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Abstract

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.

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... In the present study, the concentration of PAHs and 234 Th activities, and hence the particulate PAH/ 234 Th ratios were determined on the same QMA filter with a nominal pore size of 1 μm. The 234 Th-based particle fluxes estimates based on different size particle fractions can result in varying estimates of vertical PAHs fluxes, as reported by a number of particle flux related studies in the GOM (Guo et al., 2002;Santschi et al., 2003;Hung et al., 2004Hung et al., , 2010Xu et al., 2011). It has been reported that the smaller (1-50 μm) particles represent the major fraction of the sinking particles in the oligotrophic waters in the GOM (Guo et al., 2002;Hung et al., 2010). ...
... It has been reported that the smaller (1-50 μm) particles represent the major fraction of the sinking particles in the oligotrophic waters in the GOM (Guo et al., 2002;Hung et al., 2010). Another study by Xu et al. (2011) indicated that the intermediate-sized particles (10-50 μm) show the best agreement between POC/ 234 Th ratios in suspended and sinking particles in the northern GOM. These smaller particles can actively participate in scavenging processes and may be more important vector in exporting materials into the interior of the deep ocean than the larger particles (Baskaran et al., 1992;Guo et al., 1997Guo et al., , 2002Hung et al., 2004;Richardson and Jackson, 2007). ...
... Previous studies have suggested that a part of this difference could be methodologicalin situ pumps often under-sample the fast sinking particles while sediment traps often under-collect smaller slow sinking particles (Buesseler et al., , 2007. Another methodological issue partly contributing to such differences could be 234 Th remobilization in the collection tubes of sediment traps Hung et al., 2010;Maiti et al., 2010;Xu et al., 2011). Shorter period of trap deployment and formaldehyde solution in the brine at the bottom of the collection tubes can reduce such particle remobilization by reducing potential bacterial growth on the particles collected in the sediment traps Hung et al., 2010). ...
... At each depth, a set of sediment traps consisting of eight Plexiglas tubes filled with trap solution prepared from the mixture of filtered seawater and formaldehyde (0.3 % v/v) were snapped on a polypropylene cross frame, which is secured to a mooring line. It should be noted that radionuclide release into overlying waters in unpoisoned traps (even when deployed for less than 1 day) has been documented (Hung et al., 2010;Xu et al., 2011), which can underestimate radionuclide fluxes. The mooring array was left free drifting for 37-48 h. ...
... The I Pb is about 0.4 dpm cm −2 a −1 according to the model by Feichter et al. (1991). Based on 210 Pb chronology in ornithogenic sediments, Xu et al. (2011) estimated the atmospheric 210 Pb flux of 0.76 dpm cm −2 a −1 in the southern South China Sea. Here we assume an average I Pb of 0.6 dpm cm −2 a −1 (16.4 dpm m −2 d −1 ). ...
... where F Po is the removal flux, λ Pb is radioactive decay constant of 210 Po (= 0.005 d −1 ) and I Po is the atmospheric 210 Po deposition flux, which is assumed to be 10 % of the atmospheric 210 Pb flux (Turekian et al., 1977). Since the spatial variation of 210 Po in the surface water of the South China Sea is small Wei et al., 2011), as is the case for 234 Th, horizontal transport should be negligible. However, similar to the nutrients, the vertical profile of 210 Po t shows evidence for an increase in the thermocline layer, vertical diffusion may play a significant role in the mass balance of 210 Po in the euphotic layer (Sarin et al., 1994). ...
Article
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The time-series station, SEATS (18 • N, 116 • E) in the South China Sea was visited six times during Octo-ber 2006–December 2008 to carry out seawater sampling and floating trap deployments for the determination of distributions and fluxes of POC, PIC, PN, 234 Th, 210 Pb, and 210 Po in the upper 200 m of the water column. Radionu-clide deficiencies resulted in removal fluxes from the eu-photic layer of 1.1 × 10 3 –1.8 × 10 3 dpm m −2 d −1 and 7.1– 40.2 dpm m −2 d −1 for 234 Th and 210 Po, respectively. Due to atmospheric input, an excess of 210 Pb relative to 226 Ra is commonly observed in the upper water column. Sinking fluxes of total mass, POC, PIC, PN, 234 Th, 210 Pb, and 210 Po measured at the euphotic depth were low in summer-fall and high in winter-spring, reflecting the seasonal variability of biological pumping. Excluding the suspiciously low primary productivity data point in July 2007, a relatively high e-ratio of 0.28–0.69 was estimated by the ratio of the POC flux at the euphotic depth and the integrated primary productivity. The ratios of 234 Th, 210 Pb, and 210 Po to organic carbon, inorganic carbon, and nitrogen in the sinking particles were combined with the disequilibria of 234 Th– 238 U, 210 Pb– 226 Ra, and 210 Po– 210 Pb to estimate export fluxes of POC, PIC, and PN from the euphotic layer. Compared with measured fluxes by the sediment trap and estimated fluxes by other approaches, it is concluded that the export production in the South China Sea, ranging from 1.8 to 21.3 mmol-C m −2 d −1 , can be reasonably estimated using 234 Th, 210 Pb, and 210 Po as carbon proxies.
... At each depth, a set of sediment traps consisting of eight Plexiglas tubes filled with trap solution prepared from the mixture of filtered seawater and formaldehyde (0.3 % v/v) were snapped on a polypropylene cross frame, which is secured to a mooring line. It should be noted that radionuclide release into overlying waters in unpoisoned traps (even when deployed for less than 1 day) has been documented (Hung et al., 2010;Xu et al., 2011), which can underestimate radionuclide fluxes. The mooring array was left free drifting for 37-48 h. ...
... The I Pb is about 0.4 dpm cm −2 a −1 according to the model by Feichter et al. (1991). Based on 210 Pb chronology in ornithogenic sediments, Xu et al. (2011) estimated the atmospheric 210 Pb flux of 0.76 dpm cm −2 a −1 in the southern South China Sea. Here we assume an average I Pb of 0.6 dpm cm −2 a −1 (16.4 dpm m −2 d −1 ). ...
... where F Po is the removal flux, λ Pb is radioactive decay constant of 210 Po (= 0.005 d −1 ) and I Po is the atmospheric 210 Po deposition flux, which is assumed to be 10 % of the atmospheric 210 Pb flux (Turekian et al., 1977). Since the spatial variation of 210 Po in the surface water of the South China Sea is small Wei et al., 2011), as is the case for 234 Th, horizontal transport should be negligible. However, similar to the nutrients, the vertical profile of 210 Po t shows evidence for an increase in the thermocline layer, vertical diffusion may play a significant role in the mass balance of 210 Po in the euphotic layer (Sarin et al., 1994). ...
Article
The time-series station, SEATS (18 degrees N, 116 degrees E) in the South China Sea was visited six times during October 2006-December 2008 to carry out seawater sampling and floating trap deployments for the determination of distributions and fluxes of POC, PIC, PN, Th-234, Pb-210, and Po-210 in the upper 200m of the water column. Radionuclide deficiencies resulted in removal fluxes from the euphotic layer of 1.1 x 10(3)-1.8 x 10(3) dpmm(-2) d(-1) and 7.1-40.2 dpmm(-2) d(-1) for Th-234 and Po-210, respectively. Due to atmospheric input, an excess of Pb-210 relative to Ra-226 is commonly observed in the upper water column. Sinking fluxes of total mass, POC, PIC, PN, Th-234, Pb-210, and Po-210 measured at the euphotic depth were low in summer-fall and high in winter-spring, reflecting the seasonal variability of biological pumping. Excluding the suspiciously low primary productivity data point in July 2007, a relatively high e-ratio of 0.28-0.69 was estimated by the ratio of the POC flux at the euphotic depth and the integrated primary productivity. The ratios of Th-234, Pb-210, and Po-210 to organic carbon, inorganic carbon, and nitrogen in the sinking particles were combined with the disequilibria of Th-234-U-238, Pb-210-Ra-226, and Po-210-Pb-210 to estimate export fluxes of POC, PIC, and PN from the euphotic layer. Compared with measured fluxes by the sediment trap and estimated fluxes by other approaches, it is concluded that the export production in the South China Sea, ranging from 1.8 to 21.3 mmol-Cm-2 d(-1), can be reasonably estimated using Th-234, Pb-210, and Po-210 as carbon proxies.
... A final point to address is the extent to which POC/ 234 Th and POC/ 210 Po size fractionated ratios influence Th-POC and Po-POC fluxes. The best method for collecting sinking particles for the determination of POC/radionuclide ratios and the subsequent estimate of downward POC flux is still an open question Burd et al., 2000;Cochran et al., 2009;Lepore et al., 2009;Stewart et al., 2007a;Xu et al., 2011). ...
... In our study, POC/ 234 Th ratios in the two size fractions are scattered around the 1:1 line despite our expectation that POC/ 234 Th ratios would be lower in large than small particles. Recent work, (i.e., Xu et al., 2011 and have found a relationship between 234 Th and extracellular organic polysaccharides which may contain the refractory 234 Th-binding biopolymers indicating that the 234 Th behaviour during the formation of aggregates is not yet well known. This may explain why our POC/ 234 Th ratios are scattered around the 1:1 line, given that Riley et al. (2012) found that the particles collected were mainly aggregates, degraded material or faecal pellets. ...
... 210 Pb is only particle-surface reactive or incorporated into the biomineral lattice and is not bound to organic carbon. 234 Th and 210 Pb are both adsorbed on the walls of the cell and bounds to specific carbon group like polysaccharides (Quigley et al., 2002;Xu et al., 2011). ...
... Among these four selected radionuclides, with the exception of Pb (which has intermediate properties), all others have hard (A-type) metal ion characteristics preferring to bind to O (over N or S). For example, uronic acid content has been used as a good proxy to predict the Th flux [Guo et al., 2002b;Santschi et al., 2003;Xu et al., 2011]. More recently, hydroxamate siderophoric moieties were found to be important for the binding of Pa and Po to sinking particles , as well for the binding of Th and Pa to marine colloids [Chuang et al., 2015]. ...
... In this work, statistical results ( Table 2) further indicate that URA had the highest affinity to tested radionuclides among all tested organic molecules. It is consistent with the findings from the analysis of the combined data sets from the oligotrophic ocean that URA (or acid polysaccharide, APS) content can be used to predict the 234 Th flux [Guo et al., 2002b;Santschi et al., 2003;Xu et al., 2011] As also shown in Fig. 2, most Pa, Pb, or Be was associated in the frustules and intracellular biopolymers (Fig. 2b, c), whereas Th was mostly bound to NAEPS (Fig. 2a). ...
Article
In order to investigate the importance of biogenic silica associated biopolymers on the scavenging of radionuclides, the diatom Phaeodactylum tricornutum was incubated together with the radionuclides 234Th, 233Pa, 210Pb, and 7Be during their growth phase. Normalized affinity coefficients were determined for the radionuclides bound with different organic compound classes (i.e., proteins, total carbohydrates, uronic acids) in extracellular (non-attached and attached exopolymeric substances, EPS), intracellular (ethylene diamine tetraacetic acid, EDTA and sodium dodecyl sulfate, SDS extractable), and frustule embedded biopolymeric fractions (BF). Results indicated that radionuclides were mostly concentrated in frustule BF. Among three measured organic components, URA showed the strongest affinities to all tested radionuclides. Confirmed by spectrophotometry and 2-dimensional heteronuclear single quantum coherence-nuclear magnetic resonance (2D HSQC-NMR) analyses, the frustule BF were mainly composed of carboxyl-rich, aliphatic-phosphoproteins, which were likely responsible for the strong binding of many of the radionuclides. Results from this study provide evidence for selective absorption of radionuclides with different kinds of diatom-associated biopolymers acting in concert rather than as a single compound. This clearly indicates the importance of these diatom related biopolymers, especially frustule biopolymers, in the scavenging and fractionation of radionuclides used as particle tracers in the ocean.
... Among these four selected radionuclides, with the exception of Pb (which has intermediate properties), all others have hard (A type) metal ion characteristics preferring to bind to O (over N or S). For example, uronic acid content has been used as a good proxy to predict the Th flux [Guo et al., 2002b; Santschi et al., 2003; Xu et al., 2011]. More recently, hydroxamate siderophoric moieties were found to be important for the binding of Pa and Po to sinking particles, as well as for the binding of Th and Pa to marine colloids [Chuang et al., 2015]. ...
