234Th as a tracer of particle flux and POC export in the northern North Sea during a coccolithophore bloom

Deep-Sea Research II 49 (2002) 2965–2977

234

Th as a tracer of particle flux and POC export in the northern North Sea during a coccolithophore bloom Jane M. Foster*, Graham B. Shimmield Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, Argyll, PA37 1QA, UK Received 27 September 2001; received in revised form 8 January 2002; accepted 23 January 2002

Abstract A multidisciplinary cruise to the northern North Sea was undertaken during June 1999 to carry out a Lagrangian study of an Emiliania huxleyi bloom. During this experiment, the naturally occurring radionuclide, 234Th, was measured in the water column to estimate particle fluxes and resulting residence times. Simple steady state modelling of changes in 234 Th activity suggests an increase in scavenging efficiency over an 8 day period. The steady state 234Th flux ranges from 357 to a maximum of 1390 dpm m
2 d
1 on the 25th June 1999, with a corresponding particulate organic carbon (POC) flux from 9.5–48 mmol C m
2 d
1, estimated from POC/234Th ratios on filtered particulate material. Scavenging of the 234 Th by the bloom occurs immediately. The maximum POC fluxes follow the peak in primary production and the maximum coccolithophore cell abundance within a few days. The 234Th and POC fluxes illustrate the rapid response of export with biological activity. r 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction Factors controlling rates and magnitude of organic carbon export remain uncertain on short temporal and spatial scales, and thus a knowledge of particle export is important in predicting the fate of biologically active contaminants such as metals and hydrocarbons (Gustafsson et al., 1997; Baskaran, 2001). Such rates are also vital in the assessment of the role of CO2 transport from the atmosphere to the deep ocean and its potential negative feedback on climate change (Sarmiento and Toggweiler, 1984; Falkowski et al., 2000; Fasham et al., 2001). The superimposition of *Corresponding author. Tel.: +44-1631-559-224; fax: +441631-559-001. E-mail address: [email protected] (J.M. Foster).

bloom events onto the seasonal vertical carbon flux may have a considerable influence on the rate and magnitude of carbon sequestration in the oceans. The uptake and export of carbon and nutrients are driven by biological productivity in the euphotic zone at variable rates dependent on a range of ecosystem and state variables. For example, the particle flux across the thermocline during the lifetime of a plankton bloom can be determined by using radioactive tracers with halflives spanning appropriate timescales. The shortlived radioisotope 234Th (half-life 24.1 days) is one such particle-reactive tracer and has been used to estimate upper-ocean particulate organic carbon (POC) export in many of the world’s seas; North Atlantic (Buesseler et al., 1992), equatorial Pacific (Buesseler et al., 1995), Greenland Sea (Cochran

0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 2 ) 0 0 0 6 6 - 8

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J.M. Foster, G.B. Shimmield / Deep-Sea Research II 49 (2002) 2965–2977

et al., 1995), Arabian Sea (Buesseler et al., 1998), subtropical and equatorial Atlantic (Charette and Moran, 1999), Iberian Margin (Hall et al., 2000), Beaufort Sea (Moran and Smith, 2000), Gulf of Maine (Benitez-Nelson et al., 2000), and the Southern Ocean (van der Loeff et al., 1997; Cochran et al., 2000). The particle-reactive nature of 234Th and the disequilibrium between 234Th and conservative 238U (half-life 4.47  109 yr) in the water column allows quantitative measure of the 234 Th export on timescales of days to months. To convert the 234Th flux into a POC flux, direct measurements of 234Th and POC on filtered particles are made at discrete sample depths in the water column. In June 1999 a multidisciplinary cruise, as part of the NERC funded project dimethyl sulphide biogeochemistry within a coccolithophore bloom (DISCO), onboard the R.R.S. Discovery, to the northern North Sea (Fig. 1) investigated aspects of an Emiliania huxleyi bloom by means of a Lagrangian study. Although specifically designed to determine the role of prymnesiophyte alga in the marine sulphur cycle by measurements of dimethyl sulphide (DMS) (Burkill et al., 2002), this biogeochemical process study also investigated rates of carbon cycling and export from the surface waters using 234Th and POC measurements.

