Particulate carbon and nitrogen fluxes and compositions in the central equatorial Pacific

Mass, carbon, and nitrogen fluxes and carbon and nitrogen compositions were determined for particulate samples from plankton net tows, shallow floating sediment traps, intermediate and deep moored sediment traps, and sediment cores collected along 140°W in the central equatorial Pacific Ocean during the US JGOFS EqPac program. Mass, particulate organic carbon (POC), and particulate inorganic carbon (PIC) fluxes measured by the floating sediment traps during the Survey I (El Niño) and Survey II (non-El Niño) cruises follow essentially the same pattern as primary production: high near the equator and decreasing poleward. POC fluxes caught in free-floating traps were compared with alternative estimates of export fluxes, including ²³⁴Th models, new production, and other sediment trap studies, resulting in widely differing estimates. Applying ²³⁴Th corrections to the trap-based fluxes yielded more consistent results relative to primary production and new production. Despite factors of five differences in measured fluxes between different trap types, POC : ²³⁴Th ratios of trap material were generally within a factor of two and provided a robust means of converting modeled ²³⁴Th export fluxes to POC export fluxes. All measured fluxes decrease with depth. Trap compositional data suggest that mineral “ballasting” may be a prerequisite for POC settling. POC remineralization is most pronounced in the epipelagic zone and at the sediment–water interface, with two orders of magnitude loss at each level. Despite seawater supersaturation with respect to calcium carbonate in the upper ocean, 80% of PIC is dissolved in the epipelagic zone. Given the time-scale differences of processes throughout the water column, the contrasting environments, and the fact that only 0.01% of primary production is buried, sedimentary organic carbon accumulation rates along the transect are remarkably well correlated to primary production in the overlying surface waters. POC to particulate total nitrogen (PTN) ratios for all samples are close to Redfield values, indicating that POC and PTN are non-selectively remineralized. This constancy is somewhat surprising given conventional wisdom and previous equatorial Pacific results suggesting that particulate nitrogen is lost preferentially to organic carbon.     

The particle fluxes in sediment traps from Niulang Guyot area in the Northwest Pacific Ocean

The flux of settling particles in the ocean has been widely explored since 1980s due to its important role in biogenic elements cycling, especially in the transport of particulate organic carbon (POC) in the deep sea. However, research in the seamount area of the oligotrophic subtropical Northwest Pacific Ocean is lacking. In this work, two sediment traps were deployed at the foot and another two at the hillside of Niulang Guyot from August 2017 to July 2018. The magnitude and composition of particle fluxes were measured. The main factors influencing the spatial variations of the fluxes were evaluated. Our results indicated a low particulate flux from Niulang Guyot area in the Northwest Pacific Ocean, reflecting low primary productivity of the oligotrophic ocean. The total mass flux (TMF) decreased from 2.57 g/(m2·a) to 0.56 g/(m2·a) with increasing depth from 600 m to 4 850 m. A clear seasonal pattern of TMF was observed, with higher flux in summer than that in winter. The peak flux of 26.52 mg/(m2·d) occurred in August at 600 m, while the lowest value of 0.07 mg/(m2·d) was shown in February at 4 850 m. The settling particles at the deep layers had similar biochemical composition, with calcium carbonate (CaCO₃) accounting for up to 90%, followed by organic matter and opal, characteristics of Carbonate Ocean. The POC flux decreased more rapidly in the twilight layer because of faster decomposition, remineralization, and higher temperature. A small fraction of POC was transported into the deep ocean by biological pump. Particle fluxes were mainly controlled by the calcareous ballasts besides the primary productivity of the surface water. The advection may be another important factor affecting the flux in the seamount area. The combination of settled matters rich in foraminiferal tests with topography and currents may be the reason for regulating the local abundance of benthos on seamounts. Our results will fill in the knowledge gap of sedimentation flux, improve the understanding of ecosystem in Niulang Guyot area, and eventually provide data support for the optimization of regional ecological modeling.     

