A time-series study of particulate matter export in the North Pacific Subtropical Gyre based on 234Th : 238U disequilibrium

Depth profiles of total ²³⁴Th (dissolved+particulate) were collected at Station ALOHA (22°45N, 158°00W) in the North Pacific Subtropical Gyre during 9 cruises from April 1999 to March 2000. Samples were collected and processed by a new 2 L technique that enables more detailed depth resolution then previous ²³⁴Th studies. Significant zones of particle export (²³⁴Th deficiency) and particle remineralization (²³⁴Th excess) were measured both temporally and with depth. ²³⁴Th derived particulate carbon (PC) and nitrogen (PN) fluxes were determined with steady-state and non-steady-state models and PC/²³⁴Th and PN/²³⁴Th ratios measured with both in situ pumps and free-drifting particle interceptor traps deployed at 150 m. ²³⁴Th based export estimates of 4.0±2.3 mmol C m⁻² d⁻¹ and 0.53±0.19 mmol N m⁻² d⁻¹, were approximately 60% higher than those measured in PIT style sediment traps from the same time period, 2.4±0.2 mmol C m⁻² d⁻¹ and 0.32±0.08 mmol N m⁻² d⁻¹. Most of this difference is attributable to two large export events that occurred during October and December 1999, when traps undercollected for ²³⁴Th by a factor of 2 to 4. ²³⁴Th export (ThE) ratios based on ²³⁴Th derived PC flux/¹⁴C based primary production ranged from 4% to 22% (average=8.8%). Our results confirm the recent estimates of C export by Emerson et al. (Nature 389 (1997) 951) and Sonnerup et al. (Deep-Sea Research I 46 (1999) 777) and indicate that C export from the oligotrophic ocean must be considered when discussing C sequestration in global climate change.     

A study of particle exchange at the sediment–water interface in the Benguela upwelling area based on 234Th/238U disequilibrium

The natural isotope ²³⁴Th is used in a small-scale survey of particle transport and exchange processes at the sediment–water interface in the Benguela upwelling area. Results from water and suspended particulate matter (SPM) samples from the uppermost and lowermost water column as well as the underlying sediment of three stations are compared. The stations are situated in different sedimentological environments at 1200–1350 m water depth at the continental slope off Namibia. Highly differing extent and particle content of the bottom nepheloid layer (BNL) are determined from transmissometer data. Three models are presented, all explaining the ²³⁴Th depletion of the BNL and ²³⁴Th excess of the surface sediment that were observed. While the first model is based solely on local resuspension of surface sediment particles, the second evaluates the influence of vertical particle settling from the surface waters on the ²³⁴Th budget in the BNL. The third model explains ²³⁴Th depletion in the BNL by sedimentation of particles that were suspended in the BNL during long-range transport. Particle inventory of the BNL is highest at a depocenter of organic matter at 25.5°S, where strong deposition is presently taking place and lateral particle transport is suggested to predominate sediment accumulation. This is supported by the high settling flux of particles out of the BNL into the sediments of the depocenter, exceeding the vertical particle flux into sediment traps at intermediate depth in the same area by up to an order of magnitude. High particle residence/removal times in the BNL above the depocenter in the range of 5–9 weeks support this interpretation. Comparison with carbon mineralization rates that are known from the area reveals that, notwithstanding the large fraction of advected particles, organic carbon flux into the surface sediment is remineralized to a large extent. The deployment of a bottom water sampler served as an in situ resuspension experiment and provided the first data of ²³⁴Th activity on in situ resuspended particles. We found a mean specific activity of 86 disintegrations per minute (dpm) g⁻¹ (39–339 dpm g⁻¹), intermediate between the high values for suspended particles (in situ pump: 580–760 dpm g⁻¹; CTD rosette: 870–1560 dpm g⁻¹) and the low values measured at the sediment surface (26–37 dpm g⁻¹). This represents essential information for the modeling of ²³⁴Th exchange processes at the sediment–water interface.     

