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Western equatorial Pacific planktic foraminiferal fluxes and assemblages during a La Niña year (1999)
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Marine Micropaleontology 66 (2008) 304 – 319 www.elsevier.com/locate/marmicro
Western equatorial Pacific planktic foraminiferal fluxes and assemblages during a La Niña year (1999) Makoto Yamasaki a,⁎, Akira Sasaki b , Motoyoshi Oda c , Hanako Domitsu c a
Institute of Applied Earth Sciences, Faculty of Engineering and Resource Science, Akita University, 1-1Tegata-Gakuencho, Akita 010-8502, Japan b Cosmo Energy Exploration and Development Ltd., 1-1-1 Shibaura, Minato, Tokyo 105-8528, Japan c Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, 6-3 Aza-Aoba, Aramaki, Sendai 980-8578, Japan Received 14 March 2007; received in revised form 11 October 2007; accepted 31 October 2007
Abstract A correlation between foraminiferal community dynamics and environmental conditions may provide a basis for establishing paleoclimatic proxies. We studied planktic foraminiferal shell fluxes and assemblages in samples collected in three time-series sediment trap deployments in the western equatorial Pacific under La Niña conditions from January to November 1999. Eleven species contributed about 90% of the total flux in all traps. Two sites (MT1, MT3) in the Western Pacific Warm Pool region (WPWP) were characterized by common occurrences of the species Globigerinoides ruber, Globigerinoides sacculifer, Globigerinoides tenellus, and Neogloboquadrina dutertrei. Site MT5 farther to the east in the equatorial upwelling region had common occurrences of Globigerina bulloides, Globigerinita glutinata, and Pulleniatina obliquiloculata. Very high abundances of G. bulloides and G. glutinata at MT5 indicate that equatorial upwelling (EU) occurred during the 1999 La Niña. The two western sites have similar assemblage compositions, but MT1 (∼ 135°E) has the highest fluxes (up to ∼3800 tests m− 2 day− 1), whereas MT3 (∼ 145° E) has fluxes below ∼2200 tests m− 2 day− 1. Relatively high fluxes (up to ∼ 3000 tests m− 2 day− 1) occur at site MT5 (∼ 176° E), where upwelling occurred. The differences in faunal composition in the WPWP and EU might be attributable to differences in the way in which nutrients are supplied to the phytoplankton: large amounts of suspended material are supplied to the WPWP by advection of waters passing through the coastal region of an archipelago, whereas upwelling of nutrient-rich waters enhances primary production in the EU. At the westernmost site in the WPWP, a peak in the G. bulloides flux coincided with southward flow of the New Guinea Coastal Current (NGCC) in late February, but the highest G. ruber flux coincided with northward flow of this current in late May. Thus, the differences in species dominance at this location may be caused by monsoon-driven variability in the flow direction of the NGGC. © 2007 Elsevier B.V. All rights reserved. Keywords: Equatorial upwelling; La Niña; Monsoon; Planktic foraminifera; Western Pacific Warm Pool; Western equatorial Pacific
1. Introduction
⁎ Corresponding author. Fax: +81 18 837 0401. E-mail address: [email protected] (M. Yamasaki). 0377-8398/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2007.10.006
The Western Pacific Warm Pool (WPWP) in the equatorial Pacific has the highest sea surface water temperature (SST; N 28 °C) of the global open ocean, and is the largest single expanse of warm water on earth
M. Yamasaki et al. / Marine Micropaleontology 66 (2008) 304–319
(Yan et al., 1992). It thus represents a major source of heat transfer. The WPWP also plays an important role in the dynamics of the El Niño/Southern Oscillation (ENSO). During El Niño events, the WPWP expands toward the eastern Pacific, but it remains restricted to the western Pacific under La Niña conditions. At these times, the Indonesian Low over the WPWP and the Indonesian Archipelago generate cumulonimbus clouds which bring heavy rainfall (Brown et al., 1989). As a result of these Asian tropical monsoons, the Indonesian Archipelago and New Guinea have the highest annual precipitation in the world (N2000 mm; Nieuwolt, 1977). Under El Niño conditions, the Indonesian Low shifts toward the central Pacific, often causing droughts. The WPWP is one of the most important components of the earth's climate system, and elucidating its evolution over geologic time is necessary to enable us to develop better predictions of global environmental change. Presently, however, we do not understand the evolution of the WPWP. For instance, recent studies have yielded contradictory conclusions regarding the conditions during the period of early Pliocene warmth. Some authors argued that this warm period was
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characterized by permanent El Niño-like conditions, based on the reconstruction of a strong paleo-sea surface temperature (SST) gradient across the tropical Pacific (Wara et al., 2005), whereas others argued for cool, La Niña-like conditions in this time period (Rickaby and Halloran, 2005). A lack of detailed understanding of the modern ecological dynamics of planktic foraminifera (e.g., seasonal variability of species composition and productivity) may be at least one of the causes of this disagreement. Foraminiferal analyses using sediment trap samples have been carried out at many sites in the Atlantic, Indian, and Pacific oceans (e.g., Deuser et al., 1981; Sautter and Thunell, 1991; Curry et al., 1992; Conan and Brummer, 2000; Kincaid et al., 2000), but are limited in the WPWP region. The study by Kawahata et al. (2002) gives information on two sites (Site N1: 2°59.8′ N, 135°01.5′ E; Site N2: 4°07.5′ N, 136°16.6′ E) under El Niño conditions only. Therefore, we examined the relation between foraminiferal species composition, shell fluxes, and hydrographic conditions, using data collected on samples obtained from time-series sediment traps, in
Fig. 1. Location of sediment trap Sites MT1, MT3, and MT5, surface currents, undercurrents, and eddies in the western equatorial Pacific (modified from Fine et al., 1994). The surface currents are the Kuroshio Current, Mindanao Current (MC), North Equatorial Current (NEC), North Equatorial Countercurrent (NECC), South Equatorial Current (SEC), and New Guinea Coastal Current (NGCC; a bidirectional arrow indicates change in flow direction with the monsoon, which is eastward during winter and westward during summer). The undercurrents are the Equatorial Undercurrent (EUC), New Guinea Coastal Undercurrents (NGCUC), and Great Barrier Reef Undercurrent (GBRUC). The eddies are the Mindanao Eddy (ME) and Halmahera Eddy (HE).
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order to investigate ecological factors controlling temporal and spatial distributions of planktic foraminifera. In this paper, we present detailed findings with special emphasis on the La Niña condition as an initial step toward providing reliable paleoclimate proxies for La Niña/ El Niño conditions in the western equatorial Pacific.
(NGCC) changes flow directions with wind directions. The NGCC flows northwestward under the influence of southeasterly winds in boreal summer and southeastward with the northwesterly winds in boreal winter (Wyrtki, 1961; Kuroda, 2000).
2. Environmental settings
Data were collected in1999 after the end of the very strong El Niño of 1998. Both the NINO3 index (Fig. 2a) and the Southern Oscillation Index (SOI; Fig. 2b) indicate that the Pacific was under normal to La Niña conditions during the study period. Sites MT1 and MT3 lie within the WPWP (SST N 28 °C) throughout the year (Fig. 2c). The WPWP is characterized by overall low primary productivity, as seen in the low nitrate and chlorophyll a concentrations (e.g., Picaut et al., 1996). Relatively low SSTs (26.5–28 °C) were measured at
2.1. Ocean currents The complex system of currents affecting the study sites (Fig. 1) includes the North Equatorial Countercurrent (NECC), the South Equatorial Current (SEC), the Equatorial Undercurrent (EUC), and the New Guinea Coastal Undercurrent (NGCUC). Due to the influence of monsoons, the New Guinea Coastal Current
2.2. ENSO conditions during the study period
Fig. 2. (a) The NINO3 index and the NINO3 5-month moving average. The NINO3 index is the SST anomaly averaged over [5°S–5°N] and [150– 90°W], obtained from the Integrated Global Ocean Services System (IGOSS; Reynolds and Smith, 1994). (b) The Southern Oscillation Index (SOI) and the SOI 5-month moving average for 1999 (Japan Meteorological Agency, 2000). The SOI was calculated from air pressure differences between Tahiti and Darwin. (c) SST at the equator in the Pacific in 1999 with longitude of trap sites indicated. The SST data were taken from the Tropical Atmosphere Ocean (TAO) Project office Web site (Mangum et al., 1998).
M. Yamasaki et al. / Marine Micropaleontology 66 (2008) 304–319 Fig. 3. Variation of monthly precipitation averaged over (a) 10° N–10° S and 120°–130° E and (b) 0°–10° S and 130°–150° E in 1999. These data were based on rain gauge and satellite estimates and distributed by the National Centers for Environmental Prediction (NCEP) — Climate Prediction Center (CPC) (http://www.cpc.ncep.noaa.gov/). Dashed lines and numbers in parentheses indicate annual mean precipitation (mm day− 1). Variations in the air–sea environment, wind speed (solid line) and direction (dashed line), SST, and isotherm data in 1999 near (c) MT1, (d) MT3, and (e) MT5. Monthly wind speed and direction data were published by the NCEP — National Center for Atmospheric Research (NCAR) reanalysis project (http://www.cdc.noaa.gov/) for a 2.5° longitude–latitude grid. Weekly SST data were provided by the IGOSS (Reynolds and Smith, 1994) for a 1° longitude–latitude grid. Five-day vertical isotherm data were published by the TAO project office (Mangum et al., 1998) for the TAO buoy located in the equatorial Pacific.
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Site MT5, especially from January to April 1999. These SSTs indicate that upwelling occurred at that site, as the western end of the cold tongue extended from the eastern equatorial Pacific. After May, the site was transitional between the WPWP and the cold tongue (Fig. 2c). The three sediment trap sites thus can be categorized into two regions: the WPWP region (Sites MT1 and MT3) and the cold tongue region with equatorial upwelling (EU, Site MT5).
throughout the year. At Site MT5, water temperatures decreased sharply with water depth within the upper euphotic zone (∼0–100 m), and wind speed decreased from January to May (Fig. 3e). Strong winds present from January to May thus resulted in cooler SSTs because of substantial upwelling. 3. Materials and methods 3.1. Location and duration of sediment trap deployment
2.3. Wind and precipitation Rain gauge measurements and satellite estimates are available to document precipitation in the study area (the Climate Prediction Center; Fig. 3a–b). Monthly wind speed and direction data (Fig. 3c–e) are available online (National Center for Atmospheric Research reanalysis project; http://www.cdc.noaa.gov/). Monthly precipitation in the Indonesian Archipelago and New Guinea was high from January to April (Fig. 3a–b). At Sites MT1 and MT3, the wind direction was north to east from January to March and east to south from June to November. Sites MT1 and MT3 are located within the monsoon-impacted area, and the NGCC changes its direction together with the monsoon wind close to MT1 (Fig. 4). Site MT5 is within the trade-wind belt, thus characterized by a stable wind direction (east) and seasonal variation in water temperature and wind speed. Strong winds occurred from January to May.
