Particulate Organic Carbon Export Research Articles

Control of acid polysaccharide production and 234Th and POC export fluxes by marine organisms

Ratios of particulate organic carbon (POC) to particulate 234Th activities (POC/234Th) in the ocean are used to determine the POC export flux, and thus, the power of the biological pump. In order to understand the main reasons why this ratio frequently varies as a function of depth, size, and kind (suspended vs. sinking particles), we measured vertical profiles in a cold core ring and warm core ring in the Gulf of Mexico in May 2001. Here we show that particulate 234Th/POC ratios in the Gulf of Mexico are positively correlated to the content of different Th(IV)‐binding polysaccharide fractions (uronic acids, total acid polysaccharides, total polysaccharides) in both suspended and sinking particles as well as to prymnesiophyte abundance, but negatively correlated to bacterial production. Variations in acid polysaccharide compounds, produced by both algae and bacteria, but degraded only by bacteria, can account for observed variations in POC/234Th ratios seen in the ocean.

Implication of POC/234Th Ratios in Oceanic Particulate Matter: An Approach to Particle Aggregation

234Th has been widely applied as a tracer of particulate organic carbon (POC) fluxes in the upper ocean. Fundamental to this approach is the determination of 234Th fluxes from water column measurements of the 234Th-238U disequilibria, and the conversion of 234Th flux to POC export, using the measured POC/234Th ratio on particles. As such, POC/234Th ratios are one of the most critical factors in quantifying the carbon export flux in ocean interior when using this approach. However, the POC/234Th ratios show significant temporal and spatial variations, but cannot be predicted at this time. Therefore, it is important to elucidate factors controlling the variations of the POC/234Th ratios. To achieve this purpose, we should understand the chemical interactions between POC and 234Th. In the open ocean, POC/234Th ratios have been determined together with other oceanographic parameters. We examined here the relationship between POC/234Th and primary production. The POC/234Th ratios were linearly related to logarithmic values of primary production. Taken into account the complexation between surface ligand on particulate organic matter (POM) and 234Th, a complexation model suggests that the size of particles adsorbing 234Th is related to primary production; in the equatorial Pacific, the size of particles adsorbing 234Th apparently decreases with increasing primary production, whereas opposite phenomenon occurs in the North Atlantic. Since the POC/234Th ratios were determined in filtered particulate matter, this finding suggests that aggregation of small particles would be dominant in the equatorial Pacific, which can be explained by a chemical aggregation model.

Seasonal variation in production, retention and export of zooplankton faecal pellets in the marginal ice zone and central Barents Sea

Vertical distribution and sedimentation of faecal pellets (FPs) as well as the production rates of FPs by larger copepods were studied during three cruises to the Barents Sea in March and May 1998, and July 1999. Three to five 24-h stations were selected during each cruise, where at least one main station was located in Arctic water (ArW), one in the polar front region (PF) and one in Atlantic water (AW). A winter scenario was encountered in March with very low concentrations of FPs in the water column, most of the time well below 0.1 mg faecal pellet carbon (FPC) per cubic meter, and with sedimentation rates below 3 mg FPC m −2 day −1 at all depths and stations. Increased concentrations of FPs were observed in May and the maximum biomass of FPs was found in ArW (4.8 mg FPC m −3). This was reflected in high vertical flux of FPs in the ArW, just below the chlorophyll maximum (∼150 mg FPC m −2 day −1). FPC sedimentation explained ∼40% of the total particulate organic carbon (POC) export at 90 m depth at this station. Copepod FP production was moderate to high in May, reflecting favourable feeding conditions. Large spatial variation in the estimated retention potential of FPs was observed, ranging from 96% in AW to ∼40% in the PF region. The July scenario did not differ very much from that observed in May. The lowest suspended concentrations and vertical flux of FPs were again observed in AW, in spite of the high pellet-production rate. FPC explained 34% of the POC export out of the upper layer in ArW, 40% in the PF region, but only 8% in AW. The calculated retention potential of 70% of the produced copepod FPs in AW decreased to 60% and 47% in the PF region and ArW, respectively. Krill FPs comprised a significant fraction of both suspended and sedimented FPC throughout the central Barents Sea. The data show that spatial and temporal variations in the FP “retention filter” are extensive and evidently of importance for the patterns of vertical flux of organic matter and the regulation of pelagic–benthic coupling in the Barents Sea.

