Particle dynamics in the deep water column of Sagami Bay, Japan. I: origins of apparent flux of sinking particles

Temporal variations of sinking particle flux, together with their organic chemical properties, were monitored in the deep basin of Sagami Bay, Japan, using sediment traps with very high time resolutions from March 1997 to August 1998. At a height of 350 m above the bottom (about 1200 m water depth), the averaged total mass flux was more than 1000 mg/m²/day, which is about 10 times higher than those obtained for open ocean regions near Sagami Bay. While large amounts of phytodetritus, derived from phytoplankton blooms in the surface water, were transported downward in spring, the following extraordinary patterns in the temporal variability of sinking particle flux were also observed: (1) A sustained large flux of sinking particles during low productive periods from summer to winter in 1997. (2) An episodic increase of sinking particle flux in June 1998. (3) A difference in the temporal variability of sinking particles between the spring bloom periods of 1997 and 1998. The content of total organic carbon (TOC) and the stable carbon isotopic ratio (δ¹³C) of TOC demonstrated that the large fluxes observed in (1) and (2) could be attributed to the resuspension of phytodetritus deposited on the sea floor during the spring bloom period, and the abrupt erosion of surface sediment on the continental slope, respectively. The concentration of suspended particles in the deep water column affect the apparent flux of sinking particles. At the same time, sinking particles exported from surface waters during the spring bloom both decrease and increase suspended particle concentration through scavenging and rebound processes, respectively. Finally, the apparent difference in sinking particle flux between 1997 and 1998, (3), could be explained by differences in the extent of the scavenging process, which depend on the flux and quality of exported particles from the surface waters.     

Particulate Flux and Cd/P Ratio of Particulate Material in the Pacific Ocean

Time-series Mark 7 sediment traps were deployed at three stations at 0°N, 13°N and 48°N along 175°E to investigate seasonal and spatial variations of particulate material flux. Chemical analysis of particulate material was performed for four major chemical components, viz. opal, CaCO₃, organic material and clay minerals, Cd and P in the particulate material were also determined. We discuss the characteristics of particulate material at each site and the transportation of Cd and P to deep water by the particulate material. The total mass fluxes and variation of fluxes at each site reflect oceanographic conditions, such as biological productivity and kind of major planktonic organisms. At the northern site, large mass fluxes with a spring bloom and high ratios of opal are characteristic. Relatively small mass fluxes with high ratios of CaCO₃ are distinct, and dissolution of CaCO₃ due to sinking is recognized in the middle latitude and 0°N sites. The larger flux at the lower trap than the upper trap at the equatorial site suggests influence by lateral transport in the deep water. Distinctive decreasing Cd/P ratio and CaCO₃ concentrations in the particulate material with increasing depth suggests that the change of Cd/P ratio in the intermediate and deep water occurs through the dissolution of CaCO₃. The dissolved Cd/P ratios in the deep water are proportional to the age of the deep water in the Atlantic but not in the Pacific. This is explained by the difference of kinds of particulate material transporting Cd and P in the deep water between the oceans. That is, the major planktonic organisms are planktons of CaCO₃ tests in the Atlantic Ocean and diatoms of opal tests in the North Pacific Ocean.     

Rare earth elements and Opal/CaCO<Subscript>3</Subscript> ratio of sinking particles observed with a time-series sediment trap at the mouth of Tokyo Bay

A time-series sediment trap was deployed from December 1994 to February 2002 at the mouth of Tokyo Bay (35°03′ N, 139°40′ E; water depth, 850 m). Sinking particles were obtained with a time interval of one week at a depth of approximately 100 m above the sea floor. Observed total mass fluxes varied from 3.3 to 226.7 g/m²/day with an average of 28.0 g/m²/day. Concentrations of rare earth elements, Al, Ca and Si in particulate materials were measured. The combustible fraction at 450°C is assumed to be equivalent to the organic matter content. Contents of biogenic materials, namely organic matter, opal and calcium carbonate, were about 30% and the content of lithogenic material was about 70%. Using La/Yb ratios of particles from the sediment trap and Tama-gawa River and surface sediment of Tokyo Bay, it was estimated that about 50% of the lithogenic particles collected in the sediment trap at the mouth of Tokyo Bay originated from resuspended surface sediment in Tokyo Bay. An increasing trend of Opal/CaCO₃ ratio in the sinking particles was found in the spring season. It is suggested that the relative increase of diatoms is due to the decreasing dissolved inorganic nitrogen input into Tokyo Bay.     

