Description: 234Th-derived carbon export fluxes were measured in the Atlantic Ocean under the GEOTRACES framework to evaluate basin-scale export variability. Here, we present the results from the northern half of the GA02 transect, spanning from the equator to 64°N. As a result of limited site-specific C/234Th ratio measurements, we further combined our data with previous work to develop a basin wide C/234Th ratio depth curve. While the magnitude of organic carbon fluxes varied depending on the C/234Th ratio used, latitudinal trends were similar, with sizeable and variable organic carbon export fluxes occurring at high latitudes and low to negligible fluxes occurring in oligotrophic waters. Our results agree with previous studies, except at the boundaries between domains, where fluxes were relatively enhanced. Three different models were used to obtain satellite-derived net primary production (NPP). In general, NPP estimates had similar trends along the transect, but there were significant differences in the absolute magnitude depending on the model used. Nevertheless, organic carbon export efficiencies were generally 〈 25%, with the exception of a few stations located in the transition area between the riverine and the oligotrophic domains and between the oligotrophic and the temperate domains. Satellite-derived organic carbon export models from Dunne et al. (2005) (D05), Laws et al. (2011) (L11) and Henson et al. (2011) (H11) were also compared to our 234Th-derived carbon exports fluxes. D05 and L11 provided estimates closest to values obtained with the 234Th approach (within a 3-fold difference), but with no clear trends. The H11 model, on the other hand, consistently provided lower export estimates. The large increase in export data in the Atlantic Ocean derived from the GEOTRACES Program, combined with satellite observations and modeling efforts continue to improve the estimates of carbon export in this ocean basin and therefore reduce uncertainty in the global carbon budget. However, our results also suggest that tuning export models and including biological parameters at a regional scale is necessary for improving satellite-modeling efforts and providing export estimates that are more representative of in situ observations.

Description: AbstractWe conducted an intercomparison of methods for the determination of 234Th in seawater. Samples were collected either from a shore-based 600 m water source, or from standard bottle casts collected in deep waters off Hawaii and in the Southern Ocean. We compared large volume techniques which rely upon Mn cartridges for the collection of dissolved 234Th and its detection via gamma counting (〉200 liter samples) with small volume methods that employed either direct beta counting, or beta counting after radiochemical purification (2-20-Liter samples). Unique to this study are the presentation of a novel 2 and 5 liter 234Th methods. This new method is an adaptation of 20-liter methods which are based on a coprecipitation of thorium with Mn oxides and the direct beta counting of the precipitate. These Mn coprecipitation methods were found to be superior to other methods due to ease of sample collection and processing and low overall analytical uncertainties.

Description: 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 (half-life = 4.5 x 109 years), and its particle reactive daughter, 234Th (half-life = 24.1 days), 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.

Description: 234Th is an extremely useful radiotracer of particle dynamics in aquatic systems.Its use, however, is still limited to a relatively small number of investigators. The reasons for this may in part be due to a lack of knowledge of how this nuclide has been used in the past as well as how and where 234Th might be profitably applied in future research. The purpose of this paper, then, is to examine the variety of 234Th applications that have been used to understand natural aquatic processes in four major areas: vertical transport,particle cycling, horizontal transport, and sediment dynamics. We provide a general overview of the possible applications of 234Th in the hopes of provoking an increased interest in the inherent potential and future application of 234Th in these systems. We end this paper with a discussion of future research avenues in the context of three specific regimes: i) the upper 1000 m of the open ocean, ii) coastal sediment/water processes and iii) large freshwater lakes.

Description: The short-lived thorium isotope 234Th (half-life 24.1 days) has been used as a tracer for a variety of transport processes in aquatic systems. Its use as a tracer of oceanic export on sinking particles has stimulated a rapidly increasing number of studies that require analyses of 234Th, both in marine and in freshwater systems. The original 234Th method is labour intensive. Thus, there has been a quest for simpler techniques that require smaller sample volumes. Here, we review current methodologies in the collection and analysis of 234Th from the water column, discuss their individual strengths and weaknesses, and provide an outlook on possible further improvements and future challenges. Also included in this review are recommendations on calibration procedures and on the production of standard materials as well as a flow chart designed to help researchers find the most appropriate 234Th analytical technique for a specific aquatic regime and known sampling constraints.

Description: Intercomparision of 234Th measurements in both water and particulate samples was carried out between 15 laboratories worldwide, as a part of GEOTRACES inter-calibration program. Particulate samples from four different stations namely BATS (both shallow and deep) and shelf station (shallow) in Atlantic and SAFE (both shallow and deep) and Santa Barbara station (shallow) in Pacific were used in the effort. Particulate inter-calibration results indicate good agreement between all the participating labs with data from all labs falling within the 95% confidence interval around the mean for most instances. Filter type experiments indicate no significant differences in 234Th activities between filter types and pore sizes (0.2–0.8 μm). The only exception are the quartz filters which are associated with 10-20% higher 234Th activities attributed to sorption of dissolved 234Th. Flow rate experiments showed a trend of decreasing 234Th activities with increasing flow rates (2-9 L min-1) for 〉51μm size particles, indicating particle loss during the pumping process. No change in 234Th activities on small particles was observed with increasing flow-rates. 234Th inter-calibration results from deep water samples at SAFe station indicate a variability of 〈3% amongst labs while dissolved 234Th data from surface water at Santa Barbara Station show a less robust agreement, possibly due to the loss of 234Th from decay and large in-growth corrections as a result of long gap between sample collection and processing.

Description: The Costa Rica Dome is a picophytoplankton-dominated, open-ocean upwelling system in the Eastern Tropical Pacific that overlies the ocean's largest oxygen minimum zone. To investigate the efficiency of the biological pump in this unique area, we used shallow (90–150 m) drifting sediment traps and 234 Th: 238 U deficiency measurements to determine export fluxes of carbon, nitrogen and phosphorus in sinking particles. Simultaneous measurements of nitrate uptake and shallow water nitrification allowed us to assess the equilibrium balance of new and export production over a monthly timescale. While f -ratios (new:total production) were reasonably high (0.36 ± 0.12, mean ± standard deviation), export efficiencies were considerably lower. Sediment traps suggested e- ratios (export/ 14 C-primary production) at 90–100 m ranging from 0.053 to 0.067. ThE-ratios ( 234 Th disequilibrium-derived export) ranged from 0.038 to 0.088. C:N and N:P stoichiometries of sinking material were both greater than canonical (Redfield) ratios or measured C:N of suspended particulates, and they increased with depth, suggesting that both nitrogen and phosphorus were preferentially remineralized from sinking particles. Our results are consistent with an ecosystem in which mesozooplankton play a major role in energy transfer to higher trophic levels but are relatively inefficient in mediating vertical carbon flux to depth, leading to an imbalance between new production and sinking flux.