Carbon export during the Spring Bloom at the Antarctic Polar Front, determined with the natural tracer 234Th

Deep-Sea

Pergamon

Research

II3 Vol. 44. No. l-2, pp. 457-478, 1997 1997 Elsevier Science Ltd in Great Britain. All rights reserved 0967-0645/97 %17.00+0.00

0

Printed

PII:SO9674645(96)OOO67-7

Carbon export during the Spring Bloom at the Antarctic Polar Front, determined with the natural tracer 234Th MICHIEL

M. RUTGERS

VAN

DER LOEFF,* JANA V. BATHMANN*

(Received 3 1July 1995; in revisedform

FRIEDRICH*

and ULRICH

29 October 1995; accepted 19 June 1996)

Abstract-Profiles of particulate and dissolved 23‘?h were obtained during the JGOFS Southern Ocean expedition on R.V. Polarstern during October/November 1992.Measurements were made on three transects across the Antarctic Circumpolar Current, from the Polar Front (PF) in the north to the Weddell Sea/ACC boundary in the south. The dissolved 23?h/238U ratio in surface waters gradually decreased during the development of the plankton bloom at the Polar Front. In the period between the first two transects, the 23?h activity removed from the dissolved phase had shifted to particles that had been produced, and as a result, the total activity ratio remained unchanged. The decrease in dissolved 23?h corresponds with decreases in dissolved nutrients and pC02, and with increases in chlorophyll and plankton biomass. Only during the third transect was the total activity of 23?h clearly reduced. The 234Th activity missing in the upper 100 m amounted to approximately 6 x lo4 dpm me2 at the Polar Front. In the 22-day period between the second and third transect, the 234Th export flux averaged 3200 dpm me2 day-‘. The ratio of organic carbon to 234Th on suspended particles was lower near the Polar Front than to the south, which we attribute to the higher abundance of empty diatom frustules at the Polar Front. With an average C,,,/23?h ratio on suspended particles in surface water of about 20 pmol dpm-‘, and assuming that the C,,$234Th ratio on exported particles is 30-60% of this value, we estimate that 0.434.86 mol C m-’ had been removed over the 22-day period from the surface ocean by sinking particles. Export production was negligible in the Antarctic Zone including the ice edge, but during the later stage of the bloom in the Polar Front region, it amounted to 12-24% of primary production or 2550% of the net CO2 uptake as estimated from a COs budget. 0 1997 Elsevier Science Ltd. All rights reserved

INTRODUCTION Upwelling of Circumpolar Deep Water (CDW) forms an abundant nutrient supply to the surface waters of the Southern Ocean. The high nutrient concentrations throughout the year show that this supply exceeds the amounts required for the production of organic material (Priddle et al., 1992). The absence of nutrient limitation as a constraint for export production and corresponding drawdown of atmospheric CO2 makes the Southern Ocean a particularly important area for the study of vertical fluxes. Particle fluxes in the Southern Ocean vary greatly according to season and geographical location (Wefer et al., 1988; Wefer and Fischer, 1991). The geographical variation also is reflected in the sedimentary record. Low inventories of excess 210Pb in sediments from the central Weddell Sea (Rutgers van der Loeff and Berger, 1991) are consistent with low flux rates recorded there in sediment traps (Fischer et al., 1988). * Alfred-Wegener

Institute

for Polar and Marine

Research, 457

P.O. Box 120161, D27515 Bremerhaven,

Germany.

458

M. M. Rutgers

van der Loeff et al.

Higher productivity and fluxes occur at the margins and fronts of the Southern Ocean, like the marginal Ice Edge Zone in the Scotia Sea (EPOS), the Coastal Current (Bathmann et al., in prep.), and especially the area around the Polar Front, as shown by sediment trap (Wefer and Fischer, 1991), chlorophyll (Bathmann et al., 1997) and&O2 data (Metzl et al., 1991). It is not yet clear as to what extent the massive opaline sediments deposited in the area south of the Polar Front (Cooke and Hays, 1982) result from high local productivity and vertical fluxes, or are enhanced by sediment focussing (Francois et al., 1993). DeMaster (1979, 198 1) showed high inventories of 231Pa and 230Th. Frank et al. (1996) also observed interglacial 230TheX fluxes far in excess of the production rate in the water column, and, as excess scavenging could not be demonstrated in the water column (Rutgers van der Loeff and Berger, 1993) they interpreted these enhanced 230TheX fluxes as evidence for sediment focussing. Thus, the high accumulation rates are due, at least in part, to advection from other areas and do not necessarily prove high local vertical particle fluxes. The SO-JGOFS expedition investigated processes controlling the production of particles in the surface water of the Antarctic Circumpolar Current (ACC) and their export out of the euphotic zone. The development of an intense bloom in the area around the Polar Front was well documented by the distribution of chlorophyll a (Bathmann et al., 1997) and primary productivity (Jochem et al., 1995). As long as the export of particles can be neglected, the production of particles can be quantified by following either the depletion of dissolved CC02 and nutrients, or by the increase of particulate organic material. The uptake of CO;! was measured as a decrease in pCO2 (Bakker et al., 1997). The production of particulate organic material is monitored as an increase in particulate organic carbon (this paper) and biogenic silica (Qdguiner et al., 1997). The export of particles from the euphotic zone in principle could be derived from trap deployments, which were not done during our expedition, or from budgets of dissolved and particulate constituents. It turned out, however, that budget calculations for CC02 and nutrients were complicated severely by the variability of the source concentrations in an area as variable as the ACC, especially in proximity to the fronts (Veth et al., 1997). Indeed, in those cases where the same water mass could not be followed, budget calculations for a growth period can only be made provided that the water masses studied experienced a similar bloom history and that their initial composition is known. The advantage of 234Th as a tool for the quantification of export flux (Coale and Bruland, 1985; Eppley, 1989) is its own short time scale (24.1 days half-life), together with the long oceanic residence time and consequently conservative distribution of its parent 238U. This means that the production rate of 234Th is known irrespective of the history of the water mass. As the distribution of 234Th documents the scavenging history on a time scale of only weeks, this tracer is much better suited for a budget calculation than nutrients or X02, the concentrations of which have a history on at least a seasonal time scale. We have followed the development of particulate and dissolved 234Th on three transects across the ACC during the onset of the plankton bloom in the austral spring 1992. We calculate the export flux of 234Th from the surface mixed layer between these transects, and use particulate carbon data to make an estimate of the export flux of carbon. The importance of this flux in the carbon budget at the Antarctic Polar Front is discussed.