The bloom was located by means of an extensive survey (as satellite images were not always useful due to cloud cover) and tracked by means of a SF6 tracer which created a patch of dimensions 6  8 km. Samples for radionuclide analysis were collected on four separate occasions over a period of 8 days. Results from this work provide the first 234Th data for the northern North Sea. The calculation of 234 Th particulate fluxes and POC fluxes serves to illustrate the rapid response of export with biological activity with high flux values following periods of elevated primary production and coccolithophores in the upper water column.

2. Methods Blooms of Emiliania huxleyi are usually common in the northern North Sea during the summer period, but during June 1999, the blooms were sparse and it was necessary to perform a detailed survey of the region to locate a suitable patch. The bloom chosen was initially centred at approximately at 58.931N 02.871E and was labelled for Lagrangian tracking with 30 g of SF6 (Nightingale et al., 2000) on 16/17th June. During the study the patch drifted generally in a south-easterly direction until it was subducted, to approximately 10 m,

Fig. 1. DISCO cruise (D241) location. (K) indicate the CTD stations and numbers relate to the date in June. Depth range at stations is from 106 to 118 m.

J.M. Foster, G.B. Shimmield / Deep-Sea Research II 49 (2002) 2965–2977

under warmer, less saline Norwegian coastal water on the 23rd June. At this point, the patch was reseeded at this location on the 24th June and tracked until the 26th June.

3. Sampling Following the introduction of the SF6 patch on the 17th June, samples were collected at a position determined by the maximum concentration of SF6 for the mid-day and mid-night casts. The 20 l samples for radionuclide analysis were collected from the cast immediately after the mid-day cast. The stations for 234Th analysis were sampled on the 19th, 22nd, 25th and 27th June. Due to subduction of the original labelled patch on the 23rd June, half of the days sampled fell outside the true Lagrangian sampling period. It is for this reason that the planned use of non-steady state model for interpretation of the radionuclide profiles has not been possible. Hydrographic parameters were measured by Sea-Bird CTD sensors. Temperature was measured by the Sea-Bird fast temperature sensor, salinity (computed from conductivity), by the SeaBird conductivity sensor and fluorescence using the Aquatracka Mk III. The water depth of the studied area ranged from 106 to 118 m depth. The hydrographic data were calibrated and provided by the British Oceanographic Data Centre (BODC). Irradiance and determination of the 1% light level, i.e. the operational definition of the euphotic zone depth, was calculated from the downward irradiance measured by 2-pi PAR scalar irradiance sensors fitted to the CTD. All data presented in this paper are available at the BODC. 3.1. Radionuclide analysis Approximately 20 l seawater samples were collected using 30 l Niskin bottles attached to the Sea-Bird CTD rosette. The samples were immediately filtered to remove particulate material using 0.45 mm Asyport 142 mm diameter filters. Following the initial filtration, the samples were processed according to the method described in Pates et al. (1996) as follows. The filtrate was

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acidified with 50 ml of concentrated HCl and spiked with 50 dpm of 230Th, which acts as a yield tracer, and 10 ml of a purified iron solution, concentration 10 mg Fe ml
1 (prepared using FeCl3.6H20). The sample was shaken well and allowed to equilibrate for approximately 24 h. Fifty millilitre of concentrated ammonia solution were added to form a Fe(OH)3 precipitate. The precipitate was filtered using a 3 mm Asyport 142 mm diameter filter and purified using ion exchange chromatography, using Bio-Rad AG 1X8 100–200 mesh chloride form resin. The counting source was prepared by taking the sample to dryness in a 10 ml vial, re-dissolving in 0.5 ml of 0.1 M HCl, and mixing with 5 g of Ultima gold LLT scintillation cocktail. A Packard TriCarb 2770TR/ab liquid scintillation spectrometer (Packard Instrument Co.) was used to count the samples using two separate 200 min counts with a limit of detection of 0.04 dpm l
1. A beta counting window was selected in the multichannel analyser of 10–100 and 110–280 keV was chosen for the alpha window. A blank and a spiked sample were counted every five samples. Counting efficiency was >80%. Average yields are of the order of 50%. Errors are propagated from 1s counting errors and uncertainty due to background collections and detector calibration. 238U activity was not measured directly but estimated using the relationship as defined by Chen et al. (1986), where 238 U (dpm l
1)=salinity  0.0686. 3.2. POC and PIC analysis Seawater samples were collected from CTD casts at approximately mid-day and mid-night at 10 depths over the whole water column. Duplicate 540 ml aliquots from each depth were filtered through 25 mm pre-combusted GF/F filters, which were then frozen at –201C until return to the laboratory. One of each duplicate pair was dried at 501C for 12 h prior to analysis whilst the second filter was exposed to fuming HCl for 12 h to remove inorganic carbon before drying at 501 for 12 h. Carbon and nitrogen analyses (King et al., 1998) were made following Dumas combustion (Carlo Erba NA1500) on sub-samples of each filter calibrated against acetanilide standards. The

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J.M. Foster, G.B. Shimmield / Deep-Sea Research II 49 (2002) 2965–2977

particulate organic fraction was considered to be that carbon measured on the acid-fumed filters, whilst the inorganic fraction was considered to be the difference between measured total and organic carbon. The POC and PIC assays are reported in mM. The POC and PIC data were not available from the same casts, as the radionuclides therefore data set was obtained from the mid-day analyses.

4. Results 4.1. Hydrography The Lagrangian study was facilitated by the introduction of SF6 into the water, at a depth of between 6.5–9.5 m from the surface, and subsequent tracking over the period from 17th to 23rd June 1999. The SF6 patch, of original dimensions of 6  8 km, travelled in a south-easterly direction becoming more southerly after the 20th June until on the 22nd/23rd June it was subducted under a surface layer of Norwegian Coastal Water (NCW). The NCW was warmer and less saline, and occupied the surface 5–10 m. The general situation in the upper water column can be described from the temperature profiles. These showed a deepening of the thermocline from 15 m on the 17–19th June and, following a storm on the evening of the 19–20th June, to 23 m. The second body of water tracked (probably the NCW) showed further deepening of the thermocline layer to 30 m on 27th June. The euphotic zone depth (Table 3), taken as the depth to which 1% of surface irradiance is measured, shallowed from 46 m on 18th to 35 m on 22nd June to 31 m on the 25th June. The measured nutrients showed typical oligotrophic summertime conditions (Rees et al., 2002). Optical attenuation increased over the sampling period, with a maximum occurring on 25th June at B1.1 m
1. The maximum generally occurred in the surface layers except on the 27th June when a maximum occurred in the mid-water between 20 and 30 m. A maximum chlorophyll a concentration of 3.1 mg m
3 was observed on the 17th June in a subsurface peak at about 27 m. The subsurface chlorophyll a maximum layer persisted at about 30–35 m throughout the Lagrangian period.

4.2.

234

Th

Dissolved and particulate 234Th activities, decay corrected to the mid-point of sampling, are given in Table 1. The profiles along with selected hydrographic data are shown in Fig. 2. There is a decrease in surface dissolved 234Th activities from 2.14 dpm l
1 on the 19th June 1999 to a low of 0.94 dpm l
1 on the 25th June. For profiles taken on the 19th and 22nd June a minimum occurs at 20 m depth, whereas on the 25th the minimum is at the surface and persists down to 20 m. In the deeper waters, at 75 m, on the 19th an equilibrium value with parent 238U is reached. For the other stations the deepest sample has an activity between 1.5–2 dpm l
1, i.e. less than the 238U equilibrium activity. It is possible that the comparatively shallow depths of this part of the North Sea (B110 m) influence the 234Th disequilibrium in the deepest samples through boundary scavenging caused by sediment resuspension. Particulate 234Th activities for all profiles are greatest in the upper 20 m with a maximum value of 0.8 dpm l
1 on the 22nd June. During the four sampling periods the maximum extends from the surface samples to a depth of about 20 m. 238 U is conservative in seawater so that any deficiency of 234Th relative to 238U can be ascribed to thorium removal by particle scavenging and subsequent transport. Under equilibrium conditions the total 234Th activity is equal to the 238U activity. A 234Th/238U ratio o1 implies significant removal over a short time, certainly o2–3 times the half-life of 234Th. On the 19th June the measured profile shows that the total 234Th in the surface water is the equal to, within error, the 238 U activity and indicates that little scavenging is occurring. This would suggest that pre-bloom conditions existed at this time and that few particles are being produced to remove this particle-reactive nuclide from the water column. The decrease in total 234Th over the 8 day period is ascribed to 234Th being removed from surface waters and transferred to deeper waters. On the 22nd June a 234Th/238U ratio of 1 occurs near the surface with the value decreasing at the 20–30 m level, with an increase towards equilibrium values for the deepest samples measured. As the study

J.M. Foster, G.B. Shimmield / Deep-Sea Research II 49 (2002) 2965–2977 Table 1 Particulate and dissolved Date

Position

234

Th and

2969

U activities (dpm l
1) measured during an 8 day period in June 1999

238

Depth (m) Dissolved

Th (dpm l
1) Particulate

234

Th (dpm l
1) Total

234

Th (dpm l
1)

234

U (dpm l
1)

238

19/06/99 58145.530 N 5 03118.000 E 10 20 35 50 75

2.1470.80 1.7170.14 1.3970.11 1.8970.12 2.3070.13 1.5570.11

0.6370.07 0.6170.07 0.6270.08 0.3470.07 0.1570.06 0.2770.06

2.77 2.32 2.02 2.23 2.45 1.82

2.40 2.41 2.41 2.41 2.42 2.42

22/06/99 58125.320 N 5 03128.450 E 10 20 30 50 75

1.1570.09 1.9670.17 1.0770.08 1.5770.13 1.9970.13 1.4670.10

0.7770.07 0.5770.06 0.7070.07 0.3970.06 0.3370.06 0.3270.06

1.91 2.53 1.77 1.96 2.32 1.78

2.40 2.40 2.41 2.41 2.41 2.42

25/06/99 58116.130 N 03128.800 E

1 5 10 20 32 60

0.9470.10 1.0170.09 1.0070.09 1.0370.09 1.5970.12 1.5670.09

0.4570.06 0.6270.06 0.6070.07 0.7070.07 0.2870.06 0.2370.06

1.39 1.63 1.60 1.73 1.86 1.80

2.35 2.40 2.41 2.41 2.41 2.42

27/06/99 58141.410 N 02137.180 E

1 5 10 20 35 75

1.4670.12 1.2270.09 1.7470.16 1.4570.44 1.4370.09 1.4770.10

0.3470.06 0.2870.06 0.3970.06 0.5470.07 0.5470.07 0.2870.06

1.80 1.50 2.13 1.98 1.97 1.75

2.40 2.40 2.40 2.41 2.41 2.41

Errors based on propagation of 1s counting statistics.

progressed to the 25th June, the ratio decreased to 0.6, down the water column the values all indicate scavenging of the thorium as the number of new particles in the water column increased, with corresponding increased activity on the particulate phase. On 25th June total 234Th was just over half the equilibrium value, indicating extensive removal as the bloom progressed. Although the specific activities varied over the study period, the inventories of total 234Th varied little. The average inventory over 75 m over the 8 days was 1571.4 dpm cm
2. 4.3. POC and PIC POC concentrations (Table 2, Fig. 2) increase in the euphotic zone, with a peak occurring on the 25th June of 22 mM, corresponding with the

maximum chlorophyll peak on that day. The POC/234Thp ratio is determined using the POC values collected on GF/F filters combined with the 234 Th particulate activity collected from the 20 l water samples. Ideally, the both parameters would have been collected on the same filters, but this was not possible as the analyses were performed at different laboratories. The ratios range from 11 to 75 mM dpm
1 (Table 2), with this maximum value occurring at 32 m on the 25th June. For the first two stations the ratio values are around 17 mM dpm
1 in the upper part of the water column, increasing to a maximum at near the base of the euphotic zone and decreasing thereafter. On the 25th June the POC/234Thp ratio value is higher throughout the water column, averaging at 25 mM dpm
1 above and below the peak value of 75 mM dpm
1. On the 27th June the peak value

J.M. Foster, G.B. Shimmield / Deep-Sea Research II 49 (2002) 2965–2977

2970

19th June 234

-1

Depth (m)

