Seasonal deposition and reworking at the sediment-water interface on the northwestern Iberian margin

Seabed distributions of ²³⁴Th excess (Th_xs) were determined in the upper centimetres of 38 sediment cores from the north-western Iberian Margin, sampled from 41–44°N and from 9–12°E during five OMEX II cruises. Three main areas, a northern, and at 42°38 and 42°N, were investigated during representative seasons (winter, spring and summer). Low ²³⁴Th_xs activities in summer 1998 (18–252 Bq per kg) were similar to those measured in summer 1997. In winter ²³⁴Th also showed moderate excess. The highest values were observed in spring with surface ²³⁴Th_xs values up to 402 Bq kg⁻¹. Maximum penetration depths of ²³⁴Th_xs ranged from a few mm to 3 cm. ²³⁴Th_xs activities always showed a smooth decrease with depth, without any evidence of non-local mixing. Thus particle mixing on a short time scale can be described as an eddy diffusive process, and bioturbation rates, calculated on this basis, range from 0.02 to 3.07 cm² per year. Data (activities, inventories, bioturbation rates) are discussed in order to relate the observed surface and down-core variations to spatial and seasonal trends. Using ²³⁴Th_xs data in sediment as a substitute for sediment trap estimates, particle fluxes were calculated from ²³⁴Th_xs inventories. The range of ²³⁴Th-derived particle fluxes for the north-western Iberian Margin is 16–1418 mg.m⁻².d⁻¹. Mean values indicate a gradual decrease of mass fluxes from the shelf to the open ocean. On a 100-day scale, the northern area (43–44°N) represents a low sedimentation regime. Further south, around 42°–43°N, particle inputs are more important. On the middle slope, around 1000 to 2000 m depth, high inventories and bioturbation rates indicate enhanced, and probably organic-rich, particle fluxes to the seafloor, particularly in spring.     

Particulate barium fluxes and export production in the northwestern Mediterranean

Biogenic barium, mostly in the barite (BaSO₄) form, has been proposed as a tracer for export production in the ocean. Here we report on biogenic barium (Ba_xs) and particulate organic carbon (POC) fluxes from sediment traps deployed at the DYFAMED site in the Northwestern Mediterranean Sea. Ba_xs fluxes display average values of 37 ± 45 and 50 ± 58 μg/m²/d at 200 and 1000 m respectively, and are linearly correlated to POC fluxes (mean values of 7.9 ± 9.3 and 6.8 ± 6.8 mg C/m²/d at 200 and 1000 m). Export production estimates, calculated using published Ba_xs- or POC-based algorithms, all fall below or close to the lower limit of potential export values proposed in the literature. This work clearly demonstrates the usefulness of Ba_xs as a tracer of oceanic export production in the Northwestern Mediterranean Sea. However, development of a quantitative export production proxy requires a clear understanding of the underlying cause(s) for the observed spatial variations in the relationship between Ba_xs and POC fluxes. The present study confirms that the processes leading to barite formation differ between margin and open-ocean sites and probably account for much of the regional variability in the POC/Ba_xs ratio.     

