西北冰洋表-底层碳动力学耦合作用的同位素(13C,234Th)指示

The coupling of upper ocean-benthic carbon dynamics in the ice-free western Arctic Ocean(the Chukchi Sea and the Canada Basin) was evaluated during the late July–early September 2003 using natural stable(13C) and radioactive(238U-234Th) isotope tracers. POC export flux estimated from 234Th/238 U disequilibria and dissolved CO2 concentration([CO2(aq)]) pointed out that the strengthened biological pump in the Chukchi Shelf have significantly lowered [CO2(aq)] and altered the magnitude of isotopic(12C/13C) fractionation during carbon fixation in the surface ocean. Further, d13 C signatures of surface sediments(d13Csed) are positively correlated to those of weighted d13 CPOC in upper ocean(d13Csed =13.64+1.56×d13CPOC, r2=0.73, p0.01), suggesting that the POC isotopic signals from upper ocean have been recorded in the sediments, partly due to the rapid export of particles as evidenced by low residence times of the highly particle-reactive 234 Th from the upper water column. It is suggested that there probably exists an upper ocean-benthic coupling of carbon dynamics, which likely assures the sedimentary d13 C record an indicator of paleo-CO2 in the western Arctic Ocean.     

POC fluxes from euphotic zone estimated from 234Th deficiency in winter in the northwestern North Pacific Ocean

1IntroductionThefluxesofcarbon,nutrients,andassoci-atedelementsinvolvedinthebiogeochemicalcyclesoutoftheeuphoticzoneareimportantinthestudyofglobalCO2 change.Someworkershaveproposedthatatthesteadystatethefluxofparticulateorganiccarbonoutoftheeuphoticzoneequalsthenewproduction(EppleyandPe-terson,1979;Eppley,1989).Generallytwometh-odscanbeemployedtoobtainthefluxdata.Oneistousesedimenttrapsintheupperocean(<200m)orfloatingsedimenttraps.Thoughthesedimenttraptechnologyhasshowntobeuse-fulfortimeser…     

Particulate organic carbon export flux by 234Th/238U disequilibrium in the continental slope of the East China Sea

234Th is widely used to quantify the magnitude of upper ocean particulate organic carbon(POC)export in oceans.In the present work,the rates of particulate organic carbon export were measured based on the distribution patterns of234Th/238U disequilibrium in the water column within the continental slope of the East China Sea(ECS)during May 2011.The profiles of particulate and dissolved234Th activities at all three stations showed a relative deficit with respect to238U in the upper 100 m of the water column.The dissolved234Th scavenging rates and the particulate234Th removal rates and their residence times were calculated by a one-dimensional steady state model.The results showed that the dissolved234Th scavenging rates and the particulate234Th removal rates ranged from 12.4–61.4 dpm/(m3·d)andfrom3.8–21.8 dpm/(m3·d),respectively.The residence times of dissolved and particulate234Th were in the range of 3.4–158 d and 63.7–96.5 d,respectively.Combined with the measurement of POC/234Th ratios of suspended particles,POC export flux(calculated by carbon)from the euphotic zone was estimated in the study region,which ranged from 4.14–14.7 mmol/(m2·d),withanaverageof8.21mmol/(m2·d),occupying35%oftheprimeproductivity in the study area.The results of this study can provide new information for better understanding the carbon biogeochemical cycle within the continental slope of the ECS.     

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

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

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.     

<Superscript>234</Superscript>Th/<Superscript>238</Superscript>U disequilibrium and particulate organic carbon export in the northern South China Sea