... In this work, statistical results (Table 2) further indicate that URA had the highest affinity to tested radionuclides among all tested organic molecules. It is consistent with the findings from the analysis of the combined data sets from the oligotrophic ocean that URA (or acid polysaccharide) content can be used to predict the 234 Th flux [Guo et al., 2002b; Santschi et al., 2003; Xu et al., 2011] As also shown inFigure 2, most Pa, Pb, or Be were associated in the frustules and intracellular biopolymers (Figures 2b and 2c), whereas Th was mostly bound to NAEPS (Figure 2a ). Thus, different ratios of these biomolecules could be the main factor controlling preferential scavenging of Pa, Pb, and Be over Th by opal as observed in the field [Chase et al., 2002] and laboratory studies [Chuang et al., 2014; Guo et al., 2002a; Yang et al., 2013], in addition to redox active moieties. ...
... Before every deployment, the sediment tubes were filled with freshly filtered (1 μm pore size) seawater collected from the same depth they were to be deployed. Previous sediment trap study using no preservatives in trap tubes found significant solubilization of POC and 234 Th (Xu et al., 2011). In this study, the trap-tubes were filled with 500 mL of borate-buffered brine (S~70; 3 mM borate, pH = 8) poisoned with 37 mM formalin immediately before deployment. ...
... It has been reported that the smaller (1-50 μm) particles represent the major fraction of the sinking particles in the oligotrophic waters in the GOM (Hung et al., 2010). Another study by Xu et al. (2011) indicated that the intermediate-sized particles (10-50 μm) show the best agreement between POC/ 234 Th ratios in suspended and sinking particles in the northern GOM. Yet another study in northwestern GOM found the C/Th ratio in N53 μm size fraction to be more representative of the sinking fluxes (Hung et al., 2004) For the present study we find the C/Th ratio in traps to be in better agreement with the ratios in N51 μm size fraction than the 1-51 μm size fraction (Fig. 6). ...
Article
The northern Gulf of Mexico (NGOM) is one of the well-studied areas of global ocean, yet direct estimates of upper ocean particulate organic carbon (POC) fluxes from this region are limited. The present work reports vertical fluxes of POC from the oligotrophic region of NGOM utilizing short-lived radionuclide pairs 234Th/238U and 210Po/210Pb. In spite of the difference in time scale both 210Po and 234Th based estimates are in reasonable agreement with sinking POC fluxes, caught in sediment traps. POC flux estimates ranged between 22–41 mg C m− 2 day− 1 at 150 m and 6–40 mg C m− 2 day− 1 at 250 m. The average export efficiency at base of euphotic zone (Ez) was found to be 0.07 ± 0.03 while the export ratio (T100) at 100 m below euphotic zone was found to be 0.66 ± 0.18 indicating that most of the attenuation of NPP in this region is set in the surface layer (low Ez ratio) and there is relatively little flux attenuation in the subsurface (high T100) which is typical for oligotrophic settings. Satellite based export efficiencies predicted by the Laws and Dunne models are on average found to be two times higher than the in situ observations while estimates from the Henson model were found to be similar or lower. This is consistent with the observation on a global scale where we find export estimates from in situ data to be consistently lower than those predicted by the Laws export model for the temperature range of 20–25 °C. The discrepancy between modelled estimates and in situ measurements of POC fluxes highlights the fact that global empirical models of satellite based POC fluxes that only consider temperature are overly simple and may need further refinement for ocean biome specific scaling to accurately predict export fluxes.
... Extracellular polymeric substances composition was measured in terms of carbohydrate, protein, and uronic acid content and total EPS was calculated by summing these parameters. Particles were collected with a polycarbonate filter (0.4 µm, Millipore, United States), and the attached EPS from the particles was then extracted with 0.35 M EDTA followed by an ultrafiltration step to remove the salts and excessive EDTA (Xu et al., 2009(Xu et al., , 2011a. EPS from the dissolved phase was directly obtained by concentrating and desalting using an Amicon Ultra-15 centrifugal filter unit with ultracel-3 membrane (Millipore, 3 kDa). ...
... For POC analysis, water sample was filtered through a pre-combusted GF/F membrane (0.7 µm, Whatman, United States), and then quantified using a Perkin Elmer Series II CHNS 2400 analyzer, after HCl-fuming to remove the carbonates. Acetanilide (71.09%) was used as the analytical standard (Xu et al., 2011a). Samples from the offshore mesocosm were limited, therefore only samples from the coastal mesocosm were analyzed. ...
Article
Full-text available
Extracellular enzymes and extracellular polymeric substances (EPS) play a key role in overall microbial activity, growth and survival in the ocean. EPS, being amphiphilic in nature, can act as biological surfactant in an oil spill situation. Extracellular enzymes help microbes to digest and utilize fractions of organic matter, including EPS, which can stimulate growth and enhance microbial activity. These natural processes might have been altered during the 2010 Deepwater Horizon oil spill due to the presence of hydrocarbon and dispersant. This study aims to investigate the role of bacterial extracellular enzymes during exposure to hydrocarbons and dispersant. Mesocosm studies were conducted using a water accommodated fraction of oil mixed with the chemical dispersant, Corexit (CEWAF) in seawater collected from two different locations in the Gulf of Mexico and corresponding controls (no additions). Activities of five extracellular enzymes typically found in the EPS secreted by the microbial community – α- and β-glucosidase, lipase, alkaline phosphatase, leucine amino-peptidase – were measured using fluorogenic substrates in three different layers of the mesocosm tanks (surface, water column and bottom). Enhanced EPS production and extracellular enzyme activities were observed in the CEWAF treatment compared to the Control. Higher bacterial and micro-aggregate counts were also observed in the CEWAF treatment compared to Controls. Bacterial genera in the order Alteromonadaceae were the most abundant bacterial 16S rRNA amplicons recovered. Genomes of Alteromonadaceae commonly have alkaline phosphatase and leucine aminopeptidase, therefore they may contribute significantly to the measured enzyme activities. Only Alteromonadaceae and Pseudomonadaceae among bacteria detected here have higher percentage of genes for lipase. Piscirickettsiaceae was abundant; genomes from this order commonly have genes for leucine aminopeptidase. Overall, this study provides insights into the alteration to the microbial processes such as EPS and extracellular enzyme production, and to the microbial community, when exposed to the mixture of oil and dispersant.
... The coexistence of inorganic and organic iodine species has been reported in various environments (Baker et al., 2001;Couture and Seitz, 1983;Muramatsu, 1989;Schwehr et al., 2005a;Schwehr et al., 2005b;Xu et al., 2012a;Xu et al., 2011a;Xu et al., 2011b;Xu et al., 2011c;Xu et al., 2012b;Yuita, 1992;Yuita and Kihou, 2005). For example, methyl iodide is an important gaseous form of iodine in the marine atmosphere and in releases from nuclear fuel reprocessing facilities, while dissolved organo-I compounds comprise up to 40% of total iodine in aqueous samples from estuaries, rivers, and rain (Santschi and Schwehr, 2004). ...
Article
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... Clear relationships were found in sorption experiments with pure organic model compounds (Quigley et al., 2002;Alvarado Quiroz et al., 2006;Xu et al., 2011b) and in the field with particulate organic material Santschi et al., 2002Santschi et al., , 2003Roberts et al., 2009;Hung et al., , 2012Xu et al., 2011a). In our study, no significant correlations were found between TCHO or URA content in Table 3 Concentrations of classified amino acid groups in marine colloids and EPS samples (maximum relative standard deviations (RSD) for each analysis were 1%). ...
... Although the supernatant solution in sampling cups was not measured for 210 Po concentration, the degradation of trap particles was minimized by adding HgCl 2 preservative (Heussner et al., 1990). However, there exist some studies that reported the dissolution of particulates and the release of radionuclides during sediment trap deployment in lake setting (Schuler et al., 1991) and ocean (Hung et al., 2010;Xu et al., 2011). This warrants a caution for the measurement of 210 Po in the sediment trap experiment in the future. ...
Article
Full-text available
Vertical distributions of dissolved and particulate Pb-210 and Po-210 in the water column at the SouthEast Asian Time-series Study (SEATS, 18 degrees 00' N and 116 degrees 00' E) station in the northern South China Sea were determined from four cruises between January 2007 and June 2008. A large deficiency of Pb-210, 379 +/- 43 x 10(3) dpm m(2), from the secular equilibrium was found within the 3500 m water column. On the other hand, a smaller deficiency of Po-210, 100 +/- 21 x 10(3) dpm m(2), relative to Pb-210 was found in the water column. Time-series data showed insignificant temporal variability of the Pb-210 and Po-210 profiles. To balance these deficiencies, the removal fluxes for Pb-210 and Po-210 via particle settling ranging from 45 to 51 dpm m(-2) d(-1) and from 481 to 567 dpm m(-2) d(-1), respectively, are expected at 3500 m. The Pb-210 removal flux is comparable with, whereas the Po-210 removal flux is much higher than, the flux directly measured by moored sediment traps. The discrepancy between the modeled Po-210 flux and the measured flux suggests that sporadic events that enhance Po-210 removal via sinking ballast may occur in the water column at the site.
... waters. Signs of small particles sinking were also reported by Xu et al. [2011] who found the best agreement of POC/ 234 Th ratios with those in ST for intermediate-sized (10-50 μm) rather than larger (>50 μm) particles, suggesting that these smaller particles dominated the export flux. In that same study, the dominance of nanoplankton and pico-prymnesiophytes was proposed as the source of disagreement between POC/ 234 Th in large (>50 μm) ISP particles and ST particles. ...
Article
Summertime carbon, nitrogen, and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, particulate carbon (PC) and particulate nitrogen (PN) concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1–5 µmol dpm−1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. The highest C fixation rates were associated with diatom-dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton-dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton- and diazotroph-dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton-dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, including <50 µm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method.
... Recent field studies have recognized the important role of particle composition in the scavenging of Th and Pa as well as their fractionation in the ocean (e.g., Ku, 1999, 2004a,b;Chase et al., 2002;Chase and Anderson, 2004;Roy-Barman et al., 2005Xu et al., 2011). In addition, increased controlled laboratory experiments have shown the effect of particle composition, mostly on micro-particles or sinking particles, on the adsorption of Th and Pa in seawater (Guo et al., Marine Chemistry 162 (2014) [50][51][52][53][54][55][56][57][58][59] 2002a; Geibert and Usbeck, 2004;Roberts et al., 2009;Chuang et al., 2013). ...
... Besides sediment traps, 234 Th has been increasingly used as a tracer to estimate POC flux due to its timescales (weeks to month) in surface water (Coale and Bruland, 1985;Santschi et al., 2006) based on calculation of the product of the POC / 234 Th ratio in sinking particles and the 234 Th flux. Although both methods have their uncertainties (see review in Buesseler et al., 2006Buesseler et al., , 2007Hung and Gong, 2010;Xu et al., 2011;Hung et al., 2012), recent studies have shown that both methods are complementary means for estimating POC flux in the upper ocean in different marine environments including the ECS, the Kuroshio, the South China Sea, the Gulf of Mexico, and the oligotrophic northwestern Pacific Ocean Li, 2009;Hung et al., 2004Hung et al., , 2010cWei et al., 2011). The POC fluxes measured by sediment traps and 234 Th approaches in the Kuroshio, the Gulf of Mexico, and the South China Sea are in good agreement if the ratio of POC / 234 Th is appropriately selected. ...
Article
Full-text available
To understand carbon cycling in marginal seas better, particulate organic carbon (POC) concentrations, POC fluxes and primary production (PP) were measured in the East China Sea (ECS) in summer 2007. Higher concentrations of POC were observed in the inner shelf, and lower POC values were found in the outer shelf. Similar to POC concentrations, elevated uncorrected POC fluxes (720–7300 mg C m−2 d−1) were found in the inner shelf, and lower POC fluxes (80–150 mg C m−2 d−1) were in the outer shelf, respectively. PP values (~ 340–3380 mg C m−2 d−1) had analogous distribution patterns to POC fluxes, while some of PP values were significantly lower than POC fluxes, suggesting that contributions of resuspended particles to POC fluxes need to be appropriately corrected. A vertical mixing model was used to correct effects of bottom sediment resuspension, and the lowest and highest corrected POC fluxes were in the outer shelf (58 ± 33 mg C m−2 d−1) and the inner shelf (785 ± 438 mg C m−2 d−1), respectively. The corrected POC fluxes (486 to 785 mg C m−2 d−1) in the inner shelf could be the minimum value because we could not exactly distinguish the effect of POC flux from Changjiang influence with turbid waters. The results suggest that 27–93% of the POC flux in the ECS might be from the contribution of resuspension of bottom sediments rather than from the actual biogenic carbon sinking flux. While the vertical mixing model is not a perfect model to solve sediment resuspension because it ignores biological degradation of sinking particles, Changjiang plume (or terrestrial) inputs and lateral transport, it makes significant progress in both correcting the resuspension problem and in assessing a reasonable quantitative estimate of POC flux in a marginal sea.