MATERIAL The cruise track of the SO JGOFS

AND

expedition

METHODS with R.V. Polarstern in October-December

Carbon

export during

459

the Spring Bloom

1992 (Fig. 1) and the details of the transects made along the 6”W meridian are given in Smetacek et al. (1997). We describe data of three transects, no. 2 (Stations 868-879, II-18 October), no. 5 (Stations 886-907, 24-31 October) and no. 11 (Stations 941-969, 10-21 November and station 972 from transect 12, 22 November). About every 2” of latitude, samples were taken at 20,60, 100,200,400 and 600 m depth with 270-l Gerard bottles. Fifty to 250 1were filtered through 142~mm-diameter l-pm pore size Nuclepore filters. Thorium234 was co-precipitated from a weighed 20-l aliquot of filtrate with Fe(OH)s in the presence of 230Th yield tracer. Thorium was purified by ion exchange and plated according to the procedure of Anderson and Fleer (1982). Thorium-234 was quantified by onboard beta counting and subsequent alpha counting of 230Th in the home laboratory. Filters were digested in the presence of 230Th yield tracer and analysed following the procedure of Anderson and Fleer (I 982). Uranium-238 activity (Au, dpm 1-l) was calculated from salinity using the relationship AU = 0.0704 x salinity, based on the average uranium (238U+ 235U) concentration in seawater normalized to salinity 35 of 3.238 ng g-’ (Chen et al., 1986). Total 23?h activity (the sum of dissolved and particulate activities) in samples from below 200 m is 100 16% (SD) of 238U (Table 1). Deviations from secular equilibrium at these depths may be due both to scavenging and to mineralization (Coale and Bruland, 1987). Counting errors, uncertainties in the relative efficiencies of the alpha and beta counters and in the extent and time of separation of 23vh from its parent 238U during settling of the Fe(OH)s precipitate rather than during the subsequent ion-exchange separation, weighing and other

60”

50”

40”

30”

20”

10”

0”

10”

20”

oE ,30”

40”

-Polar

Front

50”

Bowel Isl.

60”

60”

LLezarev

Weddell sea

w

60”

70” Fig.

1.

50”

40”

30”

J

20”

Study area with cruise track and average position Stramma (1991).

10”

0”

10”

of the major fronts

Bea

/

20”

after Peterson

30” and

E

460 Table 1.

M. M. Rutgers van der Loeff et al. Dissolved and parriculate 234Th (dpm I- ‘) and total 234Th/238U ratios with propagated l-a errors, POC and PON (PM) where sampled at the same depth, and SPM (peg I-‘) from transmissometer data

Station Tr* Latitude (“S)

Date (1992)

857T

57.0

2 October

868

57.0

12 October

872

55.0

14 October

876

53.0

15 October

877

49.0

17 October

879

48.0

18 October

886

56.0

22 October

891

55.0

25 October

895

53.0

26 October

Depth 234Th dissolved (dpm 1-l) (m)

20 50 100 200 300 500 20 60 100 200 400 600 20 60 100 200 400 600 20 60 100 200 400 600 20 60 100 200 400 600 20 60 200 20 45 100 200 400 600 20 60 100 200 20 60 100 200 400

1.914&0.0671 1.734&0.061 2.086 + 0.073 2.250 kO.079 2.281 _+O.OSO 2.209_+0.077 2.354kO.082 2.183+0.076 2.157f0.076 2.557+0.090 2.428 f 0.085 2.505If:O.O88 2.346kO.082 2.116_+0.074 2.135kO.075 2.220+0.078 2.678+0.094 2.354kO.082 2.048 kO.072 2.114+0.074 2.419+0.085 2.452kO.086 2.582kO.090 2.367 k 0.083 1.974+0.069 1.890f0.066 2.201 kO.077 2.491 kO.087 2.560+0.090 2.555kO.089 1.818kO.064 1.929+0.068 2.127kO.074 2.161 kO.076 2.160+0.076 2.114f0.074 2.463kO.086 2.309_+0.081 2.304_fO.O81 2.259kO.079 2.344f0.082 2.328,0.081 2.338 +0.082 2.191 kO.077 2.200,0.077 1.977_+0.069 2.429+0.085 2.472 + 0.087

234Th particulate (dpm 1-l) 0.325*0.010$ 0.528+0.017 0.183kO.007 0.050+ 0.003 0.079+0.004 0.035_+0.003 0.165+0.017 0.303~0.018 0.052+0.003 0.110+0.012 0.059 + 0.003 0.200f0.005 0.097kO.004 0.370_+0.009 0.114f0.004 0.256_+0.006 0.062+0.003 0.299+0.006 0.108 + 0.004 0.169+0.072 0.080+0.003 0.127_+0.004 0.080f0.003 0.059 + 0.003 0.311+0.009 0.257+0.007 0.512+0.009 0.246+0.006 0.047 * 0.003 0.121_+0.003 0.336&0.008 0.397 * 0.009 0.106+0.003 0.085+0.004 0.23Oi_O.O10 0.084 k 0.005 0.074+0.003 0.034_f0.003 0.039 _+0.003 0.127kO.004 0.090+0.006 0.099 + 0.004 0.049+ 0.002 0.095 I 0.006 0.205+0.007 0.103_+0.005 0.102+0.004 0.071 k 0.003

234Th/238U total

POC PON W)

0.94On k 0.0281I 0.949 + 0.026 0.952kO.031 0.953 * 0.033 0.973*0.033 0.9212 0.032 1.049+0.035 1.035+0.033 0.917+0.031 1.099 * 0.037 1.019+0.035 1.108kO.036 1.024 + 0.034 1.042_+0.031 0.942 + 0.03 1 1.026_+0.032 1.125kO.039 1.086 * 0.034 0.902kO.030 0.955 kO.043 1.046+0.035 1.069_+0.036 1.093 * 0.037 0.993 * 0.034 0.957 kO.029 0.900 f 0.028 1.137*0.033 1.141 kO.036 1.076?rO.O37 1.100+0.037 0.903 kO.027 9.3 0.976f0.029 7.6 0.932 +0.031 2.1 0.942 + 0.032 1.002+0.032 0.919+0.031 1.048 + 0.036 0.960+0.033 0.959 _+0.033 1.001 kO.033 1.021 kO.035 1.018+0.034 0.993 * 0.034 0.957 kO.032 1.007_+0.032 0.871 kO.029 1.052+0.035 1.044+ 0.036

SPM

(PM) @g 1- ‘)

