;
Kepkay and Wells, 1992; De Baar et al., 1993; Hansell et al., 1993; Guo et al., 1994; Thomas et al., 1995; Peltzer and Hayward, 1996; Weibinga and De Baar, 1998.. Literature values for DOCrAOU molar ratios obtained from the open ocean range between 0 and y0.59 ŽTable 5.. Our results on the relationship between DOC and AOU along isopycnal surfaces agree particularly well with those of Ogura Ž1970.. He too found that DOC contributes to AOU in the upper 500 m of the water column Žat a DOCrAOU molar ratio ( 0.23., and that sinking biogenic particles contribute to the AOU balance at those depths and to most of the oxygen consumption at greater depths. The lack of correlation between TOC and AOU for deep waters Ž) 500m. reported here is in agreement with the results of Menzel Ž1964; 1970., Menzel and Ryther Ž1968. and Peltzer and Hayward Ž1996; depths ) 400 m.. The idea that organic matter can be transported at intermediate depths and support elevated rates of heterotrophic activity has been discussed for many years. Sorokin Ž1971; 1977; 1978. observed a band of water with high microbial activity lying in the upper layers of the Antarctic Intermediate Water ŽAIW. in the Pacific and Indian Oceans, and sought to explain this as a consequence of deep horizontal transport of labile organic matter. The concept was revisited by Moriarty and O’Donohue Ž1995. in the Tasman Sea who found elevated microbial growth rates in AIW at depths of 800–1200 m. The authors
Table 5 Select literature values for TOCrAOU molar ratios determined for the deep ocean Reference
Molar ratio
Data specifications
Ogura, 1970 a Kumar et al., 1990 a Druffel et al., 1992 a,b
y0.23 y0.062 y0.29 y0.59 0 y0.25 y0.068 y0.152 y0.09, y0.14 y0.074 y0.26 y0.23
upper 500 m; western North Pacific Ocean full water column; NW Indian Ocean full water column; North Atlantic Ocean - 482 m; North Pacific ) 482 m; North Pacific full water column; North Atlantic Ocean full water column; Southern California Bight - 250 m; Gulf of Mexico - 700 m; equatorial Atlantic Ocean 90–400 m; equatorial Pacific Ocean su s 23–27 in South Pacific Ocean su s 23–27 in central Indian Ocean
Kepkay and Wells, 1992 Hansell et al., 1993 a Guo et al., 1994 a Thomas et al., 1995a Peltzer and Hayward, 1996 This study a b
261
Model I regression. Ratios reported from Druffel et al. Ž1992. are those resulting from organic carbon concentrations determined by UV oxidation.
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suggested that their results supported the hypothesis for horizontal advection and mineralization of a significant pool of labile organic matter within the AIW. The core of the deep, elevated bacterial activity Moriarty and O’Donohue Ž1995. reported was located in water characterized by a minimum in salinity and a temperature of 48C–68C. On the P15S line, this water lies approximately on the 27.2 isopycnal ŽFig. 6.. We found little correlation between AOU development and TOC oxidation ŽTable 4a. in this water mass, particularly at depths ) 800 m. We do not know the reason for the elevated growth rates reported by Moriarty and O’Donohue Ž1995., but the rates cannot be assigned to significant consumption of DOC introduced isopycnally. Our results and those of Ogura Ž1970. do support Sambrotto et al. Ž1993. and Anderson and Sarmiento Ž1994. who suggested that a gradient must exit in the water column nutrient ratios between the upper and deep ocean. In the upper 500 m, a non-Redfield ratio should be anticipated because both carbon-rich nonRedfield DOM and sinking POM with a Redfield signature contribute approximately equally to the consumption of oxygen. At greater depths sinking POM with Redfield elemental ratios is the main source of reduced material supporting oxygen consumption and remineralization of the major elements. A fuller analysis to test the existence of such a gradient has yet to be performed.
5. Conclusions The maximum TOC concentrations in the western South Pacific and central Indian Ocean were observed in the warmest surface waters Ž) 278C.. The lowest surface TOC concentrations were recorded in the southernmost waters occupied Ž) 508S along 1708W and 1708E.. Deep water Ž) 1000 m. TOC concentrations were uniform across the regions studied, reflecting the relative homogeneity of deep and bottom water. TOC oxidation contributed to 30%–50% of oxygen consumption in the upper 500 m of water that ventilated north of the PFZ. At greater depths sinking biogenic matter must act as the primary source of reductant for the consumption of oxygen. The higher the latitude for ventilation of a density surface, the
greater the depth to which TOC was exported and mineralized. South of the PFZ TOC, was not seen to be exported greatly, so it’s contribution to AOU was minimal. Acknowledgements This work was conducted as a component of the NOAA Office of Global Programs’ OACES, through NOAA Award NA56GP0207 to DAH. MDD was supported from the NOAA Award NA 56GP0207, from a fellowship from the Bermuda Biological Station for Research, Grant-in-Aid program, and from ‘Direccion General de Universidades e Investigacion de la Xunta de Galicia’. Rachel Parson, Susan Becker and Tye Waterhouse performed the analyses of TOC during the cruises. We thank Calvin Mordy ŽPMEL., who provided us with inorganic nitrogen data. We gratefully acknowledge the officers, crew and technicians of the NOAA Ships DiscoÕerer and Malcom Baldrige for their assistance. Thanks to E.T. Peltzer and two anonymous reviews for their contribution to improve the manuscript. This is Contribution No. 1519 from the Bermuda Biological Station for Research. References Alvarez-Salgado, X.A., Doval, M.D., Perez, F.F., 1999. Dissolved organic matter in shelf waters of the NW Iberian upwelling system. J. Mar. Syst. 18, 383–394. Anderson, L.A., 1995. On the hydrogen and oxygen content of marine phytoplankton. Deep-Sea Res., Part I 42, 1675–1680. Anderson, L.A., Sarmiento, J.L., 1994. Redfield ratios of remineralization determined by nutrient data analysis. Global Biogeochem. Cycles 8, 65–80. Barber, R.T., 1968. Dissolved organic carbon from deep waters resist microbial oxidation. Nature 220, 274–275. Bauer, J.E., Williams, P.M., Druffel, E.R.M., 1992. 14 C activity of dissolved organic carbon fractions in the north central Pacific and Sargasso Sea. Nature 357, 667–670. Carlson, C.A., Ducklow, H.W., 1995. Dissolved organic carbon in the upper ocean of the central equatorial Pacific Ocean, 1992: daily and finescale vertical variations. Deep-Sea Res., Part II 42, 639–656. Carlson, C.A., Ducklow, H.W., Michaels, A.F., 1994. Annual flux of dissolved organic carbon from the euphotic zone in the Northwestern Sargasso Sea. Nature 371, 405–408. Carlson, C.A., Ducklow, H.W., Hansell, D.A., Smith, W.O., 1998. Differences in ecosystem dynamics between spring blooms in the Ross Sea polynya and the Sargasso Sea re-
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264 flected by contrasts in dissolved and particulate organic carbon partitioning. Limnol. Oceanogr. 43, 375–386. Copin-Montegut, C., Avril, B., 1993. Vertical distribution and ´ temporal variation of dissolved organic carbon in the north western Mediterranean Sea. Deep-Sea Res., Part I 40, 1963– 1972. Craig, H., 1971. The deep metabolism: oxygen consumption in abyssal ocean water. J. Geophys. Res. 76, 5078–5086. De Baar, H.J.W., Brussaard, C., Hegeman, J., Schijf, J., Stoll, M.H.C., 1993. Sea-trials of three different methods for measuring non-volatile dissolved organic carbon in seawater during the JGOFS North Atlantic pilot study. Mar. Chem. 41, 145–152. Doval, M.D., Alvarez-Salgado, X.A., Perez, F.F., 1997. Dissolved organic matter in a temperate embayment affected by coastal upwelling. Mar. Ecol. Prog. Ser. 157, 21–37. Druffel, E.R.M., Williams, P.M., Bauer, J.E., Ertel, J.R., 1992. Cycling of dissolved and particulate organic matter in the open ocean. J. Geophys. Res. 97, 15639–15659. Duursma, E.K., 1961. Dissolved organic carbon, nitrogen and phosphorous in the sea. Neth. J. Sea Res. 1, 1–148. Guo, L., Coleman, C.H. Jr., Santschi, P.H., 1994. The distribution of colloidal and dissolved organic carbon in the Gulf of Mexico. Mar. Chem. 45, 105–119. Hansell, D.A., Carlson, C.A., 1998a. Deep ocean gradients in concentration of dissolved organic carbon. Nature 395, 263– 266. Hansell, D.A., Carlson, C.A., 1998b. Net community production of dissolved organic carbon. Global Biogeochem. Cycles 12, 443–453. Hansell, D.A., Peltzer, E.T., 1998. Spatial and temporal variations of total organic carbon in the Arabia Sea. Deep-Sea Res., Part II 45, 2171–2193. Hansell, D.A., Waterhouse, T.Y., 1997. Controls on the distributions of organic carbon and nitrogen in the eastern Pacific Ocean. Deep-Sea Res., Part I 44, 843–857. Hansell, D.A., Williams, P.M., Ward, B.B., 1993. Measurements of DOC and DON in the Southern California Bight using oxidation by high temperature combustion. Deep-Sea Res., Part I 40, 219–234. Hansell, D.A., Bates, N.A., Carlson, C.A., 1997a. Predominance of vertical loss of carbon from surface waters of the equatorial Pacific Ocean. Nature 386, 59–61. Hansell, D.A., Carlson, C.A., Bates, N.A., Poisson, A., 1997b. A re-assessment of carbon export in the equatorial Pacific Ocean. Deep-Sea Res., Part II 44, 2115–2130. Jochem, F.J., Pollehne, F., Zeitzschel, B., 1993. Productivity regime and phytoplankton size structure in the Arabian Sea. Deep-Sea Res., Part II 40, 711–735. Karl, D.M., Christian, J.R., Dore, J.E., Letelier, R.M., 1996. Microbiological oceanography in the region west of the Antarctic Peninsula: microbial dynamics, nitrogen cycle and carbon flux. Foundations for Ecological Research West of the Antarctic Peninsula. Antarctic Research Series 70, 303-332. Kepkay, P.E., Wells, M.L., 1992. Dissolved organic carbon in North Atlantic surface waters. Mar. Ecol. Prog. Ser. 80, 275–283.