Workshop highlights iron dynamics in ocean carbon cycle

The role of iron in regulating the flux of carbon through the surface layer of the ocean has become increasingly apparent during the past 15 years. Before that time, the analytical challenges of measuring trace (parts per trillion) iron concentrations from iron ships using gear suspended on an iron wire precluded oceanographers from making accurate measurements. Laboratory experiments were invariably conducted with samples that were seriously contaminated with elevated iron concentrations. We now recognize, through greatly improved methodologies, that iron is a key regulator of phytoplankton primary production throughout the ocean. Small changes in iron concentration may produce large variations in the export of particulate organic carbon from the ocean's sunlit surface layer into deep‐sea sediments. These variations in carbon export may occur over glacial/interglacial cycles at a scale sufficient to influence the flux of carbon dioxide from the atmosphere to the ocean. Such processes have been hypothesized to be an important driver of the changes in atmospheric carbon dioxide concentration that have been recorded in ice cores over the past 400,000 years.

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

Profiles of particulate and dissolved 234Th (t1/2=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.

Biological productivity and carbon cycling in the Arctic Ocean

Primary production, bacterial production, particulate organic carbon fluxes and organic carbon burial rates were quantified during the summer period of 1999 in the Arctic Ocean via14C uptake,3H uptake,234Th/238U disequilibrium and210Pbex dating, respectively. The integrated primary production in the water column was as high as 197 mmolC/(m2 · d) in the Chukchi shelf and was 3.8 mmolC/(m2 · d) in the Canada Basin. These rates are higher than those reported previously. The ratios of bacterial production to primary production in the study region were higher than 0.5, indicating that microbial activity is not depressed but important in cold Arctic waters.234Th/238U disequilibria were evident at the station in the Canada Basin. The presence of significant234Th deficiency suggested that scavenging and removal processes are also important to biogeochemical cycles of trace elements in the Arctic Ocean. Particulate organic carbon export flux was estimated to be 1.0 mmolC/(m · d). Measurements of sediment excess Pb profile in the Chukchi shelf allowed us to estimate the amount of organic carbon buried in the bottom sediment, which ranged from 25 to 35 mmolC/(m2 · d) and represented about 59%-82% of the mean primary production in the euphotic zone. Overall, our results indicated that the Arctic Ocean has active carbon cycling and is not a biological desert as previously believed. Therefore, the Arctic Ocean may play an important role in the global carbon cycle and climate change.

Phytoplankton biomass, production and potential export in the North Water

Abstract The seasonal patterns of phytoplankton biomass and production were determined in the North Water, located between Greenland and Ellesmere Island (Canadian Arctic), in August 1997, April–July 1998, and August–September 1999. The patterns differed among the four defined regions of this large polynya, i.e. North (>77.5°N), East (>75°W), West ( 5 μm) fraction dominated the biomass and production during the bloom. During July, August, and September, biomass and production decreased over the whole region, with the highest biomass, dominated by large cells, occurring in the North. The annual particulate and dissolved phytoplankton production were the highest ever reported for the high Arctic, reaching maximum values of 254 and 123 g C m−2 yr−1, respectively, in the East. Rates in the North and West were considerably lower than in the East (ca. two- and three-fold, respectively). The f-ratios (i.e. ratio of new to total production), derived from the size structure of phytoplankton, were high north of 76°N (0.4–0.7). Regionally, this indicated a high potential export of particulate organic carbon (EPOC) from the phytoplankton community to other trophic compartments and/or downwards in the East (155 g C m−2 yr−1), with lower values in the North and West (i.e. 77 and 42 g C m−2 yr−1, respectively). The seasonal and spatial patterns of EPOC were consistent with independent estimates of potential carbon export. Phytoplankton biomass and production were generally dominated by the large size fraction, whereas EPOC seemed to be dominated by the large size fraction early in the season and by the small size fraction (

Active transport of particulate organic carbon and nitrogen by vertically migrating zooplankton in the Sargasso Sea

