Time-series measurements of 234Th in water column and sediment trap samples from the northwestern Mediterranean Sea

Cochran, J. Kirk; Miquel, Juan Carlos; Armstrong, Robert A.; Fowler, Scott W.; Masqué, Pere; Gasser, Beat; Hirschberg, David J.; Szlosek, Jennifer; Baena, Alessia M. Rodriguez y; Verdeny, E.; Stewart, Gillian

doi:10.1016/j.dsr2.2008.12.034

Cited by 27 publications

(11 citation statements)

References 46 publications

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“…Since our focus was on the POC/ 234 Th ratio of this material, the screens were rinsed immediately after collection at sea with pre-filtered seawater onto the same 25 mm diameter 1 µm pores size Ag filters used for the collection of sediment trap particles, as appropriate for 234 Th, CHN and other analyses. Losses of particulate C or Th to solution, or retention on the screen after gentle rinsing has been documented to be small (Buesseler et al, 1998;Cochran et al, 2008) and is ignored when assessing ratios of the material rinsed off of screens. Counting and calibration of these particulate samples is the same as for the sediment trap material.…”

Section: Study Sitesmentioning

confidence: 99%

Thorium-234 as a tracer of spatial, temporal and vertical variability in particle flux in the North Pacific

Buesseler

Pike

Maiti

et al. 2009

Deep Sea Research Part I: Oceanographic Research Papers

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An extensive 234 Th data set was collected at two sites in the North Pacific: ALOHA, an oligotrophic site near Hawaii, and K2, a mesotrophic HNLC site in the NW Pacific as part of the VERTIGO (VERtical Transport in the Global Ocean) study. Total 234 Th:238 U activity ratios near 1.0 indicated low particle fluxes at ALOHA, while 234 Th: 238 U ~0.6 in the euphotic zone at K2 indicated higher particle export. However, spatial variability was large at both sites-even greater than seasonal variability as reported in prior studies. This variability in space and time confounds the use of single profiles of 234 Th for sediment trap calibration purposes. At K2, there was a decrease in export flux and increase in 234 Th activities over time associated with the declining phase of a summer diatom bloom, which required the use of non-steady state models for flux predictions. This variability in space and time confounds the use of single profiles of 234 Th for sediment trap calibration purposes. High vertical resolution profiles show narrow layers (20-30 m) of excess 234 Th below the deep chlorophyll maximum at K2 associated with particle remineralization resulting in a decrease in flux at depth that may be missed with standard sampling for 234 Th and/or with sediment traps. Also, the application of 234 Th as POC flux tracer relies on accurate sampling of particulate POC/ 234 Th ratios and here the ratio is similar on sinking particles and mid-sized particles collected by in-situ filtration (>10-50 µm at ALOHA and >5-350 µm at K2). To further address variability in particle fluxes at K2, a simple model of the drawdown of 234 Th and nutrients is used to demonstrate that while coupled during export, their ratios in the water column will vary with time and depth after export. Overall these 234 Th data provide a detailed view into particle flux and remineralization in the North Pacific over time and space scales that are varying over days to weeks, and 10's to 100's km at a resolution that is difficult to obtain with other methods.

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Section: Study Sitesmentioning

confidence: 99%

Thorium-234 as a tracer of spatial, temporal and vertical variability in particle flux in the North Pacific

Buesseler

Pike

Maiti

et al. 2009

Deep Sea Research Part I: Oceanographic Research Papers

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“…Unfortunately, a trap that collects 50% of the expected 230 Th flux may, or may not, have the same collection efficiency for POC (Buesseler et al, 2007), because POC may be carried by particles with faster or slower average sinking velocities, and consequently have different hydrodynamic and geochemical properties. Also, individual trap calibrations are difficult to interpret because of differences between the scales of export being sampled by a single trap and those of the water column radionuclide budget (Cochran et al, 2009;). However, compilations of trapping efficiency do provide a general assessment of the magnitude and direction of uncertainties associated with flux methods.…”

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confidence: 99%

Assessing the apparent imbalance between geochemical and biochemical indicators of meso- and bathypelagic biological activity: What the @$♯! is wrong with present calculations of carbon budgets?

Burd

Hansell

Steinberg³

et al. 2010

Deep Sea Research Part II: Topical Studies in Oceanography

283

262

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a b s t r a c tMetabolic activity in the water column below the euphotic zone is ultimately fuelled by the vertical flux of organic material from the surface. Over time, the deep ocean is presumably at steady state, with sources and sinks balanced. But recently compiled global budgets and intensive local field studies suggest that estimates of metabolic activity in the dark ocean exceed the influx of organic substrates. This imbalance indicates either the existence of unaccounted sources of organic carbon or that metabolic activity in the dark ocean is being over-estimated. Budgets of organic carbon flux and metabolic activity in the dark ocean have uncertainties associated with environmental variability, measurement capabilities, conversion parameters, and processes that are not well sampled. We present these issues and quantify associated uncertainties where possible, using a Monte Carlo analysis of a published data set to determine the probability that the imbalance can be explained purely by uncertainties in measurements and conversion factors. A sensitivity analysis demonstrates that the bacterial growth efficiencies and assumed cell carbon contents have the greatest effects on the magnitude of the carbon imbalance. Two poorly quantified sources, lateral advection of particles and a population of slowly settling particles, are discussed as providing a means of closing regional carbon budgets. Finally, we make recommendations concerning future research directions to reduce important uncertainties and allow a better determination of the magnitude and causes of the unbalanced carbon budgets.

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“…In most models that extract export data from radiotracer profiles a steady state (SS) model is used to estimate flux, while the more difficult to treat non‐steady state (NNS) models are undertaken in situations where rapidly changing conditions render the steady state assumption invalid, such as bloom events [ Baena et al , ; Buesseler et al , ; Cochran et al , ]. The steady state assumption is less valid for 210 Po than for 234 Th due to its longer half‐life [ Bacon et al , ; Friedrich and van der Loeff , ].…”

Section: Discussionmentioning

confidence: 99%

Observations and modeling of slow‐sinking particles in the twilight zone

Villa-Alfageme

Soto

Moigne

et al. 2014

Global Biogeochemical Cycles

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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 CO 2 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 CO 2 . 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 À1 ); however, under some circumstances sinking velocity distributions are strongly bimodal with a significant fraction of total flux being carried by slowly (<10 m·d À1 ) 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 210 Pb 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 À1 at 50 m, to 75 ± 25 m·d À1 and 90 ± 20 m·d À1 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 À1. 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.

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Time-series measurements of 234Th in water column and sediment trap samples from the northwestern Mediterranean Sea

Cited by 27 publications

References 46 publications

Thorium-234 as a tracer of spatial, temporal and vertical variability in particle flux in the North Pacific

Thorium-234 as a tracer of spatial, temporal and vertical variability in particle flux in the North Pacific

Assessing the apparent imbalance between geochemical and biochemical indicators of meso- and bathypelagic biological activity: What the @$♯! is wrong with present calculations of carbon budgets?

Observations and modeling of slow‐sinking particles in the twilight zone

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