Journal of , Vol. 55, pp. 681 to 691. 1999

Seasonal Response of Planktonic Foraminifera to Surface Condition: Sediment Trap Results from the Central North Pacific Ocean

1 2 1 NOBUHISA O. EGUCHI , HODAKA KAWAHATA and ASAHIKO TAIRA

1Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japan 2Marine Geology Department, Geological Survey of Japan, Higashi 1-1, Tsukuba, Ibaraki 305-8567, Japan, and Graduate School of Science, Tohoku University, Sendai 980-8578, Japan

(Received 7 September 1998; in revised form 29 June 1999; accepted 29 June 1999)

The fluxes of planktonic foraminifera (calcareous shell producing ) were Keywords: examined in order to clarify temporal and regional variations in production in the ⋅ Sediment trap, upper ocean in relation to hydrographic conditions. Three time-series sediment traps ⋅ planktonic were deployed in the central North Pacific along 175°E for about one year, beginning foraminifera, ⋅ in June 1993. Trap sites were located in the subarctic, the transition, and the sub- organic matter, ⋅ seasonal varia- tropical water masses, from north to south. The southernmost site was under the in- tions, fluence of the transition zone in January to May. Both temporal and regional fluxes ⋅ food availability, of planktonic foraminifera showed large variations during the experiment. In the ⋅ surface ocean subarctic water mass, high total foraminiferal fluxes (TFFs) and high organic matter thermal condition, fluxes (OMFs) were observed during summer to fall, suggesting that food availability ⋅ central North is the most important factor for the production of planktonic foraminifera. Further- Pacific. more, low TFFs during winter were ascribed to low food availability and low tem- peratures. The OMFs and TFFs correlated well and increased rapidly after the dis- ruption of the seasonal thermocline during winter, peaking in late February to early March in the transition zone. In the subtropical water mass, both OMFs and TFFs remained low due to lower under oligotrophic conditions. In general, TFFs show a positive correlation with OMFs during the trap experiment, suggesting that food availability is one of the factors controlling the production of planktonic foraminifera in the central North Pacific. Relatively low TFFs during summer to fall in the subtropical water mass may be caused by the thermal structure of the upper ocean. Low SST possibly reduces the production of foraminifera during winter in the subarctic region.

1. Introduction al., 1981). In recent years the development of time-series Planktonic foraminifera have long been used for sediment traps, which allows the collection of consecu- paleoenvironmental reconstruction, based only on a gen- tive, short-duration samples (Honjo et al., 1980; Honjo eral understanding of individual species’ . How- and Doherty, 1988), has led to a better understanding of ever, their correspondence to the present-day ocean is still the sedimentation, or flux, of modern planktonic poorly understood, especially in the central North Pacific. foraminifera. Recent planktonic foraminifera studies us- A better understanding of modern assemblages of ing sediment traps have been conducted in the eastern planktonic foraminifera began with the first documenta- subarctic Pacific (Reynolds and Thunell, 1985; Sautter tion of the seasonal succession of the species composi- and Thunell, 1989), in the San Pedro Basin (Sautter and tion by Bé (1960), based on a year-long tow Thunell, 1991a, b; Thunell and Sautter, 1992), in the study of living foraminifera obtained from surface wa- Panama Basin (Curry et al., 1983; Thunell et al., 1983; ters near Bermuda. Since that time, many towing programs Thunell and Reynolds, 1984), in the Sargasso Sea (Deuser have been conducted in many oceanic regions (e.g., et al., 1981; Deuser, 1987; Deuser and Ross, 1989), in Boltovskoy, 1969; Tolderlund and Bé, 1971; Williams et the Bay of Bengal (Guptha et al., 1997), in the Arabian

681 Copyright © The Oceanographic Society of Japan. Sea (Curry et al., 1992; Steens et al., 1992) and in the 2. Method Japan Trench (Oda, 1989). These studies demonstrate a Three time-series sediment traps (PARFLUX Mark connection between seasonal successions of planktonic 7G-21: opening 0.5 m2) were deployed in the central North foraminifera and surface ocean conditions. Pacific along 175°E for about one year, beginning in June In the present study we document the seasonal vari- 1993. The traps were deployed individually at 1,412 m ations in the total fluxes of planktonic foraminifera as depth at Site 8 (46°07.2′ N, 175°01.9′ E), 1,482 m depth well as the organic matter fluxes through the water col- at Site 7 (37°24.2′ N, 174°56.7′ E) and 3,873 m depth at umn in the central North Pacific along the 175°E merid- Site 6 (30°00.1′ N, 174°59.7′ E), respectively (Fig. 1, ian and clarify the factors that regulate the production of Table 1). Each trap cup was filled with filtered deep sea planktonic foraminifera. water containing a 3% formaldehyde solution with so-

Table 1. Organic matter fluxes and total foraminiferal fluxes of three sediment trap sites in the central North Pacific Ocean. No correction is adopted for the time lag between species production and its deposition.

Sample number Trap cup Duration Estimate fluxes Open Close Organic matter Total foraminifera (days) (mg mÐ2day Ð1) (shells mÐ2day Ð1) Site 8 ET106 16 Jun. 93 30 Jun. 93 15 7.02 1451 ET107 1 Jul. 93 31 Jul. 93 31 37.31 4034 ET108 1 Aug. 93 31 Aug. 93 31 25.99 3617 ET109 1 Sep. 93 30 Sep. 93 30 17.50 2987 ET110 1 Oct. 93 31 Oct. 93 31 18.19 2828 ET111 1 Nov. 93 30 Nov. 93 30 17.58 18347 ET112 1 Dec. 93 15 Dec. 93 15 11.40 4471 ET113 16 Dec. 93 31 Dec. 93 16 5.59 2014 ET114 1 Jan. 94 15 Jan. 94 15 5.14 883 ET115 16 Jan. 94 31 Jan. 94 16 3.15 728 ET116 1 Feb. 94 15 Feb. 94 15 4.26 467 ET117 16 Feb. 94 28 Feb. 94 13 3.55 322 ET118 1 Mar. 94 15 Mar. 94 15 4.99 1156 ET119 16 Mar. 94 31 Mar. 94 16 3.62 1168 ET120 1 Apr. 94 15 Apr. 94 15 7.73 3317

Site 7 ET64 1 Jun. 93 15 Jun. 93 15 36.41 1073 ET65 16 Jun. 93 30 Jun. 93 15 41.30 800 ET66 1 Jul. 93 15 Jul. 93 15 15.91 949 ET67 16 Jul. 93 31 Jul. 93 16 8.43 582 ET68 1 Aug. 93 15 Aug. 93 15 7.57 350 ET69 16 Aug. 93 31 Aug. 93 16 10.87 ND ET70 1 Sep. 93 15 Sep. 93 15 8.25 361 ET71 16 Sep. 93 30 Sep. 93 15 3.80 196 ET72 1 Oct. 93 15 Oct. 93 15 5.00 836 ET73 16 Oct. 93 31 Oct. 93 16 7.85 806 ET74 1 Nov. 93 15 Nov. 93 15 12.05 1182 ET75 16 Nov. 93 30 Nov. 93 15 7.71 885 ET76 1 Dec. 93 15 Dec. 93 15 9.29 894 ET77 16 Dec. 93 31 Dec. 93 16 5.97 638 ET78 1 Jan. 94 15 Jan. 94 15 2.21 265 ET79 16 Jan. 94 31 Jan. 94 16 3.42 226 ET80 1 Feb. 94 15 Feb. 94 15 4.46 516 ET81 16 Feb. 94 28 Feb. 94 13 5.48 1019 ET82 1 Mar. 94 15 Mar. 94 15 12.77 418 ET83 16 Mar. 94 31 Mar. 94 16 22.93 2666 ET84 1 Apr. 94 9 Apr. 94 9 11.49 1845

682 N. O. Eguchi et al. Fig. 1. Location of three sediment traps along 175°E longitude with surface hydrography of the central North Pacific Ocean. The Kuroshio axis moves latitudinally, see text for detail.

Table 1. (continued).