Relationship between time-series diatom fluxes in the central and western equatorial Pacific and ENSO-associated migrations of the Western Pacific Warm Pool

The water masses of the central and western equatorial Pacific can be divided into two parts: the Western Pacific Warm Pool (WPWP) and the Equatorial Upwelling Region (EUR). The behavior of the WPWP plays a significant role in global climate changes such as the El Niño-Southern Oscillation (ENSO), and it drastically modifies the oceanographic conditions in the area every few years. It is important to evaluate changes in time-series diatom fluxes during both the El Niño and the La Niña events. As a part of the Global Carbon Cycle and Related Mapping based on Satellite Imagery (GCMAPS) Program, time-series sediment trap moorings were deployed and recovered along the Equator at seven stations (Sites MT1–MT7) during five R/V Mirai cruises in the central and western Pacific during January 1999–January 2003. The entire length of this study is divided into two phases depending on the oceanographic conditions: the La Niña event (1999 and 2000); and the El Niño event (2002). Site MT3 was located in the WPWP and Sites MT5–MT7 were in the EUR. Annual means of total diatom fluxes increased towards the east in each year. The fluxes observed at Sites MT4–MT6 decreased from the La Niña event to the El Niño event. However, the fluxes observed at Site MT3 in 2001 and 2002 were higher than those in 2000. Total diatom fluxes showed different seasonal patterns at all sites. The diatom assemblages in the WPWP differed from those of the EUR. Pennate diatoms (e.g., Nitzschia bicapitata, Thalassionema nitzschioides) dominated in the WPWP, while the relative abundances of centric diatoms (e.g., Rhizosolenia bergonii, Azpeitia spp., Thalassiosira spp.) were higher than those of pennate diatoms in the EUR. The diatom fluxes during the La Niña event reflected seasonal oscillation of the WPWP in spatial extent. At Site MT3 during El Niño, terrestrial materials appeared to have been transported by subsurface currents, which might be a secondary influence on total diatom fluxes. The spatial extent of the WPWP reached Site MT7 in 2002, when total diatom fluxes decreased in the sediment traps located in the eastern region including Site MT7. Therefore, we conclude that the relationships between the ENSO and diatoms fluxes in the western and central equatorial Pacific can be explained by the geographic (west–east) expansion or contraction of the WPWP.     

Influence of ENSO variability on sinking-particle fluxes in the northeastern equatorial Pacific

A time-series sediment trap was operated from July 2003 to July 2008 at a station located in the 10°N thermocline ridge of the northeastern equatorial Pacific (10°30′N, 131°20′W), with the aim of understanding variations in natural background sinking-particle flux and the influence on such fluxes of ENSO (El Niño-Southern Oscillation). Each one of weak El Niño, moderate El Niño and moderate La Niña were observed during the monitoring period. During non-ENSO periods, total mass fluxes varied from 4.1 to 36.9 mg m⁻² d⁻¹, with a distinct seasonal variation, ranging from an average flux of 14.0 mg m⁻² d⁻¹ in the warm season (June-November) to 25.3 mg m⁻² d⁻¹ in the cold season (December-May). This seasonal fluctuation was characterized by a distinct difference in CaCO₃ flux between the two seasons. The enhanced particle fluxes during the cold season are attributed to the supply of nutrient-enriched subsurface water by wind-driven vertical mixing, supported by a simultaneous reduction in sea surface temperature and enhanced trade winds. The weak El Niño event occurred in the monitoring period had no recognizable effect on particle fluxes in the study area, but the moderate El Niño event was accompanied by a significant reduction in particle fluxes to 60% of the average background value in the warm season. In contrast, particle fluxes during the moderate La Niña increased to a maximum value of 129.9 mg m⁻² d⁻¹, almost three times the average background value. Organic carbon and biogenic silica fluxes were most sensitive to the El Niño and La Niña conditions. The observed variations of particle fluxes are synchronized with those of chlorophyll-a, suggesting primary productivity for the main cause of flux change. The present data indicate that marked seasonal variability in background fluxes commonly exceeds the variability associated with ENSO and post-ENSO signals, which should be taken into account when evaluating the influence of ENSO on sinking particle fluxes in the 10°N thermocline ridge area.     

