Particulate 234Th Research Articles

Estimates of micro-, nano-, and picoplankton contributions to particle export in the northeast Pacific

Abstract. The contributions of micro-, nano-, and picoplankton to particle export were estimated from measurements of size-fractionated particulate 234Th, organic carbon, and phytoplankton indicator pigments obtained during five cruises between 2010 and 2012 along Line P in the subarctic northeast Pacific Ocean. Sinking fluxes of particulate organic carbon (POC) and indicator pigments were calculated from 234Th–238U disequilibria and, during two cruises, measured by a sediment trap at Ocean Station Papa. POC fluxes at 100 m ranged from 0.65 to 7.95 mmol m−2 d−1, similar in magnitude to previous results at Line P. Microplankton pigments dominate indicator pigment fluxes (averaging 69 ± 19% of total pigment flux), while nanoplankton pigments comprised the majority of pigment standing stocks (averaging 64 ± 23% of total pigment standing stocks). Indicator pigment loss rates (the ratio of pigment export flux to pigment standing stocks) point to preferential export of larger microplankton relative to smaller nano- and picoplankton. However, indicator pigments do not quantitatively trace particle export resulting from zooplankton grazing, which may be an important pathway for the export of small phytoplankton. These results have important implications for understanding the magnitude and mechanisms controlling the biological pump at Line P in particular, and more generally in oligotrophic gyres and high-nutrient, low-chlorophyll (HNLC) regions where small phytoplankton represent a major component of the autotrophic community.

234Th-derived particulate organic carbon export in the Prydz Bay, Antarctica

234Th activities in sea water were measured using Fe(OH)3 co-precipitation and beta counting at six stations in Prydz Bay in March 2008 during the 24th Chinese National Antarctic Research Expedition. Total 234Th activities ranged from 0.96 to 2.44 dpm L−1 with an average of 1.61 dpm L−1, showing an apparent deficit with respect to 238U due to scavenging and export with particles. With a one-dimensional steady state model, 234Th export fluxes were converted to particulate organic carbon (POC) export using bottle ratios of POC concentrations to particulate 234Th activities on suspended particles. POC fluxes at the depth of 100 m varied between 33 and 297 mmol m−2 day−1, comparable to prior work in the same region and higher than those of some other sea areas in the Southern Ocean, and indicated efficient running of biological pump in Prydz Bay. The results could be helpful to expand the knowledge of carbon cycle in seasonally ice-covered coastal regions around Antarctica.

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

234Th is widely used to quantify the magnitude of upper ocean particulate organic carbon (POC) export in oceans. In the present work, the rates of particulate organic carbon export were measured based on the distribution patterns of 234Th/238U disequilibrium in the water column within the continental slope of the East China Sea (ECS) during May 2011. The profiles of particulate and dissolved 234Th activities at all three stations showed a relative deficit with respect to 238U in the upper 100m of the water column. The dissolved 234Th scavenging rates and the particulate 234Th removal rates and their residence times were calculated by a one-dimensional steady state model. The results showed that the dissolved 234Th scavenging rates and the particulate 234Th removal rates ranged from 12.4–61.4 dpm/(m3·d) and from 3.8–21.8 dpm/(m3·d), respectively. The residence times of dissolved and particulate 234Th were in the range of 3.4–158 d and 63.7–96.5 d, respectively. Combined with the measurement of POC/234Th ratios of suspended particles, POC export flux (calculated by carbon) from the euphotic zone was estimated in the study region, which ranged from 4.14–14.7 mmol/(m2·d),with an average of 8.21 mmol/(m2·d), occupying 35% of the prime productivity in the study area. The results of this study can provide new information for better understanding the carbon biogeochemical cycle within the continental slope of the ECS.

