The Si cycle in the Pacific sector of the Southern Ocean: seasonal diatom production in the surface layer and export to the deep sea

Deep-Sea Research II 49 (2002) 1747–1763

The Si cycle in the Pacific sector of the Southern Ocean: seasonal diatom production in the surface layer and export to the deep sea Daniel E. Sigmona, David M. Nelsona,*, Mark A. Brzezinskib b

a College of Oceanic & Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA Marine Science Institute and Department of Ecology, Evolution & Marine Biology, University of California, Santa Barbara, CA 93106, USA

Abstract Dissolved silicic acid (Si(OH)4) and biogenic silica (BSiO2) concentrations were measured on four different cruises in the Southern Ocean along 1701W from late October 1997 to mid-March 1998. These data were used to construct a vertically integrated Si budget in the surface layer from 581 to 681S. Throughout that period there was a distinct, southward-moving meridional [Si(OH)4] gradient, with concentrations increasing to the south. In October the gradient was centered on the Antarctic Polar Front (APF) at 611S. Surface layer [Si(OH)4] increased from 10 to >40 mmol m
3 between 581 and 621S, while BSiO2 concentrations were o1.5 mmol Si m
3 and o125 mmol m
2 throughout the study area. As the surface mixed layer stratified and shoaled in December, diatoms bloomed in response to the more favorable light conditions, consuming Si(OH)4 in the process. By late December BSiO2 concentrations were exceptionally high for the open ocean, with maximum integrated values of 700 mmol Si m
2 around 621S, while vertically integrated [Si(OH)4] in the upper 50 m had dropped from 2500 to 870 mmol m
2, an average decrease of >30 mmol Si m
3. The diatom bloom persisted into January with maximum integrated BSiO2 of 600 mmol m
2 between 641 and 651S, while [Si(OH)4] was depleted to o2 mmol m
3 in surface waters as far south as 64.51S. The center of the [Si(OH)4] gradient had moved southward to 651S by late January, coincident with maximal diatom biomass at that time. The bloom ended by March with the disappearance of most of the diatom biomass from the surface layer, deeper mixed layers, and increased [Si(OH)4]. By late January, diatom production had removed >70% of the of Si(OH)4 initially present in the upper 50 m everywhere north of 651S, and had displaced the [Si(OH)4] gradient >400 km southward, while the APF remained stationary. Between November and late January (B80 days), an average of 2200 mmol Si m
2 had been removed from the upper 50 m between 611 and 651S. Based on changes in vertically integrated Si(OH)4 and BSiO2 concentrations between November and March, an average of 2 mol Si m
2 was exported from the upper 50 m between 611 and 651S, with 59% of that export occurring between late December and late January. The observed seasonal export is very high in comparison with other oceans, and the high export coincides spatially with a well-documented zone of high opal flux to 1000 m and high opal content in modern sediments. North of 611S BSiO2 export was limited by the low initial standing stock of Si(OH)4 in surface waters, and south of 651S export was likely limited by the shorter growing season. Thus, the combined effect of a large initial supply of Si(OH)4 in surface waters and conditions that permit efficient use of that Si by diatoms appears to be the primary cause of opal-rich sediment accumulation in the Pacific sector of the Southern Ocean. r 2002 Elsevier Science Ltd. All rights reserved. *Corresponding author. Fax: +1-541-737-2064. E-mail address: [email protected] (D.M. Nelson). 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 1 0 - 3

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D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

Re´sume´ Les concentrations en acide silicique (Si(OH)4) et en silice biog!enique (BSiO2) ont e! t!e mesur!ees au cours de quatre campagnes conduites dans l’Oc!ean Austral, le long du m!eridien 1701O, entre fin octobre 1997 et mi-mars 1998. Ces donne! es ont e! te! utilise! es pour construire un bilan inte! gre! sur la verticale des stocks de silicium dissous et particulaire dans la couche de surface entre 581 et 681S. La pr!esence d’un gradient m!eridional net de [Si(OH)4] est observ!ee, avec des concentrations qui augmentent vers le sud. Ce gradient se d!eplace vers le sud, avec, en octobre, une position centr!ee sur le Front Polaire Antarctique (FPA) a" 611S. Les concentrations de surface en Si(OH)4 augmentent de 10 a" >40 mmol m
3 entre 581S et 621S, alors que les teneurs en BSiO2 e! taient dans le m#eme temps o1.5 mmol Si m
3 et 125 mmol Si m
2 sur l’ensemble de la zone d’!etude. A mesure que la couche de m!elange se stratifie et s’amincit en d!ecembre, les diatom!ees se d!eveloppent en r!eponse aux conditions lumineuses plus favorables, consommant dans le m#eme temps le stock de Si(OH)4. Fin de! cembre, les concentrations en BSiO2 e! taient exceptionnellement e! leve! es pour l’ oc!ean ouvert, avec des valeurs int!egr!ees maximales de 700 mmol m
2 vers 621S, alors que parall!element le stock de Si(OH)4 a diminu!e, passant de 2500 a" 870 mmol m
2, soit une diminution moyenne de la concentration de>30 mmol Si m
3. La floraison de diatome! es persiste courant janvier, avec des concentrations inte! gre! es en BSiO2 qui atteignent 600 mmol m
2 entre 641S et 651S, alors que les [Si(OH)4] sont o2 mmol m
3 dans les eaux de surface jusqu’"a 64.51S. Le centre du gradient de [Si(OH)4] s’est d!eplac!e vers le sud pour atteindre 651S en janvier, co.ıncidant avec les biomasses maximales en diatom!ees. La floraison se termine en mars, avec une disparition de la plus grande partie de la biomasse en diatom!ees, un approfondissement de la couche de m!elange et une augmentation de la [Si(OH)4]. Fin janvier, la production des diatom!ees a consomm!e >70% du stock de Si(OH)4 intialement pr!esent dans les 50 premiers m!etres partout au nord de 651S, et elle a entra#ın!e un d!eplacement du gradient de [Si(OH)4] de plus de 400 km vers le sud, alors que dans le m#eme temps le FPA restait centre! sur 611S. Entre novembre et fin janvier (B80 jours), une moyenne de 2200 mmol Si m
2 a e! t!e consomm!ee dans les 50 premiers m"etres entre 611S et 651S. Sur la base de la variation des concentrations int!egr!ees de Si(OH)4 et BSiO2 entre novembre et mars, une moyenne de 2 moles Si m
2 a e! te! exporte! e au dela" des 50 premiers me! tres entre 611S et 651S, avec 59% de cette exportation se produisant entre fin d!ecembre et fin janvier. L’exportation de BSiO2 observ!ee est tr"es e! lev!ee en comparaison des autres oc!ean, et celle-ci co.ıncide avec une zone ou" non seulement on enregistre de forts flux d’opale a" 1000 m, mais e! galement ou" les se! diments sont riches en opale. Au nord de 611S, l’exportation de BSiO2 e! tait limite! e par les faibles stocks initiaux de Si(OH)4 dans les eaux de surface, et au sud de 651S, l’exportation e! tait probablement limit!ee par une saison favorable a" la croissance qui est courte. Ainsi, les effets combin!es d’un stock initial important de Si(OH)4 en surface et de conditions permettant une utilisation efficace de cet acide silicique par les diatom!ees apparaissent comme e! tant la cause principale de la pre! sence de se! diments riches en opale dans le secteur Pacifique de l’Oce! an Austral.

