Particulate Organic Carbon Export Research Articles

Flocculation and phytoplankton cell size can alter 234Th-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.

An improvement in the small-volume technique for determining thorium-234 in seawater

The recently developed 2–5 L small-volume MnO 2 coprecipitation method for determining 234Th in seawater has provided a new way to substantially increase, both temporally and spatially, the sampling resolution of 234Th and 234Th-based particulate organic carbon export estimates in the upper ocean. In this study, we further optimize the technique by reducing reagent quantities and the use of an additional water bath heating step. This optimization allows the filtration of the MnO 2 precipitate onto a 25-mm diameter, 1.0 μm pore size QMA filter to be completed within < 30 min for unfiltered waters from the South China Sea. In addition, we have modified the MnO 2 purification procedure to allow for alpha spectrometric measurements of 234Th recoveries. Results from recovery experiments suggest that reagent amounts can be reduced to 0.0375 mg KMnO 4 and 0.1 mg MnCl 2·4H 2O per liter of sample, while still maintaining high 234Th recovery. This study further confirms that the addition of a yield monitor is necessary for the application of the small-volume method.

Localized erosion affects national carbon budget

Small mountainous rivers discharge disproportionate amounts of sediment and carbon to the Earth's oceans. Our New Zealand data demonstrates that localized erosion plays a greater role in C budgets than has been recognized in national and global studies. We estimate that New Zealand's rivers export 4 ± 1 Mg C km−2 yr−1 of dissolved organic carbon (DOC) and 10 ± 3 Mg C km−2 yr−1 of particulate organic carbon (POC) (2 and 6 times the global average), which is equivalent to 40% of New Zealand's fossil fuel emissions. Under intact native vegetation in mountain‐belt hot spots, POC export greatly exceeds CO2 consumption from mineral weathering. Moreover, deforestation of fertile steepland greatly accelerates POC loss, evidenced by 1.7% of New Zealand's land area which generates 20% of exported POC. Thus, localized erosion deserves increased attention in C budgets and accounting.

Seasonal changes in POC export flux in the Chukchi Sea and implications for water column-benthic coupling in Arctic shelves

Abstract As part of the 2002 Shelf-Basin Interactions (SBI) process study, measurements of the seasonal variation in the export flux of particulate organic carbon (POC) are reported for the upper waters of the Chukchi Sea. POC fluxes were quantified by determination of 234Th/238U disequilibrium and POC/234Th ratios in large ( > 53 μ m ) aggregates collected using in situ pumps. Samples were collected at 35 stations on two cruises, one in predominantly ice-coved conditions during the spring (May 6–June 15) and the other in predominantly open water during the summer (July 17–August 26). Enhanced particle export was observed in the shelf and slope waters, particularly within Barrow Canyon, and there was a marked increase in particle export at all stations during the summer (July–August) relative to the spring (May–June). 234Th-derived POC fluxes exhibit significant seasonal and spatial variability, averaging 2.9 ± 5.3 mmol C m - 2 d - 1 ( range = 0.031 – 22 mmol C m - 2 d - 1 ) in the spring and increasing ∼ 4 -fold in the summer to an average value of 10.5 ± 9.3 mmol C m - 2 d - 1 ( range = 0.79 – 39 mmol C m - 2 d - 1 ) . The fraction of primary production exported from the upper waters increases from ∼ 15 % in the spring to ∼ 32 % in the summer. By comparison, DOC accumulation associated with net community production represented ∼ 6 % of primary production ( ∼ 2 mmol C m - 2 d - 1 ) . The majority of shelf and slope stations indicate a close agreement between POC export and benthic C respiration in the spring, whereas there is an imbalance between POC export and benthic respiration in the summer. The implication is that up to ∼ 20 % of summer production ( ∼ 6 ± 7 mmol C m - 2 d - 1 ) may be seasonally exported off-shelf in this productive shelf/slope region of the Arctic Ocean.