Relationship between sinking organic matter and minerals in the shallow zone of the western Subarctic Pacific

We used time-series sediment trap data for four major components, organic matter and ballast minerals (CaCO₃, opal, and lithogenic matter) from 150, 540, and 1000 m in the western subarctic Pacific (WSAP), where opal is the predominant mineral in sinking particles, to develop four simple models for settling particles, including the “ballast model”. The ballast model is based on the concept that most of the organic matter “rain” in the deep sea is carried by the minerals. These four models are designed to simultaneously reproduce the flux of each major component of settling particles at 540 and 1000 m by using the data for each component at 150 m as initial values. Among the four models, the ballast model, which considers the sinking velocity increase with depth, was identified as the best using the Akaike information criterion as a measure of the model fit to data. This model successfully reproduced the flux of organic matter at 540 and 1000 m, indicating that the ballast model concept works well in the shallow zone of the WSAP on a seasonal timescale. This also suggests that ballast minerals not only physically protect the organic matter from degradation during the settling process but also enhance the sinking velocity and reduce the degree of decomposition.     

Relationship between Composition of Settling Particles and Organic Carbon Flux in the Western North Pacific and the Japan Sea

Settling particles play an important role in transporting organic carbon from the surface to the deep ocean. It is known that major components of settling particles are biogenic silicates (opal), biogenic carbonate (CaCO₃), lithogenic clays and organic matter. Since each component aggregates and/or takes in organic carbon, all of these components have the ability to transport particulate organic carbon (POC) to the interior of the ocean. In this study, sediment trap experiments were carried out in four areas of the western North Pacific (including a marginal sea). Factors are proposed that correlate the composition of settling particles with POC flux. Annual mean organic carbon fluxes at 1 km depth in the western North Pacific Basin, Japan Sea, Hidaka Basin and northern Japan Trench were found to be 14.9, 18.1, 13.0 and 6.6 mg/m²/day, respectively. Organic carbon flux in the western North Pacific was greater than that in the Eastern North Pacific (7.4), the Equatorial Pacific (4.2), the Southern Ocean (5.8) and the Eastern North Atlantic (1.8). In the western North Pacific, it was calculated that 52% of POC was carried by opal particles. Opal is known to be a major component even in the Eastern North Pacific and the Southern Ocean, and the opal fluxes in these areas are similar to those in the western North Pacific. However, the organic carbon flux that was carried by opal particles (OC_opalflux) in the western North Pacific was greater than that in the Eastern North Pacific and the Southern Ocean. These results indicate that the ability of opal particles to transport POC to the deep ocean in the western North Pacific is greater than that in the other areas.     

Seasonal and vertical variations of sinking particle fluxes in the West Caroline Basin

A sediment trap experiment was carried out in the West Caroline Basin, located in the equatorial western Pacific between influences of the Asian monsoon and the open ocean. Annual mass flux at the shallow trap at Site 1 was 57.10 g m^-2 yr^-1. Generally, the higher flux of organic matter was associated with higher activities of biogenic opal-producing and carbonate-producing plankton communities. In addition, as the organic matter content increases, the organic carbon/carbonate carbon ratio shows a tendency to increase. Carbonate-producing plankton was predominant during periods 1 and 3 (May to July and November to the beginning of December), which could be due to limited silica supply to the euphotic zone. On the other hand, surface sea water was more nutrient-rich during periods 2 and 4 (August to October and the end of December to April) at Site 1. These high total mass fluxes could be stimulated by wind.The amount of biogenic components collected in the sediment traps and the accumulation in surface sediments at Site 1 could be compared with primary productivity values. Carbonate and biogenic opal fluxes were 99% and 90% less, respectively, in the surface sediments compared to those in the shallow sediment trap. This could be due to the reaction of sinking particles with undersaturated deep sea water just above the sea floor, rather than with the water column during sinking. About 20% of the organic matter was decomposed between the shallow and deep sediment traps and more than 98% between the deep sediment trap and final burial in the surface sediments. The relative amount of organic carbon preserved in surface sediments was about 0.10% of annual primary productivity.     