263
Kirchman, D.L, Lancelot, C., Fasham, M., Legendre, L., Radach, G., Scott, M., 1993. Dissolved organic material in biogeochemical models of the ocean. In: Evans, G.T., Fasham, M.J.R. ŽEds.., Towards a Model of Ocean Biogeochemical Processes. Serie I: Global Environmental Change, Vol. 10, Berlin, pp. 209–225. Kumar, M.D., Rajendran, A., Somasundar, K., Haake, B., Jenish, A., Shuo, Z., Ittekkot, Desai, B.N., 1990. Dynamics of dissolved organic carbon in the northwestern Indian. Oceanogr. Mar. Chem. 31, 299–316. Lukas, R., Lindstrom, E., 1991. The mixed layer of the western equatorial Pacific Ocean. J. Geophys. Res. 96, 3343–3357, Suppl. Martin, J.H., Fitzwater, S.E., 1992. Dissolved organic carbon in the Atlantic Southern and Pacific Oceans. Nature 356, 699– 700. McTaggart, K.E., Johnson, G.C., 1997. CTDrO2 measurements collected on a Climate and Global Change cruise ŽWOCE sections P14S and P15S. during January–March, 1996. NOAA Data Report ERL PMEL-63 ŽNTIS PB98-110158., 485 pp. Menzel, D.W., 1964. The distribution of dissolved organic carbon in the Western Indian Ocean. Deep-Sea Res. 11, 757–765. Menzel, D.W., 1970. The role in situ decomposition of organic matter on the concentration of non-conservative properties in the sea. Deep-Sea Res. 17, 751–764. Menzel, D.W., Ryther, J.R., 1968. Organic carbon and the oxygen minimum in the South Atlantic Ocean. Deep-Sea Res. 15, 327–337. Moriarty, D.J.W., O’Donohue, M.J., 1995. Organic carbon transport from the Southern Ocean and bacterial growth in the Antarctic Intermediate Water masses of the Tasman Sea. Mar. Ecol. Prog. Ser. 119, 291–297. Murray, J.W., Barber, R.T., Roman, M.R., Bacon, M.P., Feely, R.A., 1994. Physical and biological controls on carbon cycling in the equatorial Pacific. Science 266, 58–65. Ogura, N., 1970. The relation between dissolved organic carbon and apparent oxygen utilization in the Western North Pacific. Deep-Sea Res 17, 221–231. Peltola, E., Wanninkhof, R., Molinari, R., Huss, B., Feely, R., Bullister, J., Zhang, J.-Z., Chavez, F., Dickson, A., Field, A., Hansell, D., Millero, F., Quay, P., Castle, R., Thomas, G., Roddy, R., Lantry, T., Roberts, M., Chen, H., Greeley, D., Lee, K., Roche, M., Goen, J.A., Millero III, F., Buck, K., Kelly, M., Menzia, F., Huston, A., Waterhouse, T., Becker, S., Mordy, C., 1998. Chemical and hydrographic measurements during the Indian Ocean I8 report repeat cruise ŽIE8N. in September and October, 1995. NOAA Data Report ERL AMOL-34, NOAA, AOML, Miami, FL 163 pp. Peltzer, E.T., Hayward, N.A., 1996. Spatial distribution and temporal variability of total organic carbon along 140 W in the equatorial Pacific Ocean in 1992. Deep-Sea Res., Part II 43, 1155–1180. Radenac, M.-H., Rodier, M., 1996. Nitrate and chlorophyll distributions in relation to thermohaline and current structures in the western tropical Pacific during 1985–1989. Deep-Sea Res., Part II 43, 725–752. Reid, J.L., 1986. On the total geostrophic circulation of the South
264
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
Pacific Ocean: flow patterns, tracers and transports. Prog. Oceanogr. 16, 1–61. Romankevich, E.A., Ljutsarev, S.V., 1990. Dissolved organic carbon in the ocean. Mar Chem. 30, 161–178. Sambrotto, R.N., Savidge, G., Robinson, C., Boyd, P., Takahashi, T., Karl, D.M., Langdon, C., Chipman, D., Marra, J., Codisponti, L., 1993. Elevated consumption of carbon relative to nitrogen in the surface ocean. Nature 363, 248–250. Skopintsev, B.A., Timofeyeva, S.N., 1962. Organic carbon content n the Baltic and North Sea, and in tropical and subtropical regions of the North Atlantic Žin Russian.. Trudy morsk. Gudrofiz. Onst. 25, 110–117. Skopintsev, B.A., Timofeyeva, S.N., Vershinina, O.A., 1966. Organic carbon in the equatorial and southern Atlantic and Mediterranean. Okeanologiya 6, 201–210. Sokal, R.R., Rohlf, F.J., 1995. Biometry: The Principles and Practice of Statistics in Biological Research, Freeman, New York, 887 pp. Sonnerup, R.E., Quay, P.D., Bullister, J.L., 1999. Thermocline ventilation and oxygen utilization rates in the subtropical North Pacific based on CFC distributions during WOCE. Deep-Sea Res., Part I 46, 777–805. Sorokin, Y.I., 1971. On the role of bacteria in the productivity of tropical oceanic waters. Int. Rev. Gesamten Hydrobiol. 56, 1–48.
Sorokin, Y.I., 1977. Productivity and the trophic role of bacterioplankton in the region of equatorial divergence. Pol. Arch. Hydrobiol. 24, 241–259. Sorokin, Y.I., 1978. Decomposition of organic matter and nutrient regeneration. In: Kinne, O. ŽEd... Marine Ecology. Wiley, Chichester, pp. 501–616. Tanoue, E., 1993. Distributional characteristics of DOC in the central equatorial Pacific. J. Oceanogr. 49, 625–639. Thomas, C., Cauwet, G., Minster, J.-F., 1995. Dissolved organic carbon in the equatorial Atlantic Ocean. Mar. Chem. 49, 155–169. Weibinga, C.J., De Baar, H.J.W., 1998. Determination of the dissolved organic carbon in the Indian sector of the Southern Ocean. Mar. Chem. 61, 185–201. Wedborg, M., Hoppema, M., Skoog, A., 1998. On the relation between organic and inorganic carbon in the Weddell Sea. J. Mar. Syst. 17, 59–76. Williams, P.J.LeB., 1975. Biological and chemical aspects of dissolved organic material in sea water. In: Riley, J.P., Skirrow, G. ŽEds.., Chemical Oceanography, 2nd edn., Vol. 2. Academic Press, London, pp. 301–363. Williams, P.M., Druffel, E.R.M., 1987. Radiocarbon in dissolved organic matter in the central North Pacific Ocean. Nature 330, 246–248.