173.0** 166.9 50.7 6.6

66.9 60.3 35.3 26.0 22.6 18.6 118.9 110.5 98.6 59.2 48.0 43.8 60.3 52.4 32.4 8.6 4.0 1.9 208.7 197.2 140.9 21.0 10.7 6.6 1.4 205.4 1.3 183.0 0.3 22.6 77.4 73.6 36.2 17.8 12.7 10.4 60.9 55.8 47.0 11.1 48.6 51.3 47.5 13.4 6.8

461

Carbon export during the Spring Bloom Table Station Tr* Latitude (“S)

Date (1992)

899

5

51 .O

27 October

903

5

49.0

29 October

907

5

47.0

30 October

931

11

59.0

10 November

941

11

57.0

12 November

945

11

55.0

14 November

949

11

53.0

16 November

953

11

5 1.O

17 November

1. (Continued)

Depth 23?h dissolved (dpm 1-l) (m)

600 20 60 100 200 400 600 20 60 100 400 600 20 43 100 200 400 600 20 60 100 120 200 400 20 60 100 200 400 600 20 60 100 200 400 600 20 60 100 200 600

1000 20 60 100 200 400 600

2.51710.088 2.019+0.071 2.120+0.074 1.961 kO.069 2.337kO.082 2.403 + 0.084 2.310+0.081 1.612+0.056 1.6lOkO.056 1X27+0.064 2.321 &I.081 2.450+0.086 1.682f0.059 1.822+0.064 1.779+0.062 2.221+ 0.078 2.296_+0.080 2.410+0.084 2.206 + 0.077 2.331 kO.082 2.185kO.076 2.141 kO.075 2.400+0.084 2.256f0.079 2.158_fO.O76 2.236+0.078 2.177+0.076 2.347f0.082 2.414+0.084 2.193,0.077 1.996+0.070 2.128 + 0.074 2.156+0.075 2.217rtO.078 2.310_+0.081 2.161 kO.076 2.056+0.072 2.134_+0.075 1.828 f 0.064 2.261 kO.079 2.552 f 0.089 2.453 k 0.086 1.879+0.066 1.822kO.064 1.907 _+0.067 2.184+0.076 2.271 kO.079 2.088f0.073

234Th particulate (dpm I-‘)

23?h/23sU total

0.054 &-0.003 0.136f0.005 0.230+0.007 0.147+0.006 0.121+0.004 0.064 f 0.003 0.076f0.003 0.537,0.016 0.568+0.014 0.413~0.010 0.089 + 0.003 0.084_+0.003 0.565kO.016 0.694+0.018 0.362f0.013 0.156+0.005 0.097f0.003 0.06 I+ 0.002 0.082_+ 0.003 0.284+0.008 0.083 k 0.004 0.077 + 0.003 0.044 + 0.002 0.023 + 0.002 0.077 4 0.004 0.166+0.005 0.092 + 0.004 0.064 + 0.003 0.035 kO.002 0.026 + 0.002 0.095 _+0.005 0.153+0.006 0.118+0.005 0.086 _+0.004 0.053 * 0.002 0.028 f 0.002 0.077 f 0.003 0.138_+0.005 0.089 + 0.004 0.083,0.003 0.025 f 0.002 0.024 _+0.003 0.134+0.007 0.213kO.008 0.143+0.005 0.096+0.003 0.064+0.002 0.066+0.004

1.053 k 0.036 0.901+0.030 0.983f0.031 o&X2+ 0.029 1.025 + 0.034 1.014+0.035 0.978 kO.033 0.901 kO.025 0.913f0.024 0.939+0.027 0.995 f 0.034 1.041 kO.035 0.942 k 0.026 1.055+0.028 0.897+0.027 0.993 kO.033 0.989 _+0.033 1.016_+0.035 0.947,0.032 1.083 * 0.034 0.939 + 0.032 0.918kO.031 1.001~0.034 0.933 + 0.032 0.935+0.032 1.005 + 0.033 0.946 _+0.032 0.990 + 0.034 1.003 * 0.035 0.908 kO.031 0.876+0.029 0.956kO.031 0.953 kO.032 0.959+0.032 0.97o_fo.o33 0.896+0.031 0.893 _+0.030 0.951+0.031 0.803 + 0.027 0.975+0.033 1.056+0.037 1.013_+0.035 0.842_fO.O28 0.852 + 0.027 0.858 + 0.028 0.950+0.032 0.96O_fO.O33 0.883+0.030

POC PON W)

W)

4.8 6.7 6.6 4.7

0.8 1.1 1.1 0.5

14.2 14.8 7.3

2.2 2.4 1.1

12.4 11.5 8.1 3.8

2.0 1.8 1.3 0.6

6.6 6.4 3.5 2.2

0.9 1.0 0.5 0.2

4.4 3.9 3.6 3.2

0.7 0.4 1.6 0.5

9.2 8.8 5.3 3.4

1.4 1.4 0.8 0.5

SPM (pg 1-l)

4.5 130.6 138.6 117.5 38.7 21.4 10.1 299.6 298.6 186.8 27.8 24.3 259.7 249.1 216.4 41.2 18.2 9.2 135.3 135.8 128.7 114.7 54.1 49.7 145.2 146.6 110.9 56.9 66.3 49.1 116.5 114.2 110.0 40.2 31.5 31.0 105.8 105.8 68.1 42.2 33.3 31.0 141.9 144.2 120.3 58.0 42.8 37.2

462

M. M. Rutgers

Table 1. Station

van der Loeff et al.

(Continued)

Date

Depth

(“S)

(1992)

(m)

23?h dissolved (dpm I-‘)

234Th particulate (dpm I- ‘)

234Th/238U total

20 60 100 200 400 600 20 60 100 200 400 600 20 60 200 400 600 1000

1.382 + 0.048 1.364kO.048 1.724f0.060 2.421+0.085 2.217,0.078 2.416+0.085 1.053,0.037 1.322+0.046 1.567 + 0.055 2.238kO.078 2.321_+0.081 2.231 kO.078 1.094 + 0.038 1.231 kO.043 2.126+0.074 2.183kO.076 2.136+0.075 2.298 1-0.080

0.465f0.018 0.639 + 0.021 0.521 iO.012 0.155+0.004 0.067 _+0.003 0.060 + 0.002 0.421 to.012 0.628 k 0.018 0.519+0.011 0.167iO.005 0.148_+0.003 0.077 + 0.003 0.403+0.015 0.589kO.028 0.197+0.006 0.151+0.004 0.109+0.003 0.042 + 0.002

0.775 f0.022 0.840 k 0.022 0.940) 0.026 1.075 + 0.035 0.942,0.032 1.017*0.035 0.619+0.016 0.819+0.021 0.874f0.023 1.002_+0.033 1.023 _+0.034 0.951 kO.032 0.629+0.017 0.764f0.021 0.968+0.031 0.966_fO.O32 0.924*0.031 0.958 k 0.033

