Particle fluxes to the interior of the Southern Ocean in the Western Pacific sector along 170°W

Deep-Sea Research II 47 (2000) 3521}3548

Particle #uxes to the interior of the Southern Ocean in the Western Paci"c sector along 1703W Susumu Honjo *, Roger Francois , Steven Manganini , Jack Dymond
, Robert Collier
Woods Hole Oceanographic Institution Woods Hole, MA 02543, USA
Oregon State University, Corvallis, OR 97331, USA Received 3 March 1999; accepted 15 April 2000

Abstract An array of "ve bottom-tethered moorings with 19 PARFLUX time-series sediment trap at three depths (1 and 2 km below the surface, and 0.7 km above the sea-#oor) was deployed in the western Paci"c sector of the Southern Ocean, along 1703W. The "ve stations were selected to sample settling particles in the main hydrological zones of the Southern Ocean. The sampling period spanned 425 days (November 28, 1996}January 23, 1998) and was divided into 13 or 21 synchronized time intervals. A total of 174 sequential samples were recovered and analyzed to estimate #uxes of total mass (TMF), organic carbon, carbonate, biogenic silica, and lithogenic particles. The #uxes of biogenic material were higher than anticipated, challenging the notion that the Southern Ocean is a low-productivity region. Organic carbon #uxes at 1 km depth within the Polar Frontal Zone and the Antarctic Zone were relatively uniform (1.7}2.3 g m\ yr\), and about twice the estimated ocean-wide average (ca. 1 g m\ yr\). Carbonate #uxes were also high and uniform between the Subantarctic Front and ca. 643S (11}13 g m\ yr\). A large fraction of the carbonate #ux in the Antarctic Zone was due to the presence of pteropod shells. Coccoliths were found only to the north of the Polar Front, and calcium carbonate became the dominant phase in the Subantarctic Zone. In contrast, carbonate particles were nearly absent near 643S. Latitudinal variations in biogenic silica #uxes were substantial. The large opal #ux (57 g m\ yr\) measured in the Antarctic Zone suggests that opal productivity in this region has been previously underestimated and helps to explain the high sedimentary opal accumulation often found south of the Polar Front. Unlike biogenic material, #uxes of lithogenic particles were among the lowest measured in the open-ocean (0.12}0.05 g m\ yr\), re#ecting a very low dust input.  2000 Elsevier Science Ltd. All rights reserved.

* Corresponding author. 0967-0645/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 0 ) 0 0 0 7 7 - 1

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S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

1. Introduction The Southern Ocean, located between the Subtropical Front (STF), at approximately 453S, and the Antarctic continent (e.g., Gordon et al., 1977, 1981; Deacon, 1982), is a critical locus for the exchange of CO between the atmosphere and the much  larger pool of dissolved inorganic carbon sequestered in the deep ocean (e.g., Takahashi et al., 1991). As a result, biogeochemical changes a!ecting the carbon balance in the upper water column of this region could signi"cantly in#uence the level of atmospheric CO and potentially impact global climate. Indeed, several models  that call either for higher export #uxes of carbon or reduced vertical mixing in the Southern Ocean have been proposed to explain the decrease in atmospheric CO  during the last glacial maximum (e.g., Knox-Ennever and McElroy, 1985; Wenk and Siegenthaler, 1985; Toggweiler and Sarmiento, 1985; Kumar et al., 1995; Francois et al., 1997). Likewise, environmental changes that could result from a warming trend (e.g., changes in the extent of sea ice) could a!ect the carbon cycle of the Southern Ocean, potentially triggering poorly understood feedback mechanisms with as yet unforeseeable consequences. This concern is further exacerbated by predictions from GCM models that greenhouse-induced warming would be more pronounced at high latitudes (Manabe et al., 1990), and by our lack of understanding of the processes controlling the magnitude and distribution of carbon #uxes in the Southern Ocean. To better constrain the latter, we have deployed an array of sediment trap moorings as part of the US-JGOFS Antarctic Environmental Southern Ocean Process Study (AESOPS; Smith et al., 2001). Our objective was to document seasonal and spatial variations of particle composition and particle #ux to the ocean interior and to the sea #oor. Moored sediment traps provide a record of particle #ux and composition that helps establish the timing and magnitude of important upper water-column events, such as phytoplankton blooms and species succession within assemblages. They also provide a means for estimating the extent of remineralization of labile components in the deep sea and at the sediment}water interface. The capacity of time-series sediment traps that enables autonomous collection of settling particles is especially important in the Southern Ocean, where inaccessibility and ice cover prevent year-round sampling. Our goal was to integrate particle #ux data with other physical and biogeochemical measurements pertinent to the carbon cycle (surface water pCO , nutrient dynamics,  primary production, water-column density structure, and sea-ice coverage) and contrast the e!ectiveness and controls of the biological pump within each hydrologically distinct zone of the Southern Ocean (the Subantarctic Zone (SAZ), Antarctic Polar Frontal Zone (PFZ), the Antarctic Polar Front (APF), the Antarctic Zone (AZ) and Seasonal Ice Zone (SIZ); e.g., TreH guer and Jacques, 1992; Orsi et al., 1995). 2. Methods 2.1. Field program: Time-series sediment trap array We deployed from November 1996 to January 1998 along 1703W an array of time-series sediment traps that consisted of "ve bottom-tethered moorings deployed

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3523

Table 1 Summary of mooring locations, trap depths and anchor depths Station

MS-5

MS-4

MS-3

MS-2

MS-1

Hydrographic zones Km from MS-5

AZ (RSG) 0

AZ (ACC) 333

APF 652

PFZ 1029

SAZ 1546

Latitude Longitude

66310S 169340W

63309S 169354W

60317S 170303W

56354S 170310W

53302S 174344W

Depth group (km)

Trap depth; m (with open/close schedules; Table 3)

1

937 (III) 13/13 1033 (I) 14/21

1031 (III) 13/13

1003 (IV) 13/13 1103 (II) 3/21

982 (II) 21/21

986 (II) 19/21

2

1842 (I) 15/21

2026 (I) 9/21

1997 (II) 2/21

1976 (II) 0/21

1981 (II) (19/21?)

2966 (II) 4/21

3381 (II) 0/21

3 0.7 (above bottom)

2182 (I) 9/21 2885

3257 (II) 6/21 3957

4224 (II) 21/21 4924

4741 (II) 0/21

Anchor depth (m)

2311 (I) 10/21 3015

Sea #oor environment

Ridge

Ridge

Fracture zone

Fracture zone

Abyssal plain

5441

Number of successfully collected samples/expected number of samples to be collected.

north of the Antarctic Circle (Table 1). MS-5 (66310S) was covered with '80% sea ice except for a period of 3 months in summer. During austral summer the watercolumn structure is characterized by a near-freezing temperature minimum (or `nearfreezing dichothermal layera) at a depth of about 100 m and a pronounced seasonal pycnocline at ca. 50 m. MS-4 was deployed further north (63309S), where ice coverage was less extensive and persisted for a shorter time. This region still has a dichothermal structure, but the temperature minimum is well above freezing. The sharp pycnocline near 50 m remains, but is not as pronounced as at MS-5. MS-4 and MS-5 are within the Antarctic Zone (AZ) (Orsi et al., 1995). MS-5 was located within the Ross Gyre, while MS-4 was located within the Antarctic Circumpolar Current (ACC). MS-3 (60315S) was positioned within the APF, which coincides with the northward limit of the subsurface temperature minimum (e.g., Moore et al., 1999a). Here the pycnocline is deeper (ca. 100 m) and the vertical density gradient is weaker. MS-2 was located north of the APF, near the northern limit of the Polar Frontal Zone (PFZ), delineated by the Subantarctic Front (SAF). MS-1 was deployed north of the SAF and in the Subantarctic Zone (SAZ). The integrity of the part of time-series samples collected at station MS-1 was compromised by high current velocity at 1 km depth that continued for ca. half the year. Instead, particle compositions in the integrated sample available from 1981 m at MS-1, where the current velocity was insigni"cant, are reported in Table 2 and Fig. 1.