Stable carbon and nitrogen isotopes of sinking particles in the eastern bransfield strait (Antarctica)

A time-series sediment trap was deployed at 1,034 m water depth in the eastern Bransfield Strait for a complete year from December 25, 1998 to December 24, 1999. About 99% of total mass flux was trapped during an austral summer, showing distinct seasonal variation. Biogenic particles (biogenic opal, particulate organic carbon, and calcium carbonate) account for about two thirds of annual total mass flux (49.2 g m^-2), among which biogenic opal flux is the most dominant (42% of the total flux). A positive relationship (except January) between biogenic opal and total organic carbon fluxes suggests that these two variables were coupled, due to the surface-water production (mainly diatoms). The relatively low δ¹³C values of settling particles result from effects on C-fixation processes at low temperature and the high CO² availability to phytoplankton. The correspondingly low δ¹⁵N values are due to intense and steady input of nitrates into surface waters, reflecting an unlikely nitrate isotope fractionation by degree of surface-water production. The δ¹⁵N and δ¹³C values of sinking particles increased from the beginning to the end of a presumed phytoplankton bloom, except for anomalous δ¹⁵N values. Krill and the zooplankton fecal pellets, the most important carriers of sinking particles, may have contributed gradually to the increasing δ¹³C values towards the unproductive period through the biomodification of the δ¹³C values in the food web, respiring preferentially and selectively¹²C atoms. Correspondingly, the increasing δ¹⁵N values in the intermediate-water trap are likely associated with a switch in source from diatom aggregates to some remains of zooplankton, because organic matter dominated by diatom may be more liable and prone to remineralization, leading to greater isotopic alteration. In particular, the tendency for abnormally high δ¹⁵N values in February seems to be enigmatic. A specific species dominancy during the production may be suggested as a possible and speculative reason.     

Regional variations in the fluxes of foraminifera carbonate,coccolithophorid carbonate and biogenic opal in the northern Indian Ocean

Mass fluxes of diatom opal, planktonic foraminifera carbonate and coccolithophorid carbonate were measured with time-series sediment traps at six sites in the Arabian Sea, Bay of Bengal and Equatorial Indian Ocean (EIOT). The above fluxes were related to regional variations in salinity, temperature and nutrient distribution. Annual fluxes of diatom opal range between 3 and 28 g m⁻² yr⁻¹, while planktonic foraminifera carbonate fluxes range between 6 and 23 g m⁻² yr⁻¹ and coccolithophorid carbonate fluxes range between 4 and 24 g m⁻² yr⁻¹. Annual planktonic foraminifera carbonate to coccolithophorid carbonate ratios range between 0.8 and 2.2 and coccolithophorid carbonate to diatom opal ratios range between 0.5 and 3.3.In the western Arabian Sea, coccolithophorids are the major contributors to biogenic flux during periods of low nutrient concentrations. Coccolithophorid carbonate fluxes decrease and planktonic foraminiferal carbonate and diatom opal fluxes increase when nutrient-rich upwelled waters are advected over the trap site. In the oligotropic eastern Arabian Sea, coccolithophorid carbonate fluxes are high throughout the year. Planktonic foraminiferal carbonate fluxes are the major contributors to biogenic flux in the EIOT. In the northern and central Bay of Bengal, when surface salinity values drop sharply during the SW monsoon, there is a drastic reduction in planktonic foraminiferal carbonate fluxes, but coccolithophorid carbonate and diatom opal fluxes remain steady or continue to increase. Distinctly higher annual molar Si_bio/C_inorg (>1) and C_org/C_inorg (>1.5) ratios are observed in the northern and central Bay of Bengal mainly due to lower foraminiferal carbonate production as a result of sharp salinity variations. We can thus infer that the enhanced freshwater supply from rivers should increase oceanic CO₂ uptake. Its silicate supply favours the production of diatoms while the salinity drop produces conditions unfavourable for most planktonic foraminifera species.     