The influence of a mature cyclonic eddy on particle export in the lee of Hawaii

Mesoscale eddies may enhance primary production (PP) in the open ocean by bringing nutrient-rich deep waters into the euphotic zone, potentially leading to increased transport of particles to depth. This hypothesis remains controversial, however, due to a paucity of direct particle export measurements. In this study, we investigated particle dynamics using ²³⁴Th–²³⁸U disequilibria within a mesoscale cold-core eddy, Cyclone Opal, which formed in the lee of the Hawaiian Islands. ²³⁴Th samples were collected along two transects across Cyclone Opal as well as during a time-series within the eddy core during a decaying diatom bloom. Particulate carbon (PC), particulate nitrogen (PN) and biogenic silica (bSiO₂) fluxes at 150 m varied spatially and temporally within the eddy and strongly depended on the ²³⁴Th model formulation used (e.g., steady state versus non-steady state, inclusion of upwelling, etc.). Particle fluxes estimated from a steady state model assuming an upwelling rate of 2 m day⁻¹ yielded the best fit to sediment-trap data. These ²³⁴Th-derived particle fluxes ranged from 332±14 to 1719±53 μmol C m⁻² day⁻¹, 27±3 to 114±12 μmol N m⁻² day⁻¹, and 33±20 to 309±73 μmol Si m⁻² day⁻¹. Although PP rates within Cyclone Opal were elevated by a factor of 2–3, PC and PN fluxes were the same, within error, inside and outside of Cyclone Opal. The ratio of PC export to PP remained surprisingly low at <0.03 and similar to those measured in surrounding waters. In contrast, bSiO₂ fluxes within the eddy core were three times higher. Detailed analyses of ²³⁴Th depth profiles consistently showed excess ²³⁴Th at 100–175 m, associated with the remineralization and possible accumulation of suspended and dissolved organic matter from the surface. We suggest that strong microzooplankton grazing facilitated particulate organic matter recycling and resulted in the export of empty diatom frustules. Thus, while eddies may increase PP, they do not necessarily increase PC and PN export to deep waters. This may be a general characteristic of wind-driven cyclonic eddies of the North Pacific Subtropical Gyre and suggests that eddies may preferentially act as a silica pump, thereby playing an important role in promoting silicic-acid limitation in the region.     

Rapid lateral particle transport in the Argentine Basin: Molecular 14C and 230Thxs evidence

Recent studies have revealed that lateral transport and focusing of particles strongly influences the depositional patterns of organic matter in marine sediments. Transport can occur in the water column prior to initial deposition or following sediment re-suspension. In both cases, fine-grained particles and organic-rich aggregates are more susceptible to lateral transport than coarse-grained particles (e.g., foraminiferal tests) because of the slower sinking velocities of the former. This may lead to spatial and, in the case of redistribution of re-suspended sediments, temporal decoupling of organic matter from coarser sediment constituents. Prior studies from the Argentine Basin have yielded evidence that suspended particles are displaced significant distances (100–1000 km) northward and downslope by strong surface and/or bottom currents. These transport processes result in anomalously cold alkenone-derived sea-surface temperature (SST) estimates (up to 6 °C colder than measured SST) and in the presence of frustules of Antarctic diatom species in surface sediments from this area. Here we examine advective transport processes through combined measurements of compound-specific radiocarbon ages of marine phytoplankton-derived biomarkers (alkenones) from core tops and excess ²³⁰Th (²³⁰Th_xs)-derived focusing factors for late Holocene sediments from the Argentine Basin. On the continental slope, we observe ²³⁰Th_xs-based focusing factors of 1.4–3.2 at sites where alkenone-based SST estimates were 4–6 °C colder than measured values. In contrast, alkenone radiocarbon data suggest coeval deposition of marine biomarkers and planktic foraminifera, as alkenones in core tops were younger than, or similar in age to, foraminifera. We therefore infer that the transport processes leading to the lateral displacement of these sediment components are rapid, and hence probably occur in the upper water column (<1500 m).     

Impact of the Mediterranean Outflow Water on particle dynamics in intermediate waters of the Northeast Atlantic, as revealed by Th and Th