Tr* Latitude

960

11

49.0

19 November

969

11

47.0

21 November

972

12

48.7

22November

*Transect number. tLongitude 49”35’W. $Estimated error of 3.5%. §Counting error. $238U derived from salinity according /IPropagated error. **SPM derived from transmissometry

POC PON

SPM

GM) (PM) (pg 1- ‘)

19.5 14.0 7.2 2.4

3.1 5.0 1.3 0.5

17.7 12.4

2.6 1.5

418.1 407.5 203.5 103.0 69.3 47.0 558.3 550.5 273.3 127.8 109.1 104.4 543.2 377.0 107.7 85.8 76.1 62.1

to Chen et al. (1986). data and the algorithm

of Gardner

et al. (1993).

analytical errors are estimated to contribute to a propagated error of 3.5% in dissolved 234Th (Table 1). Suspended particulate matter @PM) concentrations were derived from transmissometer data. From the continuous profiles obtained with the hydrographic casts, we interpolated the transmission values at the depth of our Th samplings and calculated the suspended load using the algorithm developed by Gardner et al. (1993) for the North Atlantic Bloom Experiment. The maximum transmission (90.0%) observed at about 1000 m depth was assumed to represent the transmission of particle-free water. For POC and PON analyses, 1-2 1 of water from various depths at each station were collected by means of a Rosette sampler and filtered on to precombusted (550°C 12 h) 25mm GF/F filters. The filters were stored frozen, fumed with HCl to remove inorganic carbonates, and analysed in a Perkin Elmer CHN analyser (Strickland and Parsons, 1972). POC and PON values were calculated after blank correction and calibration with acetanilide. RESULTS All data of particulate and dissolved development of an intense phytoplankton

234Th have been listed in Table 1. The gradual bloom near the Polar Front with maximum Chl a

463

Carbon export during the Spring Bloom

concentrations of 3 mg me3 (Bathmann et al., 1997) is reflected by an increase of particulate 234Th and a corresponding decrease of dissolved 23?h (Fig. 2). In the beginning of October (transect 2), particulate 23?fh/238U, i.e. the ratio of particulate 234Th to total 238U, ranged from 0.05 to 0.10 in all surface waters of the southern ACC (south of 50.5”s and of the influence of the Polar Front). In the Polar Front region (PFr, North of 50.5”s and influenced by the Polar Front), the ratio increased to 0.15. This increase corresponded with an increase in Chl a from 0.8 to 1.2 mg m-‘. The dissolved 23?h/238U ratio ranged from 0.90 to 1.0 in the surface water of the southern ACC to 0.80 in the PFr. Total 234Th determined as the sum of dissolved and particulate 234Th, was only slightly depleted (< IboA) with respect to its parent 238U (Fig. 3). In subsequent transects, the particulate 234Th/238Uratio in the southern ACC remained unchanged, but in the PFr, it rose to 0.25 by the end of October (transect 5). Dissolved 234Th was depleted down to 70% of 238U but total 23‘?h was still near equilibrium with its parent. Increased adsorption of 23?h in the PFr was observed to a depth between 100 and 200 m. By the end of November (transect 1l), dissolved 234Th was depleted down to 44% of 238U in the surface water of the PFr. Removal by adsorption could be observed down to 200 m depth at the PFr, and extended further south than in the previous transects (Fig. 2). Only at this transect was total 234Th clearly depleted by up to 37% with respect to 238U(Fig. 3) in the surface water of the PFr. Removal greater than 10% was limited to the upper 60 m and to the area north of 52”s. The depth of the euphotic zone ranged from 126 m (200 m) in the Antarctic Zone (56-57”s) to a minimum of 57 m (80 m) in the PFr for the I % and 0.1% surface light level, respectively (Queguiner et al., 1997). The mixed-layer depth ranged from about 100 m in the Antarctic Zone to 80 m in the Polar Front region (Veth et al., 1997; 47 0

4.9

49

51

52

59

54

55

57

58

59 'S station

100

200

Fig. 2.

Section of dissolved 23?h/238U ratio for the three transects. Sampling depths are indicated by marks on the vertical lines indicating stations.

464

M. M. Rutgers

47

46

49

51

52

van der Loeff et al.

53

54

55

56

57

58

59

"S

Transect5 2z31cct

47 969

0

48

972 "&O

50

E

52

929

54

55 945

56

58

59 "S 93, Statlo"

100

200

Fig. 3.

Section of total (dissolved + particulate) 234Th/238U ratio for the three transects. depths are indicated by marks on the vertical lines indicating stations.

Sampling

Bakker et al., 1997). Integration of the 234Th deficiency to a depth, L, of 100 m (Table 2) thus includes adsorption on plankton in the euphotic zone and subsequent removal from the surface mixed layer. The seasonal development at the Polar Front is illustrated by the profiles of dissolved and total 234Th/238U ratios at the stations 877,907 and 969 (Fig. 4).

DISCUSSION The three transects 2,5 and 11 across the Antarctic Circumpolar Current showed that the spring bloom developed in the area around the Polar Front. Phytoplankton biomass increased only slightly in the area of the Marginal Ice Zone (MIZ), which coincided with the approximate position of the southern ACC Front (Orsi et aZ., 1995; Veth et al., 1997). The biogenic production of particles is reflected by the increase of Chl a values, as net output of intense primary production (Jochem et al., 1995) and an increase of particle density as measured by transmissometry. Particulate biogenic silica (Qutguiner et al., 1997), particulate carbon and particulate nitrogen also increased during the course of our study. This increased particle load in surface waters caused an enhanced adsorption of 234Th.

Scavenging

models

We use the 234Th data to calculate

adsorption

rates and removal

rates of 234Th from the

465

Carbon export during the Spring Bloom Table 2.

Station number, integrated 234Th depletion D (dpm cm-*) and steady state removal rate RTh (SS) (dpm m-’ day-‘) in the upper 100 m, using linear interpolation between sampling depths

47 48 49 50 51 52 53 54 55 56 57 58 59

Sta

D

879 877

1.04 0.71

876

&I&W

1.14

Transect 11

Transect 5

Transect 2 Latitude (“S)

296* 204

327

872 -0.36

-102

868 -0.40

-114

Sta

D

907

Transect 12

JMSS)

Sta

D

0.60

172

969

5.95

1697

903

2.06

588

960

3.95

1128

899

1.67

476

953

3.60

1027

895

0.96

275

949

2.43

693

891 -0.29 886 0.97

-83 275

945

1.83

521

941

0.83

238

931

0.003

Sta

&I&W

D

&(SS)

972 4.66

1329*

1

*CM0 m.