The downward flux of biogenic material in the NE subarctic Pacific: importance of algal sinking and mesozooplankton herbivory

In the present study we examine factors that affect the downward flux of biogenic carbon in the NE subarctic Pacific, one of the important high-nutrient-low-chlorophyll (HNLC) regions in the open ocean. We focus on the role of mesozooplankton, since their seasonal peaks in biomass and growth are in phase with the seasonal variations in the downward POC fluxes, whereas phytoplankton biomass is more-or-less uniform year-round. The relative importance of mesozooplankton and algal sinking was examined using the pigment composition of material accumulated in short-term free-drifting sediment traps positioned just below the upper stratified surface layer (ca. 100–200 m). This was compared with the phytoplankton composition in the surface waters, and with the grazing activity (gut pigments and fecal pellet production rates) of the most abundant large copepods. We also examined whether the relationships between the downward flux of carbon and pelagic processes were similar in the coastal, continental margin and offshore HNLC regions of the NE subarctic Pacific, the latter represented by Ocean Station Papa (OSP).Our results show that grazing had a variable impact on the downward flux of biogenic carbon. Carbon-transformed pheopigments (particularly pyropheophorbide a, frequently associated with copepod grazing) represented up to 13% of the total downward POC flux inshore (in May 1996) and 8–9% at OSP in May and February 1996, respectively. This flux of pheopigments was accompanied by a large potential input of fecal pellets from large copepods (as estimated from defecation rates of freshly collected animals) only in May 1996 at OSP, suggesting that pheopigments came from other sources (other herbivores, senescing algae) in February. The larger flux of pheopigments in May was probably related to the abundance of mesozooplankton at that time of the year. During summer (August 1996), both the flux of pheopigments and the potential input of fecal pellets from large copepods were negligible at OSP, consistent with more intense pelagic recycling reported in other studies. Inshore, the flux of carbon-transformed pheopigments was slightly higher than at OSP, and its contribution to the downward POC flux in May 1996 was twice that in August 1996. In contrast, the potential input of feces carbon was higher in August than in May 1996, again suggesting other sources for pheopigments found in the traps. The contribution of sinking phytoplankton to the downward biogenic flux was negligible in summer, when prymnesiophytes (indicated by the presence of 19′-hexanoyloxyfucoxanthin) and pelagophytes (19′-butanoyloxyfucoxanthin-containing) dominated in surface offshore waters. The contribution of sinking algae was maximal (9%) in winter (February 1996) at OSP, when fucoxanthin (mainly a diatom marker) dominated the carotenoid composition in the traps and when the abundance of diatoms in surface waters showed its seasonal maximum for this station. Inshore, the low contribution of diatoms (fucoxanthin) to the sinking fluxes may have resulted from inadequate sampling (i.e. the spring bloom may have been missed).Overall, we conclude that: (a) large copepods significantly influenced the downward POC flux only during spring at OSP; (b) unidentified herbivores (e.g. salps, pteropods) producing pigmented, fast-sinking fecal material likely had an important impact during winter; (c) algal sinking made a small contribution to the downward POC flux (maximum in winter); and (d) neither algal sinking nor mesozooplankton grazing had a significant influence on the downward flux of biogenic material in summer at OSP.     

The benthic silica cycle in the Northeast Atlantic: annual mass balance, seasonality, and importance of non-steady-state processes for the early diagenesis of biogenic opal in deep-sea sediments