1. Introduction Several types of single-celled oceanic organisms produce solid silica (SiO2) structures. Of these, diatoms are the only abundant primary producers that require silicon in order to grow, and are the major producers of biogenic silica (BSiO2) in the ocean (Lisitzin, 1972). Diatoms have an absolute requirement for dissolved Si, which they take up primarily in the form of silicic acid (Si(OH)4), in order to form new silicified cell walls. If there is insufficient Si dissolved in seawater to form a new frustule, diatom cell division will cease (Brzezinski et al., 1990), making Si a potentially limiting macronutrient for diatoms. Recent studies have

revealed ambient [Si(OH)4] can severely limit diatom Si-production and indicate diatom growth may be limited by Si availability in a variety of marine environments (e.g., Brzezinski and Nelson, 1996; Nelson and Dortch, 1996; Nelson et al., 2001). Diatoms are globally important primary producers, due to their ubiquitous and abundant distribution in most aquatic environments. As photosynthetic primary producers, diatoms require light and nutrients to synthesize organic compounds. Tre! guer et al. (1995) estimated that diatoms account for B40% of global oceanic primary production. Because of their relatively large size and dense frustules, diatoms are the

D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

major sources of inorganic and organic sinking flux in the ocean (Michaels and Silver, 1988; Buesseler, 1998). Diatoms also have high growth rates compared to other phytoplankton, especially under nutrient-replete conditions, and can subsequently out-grow other phytoplankton species, resulting in diatom blooms (Banse, 1982; Furnas, 1990; Tang, 1995). These blooms represent important events in the marine carbon cycle by episodically exporting BSiO2 and organic matter formed in the surface layer and supplying it to the deep ocean and sediments (Billet et al., 1983; Lampitt, 1985; Alldredge and Gotschalk, 1989; Buesseler, 1998). These properties make diatoms an important component in the marine carbon cycle. Diatoms are also the dominant phytoplankton and the primary source of deep-sea sediments around Antarctica (DeMaster, 1981). Southern Ocean sediments account for 25–50% of all biogenic silica burial worldwide (Tre! guer et al., 1995; DeMaster, 2002). Light conditions in the Southern Ocean are not favorable for phytoplankton growth most of the year due to short daylight hours, deep mixing, and ice cover. However, when conditions become favorable for growth, diatoms are able to exploit nutrient-replete conditions, and can quickly consume dissolved Si, converting it to BSiO2 (Brzezinski et al., 2001), which then supports the rapid export of Si from the surface layer to the deep ocean (Honjo et al., 2000). This process also exports diatom carbon from the surface layer to the deep ocean, making diatom blooms important in both the Si and C cycles of the Southern Ocean. The Southern Ocean includes both Antarctic and sub-Antarctic waters, where circulation is dominated by the eastward flowing Antarctic Circumpolar Current (ACC). The ACC is characterized by a series of embedded circumpolar physical and chemical fronts, which separate it from waters to the north, as well as delimiting different waters within the Southern Ocean (Belkin and Gordon, 1996). Sub-Antarctic waters are constrained by the Subtropical Front (STF) to the north and the Antarctic Polar Front (APF) to the south. Antarctic waters are found only south of the APF (Gordon, 1967). The Southern ACC

1749

Front (SACCF) separates coastal, Weddell Sea, and Ross Sea water from the ACC. The STF typically corresponds to a nitrate gradient, with values increasing to around 20 mmol m
3 south of the front, and depleted to o5 mmol m
3 in the warmer sub-tropical waters to the north (Lutjeharms et al., 1985; van Bennekom et al., 1988). The APF corresponds to a very strong [Si(OH)4] gradient in austral winter, with concentrations increasing to the south. It is not uncommon to observe an increase in [Si(OH)4] of 20– 30 mmol m
3 across 11 latitude at the APF in spring (Lutjeharms et al., 1985; van Bennekom et al., 1988; Brzezinski et al., 2001). In winter, seasonal ice cover can extend as far north as the southern edge of the APF in the Pacific sector (Moore et al., 1999). As the season progresses toward spring, light increases and the ice begins to retreat southward. By summer, the location of the [Si(OH)4] gradient can become uncoupled from the physical location of the APF, and begin to shift southward (LeJehan and Tre! guer, 1983; Brzezinski et al., 2001). The resupply of Si(OH)4 via increased mixing during winter replenishes the surface layer, restoring the gradient to the location of the APF. Resupply of nutrients is, however, a continual process since the ACC is a generalized area of upwelling (e.g., Comiso et al., 1993). It is not until biological uptake of the nutrients declines in fall or winter that net resupply to the surface layer occurs. The objective of this paper is to estimate seasonal Si(OH)4 removal and BSiO2 accumulation and export from the upper 50 m of the Southern Ocean along 1701W from 581 to 681S, based on an Si budget for the upper 50 m. This budget is constructed by calculating depth-integrated drawdown of Si(OH)4 and accumulation of BSiO2 over the growing season. These two components of the Si cycle are combined to provide an estimate of total surface-layer Si, and Si export from the surface layer based on changes in pool sizes between November 1997 and March 1998. The integrated export provides a minimum estimate of net seasonal Si export from the surface layer, and can be compared to other estimates of Si(OH)4 removal and BSiO2 export, including gross and net BSiO2 production rates, sediment

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D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

trap collection rates, 234Th-based export estimates, and seabed flux and accumulation rates.

2. Methods The data presented in this paper were collected on a series of four cruises beginning on 26 October 1997 and ending on 15 March 1998. The cruises took place in austral spring and summer, and each was B5 weeks long, with sample collection occurring for B4 of those weeks. Sampling was performed primarily in November on the first cruise; entirely during December on the second cruise; entirely during January on the third cruise; and from mid-February to midMarch on the fourth. Cruise tracks followed along 1701W and the latitudinal range of the cruises spanned from 531 to 71.51S, depending on the location and extent of sea-ice, as well as the type of cruise. This paper will focus on the area from 581 to 681S. Eight light depths were sampled at each station, corresponding to 85, 50, 25, 18, 10, 5, 1, and 0.1% of sea-surface irradiance. Samples were taken from the same sampling bottle for Si(OH)4 and BSiO2 concentrations, Si uptake rates, and silica dissolution rates. All of these measurements were made at every depth on all profile stations where light depth was known. Detailed methodology of sample collection, parameters measured, and experiments performed are detailed in Brzezinski et al. (2001) and Nelson et al. (2001). Station profile data and some surface data for [Si(OH)4] and BSiO2 were objectively contoured using Surfer 6.0. All contouring was constrained to the latitudinal range between 561S and the southernmost station sampled for each cruise. All grid files used for contouring had identical horizontal and vertical resolution. Therefore, the grid files for each cruise had the same number and distribution of grid points per meter of depth or degree latitude. The contours were visually inspected against measured concentrations to assure accurate representation of the data. Depth-integrated concentrations were calculated for Si(OH)4 and BSiO2 at every station between 581 and 681S for all four cruises. Values