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Table 2 Annual export #uxes and proportions at 1 km (a) Annual #uxes of biogeochemical components: g m\ yr\ MS-C

Depth (m)

TMF

C

5 4 3 2 1

937 1031 1003 982 1981

27.6 80.6 56.9 33.5 11.5

1.9 2.2 2.3 1.7 1.0



N 

CaCO SiO 
 

0.3 0.4 0.4 0.3 0.2

0.6 10.7 12.8 11.8 6.8

16.0 55.6 27.5 15.2 2.0

Litho 0.05 0.12 0.12 0.12 0.73

(b) Annual #uxes of biogeochemical elements, mmol m\ yr\ and mole ratios MS-C

C

5 4 3 2 1

162 183 195 139 80



C 

Si


N 

6 107 128 118 68

267 926 458 253 33

19 31 26 20 11

Al 0.15 0.38 0.37 0.37 2.2

C /C 

Si /C


Si /C


C /N  

25.7 1.7 1.5 1.2 1.2

42.3 8.7 3.6 2.2 0.5

1.7 5.1 2.4 1.8 0.4

8.4 5.8 7.4 7.0 7.0

(c) Proportion of biogeochemical components (wt%) (residual water, minor and trace constituents are not included in the total mass; see text for explanation) MS-C

CaCO 




SiO 

5 4 3 2 1

3.3 15.4 29.7 40.6 64.0

84.7 80.6 63.8 52.3 18.5

C



10.3 3.2 5.4 5.7 9.1

N 

Litho

1.4 0.6 0.9 1.0 1.5

0.3 0.2 0.3 0.4 6.9

From 1981 m trap.

Nineteen PARFLUX time-series sediment traps (Honjo and Doherty, 1988) were deployed on the "ve open-ocean moorings (Fig. 2). The shallowest traps were deployed at approximately 1 km depth, the mid-depth traps at approximately 2 km, and deepest traps approximately 700 m above the bottom. In addition, one or two Alpha-Omega2+ current meters were deployed on each of the moorings (except at MS-4), and McLane2+ recording inclinometers were deployed with the 1-km sediment traps at MS-4, MS-3 and MS-1 (Fig. 2). Current meter and trap inclination data can be obtained from the US-JGOFS Data Center AESOPS web-site http:// usjgofs.whoi.edu.jg/dir/jgofs/southern/. Each sediment trap collected material for a total of 425 days subdivided into 21 or 13 time periods. The "rst sample bottle on each trap opened at 11:00 (GMT) on November 28, 1996, and the last bottle closed at 11:00 (GMT) on January 23, 1998. The total period of collection was divided into "fty 8.5 days of `unit periodsa. Traps were opened for a unit period or a combination of unit periods, depending on

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Fig. 1. The AESOPS time-series sediment trap array; a vertical pro"le. Location of the three Mark 7G 13 traps, auxiliary instruments the optical mooring array of M. Abbott and J. Richman (pers. comm.) are also illustrated. STF: Subtropical Front; SAF: Subantarctic Front; APF: Antarctic Polar Front; SACCF: Southern Antarctic Circumpolar Current Front. The positions are approximate. Mooring con"gurations were projected on the topographic pro"le produced by narrow-beam sounding bathymetry along 1703W.

anticipated mass #uxes. The shortest sampling interval was one unit period, 8.5 days, and the longest was 136 days, a combination of 16 unit periods. The traps were thus synchronized although the sampling intervals di!ered between traps (Fig. 3).

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S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

Fig. 2. Please provide.

Two types of PARFLUX sediment trap were used in this study. The Mk7G-21 traps are mounted with 21 sampling cups and were designed to study temporal variability with high resolution. When mass #ux approaches 400}500 mg m\ d\, however, these sampling cups tend to over#ow, with settling particles that clog the bottom of the receiving conical funnel of the sediment trap, often prematurely terminating sample collection (Haake et al., 1993). We thus also deployed sediment traps with fewer (thirteen) but larger sampling cups and a conical funnel with a bottom aperture four times larger than in the former design. This design is much less susceptible to over#owing and clogging (Honjo et al., 1999) and was used to collect settling particles at 1 km depth at MS-3, MS-4 and MS-5 with a lower temporal resolution (17 days in summer). All sample containers were "lled with Southern Ocean deep-water collected before the mooring was deployed; re-distilled formalin bu!ered with sodium borate was added to the seawater to a 3% concentration (Honjo et al., 1999). Throughout the deployment of the array, the sampling cups were sealed from the ambient water except during their respective collection intervals. 2.2. Laboratory analysis After sieving particles through 5- and 1-mm Nylon mesh in seawater, the particles that passed through 1-mm mesh ((1-mm particles hereafter) were split into 10 equal aliquots using a McLane WSD-10 wet-sample divider that had a splitting error

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3527

Fig. 3. Time-series plot of the total mass #uxes (TMF) in mg m\ d\. X"no data.

of $3.4%. Three 1/10 aliquots were "ltered on 0.45-lm pre-weighed polycarbonate "lters, dried at 603C to constant weight, ground and homogenized with a pestle and mortar, and stored in a dessicator for later analysis. The 1- to 5-mm and '5-mm fractions were divided into two equal parts (Honjo and Manganini, 1993). One aliquot was rinsed and dried for analysis, while the other half was kept refrigerated in its original seawater-formalin solution for microscopic analyses.

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Total mass #ux (TMF) was obtained by dividing the dried weight by the period of sampling. Organic carbon (C ), inorganic carbon (C ), biogenic silica (Si ), and  
 aluminum or lithogenic #uxes were obtained by multiplying TMF by the concentration of each constituent measured in the dried material. Inorganic carbon was measured by coulometry, total carbon was measured on a Perkin-Elmer Elemental Analyzer, and organic carbon was estimated by di!erence. We measured contents of Si and Al Inductively-Coupled-Plasma Emission Spectrometry (ICP-ES) or ICPMass Spectrometry (Finnegan-Element ICP-MS) after total digestion of the samples in 4 ml HNO and 100 ll HF in a Te#on microwave bomb at 1503C for 15 min  (S. Manganini, in prep). Si (wt%) was estimated from total silica minus Al
 (wt%);3.42 to remove the detrital Si component (Honjo et al., 1999). Si (wt%) does
 not include water associated with biogenic opal (e.g., Mortlock and Froelich, 1989). Lithogenic #uxes were obtained from Al (wt%);12.15. Analytical errors based upon multiple analyses of reference materials (Honjo and Manganini, 1992) were estimated at 0.93% for C , 2.8% for C , 6.5% for SiO and 5.0% for Al. Annual #uxes  
  (Table 2a) equal 365 times the daily averages obtained over the 425-day period (Honjo and Manganini, 1993). The data were not corrected for dissolution during deployment. The di!erence between TMF and the sum of the constituents is mainly due to the presence of water and organic H and O that are not routinely measured in our analytical protocol. Biogenic opal was calculated from Si (wt%);1.1 (Mortlock
 and Froelich, 1989), and the sum of the "ve components was set at 100%, excluding residual water and minor/trace components from this calculation.