Vertical particle flux in the northeast Atlantic Ocean (POMME experiment)

In the framework of the Programme Océan Multidisciplinaire Méso Echelle (POMME) experiment, a 1.5 year record (February 2001–June 2002) of downward particle flux at 400 m and 1000 m was measured by sediment traps at four moorings located in the northeast Atlantic between 39°–43°N and 17°–19°W. Thorium‐230 was used to estimate sediment trap efficiency, revealing values ranging from 18.5 to 55%. The lowest trapping efficiency was observed for the trap having experienced the highest currents. Significant interannual variability between 2001 and 2002 was clearly linked to the differences observed in the mixed layer depth. At some sites, particulate organic carbon (POC) export was higher (up to a factor of 1.6) during summer than during the spring event. This could be related to the occurrence of short wind events that deepened the thermocline along with the presence of anticyclonic eddies, yielding an input of new nutrients. The average percentage of POC exported compared to the primary production of organic carbon in the surface waters ranged between 1.3 and 5.0%, with higher export efficiency during the spring. Finally, although the area was shown to present a relatively high mesoscale activity that might impact the geochemistry, POC export was rather homogeneous over the POMME area: 4.9 ± 1.6 gC m−2 yr−1 were exported below 1000 m between February 2001 and February 2002. Therefore a large fraction of the new production may be exported through convection and mode water circulation rather than by particle settling.

Ecosystem changes after the SEEDS iron fertilization in the western North Pacific simulated by a one-dimensional ecosystem model

We have developed a one-dimensional ecosystem model considering explicitly two groups of diatoms: centric and pennate. The model was applied to the iron-enrichment experiment, SEEDS (Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study) in the western North Pacific (48.5°N, 165°E). We assumed that the pennate diatoms are not stressed by the iron limitation, but that the centric diatoms are significantly stressed and their photosynthetic rate is high when released from iron limitation. The iron-enrichment experiment was conducted by changing the physiological parameters for diatoms (the initial slope of the photosynthetic rate and the potential maximum photosynthetic rate). The model successfully reproduced the time-series of CO 2 fugacity and the vertical distributions of chlorophyll- a and nutrients observed in SEEDS. The simulated results in the two-group model, especially the starting time of the diatom bloom, are better than those in the one-group model. We find that it is the transition of dominance, from pennate to centric diatoms, rather than a physiological response of each group that causes the lag between iron-enrichment and the diatom bloom. In the simulations, the added iron’s effect on the ecosystem continues for about 40 days. The model results suggest that the export flux during the observation period is only about 20% of the total export flux generated by iron-enrichment. For SEEDS, our simulated uptake of atmospheric CO 2 and the export of particulate organic carbon at 100 m depth are 0.57 and 1.29 mol C m −2, respectively, which are converted to 1720 and 3880 tC by assuming the patch’s area to be 250 km 2, and to 4.90 and 11.1 tC kg −1 Fe with the addition of 350 kg of iron.

Export fluxes of particulate organic carbon estimated from 234Th/ 238U disequilibrium during the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS 2001)

The Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study (SEEDS) was carried out in the western subarctic gyre of the North Pacific to verify the iron-limitation hypothesis. Activities of dissolved 234Th in the upper 200 m of the water column and of particulate 234Th collected in a drifting sediment trap were measured during SEEDS 2001. Dissolved 234Th activities decreased and suspended particulate 234Th increased in the upper 20 m of the water column with the formation of an iron-induced phytoplankton bloom. The calculated residence times of the dissolved and particulate 234Th in the upper 20 m of the water column ranged from 2 to 13 d and 13 to 29 d, respectively. The ratios of particulate organic carbon (POC)/ 234Th p (μmol dpm −1) in the settling particles at 200 m depth varied between 1.94 and 4.02, and these levels were 10% those for the suspended particles. No large increase of the POC export flux was observed for the drifting sediment trap deployments during the observation period. In addition, the POC export fluxes derived from 234Th with a non-steady-state model varied from 10.1 to 38.3 mmol m −2 d −1. The ratio of the estimated POC export flux to primary production, Th E, was nearly constant (0.23–0.24) from July 20 to 27 (D2 and D9 after iron-enrichment) and decreased to 0.18 on July 31 (D13 after iron-enrichment). Regardless of whether the POC export fluxes increased, there was no change in Th E. The increase of the POC export fluxes estimated from 234Th was much smaller than the increase of primary production. Thus, no large POC export fluxes are concluded to have occurred from the iron-enrichment patch, though the removal flux of 234Th in settling particles increased with particulate biogenic silica fluxes as the phytoplankton bloom progressed.

Particle fluxes and organic carbon balance across the Eastern Alboran Sea (SW Mediterranean Sea)

As part of the “Mediterranean Targeted Project II—MAss Transfer and Ecosystem Response” (MTPII-MATER) EU-funded research project, particle flux data was obtained from three instrumented arrays moored along the 1°30′ W meridian in the Eastern Alboran Sea. The mooring lines were deployed over 11 months, from July 1997 to May 1998, and were equipped with sediment trap-current metre pairs at 500–700, 1000–1200 and 2000–2200 m of water depth. The settling material was analysed to obtain total mass, lithogenic, calcium carbonate, organic carbon and opal fluxes. Integrated analysis of sediment trap and current meter data with sea-surface satellite images reveals that particle flux distribution is a function of primary production, mid-water lateral advection and near bottom nepheloid input. The spreading to the east and to the south of phytoplankton-rich water from the upwelling off the Spanish coast is controlled by the position and size of the Western and Eastern Alboran gyres, and drives the seasonal arrival of biogenic material down to the sea floor. Discrete lateral advection events unrelated to vertical entries of material can also supply particulate matter at 1000–1200 and 2000–2200 m of water depth as noted at the northern and southern stations. To achieve a better understanding of the carbon cycle in the area we have attempted to constrain the production, transfer and burial of particulate organic carbon, providing the first estimates of particulate organic carbon export in the Alboran Sea. Results suggest that sea surface circulation and associated productivity signal control the efficiency of the biological pump in the area. The export production in the Eastern Alboran Sea is higher than in other Mediterranean sites, with 0.5–0.9% of the carbon fixed during photosynthesis transferred down the water column and buried in the deep sediments. In addition, a large portion is supplied by lateral advection and through a benthic nepheloid layer, which represent a significant source of organic carbon to the deep Eastern Alboran Sea.

The role of suspended particulate matter in 234Th scavenging and 234Th-derived export fluxes of POC in the Canada Basin of the Arctic Ocean

The concentration and composition of suspended particulate matter (SPM) plays a major role in the removal of particle-reactive nuclides in the ocean. In the Arctic Ocean, SPM concentrations in the deep basins vary widely, over an order of magnitude. Unique features, such as the presence and release of algae from sea ice to the upper waters may result in differing scavenging behavior of suspended particulate matter. In this study, we collected a suite of water samples across a horizontal transect in the western Arctic Ocean, from the coast to the deep Canada Basin, and measured concentrations of SPM, particulate organic carbon (POC) and nitrogen, and activities of dissolved and particulate 234Th. Our data indicate that the concentrations and inventories of SPM is about an order of magnitude higher than those reported for the permanently ice-covered Arctic region, but comparable to the value reported recently for another deep station in the Canada Basin. There is disequilibrium between 238U and total 234Th in the upper waters (∼100 m) indicating active scavenging in the deep basins. Variations in the specific activity of 234Th in the upper ∼100 m indicate differences in the remineralization rates in the deep Canada Basin. Plots between particulate and dissolved removal rate constants and SPM indicate that the biochemical composition of particulate matter could play a significant role on the removal rates of Th. The POC export rate calculated for the upper 50 m of the deep Canada Basin was found to be 1.4 mmol m −2 day −1. The residence times and removal fluxes of particulate and dissolved 234Th calculated for the upper 50 m in our study is compared to the calculated values from published data for other regions in the Arctic Ocean.