234Th-derived surface export fluxes of POC from the Northern Barents Sea and the Eurasian sector of the Central Arctic Ocean

Settling-based surface ocean export of particulate organic carbon (POC) in the western Eurasian sector of the Arctic Ocean was investigated from the marginal ice zone (MIZ) of the northern Barents Sea to the North Pole area. Upper ocean profiles of POC were combined with corresponding dissolved and particulate 234Th activities measured with a low-volume at-sea direct beta counting protocol to constrain the 234Th-derived POC export in July and August of 2001 to 6−32mmolm−2d−1 for the Barents Sea MIZ dropping to 2–6mmolm−2d−1 for multi-year-ice (MYI) covered central Arctic stations in Nansen, Amundsen and Makarov basins. Secular equilibrium between 234Th and 238U activities in intermediate to deep waters in the Amundsen Basin (n=10) demonstrated that the at-sea measurement protocol was functioning satisfactorily. There was no distinction in POC export efficiency between the MIZ and the MYI-covered interior basins with an average ratio between 234Th-derived POC export and primary production (so-called ThE ratio) of 44%. A projected increase in primary production with retreat in areal extent of sea ice is thus likely to yield increased POC sequestration in the Arctic Ocean interior.

Distributions and removal fluxes of trace metals in the water column of the Hung-Tsai Trough off southwestern Taiwan

Vertical profiles of dissolved and particulate Mn, Fe, Ni, Cu, Zn, Cd, Pb, and 234Th were determined in the Hung-Tsai Trough off southwestern Taiwan during 19–23 November, 2004. Except in the case of Cd, the distribution coefficient (Kd) of the trace elements showed a negative correlation with the suspended particle concentration. Based on the average Kd values, the general sequence of particle affinities for the eight trace elements is, from highest to lowest, Fe>Mn≈Pb>Zn≈Th>Cd≈Cu≈Ni. The trace metal data was coupled with the particle removal flux estimated from 234Th/238U disequilibrium to investigate metal removal by particle sinking from the euphotic layer. The residence time of trace elements with respect to particle removal from the euphotic layer was estimated. A negative correlation between the residence time and the distribution coefficient for the trace metals was found.

Particulate organic carbon export fluxes and validation of steady state model of 234Th export in the Chukchi Sea

To evaluate the efficiency of biological pump and carbon sequestration content on Arctic shelf, estimations of POC export fluxes derived from 234Th/238U disequilibrium were made during the 3rd Chinese National Arctic Research Expedition (CHINARE-3) from July 12 to September 22, 2008. The 234Th analysis was based on small volume MnO2 co-precipitation and β-counting method. Great deficits of 234Th to 238U were observed widely over Chukchi shelf, with an average 234Th/238U activity ratio of 0.64±0.28, resulting from intense particle scavenging. The average POC export fluxes in the entire study area, shelf and slope area were 24.9±23.3, 29.5±23.0 and 2.1±0.5mmolCm−2d−1, respectively, resulting in that 21% of the primary production on average was exported to the benthos. Comparing with steady state model, non-steady-state model was recommended to derive a more accurate particulate 234Th export flux in a rapidly changing environment in the high latitude areas.

Insignificant enhancement of export flux in the highly productive subtropical front, east of New Zealand: a high resolution study of particle export fluxes based on <sup>234</sup>Th: <sup>238</sup>U disequilibria

Abstract. We evaluated the export fluxes of Particulate Organic Carbon (POC) in the Subtropical Frontal zone (STF) of the SW Pacific sector of the Southern Ocean. The site is characterized by enhanced primary productivity, which has been suggested to be stimulated through so-called natural iron fertilization processes where iron-depleted subantarctic water (SAW) mixes with mesotrophic, iron-replete subtropical water (STW). We adopted the small-volume 234Th method to achieve the highest possible spatial sampling resolution in austral late autumn-early winter, May–June, 2008. Inventories of chlorophyll-a, particulate 234Th and POC observed in the upper 100 m were all elevated in the mid-salinity water type (34.5 < S < 34.8), compared with low salinity waters (S < 34.5) which were of SAW origin with high macronutrients and high (S > 34.8) salinity waters which were of STW origin with low macronutrients. However, Steady-State 234Th fluxes were similar across the salinity gradient being, 25 ± 0.78 ((1.5 ± 0.047) × 103) in the mid-salinity, and 29 ± 0.53 ((1.8 ± 0.032) × 103) and 22 ± 1.1 Bq m−2 d−1 ((1.3 ± 0.066) × 103 dpm m−2 d–1) in the high and low salinity waters respectively. Bottle POC/Th ratios at the depth of 100 m were used to convert 234Th fluxes into POC export fluxes. The derived POC flux did not appear to be enhanced in mid-salinity waters where the primary productivity was inferred to be the highest at the time of sampling, with a flux of 11 ± 0.45 mmol C m−2 d−1, compared to 14 ± 0.39 mmol C m−2 d−1 in high salinity waters and 8.5 ± 0.66 mmol C m−2 d−1 in low salinity waters. This study thus implied that natural iron fertilization does not necessarily lead to an enhancement of POC export in STF regions.

