Biogenic Particle Flux Research Articles

Anthropogenic disturbances influence mineral and elemental constituents of freshwater lake sediments.

Lake sediments can provide valuable insights into anthropogenic disturbances such as intensive aquaculture and land use changes. These disturbances often manifest as elevated levels of nutrients and elements within the sediments. This paper uses several analytical techniques, i.e., FTIR (Fourier-transform infrared spectroscopy), XRD (X-ray diffraction), EDS (energy-dispersive X-ray spectroscopy), and SEM (scanning electron microscopy), to examine the elemental constituents of lake sediments, along with their relative mineral abundances and surface morphology. The selected freshwater lakes are from the Central Gangetic Plain. The analysis provides a "fingerprint" of geogenic and biogenic mineral constituents of the sediments. Physicochemical, mineralogical, and elemental analysis shows that intensive aquaculture activities in lake alter the sediment chemistry as evidenced by the increase in pH, organic carbon, organic matter, and total phosphorus which is not observed in the lake where aquaculture is prohibited. Freshwater lake sediment is characterized by a high content of biogenic silica and carbonate minerals. The variations in sediment nutrients and mineral fluxes of the selected lakes are mainly attributed to diverse anthropogenic pressures, differences in lake productivity, and the overall ecological condition of the lakes. In the selected three lakes, major variation was reported in the autochthonous sediments in comparison to the allochthonous sediments. The study concludes that catchment and biotic deposit variations in the lakes cannot be evened out by in-lake mixing mechanisms due to variations in the terrigenous and pelagic deposits of the lake. The results highlight the importance of studying annual fluctuations and spatial variations in geogenic and biogenic mineral particle fluxes in lakes. Such investigations provide valuable insights into the annual dynamics of minerals within lakes, contributing to a more comprehensive understanding of their behavior and distribution.

Role of Marginal Seas in Deep Ocean Regeneration of Dissolved Silica: A Case Study in the Marginal Seas of the Western Pacific

Deep ocean regeneration of dissolved silica (DSi) is an essential part of the ocean silica cycle and is driven by a complex series of biogeochemical processes. Here we compare the distributions of DSi and other environmental parameters in several western Pacific marginal seas to explore the role of marginal seas in deep ocean DSi regeneration. Results show that in oligotrophic marginal seas (such as the South China Sea), the DSi content in deep waters is similar to that of the adjacent Pacific waters. However, in productive marginal seas (such as the Bering Sea), the DSi content in deep waters is markedly higher than that in adjacent Pacific waters at the same depths. This is mainly due to deep ocean DSi regeneration in the marginal sea basin, which is fueled by the high biogenic particle flux from the productive surface waters. On a global scale, deep ocean DSi regeneration is accelerated in productive marginal seas, causing marginal seas such as the Bering Sea to have the highest DSi concentrations of all global waters.

Major processes of the dissolved cobalt cycle in the North and equatorial Pacific Ocean

Abstract. Over the past decade, the GEOTRACES and wider trace metal geochemical community has made substantial contributions towards constraining the marine cobalt (Co) cycle and its major biogeochemical processes. However, few Co speciation studies have been conducted in the North and equatorial Pacific Ocean, a vast portion of the world's oceans by volume and an important end-member of deep thermohaline circulation. Dissolved Co (dCo) samples, including total dissolved and labile Co, were measured at-sea during the GEOTRACES Pacific Meridional Transect (GP15) expedition along the 152∘ W longitudinal from 56∘ N to 20∘ S. Along this transect, upper-ocean dCo (σ0<26) was linearly correlated with dissolved phosphate (slope = 82±3, µmol : mol) due to phytoplankton uptake and remineralization. As depth increased, dCo concentrations became increasingly decoupled from phosphate concentrations due to co-scavenging with manganese oxide particles in the mesopelagic. The transect revealed an organically bound coastal source of dCo to the Alaskan Stream associated with low-salinity waters. An intermediate-depth hydrothermal flux of dCo was observed off the Hawaiian coast at the Loihi Seamount, and the elevated dCo was correlated with potential xs3He at and above the vent site; however, the Loihi Seamount likely did not represent a major source of Co to the Pacific basin. Elevated concentrations of dCo within oxygen minimum zones (OMZs) in the equatorial North and South Pacific were consistent with the suppression of oxidative scavenging, and we estimate that future deoxygenation could increase the OMZ dCo inventory by 18 % to 36 % over the next century. In Pacific Deep Water (PDW), a fraction of elevated ligand-bound dCo appeared protected from scavenging by the high biogenic particle flux in the North Pacific basin. This finding is counter to previous expectations of low dCo concentrations in the deep Pacific due to scavenging over thermohaline circulation. Compared to a Co global biogeochemical model, the observed transect displayed more extreme inventories and fluxes of dCo than predicted by the model, suggesting a highly dynamic Pacific Co cycle.