Sediment trap and in-situ pump size-fractionated POC/234Th ratios in the Mediterranean Sea and Northwest Atlantic: Implications for POC export

Measurements of particle size-fractionated POC/²³⁴Th ratios and ²³⁴Th and POC fluxes were conducted using surface-tethered, free-floating, sediment traps and large-volume in-situ pumps during four cruises in 2004 and 2005 to the oligotrophic eastern Mediterranean Sea and the seasonally productive western Mediterranean and northwest Atlantic. Analysis of POC/²³⁴Th ratios in sediment trap material and 10, 20, 53, 70, and 100 μm size-fractionated particles indicate, for most stations, decreasing ratios with depth, a weak dependence on particle size, and ratios that converge to ～1–5 μmol dpm^?1 below the euphotic zone (～100–150 m) throughout the contrasting biogeochemical regimes. In the oligotrophic waters of the Aegean Sea, ²³⁴Th and POC fluxes estimated using sediment traps were consistently higher than respective fluxes estimated from water-column ²³⁴Th–²³⁸U disequilibrium, observations that are attributed to terrigenous particle scavenging of ²³⁴Th. In the more productive western Mediterranean and northwest Atlantic, ²³⁴Th and POC fluxes measured by sediment trap and ²³⁴Th–²³⁸U disequilibrium agreed within a factor of 2–4 throughout the water column. An implication of these results is that estimates of POC export by sediment traps and ²³⁴Th–²³⁸U disequilibrium can be biased differently because of differential settling speeds of POC and ²³⁴Th-carrying particles.     

Particle flux in the twilight zone of the eastern Indian Ocean: A constraint from 234U–230Th and 228Ra–228Th disequilibria

Total ²³⁰Th and ²²⁸Th vertical distributions were measured in the eastern Indian Ocean. Based on disequilibria of ²³⁴U–²³⁰Th and ²²⁸Ra–²²⁸Th, we estimated the particle flux from the surface to 950 m. The estimated particle flux ranged from 190±60 (150 m in the Southern Ocean) to 8.4±2.5 mg/m²/day (950 m in the West Australian Basin). The particle fluxes correlate well with estimates of nitrate supply by vertical mixing to the euphotic zone, suggesting local control by new production. Flux attenuation at mesopelagic depths appears to be related to biogenic carbonate and silica remineralization based on comparison to alkalinity and dissolved silicate distributions, with possible additional control by lithogenic particle inputs. The particle flux estimates obtained by combining the ²³⁴U–²³⁰Th and ²²⁸Ra–²²⁸Th disequilibria offer advantages over those from ²²⁸Ra–²²⁸Th disequilibrium alone, in that they can account for some aspects of vertical mixing and thus are applicable to a wider range of oceanographic settings.     

Particulate organic carbon export fluxes and size-fractionated POC/234Th ratios in the Ligurian,Tyrrhenian and Aegean Seas

Measurements of ²³⁴Th/²³⁸U disequilibria and particle size-fractionated (1, 10, 20, 53, 70, 100 μm) organic C and ²³⁴Th were made to constrain estimates of the export flux of particulate organic C (POC) from the surface waters of the Ligurian, Tyrrhenian and Aegean Seas in March–June 2004. POC exported from the surface waters (75–100 m depth) averaged 9.2 mmol m⁻² d⁻¹ in the Ligurian and Tyrrhenian Seas (2.3±0.5–14.9±3.0 mmol m⁻² d⁻¹) and 0.9 mmol m⁻² d⁻¹ in the Aegean Sea. These results are comparable to previous measurements of ²³⁴Th-derived and sediment-trap POC fluxes from the upper 200 m in the Mediterranean Sea. Depth variations in the POC/²³⁴Th ratio suggest two possible controls. First, decreasing POC/²³⁴Th ratios with depth were attributed to preferential remineralization of organic C. Second, the occurrence of maxima or minima in the POC/²³⁴Th ratio near the DCM suggests influence by phytoplankton dynamics. To assess the accuracy of these data, the empirical ²³⁴Th-method was evaluated by quantifying the extent to which the ²³⁴Th-based estimate of POC flux, P_POC, deviates from the true flux, F_POC, defined as the p-ratio (p-ratio=P_POC/F_POC=S_Th/S_POC, where S=particle sinking rate). Estimates of the p-ratio made using Stokes’ Law and the particle size distributions of organic C and ²³⁴Th yield values ranging from 0.93–1.45. The proximity of the p-ratio to unity implies that differences in the sinking rates of POC- and ²³⁴Th-carrying particles did not bias ²³⁴Th-normalized POC fluxes by more than a factor of two.     