Shelf‐basin exchange times of Arctic surface waters estimated from 228Th/228Ra disequilibrium

The transpolar drift is strongly enriched in 228Ra accumulated on the wide Arctic shelves with subsequent rapid offshore transport. We present new data of Polarstern expeditions to the central Arctic and to the Kara and Laptev seas. Because 226Ra activities in Pacific waters are 30% higher than in Atlantic waters, we correct 226Ra for the Pacific admixture when normalizing 228Ra with 226Ra. The use of 228Ra decay as age marker critically depends on the constancy in space and time of the source activity, a condition that has not yet adequately been tested. While 228Ra decays during transit over the central basin, ingrowth of 228Th could provide an alternative age marker. The high 228Th/228Ra activity ratio (AR = 0.8–1.0) in the central basins is incompatible with a mixing model based on horizontal eddy diffusion. An advective model predicts that 228Th grows to an equilibrium AR, the value of which depends on the scavenging regime. The low AR over the Lomonosov Ridge (AR = 0.5) can be due to either rapid transport (minimum age without scavenging 1.1 year) or enhanced scavenging. Suspended particulate matter load (derived from beam transmission and particulate 234Th) and total 234Th depletion data show that scavenging, although extremely low in the central Arctic, is enhanced over the Lomonosov Ridge, making an age of 3 years more likely. The combined data of 228Ra decay and 228Th ingrowth confirm the existence of a recirculating gyre in the surface water of the eastern Eurasian Basin with a river water residence time of at least 3 years.

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

234Th was utilized as a tracer of particulate organic carbon (POC) export in the northwestern South China Sea (SCS) on the basis of the data collected at four stations during a spring cruise. Depth profiles of dissolved and particulate 234Th activities were measured in the upper 60 m, showing a significant deficit relative to 238U over the investigated stations. A stratified structure of 234Th-238U disequilibrium was in general observed in the upper 60 m water column, indicating that the euphotic zone of the northwestern SCS in this reason can be divided into two layers, an upper and lower layer, with different export rates. POC export fluxes were estimated from a one-dimensional steady state model of 234Th fluxes in combination with the measurement of POC/234Th ratios on suspended particles. The POC export in this region ranged from 8.2 to 20.0 mmol/(m2·d), with an average of 16.0 mmol/(m2·d), and was slightly higher than those previously reported in the southern SCS.

POC and particulate 234Th export fluxes estimated using 234Th/ 238U disequilibrium in an enclosed Eastern Mediterranean region (Saronikos Gulf and Elefsis Bay, Greece) in seasonal scale