Physical control of biological processes in the central equatorial Pacific Ocean

A five-component (phytoplankton, zooplankton, ammonium, nitrate, detritus) physical–biological model was developed to investigate the effects of physical processes on daily to interannual time scales, on the lower trophic levels of the central equatorial Pacific. Many of the biological processes included in the ecosystem model respond to environmental fluctuations with time scales between 1 and 10 d, which are not typically resolved by basin- to global-scale circulation models. Therefore, the equatorial Pacific ecosystem model is forced using daily information (solar radiation, velocity, temperature) from the Tropical Atmosphere Ocean (TAO) mooring array. The ecosystem model also requires vertical velocity information which necessitated the development of a method for computing daily vertical velocities from the TAO array. Much of the variability in primary production, plankton and nutrient concentrations observed in 1992 during the US Joint Global Ocean Flux Study Equatorial Pacific Process Study time-series cruises (TS1 and TS2), is well reproduced in the model simulations. Simulations demonstrate that lower primary productivities during TS1 as compared to TS2 result from the deeper thermocline that persisted during TS1 as a result of El Niño conditions; however, because of the simultaneous reduction in grazing pressure, simulated chlorophyll levels are similar for these two time periods. Simulations of this single-species ecosystem model successfully reproduce data collected both during and after the El Niño, suggesting that species composition changes are not of first-order importance when examining the effects of the 1991–92 El Niño on the equatorial Pacific ecosystem. A 60–70% increase in chlorophyll concentration and a 400% increase in the chlorophyll contribution by diatoms was associated with the passage of a tropical instability wave (20-d period) across the study site during TS2. This period of high chlorophyll concentration and diatom abundance coincided temporally with strong northward velocities and strong downwelling velocities in the upper euphotic zone. Observations and simulations suggest that this increase in chlorophyll concentration and change in species composition not only results from in situ diatom growth stimulated by increased iron concentrations, but also results from the advection of diatoms toward the convergent front located along the leading (western) edge of the instability wave. Equatorially trapped internal gravity waves can also stimulate in situ phytoplankton growth as high-frequency vertical motions introduce limiting micronutrients, such as iron, into the euphotic zone. Because iron can be taken up by the picoplankton on time scales much shorter than the wave period (6–8 days), these waves may provide a mechanism for effecting a large flux of iron into the euphotic zone. Exclusion of these high-frequency motions results in an iron flux to the euphotic zone that may be underestimated by more than 30%.     

Export fluxes of particulate organic carbon in the Chukchi Sea: A comparative study using 234Th/238U disequilibria and drifting sediment traps

Large-volume sampling of ²³⁴Th and drifting sediment trap deployments were conducted as part of the 2004 Western Arctic Shelf–Basin Interactions (SBI) spring (May 15–June 23) and summer (July 17–August 26) process cruises in the Chukchi Sea. Measurements of ²³⁴Th and particulate organic carbon (POC) export fluxes were obtained at five stations during the spring cruise and four stations during the summer cruise along Barrow Canyon (BC) and along a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. ²³⁴Th and POC fluxes obtained with in situ pumps and drifting sediment traps agreed to within a factor of 2 for 70% of the measurements. POC export fluxes measured with in situ pumps at 50 m along BC were similar in spring and summer (average = 14.0 ± 8.0 mmol C m^− 2 day^− 1 and 16.5 ± 6.5 mmol C m^− 2 day^− 1, respectively), but increased from spring to summer at the EHS transect (average = 1.9 ± 1.1 mmol C m^− 2 day^− 1 and 19.5 ± 3.3 mmol C m^− 2 day^− 1, respectively). POC fluxes measured with sediment traps at 50 m along BC were also similar in both seasons (31.3 ± 9.3 mmol C m^− 2 day^− 1 and 29.1 ± 14.2 mmol C m^− 2 day^− 1, respectively), but were approximately twice as high as POC fluxes measured with in situ pumps. Sediment trap POC fluxes measured along the EHS transect also increased from spring to summer (3.0 ± 1.9 mmol C m^− 2 day^− 1 and 13.0 ± 6.4 mmol C m^− 2 day^− 1, respectively), and these fluxes were similar to the POC fluxes obtained with in situ pumps. Discrepancies in POC export fluxes measured using in situ pumps and sediment traps may be reasonably explained by differences in the estimated POC/²³⁴Th ratios that arise from differences between the techniques, such as time-scale of measurement and size and composition of the collected particles. Despite this variability, in situ pump and sediment trap-derived POC fluxes were only significantly different at a highly productive station in BC during the spring.     