Biogeochemical responses to late-winter storms in the Sargasso Sea,III—Estimates of export production using 234Th:238U disequilibria and sediment traps

Direct measurements of new production and carbon export in the subtropical North Atlantic Ocean appear to be too low when compared to geochemical-based estimates. It has been hypothesized that episodic inputs of new nutrients into surface water via the passage of mesoscale eddies or winter storms may resolve at least some of this discrepancy. Here, we investigated particulate organic carbon (POC), particulate organic nitrogen (PON), and biogenic silica (BSiO₂) export using a combination of water column ²³⁴Th:²³⁸U disequilibria and free-floating sediment traps during and immediately following two weather systems encountered in February and March 2004. While these storms resulted in a 2–4-fold increase in mixed layer NO₃ inventories, total chlorophyll a and an increase in diatom biomass, the systems were dominated by generally low ²³⁴Th:²³⁸U disequilibria, suggesting limited particle export. Several ²³⁴Th models were tested, with only those including non-steady state and vertical upwelling processes able to describe the observed ²³⁴Th activities. Although upwelling velocities were not measured directly in this study, the ²³⁴Th model suggests reasonable rates of 2.2–3.7 m d^?1.Given the uncertainties associated with ²³⁴Th derived particle export rates and sediment traps, both were used to provide a range in sinking particle fluxes from the upper ocean during the study. ²³⁴Th particle fluxes were determined applying the more commonly used steady state, one-dimensional model with element/²³⁴Th ratios measured in sediment traps. Export fluxes at 200 m ranged from 1.91±0.20 to 4.92±1.22 mmol C m^?2 d^?1, 0.25±0.08 to 0.54±0.09 mmol N m^?2 d^?1, and 0.22±0.04 to 0.50±0.06 mmol Si m^?2 d^?1. POC export efficiencies (Primary Production/Export) were not significantly different from the annual average or from time periods without storms, although absolute POC fluxes were elevated by 1–11%. This increase was not sufficient, however, to resolve the discrepancy between our observations and geochemical-based estimates of particle export. Comparison of PON export rates with simultaneous measurements of NO₃^? uptake derived new production rates suggest that only a fraction, <35%, of new production was exported as particles to deep waters during these events. Measured bSiO₂ export rates were more than a factor of two higher (p<0.01) than the annual average, with storm events contributing as much as 50% of annual bSiO₂ export in the Sargasso Sea. Furthermore it appears that 65–95% (average 86±14%) of the total POC export measured in this study was due to diatoms.Combined these results suggest that winter storms do not significantly increase POC and PON export to depth. Rather, these storms may play a role in the export of bSiO₂ to deep waters. Given the slower remineralization rates of bSiO₂ relative to POC and PON, this transport may, over time, slowly decrease water column silicate inventories, and further drive the Sargasso Sea towards increasing silica limitation. These storm events may further affect the quality of the POC and PON exported, given the large association of this material with diatoms during these periods.     