We utilized ²³⁴Th, a naturally occurring radionuclide, to quantify the particulate organic carbon (POC) export rates in the northern South China Sea (SCS) based on data collected in July 2000 (summer), May 2001 (spring) and November 2002 (autumn). Th-234 deficit was enhanced with depth in the euphotic zone, reaching a subsurface maximum at the Chl-a maximum in most cases, as commonly observed in many oceanic regimes. Th-234 was in general in equilibrium with ²³⁸U at a depth of ∼100 m, the bottom of the euphotic zone. In this study the ²³⁴Th deficit appeared to be less significant in November than in July and May. A surface excess of ²³⁴Th relative to ²³⁸U was found in the summer over the shelf of the northern SCS, most likely due to the accumulation of suspended particles entrapped by a salinity front. Comparison of the ²³⁴Th fluxes from the upper 10 m water column between 2-D and traditional 1-D models revealed agreement within the errors of estimation, suggesting the applicability of the 1-D model to this particular shelf region. 1-D model-based ²³⁴Th fluxes were converted to POC export rates using the ratios of bottle POC to ²³⁴Th. The values ranged from 5.3 to 26.6 mmol C m⁻²d⁻¹ and were slightly higher than those in the southern SCS and other oligotrophic areas. POC export overall showed larger values in spring and summer than in autumn, the seasonality of which was, however, not significant. The highest POC export rate (26.6 mmol C m⁻²d⁻¹) appeared at the shelf break in spring (May), when Chl-a increased and the community structure changed from pico-phytoplankton (<2 μm) dominated to nano-phytoplankton (2–20 μm) and micro-phytoplankton (20–200 μm) dominated.     

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.     

234Th/238U disequilibrium and particulate organic carbon export in the northwestern South China Sea

234 Th was utilized as a tracer of particulate organic carbon (POC) export in the northwestern South China Sea (SCS) on the basis of the data collected at four stations during a spring cruise.Depth profiles of dissolved and particulate 234 Th activities were measured in the upper 60 m,showing a significant deficit relative to 238 U over the investigated stations.A stratified structure of 234 Th-238 U disequilibrium was in general observed in the upper 60 m water column,indicating that the euphotic zone of t...     

真光层的颗粒动力学──Ⅵ.南海东北部海域上层水体颗粒动力学的示踪研究

利用234Th－238U不平衡研究南海东北部海域3个站位上层水体中的颗粒动力学性质，测定了水往中溶解态及颗粒态234Th的比活度，具体讨论各相中234Th／238U）AR（放射性活度比）比值的垂直分布情况及其与水化学要素间的关系。运用稿态箱式模型计算出各站位不同水层中溶解态234Th相对于清除至颗粒物的平均停留时间和颗粒态234Th相对于迁出作用的停留时间。由模型得出的参数表明3个站位的真光层具有两种不同的层化图像，这一情形与我们在南沙群岛海域得到的结果相一致。结合POC／PTh比值，估算出3个站位从真光层输出的颗粒有机碳（POC）通量分别为4.025.0和5．4mmolC·m－3－d-1。文中进一步讨论了234Th与POC两者停留时间的关系。     

Thorium-234 as a tracer of spatial,temporal and vertical variability in particle flux in the North Pacific

An extensive ²³⁴Th data set was collected at two sites in the North Pacific: ALOHA, an oligotrophic site near Hawaii, and K2, a mesotrophic HNLC site in the NW Pacific as part of the VERTIGO (VERtical Transport In the Global Ocean) study. Total ²³⁴Th:²³⁸U activity ratios near 1.0 indicated low particle fluxes at ALOHA, while ²³⁴Th:²³⁸U ～0.6 in the euphotic zone at K2 indicated higher particle export. However, spatial variability was large at both sites—even greater than seasonal variability as reported in prior studies. This variability in space and time confounds the use of single profiles of ²³⁴Th for sediment trap calibration purposes. At K2, there was a decrease in export flux and increase in ²³⁴Th activities over time associated with the declining phase of a summer diatom bloom, which required the use of non-steady state models for flux predictions. This variability in space and time confounds the use of single profiles of ²³⁴Th for sediment trap calibration purposes. High vertical resolution profiles show narrow layers (20–30 m) of excess ²³⁴Th below the deep chlorophyll maximum at K2 associated with particle remineralization resulting in a decrease in flux at depth that may be missed with standard sampling for ²³⁴Th and/or with sediment traps. Also, the application of ²³⁴Th as POC flux tracer relies on accurate sampling of particulate POC/²³⁴Th ratios and here the ratio is similar on sinking particles and mid-sized particles collected by in-situ filtration (>10–50 μm at ALOHA and >5–350 μm at K2). To further address variability in particle fluxes at K2, a simple model of the drawdown of ²³⁴Th and nutrients is used to demonstrate that while coupled during export, their ratios in the water column will vary with time and depth after export. Overall these ²³⁴Th data provide a detailed view into particle flux and remineralization in the North Pacific over time and space scales that are varying over days to weeks, and 10's–100's km at a resolution that is difficult to obtain with other methods.     