Organic carbon and apparent oxygen utilization in the western South Pacific and the central Indian Oceans M.D. Doval ) , Dennis A. Hansell Bermuda Biological Station for Research, 17 Biological Lane, St. Georges, GE-01, Bermuda Received 11 June 1998; received in revised form 23 July 1999; accepted 23 July 1999
Abstract Samples for total organic carbon ŽTOC. analysis were collected on WOCE Line P15S Ž08 to 678S along 1708W. and from 538 to 678S along 1708E in the western South Pacific, and on Line I8 Ž58N to 438S along 808r908E. in the central Indian Ocean. TOC concentrations in the upper ocean varied greatly between the regions studied. Highest surface TOC concentrations Ž81–85 mM C and 68–73 mM C. were observed in the warmest waters Ž) 278C. of the western South Pacific and central Indian Oceans, respectively. Lowest surface TOC concentrations Ž45–65 mM C. were recorded in the southernmost waters occupied Ž) 508S along 1708W and 1708E.. Deep water Ž) 1000 m. TOC concentrations were uniform across all regions analyzed, averaging between 42.3 and 43 mM C ŽSD: "0.9 mM C.. Mixing between TOC-rich surface waters and TOC-poor deep waters was indicated by the strong correlations between TOC and temperature Ž r 2 ) 0.80, north of 458S. and TOC and density Ž r 2 ) 0.50, southernmost regions.. TOC was inversely correlated with apparent oxygen utilization ŽAOU. along isopycnal surfaces north of the Polar Frontal Zone ŽPFZ. and at depths - 500 m. The TOC:AOU molar ratios at densities of s T 23–27 ranged from y0.15 to y0.34 in the South Pacific and from y0.13 to y0.31 in the Indian Ocean. These ratios indicate that TOC oxidation was responsible for 21%–47% and 18%–43% of oxygen consumption in the upper South Pacific and Indian Oceans, respectively. At greater depths, TOC did not contribute to the development of AOU. There was no evidence for significant export of dissolved and suspended organic carbon along isopycnal surfaces that ventilate near the PFZ. q 2000 Elsevier Science B.V. All rights reserved. Keywords: total organic carbon; dissolved organic carbon; apparent oxygen utilization; Western South Pacific Ocean; Central Indian Ocean
1. Introduction The source of total organic carbon ŽTOC. in the open ocean is in situ production from several biological processes ŽCopin-Montegut and Avril, 1993; ´ ) Corresponding author. Instituto de Investigaciones Marinas, C.S.I.C., Eduardo Cabello, 6, E-36208 Vigo, Spain. Tel.: q3486-231930; Fax: q34-86-292762; E-mail: [email protected]
Kirchman et al., 1993.. DOC production mechanisms are largely influenced by the composition of the plankton community, while concentrations are largely controlled by vertical mixing processes ŽCarlson and Ducklow, 1995; Hansell and Waterhouse, 1997.. The distribution of DOC in the equatorial Pacific Ocean is mostly controlled by the physical system ŽTanoue, 1993; Carlson and Ducklow, 1995; Peltzer and Hayward, 1996; Hansell and Water-
0304-4203r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 4 2 0 3 Ž 9 9 . 0 0 0 8 1 - X
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house, 1997., while biological processes may exert greater control in areas such as the Southern Ocean ŽKarl et al., 1996; Carlson et al., 1998.. Surveys of TOC concentrations at the basin scale Žca 10 3 –10 4 km. allow an assessment of controls on distribution by the various biological and physical processes. Apart from the work by Hansell and Waterhouse Ž1997. in the eastern Pacific Ocean, there is a paucity of recent studies covering this spatial scale. Earlier surveys of DOC distribution at the basin scale include those of Duursma Ž1961., Skopintsev and Timofeyeva Ž1962., Menzel Ž1964; 1970., Skopintsev et al. Ž1966., Menzel and Ryther Ž1968., Ogura Ž1970. and Romankevich and Ljutsarev Ž1990.. The semi-labile material that accumulates in the surface ocean Ži.e., with a residence time of months; Kirchman et al., 1993; Carlson and Ducklow, 1995. can be: consumed biologically over time within surface waters; exported vertically to depth, thereby contributing to the vertical gradient of dissolved inorganic carbon in the ocean ŽCopin-Montegut ´ and Avril, 1993; Carlson et al., 1994.; or exported horizontally from sites of formation ŽPeltzer and Hayward, 1996; Hansell et al., 1997a,b.. Only in oceanic regions where winter overturn ventilates deep isopycnal layers can DOC be exported to the deep ocean. Some early studies have shown a correlation between DOC and apparent oxygen utilization ŽAOU; Ogura, 1970; Craig, 1971. in the ocean, while others have indicated that DOC does not contribute significantly to deep ocean oxygen consumption ŽMenzel and Ryther, 1968; Menzel, 1970; Williams, 1975; Hansell et al., 1993.. The biological refractiveness of DOC ŽBarber, 1968., its great age ŽWilliams and Druffel, 1987; Bauer et al., 1992. and relatively uniform deep ocean concentrations ŽMartin and Fitzwater, 1992. provided further evidence for the conservative nature of DOC. In the past few years, however, a correlation between DOC and AOU has again been demonstrated ŽGuo et al., 1994; Thomas et al., 1995; Peltzer and Hayward, 1996. and the conservative nature of deep ocean DOC has been challenged ŽHansell and Carlson, 1998a.. The discrepancies between these disparate findings have not been adequately evaluated. In this paper, we Ž1. evaluate the basin scale distributions of TOC in the western South Pacific
Ocean and the central Indian Ocean in the context of hydrographic variability in those regions; and Ž2. estimate the contribution of TOC oxidation to oxygen consumption along isopycnal surfaces in the subsurface and deep ocean.
2. Methods The work described here was performed as a component of the NOAA Climate and Global Change program Ocean–Atmosphere Carbon Exchange Study ŽOACES.. Samples for TOC analysis were collected along WOCE Line P15 Ž1708W. and 1708E on the NOAA Ship DiscoÕerer between January–March 1996, and along WOCE Line I8 Ž808E. on the NOAA Ship Malcom Baldrige between September and October of 1995 ŽFig. 1.. Cruise details are listed in Table 1. Data for potential temperature Ž u ., salinity Ž S ., potential density anomaly Ž su ., total inorganic nitroy. gen ŽTIN s NOy and AOU on the I8 line 3 q NO 2 were taken from Peltola et al. Ž1998.. Hydrographic data from the P15S line were taken from McTaggart and Johnson Ž1997. while nutrient data were provided by C. Mordy ŽPMEL.. Water samples taken for organic carbon determination were not filtered; they were collected in precleaned 40 ml glass vials or 60 ml polyethylene bottles. TOC concentrations were determined from up to 24 depths over the entire water column. TOC analysis was by a high temperature combustion method like that described by Hansell and Waterhouse Ž1997.. All measurements were referenced each analytical day against 2600 m Sargasso Sea water. To minimize the system blank in the analytical system, conditioning of the combustion tube was required prior to analysis of samples. Conditioning was performed through repeated injections of Milli-Q w water andror seawater. After conditioning, the system blank was assessed with ampoulated low carbon waters ŽLCW. that had been referenced against blank water provided by Dr. Jonathan Sharp for the 1994 DOC community intercomparison program. Typical relative standard deviations of replicate TOC analyses were ; 2%. The instrument response factor was determined two to four times daily using
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
251
Fig. 1. Map of transects occupied in Region A of the western South Pacific Ocean, Region B of the central Indian Ocean and Region C in the Southern Ocean. The dashed line across 1708W separates Regions A and C.
glucose in Milli-Q w water. Typically, the response factors on an analytical day differed by - 2% and their mean was within ; 5% of the mean of all calibrations. The instrument blank was measured every four to six samples using carbon-free distilled water. All TOC concentrations reported are corrected for the instrument blank. A portion Žf 50%. of the samples collected along the northern part of the I8 line was analyzed at the shore laboratory of Bermuda Biological Station for Research ŽBBSR.. Those samples were stored frozen in 60 ml polyethylene bottles until analysis. All other samples were analyzed at the time of collection. In the data analysis, we have employed the Model II linear regression ŽSokal and Rohlf, 1995..
