The attenuation coefficient b is one of the most common ways to describe how strong the carbon flux is attenuated throughout the water column. Therefore, b is an essential input variable in many ...carbon flux and climate models. Marsay et al. (2015, https://doi.org/10.1073/pnas.1415311112) proposed that the median surface water temperature (0–500 m) may be a predictor of b, but our observations from Arctic waters challenge this hypothesis. We found a highly variable attenuation coefficient (b = 0.43–1.84) in cold Arctic waters (<4.1 °C). Accordingly, we suggest that water temperature is not a globally valid predictor of the attenuation coefficient. We advocate instead that the phytoplankton composition and especially the relative abundance of diatoms can be used to parametrize the carbon flux attenuation in local and global carbon flux models.
Plain Language Summary
In the surface ocean, microalgae convert dissolved atmospheric CO2 into organic carbon (OC). As OC sinks in the water column, microbes and zooplankton graze on it. It has been suggested that these organisms reduce the amount of sinking OC stronger in warm waters than in cold waters. We found, however, that the amount of sinking OC is sometimes strongly and sometimes weakly reduced in cold Arctic waters (<4 °C). Therefore, we conclude that temperature seems not to be an important factor determining how strong the amount of sinking OC is reduced with depth. We instead advocate that the phytoplankton community composition is useful to predict how strong the amount of sinking OC is reduced with depth. When fast‐sinking algae form algal aggregates or are repackaged into copepod fecal pellets, their OC spends only a short time in the upper water column where hungry grazers are very abundant. The consumers have then much less time to prey on the OC than when the sinking particles consist of slowly sinking algae. Concluding, we argue that it is very important to include the phytoplankton community composition in computer simulations to correctly predict how much OC is stored in the oceans.
Key Points
Unlike previous assumptions, carbon flux attenuation can both be weak and strong in cold Arctic waters (<4 °C)
Phytoplankton community composition is a more reliable predictor of carbon flux attenuation than temperature alone
Phytoplankton and zooplankton abundance and composition should be included in carbon flux models as these parameters can modify b
Particle sinking velocity is considered to be a controlling factor for carbon transport to the deep sea and thus carbon sequestration in the oceans. The velocities of the material exported to depth ...are considered to be high in high‐latitude productive systems and low in oligotrophic distributions. We use a recently developed method based on the measurement of the radioactive pair 210Po‐210Pb to calculate particle sinking velocities in the temperate and oligotrophic North Atlantic during different bloom stages. Our estimates of average sinking velocities (ASVs) show that slowly sinking particles (<100 m d−1) contribute significantly to carbon flux at all the locations except in the temperate regions during the bloom. ASVs appear to vary strongly with season, which we propose is caused by changes in the epipelagic community structure. Our results are the first field data to confirm the long‐standing theory that particle sinking velocities increase with depth, with increases of up to 90% between 50 and 150 m depth.
Key Points
Slow‐sinking particles (<100 m d−1) contribution to particle sinking flux is ubiquitous across the North Atlantic
Strong variations in sinking velocity are found to be mostly seasonal, and the community structure appears to drive sinking velocities
In both temperate and oligotrophic regions we confirm the long‐standing hypothesis that sinking velocity increases with depth (30‐90%)
The biological carbon pump (BCP) transfers carbon from the surface ocean into the oceans' interior, mainly in the form of sinking particles with an organic component, and thereby keeps atmospheric ...CO2 at significantly lower levels than if the oceans were abiotic. The depth at which these sinking particles are remineralized is a key control over atmospheric CO2. Particle sinking speed is likely to be a critical parameter over remineralization depth. Carbon export is usually controlled by large, rapidly sinking particles (>150 m·d−1); however, under some circumstances sinking velocity distributions are strongly bimodal with a significant fraction of total flux being carried by slowly (<10 m·d−1) sinking particles. Therefore, there is an interest in determining sinking particle velocities and their variations with depth, as well as in understanding the interplay between sinking velocity distributions and carbon export. Here, we use profiles of total and particulate concentrations of the naturally occurring radionuclide pair 210Po‐210Pb from the Porcupine Abyssal Plain (PAP) site (48°N, 16.5°W) to estimate depth variation in particle sinking speed using a one‐box model and inverse techniques. Average sinking speeds increase from 60 ± 30 m·d−1 at 50 m, to 75 ± 25 m·d−1 and 90 ± 20 m·d−1 at 150 and 500 m. Furthermore, a sensitivity analysis suggests that at the PAP site the measured 210Po profiles are inconsistent with the usually assumed sinking velocities of 200 m·d−1. We hypothesize that a trend of increasing velocity with depth might be caused by a gradual loss of slow‐sinking material with depth, a factor with significant implications for regional carbon budgets.