234Th/ 238U ratio 0

.4

.6

.6

1.0

1.2

.4

.6

.0

1.0

1.2

.4

.6

.%

1 .o

1.2

100

200

E

300

f: % u 400

500

Sta

Sta 907

600

700

Fig. 4. The development of dissolved (open circles) and total (closed circles) 23‘?h/23sU near the Polar Front: station 877, 17 October; station 907, 30 October and station 969, 21 November. The hatched area corresponds with the activity on particles.

M. M. Rutgers van der Loeff et al.

466

surface water. The distribution of Th isotopes can be described with a reversible exchange model (Bacon and Anderson, 1982)

aAth --=A~xh-A~hxh-k,xA~h+k*xA;h’

(1)

at

aA;h_ -AFh X h + -at

k, X

z’ith-

kz

X

ki&,

-

PTh,

where advection and diffusion have been neglected, as will be justified below, and where Ad and AP are the activities (dpm 1-l) of dissolved and particulate Th, respectively, AU is the activity (dpm 1-l) of 238U, 2 is the decay constant (day-‘) of the Th isotope, kl and k2 are first order adsorption and desorption rate constants (day-‘) and & is the removal flux of Th on sinking particles. As the desorption rate of Th is much slower than the radioactive decay of 234Th (e.g. Bacon and Anderson, 1982), the desorption is usually neglected, giving an irreversible scavenging model (Coale and Bruland, 1985):

%=A” Xh-A$h at

Xi-&,

aAP,hA;hxi+JT,,-PTh, at

---

where &, is the net adsorption rate. If the adsorption is described as first order with respect to AdTh, then &, = kl X A&.

(3

In the original applications using 234Th to study particle cycling in the upper ocean (Coale and Bruland, 1985; Bruland and Coale, 1986), these equations were solved under the assumption of steady state. The steady-state assumption could be abandoned as time series data became available (e.g. Wei and Murray, 1992). Buesseler et al. (1992) assumed that the fluxes J and P were constant during a period t between times tl and t2 with concentrations ATh_, and ATh_*, respectively, and derived the following equations: JTh = 1

PT,, = h

Au( 1 - e-*‘) + A$h_,e-ht

- At,_,

(1 - .-A*) A”( 1 - e-*‘) + Api_,e-*’

(1 - eVit)

- A;;_*

).

(7)

With these equations, the adsorption rate can be calculated from the disequilibrium between dissolved 23?h and the parent 238U. The export rate follows from the disequilibrium between total 234Th and 238U.

Adsorption

rate of 234Th

The adsorption rate, JTh, (Equation (3)) is the net rate of transfer of 23?h from the fraction smaller than the filter pore size, in our study 1 pm, to particles retained by the filter. It includes aggregation of colloidal size material (Honeyman and Santschi, 1989), but also

Carbon

export during

467

the Spring Bloom

disaggregation and decomposition of particles. Rate constants for these individual steps in the particle dynamics can be obtained when a particle budget is made by independent measurements of particle concentrations and particle fluxes in sediment traps, as was done in the North Atlantic Bloom Experiment (NABE) (Cochran et al., 1993; Clegg and Whitfield, 1993). As we did not make sediment trap deployments during this study, we cannot make an independent particle budget, and we therefore will apply the irreversible non-steady state scavenging model used by Buesseler et al. (1992). To denote that the adsorption rate, JTh, may not simply represent an adsorption process, we call the corresponding rate constant ki (Equation (5)) a pseudo-first-order adsorption rate coefficient, following Bruland and Coale (1986). Using the equations given by Buesseler et al. (1992), we calculated the average adsorption rate in the upper 100 m of the water column in the l&12-day period between transect 2 and 5, and in the 20-22-day period between transects 5 and 11 (Table 3). Division by the average dissolved activity AdTh gives ki (Table 3, Fig. 5). In both periods, ki increased northward from the MIZ to the PFr (Fig. 5). In theory, the adsorption rate constant is dependent on the density of adsorption sites, which is roughly proportional to the concentration of suspended particulate matter (SPM). Although the expected relationship is no longer strictly linear when the rate is determined by aggregation rather than adsorption (Honeyman et al., 1988; Honeyman and Santschi, 1989), a roughly linear relationship between adsorption rate and SPM often has been reported (e.g. Bacon and Anderson, 1982). The adsorption rate constants we found in the period between the transects also correlate well with average SPM concentrations between the transects in the upper 100 m. The adsorption rate constant, normalized to total suspended material, was 1.Ox 10m4 m3 mg- ’day- ’ (Fig. 6). This value was lower than observed at NABE (2.68 f 0.7 or 2.20 + 0.7 m3 mg- ’day-’ depending on modelling, Clegg and Whitfield, 1993) but within the range of 0.5-2.38 m3 mgg’ day-‘, found in other studies (Clegg and Whitfield, 1993). The adsorption rate constant ki also has been shown to correlate with primary

Table 3. Average arlsorption rate JTh (dpm m -’ day-‘), adsorption rate constant kl(day-‘), and export rate PTh (dpm mm3 day-‘) of 234Th, and integrated carbon removal (mol mP2) in the upper 100 m as a function of latitude for the period between transects, calculated according to Buesseler et al. (1992) 2% removalrate ?% Latitude (“S)

Stations

Depth

(transect)

(In)

2

5

II

47-48 47-48

879 a79

907 907

969 969

49

877

903 899

51

JTh (dpm mm3 day-‘)

Z-II

2-5

>I1

0.0159

0.0379 0.0298

- 5.6

43.9 32.5

0.0242

0.0221

15.5

>II

2-5

60’ 100

28.5

54.3 47.4

960

100

44.1

34.2

953

100

53

876

895

949

I00

872

891

945

loo

5657

868

886

941

loo

0.01 IO

21.5 6.5 -5.1 15.8

*IO0 m-value of station 879 (transect Stations at 47 and 48”S, and stations

13.7 17.1 3.9

Integrated carbon

(dpm m-’ day-‘))

removal

(mol me*)

k, (day-‘)

2-5

55

(Pn

adsorption

0.0030 -0.W23 0.0072

2) is missing. at 56 and 573

17.9

2-5 -0.02--0.05 0.1 l-o.22

17.1

Z-11 0.35-0.69 0.43-0.86 0.23-0.45 0.21-0.43

0.0066

1.4

12.1

0.01-0.02

0.0078

- 0.2

13.0

o.o(M.OO

0.16a31

0.0018

16.5

1.6

0.1&0.20

0.02-0.04

have been put into two categories

0.15-0.30

named 47.5 and 56S”S,

468

M. M. Rutgers

van der Loeff et al

400’ ??