First long-term evidence of microplastic pollution in the deep subtropical Northeast Atlantic

No anthropogenic pollutant is more widespread in the aquatic and terrestrial environment than microplastic; however, there are large knowledge gaps regarding its origin, fate, or temporal variations in the oceans. In this study, we analyzed sediment trap material from the deep subtropical Northeast Atlantic (2000 m) in a long-term record (2003–2015) to assess the role of the deep ocean as a potential sink of microplastics. Microplastic particles were identified in all 110 analyzed samples with flux rates of 1.13–3146.81 items d−1 m−2. Calculated microplastic mass fluxes ranged between 0.10 and 1977.96 μg d−1 m−2, representing up to 8% of the particle flux. Between years, the composition of the different polymers changed significantly, dominated by polyethylene, whose amount was correlated with the lithogenic input. The correlation between polyethylene and the lithogenic fraction was attributed to an air transport pathway from northeast Africa and surrounding regions. The second most abundant polymer detected in our study was polyvinyl chloride, which is not correlated with lithogenic or biogenic particle flux fractions. Instead, we observed seasonality for polyvinyl chloride with recurring high fluxes in winter before the plankton bloom and significantly lower amounts in summer. Other polymers identified were polypropylene, polyethylene terephthalate, and lower numbers of polystyrene and polymethyl methacrylate. The average microplastic particle size for all samples and polymers was 88.44 ± 113.46 μm, with polyethylene and polyvinyl chloride having the highest proportion of small particles (<100 μm). Our findings provide first insights into temporal variations of sinking microplastics, which are crucial for understanding the fate of plastic in the oceans.

Seasonal flux patterns and carbon transport from low-oxygen eddies at the Cape Verde Ocean Observatory: lessons learned from a time series sediment trap study (2009–2016)

Abstract. Mesoscale eddies are abundant in the eastern tropical North Atlantic and act as oases for phytoplankton growth due to local enrichment of nutrients in otherwise oligotrophic waters. It is not clear whether these eddies can efficiently transfer organic carbon and other flux components to depth and if they are important for the marine carbon budget. Due to their transient and regionally restricted nature, measurements of eddies' contribution to bathypelagic particle flux are difficult to obtain. Rare observations of export flux associated with low-oxygen eddies have suggested efficient export from the surface to the deep ocean, indicating that organic carbon flux attenuation might be low. Here we report on particle flux dynamics north of the Cabo Verde islands at the oligotrophic Cape Verde Ocean Observatory (CVOO; approx. 17∘35′ N, 24∘15′ W). The CVOO site is located in the preferred pathways of highly productive eddies that ultimately originate from the Mauritanian upwelling region. Between 2009 and 2016, we collected biogenic and lithogenic particle fluxes with sediment traps moored at ca. 1 and 3 km water depths at the CVOO site. From concurrent hydrography and oxygen observations, we confirm earlier findings that highly productive eddies are characterized by colder and less saline waters and a low-oxygen signal as well. Overall, we observed quite consistent seasonal flux patterns during the passage of highly productive eddies in the winters of 2010, 2012 and 2016. We found flux increases at 3 km depth during October–November when the eddies approached CVOO and distinct flux peaks during February–March, clearly exceeding low oligotrophic background fluxes during winter 2011 and showing an enhanced particle flux seasonality. During spring, we observed a stepwise flux decrease leading to summer flux minima. The flux pattern of biogenic silicate (BSi) showed a stronger seasonality compared to organic carbon. Additionally, the deep fluxes of total mass showed an unusually higher seasonality compared to the 1 km traps. We assume that BSi and organic carbon/lithogenic material had different sources within the eddies. BSi-rich particles may originate at the eddy boundaries where large diatom aggregates are formed due to strong shear and turbulence, resulting in gravitational settling and, additionally, in an active local downward transport. Organic carbon associated with lithogenic material is assumed to originate from the interior of eddies or from mixed sources, both constituting smaller, dust-ballasted particles. Our findings suggest that the regularly passing highly productive eddies at CVOO repeatedly release characteristic flux signals to the bathypelagic zone during winter–spring seasons that are far above the oligotrophic background fluxes and sequester higher organic carbon than during oligotrophic settings. However, the reasons for a lower carbon flux attenuation below eddies remain elusive.