... This is because 238 U is the direct, very long-lived and non-particle-reactive parent nuclide of 234 Th and 234 Th is a particle-reactive element that is rapidly removed from the seawater on time scales comparable to its short half-life (t 1/2 = 24.1 days). Therefore, 234 Th/ 238 U disequilibrium is suitable for tracing biologically particles Quigley et al., 2002;Santschi et al., 2003;Quiroz et al., 2006;Xu et al., 2011), and the timescales over which they operate, in the upper ocean (Coale and Bruland, 1985;Santschi et al., 2006). The approach commonly used to estimate the POC flux is based on determining the product of the POC/ 234 Th ratio from sinking particles at a particular depth and the integrated 234 Th flux calculated from 234 Th/ 238 U ratios in the water column above it (Eq. ...
... In contrast, most of the TCHO and URA were concentrated in NAEPS biopolymers (55% and 89%, respectively), followed by the SDS-extractable intracellular (27% and 7%, respectively) and AEPS biopolymers (13% and 3%, respectively). As an indicator of relative hydrophobicity of organic matter[Xu et al., 2011b], the ratio of protein to TCHO normalized to organic carbon (Protein-C/TCHO-C) was calculated for each biopolymer fraction (Table 1 andFigure 2b). Generally, the AEPS and Fe-Mn-associated metabolites showed comparable and highest relative hydrophobicity, followed by the SDS-extractable intracellular and NAEPS biopolymers, although the abundance of organic components in Fe-Mn-associated metabolites was relatively lower. ...
Article
Laboratory incubation experiments using the coccolithophore Emiliania huxleyi were conducted in the presence of 234Th, 233Pa, 210Pb, 210Po and 7Be to differentiate radionuclide uptake to the CaCO3 coccosphere from coccolithophore-associated biopolymers. The coccosphere (biogenic calcite exterior and its associated biopolymers), extracellular (non-attached and attached exopolymeric substances), and intracellular (sodium-dodecyl-sulfate extractable and Fe-Mn associated metabolites) fractions were obtained by sequentially extraction after E. huxleyi reached its stationary growth phase. Radionuclide partitioning and the composition of different organic compound classes, including proteins, total carbohydrates (TCHO) and uronic acids (URA) were assessed. 210Po was closely associated with the more hydrophobic biopolymers (high protein/TCHO ratio, e.g. in attached exopolymeric substances), while 234Th and 233Pa showed similar partitioning behavior with most activity being distributed in URA-enriched, non-attached exopolymeric substances and intracellular biopolymers. 234Th and 233Pa were nearly undetectable in the coccosphere, with a minor abundance of organic components in the associated biopolymers. These findings provide solid evidence that biogenic calcite is not the actual main carrier phase for Th and Pa isotopes in the ocean. In contrast, both 210Pb and 7Be were found to be mostly concentrated in the CaCO3 coccosphere, likely substituting for Ca2+ during coccolith formation. Our results demonstrate that even small cells (E. huxleyi) can play an important role in the scavenging and fractionation of radionuclides. Furthermore, the distinct partitioning behavior of radionuclides in diatoms (previous studies) and coccolithophores (present study) explains the difference in the scavenging of radionuclides between diatoms- and coccolithophore-dominated marine environments.
... Although the supernatant solution in sampling cups was not measured for 210 Po concentration, the degradation of trap particles was minimized by adding HgCl 2 preservative (Heussner et al., 1990). However, there exist some studies that reported the dissolution of particulates and the release of radionuclides during sediment trap deployment in lake setting (Schuler et al., 1991) and ocean (Hung et al., 2010;Xu et al., 2011). This warrants a caution for the measurement of 210 Po in the sediment trap experiment in the future. ...
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Vertical distributions of dissolved and particulate ²¹⁰Pb and ²¹⁰Po in the water column at the SouthEast Asian Time-series Study (SEATS, 18°00' N and 116°00' E) station in the northern South China Sea were determined from 4 cruises between January 2007 and June 2008. A large deficiency of ²¹⁰Pb, 379 ± 43 × 10³ dpm m⁻², from the secular equilibrium was found within the 3500 m water column. On the other hand, a smaller deficiency of ²¹⁰Po, 100 ± 21 × 10³ dpm m⁻², relative to ²¹⁰Pb was found in the water column. Time-series data showed insignificant temporal variability of the ²¹⁰Pb and ²¹⁰Po profiles. To balance these deficiencies, the removal fluxes for ²¹⁰Pb and ²¹⁰Po via particle settling range from 45 to 51 dpm m⁻² d⁻¹ and from 481 to 567 dpm m⁻² d⁻¹, respectively, are expected at 3500 m. The ²¹⁰Pb removal flux is comparable with, whereas the ²¹⁰Po removal flux is much higher than, the flux directly measured by moored sediment traps. The discrepancy between the modeled ²¹⁰Po flux and the measured flux suggests that sporadic events that enhance ²¹⁰Po removal via sinking ballast may occur in the water column at the site.
... The major discrepancies in POC measurements like adsorption of DOC onto filter, small sample volume (1 − 2 L) and capture of live zooplankton inside Niskin sample bottle were avoided in this study by filtering 8 L seawater onto 25 mm QMA filter . There are several studies on variability of POC fluxes and the factors controlling the POC/ 234 Th ratio in suspended and sinking particles as well as its dependency on the size and biochemical processes (Hung et al., 2004;Xu et al., 2011;Wei et al., 2011). For example, based on a sediment trap study in the northwest Pacific Ocean, Hung et al. (2012) demonstrated that sinking particles contained abundant diatom cells, fecal pellets, detritus and small particle aggregates mostly smaller than 50 μm that can account for a major fraction of the bulk sinking particles in open ocean regions and 234 Th was mainly carried by the smaller (< 50 μm) sinking particles. ...
Article
Seasonal variation in POC export flux, from the upper ocean, was studied during post southwest monsoon (October–November 2013) and spring intermonsoon (May 2014) at selected locations in the central and southern Arabian Sea and during northeast monsoon (January 2014) and spring intermonsoon (March 2014) in the northern Bay of Bengal. POC fluxes estimated from two particle-reactive radionuclide flux proxies (²³⁴Th/²³⁸U and ²¹⁰Po/²¹⁰Pb) were compared. Spatial and temporal variations were observed in the POC/radionuclide ratios, net deficit flux of radionuclides and the estimated POC export fluxes in both the basins. During post southwest monsoon, the ²³⁴Th flux (1346 ± 101 dpm m− 2 d− 1), POC/²³⁴Th ratio (5.96 ± 0.29 μmol dpm− 1) and associated POC flux (8.0 ± 0.6 mmol m− 2 d− 1) were high in the southern Arabian Sea than those in the central Arabian Sea (450 ± 95 dpm m− 2 d− 1, 1.76 ± 0.07 μmol dpm− 1 and 0.8 ± 0.2 mmol m− 2 d− 1, respectively). However, during spring intermonsoon these trends were similar in both the central and southern Arabian Sea with mean ²³⁴Th fluxes, POC/²³⁴Th ratios and ²³⁴Th based POC export fluxes being 2762 ± 61 dpm m− 2 d− 1, 1.45 ± 0.05 μmol dpm− 1 and 4.0 ± 0.14 mmol m− 2 d− 1, respectively. On the other hand, the ²¹⁰Po based POC export fluxes in the southern Arabian Sea (3.37 ± 0.32 and 6.73 ± 0.47 mmol m− 2 d− 1) were higher by a factor of 2–3 than those in the central Arabian Sea (2.08 ± 0.16 and 2.12 ± 0.28 mmol m− 2 d− 1) during post southwest monsoon and spring intermonsoon, respectively. In the northern Bay of Bengal, the ²³⁴Th based POC export fluxes were similar and low (0.6–1.2 mmol m− 2 d− 1) while the ²¹⁰Po based POC export fluxes were much higher (3.75–8.48 mmol m− 2 d− 1) during both the northeast monsoon and spring intermonsoon seasons. One reason could be that more production and carbon export occurring in months prior to the period preceding sample collection due to weeks to several months difference in the mean-lives of the isotopes. The computed POC export fluxes and inferred efficiency of the biological carbon pump in the Arabian Sea and the northern Bay of Bengal are differentiated by this different span in half-lives of ²³⁴Th and ²¹⁰Po (weeks to months, respectively). Also these values are distinct due to their biogeochemical behavior and preferred affinity for specific types of particles. This would require more joint application of each tracer in time and space for various biogeochemical regimes.
... In terms of partitioning coefficients, strong positive correlations between the Th K d values and the organic carbon partitioning between particulate and dissolved phases (i.e., logK d -OC, whereby the POC concentration, divided by the total mass concentration of added HA, and the sum of COC and truly Fig. 3e) was also found. Together, these observations all consistently suggest a tight association of Th isotopes with many but specific organic compounds during its adsorption onto soil particles (i.e., particulate Th) and its exchange with surrounding rainwater or groundwater (i.e., colloidal Th), as also observed in aquatic environments 19,34,35 . ...
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Uptake of six particle-reactive and/or redox-sensitive radionuclides (210Pb, 234Th, 7Be, 59Fe, 237Np and 233Pa) by 14 humic acids (HAs) was investigated in artificial groundwater under mildly acidic conditions (pH~5.5). In HA-groundwater slurry, Pb, Be, Fe and Pa bound strongly to particulate HA (>0.45 μm), supporting their application as tracers of soil erosion. Th bound strongly to the colloidal HA (3 kDa-0.45 μm) and as such, would not be a good candidate as a tracer for monitoring soil erosion. HAs likely reduced the oxidized neptunyl form (Np(V)O4 +) to Np(IV) based on its enhanced particle-reactivity and Np uptake by particulate HAs, partially retarding the movement of anthropogenic 237Np in field polluted environments. Particulate/colloidal carbonyl/O-aryl (likely through hydroquinone/quinone) functionalities in the HA correlated to Np and Pa uptake, but only particulate O-aryl functionalities was responsible for Fe uptake. The carboxylate- and carbonyl/O-aryl-containing organic functionalities in the HA correlated strongly with Th uptake. In contrast, no significant correlations between organic parameters and Pb or Be uptake implied their predominance of uniform surface adsorption onto particles. This study provides novel insight into the binding of six radionuclides with different organic functionalities of three size fractions, as well as its possible impact on their application in the soil tracing research.
... Carbohydrates, including dissolved, colloidal, and particulate carbohydrates, are a major organic component in the biogeochemical cycling of carbon and trace elements in aquatic systems (Pakulski and Benner, 1992; Hung et al., 2003; Hassler et al., 2011; Xu et al., 2011). Dissolved carbohydrates are the prominent identified class of dissolved organic carbon (DOC) compounds, accounting for 10e30% of the DOC pool in riverine, coastal, and marine waters (Pakulski and Benner, 1994; Hedges et al., 1994; Hung et al., 2005). ...
... This is because 238 U is the direct, very long-lived and non-particle-reactive parent nuclide of 234 Th and 234 Th is a particle-reactive element that is rapidly removed from the seawater on time scales comparable to its short half-life (t 1/2 = 24.1 days). Therefore, 234 Th/ 238 U disequilibrium is suitable for tracing biologically particles Quigley et al., 2002;Santschi et al., 2003;Quiroz et al., 2006;Xu et al., 2011), and the timescales over which they operate, in the upper ocean (Coale and Bruland, 1985;Santschi et al., 2006). The approach commonly used to estimate the POC flux is based on determining the product of the POC/ 234 Th ratio from sinking particles at a particular depth and the integrated 234 Th flux calculated from 234 Th/ 238 U ratios in the water column above it (Eq. ...