Over the last decade ²³⁴Th has become increasingly used to study particle transport in the ocean on a timescale of weeks. The application of ²³⁴Th is mainly focused on the determination of particle and associated carbon fluxes from oceanic surface water. However, ²³⁴Th is also suitable for investigating particle dynamic from the upper ocean down to interface sediments, as illustrated by the present work which reports unexpected behavior of ²³⁴Th in intermediate waters associated with the Mediterranean Outflow Water (MOW). Concentration profiles of dissolved ²³⁸U and ²²⁸Ra, and dissolved and particulate ²³⁴Th and ²²⁸Th were measured in the Mediterranean Outflow Water (MOW) near the Gibraltar Straits and at two sites (36°30′N–15°35′W, Nicole; 36°27′N–10°35′W, Yseult) which had hydrographic characteristics of Meddies, i.e. MOW that propagates as eddies in the Northeastern Atlantic at intermediate depths.There are marked differences in the distribution of thorium between MOW and the surrounding Atlantic waters. At the youngest Meddy Nicole salinity maximum at 1000 m depth, ²³⁴Th(total) : ²³⁸U and ²²⁸Th(total) : ²²⁸Ra activity ratios are significantly lower than radioactive equilibrium, indicating an unusual deficit of short half-life thorium nuclides. This implies an export of thorium, presumably on particles, from intermediate Meddy Nicole waters. This process is supported by an increase of particulate thorium fluxes measured in sediment traps deployed for two weeks above and within Meddy Nicole. In contrast, offshore Meddy Yseult has more typical profiles of both thorium nuclides that are nearly in equilibrium with their parents. These results indicate that at intermediate depths, the presence of MOW affects the exchange of reactive elements between particles and dissolved forms and enhances the downward flux of particles from intermediate waters in the Northeast Atlantic.     

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

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

Radiochemical studies of deep-sea manganese nodules

Five manganese nodules collected from the Northern Mid - Pacific have been analysed for their U and Th isotopes.lt is found that the enrichment of thorium compared to uranium is universal in deep- sea manganese, nodules and in the top sides of the nodules there are large excesses of Th and Pa decreasing exponentially with depth. For each of the nodules, the concordant growth rates are obtained from three different methods: 230Thex 230Thex/232Th and 231Paex.The growth rates of manganese nodules are closely related to the chemical compositions of the nodules and the types of the underlying sediments. The growth rates of five nodules are determined to be in the range of 0.79-7.4 mm/106a, in agreement with those predicted from the chemical compositions of the nodules. By the comparison of the extrapolated 230Thex,230Thex/232 Th and 231PaeX data from the top and bottom surfaces of the nodule from Site M21 yields, the nodule turnover time is (9.83-13.7)×104a.     

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

The recently developed 2–5 L small-volume MnO₂ 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 MnO₂ 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 MnO₂ 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 KMnO₄ and 0.1 mg MnCl₂·4H₂O 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.     

Postdepositional injections of uranium-rich solutions into East Pacific Rise sediments

An 8.5 m long apparently undisturbed core from a hilltop on the crest of the East Pacific Rise has uranium and thorium isotope distributions that are very unusual. The core is very poor in ²³²Th, and very rich in U, particularly at the 500-cm level, where a value of about 150 ppm is reached. At the same depth the ²³⁰Th_xs reaches very large negative values. These facts could be accounted for if one assumes that solutions rich in U and poor in Th had been postdepositionally injected into the sediments about 90,000–110,000 years ago. The top of the sediment received much of its U from seawater, judging from the ${}^{234}\text{U}{}^{238}\text{U}$ ratio. Possibly carbonate rich solutions were the carriers of the injected uranium.     

Uncertainties in the preparation of Ra Mn fiber standards

Delayed coincidence counters (RaDeCC), used for measuring ²²³Ra and ²²⁴Ra preconcentrated from water onto MnO₂-impregnated acrylic fiber (“Mn-fiber”), require a standard Mn-fiber column that has a precisely known activity of ²²⁴Ra for calibration. This may be done by adding an aged ²²⁸Th standard solution to adsorb both ²²⁸Th and its daughter ²²⁴Ra quantitatively onto a Mn fiber. We used both seawater and deionized water (DIW) for testing the adsorption efficiency of Th and Ra onto Mn fibers. Our experimental results show that more than 50% of thorium (²³²Th and ²²⁸Th) breaks through the Mn-fiber column when DIW is used as a medium. However, near quantitative recoveries are obtained if filtered (0.45 μm) seawater is used to prepare the standard. In the case of pure DIW, the pH (initial pH  5.3) rises to > 10 after passing through the column while seawater (initial pH  7.8) changes to  7.2. Thus, the lack of thorium adsorption in DIW may be attributed to this huge increase of pH and the consequent formation of Th(OH)₄ and polyhydroxyl colloids. Based on these observations, we recommend that one should use either artificial seawater or natural seawater (which has negligible ²²⁴Ra and ²²⁸Th) as a loading solution after 0.45 μm filtration. In addition, the thorium adsorption efficiency should be confirmed either by thorium analysis of the effluent solution or long-term monitoring of the supported ²²⁴Ra on the Mn fiber using the RaDeCC. Similar cautions are likely necessary for making ²²³Ra standards by adsorption of ²²⁷Ac onto Mn fibers.     