Time-series sediment traps with bottom-tethered moorings (SMD26S-6000, 0.5 m2 collecting aperture; Nichiyu-Giken Kogyo Co. Ltd., Tokyo, Japan) were deployed at three sites (MT1: 4°2.911′ N, 135°0.019′ E; MT3: 0°0.843′ S, 145°1.580′ E; and MT5: 0°2.343′ N, 175°56.432′ E) along an E–W transect in the western equatorial Pacific from January to November 1999 (Table 1). The collection depths of the sediment traps were about 1000 m, and samples were collected at approximately 2-week intervals. The sinking velocity of planktic foraminifera (N 150 μm) was estimated to be 300–1300 m day− 1, depending on their shell weight and the presence or absence of spines (Takahashi and Bé, 1984). We did not consider the time lag between presence at the surface and arrival at the traps (6.25 days to reach about 1000 m water depth) because it was shorter than the duration of the trapping intervals. 3.2. Sample preparation
2.4. Isotherms and SST Sea surface temperatures at Sites MT1 and MT3 were constant, around 30 ± 1 °C throughout the year (Fig. 3c–d). The stratification of the water column (0–300 m) was relatively constant at Site MT3, and for site MT1 during the time interval for which data were accessible. No buoy data were available from the Tropical Atmosphere Ocean Project (TAO) for MT1 from April to October 1999, but the data for 2002, a La Niña year, show constant isotherms
Prior to settling of the sediment trap systems, polyethylene bottles were filled with filtered seawater, and a solution of 3% neutralized formalin with sodium tetraborate was added to retard bacterial activity. After recovery, the samples were stored in a cool (∼ 2–4 °C), dark place. In the laboratory, samples were wet-sieved through a 1-mm-mesh screen to remove large particles, and then smaller fractions (b1 mm) were sorted into aliquots using a precision four-head rotary splitter
Fig. 4. Summary of defined periods at each site.
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Table 1 Sediment trap mooring locations and sampling schedules MT1 Location: 4°02.911′ N, 135°00.019′ E Water depth: 4762 m Collection depth: 970 m Sample Trap cup number open S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22
– close
1–Jan–99 – 16-Jan-99 – 1-Feb-99 – 16-Feb-99 – 1-Mar-99 – 16-Mar-99 – 1-Apr-99 – 16-Apr-99 – 1-May-99 – 16-May-99 – 1-Jun-99 – 16-Jun-99 – 1-Jul-99 – 16-Jul-99 – 1-Aug-99 – 16-Aug-99 – 1-Sep-99 – 16-Sep-99 – 1-Oct-99 – 16-Oct-99 – 1-Nov-99 – 16-Nov-99 –
15-Jan-99 31-Jan-99 15-Feb-99 28-Feb-99 15-Mar-99 31-Mar-99 15-Apr-99 30-Apr-99 15-May-99 31-May-99 15-Jun-99 30-Jun-99 15-Jul-99 31-Jul-99 15-Aug-99 31-Aug-99 15-Sep-99 30-Sep-99 15-Oct-99 31-Oct-99 15-Nov-99 20-Nov-99
MT3 Location: 00°00.843′ S, 145°01.580′ E Water depth: 3680 m Collection depth: 1020 m
Duration Sample Trap cup (days) number open 15 16 15 13 15 16 15 15 15 16 15 15 15 16 15 16 15 15 15 16 15 5
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22
5-Jan-99 16-Jan-99 1-Feb-99 16-Feb-99 1-Mar-99 16-Mar-99 1-Apr-99 16-Apr-99 1-May-99 16-May-99 1-Jun-99 16-Jun-99 1-Jul-99 16-Jul-99 1-Aug-99 16-Aug-99 1-Sep-99 16-Sep-99 1-Oct-99 16-Oct-99 1-Nov-99 16-Nov-99
(WSD-4; McLane Research Laboratories, Inc.). Each sample was divided into 16 aliquots, one of which was used for planktic foraminiferal analysis. Samples were further wet-sieved into size fractions and filtered onto a membrane with 0.8-μm diameter pores. Filtered samples were dried at 39 °C. Individuals (N 125 μm) were picked, counted, and identified under a stereomicroscope. Micrographs were taken with a field emission scanning electron microscope (FE-SEM, JSM-6330F; JEOL Co. Ltd., Tokyo, Japan) at Tohoku University. Flux estimates (tests m− 2 day− 1) were made based on the sample aliquot (1/16), the duration of each collecting period, and the size of the collecting aperture of the sediment trap (0.5 m2). 4. Results 4.1. Foraminiferal flux Total foraminiferal fluxes were high during the first half of the year at Sites MT1 and MT3 (WPWP), and high throughout the year at Site MT5 (EU) (Fig. 5). The average annual fluxes were higher at Sites MT1 and MT5 than at Site MT3.
MT5 Location: 00°02.343′ N, 175°56.432′ E Water depth: 4828 m Collection depth: 1040 m
– close
Duration Sample Trap cup (days) number open
– close
– – – – – – – – – – – – – – – – – – – – – –
11 16 15 13 15 16 15 15 15 16 15 15 15 16 15 16 15 15 15 16 15 5
– – – – – – – – – – – – – – – – – – – – – –
15-Jan-99 31-Jan-99 15-Feb-99 28-Feb-99 15-Mar-99 31-Mar-99 15-Apr-99 30-Apr-99 15-May-99 31-May-99 15-Jun-99 30-Jun-99 15-Jul-99 31-Jul-99 15-Aug-99 31-Aug-99 15-Sep-99 30-Sep-99 15-Oct-99 31-Oct-99 15-Nov-99 20-Nov-99
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22
13-Jan-99 16-Jan-99 1-Feb-99 16-Feb-99 1-Mar-99 16-Mar-99 1-Apr-99 16-Apr-99 1-May-99 16-May-99 1-Jun-99 16-Jun-99 1-Jul-99 16-Jul-99 1-Aug-99 16-Aug-99 1-Sep-99 16-Sep-99 1-Oct-99 16-Oct-99 1-Nov-99 16-Nov-99
15-Jan-99 31-Jan-99 15-Feb-99 28-Feb-99 15-Mar-99 31-Mar-99 15-Apr-99 30-Apr-99 15-May-99 31-May-99 15-Jun-99 30-Jun-99 15-Jul-99 31-Jul-99 15-Aug-99 31-Aug-99 15-Sep-99 30-Sep-99 15-Oct-99 31-Oct-99 15-Nov-99 30-Nov-99
Duration (days) 3 16 15 13 15 16 15 15 15 16 15 15 15 16 15 16 15 15 15 16 15 15
At Site MT1, the mean and range of the total foraminiferal shell flux were 1040 and 80–3810 tests m− 2 day− 1 (Table 2). Flux maxima occurred in late February (3200 tests m− 2 day− 1) and late May (3810 tests m− 2 day− 1). Between these two maxima, the foraminiferal flux decreased to 531 tests m− 2 day− 1 in early April. Overall low fluxes (b 500 tests m− 2 day− 1) were recorded from June to November. At Site MT3, the mean and range of the total foraminiferal test flux were 742 and 38–2212 tests m− 2 day− 1 (Table 2). High fluxes were recorded from January to late May, and in late October. The maximum flux in early May was 2212 tests m− 2 day− 1. From June to November, fewer than 500 tests m− 2 day− 1were deposited, except in late October (Fig. 5). At Site MT5, higher fluxes were recorded from late January to May (N 1500 tests m− 2 day− 1; Fig. 5, Table 2), from July to early September, and in November. The mean and range of the total flux were 1515 and 372– 3100 tests m− 2 day− 1, respectively (Table 2). The maximum flux was reached in late April (3100 tests m− 2 day− 1). Low fluxes (b 500 tests m− 2 day− 1) were measured in two samples only, from late October to early November.
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Fig. 5. Total foraminiferal flux at Sites (a) MT1, (b) MT3, and (c) MT5.
4.2. Seasonal changes in foraminiferal assemblage composition Eleven of the 37 foraminiferal species identified in the three sediment trap samples contributed about 90% of the total flux (Table 2 and Plate I). Five species, Globigerinoides ruber, Globigerinita glutinata, Globigerinoides sacculifer, Globigerinoides tenellus, and Neogloboquadrina dutertrei, were most abundant at Sites MT1 and MT3, whereas 7 species (G. glutinata, Globigerina bulloides, G. ruber, Pulleniatina obliquiloculata, Turborotalita humilis, Globigerina rubescens, and Globigerina falconensis) were most abundant at Site MT5 (Fig. 6). At site MT1, foraminiferal fluxes show two distinct maxima: the first peak consisted predominantly of G. bulloides, and the second of G. ruber (shaded bands in Fig. 6). The G. bulloides flux peaked in late February with a small additional peak in early June. Fluxes of Globigerinoides aequilateralis, G. ruber, G. sacculifer, and G. tenellus peaked in late May. G. glutinata, N. dutertrei, and P. obliquiloculata showed two peaks, in late February and from late May to early June (Fig. 6). All 11 species showed lower fluxes from July to November.
At Site MT3, the highest fluxes of three species (G. ruber, G. sacculifer, and G. glutinata) were recorded from January to May. Although the fluxes at Site MT3 were lower than those at Site MT1, the foraminiferal species composition was similar at both sites. From June to November, extremely low fluxes were documented except in late October. At Site MT5, the dominant species were clearly different from those at Sites MT1 and MT3. Fluxes of 11 species, especially G. bulloides, G. glutinata, G. falconensis, and G. rubescens, were high from January to April (Fig. 6). These species contributed to the flux continuously throughout the year, even from May to November. 5. Discussion 5.1. Contrasting flux patterns between Sites MT1/MT3 and MT5 caused by differences in nutrition and food source The WPWP is generally seen as an area with low nutrients (Conkright et al., 1994; Levitus et al., 1994; Picaut et al., 1996; Matsumoto et al., 2004). Our data, however, indicate that foraminiferal productivity may be
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Table 2 Flux (shells m− 2 day− 1) of planktic foraminiferal species (N125 μm) at Sites MT1, MT3, and MT5 Sample number MT1 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22 Mean flux (%)⁎
G. G. G. G. G. G. bulloides falconensis rubescens aequilateralis glutinata ruber
G. G. N. P. T. Total sacculifer tenellus dutertrei obliquiloculata humilis
32.0 52.0 226.1 876.3 230.4 48.0 6.4 57.6 25.6 104.0 213.3 59.7 8.5 16.0 14.9 4.0 10.7 0.0 14.9 0.0 6.4 6.4 88.3
85.3 118.0 123.7 214.2 74.7 78.0 98.1 123.7 238.9 470.0 456.5 125.9 38.4 82.0 153.6 38.0 27.7 14.9 68.3 8.0 42.7 12.8 125.4
19.2 26.0 46.9 39.4 10.7 2.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 7.9
0.0 0.0 12.8 9.8 0.0 4.0 0.0 12.8 4.3 8.0 10.7 2.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.0 0.0 0.0 3.1
14.9 26.0 38.4 108.3 57.6 54.0 29.9 40.5 27.7 170.0 110.9 42.7 12.8 16.0 29.9 28.0 17.1 21.3 49.1 12.0 40.5 6.4 44.2
78.9 136.0 426.7 561.2 196.3 80.0 57.6 147.2 142.9 726.0 283.7 136.5 51.2 66.0 89.6 76.0 21.3 42.7 157.9 20.0 57.6 6.4 164.6
298.7 344.0 403.2 413.5 194.1 362.0 258.1 471.5 454.4 1186.0 501.3 224.0 100.3 128.0 174.9 68.0 51.2 21.3 226.1 16.0 85.3 32.0 281.4
8.5%
0.8%
0.3%
4.3%
15.8%
27.0% 12.1%
MT3 S1 116.4 S2 60.0 S3 12.8 S4 88.6 S5 72.5 S6 30.0 S7 29.9 S8 34.1 S9 44.8 S10 34.0 S11 10.7 S12 6.4 S13 6.4 S14 2.0 S15 0.0 S16 0.0 S17 4.3 S18 4.3 S19 0.0 S20 6.0 S21 0.0 S22 0.0 Mean flux 24.8 (%)⁎ 3.3%