350’ 300

o

period 2-5

.

period 511

I

250

0

0

200 1.50

.

100

0’

50

35

0

30 25 20

.P

??

??

.6 .5 .4 .5

??

0 .

??

3 0

0

.l. 0

.

r) 0

-.l47

46

49

50

51

52

53

54

55

56

57

Latitude “S

Average adsorption rate constant /cl (day-‘), export rate P (dpm me3 day-‘; both calculated according to Buesseler et al., 1992) of 23?h, and integrated removal of organic carbon (mol m-‘) in the upper 100 m as a function of latitude for the period between transects 2 and 5 (open symbols) and between transects 5 and 11 (closed symbols), compared with the average suspended particle load in these periods.

Carbon

0

50

export during

100

150

469

the Spring Bloom

200

250

300

350

400

average SPM (ug/L) Fig. 6. Adsorption rate constant (day-‘) as a function of SPM (derived from transmissometry data and the agorithm of Gardner et al., 1993) for the period between transects 2 and 5 (open symbols) and between transects 5 and 11 (closed symbols) (r2 = 0.842, n = 11).

productivity (Coale and Bruland, 1985) and export flux (Bruland and Coale, 1986). The latter correlation does not hold for our non-steady state situation. In the first period of our study, there was an appreciable adsorption flux but no export flux. Removal of 234Th from the upper water layer

The depletion of total 234Th, with respect to 238U, is due to removal of 234Th from the surface layer on large, sinking particles. A significant depletion was not observed in the first two transects (October), but during the last transect at the end of November, a significant depletion had developed (Fig. 3). The cumulative 23?h depletion in the surface water at the Polar Front at stations 969 and 972 was 5.95 and 4.66 x lo4 dpm m-‘, respectively (Table 2). The removal rate of Th depends on the timing of the removal process between the two transects. We again assume a linear development between the two transects, and use Equation (7) to calculate the removal rate in the periods between the transects (Fig. 5, Table 3). As expected from the missing 234Th (Table 2, Fig. 3), the removal rate was much higher during the second period than during the first, with a maximum of 3200 dpm rnp2 day-’ in the PFr. Although particle production reached its maximum near the PF, the area of particle export reached beyond 51’S, well into the Southern ACC. The export fluxes are high in comparison with most other open-ocean stations [57&1860 dpm me2 day-’ at VERTEX stations (Coale and Bruland, 1987); see also data compilation by Buesseler (1991)], and are comparable to the highest fluxes derived for the Equatorial Pacific (lOOO3500 dpm me2 day-’ at 100 m; Buesseler et al., 1995). The depletion of total 234Th is also large in comparison with the value of 26% observed during an intense bloom situation in the Bransfield Strait (Rutgers van der Loeff and Berger, 1991) and comparable to the depletion

470

M. M. Rutgers van der Loeff et al.

observed at the eutrophic EUMELI site off west Africa (39.5 + 3.5%, Legeleux, personal communication, 1995). However, both the depletion of total 234Th and the export fluxes remained below the values encountered during the North Atlantic Bloom Experiment (75% depletion in surface water and 309&5060 dpm me2 day- ’234Th flux at 150 m, respectively; Buesseler et al., 1992). In the equatorial Pacific, upwelling plays a significant role in the 234Th budget (Buesseler et al., 1995). In the ACC, upwelling of Circumpolar Deep Water takes place mainly south of the Polar Front. Upper Circumpolar Deep Water (UCDW) is entrained into the surface mixed layer at the southern ACC Front (Orsi et al., 1995). At the corresponding latitude, around 58’S (Veth et al., 1997), this upwelling does not affect the 23?h budget, as we did not measure a depletion or a vertical gradient in total 23‘?h activities (Fig. 3, Table 1). In order to calculate the supply of 234Th from below 100 m at the Polar Front, we follow the approach of Loscher et al. (1997), who estimated a vertical eddy diffusion coefficient of 3 x 10e5 m2 s-l for the depth range 140-250 m. With this value and a gradient for total 234Th of about 0.5 dpm 1-l per 100 m (estimated from Figs 3 and 4) we obtain a diffusive flux of 13 dpm m-’ day-‘, which is a negligible contribution to the budget at the Polar Front. North-south sections of 210Po (138 days half-life) and its parent 210Pb, made concurrently with this study, suggested that meridional advection does play a role on the time scale of 210Po (Friedrich and Rutgers van der Loeff, 1994) but from the absence of similar features in the distribution of 234Th, we expect that this advection is too slow to influence the budget of 234Th (24.1 days half-life). We can compare the 234Th export flux with the estimate that we would obtain if we had assumed steady state. The 23?h removal flux RTh (dpm m-’ day-‘) is then easily derived from the cumulative depletion in the upper 100 m, D (dpm mp2) given in Table 2. By adding Equations (3) and (4), assuming steady state, and integrating over the upper 100 m, we find that RTh = AD. At the bloom stations 969 and 972, we find a steady state 234Th removal flux respectively (Table 2). The non-steady state flux was R-rhof1300and1700dpmm-2day-‘, about twice as high, indicating that the removal rate was twice as fast as could be made up by ingrowth.

Removal

of organic carbon and silicon: export production

Eppley’s (Eppley, 1989) proposal to use the residence time of 234Th and the measured POC inventory in the surface water to calculate the rates of new production was based on the assumption that exported particles have the C/234Th ratio of SPM. Murray et al. (1989) pointed out that the residence time of organic carbon is longer than that of Th, and that consequently the procedure of Eppley overestimated export production by up to a factor of three. This was confirmed by Buesseler et al. (1992) who observed that the C/234Th ratio in sediment trap material was 20-80% lower than in SPM. Moreover, the C/234Th ratio was systematically lower in their 300 m trap than in their 150 m trap, probably resulting from mineralization, which is more rapid than radiodecay under high-flux conditions (Cochran et al., 1993). Buesseler et al. (1992) also showed that trap fluxes underestimated the actual fluxes, and Cochran et al. (1993) argued that the composition of the material responsible for the vertical flux was in-between that of SPM and material caught in the traps. An alternative approach to determine the composition of exported material is to use size-fractionated filtration and separate analysis of the large particles thought to be responsible for the vertical flux. In the EqPAC study, the C/23qh ratio on particles > 53 pm was about one-