Low particulate organic carbon export in the frontal zone of the Southern Ocean (Indian sector) revealed by 234Th

The water column deficiencies of 234Th were used to estimate the Particulate Organic Carbon (POC) fluxes in the Indian Sector of the Southern Ocean. Samples were collected in January–February 1999 in a frontal zone from 42°S to 47°S and from 60°E to 66°E during the ANTARES 4 cruise. Beta counting was used to measure the 234Th activities onboard. The 234Th export fluxes were estimated from the 234Th/238U disequilibria using a steady state 234Th model. A non-steady state model gave results close to the steady-state model in the Subtropical Zone and could not be used in the Subantarctic Zone due to a strong vertical mixing event. Small and large particles analysis indicated that the POC/234Th ratios decreased when the particle size increased. From the POC/234Th ratios on the large filtered particles, it appears that the POC export fluxes exported below 100m were very low (from 0.10 to 2.53mmolCm−2d−1) compared to those observed in the Southern Ocean and (Deep Sea Res. II 48 (2001) 4275; Deep Sea Res. II 47 (15–16) (2000) 3451; Deep Sea Res. II 44 (1997) 457) and with a strong zonal variation . It is hypothesized that the low POC export fluxes were related to the low predominance of diatoms, characteristic at the end of a bloom period. In this way, the very low POC export observed in the Subtropical Zone suggests an efficient remineralization process and/or a high bacterial activity. Otherwise, a decoupling between the primary production and the POC export derived from 234Th could also explain the low POC export.

Upper ocean ecosystem dynamics and iron cycling in a global three‐dimensional model

A global three‐dimensional marine ecosystem model with several key phytoplankton functional groups, multiple limiting nutrients, explicit iron cycling, and a mineral ballast/organic matter parameterization is run within a global ocean circulation model. The coupled biogeochemistry/ecosystem/circulation (BEC) model reproduces known basin‐scale patterns of primary and export production, biogenic silica production, calcification, chlorophyll, macronutrient and dissolved iron concentrations. The model captures observed high nitrate, low chlorophyll (HNLC) conditions in the Southern Ocean, subarctic and equatorial Pacific. Spatial distributions of nitrogen fixation are in general agreement with field data, with total N‐fixation of 55 Tg N. Diazotrophs directly account for a small fraction of primary production (0.5%) but indirectly support 10% of primary production and 8% of sinking particulate organic carbon (POC) export. Diatoms disproportionately contribute to export of POC out of surface waters, but CaCO3 from the coccolithophores is the key driver of POC flux to the deep ocean in the model. An iron source from shallow ocean sediments is found critical in preventing iron limitation in shelf regions, most notably in the Arctic Ocean, but has a relatively localized impact. In contrast, global‐scale primary production, export production, and nitrogen fixation are all sensitive to variations in atmospheric mineral dust inputs. The residence time for dissolved iron in the upper ocean is estimated to be a few years to a decade. Most of the iron utilized by phytoplankton is from subsurface sources supplied by mixing, entrainment, and ocean circulation. However, owing to the short residence time of iron in the upper ocean, this subsurface iron pool is critically dependent on continual replenishment from atmospheric dust deposition and, to a lesser extent, lateral transport from shelf regions.

Ironing Out Algal Issues in the Southern Ocean

Periods of iron supply to the ocean over geological time scales may have altered climate by increasing the amount of COfixed by anaemic (iron-limited) phytoplankton. In his perspective, Boyd comments on three papers ( Coale et al., Bishop et al., and Buesseler et al.) that report results from recent large-scale experiments in which ocean waters have been enriched with iron. By measuring how much carbon fixed by phytoplankton is sequestered into the abyss, the experiments test the hypothesis that enhanced iron supply leads to enhanced export of particulate organic carbon.

The decline and fate of an iron-induced subarctic phytoplankton bloom.