Seasonal export fluxes of particulate organic carbon from <Superscript>234</Superscript>Th/<Superscript>238</Superscript>U disequilibrium measurements in the Ulleung Basin<Superscript>1</Superscript> (Tsushima Basin) of the East Sea<Superscript>1</Superscript> (Sea of Japan)

Export fluxes of particulate organic carbon (POC) were estimated from the ²³⁴Th/²³⁸U disequilibrium in the Ulleung Basin¹ (UB) of the East/Japan Sea¹ (EJS) over four seasons. The fluxes were calculated by multiplying the average POC/²³⁴Th ratio of sinking particles larger than 0.7 μm at 100- and 200-m water depths to ²³⁴Th fluxes by the integrated ²³⁴Th/²³⁸U disequilibrium from the surface to 100-m water depth. In spring, the ²³⁴Th profiles changed dramatically with sampling time, and hence a non-steady-state ²³⁴Th model was used to estimate the ²³⁴Th fluxes. The ²³⁴Th flux estimated from the non-steady-state model was an order of magnitude higher than that estimated from the steady-state model. The ²³⁴Th fluxes estimated using the steady-state model showed distinct seasonal variation, with high values in summer and winter and low values in autumn. In spring, the phytoplankton biomass had the highest value, and primary production was higher than in summer and autumn, but the ²³⁴Th fluxes were moderate. However, these values might have been significantly underestimated, as the ²³⁴Th fluxes were estimated using the steady-state model. The POC export fluxes estimated in autumn were about four times lower than those in other seasons when they were rather similar. The annually averaged POC flux was estimated to be 161 ± 76 mgC m⁻² day⁻¹, which was somewhat lower than that in highly productive coastal areas, and higher than that in oligotrophic regions. The export/primary production (ThE) ratios ranged from 7.0 to 56.1%, with higher values in spring and summer and lower values in autumn and winter. In summer, a high ThE ratio of 48.4 ± 7.0% was measured. This may be attributed to the mass diatom sinking event following nitrate depletion. In the UB¹, the annually averaged ThE ratio was estimated to be 34.4 ± 12.9%, much higher than that in oligotrophic oceans. The high ThE ratio may have contributed to the high organic carbon accumulation in the UB¹.     

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.     

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.     

Flocculation and phytoplankton cell size can alter Th-based estimates of the vertical flux of particulate organic carbon in the sea

The deficit of ²³⁴Th relative to its radioactive parent ²³⁸U in the surface ocean can yield reliable estimates of vertical Particulate Organic Carbon (POC) fluxes to deeper waters, but only when coupled with an accurate ratio of POC concentration to activity of ²³⁴Th on sinking matter. Assuming a simple partitioning of suspended phytoplankton mass between single cells and flocs, we calculate the ratio of the POC flux estimated from ²³⁴Th deficit to the actual POC flux (p ratio, Smith, J.N., Moran, S.B., Speicher, E.A., in press. The p-ratio: a new diagnostic for evaluating the accuracy of upper ocean particulate organic carbon export fluxes estimated from ²³⁴Th/²³⁸U disequilibrium. Deep-Sea Research I.). The p ratios are calculated under the assumption that particle surface area is correlated with ²³⁴Th activity and particle volume is correlated with POC concentration. The value of the p ratio depends on the relative contributions of single cells and flocs to the vertical flux. When large single cells make up a significant fraction of the vertical flux, p ratios are less than one, meaning POC fluxes estimated from ²³⁴Th deficits underestimate actual POC fluxes. When large single cells are abundant but do not sink fast enough to contribute to vertical POC flux, p ratios are greater than one (up to 3 × overestimate). Factor analysis of the model indicates that altering the extent of flocculation in suspension and changing the density and maximum size of phytoplankton cells have the greatest effects on the p ratio. Failure to measure the properties of flocs when characterizing the ratio of POC to thorium on sinking matter potentially leads to large overestimation of the POC flux (over 20 ×). Failure to characterize the POC to thorium ratio of large particles, by, for example, destruction of phytoplankton cells in pumps, can lead to underestimation of POC flux. Estimates of POC flux should be most reliable in highly flocculated suspensions populated by small cells and rapidly sinking flocs. These conditions are often associated with intense phytoplankton blooms.     