Time-series observation of POC fluxes estimated from 234Th in the northwestern North Pacific

Time-series measurements of ²³⁴Th activities and particulate organic carbon (POC) concentrations were made at time-series stations (K1, K2, K3, and KNOT) in the northwestern North Pacific from October 2002 to August 2004. Seasonal changes in POC export fluxes from the surface layer (∼100 m) were estimated using ²³⁴Th as a tracer. POC fluxes varied seasonally from approximately 0 to 180 mg C m⁻² d⁻¹ and were higher in spring–summer than in autumn–winter. The export ratio (e-ratio) ranged from 6% to 55% and was also higher in spring–summer. Annual POC fluxes were estimated to be 31 g C m⁻² y⁻¹ in the subarctic region (station K2) and 23 g C m⁻² y⁻¹ in the region between the subarctic and subtropical gyres (station K3). POC fluxes and e-ratios in the northwestern North Pacific were much higher than those in most other oceans. The annual POC flux corresponded to 69% of annual new production estimated from the seasonal difference of the nutrient in the Western Subarctic Gyre (45 g C m⁻² y⁻¹). These results indicate that much of the organic carbon assimilated in the surface layer of the northwestern North Pacific is transferred to the deep ocean in particulate form. Our conclusions support previous reports that diatoms play an important role in the biological pump.     

Seasonal changes in thorium scavenging and particle aggregation in the western Arctic Ocean

Seasonal changes in Th scavenging and particle aggregation were determined along two shelf-basin transects in the western Arctic Ocean during the spring (May–June) and summer (July–August) of 2002 and 2004. Measurements of dissolved and particulate ²³⁴Th and ²²⁸Th activities were used to quantify Th residence times and reversible rates of Th sorption and particle aggregation. Prior to the spring bloom in 2002, ²³⁴Th and ²²⁸Th residence times were equal and Th scavenging was concordant, indicating predominately steady-state conditions. In contrast, scavenging of ²³⁴Th and ²²⁸Th in the summer of 2002 and the spring and summer of 2004 was discordant, indicating a departure in scavenging rates from steady-state conditions during periods of seasonally high biological activity and particle export. Rates of particle aggregation and disaggregation were calculated using a one-dimensional reversible exchange model and ²³⁴Th and ²²⁸Th activities in small (1–53 μm) and large (>53 μm) particles. Maximum rates were determined coincident with the chlorophyll maximum (25–100 m) and increased by an order of magnitude between periods of low and high productivity. These Th measurements provide evidence that seasonally enhanced rates of particle aggregation might increase the magnitude of the particulate organic carbon (POC) flux in this Arctic regime.     

Strong seasonality in particle dynamics of north-western Mediterranean surface waters as revealed by 234Th/238U

²³⁴Th was used to quantify sinking fluxes and residence times of particles in surface waters of the north-western Mediterranean Sea. Measurements of dissolved and particulate ²³⁴Th were made at the DYFAMED station (43°25′N–7°51′E, JGOFS-France program). Sampling covered 1 year on four cruises in 1994 (February 9, April 29, June 3, October 1) and focused on a transition period in mid-spring with six repeated profiles collected during May 1995. ²³⁴Th was nearly in equilibrium with its parent ²³⁸U most of the year, except in spring. The intensive sampling in May shows a rapid evolution throughout the month from a moderate ²³⁴Th deficit to near-equilibrium values. The time-series of ²³⁴Th were treated with steady-state and non-steady-state models. ²³⁴Th particulate fluxes clearly indicate large variability in export, with the highest values observed in spring. Particle residence times in the upper 40 m range from <10 to >250 days, and could increase by a factor of 10 within 2 weeks. POC fluxes from the upper 40 m and export ratios (ThE: ratio of ²³⁴Th-derived POC export to primary production), derived from the ²³⁴Th/²³⁸U disequilibrium in the water column and POC/²³⁴Th ratio on trapped material, decrease from about 9.5 mmol C m⁻² d⁻¹ and >22% in early May to less than 5 mmol C m⁻² d⁻¹ and 15% after mid-May. The ²³⁴Th-derived information is in agreement with the annual variations in Mediterranean Sea productivity.     