Key Points
New proxy to measure average settling velocities using 210Po‐210Pb profilesSuggest clear shallow and mesopelagic contribution of slow‐sinking particlesDepth increase in velocities suggests slow particles remineralization up to 400 m
A numerical model, previously validated with other radionuclides, was applied to simulate the dispersion of 236U released from European nuclear fuel reprocessing plants in the North Atlantic and ...Shelf Seas using a published reconstruction of Sellafield and La Hague releases. Model results are in better agreement with observations if the lowest estimation of such releases are used. This implies that approximately 40kg of 236U has been discharged from Sellafield. It was found that adsorption of 236U on bed sediments of the shallow European Shelf Seas plays an essential role in its dispersion patterns. This contrasts strongly with the more conservative behaviour of 129I in the same area. This has two important implications in the use of 236U as oceanographic tracer; i) special care must be taken in coastal areas, as sediments might act as sinks and sources of 236U; ii) the annual input function of 236U into the Arctic is not directly controlled by the annual discharges from Sellafield and La Hague, since sediments from the Irish, Celtic and North Sea modulate and smooth the signal. Only 52% of the total releases enter into the Arctic Ocean.
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•Calculated 236U concentrations in water and sediments compared with measurements.•Adsorption of 236U in bed sediments is significant.•Releases from Sellafield estimated as 40kg of 236U for period 1952–2013•Only 52% of the total releases from the three nuclear facilities are introduced into the Arctic.•Input function into the Arctic reconstructed: total of 38 ± 8kg of 236U for period 1952–2013.
Measurement of radionuclides in marine samples, specifically radioactive pairs disequilibrium, has gained interest lately due to their ability to trace cutting edge biogeochemical processes. In this ...context, we developed a fast, direct method for determining (210)Pb and (210)Po water through the use of ultra low-level liquid scintillation counting and alpha-particle spectrometry respectively and through Eichrom Sr resins for the Po-Pb separation. For (210)Pb analysis, the method uses stable lead as a yield tracer measured by a robust ICP-MS technique, and (210)Po is determined through self-deposition using the conventional (209)Po yield tracer. The improvements of the method over other techniques are: a) the analysis can be completed within 6 days, simplifying other methods, b) very low limits of detection have been achieved -0.12 and 0.005mBqL(-1) for (210)Pb and (210)Po, respectively - and c) most of the method could be carried out in on-board analysis. We applied the method to different aqueous samples and specifically to marine samples. We determined (210)Pb and (210)Po in the dissolved fraction of Mediterranean Sea water and an estuary at the South-West of Spain. We found that it can be successfully employed to marine samples but we recommend to i) use a minimum of 20L water to measure the (210)Pb in the dissolved phase by LSC and lower volumes to measure total concentrations; ii) wait for (210)Pb and (210)Bi in secular equilibrium and measure the total spectrum to minimise the limit of detection and improve accuracy.