Sample number Trap cup Duration Estimate fluxes Open Close Organic matter Total foraminifera (days) (mg mÐ2day Ð1) (shells mÐ2day Ð1) Site 6 ET43 16 Jun. 93 30 Jun. 93 15 2.61 ND ET44 1 Jul. 93 31 Jul. 93 31 2.37 265 ET45 1 Aug. 93 31 Aug. 93 31 3.00 199 ET46 1 Sep. 93 30 Sep. 93 30 1.89 265 ET47 1 Oct. 93 31 Oct. 93 31 1.57 451 ET48 1 Nov. 93 30 Nov. 93 30 1.54 300 ET49 1 Dec. 93 15 Dec. 93 15 2.47 414 ET50 16 Dec. 93 31 Dec. 93 16 3.05 396 ET51 1 Jan. 94 15 Jan. 94 15 3.77 ND ET52 16 Jan. 94 31 Jan. 94 16 8.33 898 ET53 1 Feb. 94 15 Feb. 94 15 8.97 843 ET54 16 Feb. 94 28 Feb. 94 13 8.40 604 ET55 1 Mar. 94 15 Mar. 94 15 9.23 571 ET56 16 Mar. 94 31 Mar. 94 16 13.49 1242 ET57 1 Apr. 94 15 Apr. 94 15 14.21 1860 ET58 16 Apr. 94 30 Apr. 94 15 7.49 703 ET59 1 May 94 15 May 94 15 2.80 319

Seasonal Response of Foraminifera in the Central North Pacific 683 dium borate (pH > 8). foraminifera, in terms of number of specimens per square Recovered sample bottles were immediately refrig- meter per day (shells mÐ2dayÐ1) were then made taking erated on board at approximately 2 to 4°C. In the labora- into account the sample split, the duration of each collec- tory, samples were passed through a 1 mm sieve, after tion period and the size of the opening of the sediment which the <1 mm fraction was split into aliquots with a trap (0.5 m2). In the present study we use number of rotary splitter. All flux data are based on the <1 mm size foraminifera greater than 125 µm as the total foraminifera. fraction, which dominates the flux. One half of each sample was used for chemical analy- 3. Oceanographic Settings ses to determine fluxes of carbonate, opal, organic car- The central North Pacific is characterized by two bon and lithogenics. Detailed analytical procedures used major water masses and the transition zone (e.g., Roden, for bulk analyses have been reported in Kawahata et al. 1970). The northernmost part is the subarctic water mass, (1998). The organic matter flux is derived by multiply- north of the subarctic front which is located around 40°N ing the organic carbon contents by 1.8; it is mainly pro- at 175°E (NEDO, 1995). The area described as the tran- duced by (Honjo, 1996). sition zone runs from the Kuroshio front to the subarctic A one-sixteenth to one-eighth split was used for front (e.g., Roden, 1970). The subtropical water mass lies planktonic foraminiferal analysis. The split samples were between the Kuroshio front and the North equatorial cur- gently re-sieved into three size fractions (smaller than 125 rent (NEC) (e.g., Roden, 1972) (Fig. 1). µm, 125Ð250 µm, and 250Ð1000 µm). All planktonic Although in-situ hydrographic data were only col- foraminifera greater than 125 µm were carefully picked lected in May 1994 during the sediment trap experiment out from the raw wet samples using a small, soft brush, (NEDO, 1995), weekly sea surface temperatures are avail- and then dried, counted and identified. Flux estimates of able as blended ship- and -derived SSTs for the

A

Fig. 2. Seasonal variations of (a) the total foraminiferal flux: (column) and the organic matter flux: (cross), (b) IGOSS sea surface temperature, and (c) thermal structure of the upper 300 m of the ocean. Fig. 2A: Site 8, Fig. 2B: Site 7, Fig. 2C: Site 6. Note that a one-month correction has been implemented to minimize the time lag between species production and its deposition.