Diel vertically migrating zooplankton can contribute significantly to dissolved carbon and nutrient export by respiring and excreting surface-ingested particulate organic matter below the mixed layer. Active export of particulate organic carbon (POC) and particulate organic nitrogen (PON) due to defecation at depth has rarely been considered in export budgets. We measured the gut passage time (GPT) of common migrant species at the Bermuda Atlantic Time-series Study (BATS) site, using the gut fluorescence method, to determine whether GPT is slow enough to allow active export of POC and PON to depth. Mean GPT for the copepods Pleuromamma xiphias and Euchirella messinensis was 191 and 114 min, respectively, and for the euphausiids Thysanopoda aequalis and Euphausia brevis (analyzed together) was 41 min, exceeding previously reported GPT for non- migrating zooplankton by a factor of 6. Between 18 and 81% of the initial gut pigment was retained in zooplankton guts upon descent below a mixed layer of 150 m. By comparing pigment ingestion rates (gut fluorescence technique) with total ingestion rates (CHN analysis of fecal material), we esti- mated that 71 to 85% of the migrant diet originated from non-plant material, which we included in our estimates of active POC/PON export. We applied the mean weight-specific active POC/PON export rate for the species examined to the total migratory zooplankton biomass using data from the BATS zooplankton time-series. Mean active POC (PON) flux at BATS was 0.94 mg C m -2 d -1 (0.18 mg N m -2 d -1 ) and the maximum was 5.27 mg C m -2 d -1 (1.02 mg N m -2 d -1 ), corresponding to a mean of 3% (4%) and a maximum of 18% (20%) of the mean gravitational POC (PON) flux measured by sediment traps at 150 m. Migrants also contributed significantly to passive flux via production of sinking fecal pellets during the night in surface waters. This passive flux exceeded active POC flux by ~10-fold. Freshly released feces by migrators at depth could be a valuable food source for mesopelagic organisms, in contrast to feces produced in surface waters which decompose while settling through the water column.

The Southern Ocean's biological pump during the Last Glacial Maximum

Ice core records from Antarctica show large (∼80 ppm) and regular climate-related changes in atmospheric CO2, with minimum values during glacial periods and maximum values during peak interglacials. The suggested role of the Southern Ocean in driving these changes is based on either the potential for increased utilization of surface nutrients or the potential for decreased ventilation of deep waters during glacial times. Several recent studies have invoked increased stratification of the Southern Ocean to explain lower glacial atmospheric CO2 levels in terms of reduced exchange of CO2 between the deep sea and the atmosphere. A northward displacement and/or substantial weakening of the westerly winds during glacial periods are implicit in the scenarios that invoke enhanced stratification. However, both circulation models and proxy results argue against a weakening of the westerlies. In fact, the mean flow of the Antarctic Circumpolar Current and wind-driven upwelling during the Last Glacial Maximum (LGM) are thought to be at least as vigorous as those which exist today. Given these boundary conditions, we offer two (competing) scenarios for ecosystem structure and export production of the glacial Southern Ocean. The first scenario satisfies all proxy records for nutrient utilization and phytoplankton growth rate, and requires increased (relative to today) nitrate utilization south of the Antarctic Polar Front (APF) by