5.8 8.0 4.3 2.5 2.1 2.0 0.0 6.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 4.3 0.0 0.0 0.0 1.7 0.2%
0.0 2.0 6.4 12.3 12.8 20.0 10.7 8.5 14.9 8.0 0.0 0.0 2.1 2.0 0.0 0.0 6.4 0.0 0.0 4.0 0.0 0.0 5.2 0.7%
84.4 78.0 27.7 56.6 72.5 120.0 145.1 87.5 125.9 94.0 34.1 38.4 42.7 4.0 2.1 8.0 19.2 21.3 0.0 70.0 6.4 128.0 55.1 7.4%
78.5 122.0 177.1 182.2 448.0 290.0 209.1 108.8 266.7 162.0 66.1 36.3 83.2 12.0 19.2 6.0 27.7 4.3 6.4 40.0 2.1 64.0 110.9 15.0%
0.0 22.0
21.3 178.0
202.7 562.0
MT5 S1 S2
192.0 370.0
10.7 28.0
34.1 88.0 70.4 98.5 66.1 108.0 34.1 247.5 81.1 434.0 238.9 98.1 34.1 32.0 42.7 44.0 19.2 14.9 49.1 4.0 25.6 0.0 87.9
0.0 16.0 140.8 209.2 64.0 28.0 2.1 19.2 66.1 182.0 136.5 25.6 8.5 16.0 8.5 4.0 2.1 0.0 21.3 4.0 17.1 6.4 44.5
0.0 0.0 6.4 0.0 2.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.0 2.1 0.0 0.0 0.0 0.0 2.0 0.0 0.0 0.7
736.0 930.0 2122.7 3200.0 1401.6 872.0 531.2 1198.9 1265.1 3810.0 2513.1 785.1 317.9 386.0 665.6 278.0 174.9 128.0 716.8 80.0 332.8 96.0 1040.5
7.8%
4.3%
0.1%
128.0 299.6 148.0 138.0 326.4 125.9 401.2 194.5 288.0 194.1 318.0 230.0 283.7 345.6 155.7 72.5 537.6 424.5 322.0 192.0 125.9 196.3 76.8 61.9 172.8 100.3 12.0 10.0 32.0 36.3 10.0 26.0 66.1 66.1 12.8 27.7 4.3 8.5 172.0 162.0 10.7 4.3 102.4 89.6 169.5 135.5 22.9% 18.3%
14.5 192.0 32.0 48.0 91.7 21.3 81.2 110.8 55.5 61.9 76.0 52.0 40.5 38.4 57.6 17.1 87.5 108.8 94.0 26.0 25.6 40.5 17.1 12.8 44.8 8.5 6.0 0.0 12.8 2.1 14.0 2.0 14.9 6.4 4.3 2.1 6.4 0.0 26.0 36.0 8.5 2.1 51.2 0.0 38.9 34.4 5.2% 4.6%
218.2 28.0 2.1 32.0 51.2 52.0 110.9 12.8 74.7 20.0 4.3 12.8 8.5 0.0 0.0 8.0 2.1 0.0 2.1 446.0 4.3 19.2 50.2 6.8%
2.9 4.0 2.1 4.9 10.7 18.0 12.8 57.6 21.3 28.0 8.5 6.4 8.5 0.0 2.1 0.0 2.1 0.0 0.0 2.0 0.0 0.0 9.1 1.2%
1416.7 730.0 934.4 1220.9 1386.7 1362.0 1472.0 654.9 2212.3 1096.0 573.9 288.0 497.1 50.0 115.2 74.0 324.3 89.6 38.4 1258.0 70.4 582.4 741.7
202.7 256.0
0.0 16.0
64.0 258.0
0.0 58.0
821.3 1938.0
0.0 56.0
8.4%
21.3 40.0 226.1 364.3 89.6 8.0 4.3 23.5 130.1 306.0 390.4 27.7 12.8 16.0 6.4 6.0 4.3 0.0 76.8 0.0 19.2 19.2 81.3
53.3 44.0
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Table 2 (continued) Sample number MT5 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22 Mean flux (%)⁎
G. G. G. G. G. G. bulloides falconensis rubescens aequilateralis glutinata ruber
G. G. N. P. T. Total sacculifer tenellus dutertrei obliquiloculata humilis
467.2 292.9 243.2 442.0 317.9 584.5 215.5 184.0 83.2 98.1 70.4 210.0 102.4 184.0 236.8 106.7 66.1 80.0 712.5 179.2 248.9
134.4 64.0 151.5 148.0 149.3 160.0 211.2 94.0 78.9 98.1 198.4 80.0 117.3 48.0 81.1 44.8 8.5 16.0 40.5 44.8 95.3
16.4%
64.0 36.9 44.8 24.0 25.6 12.8 4.3 4.0 6.4 4.3 6.4 0.0 8.5 22.0 4.3 2.1 6.4 26.0 2.1 21.3 17.3 1.1%
51.2 51.7 53.3 106.0 61.9 115.2 36.3 34.0 27.7 21.3 17.1 16.0 6.4 16.0 23.5 19.2 10.7 10.0 100.3 40.5 39.5 2.6%
57.6 86.2 57.6 136.0 53.3 206.9 96.0 126.0 17.1 102.4 100.3 104.0 42.7 126.0 87.5 98.1 53.3 26.0 130.1 55.5 92.2 6.1%
477.9 467.7 529.1 892.0 561.1 800.0 486.4 226.0 81.1 98.1 155.7 276.0 215.5 332.0 343.5 172.8 125.9 58.0 689.1 204.8 367.5 24.3%
356.3 226.5 328.5 338.0 224.0 593.1 409.6 366.0 138.7 196.3 187.7 262.0 115.2 194.0 266.7 119.5 46.9 72.0 360.5 134.4 247.0
16.3% 6.3%
8.5 22.2 17.1 14.0 23.5 32.0 10.7 18.0 8.5 17.1 8.5 48.0 8.5 28.0 21.3 10.7 4.3 0.0 23.5 2.1 16.2 1.1%
96.0 78.8 76.8 96.0 85.3 104.5 66.1 50.0 10.7 25.6 57.6 46.0 44.8 50.0 106.7 19.2 10.7 4.0 61.9 21.3 54.8 3.6%
243.2 189.5 243.2 244.0 157.9 236.8 153.6 106.0 27.7 64.0 113.1 90.0 279.5 260.0 232.5 185.6 87.5 58.0 238.9 204.8 173.7 11.5%
138.7 98.5 25.6 52.0 93.9 136.5 57.6 64.0 53.3 10.7 55.5 44.0 19.2 52.0 78.9 29.9 8.5 16.0 91.7 53.3 57.9
2248.5 1691.1 1870.9 2676.0 1905.1 3099.7 1881.6 1314.0 603.7 791.5 1053.9 1280.0 1079.5 1414.0 1734.4 887.5 452.3 372.0 2660.3 1032.5 1515.1
3.8%
⁎Values inidicate relative abundance of each species in each traps.
relatively high for at least some intermittent time periods. The highest foraminiferal flux was recorded from January to June at Sites MT1 and MT3 in the WPWP and from January to April 1999 at Site MT5 in the equatorial upwelling region (Fig. 5). Around Sites MT1 and MT3 in the WPWP, precipitation was higher from January to May than from June to November (Fig. 3a–b). The increase in foraminiferal flux in the region may be associated with an increase in primary productivity resulting from nutrient inputs to the ocean during the rainy period. Such terrigenous input of nutrients, however, would be only intermittent, contributing more in the first half of the year, when high precipitation on land causes increased fluvial discharge (Fig. 4). We lack quantitative data on river discharge, but Kempe and Knaack (1996) reported that lithogenic particle fluxes increased and were positively correlated to the organic carbon flux from late May to mid-July, based on their sediment trap experiment in the west Caroline Basin (5°0.060′ N, 138°49.81′ E), east of Site MT1. Their data clearly show that lithogenic particles
were delivered from surrounding islands to the offshore trap site and that these particles were related to surface ocean productivity. The flux of G. ruber increased at Sites MT1 and MT3 during the interval of high precipitation. This species was the third-most abundant species at Site MT5 during a strong upwelling period. G. ruber inhabits tropical to transitional zones (Bradshaw, 1959; Parker, 1962) and it occurs with high standing stocks in the oligotrophic region in the equatorial Pacific (Watkins et al., 1996). Our data show that G. ruber is an important assemblage component not only under lownutrient conditions, but also during intermittent eutrophic conditions when food supply is high. We suggest that the combination of warm water and abundant food causes the flux of G. ruber to increase. Thermally stratified conditions such as those in the WPWP region may favor the common occurrence of N. dutertrei. Plankton-net studies have demonstrated that N. dutertrei dominates in the middle and lower parts of the thermocline where the deep chlorophyll
Plate I. SEM micrographs of planktic foraminifera. Scale bars represent 100 μm. Hastigerina pelagica (d'Orbigny): 1a–c, Globigerina bulloides d'Orbigny: 2a–c, Globigerina falconensis Blow: 3a–c, Globigerina rubescens Hofker: 4a–c, Globigerinella aequilateralis (Brady): 5a–c, Globigerinita glutinata (Egger): 6a–c, Globigerinoides ruber (d'Orbigny): 7a–c, Globigerinoides sacculifer (Brady): 8a–c (with saclike chamber) and 9a–c (without saclike chamber), Globigerinoides tenellus Parker: 10a–c, Neogloboquadrina dutertrei (d'Orbigny): 11a–c, Pulleniatina obliquiloculata (Parker and Jones): 12a–c, and Turborotalita humilis (Brady): 13a–c.
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314
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Fig. 6. Fluxes for G. ruber, G. sacculifer, G. tenellus, N. dutertrei, G. aequilateralis, P. obliquiloculata, G. bulloides, G. glutinata, G. falconensis, G. rubescens, and T. humilis at Sites MT1, MT3, and MT5. Shaded bands show distinct maxima at Site MT1. Environmental conditions are summarized at the top of each panel.
maximum (DCM) is present (Fairbanks et al., 1982; Ravelo and Fairbanks, 1992). Based on sediment-trap data, Sautter and Thunell (1991) showed that high fluxes of N. dutertrei coincided with the development of thermal stratification within the water column. Thermally stratified conditions such as those in the WPWP region may favor abundant occurrence of N. dutertrei. However, we observed fluxes of N. dutertrei only intermittently. In November 1999, the thermocline was at a depth of 150 to 200 m in the WPWP (Matsumoto et al., 2004). A DCM does not usually develop when the thermocline is too deep to allow sufficient light penetration for photosynthesis. Although our study
provides no data on temporal variation in light intensity, DCM, or thermocline, the low flux of N. dutertrei suggests that development of a DCM at Site MT1 was very limited throughout the study period. During the La Niña period that we studied, a coldtongue-like water mass (upwelling) developed and expanded westward to the vicinity of site MT5 (Fig. 2), where conditions were characterized by stronger winds and lower SSTs from January to April (Figs. 3e and 4). Thus, the increase in foraminiferal flux may have been associated with intensified equatorial upwelling, supplying nutrients to the surface waters. G. glutinata and G. bulloides in particular were abundant at
M. Yamasaki et al. / Marine Micropaleontology 66 (2008) 304–319
315
Fig. 7. Schematic illustration of monsoon winds and ocean currents (modified from Tchernia, 1980; Fine et al., 1994, respectively). (a) NW monsoon period from December to March; (b) SE monsoon and transition period from April to November. Thin arrows indicate surface currents and thick arrows denote wind direction.
this site (Fig. 6). The former was the dominant species at Site MT5, but contributed only about 16% of the total foraminifera at Sites MT1 and MT3 (Table 2). This species has been reported to increase markedly during upwelling regions both in the Arabian Sea (Conan and Brummer, 2000) and off California (Sautter and Thunell, 1991). Our results confirm that this species prefers eutrophic conditions, such as those in the equatorial upwelling region. Furthermore, it was found to be the second- or third-most abundant species in the WPWP region. Its cosmopolitan occurrence suggests that it has wide environmental tolerances that allow it to be present in many environments. G. bulloides has often been used as an indicator of upwelling (in the Panama Basin (Thunell and Reynolds, 1984), off California (Sautter and Thunell, 1991), in the Arabian Sea (Curry et al., 1992), and off Somalia (Conan and Brummer, 2000). In the equatorial Pacific (140°W), standing stocks of G. bulloides in a planktonnet tow increased in response to accelerating primary productivity (Watkins et al., 1998). Our data on G.
