29 research outputs found
Particle fluxes associated with mesoscale eddies in the Sargasso Sea
Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 55 (2008): 1426-1444, doi:10.1016/j.dsr2.2008.02.007.We examined the impact of a cyclonic eddy and mode-water eddy on particle flux in the
Sargasso Sea. The primary method used to quantify flux was based upon measurements of the
natural radionuclide, 234Th, and these flux estimates were compared to results from sediment
traps in both eddies, and a 210Po/210Pb flux method in the mode-water eddy. Particulate organic
carbon (POC) fluxes at 150m ranged from 1 to 4 mmol C m-2 d-1 and were comparable between
methods, especially considering differences in integration times scales of each approach. Our
main conclusion is that relative to summer mean conditions at the Bermuda Atlantic Time-series
Study (BATS) site, eddy-driven changes in biogeochemistry did not enhance local POC fluxes
during this later, more mature stage of the eddy life cycle (>6 months old). The absence of an
enhancement in POC flux puts a constraint on the timing of higher POC flux events, which are
thought to have caused the local O2 minima below each eddy, and must have taken place >2
months prior to our arrival. The mode-water eddy did enhance preferentially diatom biomass in
its center where we estimated a factor of 3 times higher biogenic Si flux than the BATS summer
average. An unexpected finding in the highly depth resolved 234Th data sets are narrow layers of
particle export and remineralization within the eddy. In particular, a strong excess 234Th signal is
seen below the deep chlorophyll maxima which we attribute to remineralization of 234Th bearing
particles. At this depth below the euphotic zone, de novo particle production in the euphotic
zone has stopped, yet particle remineralization continues via consumption of labile sinking
material by bacteria and/or zooplankton. These data suggest that further study of processes in
ocean layers is warranted not only within, but below the euphotic zone.The EDDIES project was funded by the National Science Foundation Chemical, Biological, and
Physical Oceanography Programs. Additional support for HPLC pigment analysis (Dr. Charles
Trees, CHORS) was provided by NASA
VERTIGO (VERtical Transport In the Global Ocean) : a study of particle sources and flux attenuation in the North Pacific
Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 55 (2008): 1522-1539, doi:10.1016/j.dsr2.2008.04.024.The VERtical Transport In the Global Ocean (VERTIGO) study examined particle sources and
fluxes through the ocean’s “twilight zone” (defined here as depths below the euphotic zone to
1000 m). Interdisciplinary process studies were conducted at contrasting sites off Hawaii
(ALOHA) and in the NW Pacific (K2) during 3 week occupations in 2004 and 2005, respectively.
We examine in this overview paper the contrasting physical, chemical and biological settings and
how these conditions impact the source characteristics of the sinking material and the transport
efficiency through the twilight zone. A major finding in VERTIGO is the considerably lower
transfer efficiency (Teff) of particulate organic carbon (POC), POC flux 500 / 150 m, at ALOHA
(20%) vs. K2 (50%). This efficiency is higher in the diatom-dominated setting at K2 where
silica-rich particles dominate the flux at the end of a diatom bloom, and where zooplankton and
their pellets are larger. At K2, the drawdown of macronutrients is used to assess export and
suggests that shallow remineralization above our 150 m trap is significant, especially for N
relative to Si. We explore here also surface export ratios (POC flux/primary production) and
possible reasons why this ratio is higher at K2, especially during the first trap deployment. When
we compare the 500 m fluxes to deep moored traps, both sites lose about half of the sinking POC
by >4000 m, but this comparison is limited in that fluxes at depth may have both a local and
distant component. Certainly, the greatest difference in particle flux attenuation is in the
mesopelagic, and we highlight other VERTIGO papers that provide a more detailed examination
of the particle sources, flux and processes that attenuate the flux of sinking particles. Ultimately,
we contend that at least three types of processes need to be considered: heterotrophic degradation
of sinking particles, zooplankton migration and surface feeding, and lateral sources of suspended
and sinking materials. We have evidence that all of these processes impacted the net attenuation