Composition and flux of particulate amino acids and chloropigments in equatorial Pacific seawater and sediments

Compositions and fluxes of amino acids and major chloropigments were measured in the central equatorial Pacific Ocean as part of the US JGOFS EqPac program. Fluxes decreased by several orders of magnitude, from 400 to 0.03 mg amino acid m⁻² d⁻¹ and from 9 mg to 0.0004 μg chloropigment m⁻² d⁻¹, between production in the surface waters and accumulation at the sea floor. Most rapid losses were in surface waters and at the sediment interface. Losses from the mid-water column were as great as those in surface waters or at the sediment interface, but occurred over a much greater depth range. Export flux estimates based on floating sediment traps were higher near the equator and decreased poleward, similar to primary production.Little meridional difference was apparent in composition of either amino acids or pigments in exported material over the 24° of latitude sampled in spite of the large (factor of 5–6) difference in fluxes. However, pigment composition changed dramatically with depth in the water column, and considerable diagenesis occurred before particles reached the sediment. Pigment compositions suggest that suspended particles were more degraded in the northern than in the southern hemisphere, possibly due to differences in food chain structure. Compositional changes in amino acids occurred in the water column, but were most noticeable at the sediment–seawater interface. Increases in the relative proportions of aspartic acid and glycine with depth were more consistent with preferential preservation within the particulate matrix than with any inherent stability of these compounds to heterotrophic consumption. The contribution of amino acids and pigments to total organic carbon clearly shows that selective degradation of organic matter occurs with depth; this is not evident from total organic carbon data alone. Amino acids contributed about a quarter of the total organic carbon (OC) in surface waters and 16% of the OC in sediment; pigments decreased from 1% of total OC in surface waters to <0.001% in sediments. Decreases in the contribution of amino acids to total organic carbon may be due to transformation into uncharacterizeable material as well as to respiration.     

Sensitivity of 234Th export to physical processes in the central equatorial Pacific

We determined the sensitivity of the calculated sinking flux of ²³⁴Th in the central equatorial Pacific to physical processes and scavenging mechanisms by imposing a meridional and vertical advection and diffusion field on a simple dissolved and particulate ²³⁴Th cycle. We used the model to estimate the efficiency with which the ²³⁴Th deficiency relative to ²³⁸U reflected the predicted sinking flux of ²³⁴Th on particles and compared our results with ²³⁴Th data taken during the JGOFS-EqPac 1992 Survey II Cruise. ²³⁴Th deficiencies near the equator were strongly affected by both vertical advection and horizontal diffusion. The model ²³⁴Th deficiency at the equator underestimated the model ²³⁴Th sinking flux by 144% in neglecting advection and diffusion in the presence of strong upwelling at the equator. The model ²³⁴Th deficiency at the equator corrected for advection overestimated the sinking flux of ²³⁴Th by 33% in neglecting horizontal diffusion. Analysis of the scavenging mechanism suggests that, during situations of export governed by rapidly sinking particles, ²³⁴Th-based estimates of particle export are only half as sensitive to advection compared to situations of export governed by slowly sinking particles. Given that results using the mechanism of slowly sinking particles compare better with the observed ²³⁴Th deficiency and calculated meridional ²³⁴Th fluxes at the equator than the mechanism of rapidly sinking particles, we consider the mechanism of slowly sinking particle more appropriate for this region. In agreement with previous studies based on observed ²³⁴Th gradients, this study supports the incorporation of vertical advection terms in the ²³⁴Th balance to estimate particulate carbon export at the equator but suggests that this method may have overestimated the sinking flux at the equator during EqPac Survey II by 0–63% due to the role of horizontal diffusion.     

2005-2006年大亚湾大鹏澳养殖区甲藻孢囊通量的沉积捕捉器研究

于2005年3月至2006年4月在大亚湾的大鹏澳养殖区采用沉积物捕捉器,对甲藻孢囊形成和水温、盐度、营养盐含量等环境因子进行了周年调查,探索该区孢囊形成特征及其与环境因子的关系.在一年的沉积物捕捉器研究中,共分析鉴定出27种甲藻孢囊类型,锥状斯氏藻Scrippsiella trochoidea孢囊为该区最优势孢囊类群,从孢囊的组成显示了该区的污染是以养殖和生活污染为主.孢囊形成没有明显的季节分布特征,两个站年平均为1.07×10<'5>cysts·(m<'2>·d)<'-1>.高、低温都能在一定程度上促进孢囊的生成,氮限制对孢囊的形成具有明显促进作用.首次发现亚历山大藻Alexandrium孢囊在冬季形成的高峰,最高值达4.26×10<'5>cysts·(m<'2>·d)<'-1>.这说明核电站温排水冬季高水温对大亚湾生态环境有较大的影响.     