Iron supply has a key role in stimulating phytoplankton blooms in high-nitrate low-chlorophyll oceanic waters. However, the fate of the carbon fixed by these blooms, and how efficiently it is exported into the ocean's interior, remains largely unknown. Here we report on the decline and fate of an iron-stimulated diatom bloom in the Gulf of Alaska. The bloom terminated on day 18, following the depletion of iron and then silicic acid, after which mixed-layer particulate organic carbon (POC) concentrations declined over six days. Increased particulate silica export via sinking diatoms was recorded in sediment traps at depths between 50 and 125 m from day 21, yet increased POC export was not evident until day 24. Only a small proportion of the mixed-layer POC was intercepted by the traps, with more than half of the mixed-layer POC deficit attributable to bacterial remineralization and mesozooplankton grazing. The depletion of silicic acid and the inefficient transfer of iron-increased POC below the permanent thermocline have major implications both for the biogeochemical interpretation of times of greater iron supply in the geological past, and also for proposed geo-engineering schemes to increase oceanic carbon sequestration.

Particulate organic carbon fluxes estimated from 234Th deficiency in winters and springs in the northwestern North Pacific

Activities of particulate and dissolved 234Th and concentrations of particulate organic carbon (POC) were measured in the northwestern North Pacific in winters and springs. The export fluxes of POC from surface waters were estimated by using 234Th as a tracer. The POC fluxes in winter showed a wide variation (20–190 mg-C m-2d-1) and were higher in the western region than in the eastern region. The influence of the Oyashio Current and the supply of substances from the continent to the study area also appeared to be higher in the western region. Therefore, the horizontal distribution of POC fluxes in winter was influenced by continental materials. In the spring bloom, the fluxes of POC were much higher (up to 520 mg-C m-2d-1), as were the e-ratios (up to 70%). In the northwestern North Pacific, the efficiencies of export of POC from the euphotic zone were higher than in other areas of the world ocean. The increase in POC fluxes in spring was considered to depend not only on the increased abundance of phytoplankton, but also on the transition of phytoplankton species, as evidenced by the relationship between 234Th adsorption rates and the concentrations of POC.

Feedbacks between oceanic redox states and marine productivity: A model perspective focused on benthic phosphorus cycling

A new model for the marine cycles of particulate organic carbon (POC), oxygen, nitrate, and phosphorus has been developed and applied to explore the controls and constraints on marine productivity and nutrient inventories. The coupled benthic‐pelagic model uses a new approach for the simulation of the reactive phosphorus turnover (Preac corresponding to the sum of organic P, authigenic P, and adsorbed phosphate) in marine sediments. The simulated POC/Preac burial ratio in shelf, slope and rise, and deep‐sea sediments increases under strongly reducing conditions in agreement with field observation. The model runs revealed that the spread of anoxia in bottom waters may enhance the productivity of the global ocean by one order of magnitude if sufficient nitrate is provided by N2‐fixation. Thus anoxic bottom waters promote eutrophic conditions and vice versa. Additional model runs showed that the productivity and nutrient inventory of the glacial ocean were probably enhanced due to the falling sea level. Marine regression induced a narrowing of the depositional areas on the continental shelves and thereby an increase in the fraction of POC exported to the deep ocean. The accelerated POC delivery, in turn, decreased the oxygen contents of the deep water and thus favored the release of phosphate from deep‐sea and rise sediments. Enhanced recycling of phosphate at the seafloor promoted further POC export in a positive feedback loop.

