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

Seawater samples were collected in the water column of the Canada Basin and the Bering Sea from aboard the R/V Xue Long during August 1999. Activity concentrations of dissolved and particulate 234Th were measured using beta counting techniques to quantify the scavenging and residence time of 234Th and organic carbon export fluxes. Primary production (PP) and bacterial production were also determined in the study areas through in situ incubation experiments. Significant 234Th scavenging was observed in the upper 100 m of the water column in both study areas, with up to 40% of 234Th deficit found at Bering Sea stations and ~15% of 234Th deficit at the Canada Basin station. Measured PP decreased from ~12.5 µmol C/m³/h in surface water to near zero at ~100 m depth, with an integrated PP of 3.83 mmol C/m²/d in the Canada Basin. Bacterial production, on the other hand, was on the order of 2.0 mmol C/m²/d, which is up to 52% of the integrated PP. Particulate organic carbon (POC) export fluxes derived from 234Th/238U disequilibrium were ~1 mmol C/m²/d in the Canada Basin and ~10 mmol C/m²/d in the Bering Sea, with fluxes in the latter area being 5 to 10 times higher than those found in the Canada Basin. These export fluxes correspond to a ThE ratio (the ratio of 234Th-derived POC export to primary production) of 0.26 for the Canada Basin and 0.7 for the Bering Sea. The higher ThE ratios in the study areas suggest a decoupling of production and particulate export in the high-latitude ocean. Ratios of POC to particulate 234Th (µmol C/dpm) decreased consistently with increasing depth, suggesting that organic carbon is preferentially remineralized relative to 234Th. Interestingly, the profile of particulate 234Th in the Canada Basin showed a unique characteristic: particulate 234Th activities increased with increasing depth, suggesting a continuous scavenging of 234Th and a rapid settling rate of the particles.Au cours du mois d'août 1999, on a prélevé des échantillons d'eau de mer dans la colonne d'eau du bassin Canada et de la mer de Béring depuis le vaisseau RV Xue-Long. L'activité volumique du 234Th dissous et particulaire a été mesurée à l'aide de techniques de comptage au rayonnement bêta afin de quantifier le temps de balayage et le temps de séjour du 234Th et les flux d'exportation du carbone organique. On a aussi établi la production primaire (PP) et la production bactérienne dans les zones d'étude en réalisant des expériences d'incubation in situ. Dans les deux zones d'étude, on a observé un balayage marqué du 234Th dans les 100 m supérieurs de la colonne d'eau, avec jusqu'à 40 p. cent de déficit de 234Th aux postes de la mer de Béring et environ 15 p. cent au poste du bassin Canada. La PP mesurée baissait d'environ 12,5 µmol C/m³/h dans l'eau de surface à près de zéro à environ 100 m de profondeur, avec une PP intégrée de 3,83 mmol C/m²/j dans le bassin Canada. D'autre part, la production bactérienne était de l'ordre de 2,0 mmol C/m²/j, ce qui représente jusqu'à 52 p. cent de la PP intégrée. Les flux d'exportation du carbone organique particulaire (COP) calculés à partir du déséquilibre 234Th /238U étaient d'environ 1 mmol C/m²/j dans le bassin Canada et d'environ 10 mmol C/m²/j dans la mer de Béring, les flux dans cette région étant de 5 à 10 fois plus élevés que ceux trouvés dans le bassin Canada. Ces flux d'exportation correspondent à un rapport ThE (le rapport de l'exportation du COP dérivé du 234Th à la production primaire) de 0,26 pour le bassin Canada et de 0,7 pour la mer de Béring. Les rapports plus élevés de ThE dans les zones d'étude suggèrent un découplage de la production et de l'exportation de particules dans l'océan septentrional. Les rapports de COP au 234Th particulaire (µmol C/dpm) diminuaient de façon uniforme avec l'augmentation de la profondeur, ce qui suggère que le carbone organique est reminéralisé préférentiellement par rapport au 234Th. Il est à noter que le profil du 234Th particulaire dans le bassin Canada affichait une caractéristique unique: l'activité du 234Th particulaire augmentait avec la profondeur, ce qui suggère un balayage continu du 234Th et un taux rapide de sédimentation des particules

Revisiting \u3csup\u3e228\u3c/sup\u3eTh as a Tool for Determining Sedimentation and Mass Accumulation Rates

The use of 228Th has seen limited application for determining sedimentation and mass accumulation rates in coastal and marine environments. Recent analytical advances have enabled rapid, precise measurements of particle-bound 228Th using a radium delayed coincidence counting system (RaDeCC). Herein we review the 228Th cycle in the marine environment and revisit the historical use of 228Th as a tracer for determining sediment vertical accretion and mass accumulation rates in light of new measurement techniques. Case studies comparing accumulation rates from 228Th and 210Pb are presented for a micro-tidal salt marsh and a marginal sea environment. 228Th and 210Pb have been previously measured in mangrove, deltaic, continental shelf and ocean basin environments, and a literature synthesis reveals that 228Th (measured via alpha or gamma spectrometry) derived accumulation rates are generally equal to or greater than estimates derived from 210Pb, reflecting different integration periods. Use of 228Th is well-suited for shallow (\u3c15 cm) cores over decadal timescales. Application is limited to relatively homogenous sediment profiles with minor variations in grain size and minimal bioturbation. When appropriate conditions are met, complimentary use of 228Th and 210Pb can demonstrate that the upper layers of a core are undisturbed and can improve spatial coverage in mapping accumulation rates due to the higher sample throughput for sediment 228Th