Article
234Th, a surface area specific particle-reactive radionuclide has been used to estimate particulate organic carbon (POC) export fluxes, based on the POC/234Th ratio of sinking particles and the 234Th flux, in the ocean. The POC/234Th ratios from large particles (>50 μm) are conventionally considered to represent sinking particles (collected by sediment traps), but this assumption has not been thoroughly tested. More recently, Hung and Gong (2010) separated sinking particles from the northern South China Sea (where diatoms are the most dominant group) using different Nitex screens and found that small (<50 μm) sinking particles dominate the bulk 234Th and POC flux. However, the size distribution of sinking particles has seldom been measured in picoplankton-dominated oligotrophic and upwelling conditions. Here we used Nitex screens to separate sinking particles in oligotrophic and upwelling regions. Additionally, images of selected bulk sinking particles (not subject to sequential filtration) were directly taken by scanning electron microscopy (SEM) to determine the size distribution of sinking particles. Within the trap-collected 234Th pool in the upwelling area, the 1–10 and 10–50 μm fractions had, on average, the highest amounts of 234Th, while POC did not show significant variations among the different size fractions. In the oligotrophic region, the 10–50 and 1–10 μm fractions had, on average, the highest amounts of 234Th, while the >150 and 10–50 μm had elevated shares of POC. Results thus demonstrate that 234Th was mainly carried by the smaller (<50 μm) sinking particles (∼70–80% of the total). SEM images of bulk sinking particles demonstrate that sinking particles contained abundant diatom cells, fecal pellets, detritus and small particle aggregates, mostly smaller than 50 μm. These images show that particles smaller than 50 μm indeed can account for a major fraction of the bulk sinking particles in open ocean regions. While the sequentially size-fractionated filtration might have influenced the original size distribution of sinking particles to some extent, the results provide new and consistent data which suggest that the contribution of particles smaller than 50 μm to the sinking 234Th flux can be a major fraction of the total sinking flux of this isotope, and thus, particles smaller than 50 μm cannot be ignored because the POC/234Th ratio that is often used is that of large (>50 μm) particles obtained from pump systems, not traps.
... The concentration of the total suspended solids (TSS; mg L À1 ) was determined by filtering water (150 mL) through a precombusted (4008C, 5 h) and preweighed GF/F filter that was rinsed before drying at 608C for a week, at which time it was reweighed. Concentrations of dissolved and total organic carbon (DOC, TOC; mg L À1 ) were determined on a Shimadzu TOC-L (Shimadzu, Columbia, MD) using triplicates of each sample against a standard calibration curve (Guo et al., 2002;Xu et al., 2011). Certain measurements for DOC and TOC were ultimately excluded because of potential processing error. ...
Article
The subtropical estuary Galveston Bay (Texas) is influenced by frequent riverine pulses from two important rivers in its northern sector and by ocean tides from the Gulf of Mexico in its southern sector. This study examined combinations of abiotic and biotic factors that synergistically influence spatiotemporal variability of autotrophic and heterotrophic microbial plankton abundances (0.2-20 μm). Potential nutrient limitation observed in situ was supported by in vitro enrichment bioassays. Shifts in the relative in situ abundance of autotrophs and heterotrophs, stained with SYBR Green I and enumerated on a flow cytometer, were significantly related to temperature, total nitrogen, dissolved inorganic nitrogen-to-phosphorus ratios, and total organic carbon concentrations along estuarine gradients. Observations reveal a pattern of serial colimitation of the microbial plankton community. Among the variables tested, nitrogen became the principal limiting factor for growth if phosphorous was available and temperatures were warm. In vitro nutrient enrichment bioassays performed with inorganic nitrogen (as nitrate) and Pi revealed variations in the responses between autotrophic and heterotrophic microbes under nutrient-limiting conditions, particularly during co-occurring phytoplankton blooms. The measured dynamics in planktonic relationships will have important impacts on understanding estuarine nutrient processing.
... 2.5. Particulate organic carbon (POC), δ 13 C, and Δ 14 C determination Particulate organic carbon (POC) concentration in the sinking aggregates and SPM were measured using a Perkin Elmer CHNS/O 2400 analyzer, after an acid-fuming step to eliminate inorganic carbon (Xu et al., 2010(Xu et al., , 2011a. The collected sinking aggregates which were acid fumed were sent to the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility for Δ 14 C analysis (Xu et al., 2018b). ...
Article
Large amounts of oil containing mucous-like marine snow formed in surface waters adjacent to the Deepwater Horizon spill that was implicated in oil delivery to the seafloor. However, whether chemical dispersants that were used increased or decreased the oil incorporation and sedimentation efficiency, and how exopolymeric substances (EPS) are involved in this process remains unresolved. To investigate the microbial responses to oil and dispersants in different oceanic settings, indicated by EPS production, petro- and non-petro carbon sedimentation, four mesocosm (M) experiments were conducted: 1) nearshore seawater with a natural microbial consortia (M2); 2) offshore seawater with f/20 nutrients (M3); 3) coastal seawater with f/20 nutrients (M4); 4) nearshore seawater with a natural microbial consortia for a longer duration (M5). Four treatments were conducted in M2, M3 and M4 whereas only three in M5: 1) a water accommodated fraction of oil (WAF), 2) a chemically-enhanced WAF prepared with Corexit (CEWAF, not in M5), 3) a 10-fold diluted CEWAF (DCEWAF); and 4) controls. Overall, oil and dispersants input, nutrient and microbial biomass addition enhanced EPS production. Dispersant addition tended to induce the production of EPS with higher protein/carbohydrate (P/C) ratios, irrespective of oceanic regions. EPS produced in M4 was generally more hydrophobic than that produced in M3. The P/C ratio of EPS in both the aggregate and the colloidal fraction was a key factor that regulated oil contribution to sinking aggregates, based on the close correlation with %petro-carbon in these fractions. In the short term (4-5 days), both the petro and non-petro carbon sedimentation efficiencies showed decreasing trends when oil/dispersants were present. In comparison, in the longer-term (16 days), petro-carbon sedimentation efficiency was less influenced by dispersants, possibly due to biological and physicochemical changes of the components of the oil-EPS-mineral phase system, which cooperatively controlled the sinking velocities of the aggregates.
... Particulate organic carbon (POC) and particulate organic nitrogen (PON) analyses were performed by filtering water samples (n ϭ 3) through a precombusted GF/F membrane (0.7 m; Whatman, USA). After removing the carbonates by HCl fuming, quantification was performed on a Perkin-Elmer Series II CHNS 2400 analyzer with acetanilide (71.09%) as the analytical standard (54). COC and CON analysis. ...
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Microbes release exoenzymes into the environment to break down complex organic matter and nutrients into simpler forms that can be assimilated and utilized, thereby addressing their cellular carbon, nitrogen, and phosphorus requirements. Despite its importance, the factors associated with the synthesis of exoenzymes are not clearly defined, especially for the marine environment. Here, we found that exoenzymes associated with nitrogen and phosphorus acquisition were strongly correlated with inorganic nutrient levels, while those associated with carbon acquisition depended on the type of organic carbon available. We also show a linear relationship between carbon- and nitrogen-acquiring exoenzymes and a strong correlation between microbial biomass and exoenzymes, highlighting their significance to microbial productivity. Last, we show that changes in microbial community composition are not strongly associated with changes in exoenzyme activity profiles, a finding which reveals a redundancy of exoenzyme activity functions among microbial community. These findings advance our understanding of previously unknown factors associated with exoenzyme production in the marine environment.
... Bovine Serum albumin (BSA) was used as the standard and the CSP concentration was expressed as mg-BSA equivalent/L. Particulate organic carbon (POC) concentration in the sinking marine snow, marine oil snow (MOS) and SPM were measured using a Perkin Elmer CHNS/O 2400 analyzer, after an acid-fuming step to eliminate inorganic carbon (Xu et al., 2011;Xu et al., 2010). ...
Article
Extracellular polymeric substances (EPS), produced by microorganisms, are implicated for greatly influencing the fate of environmental contaminants, including oil. Transparent exopolymeric particles (TEP) are gel-like acidic polysaccharide particles that can be stained with Alcian blue, whereas Coomassie stainable particles (CSP) contain proteins and are stained with Coomassie brilliant blue. Marine microgels are reversibly formed from EPS. These terms are often used interchangeably, but they have rarely been measured simultaneously. Mesocosm and bottle experiments provided an opportunity to compare EPS, TEP, CSP and microgels in a water-accommodated fraction (WAF) of oil and seawater (control). Our results reveal that the biopolymers making up EPS, TEP and CSP consisted primarily of polysaccharides and proteins, mostly likely as proteoglycans and glycoproteins. Significant correlations were found between concentrations of TEP-C vs particulate organic carbon (POC), TEP-C vs particulate organic nitrogen (PON), TEP vs EPS, TEP vs CSP, TEP vs carbohydrates, proteins, CSP and carbohydrates, CSP vs proteins, and carbohydrates vs proteins. Chemical analysis of whole particles and colloids yielded both protein and polysaccharides concentrations higher than those in EDTA extraction, thus providing an upper limit of actual EPS contents in the particulate phase. The EPS that was electrostatically held onto particle surfaces (extractable by 1% EDTA) accounted for a minor (~4%) yet relatively constant proportion of TEP. Overall, the concentrations of the three terms ranked in the order of [gels] > [TEP] > [particulate EPS] in the water. Lastly, spectrophotometric methods have limitations in identifying complex or refractory polysaccharides, as evidenced by the comparison between NMR-quantified EPS and the total EPS determined by spectrophotometric methods. This study is the first time these terms were compared in the same sample. They provide useful information when reviewing historical TEP, CSP, EPS data collected field- and laboratory-studies, and provide linkages between them. In addition, they also demonstrate that they could provide complementary information relevant to ecosystem and flux studies.
... Although most of these tracer applications are generally resting on inferences from correlations of isotopic ratios with bulk particle properties, experimental evidence reveals organic compound-specific scavenging and partitioning for different radionuclides, either in particulate or in colloidal phases (Hayes et al., 2015a,b;Lam et al., 2015;Lin et al., 2015). Furthermore, while previous focus was on inorganic matter as the main binding agents (e.g., opal, carbonate, and lithogenic; Chase et al., 2002;Guo et al., 2002;Roy-Barman et al., 2009;Lin et al., 2014), more recent experimental studies reveal the role of specific organic matter compounds in the scavenging of different radionuclides in marine environments (Roberts et al., 2009;Xu et al., 2011a;Chuang et al., 2014Chuang et al., , 2015aLin et al., 2017). ...
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Through a combination of selective extractions and molecular characterization techniques including Isoelectric Focusing Chromatography and Electrospray Ionization Fourier-Transform Ion Cyclotron Resonance Mass spectrometry, molecular structures of diatom ( Phaeodactylum tricornutum ) and coccolithophore ( Emiliania huxleyi )-associated biopolymers that are responsible for the distinct partitioning behavior between ²¹⁰ Pb and ²¹⁰ Po were determined. Our results show that diatom-derived biopolymers have distinctive elemental grouping distributions as compared to those excreted by the coccolithophore, with the former consisting of more heterogeneous elements (i.e., nitrogen, sulfur and phosphorus-containing organic compounds). For the coccolithophore culture, two ²¹⁰ Pb-enriched biopolymers (non-attached exopolymeric substances and coccosphere shell-associated biopolymers) have a higher abundance of CHO-type compounds, suggesting CHO-only-type compounds as the main binding moieties for ²¹⁰ Pb. In contrast, such association was not evident in the diatom culture. Different with ²¹⁰ Pb, ²¹⁰ Po enrichment in coccolithophore-derived attached exopolymeric substances and Fe-Mn-associated metabolites coincided with the higher abundance of nitrogen/sulfur-containing organic compounds in these two biopolymer fractions, suggesting the strong parallel of Po with the production of nitrogen-rich organic matter as well as sulfur-containing amino acids. These different associations between ²¹⁰ Pb/ ²¹⁰ Po and organic functional groups were further explored by separating ²¹⁰ Pb or ²¹⁰ Po-labeled coccolithophore-derived biopolymers via isoelectric focusing. This technique suggests that phosphate group-containing molecules but not the other molecules that contain heterogeneous elements (e.g., CHONS, CHON, and CHOS) as the strongest binding agents for ²¹⁰ Pb, while the more hydrophobic (high protein to carbohydrate ratio) nitrogen/sulfur-enriched organic moieties acted as the main ²¹⁰ Po-binding ligands. It is concluded that the deficiency of ²¹⁰ Po with respect to ²¹⁰ Pb can be influenced by the relative abundance of nitrogen/sulfur-enriched organic moieties to the nitrogen/sulfur-depleted organic compounds in the water column. This behavior constrains the application of ²¹⁰ Po- ²¹⁰ Pb approach to quantify the particulate organic carbon (POC) export flux in the ocean. It also explains that differences in chemical binding of the ²¹⁰ Po as compared to those of other radionuclides (e.g., thorium-234) as the main factor. That suggests that differences in decay half-lives or physical factors are less important when these nuclides are applied to estimate the POC flux in the ocean.