3. Results 3.1. Hydrographic features of the ocean regions This study covered several oceanic areas: the equatorial regions in the Pacific and Indian Oceans, the oligotrophic Pacific and Indian subtropical gyres, and the Subantarctic and Antarctic PacificrSouthern Ocean. Here, in order to simplify the evaluation, these areas are grouped into three regions ŽFig. 1.: Region A — the southwest Pacific Ž08–428S, 1708W.; Region B — the central Indian Ocean Ž58N–438S, 808–958E.; and Region C — the Southern Ocean Ž428–678S, 1708W and 538–678S, 1708E.. The equatorial systems in the Pacific ŽRegion A. and Indian ŽRegion B. Oceans differed greatly dur-
Table 1 Dates of the OACES cruises ŽWOCE sections P15S in the western South Pacific Ocean including 1708E, and I8 in the central Indian Ocean; Fig. 1., stations sampled and regions considered in this study. The Stations column indicates the range of stations occupied, the number of stations occupied for TOC analysis within the range, and the number of samples analyzed Dates
Ocean section
Stations
Region
February 1–March 6, 1996 September 28–October 24, 1995 January 18–31, 1996 January 9–17, 1996
P15S, 1708W I8, 808–958E P15S, 1708W 1708E
a90–174 Ž83 stations, 705 samples. a2–101 Ž51 stations, 657 samples. a33–87 Ž51 stations, 365 samples. a5–32 Ž21 stations, 213 samples.
A Ž08–428S. B Ž58N–438S. C Ž428–678S. C Ž538–678S.
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ing the periods of sampling. Surface layer TIN concentrations were ; 5 mmolrkg in the equatorial Pacific but below limits of detection in the equatorial Indian Ocean ŽFig. 2a,b.. Warm waters Ž) 278C. characterized the upper mixed layer within the equa-
torial central Indian Ocean ŽFig. 2b., as reported previously by Jochem et al. Ž1993.. The oligotrophic gyre waters found in the western South Pacific and in the central Indian Oceans Žsouth of 108S in Regions A and B, respectively. were
Fig. 2. Distribution of mean TOC ŽmM C., temperature Ž u , 8C. and TIN Žmmolrkg. in the upper layer within Ža. Region A Župper 100 m., Žb. Region B Župper 50 m., and Žc. on line 1708W in Region C Župper 50 m. and Žd. line 1708E in Region C Župper 50 m..
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
depleted in nutrients and showed a decrease in surface temperature with increasing latitude ŽFig. 2a,b.. A limb of the low salinity, western Equatorial Pacific warm pool Ž) 278C; Radenac and Rodier, 1996. was located between 58 and 238S within region A ŽFig. 2a and Fig. 3b.. Region C Ž) 428S, 1708W and ) 508S, 1708E. covers Subantarctic Žbetween 428 and 608S. and Antarctic Žsouth of 608S. waters, with the two separated by the Polar Frontal Zone ŽPFZ.. These cold, saline waters showed relatively high nutrient concentrations ŽFig. 2c.. 3.2. Distributions of TOC The latitudinal distributions of TOC ŽFigs. 3a, 4a, 5a. showed a wide range of surface TOC concentrations between the different regions considered and within the same regions: 65–90 mM C ŽRegion A.,
253
55–80 mM C ŽRegion B. and 45–75 mM C ŽRegion C.. The variation in deep water TOC Ždepth ) 1000 m. was - 1 mM C, with mean concentrations Ž"SD. of 42.6 " 0.7 mM C in Region A, 42.2 " 0.6 mM C in Region B, and 43.0 " 0.9 mM C in Region C. All these means were statistically different Ž p - 0.05.. The distributions of TOC along 1708W and 808E ŽFig. 3a and Fig. 4a. resembled those of temperature ŽFig. 3b and Fig. 4b., broadly indicating the mixing of TOC-rich, warm surface water with TOC poor, cold deep water. TOC in the upper layer Žmean concentrations in the upper 100 m within Region A and upper 50 m within Region B. paralleled the distribution of mean temperature and inversely correlated with mean TIN concentrations ŽFig. 2a,b.. The warmest waters Ž) 278C. were those of the warm pools located at 58S–238S ŽRegion A. and 58N–108S ŽRegion B.. These waters showed the maximum TOC values in the upper layer for the entire lines
Fig. 3. Distribution of TOC ŽmM C. and temperature Ž8C. in the upper 500 m of the South Pacific Ocean.
254
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
Fig. 4. Distribution of TOC ŽmM C. and temperature Ž8C. in the upper 500 m of the central Indian Ocean.
ŽFig. 2a,b.. This trend can also be inferred from the linear regression between TOC and temperature in both areas ŽTable 2.. The average TOC concentrations within the Equatorial Pacific region were lower than those recorded within the Pacific warm pool. TOC concentrations in the surface layer of Region C were only slightly higher than values found at greater depths. The Subantarctic and Antarctic waters within the southernmost part of 1708W showed an average TOC concentration of 60.4 " 9.2 mM C in the upper 50 m, decreasing from 74.2 to 56.5 mM C from 428 to 608S and from 56.5 to 45 mM C from 608 to 678S ŽFig. 2c.. The average TOC concentration between 538 and 678S was 55.0 " 4.6 mM C along 1708W and 53.1 " 3.6 mM C along 1708E ŽFig. 2d.. Linear regression between TOC and temperature Ž p - 0.001. from all depths in Regions A and B
showed r 2 of 0.81 and 0.87, respectively ŽTable 2.. The slopes obtained were slightly higher Ž1.85 and 1.36 for Regions A and B, respectively. when only samples with temperature ) 78C were considered, but the correlation coefficients were lower Ž0.73 and 0.79 for Regions A and B, respectively; data not showed.. The narrower ranges of both TOC and temperature in the Southern Ocean ŽRegion C; 1708E; Fig. 5a and b. resulted in a low correlation Ž r 2 s 0.62 from 1708W, and 0.22 from 1708E; Table 2.. These southern waters showed a better correlation when sigma theta was considered as the hydrographic index Ž r 2 : 0.78 and 0.54, respectively, data not shown.. Linear regression showed that temperature explained 50% and 79% of the variance observed in TOC in the upper mixed layers Ždepths - 40 m, data not shown. in Regions A and B, respectively. However, the variance explained was as low as 28% and
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
255
Fig. 5. Distribution of TOC ŽmM C. and temperature Ž8C. in the upper 500 m on the 1708E line in the Southern Ocean.
32% when all thermocline waters Žtemperature) 208C, su - 25. were considered Ždata not shown.. The lack of correlation with temperature or su in the upper thermocline waters was particularly evident in the warm pools. The correlation of TOC with temperature in the upper mixed and upper thermocline waters within Region C was especially low and insignificant on 1708E Ž r 2 - 0.01, data not shown..
3.2.1. TOC r AOU relationship TOCrAOU relationships ŽModel II. are first considered for the entire water column Žwhere AOU ) 0, i.e., subsurface waters. in different areas of the western Pacific and central Indian Oceans ŽTable 3.. This analysis is similar to those performed in several earlier efforts Žsee references above., although some of the molar ratio determined using Model I regres-
Table 2 Selected linear regressions between TOC ŽmM C. and temperature Ž u , 8C. for the entire water column from the various regions studied Ž p - 0.001.. Numbers in parentheses are the standard error of the coefficients Region
Equation
r2
n
A Ž08–428S, 1708W. B Ž58N–438S, 808–958E. C Ž428–678S, 1708W. C Ž538–678S, 1708E.