Phytodetritus and the abundance and biomass of abyssal nematodes in the central,equatorial Pacific

The abundance and biomass of abyssal (4300–5000 m) nematodes were investigated along a latitudinal gradient of phytodetritus deposition from 0 to 23°N in the central, equatorial Pacific (140–158°W). Nematode abundance in the oligotrophic, central, North Pacific gyre was 50% lower than at the equator while macrofauna abundance increased 6.5 times over the same transect. Nematode abundance and biomass in the surface (0–1 cm) sediment layer were significantly higher at phytodetritus stations than at non-phytodetritus stations. Abundance and biomass were within the range recorded from other sites of comparable depth that also receive an input of phytodetritus. Abundance was also strongly correlated with microbial biomass. An increase in body size was associated with an increase in food supply. The results demonstrate that the equatorial Pacific represents an immense zone of relatively high nematode standing stock.     

Downward export of carbon by diel migrant mesozooplankton in the central equatorial Pacific

Using data collected during cruises of the JGOFS equatorial Pacific Study in March/April and October of 1992 at the equator (140°W), we examine the downward transport of carbon by three size classes of die] migrant mesozooplankton (200–500 gm, 500–1000 μm and 1000–2000 gm). In addition to respiratory carbon flux, we consider the flux due to mortality of migrators below the euphotic zone. Diel migrant mesozooplankton biomass was estimated from the difference between nighttime and daytime biomass within the euphotic zone. Except for a four-day period early in the March/April cruise, mesozooplankton nighttime biomass was significantly larger than daytime biomass within the euphotic zone during both cruises. We estimate that the downward flux of carbon from the euphotic zone due to mesozooplankton die] vertical migrators was an average of 0.6 mmol Cm⁻² d⁻¹ and 1.1 mmol C m⁻² d⁻¹ during the March/April and October cruises, respectively. Addition of this flux to the gravitational particle sinking flux estimated from²³⁴Th measurements during the same period results in a 31 % increase in the carbon export flux from the euphotic zone in the equatorial Pacific during the March/April cruise and a 44% increase in the October cruise. The migratory flux is strongly dependent on whether feeding takes place below the euphoric zone, the length of time migrators spend in the deep waters, and the mortality rate of migrators.     

Calibration of sediment traps and particulate organic carbon export using 234Th in the Barents Sea

Profiles of particulate and dissolved ²³⁴Th (t_1/2=24.1 days) in seawater and particulate ²³⁴Th collected in drifting traps were analyzed in the Barents Sea at five stations during the ALV3 cruise (from June 28 to July 12, 1999) along a transect from 78°15′N–34°09′E to 73°49′N–31°43′E. ²³⁴Th/²³⁸U disequilibrium was observed at all locations. ²³⁴Th data measured in suspended and trapped particles were used to calibrate the catchment efficiency of the sediment traps. Model-derived ²³⁴Th fluxes were similar to ²³⁴Th fluxes measured in sediment traps based on a steady-state ²³⁴Th model. This suggests that the sediment traps were not subject to large trapping efficiency problems (collection efficiency ranges from 70% to 100% for four traps). The export flux of particulate organic carbon (POC) can be calculated from the model-derived export flux of ²³⁴Th and the POC/²³⁴Th ratio. POC/²³⁴Th ratios measured in suspended and trapped particles were very different (52.0±9.9 and 5.3±2.2 μmol dpm⁻¹, respectively). The agreement between calculated and measured POC fluxes when the POC/²³⁴Th ratio of trapped particles was used confirms that the POC/²³⁴Th ratio in trap particles is representative of sinking particles. Large discrepancies were observed between calculated and measured POC fluxes when the POC/²³⁴Th ratio of suspended particles was used. In the Barents Sea, vertical POC fluxes are higher than POC fluxes estimated in the central Arctic Ocean and the Beaufort Sea and lower than those calculated in the Northeast Water Polynya and the Chukchi Sea. We suggest that the latter fluxes may have been strongly overestimated, because they were based on high POC/²³⁴Th ratios measured on suspended particles. It seems that POC fluxes cannot be reliably derived from thorium budgets without measuring the POC/²³⁴Th ratio of sediment trap material or of large filtered particles.     

JGOFS studies in the equatorial Pacific

This special issue is the third and final volume containing results from the JGOFS Process Study in the equatorial Pacific. Most of the Study in the equational Pacific. Most of the contributions evolved either from the US JGOFS workshop in 1994 on the equatorial Pacific in Scottsdale, AZ or from the NATO Advanced Research Workshop on the Carbon Cycle of the Equatorial Pacific in 1995 in Noumea, New Caledonia.     

