Vertical flux of particulate Al, Fe, Pb, and Ba from the upper ocean estimated from 234Th/238U disequilibria

Weinstein, S. E.; Moran, S. Bradley

doi:10.1016/j.dsr.2005.03.008

Cited by 34 publications

(153 citation statements)

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“…Deep-moored trap studies are common, while only a few shallow trap (less than 500 m depth) measurements of trace metals, such as Al, Fe, and Pb, had been made prior to the mid-2000s (Weinstein and Moran, 2005). These trap studies were also limited to distinct biogeochemical regions, such as the Mediterranean Sea (Migon et al, 2002;Quetel et al, 1993), the Ross Sea (Collier et al, 2000), and the Greenland Sea (Schüßler et al, 1997).…”

Section: Direct Methods For Quantifying Particulate Trace Metal Expormentioning

confidence: 99%

“…Since the first published effort that utilized 234 Th a flux proxy for particulate aluminum (Al), Fe, lead (Pb), and barium (Ba) (Weinstein and Moran, 2005), the number of studies applying variations of the 234 Th-238 U method to estimate fluxes for elements besides C, N, and Si has gradually increased. In the last decade the technique has most often been used to derive metal fluxes (see Chapter 3), although a few studies have branched into novel areas, such as polycyclic aromatic hydrocarbons (Adhikari et al, 2016) and black carbon (Fang et al, 2016).…”

Section: Th As a Tracer Of Export And Remineralizationmentioning

confidence: 99%

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An investigation of basin-scale controls on upper ocean export and remineralization

Black¹

2018

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The biological carbon pump (BCP) helps to moderate atmospheric carbon dioxide levels by bringing carbon to the deep ocean, where it can be sequestered on timescales of centuries to millennia. Climate change is predicted to decrease the efficiency of the global BCP, however, the magnitude and timescale of this shift is largely uncertain and will likely impact some areas of the global ocean more significantly than others. Therefore, it is imperative that we (1) accurately quantify surface export and remineralization of particulate organic carbon (POC) via the BCP over large regions of the global ocean, (2) examine the factors controlling these POC fluxes and their variability, which includes the cycling of biologically-relevant trace metals, and (3) establish if and how the BCP is changing over time.This thesis focuses on addressing various aspects of these objectives using the 234 Th-238 U method across basin-scale GEOTRACES transects. First, the export and remineralization of POC were examined across large gradients in productivity, upwelling, community structure, and dissolved oxygen in the southeastern tropical Pacific Ocean. Although low oxygen zones are traditionally thought to have decreased POC flux attenuation relative to other regions of the global ocean and the low oxygen Pacific locations followed this pattern, regions that were functionally anoxic had enhanced attenuation in the upper 400 m. Second, trace metal export and remineralization were quantified across the Pacific transect. Because many trace metals are necessary for the metabolic functions of marine organisms and can co-limit marine productivity, the controls on the cycling of trace metals in the upper ocean were examined. Lastly, POC export was determined across two transects in the Western Arctic Ocean, where light and nutrient availability drive the biological pump. Upper ocean export estimates in the central basin did not reflect a substantial change in the biological pump compared to studies from the last three decades, however, an extensive maximum in 234 Th relative to 238 U deeper in the water column indicated that rapid vertical transport had occurred, which could suggest a more efficient biological pump in the Arctic Ocean.

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Section: Direct Methods For Quantifying Particulate Trace Metal Expormentioning

confidence: 99%

Section: Th As a Tracer Of Export And Remineralizationmentioning

confidence: 99%

An investigation of basin-scale controls on upper ocean export and remineralization

Black¹

2018

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“…One dpm = 1 / 60th Bq. nyls; Gustafsson et al, 1997b), biogenic silica Rutgers van der Loeff et al, 2002) and trace metals (Gustafsson et al, 2000a;Weinstein and Moran, in review). Here we emphasize the use of 234 Th as a POC flux proxy.…”