Biological carbon pump efficiency enhanced by atmospheric dust deposition in the North Pacific Subtropical Gyre

We examine the flux and composition of sinking particles collected at 4500 m depth in the southeastern part of the North Pacific Subtropical Gyre (NPSG) from August 2011 to June 2012. Satellite-derived net primary production was higher in January–May 2012 compared with the rest of the study period. Both the biogenic and lithogenic particle fluxes were significantly higher in spring (March–May 2012) than at other times. The export efficiency via the transfer of produced organic carbon to the deep ocean interior (i.e., carbon sequestration) doubled during this time. Two prominent particle flux peaks were observed in March and May 2012, coinciding with the atmospheric dust-deposition peaks within the temporal resolution of the data. Coincident increases in the biogenic and lithogenic particle fluxes suggest that dust deposition in the NPSG enhances the biological carbon-pump efficiency. Strong coupling between the particulate organic carbon and biogenic opal fluxes implies that nutrient supply via atmospheric dust deposition stimulated diatom growth in this area. Dust deposition during the productive season therefore plays an important role in carbon sequestration.

Biogenic sinking particle fluxes and sediment trap collection efficiency at Ocean Station Papa

Comprehensive field observations characterizing the biological carbon pump (BCP) provide the foundation needed to constrain mechanistic models of downward particulate organic carbon (POC) flux in the ocean. Sediment traps were deployed three times during the EXport Processes in the Ocean from RemoTe Sensing campaign at Ocean Station Papa in August–September 2018. We propose a new method to correct sediment trap sample contamination by zooplankton “swimmers.” We consider the advantages of polyacrylamide gel collectors to constrain swimmer influence and estimate the magnitude of possible trap biases. Measured sediment trap fluxes of thorium-234 are compared to water column measurements to assess trap performance and estimate the possible magnitude of fluxes by vertically migrating zooplankton that bypassed traps. We found generally low fluxes of sinking POC (1.38 ± 0.77 mmol C m–2 d–1 at 100 m, n = 9) that included high and variable contributions by rare, large particles. Sinking particle sizes generally decreased between 100 and 335 m. Measured 234Th fluxes were smaller than water column 234Th fluxes by a factor of approximately 3. Much of this difference was consistent with trap undersampling of both small (&amp;lt;32 μm) and rare, large particles (&amp;gt;1 mm) and with zooplankton active migrant fluxes. The fraction of net primary production exported below the euphotic zone (0.1% light level; Ez-ratio = 0.10 ± 0.06; ratio uncertainties are propagated from measurements with n = 7–9) was consistent with prior, late summer studies at Station P, as was the fraction of material exported to 100 m below the base of the euphotic zone (T100, 0.55 ± 0.35). While both the Ez-ratio and T100 parameters varied weekly, their product, which we interpret as overall BCP efficiency, was remarkably stable (0.055 ± 0.010), suggesting a tight coupling between production and recycling at Station P.