for BSiO2 were integrated to the base of the euphotic zone, defined here as the depth where light was reduced to 0.1% of surface irradiance. Values for [Si(OH)4] were integrated between the surface and 50 m, because that was the deepest depth sampled at some stations. The only valid way to compare temporally offset, but spatially similar estimates of integrated Si(OH)4 is to integrate to a fixed and uniform depth for all stations and all times, and that uniform depth was limited by the shallowest euphotic zone (B50 m). However, BSiO2 was integrated to the base of the euphotic zone at each station in order to include biomass that was produced in the upper 50 m and sank to greater depths. The assumption that no significant biogenic silica was produced below 50 m is supported by very low 32Si production rates below 50 m (values below 50 m were always o0.075 mmol Si m
3 d
1, compared to values within the bloom of >1.5 mmol Si m
3 d
1, Brzezinski et al., 2001). In fact, for all stations with average BSiO2 concentrations >3.8 mmol m
3, Si production rates below 50 m averaged o10% of those above 50 m. Integrated BSiO2 concentrations and production rates for each station sampled are given in Brzezinski et al. (2001). The integrations reported here were performed by trapezoidal integration between the surface and either the base of the euphotic zone or 50 m. Integrated concentrations for each Si pool were binned into 11 latitude bands centered at each whole degree. Occasionally, there were no stations occupied inside a degree band during a given cruise; in those instances integrated values for each pool were interpolated by averaging the two closest stations (the closest one on either side of the latitude band in question). The two Si pools (Si(OH)4 and BSiO2) were then summed to obtain estimates of total Si present in the upper 50 for each degree band. Changes in total surface-layer Si were then used to calculate total Si export from the surface layer. Initial concentrations of BSiO2 at all degree bands under ice were assumed to be 25 mmol Si m
2, which corresponds to 0.5 mmol m
3 throughout the upper 50 m. This represents an average value for spring under-ice concentrations in the Weddell Sea (Nelson et al., 1987), and is

D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

probably an upper estimate for under-ice BSiO2 in the open ocean. Under-ice [Si(OH)4] was assumed to be 60–70 mmol m
3, or 3000–3500 mmol m
2. These values are typical of winter surface-layer concentrations in the ACC between 631 and 661S, according to concentrations in the temperature minimum layer (data available at www.usjgofs.whoi.edu). The temperature minimum layer between 631 and 661S was
1.71C, and was therefore not significantly altered from the freezing temperature at which this water was formed. A similar approach using nutrient concentrations in the temperature minimum layer (winter remnant water) has been used in the Weddell Sea (Jennings et al., 1984). Changes in the Si pools on each successive cruise in each degree band were used to estimate the removal, export or accumulation of Si(OH)4 or BSiO2 in the surface layer (Table 1–3). Changes in pool sizes were determined by subtracting the integrated Si pool size for a given month from the integrated pool size from the subsequent month, within the same latitude degree band. Therefore, negative changes indicate net loss, whereas positive changes indicate net gain in Si. Positive changes in the dissolved pool (resupply of Si(OH)4, which occurred only between January and March) were not used to estimate seasonal drawdown of Si(OH)4 or export of Si. Total surface-layer Si (defined as the sum of Si(OH)4 and BSiO2) was treated similarly; decreases in total Si were used to estimate total Si export from the surface layer.

1751

Decreases in total integrated Si were summed over the growing season, from November to March. The growing season is defined as the period during which we observed net production of BSiO2, and excludes periods before November or after March, when no data on BSiO2 production were obtained. These Si export estimates were used to compare net changes in total surface-layer Si with BSiO2 fluxes collected by sediment traps at 1000 m (Honjo et al., 2000). Therefore, the surfacelayer Si export estimate for each degree band was placed in latitudinal bins that most closely corresponded to the location of sediment traps located at 56.91, 60.21, 63.21, and 66.11S (Honjo

Table 2 Integrated biogenic silica concentrations and changes (all in mmol m
2) Latitude (1S) Concentration

Net changes

Nov Dec Jan Mar Dec–Nov Jan–Dec Mar–Jan 58 59 60 61 62 63 64 65 66 67 68

55 53 55 113 125 55 25 25 25 25 25

170 185 213 480 700 310 245 255 25 25 25

130 150 132 163 306 500 600 525 280 240 160

31 31 50 50 82 165 135 125 218 110 100

115 132 158 367 575 255 220 230 0 0 0


40
35
81
317
394 190 355 270 255 215 135


99
119
82
113
224
335
465
400
62
130
60

Table 1 Integrated silicic acid concentrations and changes (all in mmol m
2) Latitude (1S)

58 59 60 61 62 63 64 65 66 67 68

Concentration

Net changes

Seasonal removal

Nov

Dec

Jan

Mar

Dec–Nov

Jan–Dec

Mar–Jan

Amount

% of initial

550 725 850 1670 2500 3000 3250 3500 3500 3500 3500

375 390 410 920 870 2033 2650 2600 3500 3500 3500

56 80 158 148 230 450 780 1610 2550 2750 3130

240 230 330 390 520 760 900 1400 2790 3000 3300


175
335
440
750
1630
967
600
900 0 0 0


319
310
252
772
640
1583
1870
990
950
750
370

184 150 172 242 290 310 120
210 240 250 170

494 645 692 1522 2270 2550 2470 2100 950 750 370

90 89 81 91 91 85 76 60 27 21 11

D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

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Table 3 Total surface layer Si concentrations and changes (all in mmol m
2) Latitude (1S)

58 59 60 61 62 63 64 65 66 67 68

Total Si present

Net changes in total Si

Seasonal Si export

Nov

Dec

Jan

Mar

Dec–Nov

Jan–Dec

Mar–Jan

Amount

% of initial

605 778 905 1783 2625 3055 3275 3525 3525 3525 3525

566 575 623 1400 1570 2343 2895 2855 3525 3525 3525

186 230 290 311 536 950 1380 2135 2830 2990 3290

271 261 380 440 602 925 1035 1525 3008 3110 3400


39
203
282
383
1055
712
380
670 0 0 0


380
345
333
1089
1034
1393
1515
720
695
535
235

85 31 90 129 66
25
345
610 178 120 110

419 548 615 1472 2089 2130 2240 2000 720 560 260

69 70 68 83 80 70 68 57 20 16 7

et al., 2000). The latitudinal bins used to compare to the moored sediment traps were 551–581S (north of the APF), 591–611S (the APF), 621–651S (the seasonal ice zone of the ACC), and 661–681S (south of the SACCF). Therefore, the assigned latitudinal bins corresponded to distinct hydrographic zones, each of which contained one sediment trap mooring. The calculated changes in Si export for all latitudes within each bin were averaged to estimate the integrated flux of Si out of the surface layer for that latitude range (e.g., the average change between 621 and 651S is an average of the four latitude bands within the bin).

3. Results 3.1. Station profiles During the first cruise in November 1997, corresponding to austral spring, there was a pronounced meridional [Si(OH)4] gradient centered between 601 and 611S. This [Si(OH)4] gradient corresponded with the location of the APF. The southern edge of the APF, which remained relatively constant throughout the sampling period, was located between 611 and 61.51S (Moore et al., 1999). There were no vertical gradients in [Si(OH)4] observed within the upper 100 m in spring (Fig. 1a). By early summer there was clear evidence for Si(OH)4 removal. By

December the [Si(OH)4] gradient had shifted southward, and was centered between 621 and 631S. The gradient was stronger and more compressed than in November. Vertical gradients in [Si(OH)4] also became established between 611 and 631S, with concentrations increasing by B10 mmol m
3 between 40 and 50 m (Fig. 1b). During mid-summer (January 1998), severe Si(OH)4 depletion was evident, with surface concentrations o2 mmol m
3, occurring as far south as 64.51S. Consequently, the center of the [Si(OH)4] gradient was displaced farther south, to 651S, and was most compressed during this time period. Vertical gradients in [Si(OH)4] were also most pronounced during mid-summer, with the steepest gradients at 651S exceeding 30 mmol m
3 between 30 and 40 m (Fig. 1c). The [Si(OH)4] gradient was still located near 651S during the February/March cruise, but [Si(OH)4] had increased in the upper 80 m everywhere north of 651S. Silicic acid concentrations south of 661S remained relatively unchanged between January and March. Vertical [Si(OH)4] gradients were still strong in late summer between 641 and 661S, with concentrations increasing by >30 mmol m
3 between 40 and 60 m (Fig. 1d). There was very little diatom biomass in spring (November), with BSiO2 concentrations o1.5 mmol Si m
3 and o125 mmol m
2 throughout the study area. There were no vertical gradients of BSiO2 within the upper 100 m in the

D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

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Fig. 1. Contours of [Si(OH)4] in mmol m
3 from station profiles and surface sampling for each of the four cruises. The months during which the majority of sampling occurred are given. The dashed line denotes the southern edge of the Antarctic Polar Front.