bulloides for Site MT5 are consistent with previous works. 5.2. Variation in foraminiferal flux from January to June at Site MT1 related to the monsoon At site MT1, foraminiferal fluxes show two distinct maxima. The first peak consisted predominantly of G. bulloides, and the second of G. ruber (shaded bands in Fig. 6). During the study period, the flow direction of New Guinea Coastal Current (NGCC) changed with monsoon conditions (Figs. 1 and 4). We suggest that the first peak (mainly G. bulloides) occurred during the southward flow of the NGCC (Fig. 7a). The second peak (mainly G. ruber) may have occurred during northward flow of the NGCC (Fig. 7b). Such different faunal compositions of the two peaks would reflect two different patterns of the supply of nutrients advected by surface currents. In the WPWP, the surface layer is isolated from subsurface sources of nutrients by a series of shallow
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haloclines and it appears to be highly oligotrophic (Mackey et al., 1995). Thus, surface layer productivity would be strongly affected by nutrients supplied to the surface water through lateral transportation. High primary production rates observed near the northwestern margin of the Celebes Sea result from entrainment of subsurface water and vertical mixing by intense tidal flow from the Sulu Archipelago to the Celebes Sea (Takeda et al., 2007). Although no information on the distribution of living planktic foraminifera is available, surface sediments analysis revealed that G. bulloides had a high relative abundance (10–23%) in the Sulu Sea and the Celebes Sea (Pflaumann and Jian, 1999). During the NW monsoon, the NECC passes trough Site MT1 by way of the Celebes Sea while the NGCC flows southeastward (Figs. 4 and 7a). These results indicate that the NECC brought nutrient-rich and highly productive surface water, which is suitable for G.
bulloides production, and which resulted in the peak flux of G. bulloides at Site MT1. The second peak flux occurred during northwestward flow of the NGCC (Fig. 4). During the SE monsoon with southeast winds, part of the NECC arises from the NW-flowing NGCC along the northern coast of New Guinea, probably advecting some amount of nutrient loads (Fig. 7b). Kempe and Knaack (1996) suggested that particle fluxes in the Caroline Basin show a pronounced seasonality due to seasonal changes in wind and current directions, in agreement with the results of our sediment trap experiment at Site MT1. Globigerinoides ruber which contributed greatly to the second flux peak at Site MT1, had high abundance in surface sediments collected from the Solomon Sea (Pflaumann and Jian, 1999). Although we could not determine the exact water sources, the second flux peak may have resulted from advection of nutrient-rich water
Table 3 Mean and range of total foraminiferal flux in this and other region studies Trap site
North Atlantic Northwest Atlantic Indian Ocean Northeast Pacific
Northwest Pacific
Equatorial Pacific
C – CBBT NBBT PAPA Nearshore Midway Gyre SBB 50N KNOT 40N WCT-6 WCT-5 Site 8 Site 7 Site 6 Japan Basin JT WCT-7 WCT-3 WCT-2 WCT-1 JAST01 JAST02 N1 N2 MT1 MT3 MT5
Sample location
80.46°N, 11.02°W 32.1°N, 64.3°W – – 50°N, 145°W 42.1°N, 125.8°W 42.2°N, 127.6°W 41.5°N, 132°W 34.1°N, 120°W 50°N, 165°E 44°N, 155.1°E 40°N, 165°E 42°N, 155.3°E 41°N, 150°E 46.1°N, 175°E 37.4°N, 174.9°E 30°N, 175°E 39.7°N, 132.4°E 34.2°N, 142°E 36.7°N, 154.9°E 36°N, 147°E 39°N, 147°E 25°N, 137°E 27.4°N, 126.7°E 25.2°N, 127.6°E 3°N, 135°E 4.1°N, 136.3°E 4°N,135°E 0°, 145°E 0°, 175.9°E
⁎ Values show flux of N. pachyderma (sinistral).
Sampling duration
8/92–7/93 4/78–5/79 11/88–11/89 11/88–11/89 9/82–8/86 9/87–9/88 9/87–9/88 9/87–9/88 8/93–12–95 12/97–12/98 12/97–12/98 12/97–12/98 8/98–8/99 8/98–8/99 6/93–4/94 6/93–4/94 6/93–5/94 7/94–7/95 3/31–5/92 8/98–8/99 8/98–8/99 12/97–8/99 12/97–8/99 10/94–8/95 10/94–8/95 6/91–4/92 6/91–4/92 1/99–11/99 1/99–11/99 1/99–11/99
Trap depth (m) 245 3200 950 967 3800 1000 1000 1000 590 3260 2957 2986 1091 1000 1412 1482 3873 2780 3700 1191 1108 1371 917 1000 3000 1592 1796 970 1020 1040
Foraminiferal flux (shells m− 2 day− 1) Mean
Range
335 ⁎ 213 333 457 2634 781 303 130 1101 5209 3523 1454 1490 1418 3186 824 622 1519 1031 632 188 1428 207 648 606 791 484 1025 747 1492
32–803 ⁎ 115–382 38–910 99–893 11–22678 20–1938 37–725 27–279 98–3896 1128–14213 815–9338 400–3559 240–3675 12–4903 322–18347 196–2666 199–1860 3–8194 47–2130 107–1986 65–419 224–5188 5–830 5–1306 227–972 378–2387 68–1805 80–3810 38–2212 372–3100
Size (μm)
References
125 N125 N150 N150 N125 N125 N125 N125 N125 N125 N125 N125 N125 N125 N125 N125 N125 N63 N125 N125 N125 N125 N125 N125 N125 N250 N250 N125 N125 N125
Kohfeld et al. (1996) Deuser et al. (1981) Guptha et al. (1997) Guptha et al. (1997) Sautter and Thunell (1989) Ortiz and Mix (1992) Ortiz and Mix (1992) Ortiz and Mix (1992) Kincaid et al. (2000) Kuroyanagi et al. (2002) Kuroyanagi et al. (2002) Kuroyanagi et al. (2002) Mohiuddin et al. (2005) Mohiuddin et al. (2005) Eguchi et al. (1999) Eguchi et al. (1999) Eguchi et al. (1999) Park and Shin (1998) Oda and Yamasaki (2005) Mohiuddin et al. (2004) Mohiuddin et al. (2004) Mohiuddin et al. (2002) Mohiuddin et al. (2002) Xu et al. (2005) Xu et al. (2005) Kawahata et al. (2002) Kawahata et al. (2002) This study This study This study
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to the surface water around the Site MT1. Thus, the surface water mass, which is isolated from deeper nutrient-rich water, would receive high nutrient water intermittently through lateral transport by wind and surface currents. During such periods, surface- and subsurface-dwelling G. ruber and G. bulloides (e.g., Bé, 1977) would be expected to increase. At Site MT1, first- and second-peak fluxes were 3200 and 3810 tests m− 2 day− 1, respectively. Values of the flux maxima at Site MT1 are higher than in the Indian Ocean and midlatitude Pacific and Atlantic oceans, but lower than in the north Pacific and Atlantic oceans at latitudes above 44° and 50°N, respectively (Table 3). These findings indicate that the WPWP is not exactly a region of low productivity of planktic foraminifera, although foraminiferal fluxes are low from July to November. Kawahata et al. (2002) reported that five common species (G. aequilateralis, G. sacculifer, G. ruber, N. dutertrei, and P. obliquiloculata) were dominant in the planktic foraminifera fraction larger than 250 μm during El Niño (June 1991 to April 1992) at sediment trap sites near Site MT1. The flux of G. bulloides, however, was minor (b12 tests m− 2 day− 1). This difference may have originated from differences in oceanographic conditions: our study was undertaken during La Niña conditions, whereas the study of Kawahata et al. (2002) was performed under El Niño conditions. However, it is also possible that differences in analytical method contribute to this contrast. Most specimens of G. bulloides are 125 to 250 μm in our study (Fig. 6), and so the discrepancy between these two studies could reflect the difference in shell sizes analyzed. 6. Conclusions Planktic foraminiferal flux and composition were investigated in sediment trap materials from three sites in the western equatorial Pacific, deployed under La Niña conditions in 1999. High foraminiferal fluxes occurred at westernmost Site MT1 in the WPWP and at the easternmost site MT5, influenced by equatorial upwelling; fluxes were lower at Site MT3 in the WPWP. Among the major species, G. bulloides and G. glutinata were abundant at Site MT5 and G. ruber, G. sacculifer, and G. tenellus occurred in high numbers at Sites MT1 and MT3. At Site MT5, the predominance of G. bulloides and G. glutinata reflected equatorial upwelling during La Niña. We suggest that these results were likely caused by differences in supply of nutrients between these sites and Site MT5. Large amounts of suspended fluvial material might have been supplied by lateral transport to Sites MT1 and MT3, whereas
317
upwelling nutrient-rich waters would have enhanced phytoplankton production at Site MT5. At Site MT1, the G. bulloides flux shows a distinct maximum in February responding to the strengthening of the southeastward NGCC flow under the influence of the NW monsoon. Acknowledgments The manuscript was greatly improved by the critical comments of K.-Y. Wei, W. R. Howard, the reviewers, and E. Thomas, the journal editor. H. Kawahata (University of Tokyo) and Y. Tanaka (National Institute of Advanced Industrial Science and Technology) provided both the opportunity to conduct this study and valuable comments on our results. Chief scientist T. Kawano (Japan Marine Science and Technology Center; JAMSTEC) as well as the captains, crews, and onboard scientists of the MR98-K02 and MR99-K07 cruises of the R/V Mirai are commended for their substantial efforts in collecting the sediment trap material. L. P. Gupta (JAMSTEC) prepared samples and K. Sakaida (Tohoku University) made suggestions regarding the use of online environmental data. This study was supported by the Global Carbon Cycle and Related Mapping Based on Satellite Imagery Program funded by Special Coordination Funds for Promoting the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. References Bé, A.W.H., 1977. An ecological, zoogeographic and taxonomic review of recent planktonic foraminifera. In: Ramsay, A.T.S. (Ed.), Oceanic Micropaleontology, vol. 1. Academic Press, London, pp. 1–100. Bradshaw, J.S., 1959. Ecology of living planktonic foraminifera in the north and equatorial Pacific Ocean. Contributions from the Cushman Foundation for Foraminiferal Research 10, 25–64. Brown, J., Colling, A., Park, D., Phillips, J., Rothery, D., Wright, J., Bearman, G (Eds.), 1989. Ocean Circulation. The Open University, Milton Keynes, UK. 238 pp. Conan, S.M.H., Brummer, G.J.A., 2000. Fluxes of planktic foraminifera in response to monsoonal upwelling on the Somalia Basin margin. Deep-Sea Research. Part II 47, 2207–2227. Conkright, M., Levitus, S., Boyer, T., 1994. Ocean Circulation. Nutrients. NOAA Atlas NESDIS 1, vol. 1. U.S. Department of Commerce, Washington, DC. Curry, W.B., Ostermann, D.R., Guptha, M.V.S., Ittekkot, V., 1992. Foraminiferal production and monsoonal upwelling in the Arabian Sea: evidence from sediment traps. In: Summerhays, C.P., Prell, W.L., Emeis, K.C. (Eds.), Upwelling Systems: Evolution since the Early Miocene. Geological Society Special Publication, vol. 64, pp. 93–106. Deuser, W.G., Ross, E.H., Hemleben, C., Spindler, M., 1981. Seasonal changes in species composition, numbers, mass, size, and isotopic
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Marine Micropaleontology 66 (2008) 304 – 319 www.elsevier.com/locate/marmicro
Western equatorial Pacific planktic foraminiferal fluxes and assemblages during a La Niña year (1999) Makoto Yamasaki a,⁎, Akira Sasaki b , Motoyoshi Oda c , Hanako Domitsu c a