phytoplankton other than diatoms, together with a shift in the zone of maximum diatom growth from south (interglacials) to north (glacials) of the APF. The second scenario has reduced growth of all phytoplankton species south of the APF during glacials, and a shift in the zone of maximum export production to the north of the Polar Front. The principal weakness of the first scenario is that there is little sedimentary evidence to support the increased export of particulate organic carbon required by the inferred increase in nitrate utilization south of the APF. The principal weakness of the second scenario is that it requires a change in the isotopic fractionation of nitrogen by phytoplankton growing south of the Polar Front for reasons presently unknown. The Southern Ocean's biological pump at the LGM has yet to be characterized unequivocally. On observe dans les carottes de glace provenant du continent Antarctique des cycles de grandes amplitudes (∼80 ppm) dans la concentration atmosphérique du CO2, qui sont reliés aux cycles climatiques, avec des valeurs minimum durant les époques glaciaires, et des valeurs maximum durant les époques inter-glaciaires. Un rôle de l’océan austral dans ces cycles climatiques a été suggéré, compte tenu du grand surplus de nutriments dans les eaux de surface, et de la possibilité que les eaux profondes auraient été moins ventilées pendant les époques glaciaires. Dans les dernières années, plusieurs études ont associées les concentrations moins importantes de CO2 dans l’atmosphère durant les époques glaciaires à une stratification plus importante de l’océan austral, ce qui aurait réduit l’échange de CO2 entre l’atmosphère et l’océan profond. En même temps, on remarque un déplacement vers le nord, et/ou une réduction importante de la force des vents d’ouest durant les époques glaciaires. Cependant, des simulations numériques, ainsi que des données proxys, ne supportent pas l’hypothèse d’une réduction des vents d’ouest. Au contraire, pendant le Dernier Maximum Glaciaire (DMG), le forcage des vents auraient entrainé un courant antarctique circumpolaire (ACC), ainsi que des courants ascendants au moins aussi vigoureux que ceux d’aujourd’hui. En tenant compte de ces conditions aux bords, nous proposons deux scénarios possibles de l’écologie et de l’éxportation de carbone dans l’océan austral pendant la dernière époque glaciaire. Le premier scénario est en accord avec toutes les données proxys ayant attrait a l’utilisation des éléments nutritifs, ainsi que le taux de croissance des phytoplanctons. Ce scénario implique que l’utilisation de nitrate par les phytoplanctons non-diatomés est plus importante durant les époques glaciaires qu’aujourd’hui, et aussi que la zone préférée de diatomées soit au sud du Front Polaire Antarctique (FPA), durant les époques interglaciaires, et au nord du FPA durant les époques glaciaires. Le deuxième scénario, par contre, implique une réduction de la productivité de toutes les espèces de phytoplanctons au sud du FPA durant le DMG, ainsi qu’un déplacement de la zone maximale d’éxportation de carbone au nord du PFA. Le principal point faible du premier scénario est lié à l’absence de données suggérant une augmentation dans l’exportation de carbone organique particulaire, dans les sédiments au sud du PFA. Le principal point faible du deuxième scénario est qu’il requière un fractionnement isotopique de l’azote par les phytoplanctons, au sud du PFA, durant la dernière époque glaciaire qui est différent de celui d’aujourd’hui. Bref, on est toujours à la recherche d’une caracterisation non-équivoque de la “pompe biologique” dans l’océan austral durant le DMG.