of particle flux vs. depth measured in VERTIGO and would therefore need to be considered and
quantified in order to understand the magnitude and efficiency of the ocean’s biological pump.Funding for VERTIGO was provided primarily by research grants
from the US National Science Foundation Programs in Chemical and Biological Oceanography
(KOB, CHL, MWS, DKS, DAS). Additional US and non-US grants included: US Department
of Energy, Office of Science, Biological and Environmental Research Program (JKBB); the
Gordon and Betty Moore Foundation (DMK); the Australian Cooperative Research Centre
program and Australian Antarctic Division (TWT); Chinese NSFC and MOST programs (NZJ);
Research Foundation Flanders and Vrije Universiteit Brussel (FD, ME); JAMSTEC (MCH); New
Zealand Public Good Science Foundation (PWB); and internal WHOI sources and a contribution
from the John Aure and Cathryn Ann Hansen Buesseler Foundation (KOB)
Barium in twilight zone suspended matter as a potential proxy for particulate organic carbon remineralization : results for the North Pacific
Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 55 (2008): 1673-1683, doi:10.1016/j.dsr2.2008.04.020.This study focuses on the fate of exported organic carbon in the twilight zone at two
contrasting environments in the North Pacific: the oligotrophic ALOHA site (22°45'
N 158°W; Hawaii; studied during June–July 2004) and the mesotrophic Subarctic
Pacific K2 site (47°N, 161°W; studied during July-August 2005). Earlier work has
shown that non-lithogenic, excess particulate Ba (Baxs) in the mesopelagic water
column is a potential proxy of organic carbon remineralization. In general Baxs
contents were significantly larger at K2 than at ALOHA. At ALOHA the Baxs profiles
from repeated sampling (5 casts) showed remarkable consistency over a period of
three weeks, suggesting that the system was close to being at steady state. In contrast,
more variability was observed at K2 (6 casts sampled) reflecting the more dynamic
physical and biological conditions prevailing in this environment. While for both sites
Baxs concentrations increased with depth, at K2 a clear maximum was present
between the base of the mixed layer at around 50m and 500m, reflecting production
and release of Baxs. Larger mesopelagic Baxs contents and larger bacterial production
in the twilight zone at the K2 site indicate that more material was exported from the
upper mixed layer for bacterial degradation deeper, compared to the ALOHA site.
Furthermore, application of a published transfer function (Dehairs et al., 1997)
relating oxygen consumption to the observed Baxs data indicated that the latter were in
good agreement with bacterial respiration, calculated from bacterial production. These
results corroborate earlier findings highlighting the potential of Baxs as a proxy for
organic carbon remineralization.
The range of POC remineralization rates calculated from twilight zone excess
particulate Ba contents did also compare well with the depth dependent POC flux
decrease as recorded by neutrally buoyant sediment traps, except in 1 case (out of 4).
This discrepancy could indicate that differences in sinking velocities cause an
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uncoupling of the processes occurring in the fine suspended particle pool from those
affecting the larger particle pool which sustains the vertical flux, thus rendering
comparison between both approaches risky.This research was supported by Federal Science Policy
Office, Brussels through contracts EV/03/7A, SD/CA/03A, the Research Foundation
Flanders through grant G.0021.04 and Vrije Universiteit Brussel via grant GOA 22, as
well as the US National Science Foundation programs in Chemical and Biological
Oceanography
Recommended from our members
VERTIGO (VERtical Transport In the Global Ocean): A study of particle sources and flux attenuation in the North Pacific
Recommended from our members
Transformation rates and fate of dissolved, colloidal and particulate forms of organic carbon in ocean margins. Final report, May 1, 1992--April 3, 1995