... Subclasses of EPS are named depending on the method of characterization (see review of Quigg et al., 2016). If alcian blue staining is used with Xanthan as the standard, the particulate fraction (retained on a filter) is named transparent exopolymer particles (TEP) (Passow, 1995;Bar-Zeev et al., 2011;Xu et al., 2011). If alginic acid is used as a standard, the particulate material is called acid polysaccharides (APS) (Hung et al., 2003). ...
Article
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Microbes (bacteria, phytoplankton) in the ocean are responsible for the copious production of exopolymeric substances (EPS) that include transparent exopolymeric particles. These materials act as a matrix to form marine snow. After the Deepwater Horizon oil spill, marine oil snow (MOS) formed in massive quantities and influenced the fate and transport of oil in the ocean. The processes and pathways of MOS formation require further elucidation to be better understood, in particular we need to better understand how dispersants affect aggregation and degradation of oil. Toward that end, recent work has characterized EPS as a function of microbial community and environmental conditions. We present a conceptual model that incorporates recent findings in our understanding of the driving forces of MOS sedimentation and flocculent accumulation (MOSSFA) including factors that influence the scavenging of oil into MOS and the routes that promote decomposition of the oil post MOS formation. In particular, the model incorporates advances in our understanding of processes that control interactions between oil, dispersant, and EPS in producing either MOS that can sink or dispersed gels promoting microbial degradation of oil compounds. A critical element is the role of protein to carbohydrate ratios (P/C ratios) of EPS in the aggregation process of colloid and particle formation. The P/C ratio of EPS provides a chemical basis for the “stickiness” factor that is used in analytical or numerical simulations of the aggregation process. This factor also provides a relative measure for the strength of attachment of EPS to particle surfaces. Results from recent laboratory experiments demonstrate (i) the rapid formation of microbial assemblages, including their EPS, on oil droplets that is enhanced in the presence of Corexit-dispersed oil, and (ii) the subsequent rapid oil oxidation and microbial degradation in water. These findings, combined with the conceptual model, further improve our understanding of the fate of the sinking MOS (e.g., subsequent sedimentation and preservation/degradation) and expand our ability to predict the behavior and transport of spilled oil in the ocean, and the potential effects of Corexit application, specifically with respect to MOS processes (i.e., formation, fate, and half-lives) and Marine Oil Snow Sedimentation and Flocculent Accumulation.
... The coexistence of inorganic and organic iodine species has been reported in various environments (Baker et al., 2001;Couture and Seitz, 1983;Muramatsu, 1989;Schwehr et al., 2005a;Schwehr et al., 2005b;Xu et al., 2012a;Xu et al., 2011a;Xu et al., 2011b;Xu et al., 2011c;Xu et al., 2012b;Yuita, 1992;Yuita and Kihou, 2005). For example, methyl iodide is an important gaseous form of iodine in the marine atmosphere and in releases from nuclear fuel reprocessing facilities, while dissolved organo-I compounds comprise up to 40% of total iodine in aqueous samples from estuaries, rivers, and rain (Santschi and Schwehr, 2004). ...
Article
129I is commonly either the top or among the top risk drivers, along with 99Tc, at radiological waste disposal sites and contaminated groundwater sites where nuclear material fabrication or reprocessing has occurred. The risk stems largely from 129I having a high toxicity, a high bioaccumulation factor (90% of all the body's iodine concentrates in the thyroid), a high inventory at source terms (due to its high fission yield), an extremely long half-life (16M years), and rapid mobility in the subsurface environment. Another important reason that 129I is a key risk driver is that there is uncertainty regarding its biogeochemical fate and transport in the environment. We typically can define 129I mass balance and flux at sites, but cannot predict accurately its response to changes in the environment. As a consequence of some of these characteristics, 129I has a very low drinking water standard, which is set at 1 pCi/L, the lowest of all radionuclides in the Federal Register. Recently, significant advancements have been made in detecting iodine species at ambient groundwater concentrations, defining the nature of the organic matter and iodine bond, and quantifying the role of naturally occurring sediment microbes to promote iodine oxidation and reduction. These recent studies have led to a more mechanistic understanding of radioiodine biogeochemistry. The objective of this review is to describe these advances and to provide a state of the science of radioiodine biogeochemistry relevant to its fate and transport in the terrestrial environment and provide information useful for making decisions regarding the stewardship and remediation of 129I contaminated sites. As part of this review, knowledge gaps were identified that would significantly advance the goals of basic and applied research programs for accelerating 129I environmental remediation and reducing uncertainty associated with disposal of 129I waste. Together the information gained from addressing these knowledge gaps will not alter the observation that 129I is primarily mobile, but it will likely permit demonstration that the entire 129I pool in the source term is not moving at the same rate and some may be tightly bound to the sediment, thereby smearing the modeled 129I peak and reducing maximum calculated risk.
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To better understand carbon cycling in marginal seas, particulate organic carbon (POC) concentrations, POC fluxes and primary production (PP) were measured in the East China Sea (ECS) in summer 2007. Higher concentrations of POC were observed in the inner shelf and lower POC values were found in the outer shelf. Similar to POC concentrations, elevated uncorrected POC fluxes (720-7300 mg C m-2 d-1) were found in the inner shelf and lower POC fluxes (80-150 mg C m-2 d-1) were in the outer shelf, respectively. PP values (~340-3380 mg C m-2 d-1) had analogous distribution patterns to POC fluxes, while some of PP values were significantly lower than POC fluxes, suggesting that contributions of resuspended particles to POC fluxes need to be appropriately corrected. A vertical mixing model was used to correct effects of bottom sediment resuspension and the corrected POC fluxes ranging from 41 ± 20 to 956 ± 443 mg C m-2 d-1, which were indeed lower than PP values. The results suggest that 49-93% of the POC flux in the ECS might be from the contribution of resuspension of bottom sediments rather than from the actual biogenic carbon sinking flux. While the vertical mixing model is not a perfect model to solve sediment resuspension because it ignores biological degradation of sinking particles, Changjinag plume (or terrestrial) inputs and lateral transport, it makes significant progress in both correcting resuspension problem and in assessing a reasonable quantitative estimate in a marginal sea.
Article
Laboratory studies were conducted to examine the sorption of selected radionuclides (234Th, 233Pa, 210Po, 210Pb, and 7Be) onto inorganic (pure silica and acid-cleaned diatom frustules) and organic (diatom cells with or without silica frustules) particles in natural seawater and the role of templating biomolecules and exopolymeric substances (EPS) extracted from the same species of diatom, Phaeodactylum tricornutum, in the sorption process. The range of partition coefficients (Kd, reported as logKd) of radionuclides between water and the different particle types was 4.78-6.69 for 234Th, 5.23-6.71 for 233Pa, 4.44-5.86 for 210Pb, 4.47-4.92 for 210Po, and 4.93-7.23 for 7Be, similar to values reported for lab and field determinations. The sorption of all radionuclides was significantly enhanced in the presence of organic matter associated with particles, resulting in Kd one to two orders of magnitude higher than for inorganic particles only, with highest values for 7Be (logKd of 7.2). Results further indicate that EPS and frustule-embedded biomolecules in diatom cells are responsible for the sorption enhancement rather than the silica shell itself. By separating radiolabeled EPS via isoelectric focusing, we found that isoelectric points are radionuclide specific, suggesting that each radionuclide binds to specific biopolymeric functional groups, with the most efficient binding sites likely occurring in acid polysaccharides, iron hydroxides, and proteins. Further progress in evaluating the effects of diatom frustule—related biopolymers on binding, scavenging, and fractionation of radionuclides would require the application of molecular-level characterization techniques.
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We present here a global oceanic compilation of 234Th measurements that collects results from researchers and laboratories over a period exceeding 50 years. The origin of the 234Th sampling in the ocean goes back to 1967, when Bhat et al. (1969) initially studied 234Th distribution relative to its parent 238U in the Indian Ocean. However, it was the seminal work of Buesseler et al. (1992) – which proposed an empirical method to estimate export fluxes from 234Th distributions – that drove the extensive use of the 234Th–238U radioactive pair to evaluate the organic carbon export out of the surface ocean by means of the biological carbon pump. Since then, a large number of 234Th depth profiles have been collected using a variety of sampling instruments and strategies that have changed during the past 50 years. The present compilation is made of a total 223 data sets: 214 from studies published in either articles in refereed journals, PhD theses, or repositories, as well as 9 unpublished data sets. The data were compiled from over 5000 locations spanning all the oceans for total 234Th profiles, dissolved and particulate 234Th activity concentrations (in dpm L−1), and POC:234Th ratios (in µmol dpm−1) from both sediment traps and filtration methods. A total of 379 oceanographic expeditions and more than 56 600 234Th data points have been gathered in a single open-access, long-term, and dynamic repository. This paper introduces the dataset along with informative and descriptive graphics. Appropriate metadata have been compiled, including geographic location, date, and sample depth, among others. When available, we also include water temperature, salinity, 238U data (over 18 200 data points), and particulate organic nitrogen data. Data source and method information (including 238U and 234Th) is also detailed along with valuable information for future data analysis such as bloom stage and steady-/non-steady-state conditions at the sampling moment. The data are archived on the PANGAEA repository, with the dataset DOI https://doi.org/10.1594/PANGAEA.918125 (Ceballos-Romero et al., 2021). This provides a valuable resource to better understand and quantify how the contemporary oceanic carbon uptake functions and how it will change in future.
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The production of extracellular polymeric substances (EPS) by planktonic microbes can influence the fate of oil and chemical dispersants in the ocean through emulsification, degradation, dispersion, aggregation, and/or sedimentation. In turn, microbial community structure and function, including the production and character of EPS, is influenced by the concentration and chemical composition of oil and chemical dispersants. For example, the production of marine oil snow and its sedimentation and flocculent accumulation to the seafloor were observed on an expansive scale after the Deepwater Horizon oil spill in the Northern Gulf of Mexico in 2010, but little is known about the underlying control of these processes. Here, we review what we do know about microbially produced EPS, how oil and chemical dispersant can influence the production rate and chemical and physical properties of EPS, and ultimately the fate of oil in the water column. To improve our response to future oil spills, we need a better understanding of the biological and physiochemical controls of EPS production by microbes under a range of environmental conditions, and in this paper, we provide the key knowledge gaps that need to be filled to do so.
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Th (t1/2 = 24.1 d), present in seawater, is a naturally occurring particle-reactive radionuclide formed through the radioactive decay of its parent, 238U (t1/2 = 4.47 x 109 y). The 234Th:238U disequilibrium is exploited to quantify fluxes of elements moving out of the euphotic zone by attaching on to sinking particles. Under the Indian GEOTRACES programme, high-resolution sampling in the upper 300 m depth was carried out at 11 stations in the Arabian Sea and the Indian Ocean during April – May 2014 from 17°N to 16°S along 65°E transect to estimate the 234Th based particulate organic carbon (POC) export flux from the upper ocean. Average 234Th fluxes integrated to 100 m depth were 2612 and 1968 dpm m-2 d-1 for the Arabian Sea and the Indian Ocean, respectively. The estimated POC export fluxes ranged from negligible to 9.0 mmol m-2 d-1 and the 234Th based POC export efficiencies were <2 to 5%. For the same season, the Arabian Sea and the Bay of Bengal showed highly contrasting carbon export trends (mean: 4.0 and 0.8 mmol C m-2 d-1, respectively). The modeled POC export fluxes from in situ and satellite derived primary production are higher than the 234Th based values for the Laws and Dunne models and are comparable for the Henson model. The modeled POC fluxes which depend on surface temperature and primary production could be further refined for the seasonal cycle in biological productivity and associated differences in trophic structure, grazing intensity, recycling efficiency, high bacterial activity and associated DOC export.
