A high‐resolution study of particle export in the southern South China Sea based on 234Th:238U disequilibrium

Cai, Pinghe; Chen, Weifang; Dai, Minhan; Wan, Zhenwen; Wang, Dongxiao; Li, Qing; Tang, Tiantian; Lv, Dongwei

doi:10.1029/2007jc004268

Cited by 59 publications

(56 citation statements)

References 52 publications

(79 reference statements)

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“…However the higher resolution 234 Th data vs. time, space and depth have opened a wider window to examine flux variability for the two sites. Increasingly, high resolution 234 Th data sets in coastal settings (Cai et al, 2008;Giuliani et al, 2007), across mesoscale eddies 18 Maiti et al, 2008) and here in the North Pacific suggest that the spatial scales of export are more variable than the concentration of phytoplankton or NPP. Such variability on small spatial scales make it difficult to "calibrate" upper ocean sediment traps using 234 Th better than a factor of about two, as the traps collect particles from a source region that varies with sinking rate and captures only part of this spatially variable flux field.…”

Section: Discussionmentioning

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: Discussionmentioning

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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“…Cai, P., Chen, W., Dai, M., Wan, Z., Wang, D., Li, Q., Tang, T., Lv, D., 2008. A high-resolution study of particle export in the southern South China Sea based on 234 Th: 238 Figure 1: The solubility pump (right) and the biological carbon pump (left).…”

Section: This Thesismentioning

confidence: 99%

“…Due to this high affinity for particles, naturally occurring thorium isotopes have become one of the most extensively utilized and reliable means of tracking particle dynamics and export in the ocean (Cochran and Masque, 2003;Rutgers van der Loeff et al, 2006). 234 Th has a relatively long history of use as a flux proxy (Bhat et al, 1969;Buesseler et al, 1995;Cai et al, 2008;Coale and Bruland, 1987;Rutgers van der Loeff et al, 2011). The majority of 234 Th-based flux studies have focused on tracking carbon export out of the upper 200 m to 400 m (Le Moigne et al, 2013), although nitrogen (N) and silicon (Si) are two other elements of interest to which this method has been applied (e.g.…”

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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“…A further technological advantage is the use of the recently developed small-volume technique that enables high resolution sampling (Benitez-Nelson et al, 2001a;Buesseler et al, 2001a;Cai et al, 2006a). This method is essential in order to capture the particle dynamics and export flux variability in such regions as the STF, which are characterized by dynamic hydrography (e.g., Cai et al 2008;). Indeed, with high resolution sampling, Cai et al (2008) observed significant variations of 234 Th deficit in the South China Sea, with enhanced Th and POC fluxes along the western and southern boundaries of this marginal sea.…”

Section: K Zhou Et Al: Insignificant Enhancement Of Export Flux In mentioning

confidence: 99%

Insignificant enhancement of export flux in the highly productive subtropical front, east of New Zealand: a high resolution study of particle export fluxes based on 234Th: 238U disequilibria

et al. 2012

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Abstract. We evaluated the export fluxes of Particulate Organic Carbon (POC) in the Subtropical Frontal zone (STF) of the SW Pacific sector of the Southern Ocean. The site is characterized by enhanced primary productivity, which has been suggested to be stimulated through so-called natural iron fertilization processes where iron-depleted subantarctic water (SAW) mixes with mesotrophic, iron-replete subtropical water (STW). We adopted the small-volume 234 Th method to achieve the highest possible spatial sampling resolution in austral late autumn-early winter, MayJune, 2008. Inventories of chlorophyll-a, particulate 234 Th and POC observed in the upper 100 m were all elevated in the mid-salinity water type (34.5 < S < 34.8), compared with low salinity waters (S < 34.5) which were of SAW origin with high macronutrients and high (S > 34.8) salinity waters which were of STW origin with low macronutrients. However, Steady-State 234 Th fluxes were similar across the salinity gradient being, 25 ± 0.78 ((1.5 ± 0.047) × 10 3 ) in the mid-salinity, and 29 ± 0.53 ((1.8 ± 0.032)×10 3 ) and 22 ± 1.1 Bq m −2 d −1 ((1.3 ± 0.066) × 10 3 dpm m −2 d −1 ) in the high and low salinity waters respectively. Bottle POC/Th ratios at the depth of 100 m were used to convert 234 Th fluxes into POC export fluxes. The derived POC flux did not appear to be enhanced in mid-salinity waters where the primary productivity was inferred to be the highest at the time of sampling, with a flux of 11 ± 0.45 mmol C m −2 d −1 , compared to 14 ± 0.39 mmol C m −2 d −1 in high salinity waters and 8.5 ± 0.66 mmol C m −2 d −1 in low salinity waters. This study thus implied that natural iron fertilization does not necessarily lead to an enhancement of POC export in STF regions.

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A high‐resolution study of particle export in the southern South China Sea based on ²³⁴Th:²³⁸U disequilibrium

Cited by 59 publications

References 52 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

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

Insignificant enhancement of export flux in the highly productive subtropical front, east of New Zealand: a high resolution study of particle export fluxes based on <sup>234</sup>Th: <sup>238</sup>U disequilibria

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A high‐resolution study of particle export in the southern South China Sea based on 234Th:238U disequilibrium

Cited by 59 publications

References 52 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

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

Insignificant enhancement of export flux in the highly productive subtropical front, east of New Zealand: a high resolution study of particle export fluxes based on &lt;sup&gt;234&lt;/sup&gt;Th: &lt;sup&gt;238&lt;/sup&gt;U disequilibria

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A high‐resolution study of particle export in the southern South China Sea based on ²³⁴Th:²³⁸U disequilibrium

Insignificant enhancement of export flux in the highly productive subtropical front, east of New Zealand: a high resolution study of particle export fluxes based on <sup>234</sup>Th: <sup>238</sup>U disequilibria