Lateral fluxes of biogenic particles through the Mien-Hua canyon in the southern East China Sea slope

Sediment traps were deployed on the southern East China Sea (ECS) continental slope to measure particle and biogenic fluxes for deriving particulate carbon exports from the ECS continental shelf through the Mien-Hua Canyon (MHC). Particle fluxes are much higher in the MHC than in other non-canyon slopes, ranging from 4.98 g m −2 d −1 on the lower slope to 71.5 g m −2 d −1 in the MHC, indicating that the MHC is a major conduit of particle transport. More than 80% of collected particles are lithogenic, regardless of trap sites. Biogenic contents are diluted by lithogenic materials and therefore decrease with the increase in mass fluxes. However, biogenic fluxes remain significant as they are proportional to mass fluxes, ranging from 21 to 368 mg C m −2 d −1 for particulate organic carbon (POC), 12–750 mg C m −2 d −1 for particulate inorganic carbon (PIC), and 21–755 mg m −2 d −1 for opal. Although particle and biogenic fluxes decrease with distance away from the shelf break, these fluxes increase significantly with water depth, indicating the important contribution of lateral transport to measured fluxes. Most POC collected from the slope is likely derived either from the shelf or from the upper-middle slope. Short-term variabilities of collected biogenic fluxes are likely linked to episodic resuspension and lateral movement from the shelf to the slope. Such cross-shelf and off-slope-directed transport of biogenic particles appears to function via the bottom and intermediate nepheloid layers over the shelf and slope. Total shelf fluxes through the MHC are estimated to be 2.43×10 11 g C yr −1 for POC and 4.20×10 11 g C yr −1 for PIC; the POC flux accounts for 0.17% of total primary production on the shelf. If these exports are extrapolated to calculate the amount of POC and PIC export through the whole ECS slope, the exported POC and PIC may be estimated to be less than 9.35×10 12 and 16.2×10 12 g C yr −1, respectively. The shelf export of POC is equivalent to <6.4% of carbon fixed biologically on the shelf. The export of particulate nitrogen is estimated to be less than 1.18×10 12 g N yr −1, that is equivalent to <4.6% of total nitrogen required to sustain primary production on the ECS shelf. More than 70% of the nitrogen used for new production (4.11×10 12 g N yr −1) may be buried in shelf sediments.

The surface water CO 2 budget for the Baltic Proper: a new way to determine nitrogen fixation

Six cruises were performed in the eastern Gotland Sea in about monthly intervals between March and September 2001 in order to establish a surface water carbon/nitrogen budget and to derive N 2 fixation rates. Balancing the concentration changes of total CO 2 and dissolved organic carbon, the CO 2 air/sea exchange and the effect of mixed layer deepening yielded the net production of particulate organic carbon (POC) for each time interval. A total net production of 3.5 mol m −2 was obtained, which is consistent with earlier estimates of the POC export into the deep water. Measured C/N ratios in the particulate organic matter (POM) were applied to calculate the net formation of particulate organic nitrogen (PON), which was used to balance the nitrogen pools and fluxes. Taking into account the changes in dissolved organic nitrogen, the atmospheric input of NO 3 −/NH 4 +, and mixed layer deepening, the N 2 fixation was calculated for each period. A total N 2 fixation of 318±53 mmol m −2 was obtained, which substantially exceeded any previous estimate for the Baltic Proper (14–260 mmol m −2) and which corresponded to almost twice the spring bloom nitrogen consumption. The temporal distribution was characterized by an unexpected peak (108 mmol m −2) in late spring that was attributed to the activity of Aphanizomenon. A second maximum (106 mmol m −2) was found for the June–July period when Nodularia dominated the planktonic community.

Does 234Th/238U disequilibrium provide an accurate record of the export flux of particulate organic carbon from the upper ocean?

The magnitude of the flux of biogenic particulate organic carbon (POC) exported from the surface waters of the world ocean and remineralized at depth is critical to constraining models of the global carbon cycle, yet remains controversial. The use of upper ocean sediment traps is still one of the primary tools for determining this export flux, although trap fluxes have been shown to vary significantly because of hydrodynamic and sample collection biases. Over the past decade, 234Th increasingly has been used as a tracer to estimate POC export from the euphotic zone by multiplying the depth‐integrated 234Th flux by the POC/234Th ratio of sinking particles. The accuracy of this technique is highly dependent on the natural variability in the POC/234Th ratio and 234Th flux, yet the significance of this variability to estimates of POC export remains uncertain. Based on an analysis of new 234Th and POC data from the Labrador Sea and a review of 25 previous independent field studies, we report that POC export fluxes can vary 2–10 times or more solely because of variability in the POC/234Th ratio and procedures used to estimate the 234Th flux. Recommended improvements include studies of the biological, chemical, and physical mechanisms controlling 234Th‐organic matter interactions in seawater; detailed comparisons of POC/234Th ratios in size‐fractionated and sediment trap material; increased spatial and temporal sampling density of234Th; and more standardized procedures to calculate the 234Th export flux.