Latitude (°S) Fig. 2. Contours of BSiO2 in mmol m
3 from station profiles and surface sampling for each of the four cruises. The months during which the majority of sampling occurred are given. The dashed line denotes the southern edge of the Antarctic Polar Front.

study area, similar to the [Si(OH)4] profiles (Fig. 2a). By December BSiO2 concentrations had increased significantly in the upper 80 m throughout the study area, with the highest concentrations located in the upper 40 m at 621S. The BSiO2 maximum was closely associated with

the meridional [Si(OH)4] gradient (e.g., compare Figs. 1b and 2b). By January the BSiO2 maximum had shifted much farther south and was centered around 651S. The BSiO2 maximum was still located within the [Si(OH)4] gradient, which had also shifted southward. The magnitude and spatial

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D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

extent of the diatom bloom were greatest during this time. Maximal BSiO2 concentrations around 651S exceeded 17 mmol m
3, and represents the highest open ocean concentrations ever measured. There was substantial BSiO2 present from the surface to 40–50 m in the core of the bloom, slightly deeper than the observed Si(OH)4 depletion, indicative of sinking diatoms (Fig. 2c). By the end of summer, the only area of high BSiO2 was in Ross Sea Gyre water centered on 711S, and the magnitude of the maximum was much less than concentrations observed further to the north in December or January. It is unlikely that the high diatom biomass at 711S is related to the diatom bloom observed within the ACC, due to the absence of elevated biomass or significant nutrient drawdown between 651 and 711S. The decrease in BSiO2 everywhere north of 681S between late January and March indicates that a large export in diatom biomass occurred during that time (Fig. 2d). Lithogenic silica (LSiO2) concentrations were very low throughout the study area on all cruises. Average LSiO2 concentrations for the November, December, and March cruises were 0.077, 0.085, and 0.063 mmol Si m
3, respectively. The range of LSiO2 concentrations during these three cruises was 0.01–0.55 mmol Si l
1. The average LSiO2 concentrations represented 12%, 2%, and 4% of the average BSiO2 concentrations in November, December, and March, respectively. Therefore, the lithogenic phase was always small compared to the biogenic phase, and comprised o4% of total particulate Si except in early spring, before significant diatom biomass had accumulated.

Interestingly, the differences in vertically integrated Si(OH)4 across the 21 of latitude that included the [Si(OH)4] gradient were very similar in November and January (with changes of 1650 and 1770 mmol Si m
2 between 601–621S and 641– 661S, respectively). This finding differs from the observation that the strongest gradient in surface layer [Si(OH)4] based on contouring of station profiles was in January (Fig. 1). This difference can be explained by the shallow depth (B30 m) of the nutricline in January. Thus, vertical integration of [Si(OH)4] included water well below the nutricline (>50 mmol m
3 Si(OH)4) in January, but not in December. Therefore, the steepest meridional [Si(OH)4] gradient in the upper 30 m occurred in January, but when integrated down to 50 m, the steepest meridional gradient occurred in December. The largest seasonal decreases in [Si(OH)4] (1522–2550 mmol m
2) occurred between 611 and 651S, with an average removal of B2200 mmol Si m
2 between November and January (Table 1). This magnitude of removal translates to a net [Si(OH)4] drawdown of 44 mmol m
3 between November and January, at an average rate of B27.5 mmol Si m
2 d
1 or 0.55 mmol m
3 d
1. Based on integrated concentrations, maximal net Si(OH)4 removal rates of B1 mmol m
3 d
1 were observed in the 641S latitude band between December and January (removal of 1870 mmol Si m
2 in the upper 50 m in 35–40 days).

3.2. Integrated [Si(OH)4] and BSiO2 Vertically integrated concentrations of Si(OH)4 reveal the strong meridional [Si(OH)4] gradient, which was initially centered on 611S, associated with the APF (Fig. 3). By the end of summer (March), this gradient had been displaced 41 south and was centered on 651S. The meridional [Si(OH)4] gradient, based on integrated concentrations, was steepest in December, with vertically integrated [Si(OH)4] increasing from 870 to 2033 mmol Si m
2 between 621 and 631S (Table 1).

Fig. 3. Vertically integrated [Si(OH)4] in the upper 50 m. Values are in mmol Si m
2.

D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

There was a fundamental change in biological activity seen by the February/March cruise. Vertically integrated [Si(OH)4] decreased throughout the area from November through January, but by March it had increased by 120–310 mmol m
2 in all but one latitude band, which was located at 651S (Table 1). The increases observed by March correspond to [Si(OH)4] increases of 2.4–6.2 mmol m
3 relative to January concentrations. A detailed summary of the concentrations and changes in integrated [Si(OH)4] between 581 and 681S is given in Table 1. The most striking feature apparent in this analysis is that an average of 82% (range of 54–91%) of the Si(OH)4 present in the upper 50 m during November was removed by the end of January, B80 days later, from 581 to 651S (Table 1). In contrast, the net depletion of Si(OH)4 over the same time period from further south between 661 and 681S, though substantial (370–950 mmol Si m
2), represents only 11–27% of the Si(OH)4 available in this region at the end of winter. Therefore, the amount of Si(OH)4 removed from the surface layer is largely limited by the initial Si(OH)4 pool size north, but not south of 651S. The reason for this transition is likely related to meridional changes in the length of the growing season and ice cover. North of 651S stratification occurred early enough to allow near complete consumption of the available Si(OH)4, while south of 661S the shorter growing season and ice cover likely set the upper limit on the amount and fraction of initial Si(OH)4 that was removed, and kept that fraction consistently o30%. The highest percentage removal of initial Si(OH)4 (91%) occurred within the 611 and 621S latitude bands, and that removal decreased consistently to the south, with the lowest percentage removal (11%) at 681S (Table 1). Integrated BSiO2 concentrations were highest within, or just north of the [Si(OH)4] gradient during December and January. The highest concentrations of BSiO2 occurred between 611 and 651S, where integrated values exceeded 475 mmol Si m
2 in December or January (Fig. 4 and Table 2). This latitudinal range therefore comprised the main bloom region, with much lower diatom biomass measured to the north and south. The largest spatial extent and magnitude of BSiO2

1755

Fig. 4. Vertically integrated BSiO2 in the euphotic zone (0.1% light depth). Values are in mmol Si m
2.

accumulation in the euphotic zone occurred between November and December, with an average net accumulation of 330 (range of 220– 575) mmol Si m
2. Assuming B40 days between samplings within a given latitude bin results in an average BSiO2 accumulation rate of 8.25 mmol m
2 d
1 between 611 and 651S, which represents B30% of the observed Si(OH)4 drawdown in the same area. By January BSiO2 increases were less, averaging 237 mmol m
2, and occurred only south of 621S, with losses from the surface layer between 581 and 621S. The maximal integrated BSiO2 accumulation rate of 0.3 mmol Si m
3 d
1 was observed at 621S between November and December (575 mmol Si m
2 in B50 m over 40 days). The greatest export of BSiO2 from the euphotic zone occurred between January and March, with decreases observed everywhere between 581 and 681S (Table 2). Maximal seasonal export of BSiO2, averaging 450 mmol m
2, was observed between 611 and 651S, the same zone where the maximal Si(OH)4 drawdown was observed. However, these BSiO2 export estimates consider only changes in the particulate Si pool size over time. Biogenic silica was certainly being produced and exported from the surface layer throughout the growing season, and therefore these estimates of export based on net changes are minimal estimates. A detailed summary of BSiO2 concentrations and changes is found in Table 2.