234Th as a tracer of particle flux and POC export in the northern North Sea during a coccolithophore bloom

Abstract A multidisciplinary cruise to the northern North Sea was undertaken during June 1999 to carry out a Lagrangian study of an Emiliania huxleyi bloom. During this experiment, the naturally occurring radionuclide, 234 Th, was measured in the water column to estimate particle fluxes and resulting residence times. Simple steady state modelling of changes in 234 Th activity suggests an increase in scavenging efficiency over an 8 day period. The steady state 234 Th flux ranges from 357 to a maximum of 1390 dpm m −2 d −1 on the 25th June 1999, with a corresponding particulate organic carbon (POC) flux from 9.5–48 mmol C m −2 d −1 , estimated from POC/ 234 Th ratios on filtered particulate material. Scavenging of the 234 Th by the bloom occurs immediately. The maximum POC fluxes follow the peak in primary production and the maximum coccolithophore cell abundance within a few days. The 234 Th and POC fluxes illustrate the rapid response of export with biological activity.

Vertical budgets for organic carbon and biogenic silica in the Pacific sector of the Southern Ocean, 1996–1998

Abstract During the 1996–1998 Antarctic Environment and Southern Ocean Process Study (AESOPS), a component of US JGOFS, we obtained seasonal or longer-term data sets on the rates of production, vertical transport, remineralization and burial of particulate organic carbon (POC) and biogenic silica (BSiO2) in the Southern Ocean at 170°W between 55°S and 68°S. The AESOPS data records enable us to construct vertical C and Si budgets for the water column and upper sediments, with all estimates derived from direct measurement of the relevant fluxes. We constructed annual C and Si budgets for each of four ecologically distinct zonal bands within the system. For both POC and BSiO2 the greatest annual delivery to the sea floor (∼200 and 1400 mmol m−2 yr−1, respectively) and burial (∼6 and 160 mmol m−2 yr−1, respectively) were observed in the southern Antarctic Circumpolar Current (ACC) between 61.5°S and 65.5°S. That pattern is consistent with our observation that a diatom bloom propagated southward through the southern ACC during the spring and summer of 1997–1998, following the receding ice edge, and that this bloom was the main source of both POC and BSiO2 in the system on an annual basis. In the other zones the annual fluxes of POC and BSiO2 to the sea floor ranged from 19% to 67% of those in the zone traversed by the summer diatom bloom. The higher benthic fluxes of both POC and BSiO2 in the southern ACC imply that blooms similar to the one we observed in 1997–1998 occur commonly in the southern ACC, and that their high-productivity signature is transmitted to the sea floor. The data show preferential preservation of BSiO2 over POC throughout the water column and upper seabed. In the four zonal bands we consider, BSiO2 and POC are produced in mole ratios of 0.1–0.4, exported from the upper 100 m in ratios of 0.2–0.6, arrive at 1000 m in ratios of 1.5–4.5, reach the sea floor in ratios of 2.2–7.6, and are buried in ratios of 11.6–28. Despite the preferential preservation of BSiO2, accumulation of opal-rich sediments beneath the ACC does not result from unusually efficient preservation of siliceous material. The estimated BSiO2 preservation efficiency (burial-production) ranges from 1.2% to 5.5%, indistinguishable from the global average of 3%. Instead, opal-rich sediment accumulation in this region reflects very high annual rates of BSiO2 production in surface waters, along with very low accumulation rates of other sedimentary components (e.g., CaCO3 and detrital material). The observed high ratios of BSiO2 production to POC production in surface waters are consistent with the known tendency for the Si/C ratio of diatoms to increase when [Fe] is low. If greater Fe availability during the last glacial maximum permitted diatoms in the Southern Ocean to grow with lower, more normal Si/C ratios, export of diatom-produced POC could have occurred at 2–3 times its present rate. A corresponding increase in opal export or opal sediment accumulation is unlikely because silicic acid is almost totally depleted north of 65°S under present conditions. Thus, even large increases in POC production and export during glacial periods would not be reflected in the opal accumulation record.

Moored sediment trap measurements of carbon export in the Subantarctic and Polar Frontal zones of the Southern Ocean, south of Australia

Sediment trap moorings were deployed from September 21, 1997 through February 21, 1998 at three locations south of Australia along 140°E: at ∼47°S in the central Subantarctic Zone (SAZ) with traps at 1060, 2050, and 3850 m depth, at ∼51°S in the Subantarctic Front with one trap at 3080 m, and at ∼54°S in the Polar Frontal Zone (PFZ) with traps at 830 and 1580 m. Particle fluxes were high at all the sites (18–32 g m−2 yr−1 total mass and 0.5–1.4 g organic carbon m−2 yr−1 at −1000 m, assuming minimal flux outside the sampled summer period). These values are similar to other Southern Ocean results and to the median estimated for the global ocean by Lampitt and Antia [1997], and emphasize that the Southern Ocean exports considerable carbon to the deep sea despite its “high‐nutrient, low chlorophyll” characteristics. The SAZ site was dominated by carbonate (>50% of total mass) and the PFZ site by biogenic silica (>50% of total mass). Both sites exhibited high export in spring and late summer, with an intervening low flux period in December. For the 153 day collection period, particulate organic carbon export was somewhat higher in all the traps in the SAZ (range 0.57–0.84 gC m−2) than in the PFZ (range 0.31–0.53), with an intermediate value observed at the SAF (0.60). The fraction of surface organic carbon export (estimated from seasonal nutrient depletion, Lourey and Trull [2001]) reaching 1000 m was indistinguishable in the SAZ and PFZ, despite different algal communities.

Export of particulate organic carbon estimated from234Th-238U disequilibria and its temporal variation in the South China Sea

Activities of234Th and nutrient concentrations in the upper 500 m water column were measured at a time-series station in the South China Sea over a time span of 12.3 d. Results showed a reduction of dissolved234Th and an overall increase of particulate234Th during the period. Meanwhile, activities of total234Th kept fairly constant, implying rapid transformation of234Th between the dissolved and particulate forms. Vertical profiles of total234Th showed evident deficit of234Th relative to238U in the upper 500 m water column. Using an irreversible steady-state model of thorium scavenging, export fluxes of particulate organic carbon (POC) corresponding to time pointsT 1 andT 2 were estimated to be 46.5 and 13.1 mmolC · m−2 · d−1. It was demonstrated that the estimation of POC export was greatly dependent on the POC/234ThP ratios and the bias caused by the different models of234Th scavenging, however, was considered to be of minor importance.

