Particle flux in the oceans: Challenging the steady state assumption

Atmospheric carbon dioxide levels are strongly controlled by the depth at which the organic matter that sinks out of the surface ocean is remineralized. This depth is generally estimated from particle flux profiles measured using sediment traps. Inherent in this analysis is a steady state assumption; that export from the surface does not significantly change in the time it takes material to reach the deepest trap. However, recent observations suggest that a significant fraction of material in the mesopelagic zone sinks slowly enough to bring this into doubt. We use data from a study in the North Atlantic during July/August 2009 to challenge the steady state assumption. An increase in biogenic silica flux with depth was observed which we interpret, based on vertical profiles of diatom taxonomy, as representing the remnants of the spring diatom bloom sinking slowly (<40 m d-1). We were able to reproduce this behaviour using a simple model using satellite-derived export rates and literature-derived remineralization rates. We further provide a simple equation to estimate ‘additional’ (or ‘excess’) POC supply to the dark ocean during non-steady state conditions, which is not captured by traditional sediment trap deployments. In seasonal systems, mesopelagic net organic carbon supply could be wrong by as much as 25% when assuming steady state. We conclude that the steady state assumption leads to misinterpretation of particle flux profiles when input fluxes from the upper ocean vary on the order of weeks, such as in temperate and polar regions with strong seasonal cycles in export

Vertical imbalance in organic carbon budgets is indicative of a missing vertical transfer during a phytoplankton bloom near South Georgia (COMICS)

The biological carbon pump, driven principally by the surface production of sinking organic matter and its subsequent remineralization to carbon dioxide (CO2) in the deep ocean, maintains atmospheric CO2 concentrations around 200 ppm lower than they would be if the ocean were abiotic. One important driver of the magnitude of this effect is the depth to which organic matter sinks before it is remineralised, a parameter we have limited confidence in measuring given the difficulty involved in balancing sources and sinks in the ocean's interior. One solution to this imbalance might be a temporal offset in which organic carbon accumulates in the mesopelagic zone (100–1000 m depth) early in the productive season before it is consumed later. Here, we develop a novel accounting method to address non-steady state conditions by estimating fluxes of particulate organic matter into, and accumulation within, distinct vertical layers in the mesopelagic zone using high-resolution spatiotemporal vertical profiles. We apply this approach to a time series of measurements made during the declining phase of a large diatom bloom in a low-circulation region of the Southern Ocean downstream of South Georgia. Our data show that the major export event led to a significant accumulation of organic matter in the upper mesopelagic zone (100–200 m depth) which declined over the following weeks, implying that temporal offsets need to be considered when compiling budgets. However, even when accounting for this accumulation, a mismatch in the vertically resolved organic carbon budget remained, implying that there are likely widespread processes that we do not yet understand that redistribute material vertically within the mesopelagic zone

Vertical imbalance in organic carbon budgets is indicative of a missing vertical transfer during a phytoplankton bloom near South Georgia (COMICS)

The biological carbon pump, driven principally by surface production and sinking of organic matter to deep water and its subsequent remineralization to CO2 maintains atmospheric CO2 around 200 ppm lower than it would be if the ocean were abiotic. One important driver of the magnitude of this effect is the depth to which organic matter sinks before it is remineralised, a parameter we have limited confidence in measuring given the difficulty involved in balancing sources and sinks in the ocean's interior. This imbalance is due, in part, to our inability to measure respiration directly and our reliance on radiotracer-based proxies. One solution to these problems might be a temporal offset in which organic carbon accumulates in the mesopelagic zone (100–1000 m depth) early in the productive season prior to it being consumed later, a situation which could lead to a net apparent sink occurring if a steady state assumption is applied as is often the approach. In this work, we develop a novel accounting method to address this issue, independent of respiration measurements, by estimating fluxes into and accumulation within distinct vertical layers in the mesopelagic. We apply this approach to a time series of measurements of particle sinking velocities and interior organic carbon concentrations made during the declining phase of a large diatom bloom in a low-circulation region of the Southern Ocean downstream of South Georgia. Our data show that the major export event led to a significant accumulation of organic matter in the upper mesopelagic (100–200 m depth) which declined over several weeks, implying that temporal offsets need to be considered when compiling budgets. However, even when accounting for this accumulation, a mismatch in the vertically resolved organic carbon budget remained, implying that there are likely widespread processes that we do not yet understand that redistribute material vertically in the mesopelagic

Vertical imbalance in organic carbon budgets is indicative of a missing vertical transfer during a phytoplankton bloom near South Georgia (COMICS)