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D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

Due to the biological conversion of Si(OH)4 to BSiO2, net Si(OH)4 removal over time can be used to estimate BSiO2 production. Furthermore, since BSiO2 is exported from the surface layer, the relationship between net Si(OH)4 removal and BSiO2 accumulation can be used to estimate the efficiency of BSiO2 export. Thus, the ratio of BSiO2 accumulation to Si(OH)4 removal over the same time period is a measure of the amount of removal that is still present in the surface layer as BSiO2. The relationship between BSiO2 accumulation and Si(OH)4 removal within each latitude bin between 581 and 681S was tested by linear regression analysis (Fig. 5). The data presented in Fig. 5 are totals for each 11 latitude band between successive cruises during which both net Si(OH)4 removal and BSiO2 accumulation occurred. Net BSiO2 accumulation and Si(OH)4 removal between November and December occurred only from 581 to 651S (thus, 8 data points). However, both net accumulation and removal from December to January occurred only between 631 and 681S (6 data points). No BSiO2 accumulation occurred between January and March from 581 to 681S. This analysis reveals a positive linear relationship between Si(OH)4 removal and BSiO2 accumulation between November and December, with an R2 value of 0.843 and a slope of 0.3 (s.d.=0.05, po0:001). There is no significant relationship between removal and accumulation

Fig. 5. Regressions of vertically integrated BSiO2 accumulation to Si(OH)4 removal between successive cruises during which both BSiO2 increased and [Si(OH)4] decreased from 581 to 681S. Values are in mmol Si m
2.

between January and March, with an R2 value of 0.48 and a slope of 0.1 (s.d.=0.05, p ¼ 0:126), indicating that the slope of the line is indistinguishable from zero. The slopes of the two regression lines were significantly different from each other at the 95% level (Z ¼ 1:72; p ¼ 0:043). A 1:1 relationship (slope=1) between these two variables would mean that all of the observed removal of Si(OH)4 was measured as BSiO2 accumulation. However, the relationship between removal and accumulation between November and December indicates that only 30710% (slope=0.3) of the Si(OH)4 removed from the upper 50 m during that time period was still present as accumulated BSiO2 in December. Between December and January, only 10710% of the observed removal during that time period was still present as BSiO2. This analysis indicates that the other 70% and 90% of Si(OH)4 removed from November to December and from December to January, respectively, were exported as BSiO2 during that same time period. This analysis also indicates that the retention of diatom biomass in this system decreases during the growing season, with increased export in late summer compared to early summer. 3.3. Surface layer Si export Total Si is equal to the sum of the dissolved and particulate Si pools. Net Si export estimates are the changes in total surface-layer Si over time. Both total Si pool size and Si export are presented for each cruise and as the differences between subsequent cruises (Table 3). Since export is a loss term, decreases in both pools lead to larger export estimates than if one pool decreased while the other increased or remained unchanged. This method uses the initial conditions measured in November to estimate the initial total Si pool size at the beginning of the growing season. Since the dissolved pool accounted for the majority of total Si throughout the study period, export estimates closely matched changes in integrated [Si(OH)4], with an average export of B2000 mmol Si m
2 by late January between 611 and 651S (Fig. 6, Table 3). More than 57% of that seasonal export, or 1150 mmol m
2, occurred between early

D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

Fig. 6. Vertically integrated export of Si from the upper 50 m in mmol m
2 and the percentage of initial Si present in November these export values represent.

December and late January. The region of highest Si export from the surface layer matches the bloom region as expected. The seasonal (November– March) total Si export accounted for an average of 71% (range of 57–83%) of the initial total Si in the surface layer between 581 and 651S, but only 15% between 661 and 681S (Fig. 6). A detailed summary of total Si and Si export is given in Table 3.

4. Discussion Silicic acid and its dissociation products account for nearly 100% of dissolved silicon in seawater (Stumm and Morgan, 1981) and particulate silica occurs only in biogenic and lithogenic forms. Therefore, the biogeochemical cycle of Si in the ocean is relatively simple, especially compared to other nutrient cycles. Lithogenic concentrations during this study were generally o4% of total particulate Si in the euphotic zone, and therefore contribute very little to particulate Si concentrations. Consequently, the components of the Si cycle can be further simplified into Si(OH)4 and BSiO2, which comprised B100% and B95% of the dissolved and particulate Si pools, respectively, in this study. Therefore, interpretations of changes in either component of the Si cycle are more robust than if other unknown or unquantified forms, such as dissolved organic Si existed.

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Silicic acid removal in the upper 80 m propagated southward from November to January, creating a strong, southward-moving meridional gradient, and concentrations were depleted to o2 mmol m
3 in surface waters down to 651S over this time. Most removal was confined to the upper 40–60 m, with deepest removal observed north of 621S. Net surface-layer Si(OH)4 removal was greatest between December and January, with maximal removal rates of B1.0 mmol m
3 d
1. That rate is B75% of maximal gross Si production rates based on 32Si experiments measured in the peak of the bloom (Brzezinski et al., 2001). Therefore, net Si(OH)4 drawdown is consistent with the measured uptake of Si(OH)4 and subsequent production of BSiO2 by diatoms in this area. Based on changes in integrated [Si(OH)4] between 611 and 651S, an average of 2200 mmol Si m
2 was removed from the upper 50 m between November and January, corresponding to an average [Si(OH)4] decrease of 44 mmol m
3. In comparison, nitrate concentrations in the upper 50 m during this time only decreased by B6 mmol m
3 in the same area (Morrison et al., 2001). Therefore, net seasonal drawdown of Si(OH)4 was B7 times greater than the net drawdown of nitrate in the same area. Over 98% of the seasonal net Si(OH)4 removal had occurred in the study area by late January, and by March significant amounts of Si(OH)4 had been resupplied to the system, primarily via mixing with nutrient-rich subsurface waters. Si(OH)4 resupply via frustule dissolution (Brzezinski et al., 2001), mixing, and upwelling is continually occurring in most of the Southern Ocean, but the biological uptake of Si(OH)4 by diatoms was much greater than the sum of the resupply terms between November and January. The decreases in biological uptake late in the growing season, combined with more intense wind-driven mixing in early February, caused the supply of Si(OH)4 to the surface layer to exceed biological demand. Biogenic silica concentrations and changes corresponded well with both vertical and horizontal [Si(OH)4] gradients. However, the largest magnitude and spatial extent of increases were observed in December, whereas most of the

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D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

Si(OH)4 removal was observed in January. BSiO2 accumulated in significant amounts only south of 611S, and was therefore constrained to areas that had initial [Si(OH)4]>30 mmol m
3 (see Fig. 1a). The high BSiO2 concentrations measured during December and January are the highest open ocean values ever measured. Maximal BSiO2 accumulation rates observed during the study period were 0.3 mmol Si m
3 d
1, or about one third of the observed maximal Si(OH)4 removal rates. This finding is consistent with the fact that not all Si(OH)4 removal is retained in the surface layer as BSiO2, due to vertical export from the euphotic zone. The large spatial and temporal extent of the diatom bloom is illustrated by concentrations of BSiO2>475 mmol m
2 during the height of the growing season (December–January) from 611 to 651S. Changes in vertically integrated BSiO2 and Si(OH)4 can be used to estimate the fraction of net Si(OH)4 consumption that is still present in the euphotic zone as BSiO2. There was a change in the ratio of removal to accumulation within the growing season. As would be expected in spring bloom scenarios, a greater proportion of Si(OH)4 decrease was retained in surface waters as BSiO2 accumulation early in the season. Specifically, B30% of Si(OH)4 removal observed between November and December remained as accumulated BSiO2 in the euphotic zone. However, much less (B10%) of the observed Si(OH)4 depletion between December and January was measured as an increase in BSiO2. This is due to a combination of differences in removal and accumulation over this time period. The fact that high diatom biomass was already present in the surface layer minimized the observed net accumulation of BSiO2 between December and January. In addition, much more Si(OH)4 was removed between December and January, suggesting greater removal of BSiO2 from the surface layer later in summer via advection or sinking (including grazing and aggregation). Increased flux later in the growing season was also indicated by 234Th-based BSiO2 export from 100 m (Buesseler et al., 2001). Greater nutrient removal relative to accumulated biomass is also consistent with spring bloom scenarios, as heterotrophic processes and nutrient

depletion often lead to increased export late in the season. Total Si in the bloom region from 611 to 651S decreased by an average of 2000 mmol m
2 between November and March, 93% of which occurred by late January, and provides our best estimate of net Si export from the surface layer. Of this seasonal export, 57% or 1150 mmol Si m
2 occurred between December and January. The large Si export that occurred in B80 days indicates that diatom biomass rapidly sinks out of the surface layer, for BSiO2 is the only loss term for Si in the system. Si export from the surface layer based on these mass balance calculations represented an average of 71% of the initial Si present in the surface layer between 581 and 651S (see Fig. 6). Therefore, the Southern Ocean north of 661S is a highly efficient Si exporting system, with most of the surface-layer Si(OH)4 being exported as BSiO2 in less than 3 months. The magnitude of the seasonal Si export estimates (given in Table 3), based on changes in Si pools, are minimum estimates of annual export. The above analysis considers only net seasonal changes in the dissolved and particulate pools that were actually observed between the first and last cruise. Therefore, Si(OH)4 removal and BSiO2 production before November or after March is not quantified or included in this analysis. Furthermore, Si(OH)4 consumption is not quantified between late January and March, due to the net resupply of Si(OH)4 in the surface layer. Additionally, Si(OH)4 was also being resupplied to the surface layer through upwelling, diffusion, and episodic entrainment of water from below the mixed layer, each of which diminishes the observed net export. The absolute magnitude of Si(OH)4 drawdown and total Si export could thus be significantly higher than the calculated net changes, especially in the permanently ice-free degree bands (north of 621S). Nonetheless, the area of the Pacific sector of the Southern Ocean between 611 and 651S has extremely high Si(OH)4 removal and BSiO2 production and export, and is likely responsible for the opal-rich sediments found beneath the southern ACC. This conclusion is very similar to that reached by Pondaven et al. (2000) in the Indian sector of the Southern Ocean.

D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

4.1. Comparisons to other Si measurementsFgross production, flux, and burial The estimates of Si(OH)4 removal presented here compare very well with estimates of gross BSiO2 production in the same area over the same time period. Gross BSiO2 production, measured from 32Si uptake experiments, represents the rate at which Si(OH)4 is converted to BSiO2, before dissolution. Estimates of gross BSiO2 production between 621 and 651S averaged 2960 mmol m
2 (Brzezinski et al., 2001), compared to 2350 mmol m
2 of net Si(OH)4 removed from the upper 50 m. Assuming a constant dissolution rate of 6.5 mmol Si m
2 d
1 (given in Brzezinski et al., 2001) for the 80 days during which the seasonal drawdown was calculated, then net BSiO2 production (gross production–dissolution) calculated from tracer experiments would be 2440 mmol Si m
2. This estimate of net BSiO2 production obtained from tracer experiments is indistinguishable from the estimate based on observed net Si(OH)4 removal given the uncertainties inherent in vertical integration of Si standing stock and rate parameters. Furthermore, the gross BSiO2 production estimate is integrated to the base of the euphotic zone, and therefore includes areas of both high and low rates of Si uptake and dissolution. Our net Si removal estimate is confined to the upper 50 m, where most of the Si(OH)4 removal occurred, but obviously misses some areas of measurable drawdown. What is clear from this comparison is that measured rates of Si uptake are of almost exactly the right magnitude to support the observed removal of Si(OH)4 from the upper 50 m. Therefore, it is our conclusion that both methods have accurately quantified the large scale removal of Si(OH)4 from the surface layer of the Southern Ocean. A comparison of net Si(OH)4 removal rates and the subsequent export of BSiO2 can also be made with data collected in the Indian sector of the Southern Ocean. Pondaven et al. (2000) constructed a Si budget based only on seasonal [Si(OH)4] removal relative to winter remnant water concentrations over a 90 day growing season. As was discovered in our study, the region with the most net Si(OH)4 drawdown was located just

1759

south of the APF. They estimated that 2310 mmol Si m
2 was removed from the surface layer in this region during the summer growing season. This estimate is almost exactly the same as our net Si(OH)4 removal estimate of 2350 mmol m
2 in the region just south of the APF (621–651S) (Table 1). Such close agreement is surprising given the different hydrographic regimes south of the APF in the Pacific and Indian sectors of the ACC. In our study area the seasonal ice zone approaches the APF, whereas in the Indian sector, there is a large area of permanently open ocean between the APF and the seasonal ice zone. Therefore, the high production zone in our study area is influenced by melt-water stratification associated with the retreating ice edge. However, the Indian sector studied by Pondaven et al. (2000) has no such mechanism for stabilizing and shoaling the mixed layer. Yet it is apparent that in the hydrodynamic zone of the ACC directly south of the APF in both the Indian and Pacific sectors, seasonal diatom growth removes similar and large amounts of Si(OH)4 from the surface layer, and that this material is exported to the deep sea. Total Si export estimates from the upper 50 m can be compared to BSiO2 flux estimates made on similar time and space scales along 1701W. Flux of BSiO2 out of the upper 100 m was estimated using 234 Th (Buesseler et al., 2001). The 234Th flux data were obtained on each of the four cruises, and were therefore obtained for the same water masses during the same time period as the data used for the 50 m export estimates. Sediment trap collections were used to estimate BSiO2 fluxes to 1000 m (Honjo et al., 2000). Unfortunately, the sediment trap data record ended in January 1998, and therefore did not cover all of our sampling period. In fact, the majority of sediment trap data were from the previous growing season (1996–97). However, there is a record for over an entire annual cycle of fluxes. Assuming that fluxes are comparable from year to year and that the seasonal patterns of production and flux are similar interannually, the sediment trap fluxes can be used for comparison to our surface layer export estimates. BSiO2 flux to the seabed and burial in the sediments were measured at seven stations throughout the study area (Sayles et al.,

D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

1760

2001). Estimates of Si burial in sediments consider much longer time scales than processes measured in the water column, and therefore represent a long-term mean burial rate. BSiO2 export from the upper 50 m provides only a seasonal estimate of export since no constraints on Si production have been placed on time periods outside the sampling period. However, based on low Si production rates in November and March and low [Si(OH)4] and BSiO2 in surface waters by March, we can project that most of the production and export of BSiO2 occurred during our sampling period, especially south of the APF. Therefore, our seasonal estimates of Si export are assumed to be essentially equivalent to annual rates. All other flux estimates have been extrapolated to annual rates, and the values presented here (Table 4) were taken from Nelson et al. (2001). To compare the different rate estimates all Si export estimates were placed into four latitudinal bins, which spanned each sediment trap mooring spanning different hydrographic regimes. The northernmost latitude range was the sub-Antarctic zone (551–581S), the next was in the APF zone (591–611S), the third was in the southern ACC zone (621–651S), and the fourth was south of the SACC Front in the Ross Sea Gyre zone (661–681S). The Si export estimates for the described regions are presented in Table 4. Our estimates of Si export from the upper 50 m reveal that the southern ACC zone between 621 and 651S had the highest export rates. All other flux estimates of Si, including 100 m, 1000 m, seabed

fluxes, and burial in the seabed, confirm that BSiO2 export was greatest in this zone. In fact, the Si export from 50 m, flux to 1000 m and the benthos, and Si burial were more than twice as high in the southern ACC than in any other hydrographic region. Most of the southern ACC along 1701W is seasonally covered by sea ice, and it is this region where diatom biomass and production were greatest. Export based on 234Th fluxes indicated equally high values within the APF and the southern ACC, and both of these regions were more than twice as great as export estimates to the north or south. Si fluxes in the sub-Antarctic zone and the Ross Sea Gyre zone were very similar within each type of flux estimate, and represented the lowest Si export fluxes in the study area. In general, the 50 m, 100 m, 1000 m, and seabed export estimates all agree well. Even though some of the export estimates were made over very different time scales and even for different years, the overall trend is very clear, and supported by all flux estimates. Conditions within the APF could help to explain some of the discrepancies between the 234 Th fluxes and the other flux estimates in this region. The APF is an area of elevated horizontal and vertical velocities, relative to areas to the north and south (Barth et al., 2001). Therefore, the physics of the APF could have resulted in much more flux out of the upper 100 m, as evidenced by 234 Th fluxes, than could be measured as net Si removal or sedimentation to 1000 m. Specifically, the highly energetic APF zone would have higher

Table 4 Comparison of BSiO2 fluxes based on changes in total surface layer Si, 234Th fluxes, sediment trap data, flux to and burial in the seabed (all in mmol Si m
2) Annual flux toAnnual Si Seasonal Annual Th- Annual burial in export (50 m) based export sediment trap seabedn flux (1000 m)n sedimentsn (100 m)n

Latitude range (1S)

Mooring

Southern ocean subsystem

55–58 59–61 62–65

M2 M3 M4

66–68

M5

Sub-Antarctic 316 PFZ 878 Southern 2109 ACC RS Gyre 513

n

Given in Nelson et al. (this issue).

% of 50 m export reaching 1000 m

640 1420 1400

281 379 940

284 337 1325

16 48 163

89 43 45

340

206

252

16

40

D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

rates of Si(OH)4 resupply through mixing and upwelling, as was observed in December, and the higher current velocities would also transport sinking material much farther than less energetic areas to the north and south. Therefore, it is possible that the 50- and 1000-m flux estimates missed significant amounts of BSiO2 export in the APF region. Given that the APF and areas to the north are permanently ice-free, diatom production is likely to begin earlier and last longer than in areas that are seasonally ice covered. Therefore, significant amounts of BSiO2 production and export could occur that were not directly measured as changes in total Si. It is thus likely that the 50 m export estimates in the sub-Antarctic and APF zones significantly underestimate annual export due to periods of export outside the November to March sampling period. More specifically, initial total Si in the upper 50 m (as determined during the November cruise) was likely an underestimate of total Si at the beginning of the growing season within and north of the APF, and therefore caused the seasonal Si(OH)4 removal, and hence export, to be underestimated. The underestimation of 50 m BSiO2 export is most prominent in the subAntarctic zone, where [Si(OH)4] was already o10 mmol m
3 in November. BSiO2 flux to 1000 m was between 40% and 45% of our seasonal 50 m Si export estimates, except for the northernmost sub-Antarctic region. In the sub-Antarctic region, the 1000 m flux estimate was 89% of our estimated BSiO2 export from 50 m. This percentage of export caught in sediment traps is very high for open ocean systems, and likely reflects an underestimation of annual 50 m export (see above). Other than the underestimate of 50-m export in the sub-Antarctic region, export based on net changes in Si compare very well with sediment trap fluxes to 1000 m, accumulation at the seafloor, and burial in the sediments. An average of 43% of BSiO2 exported from the upper 50 m reaches 1000 m, and B50% of the 50 m export reaches the seafloor. These findings imply rapid sinking of diatom BSiO2 through the water column, with essentially 100% preservation of sinking BSiO2 between 1000 m and the seafloor.

1761

Based on the 50-m export and sediment burial rates, burial efficiency estimates, defined as the percentage of 50-m export that is buried, were calculated. An average of 5.3% of Si exported from the upper 50 m is buried in sediments beneath the ACC along 1701W between 581 and 681S. The southern ACC zone had the highest burial efficiency at 7.7% and the Ross Sea Gyre zone had the lowest at 3.1%. The 7.7% burial efficiency estimate in the southern ACC along 1701W is very similar to the 9.1% efficiency estimated by Pondaven et al. (2000) for the region south of the APF in the Indian sector of the Southern Ocean. Therefore, the hydrographic region just south of the APF supports very similar Si flux out of the surface layer, as well as burial in the deep sea sediments, in two different areas of the Southern Ocean. Furthermore, these burial efficiencies are only slightly higher than the global average of 5%, whereas the estimates of BSiO2 production and export in the Southern Ocean are B6 times higher than the global average (Tre! guer et al., 1995). This analysis indicates that BSiO2 preservation is not very different from the global mean in the two areas of the ACC for which export estimates are available. Thus it is the extremely high production and export of BSiO2 that are likely responsible for the accumulation of opal sediments beneath the ACC. High diatom biomass develops only in regions of high initial [Si(OH)4], and is therefore limited to waters south of the APF. Apparently, high concentrations of BSiO2 are also limited to waters north of the SACCF, at B651S. North of 611S the available Si(OH)4 is exported efficiently, but the initial standing stock of Si limits BSiO2 production and therefore export. The fact that high [Si(OH)4] occur south of 651S throughout the study period indicates that other factors, such as a short growing season, limit diatom production, resulting in low BSiO2 export that amounts to only a small fraction of the available Si. The region between the APF and the SACCF in the Pacific sector experiences large seasonal variations in mixed layer depths, due to the stabilizing influence of melt water from the receding sea ice (mixed layer data posted at www.usjgofs.whoi.edu), which were likely instrumental in facilitating the high

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D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763

accumulation and subsequent export of BSiO2 between 611 and 651S. A robust interpretation of the data presented here is that the very high flux of diatom BSiO2 from the surface layer, and not high burial efficiency, is responsible for the high burial rates of Si in Southern Ocean sediments, as compared to abyssal sediments in other oceanic environments. This interpretation is similar to the conclusion reached by Pondaven et al. (2000) for the Indian sector of the Southern Ocean. Specifically, it is the co-occurrence of high winter [Si(OH)4], high BSiO2 production, and efficient export of the BSiO2 that is unique to the southern ACC of the Southern Ocean, and the primary cause of the vast opal belt surrounding Antarctica.