TOC s 34.5Ž"0.3. q 1.51Ž"0.03.u TOC s 38.1Ž"0.2. q 1.2Ž"0.02.u TOC s 39.9Ž"0.3. q 1.9Ž"0.1.u TOC s 41.4Ž"0.4. q 1.8Ž"0.1.u
0.81 0.87 0.62 0.22
701 645 384 213
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
256
Table 3 Slope Žmolar ratio. of the linear regression between TOC Žmmolrkg. and AOU Žmmolrkg. and the multiple regression between TOC, temperature Ž u , 8C. and AOU when AOU) 0 ŽModel II.. Numbers in brackets are the standard error, ns number of data pairs included Region
Variables
TOCrAOU
r2
South Pacific 08–678S Ž ns 735. 08–428S Ž ns 235. 428–678S Ž ns 274.
TOCrAOU TOCru ,AOU TOCrAOU TOCru ,AOU TOCrAOU TOCru ,AOU
y0.121 Ž"0.00. y0.074 Ž"0.002. y0.13 Ž"0.005. y0.10 Ž"0.008. y0.099 Ž"0.004. y0.071 Ž"0.004.
0.44 0.75 0.68 0.81 0.44 0.69
Central Indian Ž ns 511. TOCrAOU TOCru ,AOU
y0.085 Ž"0.003. y0.030 Ž"0.001.
0.17 0.86
1708E (538– 678S) Ž ns165. TOCrAOU TOCru ,AOU
y0.074 Ž"0.003. y0.087 Ž"0.004.
0.54 0.54
sions can be underestimated. Second, an evaluation of the relationship between TOC and AOU is performed along isopycnal surfaces.
The correlation between TOC and AOU is partially due to the dependence of both variables on water mass mixing ŽMenzel and Ryther, 1968.. Therefore, when both variables show a strong correlation with temperature, the multiple correlation between TOC, temperature and AOU must be considered ŽAlvarez-Salgado X.A., personal communication.. Regressions with and without temperature considered indicate that water mass mixing tends to overestimate the contribution of TOC to AOU in all regions, except along 1708E in Region C where the correlation of both TOC and AOU with temperature is low ŽTable 3.. This overestimation is evident by the reduction in slope when temperature is included in the multiple regression. The TOCrAOU relationship for the entire water column is of limited value because it combines depths in the ocean where the relationship is strong with depths where the relationship is weak. To identify where TOC oxidation makes a substantive contribution to the development of AOU an evaluation of the two properties along isopycnal surfaces is required. The distribution of sigma theta is shown in Figs. 6 and 7 for the western South Pacific and central Indian Oceans, respectively. In Fig. 8, we
Fig. 6. Distribution of sigma theta in the upper 750 m of the South Pacific Ocean. The upper and lower dashed lines approximately indicate the shallowest and greatest depths, and the spatial extent, to which net TOC oxidation contributed to AOU development.
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
257
Fig. 7. Distribution of sigma theta in the upper 750 m of the central Indian Ocean. The dashed line approximately indicates the depth and spatial extent over which net TOC oxidation contributed to AOU development.
show TOCrAOU relationships from select density surfaces in the South Pacific. TOC and AOU concentrations from water forming the su s 25–25.5 density layer are plotted against latitude in Fig. 8a. The two variables varied little at locations ) 308S. At these latitudes, the su s 25–25.5 density layer was above the seasonal thermocline ŽFig. 6.. As the layer deepened to ) 200 m from 308S to near 128S ŽFig. 6. TOC decreased and AOU increased ŽFig. 8a.. AOU and TOC remained at fairly constant values at latitudes - 128S ŽFig. 8a.. A contribution of TOC oxidation to AOU development was largely limited to the depth and latitudinal ranges of 50–200 m and 128S–308S, respectively, in this density layer. As another example, TOC and AOU concentrations in the su s 26.5–26.75 density layer of the western South Pacific, which ventilates at a higher latitude and reaches greater depths than the su s 25– 25.5 layer ŽFig. 6., are shown in Fig. 8b. TOC concentrations decreased and AOU concentrations increased from the site of ventilation Žnear 558S. to near 208S Ž450–500 m.. At lower latitudes TOC was largely invariant but AOU continued to increase. Several density layers were evaluated in this way to determine where TOC oxidation showed a contri-
bution to AOU. The start and end points ŽTOC concentration and latitudinal ranges along each isopycnal surface. are shown in Fig. 8c for the western South Pacific Ocean. The corresponding depths for these points are approximated by the dashed lines in Figs. 6 and 7, with the upper dashed line Žabsent in Fig. 7. indicating where TOC begins contributing to AOU and the lower dashed line indicating the end of a contribution. The concentrations of TOC decreased with increasing sigma theta, both near the surface where TOC begins to contribute to AOU and at depth where the contribution ends ŽFig. 8c.. The same behavior was found in central Indian Ocean waters. The TOCrAOU molar ratios along isopycnal surfaces in the South Pacific Ocean Ž08–678S. and in the central Indian Ocean Ž58N–438S., assessed where TOC contributes to AOU as described above, are reported in Table 4a and b, respectively. In the lowest density waters Ž su - 23. TOC and AOU did not correlate because these waters reside in the surface layers where AOU is variable and near zero due to air–sea exchange processes. At intermediate densities Ž su s 23–27., the TOCrAOU molar ratio ranged from y0.15 to y0.34 Žmean of y0.26 " 0.6.
258
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
Fig. 8. Distribution of TOC ŽmM C. and AOU Žmmolrkg. along selected density surfaces in the South Pacific Ocean. Ža. su s 25– 25.5, and Žb. su s 26.5–26.75; and Žc. maxima and minima of TOC concentrations ŽmM C. on each density surface.
and y0.13 to y0.31 Žmean of y0.23 " 0.05. along 1708W and 808E, respectively. At potential densities anomalies of su ) 27 the ratio was very low, ranging from y0.03 to q0.08, and with low correlation coefficients ŽTable 4a,b.. The contribution of TOC to oxygen consumption along isopycnal surfaces can be determined after converting AOU to carbon equivalents. The ratio expected from the Redfield stoichiometry ŽC:N:O:P s 106:42:16:1, Anderson, 1995. would predict a yDCrDO 2 ratio of 0.72. The contribution of TOC to oxygen consumption at intermediate densities, in equivalent carbon units, ranged from 20.8–47.2%
with a mean of 40.4 " 8.8% in the South Pacific and from 18.0–43.0% with a mean of 32.7 " 7.8% in the central Indian Ocean ŽTable 4a and b.. These contributions change somewhat when we consider a nonRedfield CrN ratio of 12 for DOM. The average contribution of TOC to AOU development becomes 36% in the South Pacific and 29% in the central Indian Ocean. The contributions of isopycnal mixing to the horizontal gradients in AOU and TOC are difficult to ascertain. Model results from the North Pacific ŽSonnerup et al., 1999. suggest that advection dominates transport in the main thermocline of the subtropics so the contribution of diffusion is small. Taking a diffusion coefficient of 1000 m2 sy1 suggests that - 30% of the change in AOU is due to diffusive mixing. Indeed, by taking the ratio of TOC to AOU as was done here, the contributions of diffusion are to some extent negated since diffusion should affect the two terms similarly. Another uncertainty is the effect of diapycnal mixing on the ratio. This uncertainty can be minimized by grouping the data into broadly delimited density surfaces, so that most mixing is within the chosen surface and not across it. Using only the data demonstrating TOC:AOU covariance as described above, the data have been grouped as su s 23–25 Župper thermocline waters. and su ) 25.5. In the upper thermocline the molar ratios Žand the contributions of TOC to AOU in equivalent carbon units. were y0.35 Ž48.6%. and y0.21 Ž29.1%. along 1708W and 808E, respectively, while in the deeper water the slopes were y0.26 Ž36.1%. and y0.22 Ž30.5%., respectively Ždata not shown.. The molar ratio from the 808E shallow water regression has greater uncertainly since there were only 11 data points, but the ratios are in general agreement with those reported in Table 4a and b for narrower density intervals. To summarize the findings for the various density ranges, we have the following. ŽI. su - 23 Župper mixed layer from 208S to 08 within the South Pacific and surface water between 108S and 58N within the central Indian Ocean.: TOCrAOU was highly variable and the linear regression not significant. ŽII. su s 23–27, at depths - 500 m Žsubsurface waters of the western Pacific Ocean and the central
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
259
Table 4 Slope ŽModel II. between TOC Žmmolrkg. and AOU Žmmolrkg. TOCrAOU-C Ž%.