Vertical flux of hydrocarbons as measured in sediment traps in the eastern North Pacific Ocean

Sediment trap experiments were conducted at three stations (Stn. 5 at 4750 m depth, 7 at 4330 m depth and 11 at 3880 m depth) in the California Current and its adjacent areas in the eastern North Pacific from December 1982 to January 1983 to collect sinking particles, which were analyzed for organic carbon and hydrocarbons.The vertical fluxes of organic carbon, total nitrogen and lipid carbon at any depth decreased in the following order: Stn. 5 > Stn. 11 > Stn. 7, the same trend as the standing stocks of chl. a at these station, which suggests that regional variation of organic matter flux is considerably influenced by the primary productivity in the surface water layer.Hydrocarbons of the sinking particles consisted of n-C₁₅–C₂₀ with a maximum of n-C₁₇, n-C₂₁–C₃₂, n-C_21:6 and three branched C₂₅ alkenes (br-C_25:3, br-C_25:3, and br-C_25:4), but any odd or even carbon number predominance in n-C₂₁–C₃₂ was not observed. n-C_21:6 and n-C₁₇ were the most abundant throughout the depths at Stn. 5, while only n-C₁₇ was a major component of the sinking particles from the intermediate layers, but not from the deep layers, at Stns. 7 and 11. Thus, there were clear regional and vertical variabilities of hydrocarbon composition in the sinking particles.The vertical flux of various hydrocarbons at the three stations tended to decrease exponentially with depth. The attenuation constants of the hydrocarbon fluxes were calculated by analysis of the relationship between the fluxes and depths. Half-depth of the flux calculated from the attenuation constant showed extensive variability for the hydrocarbon species and the location of the station. The values of the half-depths of n-C₁₇, Σn-C_15–20, n-C_21:6 and pristane were lower than those of Σn-C_21–32 and Σbr-C₂₅, indicating that hydrocarbons derived from phytoplankton are more susceptible to biological degradation than those from zooplankton.The half-depths of the hydrocarbon fluxes tended to increase as follows: Stn. 7 < Stn. 11 < Stn. 5. This trend was the same as the size of the sinking particles collected at these stations, which suggests that the sinking rate of the particles tends to increase in the order of Stns. 7, 11 and 5. Thus, these data indicate that the sinking rate of the particles is a primary factor in determining the hydrocarbon compositon of the particles in the intermediate and deep waters.     

赤道东北太平洋沉积物间隙水中溶解有机碳的分布特征

沉积物间隙水中的溶解有机碳(DOC)是沉积物有机质矿化过程中的中间产物[1],沉积物中的有机质通过微生物水解和(厌氧)发酵等方式溶解成各类具有不同分子量的有机化合物,通常总称为溶解有机碳,并释放到沉积物间隙水中.而溶解有机碳又进一步被细菌等微生物所利用,最终被氧化为溶解无机碳,完成有机质的矿化过程.因此,沉积物间隙水中DOC的浓度是消耗和生成之间平衡的结果[1].已有的研究表明,沉积物间隙水中DOC的含量显著高于底层水体中DOC的含量,导致其向底层水体的扩散;近期的研究也表明,来自海底沉积物的DOC通量是底层水体中DOC的重要来源,是海洋有机碳储库中的重要组成之一[2~4].     

Evaluating Cenozoic equatorial sediment deposition anomalies for potential paleoceanographic and Pacific plate motion applications

If equatorial sediments form characteristic deposits around the equator, they may help to resolve the amount of northwards drift of the Pacific tectonic plate. Relevant to this issue, it has been shown that ²³⁰Th has been accumulating on the equatorial seabed faster than its production from radioactive decay in the overlying water column during the Holocene (Marcantonio et al. in Paleoceanography 16:260–267, 2001). Some researchers have argued that this reflects the deposition of particles with adsorbed ²³⁰Th carried by bottom currents towards the equator (“focusing”). If correct, this effect may combine with high pelagic productivity, which is also centered on the equator, to yield a characteristic signature of high accumulation rates marking the paleoequator in older deposits. Here we evaluate potential evidence that such an equatorial feature existed in the geological past. Seismic reflection data from seven meridional transects suggest that a band of equatorially enhanced accumulation of restricted latitude was variably developed, both spatially and temporally. It is absent in the interval 14.25–20.1 Ma but is well developed for the interval 8.55–14.25 Ma. We also examined eolian dust accumulation rate histories generated from scientific drilling data. A dust accumulation rate anomaly near the modern equator, which is not obviously related to the inter-tropical convergence zone, is interpreted as caused by focusing. Accumulation rates of Ba and P₂O₅ (proxies of export production) reveal a static equatorial signature, which suggests that the movement of the Pacific plate over the period 10–25 Ma was modest. The general transition from missing to well-developed focusing signatures around 14.25 Ma in the seismic data coincides with the mid-Miocene development of the western boundary current off New Zealand. This current supplies the Pacific with deep water from Antarctica, and could therefore imply a potential paleoceanographic or paleoclimatic origin. At 10.05–14.25 Ma, the latitudes of the seismic anomalies are up to ~2° different from the paleoequator predicted by Pacific plate-hotspot models, suggesting potentially a small change in the hotspot latitudes relative to the present day (although this inference depends on the precise form of the deposition around the equator). The Ba and P₂O₅ anomalies, on the other hand, are broadly compatible with plate models predicting slow northward plate movement over 10–25 Ma.     