Short-lived thorium isotopes (Th, Th) as indicators of POC export and particle cycling in the Ross Sea, Southern Ocean

Repeated measurements of depth profiles of ²³⁴Th (dissolved, 1–70 and >70 μm particulate) at three stations (Orca, Minke, Sei) in the Ross Sea have been used to estimate the export of Th and particulate organic carbon (POC) from the euphotic zone. Sampling was carried out on three JGOFS cruises covering the period from October 1996 (austral early spring) to April 1997 (austral fall). Deficiencies of ²³⁴Th relative to its parent ²³⁸U in the upper 100 m are small during the early spring cruise, increase to maximum values during the summer, and decrease over the course of the fall. Application of a non-steady-state model to the ²³⁴Th data shows that the flux of Th from the euphotic zone occurs principally during the summer cruise and in the interval between summer and fall. Station Minke in the southwestern Ross Sea appears to sustain significant ²³⁴Th removal for a longer period than is evident at Orca or Sei. Particulate ²³⁴Th activities and POC are greater in the 1–70 μm size fraction, except late in the summer cruise, when the >70 μm POC fraction exceeds that of the 1–70 μm fraction. The POC/²³⁴Th ratio in the >70 μm fraction exceeds that in the 1–70 μm fraction, likely due in part to the greater availability of surface sites for Th adsorption in the latter. Particulate ²³⁴Th fluxes are converted to POC fluxes by multiplying by the POC/²³⁴Th ratio of the >70 μm fraction (assumed to be representative of sinking particles). POC fluxes calculated from a steady-state Th scavenging model range from 7 to 91 mmol C m⁻² d⁻¹ during late January–early February, with the greatest flux observed at station Minke late in the cruise. Fluxes estimated with a non-steady-state Th model are 85 mmol C m⁻² d⁻¹ at Minke (1/13–2/1/97) and 50 mmol C m⁻² d⁻¹ at Orca (1/19–2/1/97). The decline in POC inventories (0–100 m) is most rapid in the southern Ross Sea during the austral summer cruise (Smith et al., 2000. The seasonal cycle of phytoplankton biomass and primary productivity in the Ross Sea, Antarctica. Deep-Sea Research II 47, 3119–3140. Gardner et al., 2000. Seasonal patterns of water column particulate organic carbon and fluxes in the Ross Sea, Antarctica. Deep-Sea Research II 47, 3423–3449), and the ²³⁴Th-derived POC fluxes indicate that the sinking flux of POC is 30–50% of the POC decrease, depending on whether steady-state or non-steady-state Th fluxes are used. Rate constants for particle POC aggregation and disaggregation rates are calculated at station Orca by coupling particulate ²³⁴Th data with ²²⁸Th data on the same samples. Late in the early spring cruise, as well as during the summer cruise, POC aggregation rates are highest in near-surface waters and decrease with depth. POC disaggregation rates during the same time generally increase to a maximum and are low at depth (>200 m). Subsurface aggregation rates increase to high values late in the summer, while disaggregation rates decrease. This trend helps explain higher values of POC in the >70 m fraction relative to the 1–70 m fraction late in the summer cruise. Increases in disaggregation rate below 100 m transfer POC from the large to small size fraction and may attenuate the flux of POC sinking out of the euphotic zone.     

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.     

POC export from ocean surface waters by means of Th/U and Po/Pb 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.     

不同潮时对厦门湾水体中234Th/238U不平衡的影响

厦门湾水体中2 3 4 Th 2 3 8U不平衡的时间序列数据表明 ,无论是溶解态、颗粒态2 3 4 Th还是总2 3 4 Th ,相对于母体2 3 8U均严重亏损 ,呈现出与开阔大洋水明显不同的特征。溶解态、颗粒态2 3 4 Th的停留时间介于 0 .5— 41d之间 ,其中低潮时停留时间比高潮时小 2— 4倍 ,证实近岸海域具有强烈的清除、迁出作用 ,且潮汐变化对海域颗粒动力学特征有重要影响。2 3 4 Th停留时间与总悬浮颗粒物浓度 (TSM)、Chl.a的关系则表明 ,近岸海域元素的清除、迁出作用主要受陆源颗粒物输送的影响 ,与生物活动关系并不密切。此外 ,非稳态与稳态清除模型结果的对比证明 ,稳态模型对于具有强烈清除、迁出作用的近岸海域是适用的     

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.