Activity concentrations of the naturally occurring, short-lived and highly particle-reactive radionuclide tracer 234Th in the dissolved and particulate phase were determined at 7 shallow-water stations (maximum depths: 30 (S.1 and S.2), 65 (S.3), 97 (S.5), 105 (S.6) and 220 m (S.4 and S.7) in Saronikos Gulf and Elefsis Bay (central Aegean Sea, Greece) during 3 seasonal cruises (summer 2008, autumn 2008 and winter 2009) to assess the time scales of the dynamics and the depositional fate of particulate matter (POC, particulate 234Th). For that reason, in situ filtrating systems were deployed in several depths of the water column consisting of GF/A disc prefilters to scavenge particulate fraction of 234Th and organic carbon and impregnated cartridges to adsorb dissolved 234Th. The obtained data showed average particulate 234Th activity concentrations of 3.7 ± 0.4 Bq m −3 in summer, 2.1 ± 0.2 Bq m −3 in autumn and 2.4 ± 0.2 Bq m −3 in winter. The respective average dissolved 234Th activity concentrations were 30.1 ± 2.8 Bq m −3 in summer, 30.2 ± 2.9 Bq m −3 in autumn and 27.4 ± 3.0 Bq m −3 in winter. The activity ratios of total 234Th and its long-lived conservative parent 238U were below unity in most of the stations indicating radioactive disequilibrium throughout the water column, thus very dynamic trace-metal scavenging and particle export from the water column. These profiles ( 234Th and 238U) were used to estimate the export fluxes and scavenging rates of 234Th, as well as their residence times in the water column. The average cumulative export fluxes of particulate 234Th were estimated to be 33 ± 4 Bq m −2 d −1 in summer, 35 ± 5 Bq m −2 d −1 in autumn and 45 ± 6 Bq m −2 d −1 in winter, whereas the respective average cumulative scavenging rates of dissolved 234Th were 39 ± 5, 33 ± 5 and 50 ± 7 Bq m −2 d −1. Moreover, the cumulative average residence times of 234Th were 25 ± 4 d in summer, 45 ± 6 d in autumn and 64 ± 7 d in winter 2009 for the dissolved fraction and 4 ± 1, 3 ± 1 and 4 ± 1 d for the particulate one, respectively. POC/ A Th p ratio profiles decreased versus depth showing a variety of marine processes, such as loss of POC due to dissolution after biological activity, impact of minerals in particle sinking and microbial remineralization. Average cumulative export fluxes of POC were 162 ± 18 mmol m −2 d −1 in summer, 107 ± 19 mmol m −2 d −1 in autumn and 157 ± 25 mmol m −2 d −1 in winter 2009. The seasonal data of POC fluxes certified the existence of phytoplankton bloom in winter for Saronikos Gulf. In addition, after evaluating the maxima of POC fluxes in Elefsis Bay (a small embayment in northern Saronikos Gulf) during summer, potential bloom of phytoplankton also concluded; this approach is in agreement with previous data of the same area. Finally, the elevated POC concentrations and fluxes in the region certify that the Gulf is still one of the most organic polluted in the Mediterranean Sea.

234Th in surface waters: Distribution of particle export flux across the Antarctic Circumpolar Current and in the Weddell Sea during the GEOTRACES expedition ZERO and DRAKE

As part of the GEOTRACES Polarstern expedition ANTXXIV/3 (ZERO and DRAKE) we have measured the vertical distribution of 234Th on sections through the Antarctic Circumpolar Current along the zero meridian and in Drake Passage and on an EW section through the Weddell Sea. Steady state export fluxes of 234Th from the upper 100 m, derived from the depletion of 234Th with respect to its parent 238U, ranged from 621±105 to 1773±90 dpm m −2 d −1. This 234Th flux was converted into an export flux of organic carbon ranging from 3.1 to 13.2 mmol C m −2 d −1 (2.1–9.0 mmol C m −2 d −1) using POC/ 234Th ratio of bulk (respectively >50 μm) suspended particles at the export depth (100 m). Non-steady state fluxes assuming zero flux under ice cover were up to 23% higher. In addition, particulate and dissolved 234Th were measured underway in high resolution in the surface water with a semi-automated procedure. Particulate 234Th in surface waters is inversely correlated with light transmission and pCO 2 and positively with fluorescence and optical backscatter and is interpreted as a proxy for algal biomass. High resolution underway mapping of particulate and dissolved 234Th in surface water shows clearly where trace elements are absorbed by plankton and where they are exported to depth. Quantitative determination of the export flux requires the full 234Th profile since surface depletion and export flux become decoupled through changes in wind mixed layer depth and in contribution to export from subsurface layers. In a zone of very low algal abundance (54–58°S at the zero meridian), confirmed by satellite Chl-a data, the lowest carbon export of the ACC was observed, allowing Fe and Mn to maintain their highest surface concentrations. An ice-edge bloom that had developed in December/January in the zone 60–65°S as studied during the previous leg had caused a high export flux at 64.5°S when we visited the area 2 months later (February/March). The ice-edge bloom had then shifted south to 65–69°S evident from uptake of CO 2 and dissolved Fe, Mn and 234Th, without causing export yet. In this way, the parallel analysis of 234Th can help to explain the scavenging behavior of other trace elements.