3. Results 3.1. Sample recovery All 19 time-series traps completed their sampling sequence as programmed, except for two traps that malfunctioned due to battery failure. However, the AESOPS sample recovery rate was considerably less than in previous "eld studies, primarily because of unexpected high particle #ux that clogged many of the Mk-21 highresolution sediment traps. Of the 375 samples that we expected to collect from the 5 deep-sea moorings, only 174 were recovered. All three Mk-13, speci"cally designed for high particle #ux, functioned as expected. Only one Mk-21 trap collected samples throughout the entire sampling period; the other traps over-"lled and clogged during austral summer when particle #ux reached a maximum, and the samples were lost (Table 1). Export #ux during austral summer exceeded 800 mg m\ d\ (Fig. 3). The dominance of "brous masses of diatom frustules in the settling material further compounded the clogging problem. 3.2. Annual export yuxes The latitudinal pro"le of TMF at 1 km depth has a maximum annual #ux at the Antarctic Circumpolar Current station in the Antarctic Zone (MS-4)

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

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Fig. 4. (a) Annual total mass #ux in g m\ yr\; (b) Fluxes of C , C and Si in mol m\ yr\; (c) Wt  
 % of the major biogeochemical components; (d) Biogeochemical molar ratios.

(80.6 g m\ yr\) and the lowest at the Ross Sea station, also in the Antarctic Zone (MS-5) (27.6 g m\ yr\; Fig. 4a). The annual #ux of biogenic silica follows a similar pattern, re#ecting the dominance of this biogenic phase in the material settling throughout the region south of the SAF. The Si #ux maximum at MS-4 was 926
 mmol Si m\ yr\, while a #ux of 458 mmol Si m\ yr\ was measured at the northern Antarctic Zone (MS-3). The Si #uxes were approximately 250 mmol Si
 m\ yr\ at MS-5 and a Polar Frontal Zone station (MS-2) (Table 2b; Fig. 4b). Compared to biogenic silica #uxes, the latitudinal variability of annual C #ux was  far smaller. Annual C #uxes ranged from 1.7 to 2.3 g m\ yr\  (139}195 mmol C m\ yr\) at the four stations (Fig. 4b). Carbonate, occurring mainly as calcite and aragonite shells and tests of planktonic organisms, was persistent at all sites. Annual #uxes of carbonate were very similar at the three stations MS-2 (11.8 g m\ yr\), MS-3 (12.8 g m\ yr\), and MS-4 (10.7 g m\ yr\), but decreased substantially (to 0.6 g m\ yr\) at MS-5 (Fig. 4b). Likewise, annual aluminum #uxes were quasi-uniform at the three northernmost mooring stations south of the SAF. Annual Al #uxes were 0.38, 0.37 and 0.37 mmol Al m\ yr\, corresponding

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to lithogenic #uxes of 0.12 g m\ yr\. At MS-5 annual Al #ux dropped signi"cantly to 0.15 mmol Al m\ yr\ (i.e. lithogenic #ux of 0.05 g m\ yr\) (Table 2a and b). Biogenic opal (SiO ) 0.4 H O) accounted for more than 50% (by weight) of the   major constituents in all 1-km traps with the exception of MS-1, and exceeded 80% at MS-4 and MS-5 (Table 2c). CaCO concentration was very low at MS-5 and  increased northward to a maximum of 64% at MS-1 (Table 2c). This trend primarily results from the gradual northward decrease in opal #uxes. Organic carbon concentration was lowest at MS-4 (3.2%), the site with the highest particle #uxes, re#ecting dilution by other phases, mainly biogenic opal (Table 2c). MS-4 stands in sharp contrast with MS-5, where a comparatively high organic carbon content (10.3%) was found in association with a high proportion of opal (84.7%; Table 2c). Re#ecting the above latitudinal changes in particle composition, the Si /C
  mole ratio decreased northward, from a high of 42 at MS-5 to about 0.5 at MS-1 (Fig. 4d). Si /C mole ratios display a pattern similar to the latitudinal transect
  of biogenic silica #ux, with a maximum of 5.1 at MS-4 and a signi"cantly lower value (1.7) at MS-5 (Fig. 4d). C /C mole ratios were also very high at MS-5 (25.7),   dropped markedly at MS-4 (1.7), and decreased gradually further north (Fig. 4d). 3.3. Seasonal variability of export yuxes Export #uxes at all stations were highly seasonal, with maxima occurring during austral summer, while very low #uxes prevailed throughout austral winter (Fig. 3). The summer onset of particle export was increasingly delayed towards the south. During austral summer 1996/97, particle #uxes were elevated at the beginning of the trap deployment (early December) at MS-2, MS-3 and MS-4. At the Ross Sea Gyre station (MS-5), however, the onset of enhanced #ux occurred later (Table 2). The southward progression of the summer bloom is also illustrated by the data collected during austral summer 1997/98. Particle #uxes started to increase in late September at station MS-2, mid-November at stations MS-3 and MS-4, and in late December at MS-5. For the latter station, the onset of summer #ux maximum during late 1997 seems to have occurred about a half month earlier relative to 1996, although this di!erence can in part be due to the coarser sampling resolution during austral summer 1997/98 (Fig. 3). Another di!erence between MS-5 and the stations further north is temporal dynamics of the summer bloom. Speci"cally, the #ux reached a maximum in February, stopped in late summer, and exhibited a long period of very low particle #ux from April to December (Fig. 3). Summer particle #uxes at MS-2, MS-3 and MS-4 often display two summer maxima separated by a half-month period with reduced #uxes (Fig. 3). Export #ux at the Ross Sea station (MS-5) is characterized by a brief but sharp rise in TMF after sea ice had receded that lasted about 2 months (Fig. 3). Total #uxes reached nearly 500 mg m\ d\ between February 4 and 21, compared to winter values (5 mg m\ d\ (Fig. 3). During the same period, #uxes of Si and C
  co-varied and reached a maximum of 3.56 and 4.31 mmol Si m\ d\, respectively
 (Figs. 5 and 6). In comparison, minimum #uxes were 20 lmol C m\ d\ for C and  75 lmol Si m\ d\ in November and early December (Table 3). C #uxes were
 

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

3531

Fig. 5. Time series of biogenic Si ( Si) #uxes in mmol Si m\ d\ at 1 km depth (4.2 km depth at MS-2).
 The extent of winter sea-ice coverage at MS-5 and 4 was estimated from the DMSP F13 Special Sensor Microwave/Imager information distributed by the University of Colorado at Boulder.

always very low at this station, but increased in the early austral summer to a maximum of 93 lmol C m\ d\ (January 9}21), 17 days earlier than the C and Si 
 maxima. The winter minimum in C #ux was (1 lmol C m\ d\ (Table 3). 

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Fig. 6. Time series of organic carbon (C ) #uxes in mmol C m\ d\ at 1 km depth (4.2 km at MS-2). 

The main characteristics of the particles exported at MS-5 are their high C /C   and Si /C (mole) ratios throughout the year (Fig. 8), particularly during the late
  stage of TMF maximum and during early winter. The maxima in Si /C and
  C /C ratios were 91 and 56, respectively, from March 10 to 27, about 1.5 months   after the maximum TMF (Fig. 3). The Si /C ratio varied within a factor of two
  throughout 1997 and stayed relatively low (1.2}2.3; Table 3), re#ecting seasonal co-variation in the #uxes of these two biogenic phases (Fig. 8). We observed at the ACC station a very large TMF at 1031 m depth, spanning a period of 4 months during austral summer, beginning in late October/early November and ending in early April (Fig. 3). We estimate that 90% of the annual #ux, about 80 g m\ yr\, was supplied during this interval. The largest daily #ux (819 mg m\ d\) occurred between February 21 and March 10. Between December 15 and March 14, the TMF was larger than 500 mg m\ d\, except for a period between January 18 and February 4 when the #ux dropped to 232 mg m\ d\ (Fig. 3).

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

3533

Flux of biogenic Si was particularly large: 926 mmol m\ yr\ of biogenic Si (56 g m\ yr\ of dehydrated opal; Table 2a). The largest Si #ux (9.4 mmol Si
 m\ d\) occurred between February 21 and March 10 (Fig. 5). The Si particles
 consisted of diverse assemblage of well-silici"ed diatom frustules. Seasonal variations in C #uxes followed that of Si , with a "rst maximum (1.5 mmol C m\ d\) 
 between December 15 and 23, and a second maximum (2.2 mmol C m\ d\) between February 21 to March 10 (Fig. 6). C #uxes followed a similar pattern, but  the two summer maxima were separated by about one month (Fig. 7). The two C #ux maxima were much higher than at MS-5 (2.08 and 1.18 mmol C m\ d\).  The main contributors to CaCO #uxes were pteropod shells and planktonic  foraminifer tests. The most common planktonic foraminifera were well-calci"ed Neogloboquadrina pachyderma and Orbulina universa. Particles exported at the Antarctic Circumpolar station (MS-4) had signi"cantly lower C /C mole ratios throughout the year relative to the Ross Sea Gyre station   (MS-5), both within the Antarctic Zone (Fig. 8). The high values found during the late stage of TMF maximum and early austral winter at MS-5 ('20) were not present at MS-4, where the C /C mole ratios essentially remained between 3 and 4. In   contrast, Si /C ratios were higher at MS-4 because of the very high opal #ux,
  notwithstanding the higher carbonate #uxes at the latter location. As in MS-5, the Si /C ratios varied by less than a factor of 2 throughout 1997. Si /C mole
 
  ratios however, were signi"cantly higher at MS-4 (5.1) compared to MS-5 (1.7; Fig. 8), pointing to signi"cant latitudinal variations in this ratio (Fig. 4d). The seasonal variations in TMF at 1003 m depth were similar to those found at MS-4. Duration of the summer maximum and the `double peaka feature were replicated at MS-3 (Fig. 3). TMF were not as high as at MS-4, however. The largest TMF at MS-3 was 443 mg m\ d\ from December 15 to January 1, 1997. From September 13 to October 17, the TMF was at a minimum of 4.7 mg m\ d\. The lower TMF at this location was primarily due to lower opal #uxes that reach a maximum (3.6 mmol Si m\ d\), which was less than half as high as that found at MS-4 (Fig. 5). Again, seasonal variations in C #uxes followed closely that of Si , 
 with maxima (1.6 mmol C m\ d\ between December 15 and 23, and 1.3 mmol C m\ d\ between February 4 and March 1) (Fig. 6). C #uxes were  also similar to those measured at MS-4 (Fig. 7). The main biogeochemical change between MS-4 to MS-3 is the reduction of the Si /C mole ratio to values closer to that of MS-5 (Fig. 4d), re#ecting the decreased
  opal #ux and the relatively unchanged C #uxes. Another characteristic of the  material settling at station MS-3 is the smaller seasonal variability in the ratios of C /C , Si /C and Si /C (Fig. 8).  
 
  The seasonal pattern and magnitude of TMF at MS-2 (982 m) were similar to those at MS-3 and 4, but the period of maximum TMF began and "nished earlier (Fig. 3). The mid-summer minimum also seems to occur earlier, just after the summer solstice. Traps deployed at intermediate depth did not (at 1976 m) or only partially (at 2966 m) operate, but one Mk G-21 trap deployed at 4224 m depth provided a complete sample set (Table 1). The #uxes recorded by the deeper trap record a similar seasonal pattern, but were generally smaller and delayed compared to the #uxes recorded at 982 m

345 443 207 372 286 81 44

312

TMF mg (mg m\ d\)

MS-3 (60317S) 1003 m (Schedule IV) 11/28/96 17 !25.0 12/15/96 17 !8.0 01/01/97 34 17.5 02/04/97 34 51.5 03/10/97 34 85.5 04/13/97 34 119.5 05/17/97 34 153.5

Accu. midday days in 1997

465 278 76 77 35 26 15 10 8 3 7 8 58 85 148 183 72 11 388

Duration days

MS-2 (56354S) 982 m (Schedule II) 11/28/96 17 !25.0 12/15/96 17 !8.0 01/01/97 17 9.0 01/18/97 17 26.0 02/04/97 17 43.0 02/21/97 17 60.0 03/10/97 17 77.0 03/27/97 17 94.0 04/13/97 17 111.0 04/30/97 34 136.5 06/03/97 51 179.0 07/24/97 34 221.0 08/27/97 17 247.0 09/13/97 17 264.0 09/30/97 17 281.0 10/17/97 17 298.0 11/03/97 17 315.0 11/20/97 17 332.0 12/07/97 17 349.0 12/24/97 17 366.0 01/10/98 17 383.0

Open date

1023 1622 657 1136 1054 305 155

903.0 811.0 740.0 1011.0 755.0 185.0 102.0

2385 3559 1314 2611 2225 711 422

3781 2335 475 311 136 82 44 39 35 29 20 39 421 591 919 787 250 23 3721

2290

C Si 
 (lmol m\ d\)

1232 1099 (No sample) 1641 1231 963 810 295 301 443 374 172 181 139 146 105 82 89 42 50 36 16 8 67 32 75 31 575 144 536 249 714 627 628 1129 246 436 49 84 1239 918

C 

1.6 2.6 1.0 1.7 2.1 1.0 0.7

3.8 2.4 1.0 1.1 0.7 0.5 0.3 0.3 0.3 0.1 0.3 0.3 1.8 1.9 2.2 1.5 0.6 0.1 3.7

2.0

Al

1.1 2.0 0.8 1.2 1.4 1.6 1.5

1.3 1.2 1.0 1.2 1.0 1.0 1.3 2.1 1.4 2.1 2.1 2.4 4.0 2.2 1.1 0.6 0.6 0.6 1.4

1.1

C /C  

1.2 2.1 0.8 1.3 1.4 1.8 1.9

3.1 2.9 1.6 0.8 0.8 0.6 0.5 0.9 1.0 3.7 0.6 1.2 2.9 2.4 1.5 0.7 0.6 0.3 4.1

C /N 

2.3 2.2 2.0 2.3 2.1 2.3 2.7

7.2 8.2 6.6 7.0 6.9 6.6 6.2

1.9 7.5 (No sample) 2.3 7.1 2.4 7.0 1.6 6.3 0.7 5.8 0.8 6.3 0.6 5.8 0.4 5.7 0.4 5.6 0.7 7.0 1.8 6.3 0.3 6.0 0.5 6.7 0.7 7.6 1.1 7.4 1.3 7.1 1.3 6.9 1.0 7.0 0.5 6.7 3.0 7.2

Si/C Si/C 
  mole ratio

2.1




Table 3 Seasonal variability in biogeochemical components and #uxes at the AESOPS Mooring stations (MIZ, AZ, APF, PFZ) at 1 km depth

3534 S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

("Sample too small for analysis.

5 15 52 253 488 106 67 56 21 3 2 9 297

26 9 5 9 179 340

MS-5 (66310S) 937 m (Schedule III) 11/28/96 17 !25.0 12/15/96 17 !8.0 01/01/97 17 9.0 01/18/97 17 26.0 02/04/97 17 43.0 02/21/97 17 60.0 03/10/97 17 77.0 03/27/97 34 102.5 04/30/97 136 187.5 09/13/97 34 272.5 10/17/97 34 306.5 11/20/97 34 340.5 12/24/97 34 374.5

196.0 238.5 272.5 306.5 340.5 374.5

468 620 736 232 760 819 610 233 43 4 8 66 154

51 34 34 34 34 34

MS-4 (63307S) 1031 m (Schedule III) 11/28/96 17 !25.0 12/15/96 17 !8.0 01/01/97 17 9.0 01/18/97 17 26.0 02/04/97 17 43.0 02/21/97 17 60.0 03/10/97 17 77.0 03/27/97 34 102.5 04/30/97 136 187.5 09/13/97 34 272.5 10/17/97 34 306.5 11/20/97 34 340.5 12/24/97 34 374.5

06/20/97 08/10/97 09/13/97 10/17/97 11/20/97 12/24/97

58 98 308 1647 3560 639 434 260 119 24 20 52 1318

1148 1533 1162 623 2217 1705 1074 549 104 34 44 163 344

99 62 36 56 740 989

4 15 25 93 85 13 8 8 8 2 1 5 47

292 597 2083 224 680 559 1181 426 60 3 6 50 68

57.0 26.0 15.0 19.0 186.0 425.0

( 144 537 2280 4314 1118 711 596 212 ( ( 75 3065

5227 7707 7242 2538 7921 9444 5929 2365 501 36 74 757 1899

251 62 ( 45 1632 3298

( 0.2 0.3 2.3 1.4 0.4 0.2 0.2 0.1 0.1 ( 0.1 1.5

4.0 3.0 2.8 1.0 2.1 2.5 2.1 1.1 0.3 0.1 0.1 0.7 0.9

0.6 0.2 0.2 0.2 0.9 1.4

15.4 6.4 12.3 17.8 41.7 48.4 56.0 30.8 15.0 11.0 22.0 10.3 28.0

3.9 2.6 0.6 2.8 3.3 3.1 0.9 1.3 1.7 4.2 2.9 3.3 5.1

1.7 2.4 2.4 3.0 4.0 2.3

( 9.5 21.4 24.6 50.5 84.9 90.6 70.6 26.7 ( ( 14.7 65.4

17.9 12.9 3.5 11.3 11.7 16.9 5.0 5.6 8.4 12.0 13.2 15.1 27.9

2.1 1.1 ( 1.1 4.1 3.6

( 1.5 1.7 1.4 1.2 1.8 1.6 2.3 1.8 ( ( 1.4 2.3

4.6 5.0 6.2 4.1 3.6 5.6 5.5 4.3 4.8 1.1 1.7 4.7 5.5

2.5 1.0 ( 0.8 2.2 3.3

( 5.4 6.8 7.4 8.7 9.5 8.1 7.4 6.9 8.1 5.6 5.5 8.4

7.2 6.8 4.3 5.9 5.7 5.4 5.7 6.5 5.1 5.9 6.2 6.4 6.8

6.6 6.3 6.8 6.5 9.0 8.5

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548 3535

3536

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

Fig. 7. Time series of inorganic carbon (C



) #uxes in mmol C m\ d\ at 1 km depth (4.2 km at MS-2).

(Figs. 5}7). Although the annual opal #ux continued to decrease (Fig. 4b), the summer maximum was similar to that of MS-3 (3.8 mmol Si m\ d\; Fig. 5). Seasonal variations in C and C #uxes followed a similar pattern and reached maxima   (1.6 and 1.2 mmol C m\ d\ between December 15 and 23, respectively) comparable to MS-3 and MS-4 (Figs. 6 and 7). The Si /C mole ratios reached
  maxima similar to MS-3 (ca. 2), but sometimes dropped to signi"cantly lower values ((1), particularly at the end of the two summer peaks. Concomitantly, there was a decrease in the Si /C mole ratios, indicating the greater
  importance of carbonate-producing plankton during these periods (Fig. 8). The dominance of CaCO in the material collected at this station (from a composite  sample recovered at 1981 m) was in sharp contrast to the material collected at the other stations further south (Fig. 4c), resulting in lower Si /C , Si /C , and
 
  C /C mole ratios (Table 2b).   3.4. Fluxes of large ('1 mm) biogenic particles at 1 km The large biogenic remains ('1 mm) are generally categorized as `swimmersa, although some of this particle could be part of the export, particularly in late summer. They consist of approximately 40% organic matter, and the rest is mostly residual water. The 1}5 mm fraction represented 3.5, 4.6 and 3.6% of the TMF at MS-4, MS-3 and MS-2, respectively (Table 4). Biogenic detritus '5 mm was rare at MS-2 and MS-3, but more abundant at MS-4 (Table 4), particularly during the late phase of the

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

3537

Fig. 8. Time series of molar biogeochemical ratios at 1 km. The Si /C ratios of MS-5 were re-plotted on
  a reduced scale (broken line) on the right vertical axis. Ratios at each period are represented by the lines that connect the mid-date of each open/close period (Table 2).

austral summer. Abundant planktonic polychaetes were caught in late summer at MS-4 in the 1031-m trap, but not in the traps deployed at 2026 and 2182 m, suggesting that they did not sink passively into the trap.