Sinking fluxes of particulate U-Th radionuclides in the East Sea (Sea of Japan)

A record of radionuclide fluxes at a deep marginal sea of the Northwest Pacific Ocean (39°40′N 132°24′ E, Japan Basin, East Sea/Sea of Japan) was obtained from analysis of a 1-year continuous collection of sediment-trap samples. The trap was placed at a depth of 2800 m, 500 m above the sea floor, and the samples were recovered at the end of one year. Concentrations of ²³⁸U, ²³⁴U, ²³²Th, ²³⁰Th and ²²⁸Th were measured in the trapped material. All of the radionuclide fluxes showed seasonal variations that were in phase with the variations in total particle flux, which were shown in earlier work to be closely tied to the primary production in the surface water. The formation of authigenic ²³⁸U appears to be less than in other open ocean regimes. The residence time of particulate thorium isotopes were of the order of a year, resulting in an average settling rate of a meter per day. Comparison of the measured ²³⁰Th_ex flux with that of the theoretical production in the overlying water column yielded about 40% surplus of ²³⁰Th, indicating that lateral advection contributes ²³⁰Th to this sampling site.     

Particle fluxes associated with mesoscale eddies in the Sargasso Sea

We examined the impact of a cyclonic eddy and mode-water eddy on particle flux in the Sargasso Sea. The primary method used to quantify flux was based on measurements of the natural radionuclide, ²³⁴Th, and these flux estimates were compared to results from sediment traps in both eddies, and a ²¹⁰Po/²¹⁰Pb flux method in the mode-water eddy. Particulate organic carbon (POC) fluxes at 150 m ranged 1–4 mmol C m⁻² d⁻¹ and were comparable between methods, especially considering differences in integration times scales of each approach. Our main conclusion is that relative to summer mean conditions at the Bermuda Atlantic Time-series Study (BATS) site, eddy-driven changes in biogeochemistry did not enhance local POC fluxes during this later, more mature stage of the eddy life cycle (>6 months old). The absence of an enhancement in POC flux puts a constraint on the timing of higher POC flux events, which are thought to have caused the local O₂ minima below each eddy, and must have taken place >2 months prior to our arrival. The mode-water eddy did enhance preferentially diatom biomass in its center, where we estimated a factor of three times higher biogenic Si flux than the BATS summer average. An unexpected finding in the highly depth-resolved ²³⁴Th data sets is narrow layers of particle export and remineralization within the eddy. In particular, a strong excess ²³⁴Th signal is seen below the deep chlorophyll maxima, which we attribute to remineralization of ²³⁴Th-bearing particles. At this depth below the euphotic zone, de novo particle production in the euphotic zone has stopped, yet particle remineralization continues via consumption of labile sinking material by bacteria and/or zooplankton. These data suggest that further study of processes in ocean layers is warranted not only within, but below the euphotic zone.     