J J

J

J

F H B HF F

H

BF

J

0

10 B B B B B B

20

20

H

10

0 20

20

40

B

2

H

Th

0

B B B B B

B

0

2

238

F

U

20 B B

F H

0

Total

10

-3

)

B B

Chl a (mg m 0

3 20

40

40

40

40

60

60

60

60

80

80

80

80 σ (kg m -3) t 26 27 28

30

B B

80

2

Th

20 B B

60

1

H

234

20

20

-3 σ t (kg m ) 26 27 28

BF

BB

20

1

2

-3

)

3

σ (kg m -3) t 26 27 28

0

0

0

0

20

20

20

20

40

40

40

40

60

60

60

60

80

80

80

Th and

B

0

20

Fig. 2. Profiles of

J

0

0

238

F H

40

Chl a (mg m

3

30

0

234

B

POC (µM)

0

80

J

J

0

σ (kg m -3) t 26 27 28

B F H H BF B FH

60

B

80

1

H

B

60

Chl a (mg m )

3

10

40

-3

Chl a (mg m ) 1

20

H

80

20

B

80 -3

0

H

BF

0

60

80

234

30

B

60

F BF

-1

Th (dpm l ) 1 2 3

J J J

POC (µM)

B B B B B B B

B

40

0

80

Particulate

0

30

0

H H H

40

POC (µM)

POC (µM) 0

BF

J

Th

234

B FH

J

80 234

J B J

60 J

H

Dissolved

-1

Th (dpm l ) 1 2 3

J B F J B F J B F

20

B F H

60

B

Depth (m)

J B

0

27th June

40

BH F

J

234

0

40

80

Depth (m)

-1

J

60

Depth (m)

25th June

Th (dpm l ) 1 2 3

J B

20

B FH

J

40

FH

F H

B

0 0

BH F

B

J

20

234

Th (dpm l ) 1 2 3

0 0

22nd June

U activities and related hydrography at four stations in June 1999.

J.M. Foster, G.B. Shimmield / Deep-Sea Research II 49 (2002) 2965–2977

2971

Table 2 POC and PIC concentrations (mM) measured during an 8 day period in June 1999 Date

Position

Depth (m)

POC (mM)

PIC (mM)

POC/234Thp (mM dpm
1)

19/06/99

58145.530 N 03118.000 E

5 10 20 35 50 75

10.9 8.9 11.0 15.6 5.3 3.4

2.05 NA 2.89
0.01 1.75 0.37

17.15 14.62 17.57 45.84 35.33 12.60

22/06/99

58125.320 N 03128.450 E

5 10 20 30 50 75

10.8 11.5 13.5 9.6 4.0 3.7

2.18 4.15 1.24 4.15 10.69 1.21

14.17 20.40 19.39 24.76 12.12 11.61

25/06/99

58116.130 N 03128.800 E

1 5 10 20 32 60

14.0 15.3 11.3 21.8 21.1 7.1

2.18 NA 4.15 1.24 10.69 1.21

31.09 24.71 18.66 31.16 75.76 30.08

27/06/99

58141.410 N 02137.180 E

1 5 10 20 35 75

13.7 16.0 15.1 14.1 9.7 3.1

7.61 4.79 8.22 5.87 3.20 0.33

39.62 56.53 38.80 26.35 17.99 11.10

occurs at 5 m depth. Values of the ratio are higher (12–15 times) than some previous studies (Buesseler et al., 1995). This could be due to GF/Fs collecting DOC along with POC (Moran et al., 1999) and because of the small sample size, this will elevate the POC value. Particulate inorganic carbon was determined by subtracting the POC from the total particulate carbon concentration. Values were in the range 1–11 mM (Table 2) over the study period and represented between 8% and 73% of the total particulate carbon concentration. The maximum value occurred at 40 m depth on the 22nd June. 4.4.