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Large amounts of mucous-rich oil-containing marine snow formed in surface waters adjacent to the Deepwater Horizon spill. This marine oil snow (MOS) was implicated in oil delivery to the seafloor. Whether the use of chemical dispersants increased or decreased MOS sedimentation remains controversial. We conducted mesocosm experiments testing the effects of oil and oil plus a dispersant on MOS formation and sedimentation, in coastal seawater. The four treatments used were a surrogate Macondo oil water accommodated fraction (“WAF”), two concentrations of chemically-enhanced WAF (“CEWAF” and diluted CEWAF, “DCEWAF”) containing a dispersant and oil, and Controls (no additions). Based on radiocarbon and ¹³C NMR results, the presence of dispersants enhanced the amounts of petro-carbon being incorporated into the sinking oil-carrying aggregates (aka MOS). However, most of the chemically-dispersed oil preferentially partitioned into the colloidal and suspended particulate fractions rather than into the rapidly forming MOS. Thus the oil and non-petro-carbon sedimentation efficiency in treatments with a dispersant was much lower, compared to those in the Control and WAF treatments, during the four-day mesocosm experiment. Formation of MOS and its subsequent sinking sequestered the oil in two stages: first via terrestrial-derived detritus containing humic compounds, and subsequently via freshly produced material, such as exopolymeric substances produced by phytoplankton and bacteria.
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The time-series station, SEATS (18° N, 116° E) in the South China Sea was visited six times during October 2006-December 2008 to carry out seawater sampling and floating trap deployments for the determination of distributions and fluxes of POC, PIC, PN, 234Th, 210Pb, and 210Po in the upper 200 m of the water column. Radionuclide deficiencies resulted in removal fluxes from the euphotic layer of 1.1 × 103-1.8 × 103 dpm m-2 d-1 and 7.1-40.2 dpm m-2 d-1 for 234Th and 210Po, respectively. Due to atmospheric input, an excess of 210Pb relative to 226Ra is commonly observed in the upper water column. Sinking fluxes of total mass, POC, PIC, PN, 234Th, 210Pb, and 210Po measured at the euphotic depth were low in summer-fall and high in winter-spring, reflecting the seasonal variability of biological pumping. Excluding the suspiciously low primary productivity data point in July 2007, a relatively high e-ratio of 0.28-0.69 was estimated by the ratio of the POC flux at the euphotic depth and the integrated primary productivity. The ratios of 234Th, 210Pb, and 210Po to organic carbon, inorganic carbon, and nitrogen in the sinking particles were combined with the disequilibria of 234Th-238U, 210Pb-226Ra, and 210Po-210Pb to estimate export fluxes of POC, PIC, and PN from the euphotic layer. Compared with measured fluxes by the sediment trap and estimated fluxes by other approaches, it is concluded that the export production in the South China Sea, ranging from 1.8 to 21.3 mmol-C m-2 d-1, can be reasonably estimated using 234Th, 210Pb, and 210Po as carbon proxies.
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Two cruises were made to the northeast Gulf of Mexico in October 1996 and August 1997. The main objectives of the cruises were to survey cetacean and seabird populations and describe their hydrographic and biological environments. An additional objective was to characterize cetacean and seabird habitats in terms of food resources. During both cruises a cyclone and an anticyclone were sampled as well as the confluence region between them. Zooplankton and micronekton were sampled both directly with nets and indirectly with a 153-kHz acoustic Doppler current profiler. Within cruises, zooplankton and micronekton biomass was higher in cyclones than in anticyclones. Biomass within the confluence was either highest or intermediate for both cruises. Between cruises, within features, August 1997 biomass was significantly higher than October 1996 biomass.
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We report primary production of organic matter and organic carbon removal from three subtropical open ocean time-series stations, two located in the Atlantic and one in the Pacific, to quantify the biological components of the oceanic carbon pump. We find that within subtropical gyres, export production varies considerably despite similar phytoplankton biomass and productivity. We provide evidence that the removal of organic carbon is linked to differences in nutrient input into the mixed layer, both from eddy induced mixing and dinitrogen fixation. These findings contribute to our knowledge of the spatial heterogeneity of the subtropical oceans, which make up more than 50% of all ocean area and are thought to spread in the course of CO2- induced global warming.
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We have performed the first known semicontinuous monitoringof particulate organic carbon (POC) fluxes and dissolved Siconcentrations delivered to the ocean during a typhoon. Samplingof the Choshui River in Taiwan during Typhoon Mindulle in 2004revealed a POC flux of 5.00 x 105 t associated with a sedimentflux of 61 Mt during a 96 h period. The linkage of high amountsof POC with sediment concentrations capable of generating ahyperpycnal plume upon reaching the ocean provides the firstknown evidence for the rapid delivery and burial of POC fromthe terrestrial system. These fluxes, when combined with storm-derivedCO2 consumption of 1.65 x 108 mol from silicate weathering,elucidate the important role of these tropical cyclone eventson small mountainous rivers as a global sink of CO2.
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Time series sediment trap samples were used to examine the rates and mechanisms of particulate organic carbon (POC) flux at three continental margin locations, Santa Barbara Basin, Cariaco Basin, and Guaymas Basin, marked by seasonal upwelling and high primary production. The mean POC flux in Santa Barbara Basin (0.096 g m−2 d−1) is nearly twice that of Cariaco Basin and 4 times higher than that in Guaymas Basin, with all three sites having POC fluxes significantly higher than the open ocean average (0.007 g m−2 d−1). In Cariaco Basin, the only site with available primary production numbers, there is no significant relationship between POC flux and monthly primary production. Rather, POC fluxes in all three areas strongly correlate with mineral (carbonate, opal, and lithogenic material) fluxes. This supports the “mineral ballast” hypothesis, where it has been suggested that higher-density minerals enhance the flux of organic matter to the deep ocean. On the basis of multiple regression analysis, the three mineral components explain 72% of the total variance in POC fluxes at the three study sites, with biogenic carbonate particles being the most effective transport mineral.
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Bacterial growth rates and carbon production rates were measured by thymidine and leucine incorporation on aggregates formed after incubation, in rolling tanks, of water samples from the River Weser, Germany. Thymidine and leucine incorporation rates per aggregate decreased with increasing pool size when several aggregates were incubated together in a vial under still conditions. The thymidine and leucine incorporation rates per aggregate were on average 5.6- to 5.3-fold lower, respectively when 5 aggregates were pooled together in vials than those of aggregates which were incubated individually and kept in suspension. The use of oxygen microelectrodes showed that, in contrast to when single aggregates were suspended, the mean diffusion distance for solutes in the surrounding water of the aggregates increased when aggregates were pooled together in vials and incubated under still conditions. Bacterial production and respiration rates were determined by combined radiotracer and microelectrode measurements on 36 individual aggregates, which were suspended during measurements. The average bacterial carbon production and respiration rates were 5.1 +/- 6.3 and 8.5 +/- 11.6 ng C agg(-1) h(-1), respectively. The average growth rate constant (mu) was 1.24 +/- 0.32 d(-1) and the average growth efficiency was 0.35 +/- 0.19. Our results demonstrate that the exchange of solutes, e.g. oxygen and radiotracers, between the aggregates and the surrounding water is reduced when aggregates are pooled in a vial as opposed to when they are suspended. Previous measurements may, therefore, have underestimated bacterial growth in aggregates due to the incubation method used.
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Phytoplankton exopolymeric substances (EPS) contribute significantly to the dissolved organic carbon (DOC) pool in the ocean, playing crucial roles in the surface ocean carbon cycle. Recent studies have demonstrated that similar to 10% of marine DOC can self-assemble as microgels through electrostatic Ca bonds providing hotspots of enriched microbial substrate. However, the question whether EPS can self-assemble and the formation mechanisms for EPS microgels have not been examined. Here we report that EPS from three representative phytoplankton species, Synechococcus, Emiliania huxleyi, and Skeletonema costatum can spontaneously self assemble in artificial seawater (ASW), forming microscopic gels of similar to 3 - 4 mu m in diameter. Different from the marine DOC polymers assembly, these EPS samples can self-assemble in Ca(2+) free ASW. Further experiments from fluorescence enhancement and chemical composition analysis confirmed the existence of fair amounts of hydrophobic domains in these EPS samples. These results suggest that hydrophobic interactions play a key role in the assembly of EPS from these three species of marine phytoplankton.
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The export flux of particulate organic carbon from the euphotic zone in the central equatorial Pacific was measured using an approach that utilizes 234Th and organic carbon analyses on water column and drifting sediment trap samples. This study was conducted as part of the U.S. Joint Global Ocean Flux Study (U.S. JGOFS) EqPac process study from 12°N to 12°S at 140°W. Samples were collected during the Survey I (February–March 1992) and Survey II (August–September 1992) cruises. The accuracy of drifting sediment traps was evaluated by comparing the measured flux of 234Th with the flux calculated from the deficiency of 234Th relative to 238U in the water column. Calculated 234Th fluxes were corrected for the effects of horizontal and vertical advection. The uncertainties on these 234Th fluxes averaged 39% for Survey I and 20% for Survey II. Comparison of measured and calculated 234Th fluxes revealed evidence for overtrapping, especially in the shallow traps (≤ 100 m). Measured and calculated 234Th fluxes agreed to within 50% for traps at 150–250 m. Good correlation was obtained between measured fluxes of organic carbon and 234Th except for some shallow samples high in organic carbon, suggesting that 234Th was a good tracer for organic carbon. The flux of particulate organic carbon (POC) was calculated as the product of the calculated flux of 234Th times the organic carbon/234Th ratio in trap samples. Assuming that the organic carbon/234Th ratio in trap samples was representative of sinking particles, we used an average value for the organic carbon/234Th ratio for each station. The variability in the station-averaged POC/234Th ratio ranged from 10% to 30%. The POC fluxes calculated using our combined 234Th-trap approach ranged from 1 to 6 mmol C m−2 day−1 during Survey I, and from 2 to 30 mmol C m−2 day−1 during Survey II. The average uncertainty for the POC fluxes was ±60%. Primary and new production integrated to the depth of the 0.1 % light level varied by factors of 2–3 for Survey I and Survey II, respectively. The export of particulate organic carbon from the euphotic zone also increased by a factor of 3. The corresponding e-ratios (POC export/primary production) ranged from 0.03 to 0.11 for Survey I, and 0.04 to 0.23 for Survey II. Annual average regional rates (10°N–10°S; 90°W–180°E) of new (0.47 Gt C year−1) and particulate export (0.42 Gt C year−1) production were in good agreement, suggesting that, on an annual basis, significant export of DOC need not be invoked to balance new and export production in this region.
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Thorium-234 is a naturally occurring radionuclide that has been widely studied in ocean sciences. These studies use the disequilibrium between the soluble parent uranium-238 (T 1/2 = 4.5·109 y), and its particle reactive daughter, 234Th (T 1/2 = 24.1 d), to quantify the in-situ removal rates of 234Th on sinking particles. Here, we present additional experiments that test a new 2-l procedure in which 234Th is co-scavenged with a MnO2 precipitate. Unlike other techniques, this method can be easily applied at-sea with an overall precision and accuracy of 5%. Our experiments have sought to elucidate the effects of delaying reagent addition and precipitate filtration, differences in sample bottle types, and issues related to sample backgrounds and 234Th particulate sampling. Most of these experiments were conducted using water collected on repeated occupations of station ALOHA (22°45.0'N, 158°00.0'W), 100 km North of Oahu, Hawaii.