The goal of this proposal was to develop new sampling and analytical techniques which could be used to understand the transformation rates and mechanisms of exchange between dissolved, colloidal, small particle and large particle size classes of organic carbon in ocean margins. To meet this goal, the authors focused on the development of cross-flow filtration (CFF) for the isolation of colloidal material from the dissolved phase in seawater. In addition, they tested and optimized high sensitivity techniques for the measurement of thorium isotopes using thermal ionization mass spectrometry (TIMS) and carbon isotopes using accelerator mass spectrometry (AMS) and stable C mass spectrometric techniques. In this final report, they highlight some of these new sampling and analytical developments as well as preliminary results from the first DOE cruise this past April. The report is broken down into 4 sections, namely (1) colloidal sampling strategies, (2) TIMS analytical developments, (3) carbon isotopic measurements and (4) results from the R/V Columbus Iselin cruise. For more detailed discussion of the findings, they have included as an appendix to this final report manuscripts which have been published or will be submitted during this funding cycle
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
Cesium-134 and 137 activities in the central North Pacific Ocean after the Fukushima Dai-ichi Nuclear Power Plant accident
Surface seawater <sup>134</sup>Cs and <sup>137</sup>Cs samples were collected in the central and western North Pacific Ocean during the 2 yr after the Fukushima Dai-ichi Nuclear Power Plant accident to monitor dispersion patterns of these radioisotopes towards the Hawaiian Islands. In the absence of other recent sources and due to its short half-life, only those parts of the Pacific Ocean would have detectable <sup>134</sup>Cs values that were impacted by Fukushima releases. Between March and May 2011, <sup>134</sup>Cs was not detected around the Hawaiian Islands and Guam. Here, most <sup>137</sup>Cs activities (1.2–1.5 Bq m<sup>–3</sup>) were in the range of expected preexisting levels. Some samples north of the Hawaiian Islands (1.6–1.8 Bq m<sup>–3</sup>) were elevated above the 23-month baseline established in surface seawater in Hawaii indicating that those might carry atmospheric fallout. The 23-month time-series analysis of surface seawater from Hawaii did not reveal any seasonal variability or trends, with an average activity of 1.46 ± 0.06 Bq m<sup>–3</sup> (Station Aloha, 18 values). In contrast, samples collected between Japan and Hawaii contained <sup>134</sup>Cs activities in the range of 1–4 Bq m<sup>–3</sup>, and <sup>137</sup>Cs levels were about 2–3 times above the preexisting activities. We found that the southern boundary of the Kuroshio and Kuroshio extension currents represented a boundary for radiation dispersion with higher activities detected within and north of the major currents. The radiation plume has not been detected over the past 2 yr at the main Hawaiian Islands due to the transport patterns across the Kuroshio and Kuroshio extension currents
A bottom-up view of the biological pump: Modeling source funnels above ocean sediment traps
<sup>234</sup>Th sorption and export models in the water column: a review
Over the past few decades, the radioisotope pair of 238U / 234Th has been widely and increasingly used to describe particle dynamics and particle export fluxes in a variety of aquatic systems. The present paper is one of five review articles dedicated to 234Th. It is focused on the models associated with 234Th whereas the companion papers (same issue) are focused on present and future methodologies and techniques (Rutgers van der Loeff et al.), C / 234Th ratios (Buesseler et al.), 234Th speciation (Santschi et al.) and present and future applications of 234Th [Waples, J.T., Benitez-Nelson, C.R., Savoye, N., Rutgers van der Loeff, M., Baskaran, M., Gustafsson, Ă–., this issue. An Introduction to the application and future use of 234Th in aquatic systems. Marine Chemistry, FATE special issue]. In this paper, we review current 234Th scavenging models and discuss the relative importance of the non-steady state and physical terms associated with the most commonly used model to estimate 234Th flux. Based on this discussion we recommend that for future work the use of models should be accompanied by a discussion of the effect that model and data uncertainty have on the model results. We also suggest that future field work incorporate repeat occupations of sample sites on time scales of 1-4 weeks in order to evaluate steady state versus non-steady state estimates of 234Th export, especially during high flux events (> ca. 800 dpm m-2 d-1). Finally, knowledge of the physical oceanography of the study area is essential, particularly in ocean margins and in areas of established upwelling (e.g., Equatorial Pacific). These suggestions will greatly enhance the application of 234Th as a tracer of particle dynamics and flux in more complicated regimes