Within the framework of the EU-funded BENGAL programme, the effects of seasonality on biogenic silica early diagenesis have been studied at the Porcupine Abyssal Plain (PAP), an abyssal locality located in the northeast Atlantic Ocean. Nine cruises were carried out between August 1996 and August 1998. Silicic acid (DSi) increased downward from 46.2 to 213 μM (mean of 27 profiles). Biogenic silica (BSi) decreased from ca. 2% near the sediment–water interface to <1% at depth. Benthic silicic acid fluxes as measured from benthic chambers were close to those estimated from non-linear DSi porewater gradients. Some 90% of the dissolution occurred within the top 5.5 cm of the sediment column, rather than at the sediment–water interface and the annual DSi efflux was close to 0.057 mol Si m⁻² yr⁻¹. Biogenic silica accumulation was close to 0.008 mol Si m⁻² yr⁻¹ and the annual opal delivery reconstructed from sedimentary fluxes, assuming steady state, was 0.065 mol Si m⁻² yr⁻¹. This is in good agreement with the mean annual opal flux determined from sediment trap samples, averaged over the last decade (0.062 mol Si m⁻² yr⁻¹). Thus ca. 12% of the opal flux delivered to the seafloor get preserved in the sediments. A simple comparison between the sedimentation rate and the dissolution rate in the uppermost 5.5 cm of the sediment column suggests that there should be no accumulation of opal in PAP sediments. However, by combining the BENGAL high sampling frequency with our experimental results on BSi dissolution, we conclude that non-steady state processes associated with the seasonal deposition of fresh biogenic particles may well play a fundamental role in the preservation of BSi in these sediments. This comes about though the way seasonal variability affects the quality of the biogenic matter reaching the seafloor. Hence it influences the intrinsic dissolution properties of the opal at the seafloor and also the part played by non-local mixing events by ensuring the rapid transport of BSi particles deep into the sediment to where saturation is reached.     

Sinking Particle Flux in the Subtropical Oligotrophic Northwestern Pacific from a Short-term Sediment Trap Experiment

Time-series sediment traps were deployed in the subtropical oligotrophic northwestern Pacific (SONP) at three depths from August to September 2015 to better understand vertical flux of sinking particles. Sinking particles were collected at 5-day intervals over the sediment trap deployment period. The average total mass flux at water depths of 400 m, 690 m, and 1,710 m was 9.1, 4.4, and 4.1 mg m^-2day^-1, respectively. CaCO₃ materials constituted 50 to 70% of sinking particles while in comparison particulate organic carbon (POC) constituted up to 20%. A synchronous variation of total mass flux was observed at the three depths, indicating that calcite-dominated particles sank from 400 to 1,710 m within a 5-day period. POC flux at these water depths was 2.4, 0.38, and 0.31 mg m^-2day^-1, respectively. Our results indicate low transfer efficiencies of 16% from 400 to 690 m and 13% for the 400 to 1,710 m depth range. The estimated transfer efficiencies were significantly lower than those observed at the K2 station in the northwest Pacific subarctic gyre, presumably because of a prevalence of pico-cyanobacteria in the SONP. Because cyanobacteria have a semi-permeable proteinaceous shell, they are more readily remineralized by bacteria than are siliceous phytoplankton in the northwest Pacific subarctic gyre. Continued surface water warming and expansion of the SONP will likely have a profound impact on ocean acidification in the northwest Pacific, possibly affecting the transfer efficiency of sinking POC to the deep-sea.     

Coupling of benthic oxygen uptake and silica release: implications for estimating biogenic particle fluxes to the seafloor

A new approach to predict biogenic particle fluxes to the seafloor is presented, which is based on diffusive oxygen uptake and, in particular, opal fluxes to the seafloor. For this purpose, we used a recently published empirical equation coupling benthic silica to oxygen fluxes, and showing a clear negative correlation between Si and O₂ fluxes. Dissolution of biogenic silica mediated by aerobic microbial activity has been inferred at 24 sites along the African and South American continental margins. Based on the assumption that these findings hold essentially for the entire Southern Atlantic Ocean, we applied the silica to oxygen flux ratio to a basin-wide grid of diffusive oxygen uptake extracted from the literature. Assuming that the silica release across the sediment-water interface equals the particulate flux of biogenic opal to the seafloor, we estimated minimum opal rain rates. We combined these calculations with published relationships between aerobic organic carbon mineralization and dissolution rates of calcite above the hydrographical lysocline, thereby assessing the calcite rain rate and particulate organic carbon flux to the seafloor. The addition of the buried fraction completes our budget of biogenic particulate rain fluxes. The combination of such empirical equations provides a powerful and convenient tool which greatly facilitates future investigations of biogenic particle fluxes to the seafloor.     