A time-series study of particulate matter export in the North Pacific Subtropical Gyre based on Th : U disequilibrium

Depth profiles of total ²³⁴Th (dissolved+particulate) were collected at Station ALOHA (22°45N, 158°00W) in the North Pacific Subtropical Gyre during 9 cruises from April 1999 to March 2000. Samples were collected and processed by a new 2 L technique that enables more detailed depth resolution then previous ²³⁴Th studies. Significant zones of particle export (²³⁴Th deficiency) and particle remineralization (²³⁴Th excess) were measured both temporally and with depth. ²³⁴Th derived particulate carbon (PC) and nitrogen (PN) fluxes were determined with steady-state and non-steady-state models and PC/²³⁴Th and PN/²³⁴Th ratios measured with both in situ pumps and free-drifting particle interceptor traps deployed at 150 m. ²³⁴Th based export estimates of 4.0±2.3 mmol C m⁻² d⁻¹ and 0.53±0.19 mmol N m⁻² d⁻¹, were approximately 60% higher than those measured in PIT style sediment traps from the same time period, 2.4±0.2 mmol C m⁻² d⁻¹ and 0.32±0.08 mmol N m⁻² d⁻¹. Most of this difference is attributable to two large export events that occurred during October and December 1999, when traps undercollected for ²³⁴Th by a factor of 2 to 4. ²³⁴Th export (ThE) ratios based on ²³⁴Th derived PC flux/¹⁴C based primary production ranged from 4% to 22% (average=8.8%). Our results confirm the recent estimates of C export by Emerson et al. (Nature 389 (1997) 951) and Sonnerup et al. (Deep-Sea Research I 46 (1999) 777) and indicate that C export from the oligotrophic ocean must be considered when discussing C sequestration in global climate change.     

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.     

Vertical flux of particulate Al,Fe, Pb,and Ba from the upper ocean estimated from 234Th/238U disequilibria

The vertical sinking flux of particulate Al, Fe, Pb, and Ba from the upper 250 m of the Labrador Sea has been estimated from measurements of ²³⁴Th/²³⁸U disequilibrium and the respective metal/²³⁴Th ratios in >53 μm size particles. ²³⁴Th-derived particulate metal fluxes include in situ scavenged metals, labile lithogenic metals, and metals derived from external input (e.g., atmospheric supply). In contrast to the POC/²³⁴Th ratio, particle size-fractionated (0.4–10 μm, 10–53 μm, and >53 μm) Al/²³⁴Th, Fe/²³⁴Th and Pb/²³⁴Th, and Ba/²³⁴Th ratios generally increase with depth and exhibit no systematic change with particle diameter. Sinking fluxes of particulate Al (2.47–22.3 μmol m⁻² d⁻¹), Fe (2.69–16.3 μmol m⁻² d⁻¹), Pb (2.85–70 nmol m⁻² d⁻¹), and Ba (0.13–2.1 μmol m⁻² d⁻¹) at 50 m (base of the euphotic zone) and 100 m (base of the mixed layer) are largely within the range of previous sediment trap results from other ocean basins. Estimates of the upper ocean residence time of Al (0.07–0.28 yr) and Pb (0.8–2.9 yr) are short compared to previously reported values. The settling rate of >53 μm particles calculated from the ²³⁴Th data ranges from 14 to 38 m d⁻¹.     

POC export from ocean surface waters by means of 234Th/238U and 210Po/210Pb disequilibria: A review of the use of two radiotracer pairs

²³⁴Th (T_1/2=24.1 d) and ²¹⁰Po (T_1/2=138.4 d) are particle reactive radioisotopes that are used as tracers for particle cycling in the upper ocean. Particulate organic carbon (POC) export has frequently been estimated using ²³⁴Th/²³⁸U disequilibrium. Recent evidence suggests that ²¹⁰Po/²¹⁰Pb disequilibrium may be used as an additional tool to examine particle export, given the direct biological uptake of ²¹⁰Po into cellular material. Differences in these two radioisotope pairs with regard to their half-lives, particle reactivity and scavenging affinity in seawater should provide complementary information to be obtained on the processes occurring in the water column. Here, we review eight different studies that have simultaneously used both approaches to estimate POC export fluxes from the surface ocean. Our aim is to provide a complete “dataset” of all the existing POC flux data derived from the coupled use of both ²³⁴Th and ²¹⁰Po and to evaluate the advantages and limitations of each tracer pair. Our analysis suggests that the simultaneous use of both radiotracers provides more useful comparative data than can be derived from the use of a single tracer alone. The difference in half-lives of ²³⁴Th and ²¹⁰Po enables the study of export production rates over different time scales. In addition, their different biogeochemical behaviour and preferred affinity for specific types of particles leads to the conclusion that ²³⁴Th is a better tracer of total mass flux, whereas ²¹⁰Po tracks POC export more specifically. The synthesis presented here is also intended to provide a basis for planning future sampling strategies and promoting further work in this field to help reveal the more specific application of each tracer under specific water column biogeochemistries.     