Downward particle fluxes within different productivity regimes off the Mauritanian upwelling zone (EUMELI program)

A 2-yr record of downward particle flux was obtained with moored sediment traps at several depths of the water column in two regions characterized by different primary production levels (mesotrophic and oligotrophic) of the eastern subtropical North Atlantic Ocean. Particle fluxes, of ∼71–78% biogenic origin (i.e. consisting of CaCO 3, organic matter and opal) on average, decrease about six-fold from the mesotrophic site (highest fluxes in the North Atlantic) nearer the Mauritanian margin (18°30′N, 21°00′W) to the remote, open-ocean, oligotrophic site (21°00′N, 31°00′W). This decrease largely reflects the difference in total primary production between the two sites, from ∼260 to ∼110 g organic C m −2 yr −1. At both sites, temporal variability of the downward particle flux seems to be linked to westward surface currents, which are likely to transport seaward biomass-rich water masses from regions nearer the coast. The influence of coastal upwelling is marked at the mesotrophic site. The large differences between the 1991 and 1992 records at that site, where carbon export is large, underscore the interest of long-term studies for export budget estimates. The different productivity regimes at the two sites seem to induce contrasting downward modes of transport of the particulate matter, as shown in particular by the faster settling rates and the higher E ratio (particulate organic carbon export versus total primary production) estimated at the mesotrophic site.

Particle transformations and export flux during an in situ iron‐stimulated algal bloom in the Southern Ocean

During the first Southern Ocean Iron RElease Experiment (SOIREE), a suite of biogeochemical measurements (water column 234Th and δ13Corg inventories, particle fluxes from sediment traps, phytoplankton sinking rates) were undertaken to test the hypothesis that the vertical export of particulate organic carbon (POC) is enhanced due to iron‐induced increases in phytoplankton production. During the 13‐days that the SOIREE bloom was monitored, export fluxes within the iron‐fertilised patch were not substantially different to those in waters outside the bloom. On days 11–13, iron enrichment may have caused particle transformations that could lead to elevated future export via particle aggregation and/or diatom chain formation. The unknown time‐lag between increased production and export, the longevity of the SOIREE bloom, and the absence of nutrient limitation over days 1–13, however, prohibit prediction of any iron‐induced export. This conclusion highlights the difficulties of fully testing the “Iron Hypothesis” and for evaluating the implications for global climate change.

Investigating the carbon cycle in the Gulf of Maine using the natural tracer thorium 234

The naturally occurring radionuclide 234Th (t1/2 = 24.1 days) was used to examine the organic carbon cycle in the Gulf of Maine. A seasonal study (March, June, and September 1995) was conducted in the central Gulf of Maine in Wilkinson and Jordan Basins, and a regional survey, which included the Scotian Shelf, was conducted during August‐September 1997. Particulate organic carbon (POC) export (particulate export production) was estimated from a three‐dimensional steady state model of 234Th flux combined with measurements of the POC/234Th ratio on >53‐μm particles. The POC export for this region was seasonally variable; average values ranged from 15 to 34 mmol C m−2 d−1, between 11% and 25% of the regionally integrated primary production. The Gulf of Maine was a net source (to the Mid‐Atlantic Bight) of dissolved organic carbon (2.4 mmol C m−2 d−1) amounting to ∼2% of carbon uptake rates. Organic carbon burial in the sediments was a minor fraction of the primary production, averaging 1.6 mmol C m−2 d−1. Though only a fraction of total export production was buried in the sediments, these estimates close the budget for organic carbon in the Gulf of Maine. An implication is that off‐shelf export may not be as important as previously estimated in this shelf region.

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.