Modelling the impact of biogenic particle flux intensity and composition on sedimentary Pa/Th

There is compelling evidence that millennial climate variability of the last glacial period was associated with significant changes in the Atlantic Meridional Overturning Circulation (AMOC). Several North Atlantic sedimentary Pa/Th records indicate a consistent and large Pa/Th increase across millennial-scale events, which has been interpreted as considerable reduction in North Atlantic Deep Water (NADW) formation. However, the use of sedimentary Pa/Th as a pure kinematic circulation proxy is challenging because Pa and Th are also highly sensitive to changes in particulate flux intensity and composition that might have occurred across these millennial scale events. In this study, we use the Pa/Th enabled iLOVECLIM Earth System Model of intermediate complexity to evaluate the impact of changes in biogenic particle flux intensity and composition on the Atlantic Pa/Th. We find that in our model, changes in Particulate Organic Carbon (POC), and to a lesser extent biogenic opal production, can significantly affect the sedimentary Pa/Th, possibly explaining up to 30% of the observed North Atlantic Pa/Th increase across Heinrich stadial 1. The sedimentary Pa/Th response is also likely sensitive to shifts in the geographical distribution of the particles, especially in high scavenging regions. Our study suggests that a decrease in opal production in the northwest Atlantic can induce a far field Pa/Th increase in a large part of the North Atlantic basin. Therefore, local monitoring of particle fluxes may not be sufficient to rule out any influence of changing particle fluxes on sedimentary Pa/Th records.

Seasonal variations in nutrients and biogenic particles in the upper and lower layers of East China Sea Shelf and their export to adjacent seas

Based on a three dimensional low-trophic ecosystem model, we revaluated budgets of nutrients and biogenic particles (phytoplankton and detritus) in the East China Sea (ECS), a continental shelf sea with high productivity that is affected by a large river (Changjiang River) and a western boundary current (Kuroshio). Following careful comparison of model results with available observation data, we calculated the monthly inventories of the nutrients and biogenic particles in the ECS and the fluxes of the nutrients and biogenic particles through the lateral and vertical interfaces of the ECS. As an improvement over previous nutrient budget calculations that treated the ECS as one box, we divided the water column into two layers: upper and lower layers corresponding to different light condition. Seasonal variations in inventories of nutrients and biogenic particles in both upper and lower layers are apparent but present different features, which are related to opposite contributions of biological processes and physical processes in the two layers. Our calculation shows the necessity of evaluating not only the horizontal fluxes of nutrients and biogenic particles into and out of the ECS through lateral boundaries with adjacent seas but also the exchange fluxes of nutrients and biogenic particles between the upper and lower layers because the latter is larger than the former. Our calculation also reveals two major export pathways of biogenic particles generated locally in the ECS: from the ECS to the Japan/East Sea through the Tsushima Strait and from the ECS to the Kuroshio region through the shelf break (200 m isobath). The export amount of biogenic particles is larger through the Tsushima Strait than through the shelf break in the water column. The depth for the biogenic particles exporting through the shelf break of the ECS is the middle layer (from ~60 m to ~160 m).

Interannual variability of vertical particle fluxes in the Ross Sea (Antarctica)

This study presents new data on biogenic and terrigenous particle fluxes collected by an oceanographic mooring (Mooring A) deployed in the south-western Ross Sea (Antarctica) in the frame of the Italian Long-Term Ecological Research network (LTER-Italy). Results from the years 2005 and 2008 document high mass fluxes during the summer and early autumn seasons, not coincident with the algal bloom. Downward particle fluxes exhibit a high inter-annual variability of both particulate fluxes and composition that seem related to the different factors as the phytoplankton increases, occurring between the beginning of February and the end of March, to the variations in the sea ice extent and to the resuspension and/or lateral advection processes. The flux variability may have been influenced by Iceberg B-15 that resided in the investigated area between 2000 and 2005. The decoupling of biogenic silica and organic carbon cycles is documented by differences in the rates of their respective key processes: biogenic silica dissolution and organic carbon degradation.