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.     

Controls of 234Th removal from the oligotrophic ocean by polyuronic acids and modification by microbial activity

To gain new insights into the variability of particulate organic carbon (POC) fluxes and to better understand the factors controlling the POC/²³⁴Th ratios in suspended and sinking particulate matter, we investigated the relationships between POC/²³⁴Th ratios and biochemical composition (uronic acids, URA; total carbohydrates, TCHO; acid polysaccharides, APS; and POC) of suspended and sinking matter from the Gulf of Mexico in 2005 and 2006. Our data show that URA/POC in sediment traps (STs), APS/POC in the suspended particles, and turnover times of particulate ²³⁴Th in the water column and those of bacteria in STs inside eddies usually increased with depth, whereas particulate POC/²³⁴Th (10–50 μm) and the sediment-trap parameters (POC flux, POC/²³⁴Th ratio, bacterial biomass, and bacterial production) decreased with depth. However, this trend was not the case for most biological parameters (e.g., phytoplankton and bacterial biomass) or for the other parameters at the edges of eddies or at coastal-upwelling sites.In general, the following relationships were observed: 1) ²³⁴Th/POC ratios in STs were correlated with APS flux, and these ratios in the 10–50 μm suspended particles also correlated with URA/POC ratios; 2) neither URA fluxes nor URA/POC ratios were significantly related to bacterial biomass; 3) the sum of two uronic acids (G2, glucuronic, and galacturonic acid, which composed most of the URA pool) was positively related to bacterial biomass; and 4) the POC/²³⁴Th ratios in intermediate-sized particles (10–50 μm) were close to those in sinking particles but much lower than those in > 50 μm particles. The results indicate that acid polysaccharides, though a minor fraction (~ 1%) of the organic carbon, act more likely as proxy compound classes that might contain the more refractory ²³⁴Th-binding biopolymer, rather than acting as the original ²³⁴Th “scavenger” compound. Moreover, these acid polysaccharides, which might first be produced by phytoplankton and then modified by bacteria, also influence the on-and-off “piggy-back” processes of organic matter and ²³⁴Th, thus causing additional variability of the POC/²³⁴Th in particles of different sizes.     

Silicon flux and distribution of biogenic silica in deep-sea sediments in the western North Pacific Ocean

We investigated biogenic silica, several biological components, and silicate in pore-water in the abyssal sediment to determine silicon flux of western North Pacific during several cruises. The surficial sediment biogenic silica content was high at high latitudes with the boundary running along the Kuroshio Extension, and maximum values (exceeding 20%) were found in the Oyashio region. In the subtropical region to the south, most stations showed less than 5% biogenic silica content. This distribution pattern reflected primary production and ocean currents in the surface layer very well. Pore-water samples were collected from 4 stations along the east coast of Japan. The highest asymptotic silicic acid concentration (670 μmol L^?1) in pore-water was observed at the junction of Kuroshio and Oyashio, followed by samples from the Oyashio region. It is at the southern station that the lowest value (450 μmol L^?1) was observed, and the primary production is low under the influence of Kuroshio there. The diffusive flux followed the same geographic trend as the asymptotic silicic acid concentrations did, ranging 77–389 mmol m^?2 yr ^?1. Multiple sampling of pore-water was conducted throughout the year at one station at high latitude. The average annual biogenic silica rain flux observed using sediment traps was 373 mmol m^?2 yr^?1; the diffusive flux and burial flux at the sediment–water interface were 305 and 9 mmol m^?2 yr^?1, respectively. We concluded that most of the settling silica particles dissolved and diffused at the sediment–water interface and approximately 3% only were preserved in this area. In addition, the obvious time lag observed between the peak rain flux and the maximum diffusive flux suggested that primary production in the surface layer has a great influence on the sedimentation environment of abyssal western North Pacific. These transitions of Si flux at the sediment–water interface were considerably greater in northwestern North Pacific than in southwestern North Pacific. In addition, a station in the Philippine Sea indicated high biogenic silica content because of Ethmodiscus ooze, which are scattered randomly on the sea floor in the subtropical region.     