Samples
r2
(a) South Pacific Ocean (08–678S) 27.9–27.5 q0.062 Ž"0.007. 27.5–27 y0.032 Ž"0.004. 27–26.75 y0.23 Ž"0.01. 26.75–26.5 y0.15 Ž"0.011. 26.5–26 y0.30 Ž"0.01. 26–25.5 y0.28 Ž"0.01. 25.5–25 y0.33 Ž"0.03. 25–24.5 y0.34 Ž"0.05. 24.5–24 y0.32Ž"0.04. 24–23.5 y0.34 Ž"0.04. 23.5–23 y0.33 Ž"0.06. 23–22.0 insignificant
– 4.4 31.9 20.8 41.7 38.9 45.8 47.2 44.4 47.2 45.8 –
08S–408, 1400–3000 m 08S–308, 520–1070 m 448–638S, 20–570 m 208S–568S, 20–470 m 208S–408S, 70–400 m 138S–408S, 50–300 m 128S–308S, 80–260 m 128S–308S, 46–210 m 98S–268S, 50–162 m 98S–218S, 50–164 m 88S–208S, 70–165 m 58S–238S, - 100 m
0.003 0.025 0.45 0.49 0.76 0.89 0.73 0.58 0.72 0.83 0.92 –
64 52 84 98 54 35 20 20 17 13 8 –
(b) Central Indian Ocean (58N–438S) 27.8–27.5 q0.08 Ž"0.004. 27.5–27 q0.039 Ž"0.004. 27–26.75 y0.13 Ž"0.01. 26.75–26.5 y0.26 Ž"0.02. 26.5–26 y0.25 Ž"0.04. 26–25.5 y0.21Ž"0.01. 25.5–25 y0.31 Ž"0.05. 25–24 y0.26 Ž"0.03. 24–23 y0.23 Ž"0.06. 23–21 insignificant
– – 18.0 36.1 34.7 29.2 43.0 36.1 31.9 –
58S–308N, 1100–2500 m 58S–458N, 500–1400 m 158S–438S, s–600 m 158S–438S, s–500 m 158S–358S, s–300 m 158S–308S, 20–270 m 158S–308S, s–200 m 178S–258S, s–120 m 58S–128N, 50–100 m 58S–108N, - 90 m
0.08 0.14 0.44 0.52 0.47 0.46 0.81 0.91 0.58 –
40 64 68 128 28 5 10 7 8 –
su
TOCrAOU Žmolar ratio.
n
Numbers in parentheses are the standard error. TOCrAOU is the molar ratio; TOCrAOU-C is calculated with AOU in carbon equivalents, Žusing a Redfield ratio yDCrDO 2 s 0.72; Anderson, 1995.. TOCrAOU-C thus represents the contribution of TOC to AOU along specific isopycnal surfaces. s s surface.
Indian Ocean.: The TOCrAOU molar ratio ranged between y0.15 and y0.34 in the South Pacific Ocean and between y0.18 and y0.31 in the central Indian Ocean. The coefficient of regression, r 2 was ) 0.4. A total of 30% to 50% of the AOU could be assigned to oxidation of suspendedrdissolved organic carbon, with the remainder due to oxidation of sinking POM. ŽIII. su ) 27 and depths ) 500 m Ždeep waters of the subtropical Pacific and central Indian Ocean.: The TOCrAOU molar ratios and the correlations were insignificant. AOU at these depths are controlled by mixing and oxidation of sinking POM. TOC was not exported to a significant extend along these density surfaces. The relative importance of TOC mineralization varied with depth and latitude. Variability in the
South Pacific region is showed on Fig. 9: the shadowed area indicates where TOC contributes significantly to oxygen consumption.
4. Discussion 4.1. TOC distribution The maximum TOC concentrations Ž) 80 mM C. were found in the western Pacific warm pool, a region characterized by high temperature Ž) 278C. and depleted nutrients. The maximum TOC concentrations in the Indian Ocean, also located in the warmest surface waters, were significantly lower at
260
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
Fig. 9. Schematic diagram depicting the locations, depths and patterns of TOC mineralization as it is exported from the upper ocean. The shadowed area indicates where TOC contributes significantly to oxygen consumption, based on observations in the western South Pacific Ocean during austral summer. The sigma theta contours 25 and 27 are showed in the diagram.
) 70 mM C ŽFigs. 3 and 4.. The Indian Ocean maximum values were similar to the surface layer concentrations reported by Hansell and Peltzer Ž1998. for the Arabian Sea during the same seasonal period Žthe Fall Intermonsoon.. They reported that the lowest concentrations on an annual basis occurred during the Fall Intermonsoon, perhaps due to a change in the composition of the microbial food web. Lower variability of TOC in Region C, especially on the most westerly line Ž1708E., agrees with the results reported for the Southern Ocean by several authors ŽCarlson et al., 1998; Wedborg et al., 1998; Weibinga and De Baar, 1998.. The uniformity of deep water TOC values Ž) 1000 m. reflects the relative homogeneity of bottom water masses in the regions studied. Deep water in Regions A and B have as their source a mixture of Antarctic Bottom Water and North Atlantic Deep Water ŽReid, 1986.. In the Southern Ocean ŽRegion C., the average deep TOC concentration was indistinguishable from the Regions A and B. These bottom TOC concentrations were very similar to those reported by Hansell and Carlson Ž1998a. for these areas. Higher surface water TOC values in the warm subtropical Pacific than in the cool Equatorial Pacific have been reported previously ŽTanoue, 1993; Peltzer and Hayward, 1996; Hansell and Waterhouse, 1997..
These authors suggested physical control as the main determinant for DOC concentrations. The highest TOC concentrations are within the tropical region where high vertical stability allows its accumulation. Net biological consumption of TOC and mixing within the subtropical gyre was indicated by the gradual decrease of the TOC in the upper 100 m from north to south. Within the Equatorial Pacific, TOC accumulation was partially prevented by the low residence time of the surface water and therefore, the high advective export of carbon ŽMurray et al., 1994.. In Region C, lower net production of DOC ŽCarlson et al., 1998; Hansell and Carlson, 1998b. and lower vertical stability resulted in low TOC concentrations. Surface TOC concentrations are controlled both by physical and biological processes ŽDoval et al., 1997; Hansell and Waterhouse, 1997; Carlson et al., 1998; Alvarez-Salgado et al., 1999.. A role for mixing between surface water rich in semi-labile DOC and deep water low in semi-labile DOC was indicated by the good correlation between TOC and temperature when the entire water column was considered in Regions A and B. However, although the maximum accumulation of TOC coincides with the warm pools, the lower correlation with temperature in the upper thermocline waters and the unequal
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
TOC concentrations in the Pacific and Indian warm pools suggest a greater biological control in this layer. The barrier layer associated with the warm pool Ži.e., the water column located between the top of thermocline and the base of the surface mixed layer; Lukas and Lindstrom, 1991., because of its inherent stabilizing effect, should restrict the vertical export of TOC from the upper mixed layer by mixing. Therefore, the high TOC concentrations present in this layer must be due mainly to in situ production and solubilization of POC. The Southern Ocean data set showed little correlation between TOC and temperature, especially along 1708E. This behavior is due in part to the reduced ranges of both TOC and temperature and the higher control by salinity on density at high latitudes. A higher biological control for the distribution of DOC in the Southern Ocean has been suggested in other studies, where DOC production, biogeochemical composition, and utilization by bacteria all strongly affect the accumulation of DOC ŽKarl et al., 1996; Carlson et al., 1998.. 4.2. Subsurface mineralization of dissolÕed organic carbon Several data sets have revealed some degree of covariance between DOC and AOU ŽOgura, 1970; Craig, 1971; Kumar et al., 1990; Druffel et al., 1992;
Kepkay and Wells, 1992; De Baar et al., 1993; Hansell et al., 1993; Guo et al., 1994; Thomas et al., 1995; Peltzer and Hayward, 1996; Weibinga and De Baar, 1998.. Literature values for DOCrAOU molar ratios obtained from the open ocean range between 0 and y0.59 ŽTable 5.. Our results on the relationship between DOC and AOU along isopycnal surfaces agree particularly well with those of Ogura Ž1970.. He too found that DOC contributes to AOU in the upper 500 m of the water column Žat a DOCrAOU molar ratio ( 0.23., and that sinking biogenic particles contribute to the AOU balance at those depths and to most of the oxygen consumption at greater depths. The lack of correlation between TOC and AOU for deep waters Ž) 500m. reported here is in agreement with the results of Menzel Ž1964; 1970., Menzel and Ryther Ž1968. and Peltzer and Hayward Ž1996; depths ) 400 m.. The idea that organic matter can be transported at intermediate depths and support elevated rates of heterotrophic activity has been discussed for many years. Sorokin Ž1971; 1977; 1978. observed a band of water with high microbial activity lying in the upper layers of the Antarctic Intermediate Water ŽAIW. in the Pacific and Indian Oceans, and sought to explain this as a consequence of deep horizontal transport of labile organic matter. The concept was revisited by Moriarty and O’Donohue Ž1995. in the Tasman Sea who found elevated microbial growth rates in AIW at depths of 800–1200 m. The authors
Table 5 Select literature values for TOCrAOU molar ratios determined for the deep ocean Reference
Molar ratio
Data specifications
Ogura, 1970 a Kumar et al., 1990 a Druffel et al., 1992 a,b
y0.23 y0.062 y0.29 y0.59 0 y0.25 y0.068 y0.152 y0.09, y0.14 y0.074 y0.26 y0.23
upper 500 m; western North Pacific Ocean full water column; NW Indian Ocean full water column; North Atlantic Ocean - 482 m; North Pacific ) 482 m; North Pacific full water column; North Atlantic Ocean full water column; Southern California Bight - 250 m; Gulf of Mexico - 700 m; equatorial Atlantic Ocean 90–400 m; equatorial Pacific Ocean su s 23–27 in South Pacific Ocean su s 23–27 in central Indian Ocean
Kepkay and Wells, 1992 Hansell et al., 1993 a Guo et al., 1994 a Thomas et al., 1995a Peltzer and Hayward, 1996 This study a b
261
Model I regression. Ratios reported from Druffel et al. Ž1992. are those resulting from organic carbon concentrations determined by UV oxidation.