684 N. O. Eguchi et al. 1° latitude-longitude grid data (weekly IGOSS NMC of (37.5°N, 174.5°E) and (37.5°N, 175.5°E) grid data of SSTs; Reynolds and Smith, 1994) (Figs. 2AÐC(b)). Long- IGOSS NMC SSTs for this site. SST varies from 12.1°C term monthly averages of vertical thermal structures for to 23.4°C (Fig. 2B), and the near-surface layer also the upper 300 m of the at each site are also changes seasonally from well-mixed to thermally strati- available from the World Ocean Atlas 1994 (Levitus and fied (Fig. 2B). During January to April, the upper part of Boyer, 1994) and are illustrated in Figs. 2 AÐC(c). the water column is essentially isothermal and the annual The surface hydrographic conditions of the trap sites SST minimum occurs in early March. These surface tem- are as follows: peratures identify the winter to spring near-surface con- Site 8 was located in the subarctic water mass ditions which are characterized by cold, nearly isother- throughout the experiment. We use linear interpolation mal, nonstratified water. In contrast, the summer to early of (45.5°N, 174.5°E), (45.5°N, 175.5°E), (46.5°N, winter, June to December, near-surface hydrography is 174.5°E) and (46.5°N, 175.5°E) grid data of IGOSS NMC characterized by thermal stratification and the annual SST SSTs for this site. The SST ranges from 3.8°C to 12.3°C maximum occurs in late August (Fig. 2B). (Fig. 2A). During winter to spring (December to May) a Site 6 was seasonally influenced by two different deep mixed layer is developed and SST ranges from 3.8 water masses. The boundary between the transition zone to 7.2°C, with the minimum occurring in late February and the subtropical water mass existed at 30°N in May (Fig. 2A). Between June and September thermal stratifi- 1994 (NEDO, 1995), when the Kuroshio axis flowed east- cation intensifies and SST increases to its yearly maxi- ward at about 32°N on 175°E line (JMA, 1994). It is sug- mum of 12.3°C. Stratification of the upper water column gested that the transition zone covers at least 2° south of diminishes between September and December, returning the Kuroshio axis. Since the Kuroshio axis moves south- to the nearly isothermal winter-to-spring conditions. ward approximately 1 to 2° during January to May 1994 Site 7 was under the influence of the transition zone (JMA, 1994), the site is influenced by the transition zone during the sampling period. We use linear interpolation during this period. By contrast, the site is placed within

B

Fig. 2. (continued).

Seasonal Response of Foraminifera in the Central North Pacific 685 the subtropical water mass during June to December as At Site 8, the OMFs vary from 3.2 to 37.3 mg the axis moves far north. We use linear interpolation of mÐ2dayÐ1 and the TFFs from 320 to 18,350 shells mÐ2 (29.5°N, 174.5°E), (29.5°N, 175.5°E), (30.5°N, 174.5°E) dayÐ1 (Table 1, Fig. 2A). The experimental interval is di- and (30.5°N, 175.5°E) grid data of IGOSS NMC SSTs vided into three periods (Periods 8A, 8B and 8C; Fig. for this site. SST ranges from 17.5°C to 27.0°C (Fig. 2C). 2A). Six species of planktonic foraminifera are observed During January to April the upper water column is nearly in this site. Period 8A (May to early November and March) isothermal and the annual SST minimum occurs in early is characterized by relatively high OMFs and high TFFs. March. The seasonal thermocline starts to appear in May The OMFs ranges between 7.0 and 37.3 mg mÐ2dayÐ1 and is well developed until October, and the annual SST whereas the TFFs range between 450 and 4,470 shells maximum is approximately 27°C in September. mÐ2dayÐ1. Globigerina quinqueloba, Tenuitellinata sp. and Neogloboquadrina pachyderma (sinistral) show a rela- 4. Results tively high flux during this period. Tenuitellinata sp. ob- The total foraminiferal fluxes (TFFs), based on served in the sediment trap samples is very similar to T. foraminifera greater than 125 µm for the three sites, are angustiumbilicata Bolli. However T. angustiumbilicata presented together with the organic matter fluxes (OMFs) has never been reported from the present ocean, thus a in Figs. 2AÐC and Table 1. All sites show remarkable precise examination is being conducting on this species. seasonal TFFs patterns during the experiment, and the On the other hand, during Period 8C (late November to experimental interval at each site can be divided into sev- February), the OMFs and TFFs show lower values, from eral periods, based upon seasonal variations in the OMFs 3.2 to 5.6 mg mÐ2dayÐ1 and 320 to 1,170 shells mÐ2dayÐ1, and TFFs. Faunal analysis of planktonic foraminifera is respectively. Period 8B corresponds to one sampling du- partly reported in Eguchi et al. (1998) and is under prepa- ration in October with a prominent TFF maximum ration for other publications. (>18,000 shells mÐ2dayÐ1). G. quinqueloba, Globigerina

C

Fig. 2. (continued).