Do bottom mixed layers influence 234Th dynamics in the abyssal near-bottom water column?

Dynamics of the natural radioactive particle tracer ²³⁴Th (half-life: 24.1 days) within the abyssal water column up to 1000 m above bottom and within surface sediments of the northeast Atlantic (Porcupine Abyssal Plain; depth: ≈4845 m) were investigated. Distributions of transmissometer voltages and potential temperature indicated a subdivision of the near-bottom water column into a benthic mixed layer (BML; thickness: ≈10–65 m) and the layer above the BML up to the upper boundary of the bottom nepheloid layer (BNL; thickness: ≈1000 m). Comparison of ²³⁴Th fluxes (dpm m⁻²d⁻¹) in sediment traps, vertical fluxes derived from ²³⁴Th/²³⁸U-disequilibrium in the near-bottom water column and excess ²³⁴Th inventories in the surface sediment provided evidence for lateral advection of ²³⁴Th-depleted water and a ‘missing sink’ for ²³⁴Th. A simple one-dimensional steady-state box-model approach was applied to investigate ²³⁴Th dynamics. Estimated residence times suggest the BML and the resuspension zone of the surface sediment to be highly dynamic systems with respect to particle cycling and sorptive reactions on a time scale of weeks. Model results indicate that, through the chemical forcing of changing particle concentration, a thickening BML results in (1) increasing residence times of particulate ²³⁴Th in the BML with respect to the net fluxes across the upper boundary of the BML and into the surface sediment; (2) declining adsorption rate constants in the BML; (3) increasing desorptive fluxes in the BML resulting in (4) enhanced ²³⁴Th decay in the BML; (5) decreasing net fluxes of particulate ²³⁴Th from the BML to the upper BNL above the BML and to the sediment. Potential consequences for carbon cycling in the water column of the deep ocean are discussed.     

Influence of Asian monsoon and ENSO events on particle fluxes in the western subtropical Pacific

A time-series sediment trap was deployed from October 2007 to May 2011 in the western subtropical Pacific with the aim of understanding the seasonal and inter-annual variability on particle flux in response to El Niño-Southern Oscillation (ENSO) events. Total mass fluxes varied from 3.04 mg m⁻² day⁻¹ to 31.1 mg m⁻² day⁻¹, with high fluxes during February–April and low fluxes during other months. This seasonal variation was also characterized by a distinct change in the CaCO₃ flux between the two periods. The marked increase in particle flux during February–April may be attributed to enhanced biological productivity in surface waters caused by strong wind-driven mixing in response to the western North Pacific monsoon system. The 2009/10 strong El Niño was accompanied by a significant reduction in particle flux, whereas the La Niña had no recognizable effect on particle flux in the subtropical Pacific. In particular, in the mature phase of the 2009/10 strong El Niño, the fluxes of organic carbon and biogenic silica decreased by 70–80% compared with those during the normal period, implying that the El Niño acted to suppress biological productivity in surface waters. The suppression of biological productivity during the 2009/10 strong El Niño is attributed to the decrease in precipitation due to the shift in the western Pacific warm pool. This finding is opposite that of other studies of the western equatorial Pacific, where El Niño events were observed to result in an increase in biological productivity and particle flux. The difference in particle flux between the western equatorial and subtropical Pacific is attributed to the regional differences in oceanic and atmospheric circulation systems generated by the strong El Niño.     