234

Th scavenging models

Several studies have used the disequilibria between 234Th and its parent, 238U, to quantify POC export. Examples are detailed in Buesseler

et al. (1992), Charette and Moran (1999), BenitezNelson et al. (2000), Cochran et al. (2000) and van der Loeff et al. (1997). These models assume that the 234Th export flux is due to scavenging particles settling through the water column. Coale and Bruland (1985, 1987) were amongst the first to estimate rates of thorium uptake using an irreversible one-dimensional vertical scavenging model. Thus the 234Th total activity balance is given by d234 Th ¼ ð238 U
234 ThÞlTh
PTh þ V ; dt

ð1Þ

where d234Th/dt is the rate of change of 234Th removal, 238U is the activity of 238U determined from salinity, 234Th is the total (i.e. dissolved and particulate activities ) measured 234Th activity, lTh is the 234Th decay constant (0.02876 d
1), and PTh is the net loss of 234Th on sinking particles. The

J.M. Foster, G.B. Shimmield / Deep-Sea Research II 49 (2002) 2965–2977

2972

term V represents physical processes such as horizontal advection and diffusion. This V term is important in regions of high upwelling velocity (Buesseler et al., 1995), and horizontal advection can be significant in near-shore sites (Gustafsson et al., 1998; Benitez-Nelson et al., 2000). The V term can be neglected due to the Lagrangian nature of this study. Eq. (1) can be separated into component parts representing steady state and non-steady state conditions. A steady state approach assumes a constant flux over short periods, i.e. days to weeks. If steady state conditions are assumed then the difference in 234Th activity from the parent 238U equilibrium activity indicates the magnitude of the 234 Th particle export flux, so the greater the deficit the greater the particulate export. In most settings, the steady state equation accounts for most of the export flux with the other terms contributing slightly (o10% according to Buesseler, 1998). The non-steady state (NSS) term, d234Th/dt, allows for any temporal changes in 234Th activity and becomes important during periods of intense scavenging, e.g., during plankton blooms (Buesseler et al., 1992; Buesseler, 1998). The primary assumption made when estimating the NSS 234Th flux is that the vertical 234Th profiles are representative of changes during the course of the bloom, rather than the water mass changing during the evolution of the bloom. However, as the water mass did not remain the same throughout the sampling period then the application of the non-steady state model was deemed inappropriate. Eq. (1) can be simplified to a 1-D vertical model by making a number of assumptions. These are that there are steady state conditions (d234Th/dt=0), with respect to production of 234Th, decay and export of particulate 234 Th; that vertical processes are more significant than horizontal and diffusion as they are usually much smaller than the production and decay terms. This results in a steady state flux that may be given by

PTh ¼ lTh

Z 0

where z is the depth at which PTh is calculated and PTh is calculated by trapezoidal integration. Export production is defined as the amount of organic carbon leaving the euphotic zone in particulate form. The POC flux is given by the empirical approach described in Buesseler (1998), which hinges on two assumptions, namely that the 234 Th flux, PTh ; is accurately estimated and that the particles analysed for both 234Th and POC are characteristic of those carrying both elements below the euphotic zone POC PPOC ¼ 234  PTh : Thp

ð3Þ

The POC/234Th ratio was calculated from values on the >0.45 mm particles, measured on seawater filtered from CTD casts taken as near in time as possible to radionuclide casts. Particulate organic carbon fluxes estimated from the 234Th disequilibrium are shown in Fig. 3 and Table 3. The total PTh flux increases from 3577447 to nearly 14007265 dpm m–2 d
1 on the 25th June (note that there is also a sub-surface flux maximum within this profile) with a corresponding increase in modelled POC flux from 9 to 48 mmol C m
2 d
1. Figs. 3a and b illustrate fluxes from the euphotic zone and the sub-euphotic zone down to 75 m. The 19th shows little export as the total 234Th is in equilibrium with the 238U within error limits. For the 19th and 22nd June the export from the euphotic zone is less than that from the remaining 40 m. On the 25th the export is divided equally between both zones. These POC fluxes fall into the same range as results given by Buesseler et al. (1992) for the North Atlantic, Charette and Moran (1999) for the equatorial Atlantic, and Benitez-Nelson et al. (2000) for the coastal shelf sea of the Gulf of Maine, and are also similar to the fluxes determined for Southern Ocean blooms (Cochran et al., 2000). Furthermore, 234Th residence times can also be calculated from the above equations, where tp ¼ ApTh =PTh