Influence of Mussel Culture on the Vertical Export of Phytoplankton Carbon in a Coastal Upwelling Embayment (Ría de Vigo, NW Iberia)

The goal of this paper is to find out whether suspended mussel culture affects the vertical fluxes of biogenic particles in the Ria de Vigo on a seasonal scale. With this aim, vertical fluxes of particulate organic carbon (POC) and the magnitude and composition of vertical export of phytoplankton carbon (Cphyto) collected in sediment traps were examined by comparing data obtained inside a mussel farming area (RaS) with those found at a reference station (ReS) not affected by mussels. Our results indicate that mussel farming has a strong impact on sedimentation fluxes under the rafts, not only increasing POC flux but also altering the magnitude and composition of Cphyto fluxes. Average POC flux at RaS (2564 ± 1936 mg m−2 day−1) was four times higher than at ReS (731 ± 276 mg m−2 day−1), and much of this increase was due to biodeposit fluxes (Cbiodep) which accounted for large proportion of POC flux (35–60 %). Indeed, because of this high Cbiodep flux, only a small proportion of the POC flux was due to Cphyto flux (3–12 %). At the same time, we observed an increased sedimentation of phytoplankton cells at RaS that could be explained by a combination of mechanisms: less energetic hydrodynamic conditions under mussel rafts, ballast effect by sinking mussel feces, and diatom aggregates. Moreover, mussel farming also altered the quality of the Cphyto flux by removing part of the predatory pressure of zooplankton and thus matching diatom composition in water column and sediment traps.

Seasonal and size-dependent variations in the phytoplankton growth and microzooplankton grazing in the southern South China Sea under the influence of the East Asian monsoon

Abstract. To examine seasonal and size-dependent variations in the phytoplankton growth and microzooplankton grazing in oligotrophic tropical waters under the influence of seasonal reversing monsoon, dilution experiments were conducted during the summer of 2009 (21 May to 9 June) and winter 2010 (9 to 18 November) in the southern South China Sea (SSCS). The results showed that environmental variables, phytoplankton biomass, phytoplankton growth rate (μ), microzooplankton grazing rate (m), and correlationship (coupling) between the μ and m, rather than the microzooplankton grazing impact on phytoplankton (m/μ) significantly varied between the two seasons. Higher relative preference index (RPI) for the larger-sized (&gt; 3 μm) phytoplankton than pico-phytoplankton (&lt; 3 μm), indicating significant size-selective grazing by microzooplankton on the larger-sized phytoplankton, were also observed. The μ and m were significantly correlated with seawater salinity and temperature, and phytoplankton biomass, which indicated that salient seasonal variations in the phytoplankton growth and microzooplankton grazing in the SSCS were closely related to the environmental variables under the influence of the East Asian monsoon. We propose that intermittent arrivals of the northeast winter monsoon could lead to the low μ and m, and the decoupling between the μ and m in the SSCS, through influencing nutrient supply to the surface water, and inducing surface seawater salinity decrease. The low m/μ (&lt; 50 % on average) indicates low remineralization of organic matter mediated by microzooplankton and mismatch between the μ and m, and thus probably accounts for part of the high vertical biogenic particle fluxes in the prevailing periods of the monsoons in the SSCS. The size-selective grazing suggests that microzooplankton grazing partially contributes to the pico-phytoplankton dominance in the oligotrophic tropical waters such as that of the SSCS.

