Settling Particles Flux in Response to El Niño/Southern. Oscillation (ENSO) in ... a significant increase in export flux in the westernmost equatorial Pacific. Ocean.
Global Environmental Change in the Ocean and on Land, Eds., M. Shiyomi et al., pp. 95–108. © by TERRAPUB, 2004.
Settling Particles Flux in Response to El Niño/Southern Oscillation (ENSO) in the Equatorial Pacific Hodaka KAWAHATA 1,2 and Lallan Prasad GUPTA 1 1
Institute for Marine Resources and Environment, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8567, Japan 2 Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan Abstract. The equatorial Pacific is an area of great concern owing to its direct involvement in the local and global climate changes through the El NiñoSouthern Oscillation (ENSO) events. Therefore, sediment trap experiments have been performed intermittently during 1991–2002. The results show that the fluxes of biogenic material (carbonate, opal and organic matter, including amino acids) and other related parameters are temporally and spatially distinct across the Western Pacific Warm Pool (WPWP). These variations resulted from the El Niño and La Niña conditions, which alternately prevailed over the equatorial Pacific Ocean during the trap mooring deployments. The westernmost WPWP (a hemipelagic region) recorded relatively high average total mass and amino acid fluxes during the El Niño event. This was in sharp contrast to the eastern part of the WPWP (oligotrophic and weak upwelling regions), which recorded higher flux values during the La Niña event. Settling particulate organic matter was rich in labile components (amino acids) during La Niña throughout the study area. Relative molar ratios of aspartic acid to β-alanine and gluatmic acid to γ-aminobutyric acid suggest that organic matter degradation was more intense during La Niña relative to that during El Niño in the WPWP. This study clearly shows that during an El Niño event the well documented decrease in export flux in the easternmost equatorial Pacific is accompanied by a significant increase in export flux in the westernmost equatorial Pacific Ocean. In addition, the results from 6 year-long sediment trap experiments conducted at 175°E on the equator show that average flux of total mass in January–April is highly correlated with southern oscillation index (SOI) and sea surface temperature (SST) anomaly (respective r = 0.75 and –0.68, p < 0.05). Total mass flux increases when the ocean conditions become more like La Niña, i.e. SOI increases and SST anomaly decreases. Keywords: ENSO cycle (El Niño/Southern Oscillation), settling particles, organic carbon, carbonate, biogenic opal, amino acid, sediment trap, WPWP, equatorial Pacific
1. INTRODUCTION
The equatorial Pacific is an area of great concern owing to its direct involvement in the local and global climate changes through the El Niño-Southern Oscillation (ENSO) events. The western sector of the equatorial Pacific is remarkably 95
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H. K AWAHATA and L. P. GUPTA Trade wind Indonesian low
ITCZ Equator Upwelling
Trade wind 130 E
70 W
Trade wind Indonesian low
ITCZ Equator Upwelling
Trade wind 70 W
130 E
ITCZ Equator D e c l i ne i n u p w e l l i ng
130 E
70 W
Fig. 1. Schematic diagram of ENSO cycle in the equatorial Pacific (modified from The Open University, 1989).
different from the rest of the Pacific in terms of sea surface temperature (SST), and is called the Western Pacific Warm Pool (WPWP). Higher SST (>28°C) and associated changes in the western Pacific have attracted a lot of attention in terms of various aspects of oceanography, climatology and biogeochemistry (e.g. Delcroix et al., 1992; Barber et al., 1996; Blanchot and Rodier, 1996; Radenac and Rodier, 1996; Mackey et al., 1997; Kawahata et al., 1998b; Sholkovitz et al., 1999; Dunne et al., 2000; Higgins and Mackey, 2000; Kawahata et al., 2000; Lee et al., 2000). The temperature and geographic extension of the WPWP fluctuate under the influence of the ENSO events. Under normal conditions, the Indonesian low exists above the WPWP around the Indonesian Maritime Continent and New Guinea. Cold water upwells in the equatorial eastern Pacific and off Peru. During
Sampling duration from to Trap depth (m) Seafloor depth (m)
Position
Trap name Region
04.6.91 27.4.92 1591 4413
2°059.8′ N 135°001.5′ E
N1 Western
04.6.91 15.4.92 1769 4888
4°007.5′ N 136°016.6′ E
N2 Western
01.01.99 21.11.99 970 4762
4°02.9′ N 135°00.0′ E
M1 Western
01.10.94 16.04.95 1164 3181
1°013.2′ N 160°033.9′ E
N10 Central
05.01.99 21.11.99 1020 3680
0°00.8′ N 145°01.6′ E
M3 Central
01.6.92 16.4.93 1357 4880
0°000.2′ N 175°009.7′ E
N3 Eastern
Table 1. Locations of the traps, water depths, mean fluxes, average compositions and various ratios of material collected at different sites. Settling particles at N1, N2, N10 and N3 were sampled during El Niño events while those at M1, M3 and M5 were obtained during La Niña events. Major component data during El Niño events are from Kawahata et al. (1998a) and Kawahata et al. (2000) and amino acid and hexosamine data are cited from Gupta and Kawahata (2000) and Gupta and Kawahata (2002).
13.01.99 12.31.03 1040 4828
0°02.3′ N 174°56.4′ E
M5 Eastern
Settling Particles Flux in Response to El Niño/Southern Oscillation (ENSO) 97
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El Niño years, anomalously cool temperatures and high pressure tend to develop over the western Pacific, with low pressure and anomalously warm temperatures in the east. In addition to this, the trade winds relax in the central and western Pacific, resulting in the warm water in the western Pacific spreading eastwards, leading to depression of the thermocline in the eastern Pacific which prevents the upwelling of cold water off Peru. In contrast, totally opposite oceanographic conditions occur during a La Niña event (Neelin et al., 1998) (Fig. 1). The WPWP, upwelling region and low pressure zone shift westward. Such cyclic climatic change may influence the efficiency of the biological pump along the equator in the Pacific. However, the role of these processes has remained largely unexplored. Therefore, we need to understand the flux and composition of settling particles in the different regions of the WPWP and to what degree and how its composition and flux change in response to the ENSO. Primary production and consequent burial of particulate organic matter (POM) in this region can be fundamental to our understanding of the global carbon cycle. A significant contribution of the net community production in the equatorial Pacific has been estimated to sink vertically in the form of settling POM (Hansell et al., 1997a, b). By estimating the labile components (e.g. amino acids) of settling POM, it becomes possible to estimate what amount of settling POM has the potential to return to fast cycling of carbon and what percentage is going to be buried in the ocean bottom for a geological time period. In order to understand the climatic and oceanographic changes, we deployed sediment traps along the equator of the western and central Pacific for four consecutive years under the GCMAPS (Global Carbon Cycle and Related Mapping Based on Satellite Imagery) program. In this paper we discuss the variations in biogenic components of settling particles at a variety of stations spread across the WPWP. The sampling sites were located in the hemipelagic region of the western WPWP (Sites N1, N2 and M1), in the oligotrophic region of the central and eastern WPWP (Sites N10 and M3) and in the equatorial upwelling region of the eastern WPWP (Sites N3, M5) (Table 1). Data for Sites N1, N2, N3 and N10 were obtained during El Niño events while those for Sites M1, M3, and M5 were obtained during a La Niña event. In addition, sediment trap experiments at Sites N3 and M5 were conducted intermittently between October 1990 and January 2003 for six years at 175°E on the equator. The data provide an opportunity to look into the impact of the ENSO on the export flux and biogeochemistry of settling POM. 2. AREAS OF INVESTIGATION AND OCEANOGRAPHIC SETTING
The study area was extended along the equator in the WPWP up to its eastern margin (Fig. 2). North equatorial current (NEC), equatorial counter current (ECC) and south equatorial current (SEC) are the three major surface ocean currents in the study area. Westbound equatorial currents (NEC and SEC) pile up warm water in the WPWP (Gordon and Fine, 1996). Besides these currents, the equatorial undercurrent (EUC), an eastbound flow, also has the potential to affect
Settling Particles Flux in Response to El Niño/Southern Oscillation (ENSO)
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Fig. 2. Locations of the mooring sites and outline of the ocean surface currents in February (simplified) in the western Pacific, NEC: North Equatorial Current; ECC: Equatorial Counter Current; SEC: South Equatorial Current. Sites N1 and M1 represent the same location while Site N2 is located approximately 140 km north-east of Sites N1 and M1. Sites N3 and M5 are situated nearly at the same point.
settling particle flux along the equator (Hansell et al., 1997a). Based on modeling studies, the oligotrophic upper layer of the WPWP can be clearly distinguished, showing nitrate depletion and low new production, from upwelled, saline, cooler, nitrate-rich waters supporting high primary production in the eastern equatorial Pacific (Stoens et al., 1999). Temporal and spatial variations in this region may occur due to solar irradiation variations, ENSO events and global warming or cooling (Yan et al., 1992). In the western equatorial Pacific the upwelling intensity is generally weaker than that in the eastern equatorial Pacific. However, there is a strong upwelling system called the Mindanao Dome in the western equatorial Pacific (7°N, 130°E) (Lukas et al., 1991; Masumoto and Yamagata, 1991; Godfrey et al., 1993), where thermocline depth is much shallower than average in the western equatorial Pacific (Peña et al., 1994). As a result, the western sector of the WPWP shows annual mean of primary production as high as 600 mg C m–2day–1, while most of the central and eastern sectors of the WPWP show medium to low primary production of 200 to 400 mg C m–2day–1 (Plates 1a–d in Antoine et al., 1996). ENSO-related sea-level changes in the WPWP affect the Indonesian Throughflow and the Leeuwin Current (Godfrey, 1996). Seasonal changes (strong northwestward and weak southwestward flows) in the Throughflow have been observed to be related to changes in the monsoon, Halmahera Eddy and Mindanao Dome (Kashino et al., 1999). Trap sites N1, N2 and M1 were located in the western sector (hemipelagic ocean) of the WPWP, where Mindanao eddy is a prominent feature. Trap sites N10 and M3 were located in the central sector (oligotrophic ocean) and Sites N3 and M5 in the eastern sector (weak equatorial upwelling region) of the WPWP.
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3. METHODOLOGY
Time-series, cone-shaped sediment traps with 0.5 m 2 aperture and microprocessor-controlled sample collectors were deployed with bottom tethered moorings. Before deployment, sample bottles were filled with filtered (0.45 µm pore size filter) deep seawater, which was collected at a depth of 3,000 m at 30°N, 175°E. Analytical grade formalin was added in seawater to make a 3% solution buffered with sodium borate in order to retard bacterial activity in the trapped material. Samples were commonly collected over 2-week intervals. Recovered samples were immediately refrigerated on board at approximately 2 to 4°C. All recognizable zooplankton “swimmers” were removed with forceps. The trap material was then carefully wet-sieved through a 1-mm screen and split into aliquots by a high precision rotary wet sample divider. The