Role of colloidal material in the removal of 234Th in the Canada basin of the Arctic Ocean

The phase partitioning of ²³⁴Th between dissolved (<10-kiloDalton, kD), colloidal (10 kD—0.4 μm), and particulate (⩾0.5 μm) matter across a horizontal transect, from a coastal station to the deep Canada Basin, and a vertical profile in the deep Canada Basin of the western Arctic Ocean was investigated. Concentrations of suspended particulate matter (SPM), dissolved, colloidal and particulate organic carbon, particulate organic nitrogen and nutrients (silicate, phosphate and nitrate) were also measured to assess transport and scavenging processes.Total ²³⁴Th (colloidal+particulate+dissolved) indicated deficiencies relative to secular equilibrium with its parent, ²³⁸U in the upper 100 m, which suggests active scavenging of ²³⁴Th onto particle surfaces. In contrast, at depths >200 m, general equilibrium existed between total ²³⁴Th and ²³⁸U. The inventory of SPM and the specific activity of particulate ²³⁴Th in the Canada Basin was about an order of magnitude higher than the profile reported for the Alpha Ridge ice camp station. This higher concentration of SPM in the southwestern Canada Basin is likely derived from ice-rafted sedimentary particles. Inventories of nutrients, and dissolved organic carbon and nitrogen in the upper 100 m of the Canada Basin are comparable to the other estimates for the central Arctic Ocean. Comparison of the mass concentrations of colloidal and filter-retained particulate matter as well as the activity of ²³⁴Th in these phases indicates that only a very small component of the colloidal material is actively involved in Th scavenging. Lower values of the conditional partition coefficient between the colloidal and dissolved phase indicate that the Arctic colloids are less reactive than colloidal material from other regions. The conditional partition coefficient between the filter-retained and dissolved phases (K_f) is generally higher than that for other regions, which is attributed to the higher complexation capacity of glacio-marine sedimentary particles in these waters. The ²³⁴Th-derived export of POC for the shelf and deep Canada Basin ranges between 5.6 and 6.5 mmol m⁻² d⁻¹, and is in agreement with other estimates reported for the central Arctic Ocean and Beaufort Sea.     

Resuspension and particle transport in the benthic nepheloid layer in and near Fram Strait in relation to faunal abundances and 234Th depletion

The West Spitsbergen Current, flowing northward through Fram Strait, causes a benthic nepheloid layer (BNL) on the western slope of the Yermak Plateau. This BNL is weaker on the eastern side of the Plateau and absent on the Greenland side of the Fram Strait, where the East Greenland Current flows south. In this BNL we find throughout a depletion of ²³⁴Th relative to its parent ²³⁸U, and we use this to study the particle dynamics in the BNL. The export flux from the ice-covered surface ocean and from a young bloom found in the ice-free waters off NE Greenland is shown to be negligible, allowing us to explain the ²³⁴Th depletion by interaction with the sediment alone. The depletion, balanced by a similar excess in the surface layer of the sediment, implies the existence of a settling-resuspension loop with an average particle residence time of 1–2 months. The asymmetry with a stronger resuspension loop on the western (80–120 mg m⁻² d⁻¹) than on the eastern side of the Yermak Plateau (1–15 mg m⁻² d⁻¹) is reflected in the numbers of species and individuals of suspension feeders in box core samples, and in epifauna densities estimated from video observations. The suspension feeders thus contribute to deposition of particles that are advected from more productive ice-free regions. This explanation is in agreement with the east–west asymmetry in the input of organic material to the sediments of the Yermak Plateau, which has been concluded from the distribution of pigments, bacterial activity and meiofauna abundances, observed in a concurrent study at the same stations. On the West Spitsbergen shelf, a very intensive BNL was monitored over 1 month with a moored filtration system. A part of the sustained high suspended load may be advected over long distances. This study illustrates how the tracer ²³⁴Th can help to determine the extent to which suspended particles are in continuous exchange with the seafloor, and where biological mediation and chemical modification can be expected.