Institute of Applied Earth Sciences, Faculty of Engineering and Resource Science, Akita University, 1-1Tegata-Gakuencho, Akita 010-8502, Japan b Cosmo Energy Exploration and Development Ltd., 1-1-1 Shibaura, Minato, Tokyo 105-8528, Japan c Institute of Geology and Paleontology, Graduate School of Science, Tohoku University, 6-3 Aza-Aoba, Aramaki, Sendai 980-8578, Japan Received 14 March 2007; received in revised form 11 October 2007; accepted 31 October 2007
Abstract A correlation between foraminiferal community dynamics and environmental conditions may provide a basis for establishing paleoclimatic proxies. We studied planktic foraminiferal shell fluxes and assemblages in samples collected in three time-series sediment trap deployments in the western equatorial Pacific under La Niña conditions from January to November 1999. Eleven species contributed about 90% of the total flux in all traps. Two sites (MT1, MT3) in the Western Pacific Warm Pool region (WPWP) were characterized by common occurrences of the species Globigerinoides ruber, Globigerinoides sacculifer, Globigerinoides tenellus, and Neogloboquadrina dutertrei. Site MT5 farther to the east in the equatorial upwelling region had common occurrences of Globigerina bulloides, Globigerinita glutinata, and Pulleniatina obliquiloculata. Very high abundances of G. bulloides and G. glutinata at MT5 indicate that equatorial upwelling (EU) occurred during the 1999 La Niña. The two western sites have similar assemblage compositions, but MT1 (∼ 135°E) has the highest fluxes (up to ∼3800 tests m− 2 day− 1), whereas MT3 (∼ 145° E) has fluxes below ∼2200 tests m− 2 day− 1. Relatively high fluxes (up to ∼ 3000 tests m− 2 day− 1) occur at site MT5 (∼ 176° E), where upwelling occurred. The differences in faunal composition in the WPWP and EU might be attributable to differences in the way in which nutrients are supplied to the phytoplankton: large amounts of suspended material are supplied to the WPWP by advection of waters passing through the coastal region of an archipelago, whereas upwelling of nutrient-rich waters enhances primary production in the EU. At the westernmost site in the WPWP, a peak in the G. bulloides flux coincided with southward flow of the New Guinea Coastal Current (NGCC) in late February, but the highest G. ruber flux coincided with northward flow of this current in late May. Thus, the differences in species dominance at this location may be caused by monsoon-driven variability in the flow direction of the NGGC. © 2007 Elsevier B.V. All rights reserved. Keywords: Equatorial upwelling; La Niña; Monsoon; Planktic foraminifera; Western Pacific Warm Pool; Western equatorial Pacific
1. Introduction
⁎ Corresponding author. Fax: +81 18 837 0401. E-mail address: [email protected] (M. Yamasaki). 0377-8398/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2007.10.006
The Western Pacific Warm Pool (WPWP) in the equatorial Pacific has the highest sea surface water temperature (SST; N 28 °C) of the global open ocean, and is the largest single expanse of warm water on earth
M. Yamasaki et al. / Marine Micropaleontology 66 (2008) 304–319
(Yan et al., 1992). It thus represents a major source of heat transfer. The WPWP also plays an important role in the dynamics of the El Niño/Southern Oscillation (ENSO). During El Niño events, the WPWP expands toward the eastern Pacific, but it remains restricted to the western Pacific under La Niña conditions. At these times, the Indonesian Low over the WPWP and the Indonesian Archipelago generate cumulonimbus clouds which bring heavy rainfall (Brown et al., 1989). As a result of these Asian tropical monsoons, the Indonesian Archipelago and New Guinea have the highest annual precipitation in the world (N2000 mm; Nieuwolt, 1977). Under El Niño conditions, the Indonesian Low shifts toward the central Pacific, often causing droughts. The WPWP is one of the most important components of the earth's climate system, and elucidating its evolution over geologic time is necessary to enable us to develop better predictions of global environmental change. Presently, however, we do not understand the evolution of the WPWP. For instance, recent studies have yielded contradictory conclusions regarding the conditions during the period of early Pliocene warmth. Some authors argued that this warm period was
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characterized by permanent El Niño-like conditions, based on the reconstruction of a strong paleo-sea surface temperature (SST) gradient across the tropical Pacific (Wara et al., 2005), whereas others argued for cool, La Niña-like conditions in this time period (Rickaby and Halloran, 2005). A lack of detailed understanding of the modern ecological dynamics of planktic foraminifera (e.g., seasonal variability of species composition and productivity) may be at least one of the causes of this disagreement. Foraminiferal analyses using sediment trap samples have been carried out at many sites in the Atlantic, Indian, and Pacific oceans (e.g., Deuser et al., 1981; Sautter and Thunell, 1991; Curry et al., 1992; Conan and Brummer, 2000; Kincaid et al., 2000), but are limited in the WPWP region. The study by Kawahata et al. (2002) gives information on two sites (Site N1: 2°59.8′ N, 135°01.5′ E; Site N2: 4°07.5′ N, 136°16.6′ E) under El Niño conditions only. Therefore, we examined the relation between foraminiferal species composition, shell fluxes, and hydrographic conditions, using data collected on samples obtained from time-series sediment traps, in
Fig. 1. Location of sediment trap Sites MT1, MT3, and MT5, surface currents, undercurrents, and eddies in the western equatorial Pacific (modified from Fine et al., 1994). The surface currents are the Kuroshio Current, Mindanao Current (MC), North Equatorial Current (NEC), North Equatorial Countercurrent (NECC), South Equatorial Current (SEC), and New Guinea Coastal Current (NGCC; a bidirectional arrow indicates change in flow direction with the monsoon, which is eastward during winter and westward during summer). The undercurrents are the Equatorial Undercurrent (EUC), New Guinea Coastal Undercurrents (NGCUC), and Great Barrier Reef Undercurrent (GBRUC). The eddies are the Mindanao Eddy (ME) and Halmahera Eddy (HE).
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order to investigate ecological factors controlling temporal and spatial distributions of planktic foraminifera. In this paper, we present detailed findings with special emphasis on the La Niña condition as an initial step toward providing reliable paleoclimate proxies for La Niña/ El Niño conditions in the western equatorial Pacific.
(NGCC) changes flow directions with wind directions. The NGCC flows northwestward under the influence of southeasterly winds in boreal summer and southeastward with the northwesterly winds in boreal winter (Wyrtki, 1961; Kuroda, 2000).
2. Environmental settings
Data were collected in1999 after the end of the very strong El Niño of 1998. Both the NINO3 index (Fig. 2a) and the Southern Oscillation Index (SOI; Fig. 2b) indicate that the Pacific was under normal to La Niña conditions during the study period. Sites MT1 and MT3 lie within the WPWP (SST N 28 °C) throughout the year (Fig. 2c). The WPWP is characterized by overall low primary productivity, as seen in the low nitrate and chlorophyll a concentrations (e.g., Picaut et al., 1996). Relatively low SSTs (26.5–28 °C) were measured at
2.1. Ocean currents The complex system of currents affecting the study sites (Fig. 1) includes the North Equatorial Countercurrent (NECC), the South Equatorial Current (SEC), the Equatorial Undercurrent (EUC), and the New Guinea Coastal Undercurrent (NGCUC). Due to the influence of monsoons, the New Guinea Coastal Current
2.2. ENSO conditions during the study period
Fig. 2. (a) The NINO3 index and the NINO3 5-month moving average. The NINO3 index is the SST anomaly averaged over [5°S–5°N] and [150– 90°W], obtained from the Integrated Global Ocean Services System (IGOSS; Reynolds and Smith, 1994). (b) The Southern Oscillation Index (SOI) and the SOI 5-month moving average for 1999 (Japan Meteorological Agency, 2000). The SOI was calculated from air pressure differences between Tahiti and Darwin. (c) SST at the equator in the Pacific in 1999 with longitude of trap sites indicated. The SST data were taken from the Tropical Atmosphere Ocean (TAO) Project office Web site (Mangum et al., 1998).
M. Yamasaki et al. / Marine Micropaleontology 66 (2008) 304–319 Fig. 3. Variation of monthly precipitation averaged over (a) 10° N–10° S and 120°–130° E and (b) 0°–10° S and 130°–150° E in 1999. These data were based on rain gauge and satellite estimates and distributed by the National Centers for Environmental Prediction (NCEP) — Climate Prediction Center (CPC) (http://www.cpc.ncep.noaa.gov/). Dashed lines and numbers in parentheses indicate annual mean precipitation (mm day− 1). Variations in the air–sea environment, wind speed (solid line) and direction (dashed line), SST, and isotherm data in 1999 near (c) MT1, (d) MT3, and (e) MT5. Monthly wind speed and direction data were published by the NCEP — National Center for Atmospheric Research (NCAR) reanalysis project (http://www.cdc.noaa.gov/) for a 2.5° longitude–latitude grid. Weekly SST data were provided by the IGOSS (Reynolds and Smith, 1994) for a 1° longitude–latitude grid. Five-day vertical isotherm data were published by the TAO project office (Mangum et al., 1998) for the TAO buoy located in the equatorial Pacific.
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Site MT5, especially from January to April 1999. These SSTs indicate that upwelling occurred at that site, as the western end of the cold tongue extended from the eastern equatorial Pacific. After May, the site was transitional between the WPWP and the cold tongue (Fig. 2c). The three sediment trap sites thus can be categorized into two regions: the WPWP region (Sites MT1 and MT3) and the cold tongue region with equatorial upwelling (EU, Site MT5).
throughout the year. At Site MT5, water temperatures decreased sharply with water depth within the upper euphotic zone (∼0–100 m), and wind speed decreased from January to May (Fig. 3e). Strong winds present from January to May thus resulted in cooler SSTs because of substantial upwelling. 3. Materials and methods 3.1. Location and duration of sediment trap deployment
2.3. Wind and precipitation Rain gauge measurements and satellite estimates are available to document precipitation in the study area (the Climate Prediction Center; Fig. 3a–b). Monthly wind speed and direction data (Fig. 3c–e) are available online (National Center for Atmospheric Research reanalysis project; http://www.cdc.noaa.gov/). Monthly precipitation in the Indonesian Archipelago and New Guinea was high from January to April (Fig. 3a–b). At Sites MT1 and MT3, the wind direction was north to east from January to March and east to south from June to November. Sites MT1 and MT3 are located within the monsoon-impacted area, and the NGCC changes its direction together with the monsoon wind close to MT1 (Fig. 4). Site MT5 is within the trade-wind belt, thus characterized by a stable wind direction (east) and seasonal variation in water temperature and wind speed. Strong winds occurred from January to May.
