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This study provides dissolved and particulate 230Th and 232Th results as well as particulate 234Th data collected during expeditions to the central Arctic Ocean (GEOTRACES, an international project to identify processes and quantify fluxes that control the distributions of trace elements; sections GN04 and GIPY11). Constructing a time series of dissolved 230Th from 1991 to 2015 enables the identification of processes that control the temporal development of 230Th distributions in the Amundsen Basin. After 2007, 230Th concentrations decreased significantly over the entire water column, particularly between 300 and 1500 m. This decrease is accompanied by a circulation change, evidenced by a concomitant increase in salinity. A potentially increased inflow of water of Atlantic origin with low dissolved 230Th concentrations leads to the observed depletion in dissolved 230Th in the central Arctic. Because atmospherically derived tracers (chlorofluorocarbon (CFC), sulfur hexafluoride (SF6)) do not reveal an increase in ventilation rate, it is suggested that these interior waters have undergone enhanced scavenging of Th during transit from Fram Strait and the Barents Sea to the central Amundsen Basin. The 230Th depletion propagates downward in the water column by settling particles and reversible scavenging.

Since the 1980–1990s, international research efforts have augmented our knowledge of the physical and chemical properties of the Arctic Ocean water masses, and recent studies have documented changes. Understanding the processes responsible for these changes is necessary to be able to forecast the local and global consequences of these property evolutions on climate. The present work investigates the distributions of geochemical tracers of particle fluxes and circulation in the Amerasian Basin and their temporal evolution over the last three decades (from stations visited between 1983 and 2015). Profiles of 230‐thorium (230Th) and 231‐protactinium (231Pa) concentrations and neodymium isotopes (expressed as εNd) measured in the Amerasian Basin prior to 2000 are compared to a new, post‐2000s data set. The comparison shows a large scale decrease in dissolved 230Th and 231Pa concentrations, suggesting intensification of scavenging by particle flux, especially in coastal areas. Higher productivity and sediment resuspension from the shelves appear responsible for the concentration decrease along the margins. In the basin interior, increased lateral exchanges with the boundary circulation also contribute to the decrease in concentration. This study illustrates how dissolved 230Th and 231Pa, with εNd support, can provide unique insights not only into changes in particle flux but also into the evolution of ocean circulation and mixing., Key Points Concentration decreases of 230Thd and 231Pad suggest an enhancement of particulate scavenging since the 2000s in the Amerasian BasinParticulate scavenging results from higher productivity and sediment resuspension from the shelvesPost‐2000s changes in intermediate and deep layers suggest enhanced lateral mixing between the margins and central areas of the basins

Demand is increasing for programs in ocean sciences, engineering, information technology, and management that address workforce development and leadership training needs to support both the traditional and emerging new blue economy. As described in this chapter, associate degrees and undergraduate programs that provide marine-related technical training represent an opportunity for growth in ocean engineering and technology workforce development. At the graduate level, new leadership programs are now being offered, such as the Blue MBA, that combine management and marine science training for students interested in developing skills applicable to the new blue economy. When considering the full spectrum of blue economy challenges and opportunities, there will likely be a growing demand for technical training and advanced academic programs in fields such as fisheries and aquaculture, ocean technology, engineering, ocean observing, renewable energy, marine biotechnology, finance and risk management. Development of new instructional programs provides an opportunity for greater collaboration between industry, government, and academia in building a diverse, equitable, and inclusive workforce that meets the needs of the burgeoning new blue economy.

This study provides dissolved and particulate 230Th and 232Th results as well as particulate 234Th data collected during expeditions to the central Arctic Ocean on ARK-XXIX/3 (2015) and ARK-XXII/2 (2007) (GEOTRACES sections GN04 and GIPY11, respectively). Constructing a time-series of dissolved 230Th from 1991 to 2015 enables the identification of processes that control the temporal development of 230Th distributions in the Amundsen Basin. After 2007, 230Th concentrations decreased significantly over the entire water column, particularly between 300 m and 1500 m. This decrease is accompanied by a circulation change, evidenced by a concomitant increase in salinity. Potentially increased inflow of water of Atlantic origin with low dissolved 230Th concentrations leads to the observed depletion in dissolved 230Th in the central Arctic. Because atmospherically derived tracers (CFC, 3He/3H) do not reveal an increase in ventilation rate, it is suggested that these interior waters have undergone enhanced scavenging of Th during transit from the Fram Strait and the Barents Sea to the central Amundsen Basin. The 230Th depletion propagates downward in the water column by settling particles and reversible scavenging. Taken together, the temporal evolution of Th distributions point to significant changes in the large-scale circulation of the Amundsen Basin.

The measurable supply of 232Th to the ocean can be used to derive the supply of other elements, which is more difficult to quantify directly. The measured inventory of an element divided by the derived supply yields a replacement time estimate, which in special circumstances is related to a residence time. As a proof of concept, Th-based supply rates imply a range in the replacement times of the rare earth elements (REEs) in the North Atlantic that is consistent with the chemical reactivity of REEs related to their ionic charge density. Similar estimates of replacement times for the bioactive trace elements (Fe, Mn, Zn, Cd, Cu and Co), ranging from 50,000 years, demonstrate the broad range of elemental reactivity in the ocean. Here, we discuss how variations in source composition, fractional solubility ratios or non continental sources such as hydrothermal vents lead to uncertainties in Th-based replacement time estimates. We show that the constraints on oceanic replacement time provided by the Th-based calculations are broadly applicable in predicting how elements are distributed in the ocean and for some elements, such as Fe, may inform us on how the carbon cycle may be impacted by trace element supply and removal.

The natural radionuclides 231Pa and 230Th are incorporated into the marine sediment record by scavenging, or adsorption to various particle types, via chemical reactions that are not fully understood. Because these isotopes have potential value in tracing several oceanographic processes, we investigate the nature of scavenging using trans-Atlantic measurements of dissolved (

Rare earth element (REE) concentrations were analyzed in surface water and submarine groundwater within the Pettaquamscutt Estuary, located on the western edge of Narragansett Bay in Rhode Island. These water samples were collected along the salinity gradient of the estuary. Rare earth element concentrations in the majority of the groundwater samples are substantially higher than their concentrations in the surface waters. In particular, Nd concentrations in groundwater range from 0.43 nmol kg − 1 up to 198 nmol kg − 1 (mean ± SD = 42.1 ± 87.2 nmol kg − 1 ), whereas Nd concentrations range between 259 pmol kg − 1 and 649 pmol kg − 1 (mean ± SD = 421 ± 149 pmol kg − 1 ) in surface waters from the estuary, which is, on average, 100 fold lower than Nd in the groundwaters. Groundwater samples all exhibit broadly similar middle REE (MREE) enriched shale-normalized REE patterns, despite the wide variation in pH of these natural waters (4.87 ≤ pH ≤ 8.13). The similarity of the shale-normalized REE patterns across the observed pH range suggests that weathering of accessory minerals, such as apatite, and/or precipitation of LREE enriched secondary phosphate minerals controls groundwater REE concentrations and fractionation patterns. More specifically, geochemical mixing models suggest that the REE fractionation patterns of the surface waters may be controlled by REE phosphate mineral precipitation during the mixing of groundwater and stream water with incoming water from the Rhode Island Sound. The estimated SGD (Submarine Groundwater Discharge) of Nd to the Pettaquamscutt Estuary is 26 ± 11 mmol Nd day − 1 , which is in reasonable agreement with the Nd flux of the primary surface water source to the estuary, the Gilbert Stuart Stream (i.e., 36 mmol day − 1 ), and of the same order of magnitude for a site in Florida.

Depth profiles of 210 Po and 210 Pb activity and phytoplankton and zooplankton abundance were collected during two cruises along the Canadian time-series Line P in the Northeast Subarctic Pacific (ranging from 48o39 N to 50o00 N and 126o40 W to 145o00 W) in August 2010 and February 2011 to evaluate connections between the planktonic community and distributions of these radionuclides in the upper 500 m of the water column. Statistical analysis indicates that 210 Po is more effectively removed from the surface ocean when large (>0.1 mg ind � 1 dry wt) zooplankton dominate, and is less effectively scavenged when the picoplankton Synechococcus is present at high concentrations (>1 � 10 5 cells ml � 1 ). While the zooplankton field data are consistent with previous lab studies and field observations, the phytoplankton results seem to conflict with recent evidence that small cells may contribute significantly to export in other oligotrophic regions. Differences in ecosystem mechanisms between the Subarctic Pacific and other oligotrophic systems that limit the contribution of small cells to sinking flux remain to be identified. 2014 Published by Elsevier Ltd.