Biogenic particle flux and benthic remineralization in the Eastern Tropical South Pacific

We studied biogenic rain to the ocean interior and sea floor in the Eastern Tropical South Pacific (ETSP), a region at the intersection of three oceanic regimes; coastal upwelling, equatorial divergence and the South Pacific oligotrophic gyre. Sediment cores from ocean depths >4000m were collected, pore water was expressed using both whole core squeezing and rhizon techniques, and profiles of nitrate and dissolved Si were modeled to estimate remineralization fluxes. Nitrate modeling was interpreted as representative of Corg remineralization assuming the oxic transformation of ammonium to nitrate. A broad range of TCO2 fluxes were determined: 0.008–0.34mmol Cm−2d−1. The range in biogenic silica (bSi) remineralization flux was also large: 0.007–0.15mmol Sim−2d−1. The pattern of TCO2 flux showed higher particulate organic carbon (POC) inputs at sites closest to coastal and equatorial upwelling and lowest fluxes at the most oligotrophic site. Moored sediment traps, suspended at ~3700m at 10°S, 100°W and 20°S, 100°W captured the annual pattern of mass and biogenic rain. The annual average mass flux was over five times greater at a 10°S site (30.8mgm−2d−1) compared to a 20°S site (5.5mgm−2d−1). The relative wt% of POC, PIC and bSi at these two stations were 4.9, 8.0, 15.5 and 6.6, 8.3, 2.7, respectively. The deep trap POC and bSi annual rain rates were within 0.5–4 times the benthic fluxes estimated from pore water models. The annual averaged surface ocean chlorophyll concentration estimated from satellites is a good predictor of POC rain to the ocean interior at the ETSP sites studied, as is 14C primary production (PP). However, the POC rain into the deep ocean at ETSP sites per unit chlorophyll or per 14C PP is significantly less than values obtained from the equatorial Pacific at 140°W or subtropical gyre station HOT. It appears that the ETSP, although underlain by an intense oxygen minimum zone, is inefficient at transferring Corg production to deeply sinking POC rain.

Seasonal dynamics in diatom and particulate export fluxes to the deep sea in the Australian sector of the southern Antarctic Zone

Particle fluxes were recorded over a one-year period (2001-02) in the southern Antarctic Zone in the Australian Sector of the Southern Ocean. Here, we present the results on the seasonal and vertical variability of biogenic particle and diatom valve fluxes. Total mass and diatom fluxes were highly seasonal, with maxima registered during the austral summer and minima during winter. Biogenic opal dominated sedimentation, followed by carbonate, and very low levels of organic carbon (annual average 1.4%). The strong correlation between opal and organic carbon at both depth levels suggests that a significant fraction of organic matter exported to the deep sea was associated with diatom sedimentation events. Seasonal diatom fluxes appear driven principally by changes in the flux of Fragilariopsis kerguelensis. The occurrence of the sea-ice affiliated diatoms Fragilariopsis cylindrus and Fragilariopsis curta in both sediment traps is considered to correspond to the sedimentation of a diatom bloom advected from an area under the influence of sea ice. Highest fluxes of the subsurface-dwelling species Thalassiothrix antarctica registered at the end of the summer bloom were linked to a drop of the light levels during the summer-autumn transition. This study provides the first annual observation on seasonal succession of diatom species in the Australian sector of the Antarctic Zone, and corresponds, in terms of magnitude and seasonality of diatom fluxes, to those in neighbouring sectors (Pacific and eastern Atlantic).

Southern-ocean and glaciogenic nutrients control diatom export production on the Chile margin

Biogenic particle flux was reconstructed using 230-Thorium normalization at two sites on the southern Chile margin. ODP Site 1233 at 41°S, 838 m depth, is at the southern limit of the Peru–Chile upwelling system, where the northern extent of the Antarctic Circumpolar Current impinges on the South American continental margin. ODP Site 1234, at 36°S, 1014 m depth, is located within the core of the coastal upwelling system near the mouths of the Bio Bio and Itata Rivers. At 41°S, opal, lithogenic and carbonate fluxes are greatest during the Last Glacial interval (26–20 ka), carbonate has a secondary peak during the mid Holocene (∼8 ka) and organic carbon fluxes increase slightly from 17 ka to the present. At 36°S, large lithogenic fluxes are observed both during the Last Glacial interval and the Holocene, and a maximum in organic carbon flux is observed during the late Holocene (∼5 ka) without an accompanying peak in opal flux. These reconstructed fluxes at 36°S and 41°S fit within a larger latitudinal pattern of a poleward increase in the magnitude of opal flux during the glacial period. The pattern of normalized opal flux, opal mass accumulation rate and opal:carbonate ratios is consistent with either i) enhanced supply of Si from the Southern Ocean, as proposed by the Silicic Acid Leakage Hypothesis or ii) enhanced Si and Fe delivery from land, driven by glacial erosion. The pattern of reconstructed export production supports our view that the appearance of more reducing conditions in the sediments upon deglaciation was most likely driven by decreased ventilation, rather than increased local productivity.