Removal fluxes of Mn and Fe from the nearshore waters along the west coast of Taiwan

Dissolved and particulate Mn and Fe in the nearshore waters were determined at 27 stations along the west coast of Taiwan during 19–23 November 2004. The latitudinal distributions of Mn and Fe were very similar; however, the concentration in the dissolved phase was lower, whereas that in the particulate phase was higher in the northern regime. The higher percentage of Fe than Mn that was associated with particles resulted in a two-order of magnitude higher particle–water partition coefficient for Fe, Kd(Fe), than that for Mn, Kd(Mn). The removal fluxes of Mn and Fe could be estimated by multiplying the particulate 234Th removal flux with the Mn/234Th and Fe/234Th ratios in suspended particles, which ranged from 0.1 to 3.8mmolm−2d−1 for Mn, and from 3.4 to 194.5mmolm−2d−1 for Fe. Residence times ranged from 1 to 20days for Mn and Fe were estimated in the nearshore.

Scavenging of 231Pa and thorium isotopes based on dissolved and size-fractionated particulate distributions at Drake Passage (ANTXXIV-3)

Drake Passage is a major route for many water masses in the strong Antarctic Circumpolar Current. During the ANTXXIV-3 expedition (in 2008), the vertical distributions of dissolved and size-fractionated particulate 231Pa and thorium isotopes ( 230Th, 232Th and 234Th) were investigated in order to better define the scavenging regimes, and the effects of the oceanic circulation on the fate of particulate material and on the Pa–Th distributions in the water column. The reversible scavenging model applied to both 230Th and 234Th, in the upper 1500 m depth, gives estimates of the particle dynamics (settling velocities S∼500–1300 m year −1, adsorption and desorption rate constants of 0.1–0.4 and 1–6 year −1, respectively). The particulate 234Th/ 230Th activity ratio decreases with increase in depth, which is in agreement with previous studies, but no relationship with particle size is found. 231Pa and thorium isotope fractionation and particle–water partition coefficients were investigated as a function of particle size vs. depth and latitude, and appeared to vary horizontally following a North–South gradient. Both the radionuclides are mostly bound to the fine suspended particles. At Drake Passage, the 230Th xs distribution is controlled by the southward upwelling of deep water (clearly visible on the vertical section of total 230Th xs, defined as dissolved+particulate concentrations) and reversible-scavenging processes (linear increase of 230Th xs with increasing depth) with north of the Southern ACC Front, higher settling velocities and less adsorption/desorption cycles than south of it. Distributions of dissolved and total 231Pa xs also reflect the influence of the north–south upwelling but this effect is limited to the upper depth (1500 m) of the water column. Below this depth, 231Pa xs vertical profiles exhibit contrasting concentrations with some highly dissolved activities in the deep water of the stations in the northern part of ACC and not to the south of ACC. These N–S differences in dissolved 231Pa xs are attributed to different origins, and the scavenging history of the deep Pacific waters flow across Drake Passage. In the north, radionuclides-rich deep water originates from the Central Pacific, while the deep water in the south is thought to be derived from the Southern Pacific, and the observed low radionuclides concentrations are attributed to high opal abundance in that region. In the southern part of the Drake Passage, highly dissolved and particulate activities of 230Th and 232Th confirmed the westward intrusion of 230Th-rich Weddell Sea Deep Water (WSDW) close to the Antarctic Peninsula.