Variability of C02 distributions and sea-air fluxes in the central and eastern equatorial Pacific during the 199–1994 El Nin˜o

As part of the U.S. JGOFS Program and the NOAA Ocean-Atmosphere Carbon Exchange Study (DACES), measurements of C02 partial pressure were made in the atmosphere and in the surface waters of the central and eastern equatorial Pacific during the boreal spring and autumn of 1992, the spring of 1993, and the spring and autumn of 1994. Surface-water pC0₂ data indicate significant diurnal, seasonal, and interannual variations. The largest variations were associated with the 1991–1994 ENSO event, which reached maximum intensity in the spring of 1992. The lower values of surface-water ΔpC0₂ observed during the 1991–1994 ENSO period were the result of the combined effects of both remotely and locally forced physical processes. The warm pool, which reached a maximum eastward extent in January-February of 1992, began in September of 1991 as a series of westerly wind events lasting about 30 days. Each wind event initiated an eastward propagating Kelvin wave which caused a deepening of the thermocline. By the end of January 1992 the thermocline was at its maximum depth, so that the upwelled water was warm and C0₂-depleted. In April of the same year, the local winds were weaker than normal, and the upwelling was from shallow depths. These changes resulted in a lower-than-normal C02 flux to the atmosphere. The results show that for the one-year period from the fall of 1991 until the fall of 1992, approximately 0.3 GtC were released to the atmosphere; 0.6 GtC were released in 1993, and 0.7 GtC in 1994, in good agreement with the model results of Ciais et al. [Science,269,1098–1102;J. Geophys. Res.,100, 5051–5070]. The net reduction of the ocean-atmosphere C02 flux during the 1991–1994 El Nifio was on the order of 0.8 – 1.2 GtC. Thus, the total amount of C02 sequestered in the equatorial oceans during the prolonged 1991–1994 El Nin˜o period was about 25% higher than the severe El Nin˜o of 1982–1983.     

西太平洋暖池水体振荡与赤道中东太平洋海表温度异常的关联性初探

赤道中东太平洋海表温度异常是研究ENSO(El Ni?o-South Oscillation)的重要指标。本文利用海洋再分析数据,着眼于西太平洋暖池暖水体三维结构的年际变异特征,分析西太平洋暖池水体变动与赤道中东太平洋海表温度异常的关联性,并从前期西太平洋暖池三维暖水体的结构演变及其在上层海洋质量与热量配置中的作用角度,探究了赤道中东太平洋关键Ni?o分区海表温度异常变化的内在一致性,以期为ENSO的预测预报提供新的思路和依据。分析表明,ENSO尺度上西太平洋暖池三维结构变异主模态表现为暖水体平均约以170oE为轴的纬向时空振荡,其可超前赤道中东太平洋海表温度异常变化约达6个月,并以在赤道中东太平洋及附近海域形成双舌状异常"暖池"为显著特征。前期暖池暖水体的纬向振荡是赤道中东太平洋海表温度异常变化的重要前兆信息之一。作为海洋中ENSO暴发的重要源区和驱动因子,西太平洋暖池纬向振荡通过在中东太平洋形成不同强度(泛指暖水量及其深度和范围等的大小)的异常"暖池",进而在上层海洋质量和热量的纬向配置中起关键性作用,并且可能与不同EI Ni?o事件的触发机制密切相关,从这种意义上讲,不同ENSO事件的发生和发展存在一定的同源一致性。