Particulate Organic Carbon Export Fluxes in The Canada Basin and Bering Sea as Derived from 234Th/238U Disequilibria

Seawater samples were collected in the water column of the Canada Basin and the Bering Sea from aboard the R/V Xue Long during August 1999. Activity concentrations of dissolved and particulate 234Th were measured using beta counting techniques to quantify the scavenging and residence time of 234Th and organic carbon export fluxes. Primary production (PP) and bacterial production were also determined in the study areas through in situ incubation experiments. Significant 234Th scavenging was observed in the upper 100 m of the water column in both study areas, with up to 40% of 234Th deficit found at Bering Sea stations and ~15% of 234Th deficit at the Canada Basin station. Measured PP decreased from ~12.5 µmol C/m³/h in surface water to near zero at ~100 m depth, with an integrated PP of 3.83 mmol C/m²/d in the Canada Basin. Bacterial production, on the other hand, was on the order of 2.0 mmol C/m²/d, which is up to 52% of the integrated PP. Particulate organic carbon (POC) export fluxes derived from 234Th/238U disequilibrium were ~1 mmol C/m²/d in the Canada Basin and ~10 mmol C/m²/d in the Bering Sea, with fluxes in the latter area being 5 to 10 times higher than those found in the Canada Basin. These export fluxes correspond to a ThE ratio (the ratio of 234Th-derived POC export to primary production) of 0.26 for the Canada Basin and 0.7 for the Bering Sea. The higher ThE ratios in the study areas suggest a decoupling of production and particulate export in the high-latitude ocean. Ratios of POC to particulate 234Th (µmol C/dpm) decreased consistently with increasing depth, suggesting that organic carbon is preferentially remineralized relative to 234Th. Interestingly, the profile of particulate 234Th in the Canada Basin showed a unique characteristic: particulate 234Th activities increased with increasing depth, suggesting a continuous scavenging of 234Th and a rapid settling rate of the particles.

Bacterioplankton distribution and production in the bathypelagic ocean: Directly coupled to particulate organic carbon export?,

A recently published evaluation of bacterioplankton abundance and productivity in the bathypelagic North Pacific suggests that these properties are generally coupled with particulate organic carbon (POC) fluxes. In that analysis, bacterial biomass and productivity were several–fold greater in subarctic than subtropical waters, consistent with the basin–scale distribution of POC flux and suggestive of a sinking POC &gt; DOC &gt; bacteria transformation of the carbon. To test this hypothesis, we sought to determine whether the very strong spatial and temporal gradients in POC flux in the Arabian Sea would force similar deep–ocean gradients in bacterial variables. On both a within and between–cruise basis, there was variability in bacterial abundance and thymidine incorporation in the deep Arabian Sea, but correspondence was equivocal between these variables and several correlates to export: flux of biogenic carbon from the euphotic zone, state of the monsoon, and proximity to productive coastal upwelling zones. However, when annual mean bacterial abundance at 2,000 m was compared with annual POC flux at that depth, a strong correspondence emerged: high annual flux supported high bacterial abundance (such a correspondence was not found for bacterial productivity). This finding suggests that bathypelagic bacterial abundance responds to the long–term mean input of organic matter and less to episodic inputs. A comparative evaluation of the North Pacific revealed that although the bathypelagic bacteria there showed correspondence to deep POC flux, that variable alone would not account for the wide meridional variations in bacterial abundance that have been reported.