Time-series sediment traps with bottom-tethered moorings (SMD26S-6000, 0.5 m2 collecting aperture; Nichiyu-Giken Kogyo Co. Ltd., Tokyo, Japan) were deployed at three sites (MT1: 4°2.911′ N, 135°0.019′ E; MT3: 0°0.843′ S, 145°1.580′ E; and MT5: 0°2.343′ N, 175°56.432′ E) along an E–W transect in the western equatorial Pacific from January to November 1999 (Table 1). The collection depths of the sediment traps were about 1000 m, and samples were collected at approximately 2-week intervals. The sinking velocity of planktic foraminifera (N 150 μm) was estimated to be 300–1300 m day− 1, depending on their shell weight and the presence or absence of spines (Takahashi and Bé, 1984). We did not consider the time lag between presence at the surface and arrival at the traps (6.25 days to reach about 1000 m water depth) because it was shorter than the duration of the trapping intervals. 3.2. Sample preparation
2.4. Isotherms and SST Sea surface temperatures at Sites MT1 and MT3 were constant, around 30 ± 1 °C throughout the year (Fig. 3c–d). The stratification of the water column (0–300 m) was relatively constant at Site MT3, and for site MT1 during the time interval for which data were accessible. No buoy data were available from the Tropical Atmosphere Ocean Project (TAO) for MT1 from April to October 1999, but the data for 2002, a La Niña year, show constant isotherms
Prior to settling of the sediment trap systems, polyethylene bottles were filled with filtered seawater, and a solution of 3% neutralized formalin with sodium tetraborate was added to retard bacterial activity. After recovery, the samples were stored in a cool (∼ 2–4 °C), dark place. In the laboratory, samples were wet-sieved through a 1-mm-mesh screen to remove large particles, and then smaller fractions (b1 mm) were sorted into aliquots using a precision four-head rotary splitter
Fig. 4. Summary of defined periods at each site.
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Table 1 Sediment trap mooring locations and sampling schedules MT1 Location: 4°02.911′ N, 135°00.019′ E Water depth: 4762 m Collection depth: 970 m Sample Trap cup number open S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22
– close
1–Jan–99 – 16-Jan-99 – 1-Feb-99 – 16-Feb-99 – 1-Mar-99 – 16-Mar-99 – 1-Apr-99 – 16-Apr-99 – 1-May-99 – 16-May-99 – 1-Jun-99 – 16-Jun-99 – 1-Jul-99 – 16-Jul-99 – 1-Aug-99 – 16-Aug-99 – 1-Sep-99 – 16-Sep-99 – 1-Oct-99 – 16-Oct-99 – 1-Nov-99 – 16-Nov-99 –
15-Jan-99 31-Jan-99 15-Feb-99 28-Feb-99 15-Mar-99 31-Mar-99 15-Apr-99 30-Apr-99 15-May-99 31-May-99 15-Jun-99 30-Jun-99 15-Jul-99 31-Jul-99 15-Aug-99 31-Aug-99 15-Sep-99 30-Sep-99 15-Oct-99 31-Oct-99 15-Nov-99 20-Nov-99
MT3 Location: 00°00.843′ S, 145°01.580′ E Water depth: 3680 m Collection depth: 1020 m
Duration Sample Trap cup (days) number open 15 16 15 13 15 16 15 15 15 16 15 15 15 16 15 16 15 15 15 16 15 5
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22
5-Jan-99 16-Jan-99 1-Feb-99 16-Feb-99 1-Mar-99 16-Mar-99 1-Apr-99 16-Apr-99 1-May-99 16-May-99 1-Jun-99 16-Jun-99 1-Jul-99 16-Jul-99 1-Aug-99 16-Aug-99 1-Sep-99 16-Sep-99 1-Oct-99 16-Oct-99 1-Nov-99 16-Nov-99
(WSD-4; McLane Research Laboratories, Inc.). Each sample was divided into 16 aliquots, one of which was used for planktic foraminiferal analysis. Samples were further wet-sieved into size fractions and filtered onto a membrane with 0.8-μm diameter pores. Filtered samples were dried at 39 °C. Individuals (N 125 μm) were picked, counted, and identified under a stereomicroscope. Micrographs were taken with a field emission scanning electron microscope (FE-SEM, JSM-6330F; JEOL Co. Ltd., Tokyo, Japan) at Tohoku University. Flux estimates (tests m− 2 day− 1) were made based on the sample aliquot (1/16), the duration of each collecting period, and the size of the collecting aperture of the sediment trap (0.5 m2). 4. Results 4.1. Foraminiferal flux Total foraminiferal fluxes were high during the first half of the year at Sites MT1 and MT3 (WPWP), and high throughout the year at Site MT5 (EU) (Fig. 5). The average annual fluxes were higher at Sites MT1 and MT5 than at Site MT3.
MT5 Location: 00°02.343′ N, 175°56.432′ E Water depth: 4828 m Collection depth: 1040 m
– close
Duration Sample Trap cup (days) number open
– close
– – – – – – – – – – – – – – – – – – – – – –
11 16 15 13 15 16 15 15 15 16 15 15 15 16 15 16 15 15 15 16 15 5
– – – – – – – – – – – – – – – – – – – – – –
15-Jan-99 31-Jan-99 15-Feb-99 28-Feb-99 15-Mar-99 31-Mar-99 15-Apr-99 30-Apr-99 15-May-99 31-May-99 15-Jun-99 30-Jun-99 15-Jul-99 31-Jul-99 15-Aug-99 31-Aug-99 15-Sep-99 30-Sep-99 15-Oct-99 31-Oct-99 15-Nov-99 20-Nov-99
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22
13-Jan-99 16-Jan-99 1-Feb-99 16-Feb-99 1-Mar-99 16-Mar-99 1-Apr-99 16-Apr-99 1-May-99 16-May-99 1-Jun-99 16-Jun-99 1-Jul-99 16-Jul-99 1-Aug-99 16-Aug-99 1-Sep-99 16-Sep-99 1-Oct-99 16-Oct-99 1-Nov-99 16-Nov-99
15-Jan-99 31-Jan-99 15-Feb-99 28-Feb-99 15-Mar-99 31-Mar-99 15-Apr-99 30-Apr-99 15-May-99 31-May-99 15-Jun-99 30-Jun-99 15-Jul-99 31-Jul-99 15-Aug-99 31-Aug-99 15-Sep-99 30-Sep-99 15-Oct-99 31-Oct-99 15-Nov-99 30-Nov-99
Duration (days) 3 16 15 13 15 16 15 15 15 16 15 15 15 16 15 16 15 15 15 16 15 15
At Site MT1, the mean and range of the total foraminiferal shell flux were 1040 and 80–3810 tests m− 2 day− 1 (Table 2). Flux maxima occurred in late February (3200 tests m− 2 day− 1) and late May (3810 tests m− 2 day− 1). Between these two maxima, the foraminiferal flux decreased to 531 tests m− 2 day− 1 in early April. Overall low fluxes (b 500 tests m− 2 day− 1) were recorded from June to November. At Site MT3, the mean and range of the total foraminiferal test flux were 742 and 38–2212 tests m− 2 day− 1 (Table 2). High fluxes were recorded from January to late May, and in late October. The maximum flux in early May was 2212 tests m− 2 day− 1. From June to November, fewer than 500 tests m− 2 day− 1were deposited, except in late October (Fig. 5). At Site MT5, higher fluxes were recorded from late January to May (N 1500 tests m− 2 day− 1; Fig. 5, Table 2), from July to early September, and in November. The mean and range of the total flux were 1515 and 372– 3100 tests m− 2 day− 1, respectively (Table 2). The maximum flux was reached in late April (3100 tests m− 2 day− 1). Low fluxes (b 500 tests m− 2 day− 1) were measured in two samples only, from late October to early November.
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Fig. 5. Total foraminiferal flux at Sites (a) MT1, (b) MT3, and (c) MT5.
4.2. Seasonal changes in foraminiferal assemblage composition Eleven of the 37 foraminiferal species identified in the three sediment trap samples contributed about 90% of the total flux (Table 2 and Plate I). Five species, Globigerinoides ruber, Globigerinita glutinata, Globigerinoides sacculifer, Globigerinoides tenellus, and Neogloboquadrina dutertrei, were most abundant at Sites MT1 and MT3, whereas 7 species (G. glutinata, Globigerina bulloides, G. ruber, Pulleniatina obliquiloculata, Turborotalita humilis, Globigerina rubescens, and Globigerina falconensis) were most abundant at Site MT5 (Fig. 6). At site MT1, foraminiferal fluxes show two distinct maxima: the first peak consisted predominantly of G. bulloides, and the second of G. ruber (shaded bands in Fig. 6). The G. bulloides flux peaked in late February with a small additional peak in early June. Fluxes of Globigerinoides aequilateralis, G. ruber, G. sacculifer, and G. tenellus peaked in late May. G. glutinata, N. dutertrei, and P. obliquiloculata showed two peaks, in late February and from late May to early June (Fig. 6). All 11 species showed lower fluxes from July to November.
At Site MT3, the highest fluxes of three species (G. ruber, G. sacculifer, and G. glutinata) were recorded from January to May. Although the fluxes at Site MT3 were lower than those at Site MT1, the foraminiferal species composition was similar at both sites. From June to November, extremely low fluxes were documented except in late October. At Site MT5, the dominant species were clearly different from those at Sites MT1 and MT3. Fluxes of 11 species, especially G. bulloides, G. glutinata, G. falconensis, and G. rubescens, were high from January to April (Fig. 6). These species contributed to the flux continuously throughout the year, even from May to November. 5. Discussion 5.1. Contrasting flux patterns between Sites MT1/MT3 and MT5 caused by differences in nutrition and food source The WPWP is generally seen as an area with low nutrients (Conkright et al., 1994; Levitus et al., 1994; Picaut et al., 1996; Matsumoto et al., 2004). Our data, however, indicate that foraminiferal productivity may be
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Table 2 Flux (shells m− 2 day− 1) of planktic foraminiferal species (N125 μm) at Sites MT1, MT3, and MT5 Sample number MT1 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22 Mean flux (%)⁎
G. G. G. G. G. G. bulloides falconensis rubescens aequilateralis glutinata ruber
G. G. N. P. T. Total sacculifer tenellus dutertrei obliquiloculata humilis
32.0 52.0 226.1 876.3 230.4 48.0 6.4 57.6 25.6 104.0 213.3 59.7 8.5 16.0 14.9 4.0 10.7 0.0 14.9 0.0 6.4 6.4 88.3
85.3 118.0 123.7 214.2 74.7 78.0 98.1 123.7 238.9 470.0 456.5 125.9 38.4 82.0 153.6 38.0 27.7 14.9 68.3 8.0 42.7 12.8 125.4
19.2 26.0 46.9 39.4 10.7 2.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 7.9
0.0 0.0 12.8 9.8 0.0 4.0 0.0 12.8 4.3 8.0 10.7 2.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.0 0.0 0.0 3.1
14.9 26.0 38.4 108.3 57.6 54.0 29.9 40.5 27.7 170.0 110.9 42.7 12.8 16.0 29.9 28.0 17.1 21.3 49.1 12.0 40.5 6.4 44.2
78.9 136.0 426.7 561.2 196.3 80.0 57.6 147.2 142.9 726.0 283.7 136.5 51.2 66.0 89.6 76.0 21.3 42.7 157.9 20.0 57.6 6.4 164.6
298.7 344.0 403.2 413.5 194.1 362.0 258.1 471.5 454.4 1186.0 501.3 224.0 100.3 128.0 174.9 68.0 51.2 21.3 226.1 16.0 85.3 32.0 281.4
8.5%
0.8%
0.3%
4.3%
15.8%
27.0% 12.1%