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

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

High particle export over the continental shelf of the west Antarctic Peninsula

Drifting cylindrical traps and the flux proxy 234Th indicate more than an order of magnitude higher sinking fluxes of particulate carbon and 234Th in January 2009 than measured by a time‐series conical trap used regularly on the shelf of the west Antarctic Peninsula (WAP). The higher fluxes measured in this study have several implications for our understanding of the WAP ecosystem. Larger sinking fluxes result in a revised export efficiency of at least 10% (C flux/net primary production) and a requisite lower regeneration efficiency in surface waters. High fluxes also result in a large supply of sinking organic matter to support subsurface and benthic food webs on the continental shelf. These new findings call into question the magnitude of seasonal and interannual variability in particle flux and reaffirm the difficulty of using moored conical traps as a quantitative flux collector in shallow waters.

Sinking of particulates 234Thxs, 7Be and 210Pbxs in the Changjiang estuary, China

The time scale of transport and reaction processes in the ocean can be evaluated using natural radionuclides of different half-lives. In the present study, we evaluated the residence times of particulate reactive radionuclides 7Be, 210Pbxs and 234Th in the Changjiang (Yangtze River) estuary, which is a high turbidity estuary, in June, 2006. The residence times of particulate 7Be, 234Thxs and 210Pbxs in the Changjiang estuary were calculated to be 0.7–20.4, 0.18–4.1 and 0.6–89.8 d, respectively. The obtained residence times of those nuclides are comparable with those of other estuaries and bays. For 234Thxs and 7Be, the residence times decreased with increasing concentrations of suspended particulate matter (SPM); however, those of 210Pbxs increased. Moreover, scavenging residence times of 234Th and 7Be increased from coast to offshore, while those of 210Pbxs decreased.

Comparative evaluation of sediment trap and 234Th-derived POC fluxes from the upper oligotrophic waters of the Gulf of Mexico and the subtropical northwestern Pacific Ocean

To better understand the inter-relationships between particulate organic carbon (POC) fluxes and phytoplankton and bacteria biomass and production, we compared POC fluxes determined in sediment traps and approaches based on size-fractionated (1–10, 10–50, 50–150 and > 150 μm) suspended particulate 234Th and POC concentrations in oligotrophic sections of the Gulf of Mexico during August 2005 and May 2006 and in the oligotrophic northwestern Pacific Ocean during 2009. In 2005, the sediment trap POC flux near the bottom of the euphotic zone (120 m) ranged from 71 to 94 mg C m − 2 day − 1 , while 234Th-derived POC fluxes using POC/ 234Th ratios in the 10–50 µm and 50–150 µm varied from 71 to 150 mg C m − 2 day − 1 . In 2006, the sediment trap POC flux at 120 m ranged from 24 to 67 mg C m − 2 day − 1 , while the 234Th-derived POC fluxes in the 10–50 µm fraction were comparable or somewhat higher, ranging from 71 to 119 mg C m − 2 day − 1 . The POC fluxes in 2006, calculated by using POC/ 234Th ratios in the 1–10 µm and the 50–150 µm fractions were much higher, ranging from 847 to 1369 mg C m − 2 day − 1 . Correlations with biological and chemical parameters support a likely mechanism of sinking aggregates of haptophytes (0.2–20 µm) of higher density held together by Th-complexing and uronic acid containing exopolymeric substances. The observations that 234Th (and POC) is mainly associated with medium-sized (10–15 µm) suspended particles rather than larger (50–150 µm) ones may be caused by the use of a one-filter method and standard filtration and processing procedures that were applied here for collecting suspended particles. This then raises the question of what constitutes representative material from the ocean that settles on the characteristic time scale of 234Th. As a comparison, size-fractionated trap-collected particles in the oligotrophic northwestern Pacific Ocean showed that the 10–50 μm fraction contained the largest proportion of POC (22–41%), followed by the 50–150 μm (22–37%), the > 150 μm (15–27%), and the 1–10 μm (17–23%) fraction. The partitioning of 234Th in trap-collected particles was slightly different from that of POC, with the 1–10 μm fraction representing the largest proportion (27–48%) of 234Th flux. Together, the < 50 μm particles contributed, on average, 52 ± 6% of POC, which suggests that the POC/ 234Th ratios traditionally derived from large (> 50 µm) pump-collected particles may not accurately reflect the majority of sinking particles. Therefore, estimated POC fluxes may be significantly biased using a conventional 234Th based approach, i.e., using POC/ 234Th ratios from a single filter obtained from large (> 50 µm) pump-collected particles.