686 N. O. Eguchi et al. umbilicata and N. pachyderma (sinistral) show their maxi- and Hutson, 1977), seasonal successions of the produc- mum shell production at this period and among them, N. tion of phytoplankton affects the planktonic foraminifera’s pachyderma (sinistral) occupies more than 50% of the diet. Thus, in the present context, observed OMF is re- flux. garded as a proxy of food availability for planktonic At Site 7, the OMFs and TFFs range from 2.2 to 41.3 foraminifera. mg mÐ2dayÐ1 and 230 to 2,670 shells mÐ2dayÐ1, respec- The depth of Site 6 (approximately 3,800 tively (Table 1, Fig. 2B). A total of 28 species of plank- m) is different from those of Sites 7 and 8 (approximately tonic foraminifera have been identified in this site. Pe- 1,400 m). This difference of mooring depth may affect riod 7A (May to July) corresponds to a period when OMFs the deposition of OMF and TFF during descent by disso- are high (between 7.6 and 41.3 mg mÐ2dayÐ1). However, lution and phase lag. However, Honjo et al. (1982) dem- the TFFs show intermediate values between 350 and 1,070 onstrated a slower rate of dissolution of organic carbon shells mÐ2dayÐ1. Neogloboquadrina dutertrei and G. with depth within the bathypelagic layer. Furthermore, quinqueloba show their maximum shell production dur- several investigators, such as Berger and Piper (1972), ing Period 7A. Period 7B (August to December) is char- Adelseck and Berger (1975), Honjo (1977), and Thunell acterized by stratified surface water. The OMFs and TFFs and Honjo (1981), demonstrated that very little or no dis- vary between 2.2 and 12.4 mg mÐ2dayÐ1 and between 230 solution of carbonate takes place during settlement and 1,180 shells mÐ2dayÐ1, respectively. The foraminiferal through the water column. Therefore, in the present pa- assemblage is characterized by Globigerinoides ruber per, we do not consider the dissolution effect for both during this period. The OMFs are at an intermediate level OMF and TFF between different mooring depths. In ad- (4.5 to 22.9 mg mÐ2dayÐ1), while the TFFs show higher dition to these facts, previous observation suggests that values, ranging from 420 to 2,670 shells mÐ2dayÐ1 during foraminiferal shells sink rapidly and calculations show Period 7C (January to March). Globorotalia that it takes three to twelve days to reach a 3,800 m depth truncatulinoides, N. pachyderma (dextral), Globigerinita from the ocean surface, based on the reported sinking glutinata, Globorotalia inflata, Globorotalia hirsuta and speed (Takahashi and Bé, 1984). This observation implies Globigerina falconensis display their maximum shell pro- that sinking foraminifera shells take one to eight days to duction during Period 7C. reach 3,800 m from 1,400 m depth. In addition to that, At Site 6, the OMFs and TFFs vary from 1.5 to 14.2 Honjo and Manganini (1993) reported that there was no mg mÐ2dayÐ1 and 200 to 1,860 shells mÐ2dayÐ1, respec- offset of arrival time for foraminiferal shell fluxes at tively (Table 1, Fig. 2C). A total of thirty species of plank- 1,000, 2,000 and 3,400 m depth traps in the North Atlan- tonic foraminifera are identified in this site (Eguchi et tic, indicating their rapid settling speed. Since the sam- al., 1998). Period 6A (June to November) is character- pling duration of all samples is 13 to 31 days, and taking ized by low OMFs and low TFFs, ranging from 1.5 to 3.1 these earlier results into consideration, there would be no mg mÐ2dayÐ1 and 200 to 450 shells mÐ2dayÐ1, respectively. offset of arrival time between among the three sites. The foraminiferal assemblage is dominated by G. ruber, However, we expect that some significant time pe- Globigerinoides sacculifer and Globigerinoides tenellus riod exists from a specimen’s initial production at the during Period 6A. By contrast, both the OMFs and TFFs surface water and its final deposition in the trap. Depend- show higher values, ranging from 2.8 to 14.2 mg mÐ2 ing on the life cycle of the individual, a life cycle period dayÐ1 and 320 and 1,860 shells mÐ2dayÐ1 during Period can be on the order of two to four weeks (Sautter and 6B (December to February), respectively. G. Thunell, 1991a). The entire span of the time required thus truncatulinoides, G. inflata, G. hirsuta, G. aequilateralis, depends on the duration of shell production, survival time G. glutinata and G. ruber show their optimum produc- and sinking time, all of which may thus vary between tion during Period 6B. Especially, G. truncatulinoides, approximately two and five weeks. Therefore in the fol- G. inflata and G. hirsuta display their maximum produc- lowing discussion, the hydrographic data (IGOSS SST tion in early March. and LEVITUS 94) should be adjusted by one month in order to correlate the periods of production in the photic 5. Discussion zone with the deposition in the sediment trap. As a first-order approximation, it has been suggested that organic matter flux (OMF) in the sediment trap can 5.1 Subarctic (Site 8) be closely correlated to the export production from the Food availability, thermal structure and SST affect upper ocean (Deuser and Ross, 1980) and is primarily the production of planktonic foraminifera (Bé and Hutson, controlled by phytoplankton production (Honjo, 1996). 1977) and phytoplankton production in the surface ocean Because some species of planktonic foraminifera are pri- is mainly controlled by nutrient concentration and solar mary consumers (herbivores), others are secondary con- light intensity in the upper ocean (Lalli and Parsons, sumers (carnivores) and still others are omnivores (Bé 1997). Although the nutrient supply to the surface ocean