Anomalously low alkenone temperatures caused by lateral particle and sediment transport in the Malvinas Current region, western Argentine Basin

We analysed the alkenone unsaturation ratio (U^K′₃₇) in 87 surface sediment samples from the western South Atlantic (5°N–50°S) in order to evaluate its applicability as a paleotemperature tool for this part of the ocean. The measured U^K′₃₇ ratios were converted into temperature using the global core-top calibration of Müller et al. (1998) and compared with annual mean atlas sea-surface temperatures (SSTs) of overlying surface waters. The results reveal a close correspondence (<1.5°C) between atlas and alkenone temperatures for the Western Tropical Atlantic and the Brazil Current region north of 32°S, but deviating low alkenone temperatures by −2° to −6°C are found in the regions of the Brazil–Malvinas Confluence (35–39°S) and the Malvinas Current (41–48°S). From the oceanographic evidence these low U^K′₃₇ values cannot be explained by preferential alkenone production below the mixed layer or during the cold season. Higher nutrient availability and algal growth rates are also unlikely causes. Instead, our results imply that lateral displacement of suspended particles and sediments, caused by strong surface and bottom currents, benthic storms, and downslope processes is responsible for the deviating U^K′₃₇ temperatures. In this way, particles and sediments carrying a cold water U^K′₃₇ signal of coastal or southern origin are transported northward and offshore into areas with warmer surface waters. In the northern Argentine Basin the depth between displaced and unaffected sediments appears to coincide with the boundary between the northward flowing Lower Circumpolar Deep Water (LCDW) and the southward flowing North Atlantic Deep Water (NADW) at about 4000 m.     

Time-series measurements of Th in water column and sediment trap samples from the northwestern Mediterranean Sea

Disequilibrium between ²³⁴Th and ²³⁸U in water column profiles has been used to estimate the settling flux of Th (and, by proxy, of particulate organic carbon); yet potentially major non-steady-state influences on ²³⁴Th profiles are often not able to be considered in estimations of flux. We have compared temporal series of ²³⁴Th distributions in the upper water column at both coastal and deep-water sites in the northwestern Mediterranean Sea to coeval sediment trap records at the same sites. We have used sediment trap records of ²³⁴Th fluxes to predict temporal changes in water column ²³⁴Th deficits and have compared the predicted deficits to those measured to determine whether the time-evolution of the two coincide. At the coastal site (327 m water depth), trends in the two estimates of water column ²³⁴Th deficits are in fairly close agreement over the 1-month deployment during the spring bloom in 1999. In contrast, the pattern of water column ²³⁴Th deficits is poorly predicted by sediment trap records at the deep-water site (DYFAMED, 2300 m water depth) in both 2003 and 2005. In particular, the transition from a mesotrophic to an oligotrophic system, clearly seen in trap fluxes, is not evident in water column ²³⁴Th profiles, which show high-frequency variability. Allowing trapping efficiencies to vary from 100% does not reconcile the differences between trap and water column deficit observations; we conclude that substantial lateral and vertical advective influences must be invoked to account for the differences.Advective influences are potentially greater on ²³⁴Th fluxes derived from water column deficits relative to those obtained from traps because the calculation of deficits in open-ocean settings is dominated by the magnitude of the “dissolved” ²³⁴Th fraction. For observed current velocities of 5–20 cm s⁻¹, in one radioactive mean-life of ²³⁴Th, the water column at the DYFAMED site can reflect ²³⁴Th scavenging produced tens to hundreds of kilometers away. In contrast, most of the ²³⁴Th flux collected in shallow sediment traps at the DFYFAMED site was in the fraction settling >200 m d⁻¹; in effect the sediment trap can integrate the ²³⁴Th flux over distances 40-fold less than water column ²³⁴Th distributions. In some sense, sediment trap and water column sampling for ²³⁴Th provide complementary pictures of ²³⁴Th export. However, because the two methods can be dominated by different processes and are subject to different biases, their comparison must be treated with caution.     