In this work data of salinity, temperature, suspended particulate matter (SPM), chlorophyll-a, and phytoplankton concentration in the Gibraltar Strait coast, in the confluence of the Mediterranean ...Sean and the Atlantic Ocean, were analyzed together with 210Po, 40K, 210Pb and 234Th activity concentration in different types of bivalve molluscs at the same time-period. The physicochemical parameters were evaluated using the Copernicus Marine Environment Monitoring Service (CMEMS) products based on satellite observations. A multivariate statistical analysis, including contrasted natural radioactivity contents, allowed the discrimination of bivalve molluscs from Atlantic and Mediterranean coasts. Additionally, a cluster analysis determined a highly significant negative correlation 210Po concentration in molluscs and phytoplankton concentration suggesting that phytoplankton concentration in the water column is a determinant factor to regulate 210Po concentration in those animals. These results introduce a useful tool to calculate the radiological doses in seafood from chlorophyll satellite image.
•First time that physicochemical parameters based on satellite observations were analyzed with activity concentration.•A significant negative correlation was shown between 210Po concentration in molluscs and phytoplankton concentration.•A multivariate statistical analysis allowed the discrimination of bivalve molluscs from Atlantic and Mediterranean coasts.•Radiological doses in seafood were obtained through chlorophyll satellite image.
234Th measurements are widely used to estimate the downward carbon flux of particles via the oceanic Biological Pump. Carbon export is evaluated from 234Th‐238U disequilibrium assuming either steady ...state (SS) conditions, or including a non‐SS (NSS) correction. We use a novel stochastic simulation to quantify the temporal variation of vertical carbon and 234Th (dissolved and particulate) concentration profiles with high temporal resolution. We calculate seasonal export as if in situ measured with sediment trap and SS‐ and NSS‐234Th approaches and quantify the periods of validity for SS/NSS conditions defined in previous works. The SS approach is valid throughout the entire season in oligotrophic regions. In temperate regions, the SS introduces a bias in the export if sampling takes place outside specific temporal windows. Windows of validity range from days in short blooms of ~15‐day duration to weeks in blooms longer than ~30 days.
Plain Language Summary
Climate change driven by anthropogenic CO2 emissions has a significant impact on mankind. An important component of Earth's carbon cycle is the ocean's biological carbon pump (BCP), which buffers the air‐sea balance of CO2. It starts with production of organic matter in the surface ocean, continues with its sinking into the dark ocean, and finishes with the storage of a small fraction below 1,000 m. Without the BCP, atmospheric CO2 concentration would be 50% higher than it currently is. Understanding the BCP helps to predict the future CO2 that will be stored by the oceans and how that will affect the global carbon cycle. The photosynthetically fixed carbon is mainly transferred as large particles. Among the methods currently used to estimate the downward flux of particles, sediment traps, and 234Th‐238U radioactive pair are the most accepted ones. However, the simultaneous use of these two techniques does not always provide comparable flux estimates. This questions the consistency of our current calculations of the magnitude of the carbon flux and the accuracy of the global predictions. In this study, we assess the accuracy of the 234Th method in relation to the traps and explain the source of the discrepancies between them.
Key Points
We assess downward carbon flux temporal evolution along the season from simulated sinking carbon and dissolved and particulate 234Th depth profiles
We analyze the accuracy of the steady/non‐steady state approaches to calculate the 234Th‐derived carbon flux in relation to the trap flux
We quantify temporal windows during which the steady state approach results accurate and introduce the concept of “window of success”
This work presents a physical, mechanical, durability, leaching, and radiological assay of three wastes (egg and scallop shells and olive pomace ash) as road-base binders. Two different ...waste/Portland-cement ratios (7.5/92.5 and 80/20) were studied. Density and compressive strength decreased when different wastes were added in every proportion. Additions of 7.5% of both shells reduce the density to about 2.5% and the compressive strength to 20%, while 80% reduces the density to 20% and the compressive strength to 90%, while the addition of biomass fly ash decreases the density and compressive strength in a higher proportion than shells. The durability against acid attack is increased when the three wastes are used, and this increase is higher when the waste dosage is increased (up to 15 times more when 80% biomass ash is used). With respect to leaching, scallop and eggshells can be used as a component of hydraulic road binder, but olive pomace ash presents leaching values higher than the limits of different regulations (Se, Pb, Ni, Mo, Cu, and As). From a radiological perspective, all road-base binders present an activity concentration index lower than 1, except when olive pomace ash was used, and the binders showed higher values of 40K due to the high potassium content of fly ash.
•First time a sequential method for Sr-90 and Po-210 was developed.•A low quantification limit was obtained combining lyophilisation and acid leaching.•The method was applied to seafood samples.•The ...method was fully validated through certified reference materials and intercomparison exercise.•Good chemical yields were obtained.
The latest EU Council Regulation 2016/52/Euratom updates the emergency limits on radionuclides in foods including 210Po and 90Sr, two of the most important radionuclides for radiological dose from the ingestion pathway. A novel and straightforward method has been developed for sequential determination of 90Sr and 210Po in food samples using ultra low-level liquid scintillation counting and alpha-particle spectrometry. For 90Sr analysis, the method makes use of stable strontium as yield tracer, and 210Po is determined through self-deposition using 209Po as a yield tracer. The quantification limit for this method is 25.0 and 2.0Bqkg−1 for 90Sr and 210Po, respectively. The proposed radiochemical separation can be completed within 2days for a batch of 12 samples. The radiochemical procedure was validated by its application for the measurement of IAEA certified reference materials, and through participation in a national intercomparison exercise. Results are also presented in seafood from the Mediterranean coast.
Abstract
234
Th measurements are widely used to estimate the downward carbon flux of particles via the oceanic Biological Pump. Carbon export is evaluated from
234
Th‐
238
U disequilibrium assuming ...either steady state (SS) conditions, or including a non‐SS (NSS) correction. We use a novel stochastic simulation to quantify the temporal variation of vertical carbon and
234
Th (dissolved and particulate) concentration profiles with high temporal resolution. We calculate seasonal export as if in situ measured with sediment trap and SS‐ and NSS‐
234
Th approaches and quantify the periods of validity for SS/NSS conditions defined in previous works. The SS approach is valid throughout the entire season in oligotrophic regions. In temperate regions, the SS introduces a bias in the export if sampling takes place outside specific temporal windows. Windows of validity range from days in short blooms of ~15‐day duration to weeks in blooms longer than ~30 days.
Plain Language Summary
Climate change driven by anthropogenic CO
2
emissions has a significant impact on mankind. An important component of Earth's carbon cycle is the ocean's biological carbon pump (BCP), which buffers the air‐sea balance of CO
2
. It starts with production of organic matter in the surface ocean, continues with its sinking into the dark ocean, and finishes with the storage of a small fraction below 1,000 m. Without the BCP, atmospheric CO
2
concentration would be 50% higher than it currently is. Understanding the BCP helps to predict the future CO
2
that will be stored by the oceans and how that will affect the global carbon cycle. The photosynthetically fixed carbon is mainly transferred as large particles. Among the methods currently used to estimate the downward flux of particles, sediment traps, and
234
Th‐
238
U radioactive pair are the most accepted ones. However, the simultaneous use of these two techniques does not always provide comparable flux estimates. This questions the consistency of our current calculations of the magnitude of the carbon flux and the accuracy of the global predictions. In this study, we assess the accuracy of the
234
Th method in relation to the traps and explain the source of the discrepancies between them.
Key Points
We assess downward carbon flux temporal evolution along the season from simulated sinking carbon and dissolved and particulate
234
Th depth profiles
We analyze the accuracy of the steady/non‐steady state approaches to calculate the
234
Th‐derived carbon flux in relation to the trap flux
We quantify temporal windows during which the steady state approach results accurate and introduce the concept of “window of success”
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