MT3 S1 116.4 S2 60.0 S3 12.8 S4 88.6 S5 72.5 S6 30.0 S7 29.9 S8 34.1 S9 44.8 S10 34.0 S11 10.7 S12 6.4 S13 6.4 S14 2.0 S15 0.0 S16 0.0 S17 4.3 S18 4.3 S19 0.0 S20 6.0 S21 0.0 S22 0.0 Mean flux 24.8 (%)⁎ 3.3%
5.8 8.0 4.3 2.5 2.1 2.0 0.0 6.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 4.3 0.0 0.0 0.0 1.7 0.2%
0.0 2.0 6.4 12.3 12.8 20.0 10.7 8.5 14.9 8.0 0.0 0.0 2.1 2.0 0.0 0.0 6.4 0.0 0.0 4.0 0.0 0.0 5.2 0.7%
84.4 78.0 27.7 56.6 72.5 120.0 145.1 87.5 125.9 94.0 34.1 38.4 42.7 4.0 2.1 8.0 19.2 21.3 0.0 70.0 6.4 128.0 55.1 7.4%
78.5 122.0 177.1 182.2 448.0 290.0 209.1 108.8 266.7 162.0 66.1 36.3 83.2 12.0 19.2 6.0 27.7 4.3 6.4 40.0 2.1 64.0 110.9 15.0%
0.0 22.0
21.3 178.0
202.7 562.0
MT5 S1 S2
192.0 370.0
10.7 28.0
34.1 88.0 70.4 98.5 66.1 108.0 34.1 247.5 81.1 434.0 238.9 98.1 34.1 32.0 42.7 44.0 19.2 14.9 49.1 4.0 25.6 0.0 87.9
0.0 16.0 140.8 209.2 64.0 28.0 2.1 19.2 66.1 182.0 136.5 25.6 8.5 16.0 8.5 4.0 2.1 0.0 21.3 4.0 17.1 6.4 44.5
0.0 0.0 6.4 0.0 2.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.0 2.1 0.0 0.0 0.0 0.0 2.0 0.0 0.0 0.7
736.0 930.0 2122.7 3200.0 1401.6 872.0 531.2 1198.9 1265.1 3810.0 2513.1 785.1 317.9 386.0 665.6 278.0 174.9 128.0 716.8 80.0 332.8 96.0 1040.5
7.8%
4.3%
0.1%
128.0 299.6 148.0 138.0 326.4 125.9 401.2 194.5 288.0 194.1 318.0 230.0 283.7 345.6 155.7 72.5 537.6 424.5 322.0 192.0 125.9 196.3 76.8 61.9 172.8 100.3 12.0 10.0 32.0 36.3 10.0 26.0 66.1 66.1 12.8 27.7 4.3 8.5 172.0 162.0 10.7 4.3 102.4 89.6 169.5 135.5 22.9% 18.3%
14.5 192.0 32.0 48.0 91.7 21.3 81.2 110.8 55.5 61.9 76.0 52.0 40.5 38.4 57.6 17.1 87.5 108.8 94.0 26.0 25.6 40.5 17.1 12.8 44.8 8.5 6.0 0.0 12.8 2.1 14.0 2.0 14.9 6.4 4.3 2.1 6.4 0.0 26.0 36.0 8.5 2.1 51.2 0.0 38.9 34.4 5.2% 4.6%
218.2 28.0 2.1 32.0 51.2 52.0 110.9 12.8 74.7 20.0 4.3 12.8 8.5 0.0 0.0 8.0 2.1 0.0 2.1 446.0 4.3 19.2 50.2 6.8%
2.9 4.0 2.1 4.9 10.7 18.0 12.8 57.6 21.3 28.0 8.5 6.4 8.5 0.0 2.1 0.0 2.1 0.0 0.0 2.0 0.0 0.0 9.1 1.2%
1416.7 730.0 934.4 1220.9 1386.7 1362.0 1472.0 654.9 2212.3 1096.0 573.9 288.0 497.1 50.0 115.2 74.0 324.3 89.6 38.4 1258.0 70.4 582.4 741.7
202.7 256.0
0.0 16.0
64.0 258.0
0.0 58.0
821.3 1938.0
0.0 56.0
8.4%
21.3 40.0 226.1 364.3 89.6 8.0 4.3 23.5 130.1 306.0 390.4 27.7 12.8 16.0 6.4 6.0 4.3 0.0 76.8 0.0 19.2 19.2 81.3
53.3 44.0
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Table 2 (continued) Sample number MT5 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22 Mean flux (%)⁎
G. G. G. G. G. G. bulloides falconensis rubescens aequilateralis glutinata ruber
G. G. N. P. T. Total sacculifer tenellus dutertrei obliquiloculata humilis
467.2 292.9 243.2 442.0 317.9 584.5 215.5 184.0 83.2 98.1 70.4 210.0 102.4 184.0 236.8 106.7 66.1 80.0 712.5 179.2 248.9
134.4 64.0 151.5 148.0 149.3 160.0 211.2 94.0 78.9 98.1 198.4 80.0 117.3 48.0 81.1 44.8 8.5 16.0 40.5 44.8 95.3
16.4%
64.0 36.9 44.8 24.0 25.6 12.8 4.3 4.0 6.4 4.3 6.4 0.0 8.5 22.0 4.3 2.1 6.4 26.0 2.1 21.3 17.3 1.1%
51.2 51.7 53.3 106.0 61.9 115.2 36.3 34.0 27.7 21.3 17.1 16.0 6.4 16.0 23.5 19.2 10.7 10.0 100.3 40.5 39.5 2.6%
57.6 86.2 57.6 136.0 53.3 206.9 96.0 126.0 17.1 102.4 100.3 104.0 42.7 126.0 87.5 98.1 53.3 26.0 130.1 55.5 92.2 6.1%
477.9 467.7 529.1 892.0 561.1 800.0 486.4 226.0 81.1 98.1 155.7 276.0 215.5 332.0 343.5 172.8 125.9 58.0 689.1 204.8 367.5 24.3%
356.3 226.5 328.5 338.0 224.0 593.1 409.6 366.0 138.7 196.3 187.7 262.0 115.2 194.0 266.7 119.5 46.9 72.0 360.5 134.4 247.0
16.3% 6.3%
8.5 22.2 17.1 14.0 23.5 32.0 10.7 18.0 8.5 17.1 8.5 48.0 8.5 28.0 21.3 10.7 4.3 0.0 23.5 2.1 16.2 1.1%
96.0 78.8 76.8 96.0 85.3 104.5 66.1 50.0 10.7 25.6 57.6 46.0 44.8 50.0 106.7 19.2 10.7 4.0 61.9 21.3 54.8 3.6%
243.2 189.5 243.2 244.0 157.9 236.8 153.6 106.0 27.7 64.0 113.1 90.0 279.5 260.0 232.5 185.6 87.5 58.0 238.9 204.8 173.7 11.5%
138.7 98.5 25.6 52.0 93.9 136.5 57.6 64.0 53.3 10.7 55.5 44.0 19.2 52.0 78.9 29.9 8.5 16.0 91.7 53.3 57.9
2248.5 1691.1 1870.9 2676.0 1905.1 3099.7 1881.6 1314.0 603.7 791.5 1053.9 1280.0 1079.5 1414.0 1734.4 887.5 452.3 372.0 2660.3 1032.5 1515.1
3.8%
⁎Values inidicate relative abundance of each species in each traps.
relatively high for at least some intermittent time periods. The highest foraminiferal flux was recorded from January to June at Sites MT1 and MT3 in the WPWP and from January to April 1999 at Site MT5 in the equatorial upwelling region (Fig. 5). Around Sites MT1 and MT3 in the WPWP, precipitation was higher from January to May than from June to November (Fig. 3a–b). The increase in foraminiferal flux in the region may be associated with an increase in primary productivity resulting from nutrient inputs to the ocean during the rainy period. Such terrigenous input of nutrients, however, would be only intermittent, contributing more in the first half of the year, when high precipitation on land causes increased fluvial discharge (Fig. 4). We lack quantitative data on river discharge, but Kempe and Knaack (1996) reported that lithogenic particle fluxes increased and were positively correlated to the organic carbon flux from late May to mid-July, based on their sediment trap experiment in the west Caroline Basin (5°0.060′ N, 138°49.81′ E), east of Site MT1. Their data clearly show that lithogenic particles
were delivered from surrounding islands to the offshore trap site and that these particles were related to surface ocean productivity. The flux of G. ruber increased at Sites MT1 and MT3 during the interval of high precipitation. This species was the third-most abundant species at Site MT5 during a strong upwelling period. G. ruber inhabits tropical to transitional zones (Bradshaw, 1959; Parker, 1962) and it occurs with high standing stocks in the oligotrophic region in the equatorial Pacific (Watkins et al., 1996). Our data show that G. ruber is an important assemblage component not only under lownutrient conditions, but also during intermittent eutrophic conditions when food supply is high. We suggest that the combination of warm water and abundant food causes the flux of G. ruber to increase. Thermally stratified conditions such as those in the WPWP region may favor the common occurrence of N. dutertrei. Plankton-net studies have demonstrated that N. dutertrei dominates in the middle and lower parts of the thermocline where the deep chlorophyll
Plate I. SEM micrographs of planktic foraminifera. Scale bars represent 100 μm. Hastigerina pelagica (d'Orbigny): 1a–c, Globigerina bulloides d'Orbigny: 2a–c, Globigerina falconensis Blow: 3a–c, Globigerina rubescens Hofker: 4a–c, Globigerinella aequilateralis (Brady): 5a–c, Globigerinita glutinata (Egger): 6a–c, Globigerinoides ruber (d'Orbigny): 7a–c, Globigerinoides sacculifer (Brady): 8a–c (with saclike chamber) and 9a–c (without saclike chamber), Globigerinoides tenellus Parker: 10a–c, Neogloboquadrina dutertrei (d'Orbigny): 11a–c, Pulleniatina obliquiloculata (Parker and Jones): 12a–c, and Turborotalita humilis (Brady): 13a–c.
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Fig. 6. Fluxes for G. ruber, G. sacculifer, G. tenellus, N. dutertrei, G. aequilateralis, P. obliquiloculata, G. bulloides, G. glutinata, G. falconensis, G. rubescens, and T. humilis at Sites MT1, MT3, and MT5. Shaded bands show distinct maxima at Site MT1. Environmental conditions are summarized at the top of each panel.
maximum (DCM) is present (Fairbanks et al., 1982; Ravelo and Fairbanks, 1992). Based on sediment-trap data, Sautter and Thunell (1991) showed that high fluxes of N. dutertrei coincided with the development of thermal stratification within the water column. Thermally stratified conditions such as those in the WPWP region may favor abundant occurrence of N. dutertrei. However, we observed fluxes of N. dutertrei only intermittently. In November 1999, the thermocline was at a depth of 150 to 200 m in the WPWP (Matsumoto et al., 2004). A DCM does not usually develop when the thermocline is too deep to allow sufficient light penetration for photosynthesis. Although our study
provides no data on temporal variation in light intensity, DCM, or thermocline, the low flux of N. dutertrei suggests that development of a DCM at Site MT1 was very limited throughout the study period. During the La Niña period that we studied, a coldtongue-like water mass (upwelling) developed and expanded westward to the vicinity of site MT5 (Fig. 2), where conditions were characterized by stronger winds and lower SSTs from January to April (Figs. 3e and 4). Thus, the increase in foraminiferal flux may have been associated with intensified equatorial upwelling, supplying nutrients to the surface waters. G. glutinata and G. bulloides in particular were abundant at
M. Yamasaki et al. / Marine Micropaleontology 66 (2008) 304–319
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Fig. 7. Schematic illustration of monsoon winds and ocean currents (modified from Tchernia, 1980; Fine et al., 1994, respectively). (a) NW monsoon period from December to March; (b) SE monsoon and transition period from April to November. Thin arrows indicate surface currents and thick arrows denote wind direction.
this site (Fig. 6). The former was the dominant species at Site MT5, but contributed only about 16% of the total foraminifera at Sites MT1 and MT3 (Table 2). This species has been reported to increase markedly during upwelling regions both in the Arabian Sea (Conan and Brummer, 2000) and off California (Sautter and Thunell, 1991). Our results confirm that this species prefers eutrophic conditions, such as those in the equatorial upwelling region. Furthermore, it was found to be the second- or third-most abundant species in the WPWP region. Its cosmopolitan occurrence suggests that it has wide environmental tolerances that allow it to be present in many environments. G. bulloides has often been used as an indicator of upwelling (in the Panama Basin (Thunell and Reynolds, 1984), off California (Sautter and Thunell, 1991), in the Arabian Sea (Curry et al., 1992), and off Somalia (Conan and Brummer, 2000). In the equatorial Pacific (140°W), standing stocks of G. bulloides in a planktonnet tow increased in response to accelerating primary productivity (Watkins et al., 1998). Our data on G.