262
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
suggested that their results supported the hypothesis for horizontal advection and mineralization of a significant pool of labile organic matter within the AIW. The core of the deep, elevated bacterial activity Moriarty and O’Donohue Ž1995. reported was located in water characterized by a minimum in salinity and a temperature of 48C–68C. On the P15S line, this water lies approximately on the 27.2 isopycnal ŽFig. 6.. We found little correlation between AOU development and TOC oxidation ŽTable 4a. in this water mass, particularly at depths ) 800 m. We do not know the reason for the elevated growth rates reported by Moriarty and O’Donohue Ž1995., but the rates cannot be assigned to significant consumption of DOC introduced isopycnally. Our results and those of Ogura Ž1970. do support Sambrotto et al. Ž1993. and Anderson and Sarmiento Ž1994. who suggested that a gradient must exit in the water column nutrient ratios between the upper and deep ocean. In the upper 500 m, a non-Redfield ratio should be anticipated because both carbon-rich nonRedfield DOM and sinking POM with a Redfield signature contribute approximately equally to the consumption of oxygen. At greater depths sinking POM with Redfield elemental ratios is the main source of reduced material supporting oxygen consumption and remineralization of the major elements. A fuller analysis to test the existence of such a gradient has yet to be performed.
5. Conclusions The maximum TOC concentrations in the western South Pacific and central Indian Ocean were observed in the warmest surface waters Ž) 278C.. The lowest surface TOC concentrations were recorded in the southernmost waters occupied Ž) 508S along 1708W and 1708E.. Deep water Ž) 1000 m. TOC concentrations were uniform across the regions studied, reflecting the relative homogeneity of deep and bottom water. TOC oxidation contributed to 30%–50% of oxygen consumption in the upper 500 m of water that ventilated north of the PFZ. At greater depths sinking biogenic matter must act as the primary source of reductant for the consumption of oxygen. The higher the latitude for ventilation of a density surface, the
greater the depth to which TOC was exported and mineralized. South of the PFZ TOC, was not seen to be exported greatly, so it’s contribution to AOU was minimal. Acknowledgements This work was conducted as a component of the NOAA Office of Global Programs’ OACES, through NOAA Award NA56GP0207 to DAH. MDD was supported from the NOAA Award NA 56GP0207, from a fellowship from the Bermuda Biological Station for Research, Grant-in-Aid program, and from ‘Direccion General de Universidades e Investigacion de la Xunta de Galicia’. Rachel Parson, Susan Becker and Tye Waterhouse performed the analyses of TOC during the cruises. We thank Calvin Mordy ŽPMEL., who provided us with inorganic nitrogen data. We gratefully acknowledge the officers, crew and technicians of the NOAA Ships DiscoÕerer and Malcom Baldrige for their assistance. Thanks to E.T. Peltzer and two anonymous reviews for their contribution to improve the manuscript. This is Contribution No. 1519 from the Bermuda Biological Station for Research. References Alvarez-Salgado, X.A., Doval, M.D., Perez, F.F., 1999. Dissolved organic matter in shelf waters of the NW Iberian upwelling system. J. Mar. Syst. 18, 383–394. Anderson, L.A., 1995. On the hydrogen and oxygen content of marine phytoplankton. Deep-Sea Res., Part I 42, 1675–1680. Anderson, L.A., Sarmiento, J.L., 1994. Redfield ratios of remineralization determined by nutrient data analysis. Global Biogeochem. Cycles 8, 65–80. Barber, R.T., 1968. Dissolved organic carbon from deep waters resist microbial oxidation. Nature 220, 274–275. Bauer, J.E., Williams, P.M., Druffel, E.R.M., 1992. 14 C activity of dissolved organic carbon fractions in the north central Pacific and Sargasso Sea. Nature 357, 667–670. Carlson, C.A., Ducklow, H.W., 1995. Dissolved organic carbon in the upper ocean of the central equatorial Pacific Ocean, 1992: daily and finescale vertical variations. Deep-Sea Res., Part II 42, 639–656. Carlson, C.A., Ducklow, H.W., Michaels, A.F., 1994. Annual flux of dissolved organic carbon from the euphotic zone in the Northwestern Sargasso Sea. Nature 371, 405–408. Carlson, C.A., Ducklow, H.W., Hansell, D.A., Smith, W.O., 1998. Differences in ecosystem dynamics between spring blooms in the Ross Sea polynya and the Sargasso Sea re-
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264 flected by contrasts in dissolved and particulate organic carbon partitioning. Limnol. Oceanogr. 43, 375–386. Copin-Montegut, C., Avril, B., 1993. Vertical distribution and ´ temporal variation of dissolved organic carbon in the north western Mediterranean Sea. Deep-Sea Res., Part I 40, 1963– 1972. Craig, H., 1971. The deep metabolism: oxygen consumption in abyssal ocean water. J. Geophys. Res. 76, 5078–5086. De Baar, H.J.W., Brussaard, C., Hegeman, J., Schijf, J., Stoll, M.H.C., 1993. Sea-trials of three different methods for measuring non-volatile dissolved organic carbon in seawater during the JGOFS North Atlantic pilot study. Mar. Chem. 41, 145–152. Doval, M.D., Alvarez-Salgado, X.A., Perez, F.F., 1997. Dissolved organic matter in a temperate embayment affected by coastal upwelling. Mar. Ecol. Prog. Ser. 157, 21–37. Druffel, E.R.M., Williams, P.M., Bauer, J.E., Ertel, J.R., 1992. Cycling of dissolved and particulate organic matter in the open ocean. J. Geophys. Res. 97, 15639–15659. Duursma, E.K., 1961. Dissolved organic carbon, nitrogen and phosphorous in the sea. Neth. J. Sea Res. 1, 1–148. Guo, L., Coleman, C.H. Jr., Santschi, P.H., 1994. The distribution of colloidal and dissolved organic carbon in the Gulf of Mexico. Mar. Chem. 45, 105–119. Hansell, D.A., Carlson, C.A., 1998a. Deep ocean gradients in concentration of dissolved organic carbon. Nature 395, 263– 266. Hansell, D.A., Carlson, C.A., 1998b. Net community production of dissolved organic carbon. Global Biogeochem. Cycles 12, 443–453. Hansell, D.A., Peltzer, E.T., 1998. Spatial and temporal variations of total organic carbon in the Arabia Sea. Deep-Sea Res., Part II 45, 2171–2193. Hansell, D.A., Waterhouse, T.Y., 1997. Controls on the distributions of organic carbon and nitrogen in the eastern Pacific Ocean. Deep-Sea Res., Part I 44, 843–857. Hansell, D.A., Williams, P.M., Ward, B.B., 1993. Measurements of DOC and DON in the Southern California Bight using oxidation by high temperature combustion. Deep-Sea Res., Part I 40, 219–234. Hansell, D.A., Bates, N.A., Carlson, C.A., 1997a. Predominance of vertical loss of carbon from surface waters of the equatorial Pacific Ocean. Nature 386, 59–61. Hansell, D.A., Carlson, C.A., Bates, N.A., Poisson, A., 1997b. A re-assessment of carbon export in the equatorial Pacific Ocean. Deep-Sea Res., Part II 44, 2115–2130. Jochem, F.J., Pollehne, F., Zeitzschel, B., 1993. Productivity regime and phytoplankton size structure in the Arabian Sea. Deep-Sea Res., Part II 40, 711–735. Karl, D.M., Christian, J.R., Dore, J.E., Letelier, R.M., 1996. Microbiological oceanography in the region west of the Antarctic Peninsula: microbial dynamics, nitrogen cycle and carbon flux. Foundations for Ecological Research West of the Antarctic Peninsula. Antarctic Research Series 70, 303-332. Kepkay, P.E., Wells, M.L., 1992. Dissolved organic carbon in North Atlantic surface waters. Mar. Ecol. Prog. Ser. 80, 275–283.