Seasonal Response of Foraminifera in the Central North Pacific 687 is generally higher in the high latitudes, solar radiation is foraminifera, low food availability prohibits foraminiferal much more limited during winter time (Lalli and Parsons, production, resulting in low TFFs during this period. A 1997). A well developed mixed layer can supply nutri- similar observation has been reported from the eastern ents to the surface ocean during winter to early spring subarctic Pacific (Thunell and Honjo, 1987). (Period 8C). However, both the OMFs and TFFs remain Another factor might affect the flux of planktonic low throughout this period (Fig. 2A). One of the plausi- foraminifera during this period; the optimum tempera- ble reasons is limitation of phytoplankton production by ture range for the subpolar assemblage of planktonic a light intensity that is too weak for photosynthesis. Since foraminifera (e.g., G. bulloides, G. quinqueloba and N. phytoplankton is the main food source for planktonic pachyderma) is considered to be 5Ð10°C (Bé, 1977). SST during this period was 4.0Ð5.7°C, which was definitely lower than the proposed optimum temperatures. There- fore these SSTs possibly caused the production of plank- tonic foraminifera to decline. The TFFs and OMFs shows relatively high values during Periods 8A and 8B. In general, the TFFs are well correlated with OMFs, except for the prominent TFF peak during Period 8B (Fig. 3). In the subarctic region, diatom production often plays an important role in primary pro- duction. Since the subarctic species (e.g., N. pachyderma, G. glutinata) prefer to prey on phytoplankton (Hemleben et al., 1989), the food availability probably controls the production of foraminifera during Period 8A and 8B in the subarctic region. During Period 8B, TFF shows a very high peak of up to 18,000 shells mÐ2dayÐ1. Although there are no large differences in OMFs, a deepening of the surface mixed layer and a surface temperature of 8Ð10°C might have expanded the optimum habitat of the planktonic foraminiferal assemblage during this period. There is a difference in TFF variation pattern between central and eastern subarctic sites during summer. TFF is maintained at Site 8, while it decreases rapidly in the east- ern subarctic Pacific site (50°N, 145°E) (Reynolds and Thunell, 1985). Thunell and Honjo (1987) suggested that this rapid decrease was due to the development of a sea- sonal thermocline and an increase in SST during sum- mer. The eastern subarctic site is under the influence of the North Pacific Drift, which falls within a transitional water mass during summer. A non-subarctic foraminiferal species, Orbulina universa, is reported in the eastern subarctic Pacific site during summer (Sautter and Thunell, 1989), while only subarctic species were observed at Site 8 throughout the experiment. Compared with the transi- tion water mass, the subarctic region shows relatively higher , which could enhance food availability and the production of foraminifera during summer in the central North Pacific site, even when the seasonal thermocline is developed and SST increases.