bulloides for Site MT5 are consistent with previous works. 5.2. Variation in foraminiferal flux from January to June at Site MT1 related to the monsoon At site MT1, foraminiferal fluxes show two distinct maxima. The first peak consisted predominantly of G. bulloides, and the second of G. ruber (shaded bands in Fig. 6). During the study period, the flow direction of New Guinea Coastal Current (NGCC) changed with monsoon conditions (Figs. 1 and 4). We suggest that the first peak (mainly G. bulloides) occurred during the southward flow of the NGCC (Fig. 7a). The second peak (mainly G. ruber) may have occurred during northward flow of the NGCC (Fig. 7b). Such different faunal compositions of the two peaks would reflect two different patterns of the supply of nutrients advected by surface currents. In the WPWP, the surface layer is isolated from subsurface sources of nutrients by a series of shallow
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haloclines and it appears to be highly oligotrophic (Mackey et al., 1995). Thus, surface layer productivity would be strongly affected by nutrients supplied to the surface water through lateral transportation. High primary production rates observed near the northwestern margin of the Celebes Sea result from entrainment of subsurface water and vertical mixing by intense tidal flow from the Sulu Archipelago to the Celebes Sea (Takeda et al., 2007). Although no information on the distribution of living planktic foraminifera is available, surface sediments analysis revealed that G. bulloides had a high relative abundance (10–23%) in the Sulu Sea and the Celebes Sea (Pflaumann and Jian, 1999). During the NW monsoon, the NECC passes trough Site MT1 by way of the Celebes Sea while the NGCC flows southeastward (Figs. 4 and 7a). These results indicate that the NECC brought nutrient-rich and highly productive surface water, which is suitable for G.
bulloides production, and which resulted in the peak flux of G. bulloides at Site MT1. The second peak flux occurred during northwestward flow of the NGCC (Fig. 4). During the SE monsoon with southeast winds, part of the NECC arises from the NW-flowing NGCC along the northern coast of New Guinea, probably advecting some amount of nutrient loads (Fig. 7b). Kempe and Knaack (1996) suggested that particle fluxes in the Caroline Basin show a pronounced seasonality due to seasonal changes in wind and current directions, in agreement with the results of our sediment trap experiment at Site MT1. Globigerinoides ruber which contributed greatly to the second flux peak at Site MT1, had high abundance in surface sediments collected from the Solomon Sea (Pflaumann and Jian, 1999). Although we could not determine the exact water sources, the second flux peak may have resulted from advection of nutrient-rich water
Table 3 Mean and range of total foraminiferal flux in this and other region studies Trap site
North Atlantic Northwest Atlantic Indian Ocean Northeast Pacific
Northwest Pacific
Equatorial Pacific
C – CBBT NBBT PAPA Nearshore Midway Gyre SBB 50N KNOT 40N WCT-6 WCT-5 Site 8 Site 7 Site 6 Japan Basin JT WCT-7 WCT-3 WCT-2 WCT-1 JAST01 JAST02 N1 N2 MT1 MT3 MT5
Sample location
80.46°N, 11.02°W 32.1°N, 64.3°W – – 50°N, 145°W 42.1°N, 125.8°W 42.2°N, 127.6°W 41.5°N, 132°W 34.1°N, 120°W 50°N, 165°E 44°N, 155.1°E 40°N, 165°E 42°N, 155.3°E 41°N, 150°E 46.1°N, 175°E 37.4°N, 174.9°E 30°N, 175°E 39.7°N, 132.4°E 34.2°N, 142°E 36.7°N, 154.9°E 36°N, 147°E 39°N, 147°E 25°N, 137°E 27.4°N, 126.7°E 25.2°N, 127.6°E 3°N, 135°E 4.1°N, 136.3°E 4°N,135°E 0°, 145°E 0°, 175.9°E
⁎ Values show flux of N. pachyderma (sinistral).
Sampling duration
8/92–7/93 4/78–5/79 11/88–11/89 11/88–11/89 9/82–8/86 9/87–9/88 9/87–9/88 9/87–9/88 8/93–12–95 12/97–12/98 12/97–12/98 12/97–12/98 8/98–8/99 8/98–8/99 6/93–4/94 6/93–4/94 6/93–5/94 7/94–7/95 3/31–5/92 8/98–8/99 8/98–8/99 12/97–8/99 12/97–8/99 10/94–8/95 10/94–8/95 6/91–4/92 6/91–4/92 1/99–11/99 1/99–11/99 1/99–11/99
Trap depth (m) 245 3200 950 967 3800 1000 1000 1000 590 3260 2957 2986 1091 1000 1412 1482 3873 2780 3700 1191 1108 1371 917 1000 3000 1592 1796 970 1020 1040
Foraminiferal flux (shells m− 2 day− 1) Mean
Range
335 ⁎ 213 333 457 2634 781 303 130 1101 5209 3523 1454 1490 1418 3186 824 622 1519 1031 632 188 1428 207 648 606 791 484 1025 747 1492
32–803 ⁎ 115–382 38–910 99–893 11–22678 20–1938 37–725 27–279 98–3896 1128–14213 815–9338 400–3559 240–3675 12–4903 322–18347 196–2666 199–1860 3–8194 47–2130 107–1986 65–419 224–5188 5–830 5–1306 227–972 378–2387 68–1805 80–3810 38–2212 372–3100
Size (μm)
References
125 N125 N150 N150 N125 N125 N125 N125 N125 N125 N125 N125 N125 N125 N125 N125 N125 N63 N125 N125 N125 N125 N125 N125 N125 N250 N250 N125 N125 N125
Kohfeld et al. (1996) Deuser et al. (1981) Guptha et al. (1997) Guptha et al. (1997) Sautter and Thunell (1989) Ortiz and Mix (1992) Ortiz and Mix (1992) Ortiz and Mix (1992) Kincaid et al. (2000) Kuroyanagi et al. (2002) Kuroyanagi et al. (2002) Kuroyanagi et al. (2002) Mohiuddin et al. (2005) Mohiuddin et al. (2005) Eguchi et al. (1999) Eguchi et al. (1999) Eguchi et al. (1999) Park and Shin (1998) Oda and Yamasaki (2005) Mohiuddin et al. (2004) Mohiuddin et al. (2004) Mohiuddin et al. (2002) Mohiuddin et al. (2002) Xu et al. (2005) Xu et al. (2005) Kawahata et al. (2002) Kawahata et al. (2002) This study This study This study
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to the surface water around the Site MT1. Thus, the surface water mass, which is isolated from deeper nutrient-rich water, would receive high nutrient water intermittently through lateral transport by wind and surface currents. During such periods, surface- and subsurface-dwelling G. ruber and G. bulloides (e.g., Bé, 1977) would be expected to increase. At Site MT1, first- and second-peak fluxes were 3200 and 3810 tests m− 2 day− 1, respectively. Values of the flux maxima at Site MT1 are higher than in the Indian Ocean and midlatitude Pacific and Atlantic oceans, but lower than in the north Pacific and Atlantic oceans at latitudes above 44° and 50°N, respectively (Table 3). These findings indicate that the WPWP is not exactly a region of low productivity of planktic foraminifera, although foraminiferal fluxes are low from July to November. Kawahata et al. (2002) reported that five common species (G. aequilateralis, G. sacculifer, G. ruber, N. dutertrei, and P. obliquiloculata) were dominant in the planktic foraminifera fraction larger than 250 μm during El Niño (June 1991 to April 1992) at sediment trap sites near Site MT1. The flux of G. bulloides, however, was minor (b12 tests m− 2 day− 1). This difference may have originated from differences in oceanographic conditions: our study was undertaken during La Niña conditions, whereas the study of Kawahata et al. (2002) was performed under El Niño conditions. However, it is also possible that differences in analytical method contribute to this contrast. Most specimens of G. bulloides are 125 to 250 μm in our study (Fig. 6), and so the discrepancy between these two studies could reflect the difference in shell sizes analyzed. 6. Conclusions Planktic foraminiferal flux and composition were investigated in sediment trap materials from three sites in the western equatorial Pacific, deployed under La Niña conditions in 1999. High foraminiferal fluxes occurred at westernmost Site MT1 in the WPWP and at the easternmost site MT5, influenced by equatorial upwelling; fluxes were lower at Site MT3 in the WPWP. Among the major species, G. bulloides and G. glutinata were abundant at Site MT5 and G. ruber, G. sacculifer, and G. tenellus occurred in high numbers at Sites MT1 and MT3. At Site MT5, the predominance of G. bulloides and G. glutinata reflected equatorial upwelling during La Niña. We suggest that these results were likely caused by differences in supply of nutrients between these sites and Site MT5. Large amounts of suspended fluvial material might have been supplied by lateral transport to Sites MT1 and MT3, whereas
317
upwelling nutrient-rich waters would have enhanced phytoplankton production at Site MT5. At Site MT1, the G. bulloides flux shows a distinct maximum in February responding to the strengthening of the southeastward NGCC flow under the influence of the NW monsoon. Acknowledgments The manuscript was greatly improved by the critical comments of K.-Y. Wei, W. R. Howard, the reviewers, and E. Thomas, the journal editor. H. Kawahata (University of Tokyo) and Y. Tanaka (National Institute of Advanced Industrial Science and Technology) provided both the opportunity to conduct this study and valuable comments on our results. Chief scientist T. Kawano (Japan Marine Science and Technology Center; JAMSTEC) as well as the captains, crews, and onboard scientists of the MR98-K02 and MR99-K07 cruises of the R/V Mirai are commended for their substantial efforts in collecting the sediment trap material. L. P. Gupta (JAMSTEC) prepared samples and K. Sakaida (Tohoku University) made suggestions regarding the use of online environmental data. This study was supported by the Global Carbon Cycle and Related Mapping Based on Satellite Imagery Program funded by Special Coordination Funds for Promoting the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. References Bé, A.W.H., 1977. An ecological, zoogeographic and taxonomic review of recent planktonic foraminifera. In: Ramsay, A.T.S. (Ed.), Oceanic Micropaleontology, vol. 1. Academic Press, London, pp. 1–100. Bradshaw, J.S., 1959. Ecology of living planktonic foraminifera in the north and equatorial Pacific Ocean. Contributions from the Cushman Foundation for Foraminiferal Research 10, 25–64. Brown, J., Colling, A., Park, D., Phillips, J., Rothery, D., Wright, J., Bearman, G (Eds.), 1989. Ocean Circulation. The Open University, Milton Keynes, UK. 238 pp. Conan, S.M.H., Brummer, G.J.A., 2000. Fluxes of planktic foraminifera in response to monsoonal upwelling on the Somalia Basin margin. Deep-Sea Research. Part II 47, 2207–2227. Conkright, M., Levitus, S., Boyer, T., 1994. Ocean Circulation. Nutrients. NOAA Atlas NESDIS 1, vol. 1. U.S. Department of Commerce, Washington, DC. Curry, W.B., Ostermann, D.R., Guptha, M.V.S., Ittekkot, V., 1992. Foraminiferal production and monsoonal upwelling in the Arabian Sea: evidence from sediment traps. In: Summerhays, C.P., Prell, W.L., Emeis, K.C. (Eds.), Upwelling Systems: Evolution since the Early Miocene. Geological Society Special Publication, vol. 64, pp. 93–106. Deuser, W.G., Ross, E.H., Hemleben, C., Spindler, M., 1981. Seasonal changes in species composition, numbers, mass, size, and isotopic
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