The biological carbon pump, driven principally by the surface production of sinking organic matter and its subsequent remineralization to carbon dioxide (CO2) in the deep ocean, maintains atmospheric CO2 concentrations around 200 ppm lower than they would be if the ocean were abiotic. One important driver of the magnitude of this effect is the depth to which organic matter sinks before it is remineralised, a parameter we have limited confidence in measuring given the difficulty involved in balancing sources and sinks in the ocean's interior. One solution to this imbalance might be a temporal offset in which organic carbon accumulates in the mesopelagic zone (100–1000 m depth) early in the productive season before it is consumed later. Here, we develop a novel accounting method to address non-steady state conditions by estimating fluxes of particulate organic matter into, and accumulation within, distinct vertical layers in the mesopelagic zone using high-resolution spatiotemporal vertical profiles. We apply this approach to a time series of measurements made during the declining phase of a large diatom bloom in a low-circulation region of the Southern Ocean downstream of South Georgia. Our data show that the major export event led to a significant accumulation of organic matter in the upper mesopelagic zone (100–200 m depth) which declined over the following weeks, implying that temporal offsets need to be considered when compiling budgets. However, even when accounting for this accumulation, a mismatch in the vertically resolved organic carbon budget remained, implying that there are likely widespread processes that we do not yet understand that redistribute material vertically within the mesopelagic zone

Recent evolution of 129I levels in the Arctic and North Atlantic Oceans

Resumen del trabajo presentado a la "Thirteenth International Conference on Accelerator Mass Spectrometry" celebrada en Aix-en-Provence (Francia) del 24 al 29 de agosto de 2014.-- et al.129I is mainly released into the marine system through liquid discharge from the nuclear fuel reprocessing plants (NFRP) at Sellafield and La Hague. This radioisotope is transported around the European shelf and northwards into the Nordic Seas and Arctic Ocean. Recent studies (Casacuberta, this conference) point out to an increase of 129I concentrations in the Arctic during the last decade. In this work, the Irminger and Iceland Basins (IrB and IB), the transects Glasgow-Reykjavik and Sellafield-Porcupine Abyssal Plain (PAP), Norwegian and Barent Seas, Fram Strait and Greenland Sea were sampled. An outline of updated 129I concentrations in key locations of the North Atlantic and the Arctic Oceans is presented. An enduring increase of 129I concentrations throughout the North Atlantic and Arctic oceans was found, up to an order of magnitude in some areas. Our results show a week correlation surface 129I - latitude, previously reported. We have evaluated if this lost might be due to the sinking of 129I of the Atlantic surface waters during the formation of the North Atlantic Deep Water. For that, 129I inventories were estimated at the Nordic Seas and IrB. They showed a strong positive correlation with latitude that might be associated to the sinking of 129I. Finally, high 129I concentrations were also detected in Southern and North-western positions (IB and PAP site) ; this suggests that 129I from Sellafield might not be exclusively transported by the North Atlantic Current and Norwegian Coastal Current into the North and Nordic Seas ; the amount of 129I split from the main Northern branch might have implications in the evaluation of the 129I input function into the Arctic and its use as a water masses tracer, so further investigations would be necessary.Peer reviewe

129I concentrations in a transect from the Labrador Sea to the North-Eastern Atlantic

Trabajo presentado en la 14th International conference on Accelerator Mass Spectrometry, celebrada en Ottawa (Canadá), del 14 al 18 de agosto de 2017Peer reviewe

Uranium immobilization by FEBEX bentonite and steel barriers in hydrothermal conditions

FEBEX clay is considered a reference material in engineered barriers for safe storage of nuclear waste and uranium is a minor component of high-level radioactive waste (HLRW) and a main component of the spent nuclear fuel (SNF). Here, the kinetics of reaction of uranium with FEBEX was investigated in addition to the uranium immobilisation ability and the structural analysis of the reaction products. Hydrothermal treatments were accomplished with UO22+ and tetravalent actinide simulator ZrO2+, also present in HLRW. The quantification of the reaction was performed through gamma spectrometry of uranium. Two mechanisms for UO22+ retention by FEBEX were detected: adsorption and formation of stable and insoluble new phases. The structural analyses performed using ZrO2+, confirmed the uranium adsorption and the presence of new phases, ZrO2 and Zr(SiO4), that emphasise the existence of a chemical reaction with the bentonite. The analysis of the velocity of reaction uranium-clay minerals revealed temperature dependence. An exponential fitting suggested that the removal of uranium from solution at temperatures over 200 °C could be completed in less than a year. For lower temperatures, several years are needed. Milliequivalents of UO22+ immobilised by the clay depended on temperature and time and were over cation exchange capacity (CEC) of FEBEX even at 100 °C (reaching 600% of CEC). The reaction with steel, also temperature dependent, was finally analysed. At 200 °C 40–70% of uranium reacted with steel. But only 30–15% reacted at 300 °C and 100 °C. The reactions provide a stable immobilisation mechanism for uranium even when its sorption and swelling capacities fail. Our experiments will be of particular interest for very deep borehole disposals were higher temperatures and pressures are expected.Peer Reviewe