Decadal shift of biogenic sinking particle flux in the western North Pacific subpolar region

AbstractTime series of biogenic sinking particle flux in the western North Pacific subpolar region over two decades (1989–2008) revealed that the biogenic CaCO3 (CC) flux has shown a significant decreasing trend of 2.7% year−1 (annual average, −0.88 ± 0.13 mg m2 day−1 year−1) along with the decreasing particulate organic carbon (POC) flux of 0.7% year−1, while the biogenic opal (OP) flux had no long‐term trend. Comparing these results with the decreasing rate of satellite‐derived surface CC with −0.7% year−1, we concluded that three fourths of the decreasing trend of CC flux was derived from the strengthening of CaCO3 dissolution through seawater column due to the weakening of water ventilation and the rest was from the decline of CaCO3‐shelled species, indicating the enhancement of the efficiency in oceanic sequestration of atmospheric CO2 in the sea surface of this region due to the increase of OP/CC ratio.

Present-day fluxes of coccolithophores and diatoms in the pelagic Ionian Sea

Biogenic fluxes from two sediment traps in the Ionian sea (35°13′N, 21°30′E) at 500 and 2800m water depth are discussed in relation with the main oceanographic and external forcing and compared with previous data from a nearby location. This study is part of a multi-year sediment trap deployment, aimed at assessing seasonality and interannual variability of biogenic and abiogenic fluxes. Here, we focus on fluxes related to two main phytoplankton groups: coccolithophores and diatoms.At our mooring site, high-coccolithophore and low-diatom fluxes confirm the oligotrophic character of the pelagic eastern Mediterranean year-round. Coccolithophore assemblages are dominated by the cosmopolitan species Emiliania huxleyi, followed by the deep-dwelling Florisphaera profunda and by several minor species. Diatom assemblages are dominated by Thalassionema bacillare and Nitzschia interruptestriata, with ~100 common and minor species. Overall, the combined flux pattern of coccolithophores and diatoms shows a clear seasonality throughout the study period, which can be related to changing oceanographic conditions and a different depth of production within the photic zone. Worth to note is the extremely high abundance, with respect to previous pelagic records, of species indicative of an intense deep chlorophyll maximum (DCM), which could indicate a shoaling of the nutricline. This feature can be related to the variability that affects surface hydrography and the deep water masses.Additionally, the occurrence of neritic, benthic and brackish to fresh-water diatom species, mainly in the deeper trap, could be linked to either lateral transport within the water column or the atmospheric input of Saharan dust, which is known to be common over the eastern Mediterranean especially during spring time.Finally, correlation of the flux pattern at different depths allowed sinking speeds for total and biogenic particles to be calculated, which ranged from 70 to >200mday−1. Three possible mechanisms can explain such high sinking rates and the coincidence of biogenic and total particle fluxes in the traps: a) the ballast effect of coccoliths; b) the packaging effect of fecal pellets related to the grazing activity of zooplankton; and c) the ballast effect of dust particles.