3538

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

Table 4 Flux of particles that remained on 1,5-mm mesh at 1 km MS-C

5 4 3 2

Trap depth (m)

1}5 mm

'5 mm

% #ux/yr Ave. #ux in TMF (mg m\ d\)

Y-#ux (g m\ yr\)

% #ux/yr Ave. #ux in TMF (mg m\ d\)

Y-#ux (g m\ yr\)

1030 1031 3257 983

0.2 3.5 4.6 3.6

0.14 2.93 2.86 1.19

0.00 0.86 0.06 0.54

0.00 0.72 0.04 0.19

0.38 8.51 8.11 3.27

0.00 1.99 0.10 0.53

4. Discussion 4.1. Latitudinal variations in particle composition and comparison with the underlying sediment The particles collected by the sediment trap array document the transition in sediments from carbonate to opal-rich material that has long been recognized. Southern Ocean surface sediments north of the SAF typically have high carbonate content ('80%) above the lysocline, and opal content close to detection limits. Carbonate content gradually decreases, while opal content gradually increases towards the APF, where opal becomes the dominant phase. South of the APF, opal is the principal sedimentary constituent. Further south, opal concentrations decrease again, this time due to dilution by crustal materials (e.g. Lorenzo and Hays, 1976; Cooke, 1978; Cooke and Hays, 1982; Burckle et al., 1982; Yu, 1994). The mean annual composition of the settling material intercepted at 1 km (Fig. 4c; expressed in % of the "ve major constituents) is generally consistent with these trends. In the SAZ we found 64% CaCO and 18% opal (at 2 km depth), whereas near the SAF, carbonate content  decreased to 41% and opal content increased to 52%. Near the APF opal content further increased to 64%, but the carbonate content was still signi"cant (30%). This trend continued in the AZ where opal content rose to 81%, while carbonate content dropped to 15%. South of the SACCF opal content was similarly high (85%), while carbonate dropped to a very low concentration (3%). Although there was a general agreement between settling particle and sediment composition, several di!erences were observed. First, % opal in the material collected in the SAZ appears signi"cantly higher than % opal typically measured in Subantarctic sediments (e.g., Charles et al., 1991; Yu, 1994), suggesting that a signi"cant #ux of biogenic silica may occur in the SAZ, but the particles are poorly preserved in the sediments. Likewise, the carbonate content of settling particles remained surprisingly high well south of the APF and into the AZ. This di!ers from the sediments, as they generally contain very little carbonate south of the APF. If the carbonate #uxes measured here are typical, they suggest signi"cant export of alkalinity from the upper water column and carbonate dissolution on the sea #oor. Carbonate preservation is

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

3539

likely to be better on the sea #oor of the AZ at 1703W, however, due to relatively shallow depth ((3000 m). Finally, the lithology of Southern Ocean sediments typically shifts near the Antarctic continent from siliceous ooze to siliceous silty clay or clayey silts, with lower opal concentrations due to dilution by lithogenic material (Cooke, 1978). This transition has been taken as an indication of sea ice extent, based on the assumption that sea ice inhibits diatom production and collects dust particles that are released upon melting (Burckle et al., 1982). The low lithogenic #ux measured at MS-5, however, does not corroborate the purported increase in terrigenous #ux associated with a receding ice edge. This observation brings into question whether this lithological boundary is directly linked to receding sea ice and whether it can be used to reconstruct past changes in sea ice extent. 4.2. The carbonate/silica biogeochemical front Biogeochemical fronts between carbonate and opal-dominated areas are often found separating oceanic regions of contrasting productivity regime. The settling material collected in coccolith-dominated, low-productivity regions have Si /C
  mole ratios ;1, while material collected in more productive, diatom-dominated regions have Si /C mole ratios <1 (Honjo, 1997). In the Southern Ocean
  this biogeochemical front is particularly important, as down-core variations in sediment concentration of opal vs. carbonate have been used to deduce past changes in the position of the Southern Ocean frontal system (e.g., Mortlock et al., 1991; Howard and Prell, 1992). Along 1703W the transition from carbonate to opal dominance occurs within the PFZ, between MS-1, where Si /C "0.2, and MS-3,
  where Si /C "3.6. The #ux of carbonate in the SAZ was dominated by
  coccoliths, as in low-latitude, low-productivity, areas of the ocean (Honjo, 1997). The SAF was a transitory station (Si /C "2.2), with characteristics from both
  oceanic provinces that recorded signi"cant #uxes of both coccoliths and diatoms (Table 2). In contrast, coccoliths were not found at the APF or further south, where more than half of C #uxes was supplied by pteropod shells. The remainder  consisted of planktonic foraminifera tests, mostly N. pachyderma, although the foraminifer community was not as mono speci"c as found in many other high latitude oceans. 4.3. Carbonate yuxes The #uxes of carbonate were surprisingly invariant in the PFZ (11.8 g m\ yr\), the vicinity of the APF (12.8 g m\ yr\), and the northern AZ (10.7 g m\ yr\) (Table 2a), the northward increase in % CaCO being primarily a result of decreasing  opal #ux (Fig. 4b). These carbonate #uxes are substantial and equivalent to those measured in the mid-latitude North Atlantic Ocean (Honjo and Manganini, 1993; Newton et al., 1994). Although calcite (coccoliths and planktonic foraminifera tests) is generally believed to be una!ected by dissolution while sinking to the sea #oor (Honjo and Roman, 1978; Honjo and Weller, 1998), there is increasing evidence for signi"cant carbonate dissolution in the water column above the calcite saturation horizon

3540

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

(Dymond and Lyle, 1985; Nozaki and Oba, 1995; Archer, 1996; Milliman et al., 1999; Yu et al., 2000). This could occur more readily south of the PFZ, where a signi"cant fraction of the carbonate #ux consists of the more soluble aragonite (pteropod shells). We have found direct microscopic evidence for aragonite dissolution in our samples. Small pteropods shells ((1 mm) showed clear signs of pitting and partial dissolution. In addition, larger specimens had a `frosteda appearance that contrasted with the transparency of the shells of living pteropods captured by net tow. This change in appearance also can be taken as an indication for partial dissolution (Berner and Honjo, 1981). Such dissolution is unlikely to have occurred in the sampling cups during deployment. Dissolution due to CO production from  decay of organic matter can be discounted based on pH in the cups after recovery of the traps that was invariably '8.1. Since the cups were sealed from ambient water after their respective collection interval, any carbonate dissolution that proceeded in the cup after collection would rapidly saturate the enclosed seawater, stopping further dissolution (Honjo and Manganini, 1993). Calcite and aragonite production in the surface water of the Southern Ocean and dissolution of aragonite in intermediate waters could account for the carbonate alkalinity depletion found in surface Antarctic waters (Broecker et al., 1982). The potential impact of calcite and aragonite formation and dissolution on the alkalinity of Southern Ocean water warrants further study. 4.4. Biogenic silica yuxes Fluxes of Si were high but quite variable, with a clear maximum in the AZ
 (0.9 mol Si m\ yr\; Fig. 4b). This maximum coincides with a maximum in the mean SeaWiFS-derived surface chlorophyll concentrations (Moore et al., 1999b). These observations are also consistent with sediment data. Th-normalized opal #uxes reach a maximum just south of the APF in the Atlantic and the central Indian sector of the Southern Ocean that is typically 0.15}0.30 mol Si m\ yr\ (Kumar et al., 1995; Francois et al., 1997). Preliminary Th measurements in the sediments of the Paci"c sector at 1703W indicate a similar, although somewhat lower, maximum in normalized opal #uxes (R. F. Anderson et al., pers. comm.) Because the AZ was generally considered to be a low-productivity region, the occurrence of very high sedimentary opal accumulation rates south of the APF was considered puzzling. Enhanced opal preservation in the water column has been invoked to mitigate this apparent paradox (Nelson et al., 1995), but no clear mechanisms that could explain higher opal preservation in this region could be put forth. These sediment trap data add important constraints to this debate. The opal #ux measured at 1 km depth at MS-4 (0.9 mol Si m\ yr\) is equivalent to the upper limit of opal production rate proposed by Nelson et al. (1995), based on measurements from the Weddell Sea (0.4}1 mol Si m\ yr\; Leynaert et al., 1993). Considering that 20}60% of the opal produced dissolves in the upper water column (Nelson et al., 1995), sustaining a #ux of 1 mol Si m\ yr\ at 1 km would require opal production rates 2}5 times higher. This calculation suggests that opal productivity in the AZ has been signi"cantly underestimated.