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A large pool of organic carbon resides in the world's oceans in the form of dissolved organic matter (DOM) *RF 1,2*.DOM is operationally defined as the fraction of organic matter that passes through a filter with a given pore size (which can range from less than 0.1 micro m to 0.46 micro m). This fraction has a longer oceanic residence time-and is generally less biodegradable-than particulate organic matter (POM) [1-4]. Processes transforming DOM into POM are therefore crucial for our understanding of the cycling of organic material in the oceans [1-4]. The aggregation of marine colloids, which constitute 10-40% of DOM [2,3,5], is thought to be an important step in the transformation of DOM into POM [3]. It has been suggested that colloids, as well as transparent exopolymer particles and large aggregates ('marine snow') can be viewed as polymer gels [6-8]. Whether free DOM polymers can indeed spontaneously assemble to form polymer gels has, however, not yet been shown. Here we present experimental obse
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Picoplankton, i.e., cells smaller than 2-3 mu m, dominate in most open oceanic regions, such as in the Pacific Ocean. In these areas, the dominant carotenoid of photosynthetic eukaryotes is 19'-hexanoyloxyfucoxanthin (19HF), considered to be a diagnostic marker for prymnesiophytes. This suggests that this class could be a major component of eukaryotic picoplankton, despite the fact that virtually no prymnesiophyte has been described to date from this size class. To address this question, we assessed prymnesiophyte diversity and abundance in natural picoplankton communities, using a molecular approach. Total genomic DNA was isolated from 3- mu m-filtered samples collected in the Pacific Ocean. Small subunit (18S) ribosomal RNA genes (rDNA) were amplified by the polymerase chain reaction (PCR) using universal eukaryotic primers. The relative abundance of 18S rDNA from prymnesiophytes was quantified using group-specific and eukaryotic 18S rDNA probes. The percentage of the prymnesiophyte versus total 18S rDNA was much lower than the percentage of prymnesiophytes calculated on the basis of pigment analyses for the same samples. 18S rDNA libraries from five samples were screened using a prymnesiophyte-specific oligonucleotide probe, and 14 nearly complete 18S rDNA sequences were retrieved. Phylogenetic analysis of these sequences established the presence of several prymnesiophyte lineages with no equivalent among cultivated species.
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Rapid mass sinking of cells following diatom blooms, observed in lakes and the sea, is argued here to represent the transition from a growing to a resting stage in the life histories of these algae. Mass sinking is of survival value in those bloom diatoms that retain viability over long periods in cold, dark water but not in warm, nutrient-depleted surface water. Mechanisms for accelerating sinking speed of populations entering a resting or seeding mode are proposed. Previously unexplained features of diatom form and behaviour take on a new meaning in this context of diatom seeding strategies. Diatoms have physiological control over buoyancy as declining growth is accompanied by increasing sinking rates, where the frustule acts as ballast. Increased mucous secretion in conjunction with the cell protuberances characteristic of bloom diatoms leads to entanglement and aggregate formation during sinking; the sticky aggregates scavenge mineral and other particles during descent which further accelerates the sinking rate. Such diatom flocs will have sinking rates of 100 m d-1 or more. This is corroborated by recent observations of mass phytoplankton sedimentation to the deep sea. This mechanism would explain the origin of marine snow flocs containing diatoms in high productivity areas and also the well-known presence of a viable deep sea flora. That mortality is high in such a seeding strategy is not surprising. A number of species-specific variables pertaining to size, morphology, physiology, spore formation and frustule dissolution rate will determine the sinking behaviour and thus control positioning of resting stages in the water column or on the bottom. It is argued that sinking behaviour patterns will be environmentally selected and that some baffling aspects of diatom form and distribution can be explained in this light. Rapid diatom sedimentation is currently believed to be mediated by zooplankton faecal pellets, particularly those of copepods. This view is not supported by recently published observations. I speculate that copepod grazing actually retards rather than accelerates vertical flux, because faecal pellets tend to be recycled within the surface layer by the common herbivorous copepods. Egestion of undigested food by copepods during blooms acts as a storage mechanism, as ungrazed cells are likely to initiate mass precipitation and depletion of the surface layer in essential elements. Unique features of diatoms are discussed in the light of their possible evolution from resting spores of other algae. An evolutionary ecology of pelagic bloom diatoms is deduced from behavioural and morphological characteristics of meroplanktonic and tychopelagic forms. Other shell-bearing protistan plankters share common features with diatoms. Similar life-history patterns are likely to be present in species from all these groups. The geological significance of mass diatom sinking in rapidly affecting transfer of biogenic and mineral particles to the sea floor is pointed out.
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In simulation studies of the ocean's role in the global carbon cycle, predicting the depth-distribution for remineralization of particulate organic carbon (POC) is of particular importance. Following Sarmiento et al. (Global Biogeochemical Cycles 7 (1993) 417), most simulation models have the power-law curve of Martin et al. (Deep-Sea Research 34 (1987) 267) for this purpose. The Martin et al. curve is an empirical fit to data, most of which is from shallow floating sediment traps. Using such a fit implies that all the information necessary for prediction is contained in the carbon flux itself, so that the organic-carbon flux FOC(z) at any depth z can be predicted from the flux of organic carbon FOC(z0) at some near-surface depth z0. Here, we challenge this basic premise, arguing that fluxes of ballast minerals (silicate and carbonate biominerals, and dust) determine deep-water POC fluxes, so that a mechanism-based model of POC flux must simultaneously predict fluxes of both POC and ballast minerals. This assertion is based on the empirical observation that POC fluxes are tightly linked quantitatively to fluxes of ballast minerals in the deep ocean. Here, we develop a model structure that incorporates this observation, and fit this model to US JGOFS EqPac data. This model structure, plus the preliminary parameter estimates we have obtained, can be used to explore the implications of our model in studies of the ocean carbon cycle.
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In this book, we describe our current state of knowledge on circulation in the Gulf of Mexico through 22 papers at the forefront of research. The view is selective rather than all-inclusive, with primary focus on circulation at the sea surface and at depth, including nearshore flow. In choosing topics, we have attempted to avoid the distinction between observations and models, as both are essential to advance our understanding of this exceptional body of water. We have also written for specialists and non-specialists alike in both science and industry: for those who work directly on the science associated with the Gulf, and for those whose work depends on the Gulf. And we refer here to physical oceanography, marine geology, sedimentology, coastal and estuarine science, as well as to the petrochemical and fishing industries.
Article
The lipid class, fatty acid molecular assemblage and the compound-specific δ13C composition of individual fatty acids were examined as part of a study to elucidate the transfer of primary photosynthate from the euphotic zone to benthic/hyperbenthic environments in Conception Bay (Newfoundland, Canada). Total lipid and lipid classes from rapidly settling particles showed a decrease in fluxes between the sediment traps collecting at 80 and 220 m depths. The flux difference is most pronounced in the wax and steryl esters fraction where hardly any discernible amounts are recorded in the 220 m traps. The difference in total lipid fluxes with depth likely reflects active heterotrophic utilization at the intervening depths, but with the wax and steryl esters fraction also influenced by zooplankton activity at the trap deployment depths. The latter is most clearly apparent when poisoned and non-poisoned trap fluxes of wax and steryl esters are compared indicating significant contribution from zooplankton (swimmers) in the poisoned traps. Temporal variations in fatty acid molecular signatures agreed with previously recognized shifts in predominant phytoplankton taxonomy during spring blooms in cold ocean environments; i.e. the diatom contribution maximized at the very early stages of the spring bloom and dinoflagellate input was highest at the latest stages of the bloom. Compound-specific fatty acid δ13C values are highest at the peak of the bloom, and become 13C-depleted towards the end of the bloom. The δ13C temporal shift is consistent with the severe limitation in the availability of dissolved inorganic carbon substrate in the euphotic zone at the height of the bloom. However, the specific effects of growth rate and geometric parameters in fatty acid isotopic composition are also likely reflected in the observed δ13C temporal trend.
Article
In order to study the role of polysaccharides (PS) in the colloidal organic matter (COM) pool for complexing 234 Th, controlled laboratory experiments were carried out to determine the chemical nature of the strong Th(IV) binding to macromolecular organic ligands (.1 kDa). The partition coefficient of 234 Th between marine COM and solution, Kc, is higher than that for any known marine mineral sorbent. PS-enriched fractions of COM had the highest partition coefficient (Kc) of any sorbent for 234 Th. Kc of 234 Th and other metals, including Fe, Mn, Zn, Pb, and Pu, were up to an order of magnitude higher than that for bulk COM. Most importantly, log Kc values correlated linearly with the fraction of PS-enriched carbon ( f PS) of marine COM, as Kc 5 Kc(0) 3 The log Kc value 2.2 f PS 10 . of ;7.9 of the pure PS end-member fraction was very similar to the highest values obtained for model acid PS (log Kc ; 8). A value for the conditional stability constant for Th binding to the pure PS end member of 10 7.8 could be determined from a concentration of strong acid binding sites (with a pKa #3) of 1.3 mmol g 21 COM. Through the novel use of gradient gel electrophoresis (including two-dimensional polyacrylamide gel electropho- resis), the strong Th(IV) binding ligand was shown to be ;13 kDa in size and to have strong acidic functional groups. We propose that the observed variability of OC : 234 Th ratios in suspended, and sinking matter in the ocean might be caused by the variability of PS content.
Article
The transfer of terrestrially derived (yet poorly quantified) organic carbon to the oceans is an important yet unagreed upon proportion of the world's carbon budget. The few data that exist suggest that high-standing oceanic islands in the southwest Pacific may make important contributions to the overall terrestrial flux of particulate organic carbon (POC) to the oceans. We have determined the POC flux from several streams in New Zealand. With those data and previous measurements from Taiwan and Papua New Guinea, we estimate the POC flux from high-standing islands in the southwest Pacific to be 48 × 1012 g C yr-1, a value slightly lower than previous estimates. These islands, which make up only ˜3% of Earth's landmass, contribute 17% 35% of the estimated POC entering the world's oceans annually and thus may be important sources of terrestrial carbon to the ocean. Anthropogenic activities, especially deforestation and forest harvesting, have probably exacerbated the natural flux. Few to no data exist for many of these islands and thus a more detailed assessment awaits further measurements.
Article
Argos drift buoy trajectory data showed that a region of anticyclonic circulation about 100 km in diameter was present over the upper continental slope of the NW corner of the Gulf of Mexico in September-October, 1988. Guided by these data, Texas A&M University scientists joined by colleagues from Mexico's Direccion General de Oceanografia Naval surveyed the area from October 17-22 on R/V Gyre cruise 88G-05 with a dense grid of conductivity-temperature-depth and expendable bathythermograph stations. The presence of a subsurface salinity maximum greater than 36.5 psu within the upper 150 m of this anticyclone indicated that it had originated as a warm-core eddy of the Loop Current; however, a maximum of only 36.54 psu at sigmat=25.5 in contrast to as much as 36.88 psu at this density surface in a ``fresh'' ring indicated that this feature had spent many months in the western gulf since its separation from the Loop Current. Biologically, the warm-core ring was oligotrophic: its surface waters were generally depleted in nitrate to depths of more than 100 m, and chlorophyll standing stocks (
Article
Reveals deficiencies relative to equilibrium with 238U that are related to uptake onto sinking particles. The deficiencies are generally enhanced in the upper 50 m of the water column (Polar Water), a zone of high suspended particle, particulate organic carbon, and pigment concentrations. Removal of 234Th from the water column is balanced, within the uncertainties, by inventories of excess 234Th in bottom sediments, with both water column deficiences and sediment inventories greater in 1993 than 1992. This indicates that on the timescale of several half-lives of 234Th (~100 days) the polynya is in balance with respect to 234Th (and probably the particles with which it is associated). -from Authors
Article
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.
Article
Carbohydrates are an important organic compound class in seawater and play an active role in the biogeochemical cycling of organic carbon and trace elements in the ocean, but are poorly characterized. To better understand the sources and role of carbohydrate species in marine environments, the concentrations and fluxes of particulate carbohydrates (CHO), total acid polysaccharides (APS), uronic acids (URA), phytoplankton composition and bacterial production were measured in the Gulf of Mexico in 2000 and 2001. A strong positive correlation between APS concentration and cyanobacteria abundance was found in 2000. In 2001, prymnesiophyte abundance correlated well with both concentrations of APS and URA. Bacterial production data, measured simultaneously in 2001, showed significant positive relationships with particulate organic carbon (POC), CHO, APS and URA concentrations, respectively. The average fluxes out of the euphotic zone of CHO, APS and URA in 2000 were 8.1, 1.3, and 0.7 mg C m-2 d-1, respectively. In 2001, the average fluxes of CHO, APS and URA were about 3 times higher than those in 2000, which was a time of lower nutrient concentrations, indicating that the fluxes of carbohydrate species are related to the nutrient status and phytoplankton composition. The results suggest that APS in the upper water column can be produced by cyanobacteria, prymnesiophytes, and heterotrophic bacteria. Most importantly, our data indicate that APS and CHO compounds are more resistant to biological degradation than other organic compounds, suggesting that the role of CHO compounds in carbon cycling in the ocean is more complex than previously thought.
Article
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.