5.2 Transition (Site 6 during winter to spring, and Site 7) The transition water mass covers Site 7 throughout Fig. 3. Correlation between the total foraminiferal flux and the the experiment and Site 6 during winter to spring when organic matter flux at three sites. Note that vertical axis is different in Site 8. the Kuroshio axis moves southward.

688 N. O. Eguchi et al. The OMFs and TFFs increase rapidly after the dis- 5.4 Factors controlling the production of planktonic ruption of the seasonal thermocline during Period 7C and foraminifera then peak in late February to early March. The produc- In general, TFFs show a positive correlation with tion of foraminifera is related to the expansion of the OMFs in the central North Pacific during the trap experi- mixed layer, as is also observed in the Sargasso Sea ment at each site (Fig. 3), although the amplitude of TFFs (Deuser, 1986). In the Sargasso Sea, the mixing processes are different among the sites. This suggests that the food reaches their maximum extent by February/March, result- availability may be one of the controlling factors for the ing in high OMF and TFF (Deuser et al., 1981; Deuser, production of planktonic foraminifera in the upper ocean. 1986). The TFF maximum in Period 7C is caused by a However, larger and smaller productions of planktonic large contribution from herbivorous species, such as G. foraminifera than expected from the OMF-TFF relation- hirsuta, G. inflata, G. truncatulinoides and N. pachyderma ship, although in a small range, were observed during (dextral). Period 6B exhibits one broad maximum with summer to fall in the subtropical water mass (Period 6A) relatively intermediate values in the OMFs and TFFs. and in winter in the subarctic (Period 8C), respectively. During Period 6B, Site 6 is under the influence of the During Period 6A, the foraminiferal assemblage was re- transition water mass. Therefore variations in OMFs, TFFs stricted to the surface dwelling warm stratified water pre- and surface ocean conditions are similar to those during ferred species (G. ruber, G. sacculifer and G. tenellus). 7C. There are good correlations between OMFs and TFFs These species are symbiont-bearing species (Hemleben during Period 6B and Period 7C (Fig. 3). Therefore food et al., 1989), thus the production of these species are in- availability also plays an important role for foraminiferal dependent of phytoplankton production. The abundant production during these periods, and the same winter TFF production of these species may caused the discrepancy maximum, which responded to a diatom winter bloom- between OMF and TFF during Period 6A. In addition to ing, is also reported from the San Pedro Basin (Sautter the low food availability, judging from low OMFs, low and Thunell, 1991a). SST also prohibited the production of foraminifera dur- Period 7B is characterized by relatively low OMFs, ing Period 8C. high SST and a developed thermocline, which can reduce The OMF-TFF relationship shows a positive corre- the supply of nutrients to the upper ocean. Therefore, lation at three sites; however, different slopes were ob- phytoplankton is less abundant in the upper ocean. The served at each period (Fig. 3). It is considered that this TFFs are almost at the same level as during Period 7A, difference is due to the production of a different faunal despite the occurrence of different faunal groups between assemblage during each period. Thus, further investiga- Periods 7A and 7B. The shallow dwelling, symbiont bear- tion of the foraminiferal assemblages may illuminate these ing warm water species G. ruber is abundant and com- difference of TFFs. prises up to 68% of TFFs during Period 7B, whereas N. dutertrei and G. quinqueloba dominate during Period 7A. 6. Summary and Conclusions A similar occurrence of G. ruber is reported from the San Three time-series sediment traps moored in differ- Pedro Basin (Sautter and Thunell, 1991a) and the Panama ent oceanic regions in the central North Pacific Ocean Basin (Thunell and Reynolds, 1984), and is described as provides good examples of seasonal and regional varia- a result of increased thermal stratification and warm sur- tions in planktonic foraminiferal fluxes as well as organic face water. Also, as Hemleben et al. (1989) pointed out, matter fluxes as a function of changing hydrographic con- the symbiont activity might play an important role in the ditions in the regions. abundance of symbiont-bearing foraminifera during Pe- (1) In the subarctic, high OMFs and high TFFs dur- riod 7B. ing summer suggest that food availability is the most important factor governing the production of planktonic 5.3 Subtropical (summer to fall at Site 6) foraminifera. By contrast, low TFFs during the winter are Both OMFs and TFFs remain low during Period 6A attributed to low food availability and low temperatures. (Fig. 2C). This is attributed to the influence of the sub- (2) In the transition zone, the OMFs and TFFs in- tropical water mass, marked by an oligotrophic condi- creased rapidly after the disruption of the seasonal tion (Longhurst et al., 1995). The planktonic foraminiferal thermocline during winter and then peaked in late Febru- assemblages during Period 6A mainly consist of shallow ary to early March. Summer was characterized by rela- dwelling, symbiont-bearing warm water species such as tively low OMFs, high SST and a developed thermocline. G. ruber, G. sacculifer and G. tenellus. This observation Shallow dwelling, symbiont bearing warm water species is consistent with relatively high SST and well developed were abundant during this period. seasonal thermocline in the surface water. (3) Both OMFs and TFFs remained low due to lower productivity in oligotrophic conditions.