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

3541

4.5. Decoupling between Si and C yuxes
  Unlike the biogenic opal #uxes, C export #uxes varied little and ranged from  139 mmol C m\ yr\ at MS-2 to 195 mmol C m\ yr\ at MS-5, producing relatively large latitudinal variations in Si /C mole ratio (Fig. 4d). While the compar
  atively low Si /C mole ratios at MS-2 (1.8) and MS-3 (2.4) could re#ect the
  contribution of C associated with carbonate-producing plankton, this does not  explain the low Si /C mole ratio (1.7) measured at MS-5 in the Ross gyre or the
  high Si /C mole ratio (5.1) measured north of the SACCF (Table 2b). Several
  possibilities could explain this sharp di!erence in Si /C . Higher Si /C at MS-4
 
  could re#ect higher Si /C in diatoms growing in this region. Alternatively, it could
  also re#ect better biogenic silica preservation, or higher rates of organic matter remineralization in the upper 1 km. On the other hand, lower Si /C at MS-5 could
  re#ect lower Si /C in diatoms or production by phytoplankton species that do not
  produce opal. It also could re#ect higher rates of biogenic silica dissolution, or lower rates of organic matter remineralization in the upper 1 km. It has recently been discovered that Fe-limitation can induce a two-fold increase in the Si /C of diatoms (Hutchins and Bruland, 1998; Takeda, 1998), and studies
  suggest that primary production in surface waters south of the PFZ is likely Felimited for signi"cant portions of the growing season (Franck et al., 2001; Olson et al., 2000). Fe-limitation also produces thicker diatom frustules that sink faster (Muggli et al., 1996; Hutchins and Bruland, 1998), and which are less likely to dissolve in the upper water column (Boyle, 1998). The four-fold di!erence in Si /C between MS-4
  and 5 would thus require Fe limitation in the AZ north of the SACCF, and Fe-replete conditions south of the SACCF. Fe could be added to the surface water at MS-5 from dust released by melting sea ice, but that is unlikely since lithogenic #uxes at the MS-5 are lower than in the MS-4 (Table 2a). Similar C #uxes at both stations (Fig. 4b) are  also inconsistent with Fe-replete conditions at MS-5. Sea-ice melting was also found to be a negligible source of Fe in the Atlantic sector (Lescher et al., 1997). It seems unlikely that di!erences in Fe limitation can explain the di!erence in the Si /C of
  the particle #ux in these two regions. If the two regions are similarly Fe-limited, then di!erences in the Si /C at 1 km
  cannot be due to di!erence in diatom silici"cation. Instead, we must invoke higher organic carbon #uxes at 1 km at MS-5, either because of better preservation in the upper water column, or contribution from phytoplankton that do not produce biogenic silica. The Si /C mole ratio of Fe-starved diatoms is ca. 0.4 (Hutchins and
  Bruland, 1998; Takeda, 1998). That would be the Si /C production ratio, if
  Fe-stressed diatoms were the only phytoplankton growing at these sites. Assuming 20}60% opal dissolution in the upper water column (Nelson et al., 1995), Si /C at
  MS-4 (5.1) would imply 94}97% remineralization of diatom carbon by the time it reaches 1000 m (1!+[0.4/5.1];[0.4}0.8],) due to food chain processing and bacterial decomposition. A similar calculation at MS-5 (Si /C "1.7) implies only
  80}90% remineralization. The situation at MS-4 seems typical for open-ocean waters. It has been estimated that organic carbon remineralization between the bottom of the euphotic zone and 1 km is about 90% (Honjo, 1996; Buesseler et al., 1998). f-ratios

3542

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

of 0.2}0.4, as typically found south of the APF (Sambrotto and Mace, 2001) would thus result in 96}98% remineralization of net primary production (1!+[1}0.9];f,). The apparently lower remineralization of organic matter in the upper water column at MS-5 is consistent with the lack of evidence for abundant grazers at this site. Pteropod shells and soft-tissue `swimmersa were absent in the MS-5 traps (Table 4). The only noticeable zooplankton was a very small planktonic foraminifera community consisting primarily of dwarfed tests that may represent a stressed community. Absence of pteropods shells and low foraminifera abundance are re#ected in the very small annual C export #ux measured at this station (6 mmol C m\ yr\). The   situation was very di!erent at MS-4, where a mean annual #ux greater than 100 mmol C m\ yr\ was recorded (Table 2b), re#ecting the abundance of shells  from the pteropod Limacina helicina, a known diatom grazer. 4.6. Short and truncated export season south of the SACCF The period of maximum export #ux at MS-5 was brief and truncated in late summer (Fig. 3), in general agreement with seasonal variations in surface chlorophyll concentrations estimated from SeaWiFS satellite imagery (Moore et al., 1999b). MS-5 was the only deep-sea station that was covered by extensive ('80%) seasonal sea-ice cover for most of the year (Fig. 5). The delayed onset of increased summer #ux at this station compared to MS-4 and MS-3 (Fig. 3) coincided with the timing of sea-ice retreat, as in earlier observations from the Weddell Sea (Fischer et al., 1988; Honjo, 1990). The late-summer cessation of export at MS-5 and the resulting decrease in mean annual #ux are more di$cult to explain. This truncation of #ux was not a result of the renewed extension of winter sea ice, since the drop in export #ux occurred one month before ice cover typically reappears at MS-5 (Fig. 5). In addition, major nutrients (nitrate, phosphate and silicate) were only partially drawn down (Smith et al., 2001), and could not have limited export production. Station MS-5 in the Ross Sea Gyre is characterized by a `near-freezing dichothermal layera (Toole, 1981; Craig et al., 1981; Gordon et al., 1981; Yang and Honjo, 1996;). Such vertical temperature structure produces a strong seasonal pycnocline that reduces substantially vertical mixing in summer. In view of the low #ux of lithogenic material at this site (Table 2a), it suggests that the main source of Fe to the euphotic zone is vertical mixing. If export production south of the APF is iron-limited, the lower export #ux of opal at MS-5 compared to MS-4 could re#ect lower Fe supply due to enhanced strati"cation (Fig. 9). Late-summer truncation of export #ux at MS-5 would thus re#ect complete utilization of dissolved Fe supplied to the mixed layer by winter mixing that would limit diatom production in this area. Deeper winter mixing at MS-4, owing to the absence of a near-freezing dichothermal layer, would supply comparatively more dissolved Fe to the surface water of this station, sustaining a longer summer production period. 4.7. Removal of carbon to the deeper ocean interior Flux data from traps deployed in deeper waters are useful to obtain rough estimates of particle sinking rates (Honjo, 1984; Honjo et al., 1995) and to evaluate the extent of