Particulate organic matter and ballast fluxes measured using time-series and settling velocity sediment traps in the northwestern Mediterranean Sea

Prompted by recent data analyses suggesting that the flux of particulate organic carbon sinking into deep waters is determined by fluxes of mineral ballasts, we undertook a study of the relationships among organic matter (OM), calcium carbonate, opal, lithogenic material, and excess aluminum fluxes as part of the MedFlux project. We measured fluxes of particulate components during Spring and Summer of 2003, and Spring of 2005, using a swimmer-excluding sediment trap design capable of measuring fluxes both in a time-series (TS) mode and in a configuration for obtaining particle settling velocity (SV) profiles. On the basis of these studies, we suggest that distinct OM–ballast associations observed in particles sinking at a depth of 200 m imply that the mechanistic basis of the organic matter–ballast association is set in the upper water column above the Twilight Zone, and that the importance of different ballast types follows the seasonal succession of phytoplankton. As in other studies, carbonate appears to enhance the flux of organic matter over opal. Particles must be at least half organic matter before their settling velocity is affected by ballast concentration. This lack of change in ballast composition with SV in particles with <40% OM content suggests that particle SV reaches a maximum because of the increasing importance of inertial drag. Relative amounts of OM and opal decrease with depth due to decomposition and dissolution; carbonates and lithogenic material contribute about the same amount to total mass, or increase slightly, throughout the water column. The high proportion of excess Al cannot be explained by its incorporation into diatom opal or reverse weathering, so Al is most likely adsorbed to particulate oxides. On shorter time scales, dust appears to increase particle flux through its role in aggregation rather than by nutrient inputs enhancing productivity. We suggest that the role of dust as a catalyst in particle formation may be a central mechanism in flux formation in this region, particularly when zooplankton fecal pellet production is low.     

Possibility of recent changes in vertical distribution and size composition of chlorophyll-a in the western North Pacific region

Our analysis of the last three decades of retrospective data of vertical distributions and size composition of chlorophyll-a (Chl-a) over the western North Pacific has revealed significant changes of three indices related to Chl-a during summer season, as follows: (1) decreasing linear trend of the proportion of Chl-a in surface layer to that of the whole water column by 0.4 and 2.3% year⁻¹ in the subtropical area along 137°E (STA₁₃₇) during 1972 to 1997 and in the Kuroshio Extension area along 175°E (KEA₁₇₅) during 1990 to 2001; (2) increasing linear trend of the depth of subsurface Chl-a maximum (DCM) by 0.4 and 2.6 m year⁻¹ in STA₁₃₇ and KEA₁₇₅; and (3) decreasing linear trend of larger-size Chl-a (>3 μm) by 0.1 and 2.5% year⁻¹ in STA₁₃₇ and KEA₁₇₅, respectively. Water density (σ _θ ) at 75 m depth had also decreased by 0.006 and 0.05 year⁻¹ in STA₁₃₇ and KEA₁₇₅, respectively. The ratio of biogenic opal to biogenic CaCO₃ in the sinking flux decreased by 0.015 year⁻¹ in the subtropical region from 1997 to 2005. These findings may indicate that the subsurface chlorophyll maximum is deepening and larger phytoplankton such as diatoms has been decreasing during the past decade, associated with the decreasing density of surface water caused by warming in the western North Pacific, especially in the summer.     

Comparison of sinking particles in the upper 200 m between subarctic station K2 and subtropical station S1 based on drifting sediment trap experiments

Drifting sediment trap experiments were conducted during various seasons to elucidate the characteristics of particles sinking through the upper 200 m of the water column in the western Pacific at subarctic station K2 and subtropical station S1. The sinking particle flux increased when primary productivity was high at each station, during June–July at K2 and during February at S1. Biogenic opal (Opal) and CaCO₃ were the major components of the fluxes at K2 and S1, respectively. Contrary to the expectation of a high flux at the eutrophic station K2 and low flux at the oligotrophic station S1, the annual average organic carbon fluxes at 100 m were comparable at both stations: 62.7 mg C m^?2 day^?1 at K2 and 56.1 mg C m^?2 day^?1 at S1. The similarity of the fluxes was perhaps a reflection of the unexpectedly high primary production at S1. At K2, the organic carbon export ratio (organic carbon flux/primary productivity) was significantly and negatively correlated with primary production and tended to decrease with depth. The magnitude of the rate of attenuation of the organic carbon flux with depth was larger at S1 than at K2. This rate of attenuation tended to decrease and increase with primary production at K2 and S1, respectively. The explanation for these patterns may be that the flux of labile organic carbon at relatively shallow depths decreased with increasing primary production at K2, and zooplankton grazing pressure increased with increasing primary productivity at S1.     