ð4Þ

and

z

½238 U
234 Th dz;

ð2Þ

JTh ¼ ð238 U
AdTh Þ lTh ;

ð5Þ

ApTh

ð6Þ 234

and AdTh are the Th activities of the where particulate and dissolved phases, respectively. The residence times are calculated using Eqs. (4) and (6). The residence times for both phases decreases from the 19th to 25th June, with the dissolved residence time decreasing from 83 to 38 days and the particulate residence times from 94 to 22 days. The abundance of Emiliania huxleyi in the surface waters over this period increased from 945 cells ml
1 on the 19th June to 1450 cells ml
1 on the 22nd June to 3026 cells ml
1 on the 26th June (Burkill et al., 2002), confirming the existence of newly produced particle surfaces. The concentration of coccolithophores down through

0.46 48 22.2974.89 8.24 14.06

0.96 50 48.14713.55

0.14

0.19 50

91 12.3572.20 4.26

22.90 25.24 1386

1116 703 413

td ¼ AdTh =JTh ;

30

so

27/06/99

Fig. 3. (a) Particulate 234Th flux, in dpm m
2 d
1, and (b) POC export flux, in mmol C m
2 d
1, for an 8 day period in June 1999.

737

19

649

0

31

5

25/06/99

10

8.09

15

747

20

363

25

384

30

35

Sub-euphotic zone

35

22/06/99

_2

POC flux (m mol C m

40

9.5473.11

27

Euphotic zone

_

d 1)

45

2.73

22 25 Date in Ju ne

50

6.81

27

357

25

210

22

147

19

43

0

58145.530 N 03118.000 E 58125.320 N 03128.450 E 58116.130 N 03128.800 E 58141.410 N 02137.180 E

200

19/06/99

400

Sub-euphotic— Total 75 m

600

Euphotic zone

800

Primary prodn ThE ratio mmol C m–2 d–1 Euphotic Euphotic zone zone

Sub-euphotic zone

POC export flux mmol C m
2 d
1

1000

Euphotic PTh dpm m
2 d
1 zone depth (m) Euphotic Sub-euphotic— Total zone 75 m

Euphotic zone

Position

1200

2973

Date

PTh (dpm m

_2

_

d 1)

1400

Table 3 234 Th estimated particulate flux, PTh ; in dpm m
2 d
1, and POC flux, in mmol C m
2 d
1, for a 75 m water column. Primary production values in mmol C m
2 d
1 and the ThE ratio (234Th derived export flux/14C derived primary production) are shown

J.M. Foster, G.B. Shimmield / Deep-Sea Research II 49 (2002) 2965–2977

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Fig. 4 shows the coccolithophore abundance at discrete depths within the euphotic zone plotted against the 234Th deficiency, that is the total 234Th activity subtracted from the parent 238U, at the same depths. The trend in the surface waters is for the 234Th deficiency to increase as the cell abundance increases as the 234Th is scavenged from the upper water column. Integrated coccolithophore abundance for a 75 m water column for the period 19–27th June are shown in Fig. 5 with the POC and PIC fluxes. This clearly illustrates that the peak in POC export lags the maximum coccolithophore abundance by a number of days. This corresponds with the fact that the 234Th export is associated with the sinking of material from the euphotic zone. The PIC flux rises steadily to a maximum value on the 27th June, which would suggest that there has been a bloom at this location that has crashed, giving rise to these high values.

1.20

234

-1

Th deficiency (dpm l )

the water column decreased significantly below the euphotic zone depth on each day sampled. The water body sampled on the 27th June, now a component of NCW, had an abundance of coccoliths in the surface water of 469 cells ml
1, although values of >1000 cells ml
1 were observed at depths of 20–30 m.

1.00 0.80

19th June

0.60 0.40

22nd J une

0.20 0.00

27th June

-0.20 0 -0.40

25th June

500

1000

1500

2000

2500 -1

Coccolithophore abundance (cells ml )

Fig. 4. Coccolithophore abundance plotted against 234Th deficiency (238U—total 234Th activity) for euphotic zone samples for the four dates in June.