Section: Introductionmentioning

confidence: 99%

An assessment of particulate organic carbon to thorium-234 ratios in the ocean and their impact on the application of 234Th as a POC flux proxy

et al. 2006

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Thorium-234 is increasingly used as a tracer of ocean particle flux, primarily as a means to estimate particulate organic carbon export from the surface ocean. This requires determination of both the 234 Th activity distribution (in order to calculate 234 Th fluxes) and an estimate of the C / 234 Th ratio on sinking particles, to empirically derive C fluxes. In reviewing C / 234 Th variability, results obtained using a single sampling method show the most predictable behavior. For example, in most studies that employ in situ pumps to collect size fractionated particles, C / 234 Th either increases or is relatively invariant with increasing particle size (size classes N 1 to 100s Am). Observations also suggest that C / 234 Th decreases with depth and can vary significantly between regions (highest in blooms of large diatoms and highly productive coastal settings). Comparisons of C fluxes derived from 234 Th show good agreement with independent estimates of C flux, including mass balances of C and nutrients over appropriate space and time scales (within factors of 2-3). We recommend sampling for C / 234 Th from a standard depth of 100 m, or at least one depth below www.elsevier.com/locate/marchem the mixed layer using either large volume size fractionated filtration to capture the rarer large particles, or a sediment trap or other device to collect sinking particles. We also recommend collection of multiple 234 Th profiles and C / 234 Th samples during the course of longer observation periods to better sample temporal variations in both 234 Th flux and the characteristic of sinking particles. We are encouraged by new technologies which are optimized to more reliably sample truly settling particles, and expect the utility of this tracer to increase, not just for upper ocean C fluxes but for other elements and processes deeper in the water column. D

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“…In its most basic sense, the difference between the 234 Th produced from 238 U and what is actually measured in the water, can be assumed to be due to removal by sinking particles (Savoye et al, 2006-this issue). This 234 Th flux is then used with a site and time specific carbon (C)/ 234 Th ratio to convert 234 Th fluxes into those of C. Other elemental and compound fluxes can be calculated using the same approach (e.g., Bacon et al, 1996;Gustafsson et al, 1997a,b;Rutgers van der Loeff et al, 2002;Weinstein and Moran, 2005). Measured C/ 234 Th ratios vary significantly with region, season, depth and sample type, and reasons for this variability remain poorly understood (see Buesseler et al, 2006-this issue).…”

Section: Introductionmentioning

confidence: 99%

Thorium speciation in seawater

et al. 2006

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Since the 1960s, thorium isotopes occupy a special place in the oceanographer's toolbox as tracers for determining rates and mechanisms of oceanic scavenging, particle dynamics, and carbon fluxes. Due to their unique and constant production rates from soluble parent nuclides of uranium and radium, their disequilibrium can be used to calculate rates and time scales of sinking particles. In addition, by ratio-ing particulate 234 Th (as well, in principle, other Th-nuclides) to carbon (and other elements), and linking this ratio to the parent-daughter disequilibrium in the water column, it is possible to calculate fluxes of carbon and other elements. Most of these applications are possible with little knowledge of the dissolved chemical properties of thorium, other than its oxidation state (IV) and tendency to strongly sorb to surfaces, i.e., its bparticle-or surface-activityQ. However, the use of any tracer is hindered by a lack of knowledge of its chemical properties. Recent observations in the variability of carbon to 234 Th ratios in different particle types, as well as of associations of Th(IV) with various marine organic biomolecules has led to the need for a review of current knowledge and what future endeavors should be taken to understand the marine chemistry of thorium. D

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Vertical flux of particulate Al, Fe, Pb, and Ba from the upper ocean estimated from 234Th/238U disequilibria

Cited by 34 publications

References 50 publications

An investigation of basin-scale controls on upper ocean export and remineralization

An investigation of basin-scale controls on upper ocean export and remineralization

An assessment of particulate organic carbon to thorium-234 ratios in the ocean and their impact on the application of 234Th as a POC flux proxy

Thorium speciation in seawater

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