Seasonal evolution of hydrographic properties in the Antarctic circumpolar current at 170°W during 1997–1998

This paper discusses the seasonal evolution of the hydrographic and biogeochemical properties in the Antarctic Circumpolar Current (ACC) during the US Joint Global Ocean Flux (JGOFS) Antarctic Environment and Southern Ocean Process Study (AESOPS) in 1997–1998. The location of the study region south of New Zealand along 170°W was selected based on the zonal orientation and meridional separation of the physical and chemical fronts found in that region. Here we endeavor to describe the seasonal changes of the macronutrients, fluorescence chlorophyll_, particulate organic carbon (POC), and carbon dioxide (CO₂) in the upper 400 m of the ACC during the evolution of the seasonal phytoplankton bloom found in this area. While the ACC has extreme variability in the meridional sense (due to fronts, etc.), it appears to be actually quite uniform in the zonal sense. This is reflected by the fact that a good deal of the seasonal zonal changes in nutrients distributions at 170°W follow a pattern that reflects what would be expected if the changes are associated with seasonal biological productivity. Also at 170°W, the productivity of the upper waters does not appear to be limited by availability of phosphate or nitrate. While there is a significant decrease (or uptake) of inorganic nitrogen, phosphate and silicate associated with the seasonal phytoplankton bloom, none of the nutrients, except perhaps silicate (north of the silicate front) are actually depleted within the euphotic zone. At the end of the growing season, nutrient concentrations rapidly approached their pre-bloom levels. Inspection of the ratios of apparent nutrient drawdown near 64°S suggests N/P apparent drawdowns to have a ratio of 10 and N/Si apparent drawdowns to have a ratio of >4. These ratios suggest a bloom that was dominated by Fe limited diatoms. In addition, the surface water in the Polar Front (PF) and the Antarctic Zone (AZ) just to the south of the PF take up atmospheric CO₂ at a rate 2–3 times as fast as the mean global ocean rate during the summer season but nearly zero during the rest of year. This represents an important process for the transport of atmospheric CO₂ into the deep ocean interior. Finally, the net CO₂ utilization or the net community production during the 2.5 growing months between the initiation of phytoplankton blooms and mid-January increase southward from 1.5 mol C m⁻² at 55°S to 2.2 mol C m⁻² to 65°S across the Polar Frontal Zone (PFZ) into the AZ.     

Dissolved organic carbon in waters of the Polar Frontal Zone of the Atlantic Antarctic in the spring-summer season of 1988–1989

Samples were collected and preserved for onshore measurement of dissolved organic carbon (DOC) in the area extending over the Polar Frontal Zone (PFZ) (26–40°W, 48–52°S) in September-December 1988. The highest concentrations of DOC were found in the pycnocline layer, near the southern boundary of the PFZ and in dynamically active areas, such as meanders and mesoscale eddies, where the DOC concentrations amounted to 6–8 mg l⁻¹. To the north and south of the PFZ, concentration of DOC diminished considerably, as a result of less intensive processes of organic matter production.     

Anomalous Large Scavenging of 230Th and 231Pa Controlled by Particle Composition in the Northwestern North Pacific

We report the role of particle composition and lateral particle movement that influences the oceanic distribution of ²³¹Pa and ²³⁰Th. Settling particles were collected during sediment trap experiments. These and surface sediments were obtained from five stations along 38 to 44°N in the northwestern North Pacific. The high total mass flux and seasonal variations in the marginal area of the western North Pacific are controlled by the supply of lithogenic materials and primary productivity. The high content of the lithogenic material in the settling particles in this area contributes to ²³⁰Th_ex fluxes that exceed the local rate of supply. The lithogenic materials are important as a carrier of ²³⁰Th and contribute to the fractionation between ²³⁰Th and ²³¹Pa in the ocean, as the ²³¹Pa_ex/²³⁰Th_ex ratio in the settling particles decreases with increasing ²³²Th concentration. The ²³¹Pa_ex/²³⁰Th_ex ratio in the settling particles collected in the abyssal basin decreases with water depth, which indicates that lateral transport of the lithogenic particles from the marginal area and/or shallower depth plays an important role in determining the ²³¹Pa_ex/²³⁰Th_ex ratio in a population of settling particles and remineralization. This indicates that lateral redistribution of particles and sediment focusing influence the ²³¹Pa_ex/²³⁰Th_ex ratios in surface sediments. Thus, the observations reported here mean that the use of the sediment ²³¹Pa_ex/²³⁰Th_ex ratio as a paleoproductivity proxy will be problematic in the northwestern North Pacific. This revised version was published online in July 2006 with corrections to the Cover Date.     