263
Kirchman, D.L, Lancelot, C., Fasham, M., Legendre, L., Radach, G., Scott, M., 1993. Dissolved organic material in biogeochemical models of the ocean. In: Evans, G.T., Fasham, M.J.R. ŽEds.., Towards a Model of Ocean Biogeochemical Processes. Serie I: Global Environmental Change, Vol. 10, Berlin, pp. 209–225. Kumar, M.D., Rajendran, A., Somasundar, K., Haake, B., Jenish, A., Shuo, Z., Ittekkot, Desai, B.N., 1990. Dynamics of dissolved organic carbon in the northwestern Indian. Oceanogr. Mar. Chem. 31, 299–316. Lukas, R., Lindstrom, E., 1991. The mixed layer of the western equatorial Pacific Ocean. J. Geophys. Res. 96, 3343–3357, Suppl. Martin, J.H., Fitzwater, S.E., 1992. Dissolved organic carbon in the Atlantic Southern and Pacific Oceans. Nature 356, 699– 700. McTaggart, K.E., Johnson, G.C., 1997. CTDrO2 measurements collected on a Climate and Global Change cruise ŽWOCE sections P14S and P15S. during January–March, 1996. NOAA Data Report ERL PMEL-63 ŽNTIS PB98-110158., 485 pp. Menzel, D.W., 1964. The distribution of dissolved organic carbon in the Western Indian Ocean. Deep-Sea Res. 11, 757–765. Menzel, D.W., 1970. The role in situ decomposition of organic matter on the concentration of non-conservative properties in the sea. Deep-Sea Res. 17, 751–764. Menzel, D.W., Ryther, J.R., 1968. Organic carbon and the oxygen minimum in the South Atlantic Ocean. Deep-Sea Res. 15, 327–337. Moriarty, D.J.W., O’Donohue, M.J., 1995. Organic carbon transport from the Southern Ocean and bacterial growth in the Antarctic Intermediate Water masses of the Tasman Sea. Mar. Ecol. Prog. Ser. 119, 291–297. Murray, J.W., Barber, R.T., Roman, M.R., Bacon, M.P., Feely, R.A., 1994. Physical and biological controls on carbon cycling in the equatorial Pacific. Science 266, 58–65. Ogura, N., 1970. The relation between dissolved organic carbon and apparent oxygen utilization in the Western North Pacific. Deep-Sea Res 17, 221–231. Peltola, E., Wanninkhof, R., Molinari, R., Huss, B., Feely, R., Bullister, J., Zhang, J.-Z., Chavez, F., Dickson, A., Field, A., Hansell, D., Millero, F., Quay, P., Castle, R., Thomas, G., Roddy, R., Lantry, T., Roberts, M., Chen, H., Greeley, D., Lee, K., Roche, M., Goen, J.A., Millero III, F., Buck, K., Kelly, M., Menzia, F., Huston, A., Waterhouse, T., Becker, S., Mordy, C., 1998. Chemical and hydrographic measurements during the Indian Ocean I8 report repeat cruise ŽIE8N. in September and October, 1995. NOAA Data Report ERL AMOL-34, NOAA, AOML, Miami, FL 163 pp. Peltzer, E.T., Hayward, N.A., 1996. Spatial distribution and temporal variability of total organic carbon along 140 W in the equatorial Pacific Ocean in 1992. Deep-Sea Res., Part II 43, 1155–1180. Radenac, M.-H., Rodier, M., 1996. Nitrate and chlorophyll distributions in relation to thermohaline and current structures in the western tropical Pacific during 1985–1989. Deep-Sea Res., Part II 43, 725–752. Reid, J.L., 1986. On the total geostrophic circulation of the South
264
M.D. DoÕal, D.A. Hansellr Marine Chemistry 68 (2000) 249–264
Pacific Ocean: flow patterns, tracers and transports. Prog. Oceanogr. 16, 1–61. Romankevich, E.A., Ljutsarev, S.V., 1990. Dissolved organic carbon in the ocean. Mar Chem. 30, 161–178. Sambrotto, R.N., Savidge, G., Robinson, C., Boyd, P., Takahashi, T., Karl, D.M., Langdon, C., Chipman, D., Marra, J., Codisponti, L., 1993. Elevated consumption of carbon relative to nitrogen in the surface ocean. Nature 363, 248–250. Skopintsev, B.A., Timofeyeva, S.N., 1962. Organic carbon content n the Baltic and North Sea, and in tropical and subtropical regions of the North Atlantic Žin Russian.. Trudy morsk. Gudrofiz. Onst. 25, 110–117. Skopintsev, B.A., Timofeyeva, S.N., Vershinina, O.A., 1966. Organic carbon in the equatorial and southern Atlantic and Mediterranean. Okeanologiya 6, 201–210. Sokal, R.R., Rohlf, F.J., 1995. Biometry: The Principles and Practice of Statistics in Biological Research, Freeman, New York, 887 pp. Sonnerup, R.E., Quay, P.D., Bullister, J.L., 1999. Thermocline ventilation and oxygen utilization rates in the subtropical North Pacific based on CFC distributions during WOCE. Deep-Sea Res., Part I 46, 777–805. Sorokin, Y.I., 1971. On the role of bacteria in the productivity of tropical oceanic waters. Int. Rev. Gesamten Hydrobiol. 56, 1–48.
Sorokin, Y.I., 1977. Productivity and the trophic role of bacterioplankton in the region of equatorial divergence. Pol. Arch. Hydrobiol. 24, 241–259. Sorokin, Y.I., 1978. Decomposition of organic matter and nutrient regeneration. In: Kinne, O. ŽEd... Marine Ecology. Wiley, Chichester, pp. 501–616. Tanoue, E., 1993. Distributional characteristics of DOC in the central equatorial Pacific. J. Oceanogr. 49, 625–639. Thomas, C., Cauwet, G., Minster, J.-F., 1995. Dissolved organic carbon in the equatorial Atlantic Ocean. Mar. Chem. 49, 155–169. Weibinga, C.J., De Baar, H.J.W., 1998. Determination of the dissolved organic carbon in the Indian sector of the Southern Ocean. Mar. Chem. 61, 185–201. Wedborg, M., Hoppema, M., Skoog, A., 1998. On the relation between organic and inorganic carbon in the Weddell Sea. J. Mar. Syst. 17, 59–76. Williams, P.J.LeB., 1975. Biological and chemical aspects of dissolved organic material in sea water. In: Riley, J.P., Skirrow, G. ŽEds.., Chemical Oceanography, 2nd edn., Vol. 2. Academic Press, London, pp. 301–363. Williams, P.M., Druffel, E.R.M., 1987. Radiocarbon in dissolved organic matter in the central North Pacific Ocean. Nature 330, 246–248.