南极普里兹湾外海沉降颗粒物通量、组成变化及其与罗斯海对比研究

在南极普里兹湾外开阔海域布放时间系列沉积物捕获器,研究沉降颗粒物中生源组分(生物硅、有机质、碳酸钙)通量、组成、来源及元素的摩尔比值的生物地球化学意义,结果表明Ⅲ-1+站在1000m处颗粒物总通量的变化为13.00~334.59mg/(d·m2),颗粒物中以生物硅为主,占总通量的80%以上;各组分通量呈现明显的季节性变化.结合罗斯海沉降颗粒物通量和元素的摩尔比值的对比研究表明,研究海域1000m深的沉降颗粒物中生物硅与有机碳元素摩尔比值、无机碳与有机碳的元素摩尔比值较高,表明研究海域生物硅与有机碳生物地球化学循环过程是非耦合的,生物活动有效地将CO₂由表层水中移出.     

Alkaline leaching characteristics of biogenic opal in Eocene sediments from the central Arctic Ocean: a case study in the ACEX cores

Accurate knowledge of the extent of biogenic opal preservation in marine sediment cores is important for paleoceanographic reconstructions. The alkaline leaching method is widely employed for %biogenic opal analysis due to its ease and speed. In this study, a revised method for measuring %biogenic opal in sediment from arctic coring expedition samples was suggested. The studied middle Eocene sediments from the central Arctic Ocean presented a problem in insufficiently leaching biogenic opal with a Na₂CO₃ solution. Based on XRD analysis, it was suggested that such an alkaline resistance results from slight diagenesis of biogenic opal. In order to solve this problem, an alkaline leaching method utilizing a 2 M NaOH solution was suggested for the accurate measurement of %biogenic opal in the Eocene sediments from the central Arctic. Furthermore, dissolution rates from lithogenic matter by NaOH solution were measured in order to correct the %biogenic opal values.     

Seasonal and interannual variability in particle fluxes of carbon,nitrogen and silicon from time series of sediment traps at Ocean Station P, 1982–1993: relationship to changes in subarctic primary productivity

An extended time series of particle fluxes at 3800 m was recorded using automated sediment traps moored at Ocean Station Papa (OSP, 50°N, 145°W) in the northeast Pacific Ocean for more than a decade (1982–1993). Time-series observations at 200 and 1000 m, and short-term measurements using surface-tethered free-drifting sediment traps also were made intermittently. We present data for fluxes of total mass (dry weight), particulate organic carbon (POC), particulate organic nitrogen (PON), biogenic Si (BSi), and particulate inorganic carbon (PIC) in calcium carbonate. Mean monthly fluxes at 3800 m showed distinct seasonality with an annual minimum during winter months (December–March), and maximum during summer and fall (April–November). Fluxes of total mass, POC, PIC and BSi showed 4-, 10-, 7- and 5-fold increases between extreme months, respectively. Mean monthly fluxes of PIC often showed two plateaus, one in May–August dominated by <63 μm particles and one in October–November, which was mainly >63 μm particles. Dominant components of the mass flux throughout the year were CaCO₃ and opal in equal amounts. The mean annual fluxes at 3800 m were 32±9 g dry weight g m⁻² yr⁻¹, 1.1±0.5 g POC m⁻² yr⁻¹, 0.15±0.07 g PON m⁻² yr⁻¹, 5.9±2.0 g BSi m⁻² yr⁻¹ and 1.7±0.6 g PIC m⁻² yr⁻¹. These biogenic fluxes clearly decreased with depth, and increased during “warm” years (1983 and 1987) of the El Niño, Southern Oscillation cycle (ENSO). Enhancement of annual mass flux rates to 3800 m was 49% in 1983 and 36% in 1987 above the decadal average, and was especially rich in biogenic Si. Biological events allowed estimates of sinking rates of detritus that range from 175 to 300 m d⁻¹, and demonstrate that, during periods of high productivity, particles sink quickly to deep ocean with less loss of organic components. Average POC flux into the deep ocean approximated the “canonical” 1% of the surface primary production.     