Article
Procedures have been developed for the analyses of dissolved and particulate 234Th, 228Th, 230Th and 232Th in seawater. Large volume samples (>1000 1) are collected using in situ pumps. Seawater is pumped sequentially through a filter cartridge and two MnO2 adsorbers for the collection of particulate and dissolved Th, respectively. Both filters adsorbers are analysed for 234Th using a simple gamma counting technique. This newly developed 234Th procedure can be conducted at sea, and thus provides an easy and efficient method for 234Th analyses on large volume samples. Subsequent radiochemical purification procedures and low-level alpha counting techniques are used in the laboratory for the analyses of 228Th, 230Th and 232Th on these same samples.
Article
Oceanographers now recognize two distinct classes of particles in seawater, broadly categorized as suspended and sinking. The former class dominates the standing stock of particulate matter in the ocean and the latter class dominates the exchange between the surface waters and greater ocean depths1. The downward vertical flux of particulate organic matter (POM) in the open ocean exhibits a non-linear decrease with increasing water depth2-6, and greater than 75% of the net POM loss occurs in the upper 500 m of the water column6. Because sinking particles contain viable, metabolically active microorganisms7-12, the process of microbial decomposition is considered to be an important mechanism controlling POM flux. This model is consistent with the observed correspondence between POM flux and dissolved inorganic carbon concentrations13, and with the reported selective loss of biochemically labile compounds from sinking particles14-17. From our experiments, however, we conclude that the large sinking particles are, in general, poor habitats for bacterial growth and therefore unlikely sites for the active remineralization of organic matter. Our results require a shift in the emphasis of current ideas of particle decomposition from microbes attached to rapidly sinking particles to the microbial populations which are either free-living in the water column or attached to suspended (non-sinking) particulate matter.
Article
Large-volume sampling of 234Th was conducted to estimate particulate organic carbon (POC) export in conjunction with drifting sediment trap deployments in the northern Barents Sea in July 2003 and May 2005. 234Th-derived POC fluxes averaged 42.3 ± 39.7 mmol C m− 2 d− 1 in 2003 and 47.1 ± 30.6 mmol C m− 2 d− 1 in 2005. Sediment trap POC fluxes averaged 13.1 ± 8.2 mmol C m− 2 d− 1 in 2003 and 17.3 ± 11.4 mmol C m− 2 d− 1 in 2005, but better reflected the transient bloom conditions that were observed at each station within a season. Although 234Th fluxes agreed within a factor 2 at most stations and depths sampled, sediment trap POC fluxes were lower than large-volume POC flux estimates at almost every station. This may represent an under-collection of POC by the drifting sediment traps or, conversely, an over-collection of POC by the large-volume sampling of 234Th. It is hypothesized that the offset between the two methods is partly due to the presence of the prymnesiophyte Phaeocystis pouchetii, which potentially causes a large variation in > 53-μm POC/234Th ratios. Due to the large proportion of dissolved carbon or mucilage released by P. pouchetii, and because it is thought that P. pouchetii does not contribute significantly to the vertical export of biogenic matter in the Barents Sea, the application of large-volume sampling of 234Th may yield relatively high, and possibly inaccurate POC/234Th ratios. Hence, POC fluxes derived from 234Th sampling may be inappropriate and drifting sediment traps might be a more reliable method to measure the vertical export of biogenic matter in regions that have recurrent P. pouchetii blooms, such as the Barents Sea.
Article
A new matrix factorization program 'CHEMTAX' was used to interpret high- performance liquid chromatography (HPLC) pigment data from a transect between Prydz Bay, Antarctica, and Australia during March 1987. The program calculated the abundance of diatoms, dinoflagellates, haptophytes resembling Emiliania huxleyi, haptophytes resembling Phaeocystis antarctica, cyanobacteria, prasinophytes, chlorophytes and cryptophytes along the transect. The results were compared with those of microscopy and particle size analysis. The transect was dominated by small cells: particle size analysis showed that particles <2 μm represented 27 to 44% of the total by number while particles 2 to 20 μm represented 55 to 68%. Particles >20 μm never represented more than 3% by number but constituted 57 to 93% of the total volume. Microscopic analysis showed that small flagellates were the most abundant cells along the transect, with a 5-fold increase in abundance at 47°S. Numbers of diatoms (most <20 μm in size) increased markedly south of the Polar Front, correlating with the concentration of silica. Dinoflagellate numbers were relatively constant along the transect, although somewhat higher north of 50°S. Those <20 μm in size were most numerous and accounted for most of the latitudinal variation. Interpretation of HPLC pigment data using the CHEMTAX program was consistent with microscopical analysis. The computed abundances of diatoms and dinoflagellates correlated more strongly with the numbers of small (<20 μm) diatoms and dinoflagellates, respectively, than with large ones. Computed cyanobacterial abundances correlated well with microscopical observations except for small errors where cyanobacteria were absent, probably due to misallocation of zeaxanthin from chlorophytes and prasinophytes. The program was able to distinguish 2 populations of haptophytes along the transect, representing Phaeocystis antarctica and coccolithophorids, even though their pigment compositions were qualitatively (though not quantitatively) identical. It also indicated the separate distributions of chlorophytes and prasinoxanthin- containing prasinophytes, and showed the presence of cryptophytes where none were observed by microscopy.
Article
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 >150μm) 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 (120m) ranged from 71 to 94 mg Cm−2day−1, while 234Th-derived POC fluxes using POC/234Th ratios in the 10–50µm and 50–150µm varied from 71 to 150mgCm−2day−1. In 2006, the sediment trap POC flux at 120m ranged from 24 to 67mgCm−2day−1, while the 234Th-derived POC fluxes in the 10–50µm fraction were comparable or somewhat higher, ranging from 71 to 119mgCm−2day−1. The POC fluxes in 2006, calculated by using POC/234Th ratios in the 1–10µm and the 50–150µm fractions were much higher, ranging from 847 to 1369mgCm−2day−1. Correlations with biological and chemical parameters support a likely mechanism of sinking aggregates of haptophytes (0.2–20µm) 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 (10–15µm) suspended particles rather than larger (50–150µm) 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 10–50μm fraction contained the largest proportion of POC (22–41%), followed by the 50–150μm (22–37%), the >150μm (15–27%), and the 1–10μm (17–23%) fraction. The partitioning of 234Th in trap-collected particles was slightly different from that of POC, with the 1–10μm fraction representing the largest proportion (27–48%) of 234Th flux. Together, the 50µm) 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 (>50µm) pump-collected particles.
Article
The partitioning of 234Th to natural organic matter (NOM) in the colloidal size range (1 kDa–0.1 μm) was evaluated in order to examine the sorption and coagulation behavior of marine colloidal organic matter. Colloids were isolated using large volume cross-flow ultrafiltration and the partitioning of 234Th was quantified using stirred cell ultrafiltration and radioactive assay. The uptake of 234Th by NOM is irreversible over a period of 5 days, implying that over the mean life of 234Th, very little release of 234Th would occur after binding to NOM. Furthermore, the Th–NOM complex is stronger than the Th–EDTA complex, as EDTA was unable to displace the 234Th from its association with NOM. The extent of the initial partitioning of 234Th to suspended matter and colloids is similar and independent of pH in the range from 3 to 9. Coagulation experiments show that 234Th complexed with low molecular weight (1–10 kDa) colloids is transferred to larger (>0.1 μm) filter retained fractions. However, 234Th is transferred to a greater extent than is organic matter and this results in greater partitioning coefficients for 234Th onto particle phases with time. The final equilibrium between 0.1 μm filter-retained and filter-passing 234Th activity is the same regardless of whether the Th was tagged initially to colloidal or suspended matter fractions. The coagulation of colloidal organic matter, COM, consists of both fast and slow steps, with kinetic rate constants on the order of 0.02–0.03 and 0.003–0.007 h−1, respectively. The stickiness (or collision efficiency) factor, α, for COM was experimentally determined to be 0.7(−) for seawater conditions. Using the colloidal pumping model of Honeyman and Santschi [J. Mar. Res. 47 (1989): 951], the ‘predicted’ “fast-phase” coagulation rate coefficient is 0.03 h−1 in our coagulation experiments when the measured α value and the experimental conditions are used for model inputs. These experiments demonstrate that coagulation is the dominant step in the transport of 234Th to the particulate phase.
Article
Size-fractionated particulate 234Th and acid polysaccharides (APS) were collected from stations along a transect in the Gulf of Mexico, in order to examine the role of APS content in controlling the extent and rates of 234Th scavenging in the ocean and to explore, for the first time, the relationship between Th scavenging and biochemical composition of particulate matter. Oceanographically consistent vertical profiles of dissolved and particulate 234Th concentrations were observed, with a considerable 234Th deficit relative to 238U in the upper water column and in benthic nepheloid layers, but reaching secular equilibria between 234Th and 238U in intermediate waters. Within the total particulate 234Th pool (>0.5 μm), the 10–53 μm fraction had the largest share of 234Th (37–57%), followed by the >53 μm (13–36%), the 1–10 μm (10–21%), and the 0.5–1 μm (8–17%) fractions, resulting in a decrease of POC/234Th ratios with increasing particle size. Residence times of 234Th in size-fractionated particles, calculated with a serial multi-box model, were, as expected, consistently shorter than those for total particulate 234Th, with the shortest residence times (53 μm. These results suggest that submicron and micron-sized particles are the most important intermediary in the Th scavenging and that 234Th on smaller particles (
Article
Size-fractionated particulate 234 Th and particulate organic carbon (POC) fluxes were measured in the Gulf of Mexico during 2000 and 2001 in order to obtain a better estimation of upper ocean organic carbon export out of the euphotic zone within cold core and warm core rings, and to assess the relative merit of sediment trap and POC/ 234 Th methods. In 2000, the flux of POC measured by sediment traps at 120 m ranged from 60 to 148 mg C m –2 d –1 , while 234 Th-derived POC fluxes in large particles (>53 mm) varied from 18 to 61 mg C m –2 d –1 using the ratio of POC/ 234 Th at 120 m, and from 51 to 163 mg C m –2 d –1 using an average ratio of POC/ 234 Th for the upper 120 m water column. In 2001, the fluxes of POC measured by traps deployed at 120 m water depth ranged from 39 to 48 mg C m –2 d –1 , while the 234 Th-derived POC fluxes in large particles (>53 mm) varied from 7 to 37 mg C m –2 d –1 using a ratio of POC/ 234 Th at 120 m, and from 37 to 45 mg C m –2 d –1 using an average ratio of POC/ 234 Th within the 0–120 m interval. The results show that POC fluxes estimated by the 234 Th method using the average ratio of POC/ 234 Th within the euphotic zone are similar to those measured by sediment traps. Furthermore, the results demonstrate that the variability in POC export fluxes estimated by the 234 Th/ 238 U disequilibrium approach is strongly related to the ratio of POC/ 234 Th that is taken, and for which we have independent evidence that it may be controlled by the chemical composition of the suspended particles. The results also reveal that using POC/ 234 Th ratios in small particles may result in an estimate of the POC export flux that is considerably higher than when using POC/ 234 Th ratios in large particles (>53 mm). The POC flux calculated from ratios in large particles is, however, more compa-rable to the POC flux determined directly by sediment traps, but both of these estimates are much lower than that deter-mined by using the POC/ 234 Th ratios in sinking particles. Therefore, without reliable flux values to compare with, 234 Th-based and sediment trap approaches are complementary methods for estimating upper ocean POC export, with compara-ble uncertainties for both of these approaches. different marine environments (e.g., Buesseler et al., 1992, 1995; Cochran et al., 1995; Murray et al., 1996). POC fluxes out of the euphotic zone are therefore deter-mined by the product of the POC/ 234 Th ratio in sinking particles and the depth-integrated 234 Th flux (Buesseler, 1998 and references therein). One important assumption in 234 Th-derived POC flux assessments is that the ratio of POC/ 234 Th in particles should be relatively constant in time and space. However, recent studies have shown that ratios of POC/ 234 Th could vary with water depth, particle size fractions, plankton species, and different hydrographic regimes (e.g., Cochran et al., 1995; Buesseler, 1998; Santschi et al., 1999, 2003; Moran et al., 2003). The ratio of POC/ 234 Th is a crucial parameter in marine biogeochemical studies. The mechanisms that control the POC/ 234 Th ratio are, however, still unresolved (Bacon et al., 1996; Murray et al., 1996; Buesseler, 1998; Ducklow et al., 2001; Benitez-Nelson et al., 2001; Coppola et al., 2002; Chen et al., 2003; Moran et al., 2003; Santschi et al., 2003).