High-resolution measurement of Southern Ocean CO2 and O2/Ar by membrane inlet mass spectrometry

This paper evaluates the simultaneous measurement of dissolved gases (CO₂ and O₂/Ar ratios) by membrane inlet mass spectrometry (MIMS) along the 180° meridian in the Southern Ocean. The calibration of pCO₂ measurements by MIMS is reported for the first time using two independent methods of temperature correction. Multiple calibrations and method comparison exercises conducted in the Southern Ocean between New Zealand and the Ross Sea showed that the MIMS method provides pCO₂ measurements that are consistent with those obtained by standard techniques (i.e. headspace equilibrator equipped with a Li–Cor NDIR analyser). The overall MIMS accuracy compared to Li–Cor measurements was 0.8 μatm. The O₂/Ar ratio measurements were calibrated with air-equilibrated seawater standards stored at constant temperature (0 ± 1 °C). The reproducibility of the O₂/Ar standards was better than 0.07% during the 9 days of transect between New Zealand and the Ross Sea.The high frequency, real-time measurements of dissolved gases with MIMS revealed significant small-scale heterogeneity in the distribution of pCO₂ and biologically-induced O₂ supersaturation (ΔO₂/Ar). North of 65°S several prominent thermal fronts influenced CO₂ concentrations, with biological factors also contributing to local variability. In contrast, the spatial variation of pCO₂ in the Ross Sea gyre was almost entirely attributed to the biological utilization of CO_2, with only small temperature effects. This high productivity region showed a strong inverse relationship between pCO₂ and biologically-induced O₂ disequilibria (r² = 0.93). The daily sea air CO₂ flux ranged from − 0.2 mmol/m² in the Northern Sub-Antarctic Front to − 6.4 mmol/m² on the Ross Sea shelves where the maximum CO₂ influx reached values up to − 13.9 mmol/m². This suggests that the Southern Ocean water (south of 58°S) acts as a seasonal sink for atmospheric CO₂ at the time of our field study.     

The POC / Th ratio of settling particles isolated using split flow-thin cell fractionation (SPLITT)

The common assumption that the ratio between particulate organic carbon (POC) and particulate ²³⁴Th obtained from shallow sediment traps and filterable particles are representative of the ratio in the total particle settling flux should be treated with caution in view of well-known biases associated with tethered shallow sediment traps and the decoupling between size and settling velocity of many natural particle regimes. To make progress toward reliably constraining the POC / ²³⁴Th ratio on truly settling particles, we have tested here a settling collection technique designed to remove any hydrodynamic bias; split flow-thin cell fractionation (SPLITT). These first results from a North Sea fjord and an open Baltic Sea time-series station indicates that the POC / ²³⁴Th ratio on the more complete particle-settling spectrum, isolated with SPLITT, was higher than the POC / ²³⁴Th ratio obtained simultaneously from tethered shallow sediment traps in seven out of seven parallel deployments with an average factor of 210%. The POC / ²³⁴Th ratio from the SPLITT was either in the same range or higher than that obtained on filtered “bulk” particles. To explain this novel data we hypothesize that the slowest settling fraction is organic-matter rich and does not strongly complex ²³⁴Th (i.e., high POC / ²³⁴Th). We suggest that this ultra-slow sinking fraction is better collected by SPLITT than with tethered sediment traps because of minimized hydrodynamic bias.This was tested using the ratio of POC / Al as a tracer of detrital mineral-ballast influenced settling velocity. The higher POC / Al ratios in SPLITT samples relative to in traps is consistent with the hypothesis that SPLITT is better suited for collecting also the slow-settling component of sinking particles. This important slow-settling component appears to here consist primarily of non-APS/TEP components of plankton exudates or other less-strongly ²³⁴Th-complexing organic matter. Further applications of the SPLITT technique are likely to return increasingly new insights on the composition (including “truly settling” POC / ²³⁴Th) of the total spectrum of particles settling out of the upper ocean.