Carbon

export during

471

the Spring Bloom

half the ratio on suspended particles > 0.7 pm (Buesseler et al., 1995). As we did not make sediment trap deployments or separate analyses of large particles, we base our estimate of the C/234Th ratio in exported particles on measurements of suspended material. POC correlates well with SPM (Fig. 7), giving a carbon content in SPM of 40.2 f 2.7% (r2 = 0.874). This value appears rather high compared to 31 f 1.3% in the NABE experiment (Clegg and Whitfield, 1993). Moreover, as carbon constitutes normally about 40% of organic matter, our value is in conflict with the observation of Qutguiner et al. (1997) that biogenic opal sometimes exceeded organic matter in the bloom. One explanation may be that the contribution to SPM of the abundant empty frustules of Corethron (Crawford, 1995) is underestimated by transmissometry. The algorithm that we used to convert transmissometer data in SPM values, which was based by Gardner et al. (1993) on the NABE experiment, is apparently not directly applicable to the particle composition of the Southern Ocean. We have considered as an alternative to use the algorithm developed for the Equatorial Pacific (Gardner et al., 1995). However, the particle population during the spring bloom situation in the Polar Frontal region was characterized by large particlesmainly diatoms-and probably can be better compared with the situation of NABE than with the small particles prevailing in the Equatorial Pacific. Without actual weighing data, we cannot improve our SPM estimates. It should be realized, however, that we use SPM values only to illustrate correlations between Th adsorption, SPM and POC. Errors in the absolute SPM values do not influence our estimates of 234Th or carbon export rates. During the first two transects, particulate 234Th also was linearly correlated with SPM, but in contrast to POC (Fig. 7), particles observed after the export had started during the last transect were low in 23vh [Fig. 8(a)]. These low values are in part explained by the low dissolved activities, which cause lower adsorption rates. In steady state and in the absence of

20 18 16 14

OJ 0

.

1

100

200

300 SPM

Fig. 7.

POC (PM) as a function

400

500

600

(NJ/L)

of SPM (pg I-‘), yielding a carbon (? = 0.874, n = 34).

content

in SPM of 40.2 f 2.7%

472

M. M. Rutgers

van der Loeff et al.

6001”

’

+

b) 0 0 particulate

234 Th (dpmll)

.05

.I

.I5

.2

.25

.3

.35

4

.45

.5

AplAd

Fig. 8. Suspended particulate matter as a function of (a) particulate 234Th (dpm 1-l) and (b) P/Ad, the ratio of particulate to dissolved 23?h. Stations 960, 969 and 972, influenced by the export, represented by open symbols. Regression lines for pre-export samples drawn by hand.

export flux, we would expect the Ap/Ad ratio to be constant (from Equations (4) and (5)). However, at high values of SPM, even Ap/Ad ratios fall off the correlation [Fig. 8(b)], indicating that adsorption and aggregation rates are slow compared to the loss of 234Th by sinking of large particles. During our expedition, we observed large variations in the C/234Th ratio in suspended material [Fig. 9(a)]. First, there was an increase with time during the development of the bloom. The low specific activity on suspended particles in the bloom [Fig. 8(a)] is reflected in high POC/234Th ratios in surface waters at stations 960 and 972 [Fig. 9(a)]. Similar to Fig. 8(b), the effect of supply limitation can be removed by plotting POC versus Ap/Ad, which brings the bloom stations closer to the pre-bloom stations [Fig. 9(b)]. Second, there is a decrease of POC/234Th with depth (Fig. lo), in agreement with the more rapid turnover of C compared to Th noticed before (Murray et al., 1989). Third, and unexpected, the POC/234Th ratio is clearly higher in the Southern ACC than in the Polar Front region [Figs 9(a) and lo)]. This difference originates especially at 100 and 200 m depth, where particulate 234Th is significantly higher in the PFr than in the southern ACC, whereas POC is only barely higher at 100 m and similar at 200 m (Table 1). We attribute this difference to the different composition of the suspended particles with abundant empty diatom frustules in the PFr (Crawford, 1993, illustrated by an unusually high ratio of biogenic silica to Corg (Queguiner et al., 1997). These frustules are poor in carbon but yield ample surface area for Th adsorption. An alternative explanation is the availability of Fe oxyhydroxide surfaces at the Polar Front. Freshly precipitated Fe-oxyhydroxides provide strong binding sites for hydrolyzable elements like Th, either by co-precipitation or by subsequent adsorption (e.g. Balistrieri et al., 1981). De Baar et al. (1995) showed a strong gradient of dissolved Fe across the ACC with lower values south of the PF. Fe is efficiently scavenged in the high particle concentrations found during the bloom at the Front (Fig. 5) as was demonstrated by the decrease in dissolved Fe from transects 5 to 11. Leachable particulate Fe has been

473

Carbon export during the Spring Bloom

0

.I

.2

.3 particulate

.4

.5

234 Th (@m/L)

.6

.7

.6

0

.05

.I

.I5

.2

.25

.3

.35

.4

.45

.5

ApiAd

Fig. 9. POC (PM) as a function of (a) particulate 23‘rTh(dpm 1-l) and (b) Ap/Ad for the southern ACC (open symbols) and for the Polar Front region (closed symbols), with export stations 960 and 972 indicated by small symbols and connected by broken lines. Symbols corresponding to stations as indicated in (b). Regression of pre-export data at the PFr in (a) yields a C,,/23‘?h ratio of 20.9* 3.1 pmol dpm- ’(? = 0.832, n = 11).In the southern ACC, the correlation is worse (rs = 0.45, n = 16). Regression lines drawn by hand through origin.

measured during transect 5, reaching its maximum value near the Polar Front at 49”s (station 903, Lijscher et al., 1997). At 40 m depth, these authors measured 3.8 nM leachable particulate Fe, 1.3 nM of which was leachable in acetic acid and called reactive iron. Farther south in the southern ACC (51”S, station 899), only 0.7 nM leachable Fe was found, less than 0.1 nM of which was reactive. Another maximum in leachable iron was observed at the southern ACC Front around 55”S, but we have no corresponding data on C/234Th ratios. Fresh oxyhydroxides thus may scavenge particulate 234Th near the Polar Front, but as they are also effective scavengers of DOC (Powell et al., in press), the effect on the particulate C/234Th ratio cannot be predicted. In order to obtain representative values of the POC/23?h ratio, in Fig. 9(a), we distinguish the values from the southern ACC and from the PFr. The correlations between particulate 234Th and POC are fair, considering that the samples have been obtained in separate casts with different water samplers and different filtration equipment. As most of the particulate Th activity is on particles larger than 1 pm (Moore and Hunter, 1985), we consider the bias in the C/234Th ratio introduced by comparing filters with different pore sizes for POC (GFF filters, > 0.7 mm) and particulate 234Th (l-pm Nuclepore) to be minor. For the PFr, before the particulate 234Th became supply limited, we find an average POC/234Th ratio of 20.9 + 3.1 pmol dpm-’ [Fig. 9(a)], which is at the upper end of the range of 8-20 pmol dpm- ’ observed in the NABE experiment (Buesseler et al., 1992), but much higher than the values observed in the JGOFS Equatorial Pacific (EqPac) experiment (24 pmol dpm -i in material > 0.7 pm; Buesseler et al., 1995). In the southern ACC, the average POC/23?h ratio was 40 pmol dpm-‘. This remains within the range of 13120 pmol dpm-‘, reported by Cochran et al. (1995) for the Northeast water Polynya in Greenland.