Seasonal variability of the advective transport of particulate matter and organic carbon in the Gulf of Lion (NW Mediterranean)

Nine hydrographic cruises were performed on the Gulf of Lion continental margin between June 1993 and July 1996. These observations are analysed to quantify the fluxes of particulate matter and organic carbon transported along the slope by the Northern Current and to characterise their seasonal variability. Concentration of particulate matter and organic carbon are derived from light-transmission data and water sample analyses. The circulation is estimated from the geostrophic current field. The uncertainty on the transport estimate, related to the error on the prediction of particle concentrations from light-transmission data and the error on velocities, is assessed. The particulate matter inflow entering the Gulf of Lion off Marseille is comparable to the Rhône River input and varies seasonally with a maximum transport between autumn and spring. These modifications result from variations of the water flux rather than variations of the particulate matter concentration. Residual transports of particulate matter and organic carbon across the entire Gulf of Lion are calculated for two cruises enclosing the domain that were performed in February 1995 and July 1996. The particulate matter budgets indicate a larger export from the shelf to deep ocean in February 1995 (110 ± 20 kg s –1) than in July 1996 (25 ± 18 kg s –1). Likewise, the mean particulate organic carbon export is 12.8 ± 0.5 kg s –1 in February 1995 and 0.8 ± 0.2 kg s –1 in July 1996. This winter increase is due to larger allochthonous and autochthonous inputs and also to enhanced shelf–slope exchange processes, in particular the cascading of cold water from the shelf. The export of particulate matter by the horizontal currents is moreover two orders of magnitude larger than the vertical particulate fluxes measured at the same time with sediment traps on the continental slope.

Modeling the bloom evolution and carbon flows during SOIREE: Implications for future in situ iron-enrichments in the Southern Ocean

The impact of a mesoscale in situ iron-enrichment experiment (SOIREE) on the planktonic ecosystem and biological pump in the Australasian-Pacic sector of the Southern Ocean was investigated through model simulations over a period of 60-d following an initial iron infusion. For this purpose we used a revised version of the biogeochemical SWAMCO model (Lancelot et al., 2000), which describes the cycling of C, N, P, Si, Fe through aggregated chemical and biological components of the planktonic ecosystem in the high nitrate low chlorophyll (HNLC) waters of the Southern Ocean. Model runs were conducted for both the iron-fertilized waters and the surrounding HNLC waters, using in situ meteorological forcing. Validation was performed by comparing model predictions with observations recorded during the 13-d site occupation of SOIREE. Considerable agreement was found for the magnitude and temporal trends in most chemical and biological variables (the microbial food web excepted). Comparison of simulations run for 13- and 60-d showed that the e!ects of iron fertilization on the biota were incomplete over the 13-d monitoring of the SOIREE bloom. The model results indicate that after the vessel departed the SOIREE site there were further iron-mediated increases in properties such as phytoplankton biomass, production, export production, and uptake of atmospheric CO , which peaked 20}30 days after the initial iron infusion. Based on model simulations, the increase in net carbon production at the scale of the fertilized patch (assuming an area of 150 km) was estimated to 9725 t C by day 60. Much of this production accumulated in the upper ocean, so that the predicted downward export of particulate organic carbon (POC) only represented 22% of the accumulated C in the upper ocean. Further model runs that implemented improved parameterization of diatom sedimentation (i.e. including iron-mediated diatom sinking rate, diatom chain-forming and aggregation) suggested that the downward POC #ux predicted by the standard run might have been underestimated by a factor

Upper ocean export of particulate organic carbon and biogenic silica in the Southern Ocean along 170°W

Abstract Upper-ocean fluxes of particulate organic carbon (POC) and biogenic silica (bSi) are calculated from four US JGOFS cruises along 170°W using a thorium-234 based approach. Both POC and bSi fluxes exhibit large variability vs. latitude during the seasonal progression of diatom dominated blooms. POC fluxes at 100 m of up to 50 mmol C m −2 d −1 are found late in the bloom, and farthest south near the Ross Sea Gyre. Biogenic Si fluxes also peak late in the bloom as high as 15 mmol Si m −2 d −1 , but this flux peak occurs at a different latitude, just south of the Antarctic Polar Front (APF), which is centered around 60°S along this cruise track. The ratios of both POC and bSi export relative to their production rates are large, suggesting an efficient biological pump at these latitudes. The highest relative bSi/POC flux ratios at 100 m are found just south of the APF, coincident with a bSi/POC flux peak seen in 1000 m traps during this same program by Deep-Sea Research II (Honjo et al., Deep-Sea Research II 47, 3521–3548). These data suggest that efficient export at these latitudes can support the high accumulation rates of bSi found in the sediments under and south of the APF, despite the generally low biomass and productivity levels in this region.