Sediment &amp;lt;sup&amp;gt;231&amp;lt;/sup&amp;gt;Pa/&amp;lt;sup&amp;gt;230&amp;lt;/sup&amp;gt;Th as a recorder of the rate of the Atlantic meridional overturning circulation: insights from a 2-D model

Abstract. A two dimensional scavenging model is used to investigate the patterns of sediment 231Pa/230Th generated by the Atlantic Meridional Overturning Circulation (AMOC) and further advance the application of this proxy for ocean paleocirculation studies. The scavenging parameters and the geometry of the overturning circulation cell have been chosen so that the model generates meridional sections of dissolved 230Th and 231Pa consistent with published water column profiles and an additional 12 previously unpublished profiles measured in the North and Equatorial Atlantic. The processes that generate the meridional sections of dissolved and particulate 230Th, dissolved and particulate 231Pa, dissolved and particulate 231Pa/230Th, and sediment 231Pa/230Th are discussed in detail. The results indicate that the relationship between sediment 231Pa/230Th at any given site and the overturning circulation is very complex. They clearly show that constraining past changes in the strength and geometry of the AMOC requires an extensive data set and they suggest strategies to maximize information from a limited number of samples.

Nearshore scavenging phenomenon elucidated by 234th/238u disequilibrium in the coastal waters off Western Taiwan

Dissolved and particulate 234Th activities in surface seawater were determined at 27 stations along the coastline of western Taiwan during 19–23 November 2004. Contrasting scavenging settings were observed between the northern and southern regimes of the nearshore water off western Taiwan, separated by the Cho-Shui River. The northern regime is characterized by a large quantity of suspended load contributed by northward transport of a suspension plume from the Cho-Shui River, while the southern regime, low in suspended load and high in chlorophyll concentration, is a system controlled by biological activity. A scavenging model that takes account of the physical transport was used to estimate the 234Th budget in order to estimate the scavenging and removal rates from the nearshore water. The scavenging and removal rates ranged from 21 to 127 dpm m−3d−1 and from 36 to 525 dpm m−3d−1, for dissolved and particulate 234Th, respectively. The removal fluxes of particulate organic carbon (POC) and particulate organic nitrogen (PON) were estimated by multiplying the particulate 234Th removal flux to the organic carbon/234Th and nitrogen/234Th ratios in suspended particles, which ranged from 4.5 to 275.2 mmol-C m−2d−1 and from 1.3 to 50.1 mmol-N m−2d−1, respectively. These fluxes resulted in residence times of 1∼20 days for the POC in the surface water of nearshore water off western Taiwan.

Vertical flux of particulate organic carbon in the central South China Sea estimated from 234Th-238U disequilibria

234Th-238U disequilibria were applied to examine the particle dynamics in the euphotic zone of the central South China Sea during the spring 2002 cruise. The particulate organic carbon (POC), 234Th (including both dissolved and particulate) and 238U in the water column at three stations were determined. The profiles of 234Th/238U activity ratio at the three stations all showed consistent 234Th deficit as compared to 238U in the upper 100 m water column. Based on the profiles of the dissolved and particulate 234Th and a steady state box model, the dissolved 234Th scavenging rates, the particulate 234Th removal rates and their resident times were quantified. It was found that the POC downward export fluxes out of the upper 100 m euphotic zone ranged from 9.40 to 14.78 mmol·m−2·d−1. The results from this study provide new information for our understanding of carbon biogeochemical cycle in the South China Sea.