δ15N values of settling biogenic particles in the eastern Bransfield Basin (west Antarctic) and their records for the surface-water condition

Sediment trap deployments in the eastern Brans-field Basin (west Antarctic) show seasonal and inter-annual variability in particle fluxes, with most of total annual flux at mid-depth occurring during a short period. High seasonality in particle flux may be associated with biological processes (the timing of phytoplankton blooms) and physical factor (sea-ice cover). Inter-annual variability of particle flux may also reflect inter-annual variation of sea-ice cover related to the biological processes. Most notable is that year-round high particle flux is observed in sediment traps moored near the seabed, which is not unexpected, emphasizing the contribution of supplementary transport of lithogenic particles particularly during the unproductive season. In this case, particles in relatively rapid vertical transit, such as aggregates and fecal pellets, are presumably mixed with laterally-transported particles. The relationship between δ15N values and biogenic particle flux highlights the contribution of particles in lateral transit to the vertical setting particles downward to the near-bottom sediment trap. Mid-depth sediment trap shows clear seasonal δ15N variability, whereas the near-bottom sediment trap shows no distinct seasonal trend, indicating the contamination of low δ15N, fresh organic material by high δ15N, regenerated and more refractory material resuspended from shallow continental margin. However, judging from the flux-weight δ15N values between the mid-depth and near-bottom sediment traps, biogeochemical components contained in the horizontally delivered particles are not critical to prevent the surface-water condition preserved in those of downward sinking particles. Our results provide the convincing implications from the recovery of paleoclimatic information using sediment cores from the eastern Bransfield Basin (west Antarctic).

234Th balance and implications for seasonal particle retention in the eastern Bering Sea

As part of the Bering Ecosystem Study-Bering Sea Integrated Ecosystem Research Program (BEST-BSIERP), distributions of 234Th in the water column and sediments were measured at approximately 60 stations over the shelf and slope/oceanic regions of the eastern Bering Sea during spring and summer of 2009 and 2010. During this study period, the inventory of sediment excess 234Th (234Thxs) on the shelf was determined to be ∼1/3 of the total 234Th deficit in the overlying water column; the average focusing factor (FFTh=sediment 234Thxs/water column 234Th deficit) was 0.34±0.23. In addition, the export flux of 234Th from the shelf to the slope/oceanic region was determined to be on average ∼30% of the total production of 234Th throughout the shelf. Taken together, these results indicate that 234Th and associated particles are largely retained on the shelf, and that shelf sediments represent a sink for particles. In contrast, the FFTh in the slope/oceanic region was higher, averaging 1.52±1.34. The higher FFTh at the slope is attributed primarily to enhanced scavenging removal and sediment deposition of 234Th associated with periods of high biogenic particle flux at the marginal ice zone during the spring sea-ice retreat. A more general conclusion is that the ocean margin of the eastern Bering Sea may serve as an accumulation area for particles and associated reactive chemicals.

230Th and 232Th distributions in mid-latitudes of the North Pacific Ocean: Effect of bottom scavenging

Vertical distributions of total 230Th and dissolved 232Th were determined in the mid-latitudes of the North Pacific Ocean (8°N, 95°W–40°N, 160°E). The observed 232Th concentrations are some of the lowest values reported for oceans. Because of low biogenic particle flux, high 230Th activity is exhibited in the central gyre. Most vertical distributions of 230Th in the Pacific Ocean have been described well using the reversible-scavenging model. However, we observed the depletion of total 230Th compared with a reversible scavenging model prediction for the deep layers in the mid-latitudes of the North Pacific Ocean. Considering the time-scale of the scavenging residence time of Th, horizontal eddy diffusion from the higher particle flux region could not have a major influence on 230Th depletion in deep waters. The observed depletion is partly explained by lateral advection, adopting the scavenging-mixing model. Additionally, a bottom scavenging process above the seafloor and a diffusion process are considered to explain the 230Th deficit. Taking into account the time-scale of the scavenging residence time of Th in deep waters (<100yr), the vertical diffusion process could reach 800–1600m above the seafloor. The enhanced vertical eddy diffusion derived from the complex topography of the western Pacific Ocean might promote bottom scavenging and transport of the low 230Th in deep water to the upper layers.