Seasonal Response of Foraminifera in the Central North Pacific 689 In general, TFFs shows positive correlation with (1992): Foraminiferal production and monsoonal OMFs in the central North Pacific and suggests that the in the Arabian Sea: evidence from sediment traps. p. 93Ð food availability is one of the controlling factors for the 106. In Upwelling Systems: Evolution since the Early production of planktonic foraminifera in the upper ocean. Miocene, ed. by C. P. Summerhayes, W. L. Prell and K. C. Furthermore, the thermal structure of upper ocean also Emeis, Geological Society Special Publication 64, The Geo- logical Society, London. has an influence on foraminiferal production during a low Deuser, W. G. (1986): Seasonal and interannual variations in OMF period. Low SST also reduces the production of deep-water particle fluxes in the Sargasso Sea and their re- foraminifera in the subarctic region. lation to surface hydrography. Deep-Sea Res., 33(2), 225Ð 246. Acknowledgements Deuser, W. G. (1987): Seasonal variations in isotopic composi- The authors would like to express their sincere ap- tion and deep-water fluxes of the tests of perennially abun- preciation to Prof. H. Ujiié (Takusyoku University) for dant planktonic foraminifera of the Sargasso Sea: results instructive suggestions and for faunal analysis of plank- from sediment-trap collections and their paleoceanographic tonic foraminifera, to three anonymous reviewers for con- significance. J. Foraminiferal Res., 17(1), 14Ð27. structive comments and suggestions which helped im- Deuser, W. G. and E. H. Ross (1980): Seasonal change in the prove the manuscript, and to Dr. I. Motoyama (Univer- flux of organic carbon to the deep Sargasso Sea. Nature, 283, 364Ð365. sity of the Ryukyus) for permission to use a location map Deuser, W. G. and E. H. Ross (1989): Seasonally abundant of the northwestern Pacific ocean. Their appreciation is planktonic foraminifera of the Sargasso Sea: succession, also extended to Prof. H. Tokuyama (Ocean Research deep-water fluxes, isotopic compositions, and Institute, University of Tokyo), Dr. A. Nishimura (Geo- paleoceanographic implications. J. Foraminiferal Res., logical Survey of Japan), and Dr. Y. Tanaka (Geological 19(4), 268Ð293. Survey of Japan). This study was supported by the fol- Deuser, W. G., E. H. Ross, C. Hemleben and M. Spindler (1981): lowing research programs; “Northwest Pacific Carbon Seasonal changes in species composition, numbers, mass, Cycle Study” consigned to the Kansai Environmental size, and isotopic composition of planktonic foraminifera Engineering Center Co., Ltd. by the New Energy and In- settling into the deep Sargasso Sea. Palaeogeogr., dustrial Technology Development Organization and Palaeoclimatol., Palaeoecol., 33, 103Ð127. “Study on Paleoceanography” funded by Geological Sur- Eguchi, N. O., H. Kawahata and A. 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