60

60

50

50 40 40 30 30 20 20 10

10

0

POC and PIC exportflux (mmol C m -2 d -1)

Coccolithophore abundance (109 cells m -2)

70

0 19

21

23

25

27

Date in June Coccolithphore abundance

POC export flux

PIC export flux

Fig. 5. Coccolithophore abundance (109 cell m
2), PIC and POC (mmol C m
2 d
1) fluxes between 19th and 27th June.

J.M. Foster, G.B. Shimmield / Deep-Sea Research II 49 (2002) 2965–2977

5. Discussion The aim of the DISCO project was to determine the routes, rates and controls on biogeochemical cycling of DMS within a bloom. The roles of viruses, bacteria, phytoplankton and zooplankton were assessed along with the dynamics of the production, respiration, grazing and sedimentation. DMS is an important biogenic sulphur compound in the marine environment accounting for more than 50% of the total biogenic sulphur entering the atmosphere annually (Andreae, 1990). DMS plays a major role in the marine sulphur cycle and has influences on the climate due to formation of cloud condensation nuclei (Charlson et al., 1987). In addition, DMS and its precursors represent important sources of carbon for bacterioplankton (Kiene et al., 2000). Coccolithophore blooms are an important source of DMS and have received considerable attention in the past because of their potential for export of particulate carbon to the ocean sediments following the shedding of liths (Fernandez et al., 1993). Such blooms result in the loss of both POC and PIC to the deeper ocean. The results from the hydrographical parameters illustrate that the Lagrangian experiment persisted for only part of the sampling period. It is apparent from the temperature salinity and sigma-t profiles that the samples taken on the 19th and 22nd June were from the same body of water, which was initially labelled with the SF6 tracer, and that those taken on the 25th and 27th June were a mixture of the original patch and the NCW, with the final station being most influenced by NCW. This change in water masses is reflected by other parameters such as coccolithophore cell numbers and is reflected in the 234Th profiles and flux calculations. The pattern of 234Th distributions, coccolithophore abundances and POC fluxes relate to the evolution of a bloom. The peak in biological activity and coccolithophore production is reached on the 22nd June, followed by a peak in POC export on the 25th June (Fig. 5), although it is uncertain whether this profile belongs to the initially selected body of water, as indicated above. However, it is clear that a bloom has developed at

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this location immediately prior to sampling. The final station is unrelated to this bloom as the temperature and salinity measured clearly indicate a water mass of different character. Primary production rates are detailed in Table 3 (Rees et al., 2002). At its peak, production rates of 91 mmol C m
2 d
1 were measured on the 22nd June. These decreased afterwards to approximately 50 mmol C m
2 d
1. The percentage of primary production exported, ThE=234Th derived export flux/14C derived primary productivity (Buesseler, 1998), increases over the period from values of 7% to a maximum of 96% on the 25th June assuming that the carbon produced by primary production is exported on the same day. The 234Th profiles and calculated export fluxes illustrates the rapid scavenging of 234Th by sinking cells following bloom production. The high export values follow a few days after periods of high coccolithophore concentrations (Fig. 5) and the peak in primary production in the water column, and hence are associated with the sinking of phytodetritus and not the actual formation of the bloom itself.

6. Conclusions Over an 8 day period samples were collected for Th and POC and PIC analysis. The 234Th profiles show increasing scavenging during the Lagrangian study of a phytoplankton bloom. This is coincident with increased particulate 234Th flux and POC export, and these fluxes follow the peak in measured primary production and coccolithophore cell abundance. The results demonstrate the rapid response of 234Th and POC export with biological activity.

234

Acknowledgements We are grateful to Captain Robin Plumley and the officers and crew of R.R.S. Discovery for ensuring the smooth running of this cruise. We are indebted to Jackie Pates for her significant assistance with the 234Th method. Thanks go to Gordon Cook and Robert Anderson at the

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Scottish Research and Reactor Centre for counting the 234Th samples on their liquid scintillation counter. I am also grateful to Andy Rees, Malcolm Woodward, Denise Cummings, Claire Widdicombe and Glen Tarran at Plymouth Marine Laboratory for making their data, including nutrient and organic carbon data, available. Funding for this project came from the CCMS DYME programme.

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