Long-term biogenic particle fluxes in the Bering Sea and the central subarctic Pacific Ocean, 1990–1995

Time-series sediment traps were deployed for five consecutive years in two distinctively different subarctic marine environments. The centrally located subarctic pelagic Station SA (49°N, 174°W; water depth 5406 m) was simultaneously studied along with the marginal sea Station AB (53.5°N, 177°W; water depth 3788 m) in the Aleutian Basin of the Bering Sea. A mooring system was tethered to the sea-floor with a PARFLUX type trap with 13 sample bottles, which was placed at 600 m above the sea-floor at each of the two stations. Sampling intervals were synchronized at the stations, and they were generally set for 20 days during highly productive seasons, spring through fall, and 56 days during winter months of low productivity. Total mass fluxes, which consisted of mainly biogenic phases, were significantly greater at the marginal sea Station AB than at the pelagic Station SA for the first four years and moderately greater for the last year of the observations. This reflects the generally recognized higher productivity in the Bering Sea. Temporal excursion patterns of the mass fluxes at the two stations generally were in parallel, implying that temporal changes in their biological productivity are strongly governed by a large-scale seasonal climatic variability over the region rather than local phenomena. The primary reason for the difference in total mass flux at the two stations stems mainly from varying contributions of siliceous and calcareous planktonic assemblages. A significantly higher opal contribution at Station AB than at Station SA was mainly due to diatoms. Diatom fluxes at the marginal sea station were about twice those observed at the pelagic station, resulting in a very high opal contribution at Station AB. In contrast to the opal fluxes, CaCO₃ fluxes at Station AB were slightly lower than at Station SA. The ratios of C_org/C_inorg were usually significantly greater than one in both regions, suggesting that preferentially greater organic carbon from cytoplasm than skeletal inorganic carbon was exported from the surface layers. Such a process, known as the biological pump, leads to a carbon sink which effectively lowers p CO₂ in the surface layers and then allows a net flux of atmospheric CO₂ into the surface layer. The efficiency of the biological pump is greater in the Bering Sea than at the open-ocean station.     

Long-term monitoring of the sedimentary processes in the central part of Sagami Bay, Japan: rationale, logistics and overview of results

Deep-sea benthic ecosystems are mainly sustained by sinking organic materials that are produced in the euphotic zone. “Benthic-pelagic coupling” is the key to understanding both material cycles and benthic ecology in deep-sea environments, in particular in topographically flat open oceanic settings. However, it remains unclear whether “benthic-pelagic coupling” exists in eutrophic deep-sea environments at the ocean margins where areas of undulating and steep bottom topography are partly closely surrounded by land. Land-locked deep-sea settings may be characterized by different particle behaviors both in the water column and in relation to submarine topography. Mechanisms of particle accumulation may be different from those found in open ocean sedimentary systems. An interdisciplinary programme, “Project Sagami”, was carried out to understand seasonal carbon cycling in a eutrophic deep-sea environment (Sagami Bay) with steep bottom topography along the western margin of the Pacific, off central Japan. We collected data from ocean color photographs obtained using a sea observation satellite, surface water samples, hydrographic casts with turbidity sensor, sediment trap moorings and multiple core samplings at a permanent station in the central part of Sagami Bay between 1997 and 1998. Bottom nepheloid layers were also observed in video images recorded at a real-time, sea-floor observatory off Hatsushima in Sagami Bay. Distinct spring blooms were observed during mid-February through May in 1997. Mass flux deposited in sediment traps did not show a distinct spring bloom signal because of the influence of resuspended materials. However, dense clouds of suspended particles were observed only in the spring in the benthic nepheloid layer. This phenomenon corresponds well to the increased deposition of phytodetritus after the spring bloom. A phytodetrital layer started to form on the sediment surface about two weeks after the start of the spring bloom. Chlorophyll-a was detected in the top 2 cm of the sediment only when a phytodetritus layer was present. Protozoan and metazoan meiobenthos increased in density after phytodetritus deposition. Thus, “benthic-pelagic coupling” was certainly observed even in a marginal ocean environment with undulated bottom topography. Seasonal changes in features of the sediment-water interface were also documented.