Revisiting 228Th as a tool for determining sedimentation and mass accumulation rates

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Tamborski, J., Cai, P., Eagle, M., Henderson, P., & Charette, M. Revisiting 228Th as a tool for determining sedimentation and mass accumulation rates. Chemical Geology, 607, (2022): 121006, https://doi.org/10.1016/j.chemgeo.2022.121006.The use of 228Th has seen limited application for determining sedimentation and mass accumulation rates in coastal and marine environments. Recent analytical advances have enabled rapid, precise measurements of particle-bound 228Th using a radium delayed coincidence counting system (RaDeCC). Herein we review the 228Th cycle in the marine environment and revisit the historical use of 228Th as a tracer for determining sediment vertical accretion and mass accumulation rates in light of new measurement techniques. Case studies comparing accumulation rates from 228Th and 210Pb are presented for a micro-tidal salt marsh and a marginal sea environment. 228Th and 210Pb have been previously measured in mangrove, deltaic, continental shelf and ocean basin environments, and a literature synthesis reveals that 228Th (measured via alpha or gamma spectrometry) derived accumulation rates are generally equal to or greater than estimates derived from 210Pb, reflecting different integration periods. Use of 228Th is well-suited for shallow (<15 cm) cores over decadal timescales. Application is limited to relatively homogenous sediment profiles with minor variations in grain size and minimal bioturbation. When appropriate conditions are met, complimentary use of 228Th and 210Pb can demonstrate that the upper layers of a core are undisturbed and can improve spatial coverage in mapping accumulation rates due to the higher sample throughput for sediment 228Th.This research was undertaken thanks in part to funding from the Canada First Research Excellence Fund, through the Ocean Frontier Institute. This project was supported by U.S. Geological Survey Coastal and Marine Hazards and Resources Program. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. PC acknowledges the support of the Natural Science Foundation of China (NSFC) through Grants No. 92058205

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

Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Chemistry 100 (2006): 282-288, doi:10.1016/j.marchem.2005.10.016.The recently developed 2‐5 L small‐volume MnO2 coprecipitation method for determining 234Th in seawater has provided a new way to substantially increase, both temporally and spatially, the sampling resolution of 234Th and 234Th‐based particulate organic carbon export estimates in the upper ocean. In this study, we further optimize the technique by reducing reagent quantities and the use of an additional water bath heating step. This optimization allows the filtration of the MnO2 precipitate onto a 25‐mm diameter, 1.0 μm pore size QMA filter to be completed within < 30 minutes for unfiltered waters from the South China Sea. In addition, we have modified the MnO2 purification procedure to allow for alpha spetrometric measurements of 234Th recoveries. Results from recovery experiments suggest that reagent amounts can be reduced to 0.0375 mg KMnO4 and 0.1 mg MnCl2∙4H2O per liter of sample, while still maintaining high 234Th recovery. This study further confirms that the addition of a yield monitor is necessary for the application of this small‐volume method.Support for this work came from the Natural Science Foundation of China through grants #40206011, #49825162, and #90211020. This study was also supported by the China Ministry of Education through a program for Changjiang Scholars and Innovative Research Teams in Universities (PRSIRT)

A high-resolution study of particle export in the southern South China Sea based on Th-234 : U-238 disequilibrium

During a spring intermonsoon cruise in 2004, depth profiles of total and particulate Th-234 in the upper 100 m were collected at 36 stations in the southern South China Sea (SCS), covering a surface area of similar to 1.0 x 10(6) km(2). Thorium-234 was sampled by using a modified small-volume MnO2 co-precipitation technique, which allows mapping the Th-234 distribution with a high spatial resolution. A stratified structure of Th-234/U-238 disequilibria was generally observed in the upper 100 m water column, suggesting that the euphotic zone of the southern SCS in this season can be separated into two layers: an upper layer with low export production rates and a lower layer with high export production rates. At the same time, we observed extensive zones of Th-234 excess within the euphotic layer, which is possibly due to intense remineralization of particulate matter. Particulate organic carbon (POC) export was estimated from a three-dimensional steady state model of Th-234 fluxes combined with measurements of the POC/Th-234 ratio on suspended particles. The POC export for this region varied from a low of -10.7 +/- 1.5 mmolC m(-2) d(-1) to a high of 12.6 +/- 1.1 mmolC m(-2) d(-1), with an average of 3.8 +/- 4.0 mmolC m(-2) d(-1). A negative flux of POC export is interpreted as the result of lateral input of particulate matter from nearby waters. Regional patterns in POC export show enhanced fluxes along the western and southern boundaries of the study region, and a "tongue'' of low export extending northwestward from similar to 7 degrees N 116 degrees E to similar to 10 degrees N 111 degrees E. This geographic distribution is consistent with the overall surface circulation pattern of the southern SCS in this season

The GEOTRACES Intermediate Data Product 2014

The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEIs) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-? data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes