Buesseler Kenneth O.

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Buesseler

First Name

Kenneth O.

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0000-0001-7362-8796

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Kinetics of thorium and particle cycling along the U.S. GEOTRACES North Atlantic Transect

( 2017-05) Lerner, Paul ; Marchal, Olivier ; Lam, Phoebe J. ; Buesseler, Ken O. ; Charette, Matthew A.

The high particle reactivity of thorium has resulted in its widespread use in tracing processes impacting marine particles and their chemical constituents. The use of thorium isotopes as tracers of particle dynamics, however, largely relies on our understanding of how the element scavenges onto particles. Here, we estimate apparent rate constants of Th adsorption (k1), Th desorption (k−1), bulk particle degradation (β-1), and bulk particle sinking speed (w) along the water column at 11 open-ocean stations occupied during the GEOTRACES North Atlantic Section (GA03). First, we provide evidence that the budgets of Th isotopes and particles at these stations appear to be generally dominated by radioactive production and decay sorption reactions, particle degradation, and particle sinking. Rate parameters are then estimated by fitting a Th and particle cycling model to data of dissolved and particulate 228,230,234Th, 228Ra, particle concentrations, and 234,238U estimates based on salinity, using a nonlinear programming technique. We find that the adsorption rate constant (k1) generally decreases with depth across the section: broadly, the time scale 1/k1 averages 1.0 yr in the upper 1000 m and (1.4–1.5) yr below. A positive relationship between k1 and particle concentration (P) is found, i.e., , k1 ∝ Pb where b ≥ 1, consistent with the notion that k1 increases with the number of surface sites available for adsorption. The rate constant ratio, K = k1/(k-1 + β-1), which measures the collective influence of rate parameters on Th scavenging, averages 0.2 for most stations and most depths. We clarify the conditions under which K/P is equivalent to the distribution coefficient, KD, test that the conditions are met at the stations, and find that decreases with P, in line with a particle concentration effect (dKD/dP < 0). In contrast to the influence of colloids as envisioned by the Brownian pumping hypothesis, we provide evidence that the particle concentration effect arises from the joint effect of P on the rate constants for thorium attachment to, and detachment from, particles.
Particle export during the Southern Ocean Iron Experiment (SOFeX)

( 2004-07-26) Buesseler, Ken O. ; Andrews, J. E. ; Pike, Steven M. ; Charette, Matthew A. ; Goldson, Laura E. ; Brzezinski, Mark A. ; Lance, V. P.

We studied the effect of iron addition on particle export in the Southern Ocean by measuring changes in the distribution of thorium-234 during a 4 week Fe enrichment experiment conducted in the high-silicate high-nitrate waters just south of the Southern Antarctic Circumpolar Current Front at 172.5°W. Decreases in 234Th activity with time in the fertilized mixed layer (0-50m) exceeded those in unfertilized waters, indicating enhanced export of 234Th on sinking particles after Fe enrichment. The addition of Fe also affected export below the fertilized patch by increasing the efficiency of particle export through the 100 m depth horizon. Extensive temporal and vertical Lagrangian sampling allowed us to make a detailed examination of the 234Th flux model, which was used to quantify the fluxes of particulate organic carbon (POC) and biogenic silica (bSiO2). Iron addition increased the flux of both POC and bSiO2 out of the mixed layer by about 300%. The flux at 100 m increased by more than 700% and 600% for POC and bSiO2, respectively. The absolute magnitude of the POC and bSiO2 fluxes were not large relative to natural blooms at these latitudes, or to those found in association with the termination of blooms in other ocean regions. Our results support the hypothesis that Fe addition leads directly to significant particle export and sequestration of C in the deep ocean. This is a key link between ocean Fe inputs and past changes in atmospheric CO2 and climate.
Testing models of thorium and particle cycling in the ocean using data from station GT11-22 of the U.S. GEOTRACES North Atlantic section

( 2016-03-31) Lerner, Paul ; Marchal, Olivier ; Lam, Phoebe J. ; Anderson, Robert F. ; Buesseler, Ken O. ; Charette, Matthew A. ; Edwards, R. Lawrence ; Hayes, Christopher T. ; Huang, Kuo-Fang ; Lu, Yanbin ; Robinson, Laura F. ; Solow, Andrew R.

Thorium is a highly particle-reactive element that possesses different measurable radio-isotopes in seawater, with well-constrained production rates and very distinct half-lives. As a result, Th has emerged as a key tracer for the cycling of marine particles and of their chemical constituents, including particulate organic carbon. Here two different versions of a model of Th and particle cycling in the ocean are tested using an unprecedented data set from station GT11-22 of the U.S. GEOTRACES North Atlantic Section: (i) 21 228;230;234Th activities of dissolved and particulate fractions, (ii) 228Ra activities, (iii) 234;238U activities estimated from salinity data and an assumed 234U/238U ratio, and (iv) particle concentrations, below a depth of 125 m. The two model versions assume a single class of particles but rely on different assumptions about the rate parameters for sorption reactions and particle processes: a first version (V1) assumes vertically uniform parameters (a popular description), whereas the second (V2) does not. Both versions are tested by fitting to the GT11-22 data using generalized nonlinear least squares and by analyzing residuals normalized to the data errors. We find that model V2 displays a significantly better fit to the data than model V1. Thus, the mere allowance of vertical variations in the rate parameters can lead to a significantly better fit to the data, without the need to modify the structure or add any new processes to the model. To understand how the better fit is achieved we consider two parameters, K = k1=(k-1 + β-1) and K/P, where k1 is the adsorption rate constant, k-1 the desorption rate constant, β-1 the remineralization rate constant, and P the particle concentration. We find that the rate constant ratio K is large (≥0.2) in the upper 1000 m and decreases to a nearly uniform value of ca. 0.12 below 2000 m, implying that the specific rate at which Th attaches to particles relative to that at which it is released from particles is higher in the upper ocean than in the deep ocean. In contrast, K/P increases with depth below 500 m. The parameters K and K/P display significant positive and negative monotonic relationship with P, respectively, which is collectively consistent with a particle concentration effect.
Size-fractionated labile trace elements in the Northwest Pacific and Southern Oceans

( 2011-04-11) Baeyens, W. ; Bowie, Andrew R. ; Buesseler, Ken O. ; Gao, Y. ; Lamborg, Carl H. ; Remenyi, Tomas A. ; Zhang, H.

Photosynthesis by marine phytoplankton requires bioavailable forms of several trace elements that are found in extremely low concentrations in the open ocean. We have compared the concentration, lability and size distribution (< 1 nm and < 10 nm) of a suite of trace elements that are thought to be limiting to primary productivity as well as a toxic element (Pb) in two High Nutrient Low Chlorophyll (HNLC) regions using a new dynamic speciation technique, Diffusive Gradients in Thin-film (DGT). The labile species trapped within the DGT probes have a size that is smaller or similar than the pore size of algal cell walls and thus present a proxy for bioavailable species. Total Dissolvable trace element concentrations (TD concentration) varied between 0.05 nM (Co) and 4.0 nM (Ni) at K2 (Northwest Pacific Ocean) and between 0.026 nM (Co) and 4.7 nM (Ni) in the Southern Ocean. The smallest size fractionated labile concentrations (< 1 nm) observed at Southern Ocean sampling stations ranged between 0.002 nM (Co) and 2.1 nM (Ni). Moreover, large differences in bioavailable fractions (ratio of labile to TD concentration) were observed between the trace elements. In the Northwest Pacific Ocean Fe, Cu and Mn had lower labile fractions (between 10 and 44%) than Co, Cd, Ni and Pb (between 80 and 100%). In the Southern Ocean a similar trend was observed, and in addition: (1) Co, Cd, Ni and Pb have lower labile fractions in the Southern Ocean than in the Northwest Pacific and (2) the ratios of <1nm to dissolvable element concentrations at some Southern Ocean stations were very low and varied between 4 and16 %.
Fishing for answers

( 2012-08-30) Buesseler, Ken O.

The triple disaster of the March 11, 2011, earthquake, tsunami, and subsequent radiation releases at Fukushima Dai-ichi were, and continue to be, unprecedented events for the ocean and for society. More than 80% of the radioactivity from Fukushima was either blown offshore or directly discharged in to the ocean from waters used to cool the nuclear power plants (1). Although offshore waters are safe with respect to international standards for radionuclides in the ocean (2), the nuclear power plants continue to leak radioactive contaminants into the ocean (3), and many near-shore fisheries remain closed.
Changes in fecal pellet characteristics with depth as indicators of zooplankton repackaging of particles in the mesopelagic zone of the subtropical and subarctic North Pacific Ocean

( 2008-01-31) Wilson, Stephanie E. ; Steinberg, Deborah K. ; Buesseler, Ken O.

We investigated how fecal pellet characteristics change with depth in order to quantify the extent of particle repackaging by mesopelagic zooplankton in two contrasting open-ocean systems. Material from neutrally buoyant sediment traps deployed in the summer of 2004 and 2005 at 150, 300, and 500 m was analyzed from both a mesotrophic (Japanese time-series station K2) and an oligotrophic (Hawaii Ocean Time series-HOT station ALOHA) environment in the Pacific Ocean as part of the VERtical Transport In the Global Ocean (VERTIGO) project. We quantified changes in the flux, size, shape, and color of particles recognizable as zooplankton fecal pellets to determine how these parameters varied with depth and location. Flux of K2 fecal pellet particulate organic carbon (POC) at 150 and 300 m was 4-5 times higher than at ALOHA, and at all depths, fecal pellets were 2-5 times larger at K2, reflective of the disparate zooplankton community structure at the two sites. At K2, the proportion of POC flux that consisted of fecal pellets generally decreased with depth from 20% at 150 m to 5% at 500 m, whereas at ALOHA this proportion increased with depth (and was more variable) from 14% to 35%. This difference in the fecal fraction of POC with increasing depth is hypothesized to be due to differences in the extent of zooplankton-mediated fragmentation (coprohexy) and in zooplankton community structure between the two locations. Both regions provided indications of sinking particle repackaging and zooplankton carnivory in the mesopelagic. At ALOHA this was reflected in a significant increase in the mean flux of larvacean fecal pellets from 150 to 500 m of 3 to 46 μg C m-2 d-1, respectively, and at K2 a large peak in larvacean mean pellet flux at 300 m of 3.1 mg C m-2 d-1. Peaks in red pellets produced by carnivores occurred at 300 m at K2, and a variety of other fecal pellet classes showed significant changes in their distribution with depth. There was also evidence of substantially higher pellet fragmentation at K2 with nearly double the ratio of broken:intact pellets at 150 and 300 m (mean of 67% and 64%, respectively ) than at ALOHA where the proportion of broken pellets remained constant with depth (mean 35%). Variations in zooplankton size and community structure within the mesopelagic zone can thus differentially alter the transfer efficiency of sinking POC.
A review of present techniques and methodological advances in analyzing Th-234 in aquatic systems

( 2005-10-10) Rutgers van der Loeff, Michiel M. ; Sarin, Manmohan M. ; Baskaran, Mark ; Benitez-Nelson, Claudia R. ; Buesseler, Ken O. ; Charette, Matthew A. ; Dai, Minhan ; Gustafsson, Orjan ; Masqué, Pere ; Morris, Paul J. ; Orlandini, Kent ; Rodriguez y Baena, Alessia ; Savoye, Nicolas ; Schmidt, Sabine ; Turnewitsch, Robert ; Voge, Ingrid ; Waples, James T.

The short-lived thorium isotope 234Th (half-life 24.1 days) has been used as a tracer for a variety of transport processes in aquatic systems. Its use as a tracer of oceanic export via sinking particles has stimulated a rapidly increasing number of studies that require analyses of 234Th in both marine and freshwater systems. The original 234Th method is labour intensive. Thus, there has been a quest for simpler techniques that require smaller sample volumes. Here, we review current methodologies in the collection and analysis of 234Th from the water column, discuss their individual strengths and weaknesses, and provide an outlook on possible further improvements and future challenges. Also included in this review are recommendations on calibration procedures and the production of standard reference materials as well as a flow chart designed to help researchers find the most appropriate 234Th analytical technique for a specific aquatic regime and known sampling constraints.
Th-234 as a tracer of particulate export and remineralization in the southeastern tropical Pacific

( 2017-06) Black, Erin E. ; Buesseler, Ken O. ; Pike, Steven M. ; Lam, Phoebe J.

Oxygen minimum zones (OMZs) are thought to be regions of decreased carbon attenuation in the upper ocean with a biological pump that could deliver a greater percentage of exported carbon to the mesopelagic relative to surrounding waters. However, much is still unknown about carbon cycling through these zones and the areas of extreme oxygen minima (nM O2) or oxygen deficient zones (ODZs) within the OMZs. Paired sampling for 234Th (t½ ~24.1 days) and particulate organic carbon (POC) was performed along a zonal transect between 77° W and 152° W during the U.S. GEOTRACES Southeastern Tropical Pacific campaign in 2013 in order to constrain the magnitude of carbon export and remineralization through the Peruvian OMZ. POC export varied by an order of magnitude from the coast to 152° W, reflecting a decrease in POC:234Th ratios (>51 μm) with distance offshore and the influence of upwelling at the coast. Modeling indicated that 234Th fluxes could be underestimated at coastal stations by up to 4-fold without adjustment for the impact of upwelling, which in turn would produce much lower carbon export estimates. Low carbon Export:NPP ratios (<0.15) at the base of the euphotic zone (Ez) in the gyre support previous findings of inefficient surface export via the biological pump in the southeastern tropical Pacific. A broad remineralization feature beginning at Ez was observed across >7500 km that resulted in, on average, 3% of the POC exported from the euphotic zone reaching 100 m below Ez. Although the highest percentages (>10%) of total exported POC at 100 m below Ez were observed in the coastal ODZ region, the observed remineralization was also most pronounced these stations. While an average of 75% of the carbon export from the euphotic zone remained at Ez +100 m in the gyre, a range of 10% to 50% was observed at ODZ stations, reflecting increased attenuation. Local subsurface minima in light transmission and maxima in fluorescence were observed in the regions of greatest remineralization at the upper ODZ boundary, suggesting that complex bacterial community dynamics play a role in increased attenuation through these zones. With ODZs and OMZs predicted to grow worldwide with climate change, these areas require further large-scale and seasonal studies to assess the permanency of these attenuation features and the impact of high Gyre and lower ODZ transfer of POC on the overall efficiency of carbon export in the Pacific.
Plutonium in groundwater at the 100K-Area of the U.S. DOE Hanford Site

( 2004-01-27) Dai, Minhan ; Buesseler, Ken O. ; Pike, Steven M.

We examined the concentration, size distribution, redox state and isotopic composition of plutonium (Pu) in groundwater at the 100K-Area at the US Department of Energy’s (DOE) Hanford Site. Total concentrations of Pu isotopes were extremely low (10-4 to 10-6 pCi/kg, ≈ 104 to 106 atoms/kg), but measurable for the first time in the 100K-Area wells using mass spectrometric analyses that are much more sensitive than alpha spectroscopy methods used previously. Size fractionation data from two wells suggests that 7-29% of the Pu is associated with colloids, operationally defined here as particles between 1 kDa – 0.2 μm in size. These colloids were collected using a 1 kDa cross-flow ultrafiltration system developed specifically for groundwater actinide studies to include careful controls both in the field and during processing to ensure in-situ geochemical conditions are maintained and size separations can be well characterized. Pu in this colloidal fraction was exclusively in the more reduced Pu(III/IV) form, consistent with the higher affinity of Pu for particle surfaces in the lower oxidation states. While the overall concentrations of Pu were low, the Pu isotopic composition suggests at least two local sources of groundwater Pu, namely local Hanford reactor operations at the 100K-Area, and spent nuclear fuel from the N reactor, which was stored in concrete pools at this site. Differences between this site and the Savannah River Site (SRS) are noted, since groundwater Pu at the F-Area seepage basin at SRS has been found using these same 2 methods, to be characterized by much lower colloidal abundances and higher oxidation states. This difference is not directly attributable to groundwater redox potential or geochemical conditions, but rather the physical-chemical difference in Pu sources, which at SRS appear to be dominated downstream from the seepage basins by decay of 244Cm, resulting in more oxidized forms of 240Pu. There is no clear evidence for colloid facilitated transport of Pu in groundwater at this site, since downstream wells have both an order of magnitude lower concentrations of Pu, but also a lower fractional colloidal distribution.
Fukushima 137Cs at the base of planktonic food webs off Japan

( 2015-09-18) Baumann, Zofia ; Fisher, Nicholas S. ; Gobler, Christopher J. ; Buesseler, Ken O. ; George, J. A. ; Breier, Crystaline F. ; Nishikawa, Jun

The potential bioaccumulation of 137Cs in marine food webs off Japan became a concern following the release of radioactive contaminants from the damaged Fukushima nuclear power plant into the coastal ocean. Previous studies suggest that 137Cs activities increase with trophic level in pelagic food webs, however, the bioaccumulation of 137Cs from seawater to primary producers, to zooplankton has not been evaluated in the field. Since phytoplankton are frequently the largest component of SPM (suspended particulate matter) we used SPM concentrations and particle-associated 137Cs to understand bioaccumulation of 137Cs in through trophic pathways in the field. We determined particle-associated 137Cs for samples collected at 20 m depth from six stations off Japan three months after the initial release from the Fukushima nuclear power plant. At 20 m SPM ranged from 0.65 to 1.60 mg L-1 and rapidly declined with depth. The ratios of particulate organic carbon to chlorophyll a suggested that phytoplankton comprised much of the SPM in these samples. 137Cs activities on particles accounted for on average 0.04% of the total 137Cs in seawater samples, and measured concentration factors of 137Cs on small suspended particles were comparatively low (~102). However, when 137Cs in crustacean zooplankton was derived based only on modeling dietary 137Cs uptake, we found predicted and measured 137Cs concentrations in good agreement. We therefore postulate the possibility that the dietary route of 137Cs bioaccumulation (i.e., phytoplankton ingestion) could be largely responsible for the measured levels in the copepod-dominated (%) zooplankton assemblages in Japanese coastal waters. Finally, our data did not support the notion that zooplankton grazing on phytoplankton results in a biomagnification of 137Cs.
Particle fluxes associated with mesoscale eddies in the Sargasso Sea

( 2007-09-25) Buesseler, Ken O. ; Lamborg, Carl H. ; Cai, Pinghe ; Escoube, Raphaelle ; Johnson, Rodney J. ; Pike, Steven M. ; Masqué, Pere ; McGillicuddy, Dennis J. ; Verdeny, Elisabet

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.
Primary, new and export production in the NW Pacific subarctic gyre during the vertigo K2 experiments

( 2008-01-28) Elskens, Marc ; Brion, N. ; Buesseler, Ken O. ; Van Mooy, Benjamin A. S. ; Boyd, Philip W. ; Dehairs, Frank ; Savoye, Nicolas ; Baeyens, W.

This paper presents results on tracer experiments using 13C and 15N to estimate uptake rates of dissolved inorganic carbon (DIC) and nitrogen (DIN). Experiments were carried out at station K2 (47°N, 161°E) in the NW Pacific subarctic gyre during July-August 2005. Our goal was to investigate relationships between new and export production. New production was inferred from the tracer experiments using the f ratio concept (0-50m); while export production was assessed with neutrally buoyant sediment traps (NBSTs) and the e ratio concept (at 150m). During trap deployments, K2 was characterized both by changes in primary production (523 to 404 mg C m-2 d-1), new production (119 to 67 mg C m-2 d-1), export production (68 to 24 mg C m-2 d-1) and phytoplankton composition (high to low proportion of diatoms). The data indicate that 17 to 23% of primary production is exportable to deeper layers (f ratio) but only 6 to 13% collected as a sinking particle flux at 150m (e ratio). Accordingly, > 80% of the carbon fixed by phytoplankton would be mineralized in the upper 50m (1 – f), while < 11% would be within 50-150m (f – e). DIN uptake flux amounted to 0.5 mM m-2 h-1, which was equivalent to about 95% particulate nitrogen (PN) remineralized and/or grazed within the upper 150 m. Most of the shallow PN remineralization occurred just above the depth of the deep chlorophyll maximum (DCM), where a net ammonium production was measured. Below the DCM, while nitrate uptake rates became negligible because of light limitation, ammonium uptake did continue to be significant. The uptake of ammonium by heterotrophic bacteria was estimated to be 14-17% of the DIN assimilation. Less clear are the consequences of this uptake on the phytoplankton community and biogeochemical processes, e.g. new production. It was suggested that competition for ammonium could select for small cells and may force large diatoms to use nitrate. This implies that under Fe stress as observed here, ammonium uptake is preferred and new production progressively suppressed despite the surplus of nitrate.
Beryllium-7 analyses in seawater by low background gamma-spectroscopy

( 2006-10-11) Andrews, J. E. ; Hartin, C. ; Buesseler, Ken O.

7Be is a cosmogenic isotope produced in the stratosphere and troposphere. 7Be has a half-life of 53.4 days and decays to 7Li emitting a 477 keV gamma line with a branching ratio of 0.104. It is predominantly washed out of the atmosphere through wet deposition. It is a tool for oceanographers to study air sea interaction and water mass mixing. Beryllium’s largely non-reactive nature in the open ocean makes it an excellent conservative tracer. Its conservative nature and extreme dilution in seawater also makes it difficult to concentrate and analyze. Early experiments at WHOI with Fe(OH)3 cartridges to directly collect 7Be by insitu underwater pumps proved ineffective. Collection efficiencies of the cartridges were too low to be consistently useful. At sea chemistry of whole water samples became the method of choice. The use of stable 9Be as a yield monitor further improved the accuracy of the procedure. The method was optimized at WHOI in 2005 using a seawater line that enters WHOI’s coastal research lab. The procedure was then used on an oceanographic cruise on the R/V Oceanus out of Bermuda in the oligotrophic Sargasso Sea.
Unexpected source of Fukushima-derived radiocesium to the coastal ocean of Japan

( 2017-08) Sanial, Virginie ; Buesseler, Ken O. ; Charette, Matthew A. ; Nagao, Seiya

There are 440 operational nuclear reactors in the world, with approximately half situated along the coastline. This includes the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), which experienced multiple reactor meltdowns in March 2011 followed by the release of radioactivity to the marine environment. While surface inputs to the ocean via atmospheric deposition and rivers are usually well monitored after a nuclear accident, no study has focused on subterranean pathways. During our study period, we found the highest cesium-137 (137Cs) levels (up to 23,000 Bq m-3) outside of the FDNPP site not in the ocean, rivers or potable groundwater, but in groundwater beneath sand beaches over tens of kilometers away from the FDNPP. Here, we present evidence of a previously unknown, ongoing source of Fukushima-derived 137Cs to the coastal ocean. We postulate that these beach sands were contaminated in 2011 through wave and tide driven exchange and sorption of highly radioactive Cs from seawater. Subsequent desorption of 137Cs and fluid exchange from the beach sands was quantified using naturally occurring radium isotopes. This estimated ocean 137Cs source (0.6 TBq y-1) is of similar magnitude as the ongoing releases of 137Cs from the FDNPP site for 2013-2016, as well as the input of Fukushima-derived dissolved 137Cs via rivers. Though this ongoing source is not at present a public health issue for Japan, the release of Cs of this type and scale needs to be considered in NPP monitoring and scenarios involving future accidents.
VERTIGO (VERtical Transport In the Global Ocean) : a study of particle sources and flux attenuation in the North Pacific

( 2008-03-21) Buesseler, Ken O. ; Trull, Thomas W. ; Steinberg, Deborah K. ; Silver, Mary W. ; Siegel, David A. ; Saitoh, S.-I. ; Lamborg, Carl H. ; Lam, Phoebe J. ; Karl, David M. ; Jiao, N. Z. ; Honda, Makio C. ; Elskens, Marc ; Dehairs, Frank ; Brown, S. I. ; Boyd, Philip W. ; Bishop, James K. B. ; Bidigare, Robert R.

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.
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

( 2005-06-18) Buesseler, Ken O. ; Benitez-Nelson, Claudia R. ; Burd, Adrian B. ; Charette, Matthew A. ; Cochran, J. Kirk ; Coppola, L. ; Fisher, Nicholas S. ; Fowler, Scott W. ; Gardner, Wilford D. ; Guo, L. D. ; Gustafsson, Orjan ; Lamborg, Carl H. ; Masqué, Pere ; Miquel, Juan Carlos ; Passow, Uta ; Santschi, Peter H. ; Savoye, Nicolas ; Stewart, G. ; Trull, Thomas W.

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 234Th activity distribution (in order to calculate 234Th fluxes) and an estimate of the C/234Th ratio on sinking particles, to empirically derive C fluxes. In reviewing C/234Th 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/234Th either increases or is relatively invariant with increasing particle size (size classes >1 to 100’s μm). Observations also suggest that C/234Th 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 234Th 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/234Th from a standard depth of 100 m, or at least one depth below 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 234Th profiles and C/234Th samples during the course of longer observation periods to better sample temporal variations in both 234Th 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.
Th-234 sorption and export models in the water column : a review

( 2005-10-10) Savoye, Nicolas ; Benitez-Nelson, Claudia R. ; Burd, Adrian B. ; Cochran, J. Kirk ; Charette, Matthew A. ; Buesseler, Ken O. ; Jackson, George A. ; Roy-Barman, Matthieu ; Schmidt, Sabine ; Elskens, Marc

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

( 2009-03-27) Buesseler, Ken O. ; Pike, Steven M. ; Maiti, Kanchan ; Lamborg, Carl H. ; Siegel, David A. ; Trull, Thomas W.

An extensive 234Th 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 234Th:238U activity ratios near 1.0 indicated low particle fluxes at ALOHA, while 234Th:238U ~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 234Th for sediment trap calibration purposes. At K2, there was a decrease in export flux and increase in 234Th 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 234Th for sediment trap calibration purposes. High vertical resolution profiles show narrow layers (20-30 m) of excess 234Th 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 234Th and/or with sediment traps. Also, the application of 234Th as POC flux tracer relies on accurate sampling of particulate POC/234Th 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 234Th 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 234Th 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.
Marine radioecology after the Fukushima Dai-ichi nuclear accident : are we better positioned to understand the impact of radionuclides in marine ecosystems?

( 2017-10-31) Vives i Batlle, Jordi ; Aoyama, Michio ; Bradshaw, Clare ; Brown, Justin E. ; Buesseler, Ken O. ; Casacuberta, Nuria ; Christl, Marcus ; Duffa, Celine ; Impens, Nathalie R. E. N. ; Iosjpe, Mikhail ; Masqué, Pere ; Nishikawa, Jun

This paper focuses on how a community of researchers under the COMET (CO-ordination and iMplementation of a pan European projecT for radioecology) project has improved the capacity of marine radioecology to understand at the process level the behaviour of radionuclides in the marine environment, uptake by organisms and the resulting doses after the Fukushima Dai-ichi nuclear accident occurred in 2011. We present new radioecological understanding of the processes involved, such as the interaction of waterborne radionuclides with suspended particles and sediments or the biological uptake and turnover of radionuclides, which have been better quantified and mathematically described. We demonstrate that biokinetic models can better represent radionuclide transfer to biota in non-equilibrium situations, bringing more realism to predictions, especially when combining physical, chemical and biological interactions that occur in such an open and dynamic environment as the ocean. As a result, we are readier now than we were before the FDNPP accident in terms of having models that can be applied to dynamic situations. The paper concludes with our vision for marine radioecology as a fundamental research discipline and we present a strategy for our discipline at the European and international levels. The lessons learned are presented along with their possible applicability to assess/reduce the environmental consequences of future accidents to the marine environment and guidance for future research, as well as to assure sustainability of marine radioecology in Europe and globally. This guidance necessarily reflects on why and where further research funding is needed, signalling the way for future investigations.
Introduction to “Understanding the Ocean's biological pump : results from VERTIGO”

( 2008-04-04) Buesseler, Ken O. ; Lampitt, Richard S.

Unknown and unexplained aspects of the human condition were the focus of a television series in the USA created by Rod Serling in the early 1960s. The term “The Twilight Zone” used in his show is even more pertinent to the mysterious region between 100 and 1000m in the great oceans of the world, the “middle ground between light and shadow”. It is here where the sunlight at the ocean surface is finally extinguished and replaced by occasional flashes of biological light. This mesopelagic zone, as it is more formally called, is a region of immense change with depth and it is here that most of the biogenic material that settles out of the sunlit or euphotic zone is broken down and returned to the dissolved state. The gravitational downward flux of particles thus decreases with depth in general, and the animals that traverse this great depth, some each and every day, exert a powerful influence on the distribution of many types of materials. The extent of mixing also declines dramatically with depth, such that the water at 1000m is isolated from the atmosphere for many decades to centuries, and this has great significance when considering the influence of the oceans on the overlying atmosphere.

Buesseler Kenneth O.

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