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

3543

Fig. 9. Hydrographic pro"le of upper 400 m at MS-5, 4, 3 and 2 to illustrate the temperature minimum zone and dichothermal layer at MS-5 and the resulting density strati"cation. Data from R/V N. B. Palmer cruise NBP-02-98.

dissolution or remineralization of biogenic particles in the deep sea (Yu et al., 2000). Unfortunately, only one high-resolution Mk-21 trap, deployed at 4224 m depth in the PFZ, recorded the entire sampling sequence. Nonetheless, several traps did record the beginning of the summer bloom (3 km depth at MS-3; 2 and 3 km depth at MS-4; and 2 km depth at MS-5), providing some indication on the sinking rates of particles. The #uxes of C , C and Si measured with the high-resolution traps during  
 the "rst two months were similar to the #uxes at 1 km. For example, at MS-4 the #ux

3544

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

of C , C , Si at 1031 m from November 28 to February 4 was almost identical  
 to the #uxes at 2026 and 2182 m. The C and Si #ux maxima were delayed by 
 a 17-day period, indicating a `bulk settling ratea (Honjo, 1984) for C and Si -rich 
 particles of approximately 200 m d\. This rate is equal to or greater than the rates estimated from other high-export-#ux ocean areas at lower latitudes (Honjo, 1996). No time-lag was observed for the C , indicating that pteropod shells and plank  tonic foraminifer settle even more rapidly. Noriki et al. (1985) made similar observations during austral summer 1984 at 61333S, 150327E.

5. Summary and Conclusions The latitudinal pro"le of annual TMF at 1 km depth (Fig. 4a) exhibit a maximum of 81 g m\ yr\ south of the APF within the AZ. Annual export #uxes measured at the four stations south of the SAF exceeded 25 g m\ yr\. These #uxes are larger than in most low-latitude open-ocean locations studied to date. The material exported was dominated by biogenic silica produced by a diversi"ed diatom community. Opal #ux reached a maximum of 926 mmol Si m\ yr\ at MS-4. This large #ux of biogenic silica suggests that opal production in the AZ is higher than previously thought (Nelson et al., 1995) and helps explain the high sedimentary opal accumulation observed south of the APF. In contrast to biogenic silica #uxes, #uxes of lithogenic particles were among the lowest measured in the open-ocean. They ranged from 0.12 g m\ yr\ from the PFZ to the AZ north of the SACCF, and decreased further (0.05 g m\ yr\) south of the SACCF. They re#ect a very low dust input, which could contribute to Fe limitation of phytoplankton growth in this region. Carbonate #uxes were higher than expected south of the SAF (107}128 mmol C m\ yr\), a level of carbonate export equivalent to that of the mid-latitude North Atlantic Ocean. They did not decrease substantially before reaching the Ross Gyre, where they dropped to 6 mmol C m\ yr\. A large fraction of the carbonate #ux at the southern stations was due to pteropods, which were partially dissolved at 1 km. Coccoliths were found only in the PFZ, and especially in the SAZ where calcium carbonate is the dominant phase. The biogeochemical front marking the transition from a carbonate-dominated to an opaldominated settling #ux occurs in the vicinity of the SAF and MS-2, where the Si /C mole ratio oscillates around 1.0. The #uxes of organic carbon at 1 km
  depth varied little between the PFZ and the AZ, and were relatively high (139}195 mmol C m\ yr\), about twice the estimated ocean-wide average (ca. 80 mmol C m\ yr\). That "nding, along with the high biogenic opal #uxes that we measured in the AZ, challenges the notion that this is a low-productivity region. Fluxes of biogenic material were maximum in austral summer and minimum in austral winter. The onset of maximum summer #ux was increasingly delayed to the south and appeared linked to the location of the receding ice edge (Fig. 5). It was followed by a brief #ux maximum that was terminated in late summer. This early termination was not associated with winter sea-ice expansion or with depletion of major nutrients in surface waters. We suggest that it was caused by depletion of

S. Honjo et al. / Deep-Sea Research II 47 (2000) 3521}3548

3545

a micronutrient, possibly iron, as a result of the strong summer strati"cation that results from sea-ice formation and summer melting. Material settling in MS-4 and MS-5 had similar opal content (81 and 85%, respectively), but very di!erent Si /C mole ratio (5.1 and 1.7). Lower ratios at
  MS-5 could result from better organic carbon preservation in the upper km due to a reduced zooplankton community, or formation of organic matter by nondiatomaceous phytoplankton. Such geographic variability in the Si /C rain ratio
  contrasts with seasonal co-variations in opal and C #ux at each station, and must  be considered when using sedimentary opal accumulation to constrain past changes in ocean productivity. Acknowledgements We are deeply indebted to R.F. Anderson and W.O. Smith who organized AESOPS under the auspices of US JGOFS and ONR. We thank M. Abbott, P. Froelich, C. Measures, K. Moore, D. Nelson, S. Rubin, C. Sweeney and T. Takahashi for valuable suggestions and discussions. We thank Master J. Borkowski, J. Alberts and the crew of Ice-breaker Research Vessel N.B. Palmer, National Science Foundation, who maintained high standards of professionalism during one of the most di$cult but successful sediment trap mooring array cruises. Without the dedicated and highly skilled joint OSU-WHOI mooring team, this program would never have materialized: They were C. Moser, who served as the mooring master for the recovery cruise, J. Billings, M. Bowels, C. Brooksforce, L. Costello, T. Gann, R. Krish"eld and D. Root. We are indebted to S. O'Hara who was in charge of CTD operation, narrowbeam sounding and assisted us in precision navigation for array deployment and recovery. The joint OSU-WHOI sample-analysis team, including L. Ball, P. Collier, C. Meredith and J. Palmieri completed the formidable analytical task with high standards. K. Brown's assistance in editing the manuscript and data tables was especially helpful. This research was supported by the O$ce of Polar Programs, National Science Foundation under grant number OPP-9530300. This is US JGOFS contribution number 510 and Woods Hole Oceanographic Institution contribution number 10025. References Archer, D., 1996. A data-driven model of the global calcite lysocline. Global Biogeochemical Cycles 10, 511}526. Berner, R.A., Honjo, S., 1981. Pelagic sedimentation of aragonite: its geochemical signi"cance. Science 211, 940}942. Boyle, E.A., 1998. Pumping iron makes thinner diatoms. Nature 393, 733}734. Broecker, W.S., Spencer, D.W., Craig, H. (Eds.), (1982). GEOSECS Paci"c Expedition, Vol. 3. IDOSNational Science Foundation, Washington DC, U.S. Government Printing O$ce, Washington, DC. Buesseler, K.O., Ball, L., Andrews, J., Benitez-Nelson, C., Belastock, R., Chia, F., Chao, Y., 1998. Upper ocean export of particulate organic carbon in the Arabian Sea derived from Thorium-234. Deep-Sea Research II 45, 2461}2487.

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