474

van der Loeff et al.

M. M. Rutgers C/ 234Th 0

lo

20

30

40

(pmol/dpm) 50

60

70

80

90

60. 80

500 1I

-

&20140

-

411: 180 160

I

1 Fig. 10.

C,,,/23?h

versus depth for stations in the PFr (closed symbols) (open symbols).

and in the southern

ACC

We will base our further estimates on the POC/234Th ratio observed in the PFr, since this is the region where the major export occurred. As all previous studies show that the POC/23?h ratio on large particles is lower than on average suspended material, the POC export flux must be smaller than derived from the 234Th removal rate and the suspended particulate POC/234Th ratio (20 pmol dpm- ‘). This yields an upper limit of 1.43 mol C m-* for the POC export flux in the 22-day period between transects 5 and 11. Based on the literature data on Corg and 23?h in suspended matter and trap material cited above, we estimate that the C/234Th ratio of material exported from the euphotic zone amounts to 3&60% of the ratio in surface water SPM, or 6-12 pmol dpm-‘. With these values, the carbon export in the PFr (4748”s) between transects 5 and 11 becomes 0.430.86 mol C m-‘. As a large range of values for the C/234Th ratio and for the difference in composition between large and small particles has been observed, these estimates remain uncertain until either size-fractionated data or sediment trap data become available. The export fluxes now can be compared with other components of the carbon budget at the Polar Front region during the period of 18 October to 21 November (Table 4). Primary increased from 0.3-l .O gC m-* day-’ production (PP) measured with 14C incubation 11 (Jochem et al., 1995). during transect 5 to 1.1-3.0 gC m-* day-’ during transect Assuming 0.65 gC m-* day-’ for the 12-day period between transects 2 and 5, and 1.35 gC m-’ day-’ for the 22-day period between transects 5 and 11, we find a total primary production of 3.12 mol C m-* in the 34-day period.

475

Carbon export during the Spring Bloom Table 4.

Carbonjkes

(mol C mv2) near the Polar Front in the SE Atlantic Flux (mol C rn-=)

18 October-21 November 1992 ANTX/6,6” W 14Cprimary production Net CO2 incorporation in biomass Increase in biomass standing stock O-200 m Export production (23‘iTh) Export production upper limit (23?h) Sediment trap deployment 1987-1988,6”E Annual flux at 100 m

Reference

3.12 1.57 0.52 0.434.86 1.43

Jochem et al. (1995) Bakker et al. (1997) Queguiner et al. (1997) This paper This paper

1.27

Wefer and Fischer (199 1)

The net incorporation of COz into organic material can be estimated from the measurements of the carbonate system, if exchange with other water masses is neglected. fCO;! decreased over the investigation period by 0.67 patm day-’ (Bakker et al., 1997), but the same authors show that the residual fCOz change, corrected for the effect of temperature on CO1 solubility and for air-sea exchange, was about twice as high at 1.38 patm day-‘, which converts with their equation (5) to a XC02 uptake of 0.58 PM day-’ or with a mixedlayer depth of 80 m to 1.57 mol C m-’ in 34 days. POC has been measured by Qutguiner et al. in higher resolution than in our own study. They report a POC inventory in the upper 200 m of the Polar Front region at the end of the growth period of 1.24 mol C me2 (average of four stations). If we assume that the inventory before the bloom was comparable to the inventory in the southern ACC (0.72 mol C m-‘, average of 10 stations), the POC inventory increased by 0.52 mol C m-‘. CONCLUSIONS From the low initial depletion in total 234Th, we can conclude that there had been little export production in the months before we arrived until the beginning of November. Significant export occurred in the second period (November) in the area north of about 53”s. In terms of 234Th, the export from the upper 100 m is estimated as 7.1+ 1.Odpm cmp2. Larger errors are associated with estimates of the export of organic components because of an uncertainty in the ratio of organic carbon to 234Th on large sinking particles. About 3.1 mol C m-’ was taken up over a 34-day period by primary producers during the bloom at the Polar Front. Half of this was converted to biomass, the other half being remineralized. Of this net biomass production of 1.57 mol C m-‘, 0.43-0.86 (upper limit: 1.43) mol C m-’ was exported in the later stage of the bloom, and about 0.52 mol C mm2 was still present in the water column, close to a closed budget in view of the large errors. Because a large part of the annual export occurs in only a few major blooms (Mathot et al., submitted), the export flux compares well with the annual flux estimate of 1.27 mol C me2 at 100 m, based on a sediment trap deployment in 1987 (Wefer and Fischer, 1991). Export production in the bloom amounted to 12-24% of primary production or 25-50% of the net CO2 conversion to biomass as estimated from a CO2 budget (Bakker et al., 1997). In the southern ACC, primary production amounted to 0.2-0.3 gC m-’ day-’ (Jochem et al., 1995) or 0.71 mol C mm2 in 34 days. Export south of 53”s was 0.0990.17 mol C m-’ (Table 3), corresponding to the same export percentage (12-24%, which in steady state would correspond to an&ratio of 0.12-0.24) as in the Polar Frontal region. Thus, our data

476

M. M. Rutgers

van der Loeff et al.

cannot confirm that the exported percentage situation of modest continuous growth.

is higher

in a bloom

situation

than

in

a

Acknowledgements-We thank captain Suhrmeyer and his crew for their help. The oceanography team of Cornelis Veth, Sven Ober and Ronald de Koster provided the hydrographical basis and the transmissometer data. We thank Heike Hijltzen for her help with the Gerard sampling and thorium analyses, and Corinna Dubischar for the POC and PON analyses. We thank Victor Smetacek and Hein de Baar for their stimulating role in the SO-JGOFS project, and acknowledge constructive reviews of Bob Anderson, Ken Buesseler, Kirk Cochran and James Murray. This is AWI contribution no. 974, and SFB 261 contribution no. 116.

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