Acknowledgements We would like to thank Valerie Franck, Julie Arrington, and Bill Golden for their admirable efforts to help collect samples and data for this project. We also thank the crew of the R./V. Revelle for allowing us to perform our work as smoothly as possible. This work was funded by the National Science Foundation through grants OPP-9530677 and OCE-9911312 to Oregon State University and OPP-9531982 to the University of California, Santa Barbara.

References Alldredge, A.L., Gotschalk, C.D., 1988. In situ settling behavior of marine snow. Limnology and Oceanography 33, 339–351. Banse, K., 1982. Cell volumes, maximal growth rates of unicellular algae and ciliates, and the role of ciliates in the marine pelagial. Limnology and Oceanography 27, 1059–1071. Barth, J.A., Cowles, T.J., Pierce, S.D. Mesoscale physical and bio-optical structure of the Antarctic Polar Front near 1701W during spring. Journal of Geophysical Research 106, 13879–13902. Belkin, I.M., Gordon, A.L., 1996. Southern Ocean fronts from the Greenwich meridian to Tasmania. Journal of Geophysical Research 101, 3675–3696. Billet, D.S.M., Lampitt, R.S., Rice, A.L, Mantoura, R.F.C., 1983. Seasonal sedimentation of phytoplankton to the deep sea benthos. Nature 302, 520–522.

Brzezinski, M.A., Nelson, D.M., 1996. Chronic substrate limitation of silica production in the Sargasso Sea. DeepSea Research II 43, 437–453. Brzezinski, M.A., Olson, R.J., Chisholm, S.W., 1990. Silicon availability and cell-cycle progression in marine diatoms. Marine Ecology Progress Series 67, 83–96. Brzezinski, M.A., Nelson, D.M., Franck, V.M., Sigmon, D.E., 2001. Silicon dynamics within an intense open-ocean diatom bloom in the Pacific sector of the Southern Ocean. Deep-Sea Research II 48, 3997–4018. Buesseler, K.O., 1998. The decoupling of production and particulate export in the surface ocean. Global Biogeochemical Cycles 12, 297–310. Buesseler, K.O., Ball, L., Andrews, J., Cochran, J.K., Hirschberg, D.J., Bacon, M.P., Fleer, A., Brzezinski, M., 2001. Upper ocean export of particulate organic carbon and biogenic silica in the Southern Ocean along 1701W. DeepSea Research II 48, 4247–4273. Comiso, J.C., McClain, C.R., Sullivan, C.W., Ryan, J.P., Leonard, C.L., 1993. Coastal zone color scanner pigment concentrations in the Southern Ocean and relationship to geophysical surface features. Journal of Geophysical Research 98, 2419–2451. DeMaster, D.J., 1981. The supply and accumulation of silica in the marine environment. Geochimica et Cosmochimica Acta 45, 1715–1732. DeMaster, D.J., 2002. Continental margin biogenic silica accumulation: coupling of the marine cycles of organic matter and biogenic silica. Deep-Sea Research II in press. Furnas, M., 1990. In situ growth rates of marine phytoplankton: approaches to measurement, community and species growth rates. Journal of Plankton Research 12, 1117–1151. Gordon, A.L., 1967. Structure of Antarctic waters between 201W and 1701W. In: Bushnell, V.C. (Ed.), Antarctic Map Foli Service folio 6. American Geographical Society, New York. 10pp. Honjo, .S, Collier, R., Francois, R., Manganini, S., Dymond, J., 2000. Particle fluxes to the interior of the Southern Ocean in the western Pacific sector along 1701W. Deep-Sea Research II 15–16, 3521–3548. Jennings Jr, J.C., Gordon, L.I., Nelson, D.M., 1984. Nutrient depletion indicates high primary productivity in the Weddell Sea. Nature 308, 51–54. Lampitt, R.S., 1985. Evidence for the seasonal deposition of detritus to the deep-sea floor and its subsequent resuspension. Deep-Sea Research I 32, 885–897. LeJehan, S., Tr!eguer, P., 1983. Uptake and regeneration Si/N/P ratios in the Indian sector of the Southern Ocean: originality of the biological cycle of silicon. Polar Biology 2, 127–136. Lisitzin, A.P., 1972. Sedimentation in the World Ocean. Society Economic Paleontologists and Mineralogists. Special Publication #17, 218pp. Lutjeharms, J.R.E., Walters, N.M., Allanson, B.R., 1985. Oceanic frontal systems and biological enhancement. In: Siegfried, W.R., Condy, P.R., Laws, R.M. (Eds.), Antarctic Nutrient Cycles and Food Webs. Springer, Berlin, pp. 11–21.

D.E. Sigmon et al. / Deep-Sea Research II 49 (2002) 1747–1763 Michaels, A.F., Silver, M.W., 1988. Primary production, sinking fluxes and the microbial food web. Deep-Sea Research I 35, 473–490. Moore, J.K., Abbott, M.R., Richman, J.G., 1999. Location and dynamics of the Antarctic Polar Front from satellite sea surface temperature data. Journal of Geophysical Research 104, 3059–3073. Morrison, J.M., Gaurin, S., Codispoti, L.A., Millero, F.J., Takahashi, T., Gardner, W.D., Richardson, M.J., 2001. Seasonal evolution of the hydrographic properties during the US JGOFS AESOPS expedition to the Antarctic Polar Frontal Zone, 1997–1998. Deep-Sea Research II 48, 3943–3972. Nelson, D.M., Dortch, Q., 1996. Silicic acid depletion and silicon limitation in the plume of the Mississippi River: evidence from kinetic studies in spring and summer. Marine Ecology Progressive Series 136, 163–178. Nelson, D.M., Smith Jr., W.O., Gordon, L.I., Huber, B.A., 1987. Spring distributions of density, nutrients and phytoplankton biomass in the ice-edge zone of the Weddell-Scotia Sea. Journal of Geophysical Research 92, 7181–7190. Nelson, D.M., Brzezinski, M.A., Sigmon, D.E., Franck, V.M., 2001. A seasonal progression of Si limitation in the Pacific sector of the Southern Ocean. Deep-Sea Research II 48, 3973–3995.

1763

Nelson, D.M., et al. Vertical budgets for organic carbon and biogenic silica in the Pacific sector of the Southern Ocean, 1996–1998. Deep-Sea Research II 49, 1645–1673. Pondaven, P., Ragueneau, O., Treguer, P., Hauvespre, A., Dezileau, L., Reyss, J.L., 2000. Resolving the ‘opal paradox’ in the Southern Ocean. Nature 405, 168–172. Sayles, F.L., Martin, W.R., Chase, Z., Anderson, R.F., 2001. Benthic remineralization and burial of biogenic SiO2, CaCO3, and organic carbon and detrital material I the Southern Ocean along a transect at 1701W. Deep-Sea Research II 48, 4323–4383. Stumm, W., Morgan, J.J., 1981. Aquatic Chemistry, 2nd Editioin. Wiley, New York, 780pp. Tang, Y., 1995. The allometry of algal growth rates. Journal of Plankton Research 17, 1325–1335. Tr!eguer, P., Nelson, D.M., van Bennekom, A.J., DeMaster, D.J., Leynaert, A., Qu!eguiner, B., 1995. The silica balance in the world ocean: a re-estimate. Science 268, 375–379. van Bennekom, A.J., Berger, G.W., van der Gaast, S.J., de Vries, R.T.P., 1988. Primary productivity and the silica in the Southern Ocean (Atlantic sector). In: Olansson E. (Ed.), The Polar Ocean: the Antarctic: Present and Past; Palaeogeography Palaeoclimatology, Palaeoecology, Special Issue 66, 19–30.