Marshes in the urban Jamaica Bay Estuary, New York, USA are disappearing at an average rate of 13 ha/yr, and multiple stressors (e.g., wastewater inputs, dredging activities, groundwater removal, and global warming) may be contributing to marsh losses. Among these stressors, wastewater nutrients are suspected to be an important contributing cause of marsh deterioration. We used census data, radiometric dating, stable nitrogen isotopes, and soil surveys to examine the temporal relationships between human population growth and soil nitrogen; and we evaluated soil structure with computer-aided tomography, surface elevation and sediment accretion trends, carbon dioxide emissions, and soil shear strength to examine differences among disappearing (Black Bank and Big Egg) and stable marshes (JoCo). Radiometric dating and nitrogen isotope analyses suggested a rapid increase in human wastewater nutrients beginning in the late 1840s, and a tapering off beginning in the 1930s when wastewater treatment plants (WWTPs) were first installed. Current WWTPs nutrient loads to Jamaica Bay are approximately 13 995 kg N/d and 2767 kg P/d. At Black Bank, the biomass and abundance of roots and rhizomes and percentage of organic matter on soil were significantly lower, rhizomes larger in diameter, carbon dioxide emission rates and peat particle density significantly greater, and soil strength significantly lower compared to the stable JoCo Marsh, suggesting Black Bank has elevated decomposition rates, more decomposed peat, and highly waterlogged peat. Despite these differences, the rates of accretion and surface elevation change were similar for both marshes, and the rates of elevation change approximated the long-term relative rate of sea level rise estimated from tide gauge data at nearby Sandy Hook, New Jersey. We hypothesize that Black Bank marsh kept pace with sea level rise by the accretion of material on the marsh surface, and the maintenance of soil volume through production of larger diameter rhizomes and swelling (dilation) of waterlogged peat. JoCo Marsh kept pace with sea-level rise through surface accretion and soil organic matter accumulation. Understanding the effects of multiple stressors, including nutrient enrichment, on soil structure, organic matter accumulation, and elevation change will better inform management decisions aimed at maintaining and restoring coastal marshes.

Regional patterns of iron (Fe) and manganese (Mn) reduction rates across the shelf and slope of the southeastern Bering Sea, as well as the relative importance of these pathways in sedimentary organic matter remineralization, were investigated during the spring and summer of 2009. Reduction rates of Fe and Mn were calculated using depth pro! les of solid-phase iron and manganese oxide concentrations and bioturbation coef! cients, Db, determined from pro! les of excess 234 Th. Iron reduction was found to be a signi! cant pathway for carbon mineralization across the shelf, with an average rate of 1.74 mmol m ! 2 d ! 1 . However, Fe reduction rates higher than 6 mmol m ! 2 d ! 1 were calculated, and a signi! cant regional pattern was observed, with highest rates found on the northern shelf, and dropping toward the south and offshore. Conversely, Mn oxide reduction was found to be of minor signi! cance, with low reduction rates in all regions, averaging only 0.09 mmol m ! 2 d ! 1 across the shelf, and accounting for no more than 5% of total carbon oxidation in any region. These results indicate that Fe oxide reduction is a signi! cant pathway for carbon remineralization in the northern and middle-shelf regions, where organic matter deposition rates and benthic biomass are high. Additionally, this work provides insight into the potential role of sedimentary iron reduction as a source of bioavailable Fe in this region.

Intercomparision of 234 Th measurements in both water and particulate samples was carried out between 15 laboratories worldwide, as a part of GEOTRACES inter-calibration program. Particulate samples from four dif ferent stations namely BATS (both shallow and deep) and shelf station (shallow) in Atlantic and SAFE (both shallow and deep) and Santa Barbara station (shallow) in Pacific were used in the effort. Particulate intercali bration results indicate good agreement between all the participating labs with data from all labs falling with in the 95% confidence interval around the mean for most instances. Filter type experiments indicate no signif icant differences in 234 Th activities between filter types and pore sizes (0.2‐0.8 µm). The only exception are the

Nineteen labs representing nine nations participated in the GEOTRACES intercalibration initiative that determined concentrations of 232Th, 230Th, 231Pa or 10Be in seawater, suspended particles or sediments. Results generally demonstrated good agreement among labs that analyzed marine sediments. Two sets of seawater samples, aliquots of particulate material filtered in situ, and/or aliquots of biogenic sediments were distributed to participating labs. Internal consistency among participating labs improved substantially between the first and second set of seawater samples. Contamination was a serious problem for 232Th. Standard Niskin™ bottles introduced no detectable contamination, whereas sample containers, reagents and labware were implicated as sources of contamination. No detectable differences in concentrations of dissolved 232Th, 230Th or 231Pa were observed among samples of seawater filtered through Nuclepore ™, Supor ™ or QMA™ (quartz) filters with pore diameters ranging between 0.4 and 1.0 μm. Isotope yield monitors equilibrate with dissolved Th in seawater on a time scale of much less than one day. Samples of filtered seawater acidified to a pH between 1.7 and 1.8 experienced no detectable loss of dissolved Th or Pa during storage for up to three years. The Bermuda Atlantic Time Series station will serve as a GEOTRACES baseline station for future intercalibration of 232Th and 230Th concentrations in seawater. Efforts to improve blanks and standard calibration are ongoing, as is the development of methods to determine concentrations of particulate nuclides, tests of different filtration methods, and an increasing awareness of the need to define protocols for reporting uncertainties.

Submarine groundwater discharge (SGD) represents a significant flux of water and chemical constituents to coastal regions. Because 226Ra and 228Ra are highly concentrated in groundwater relative to surface water, and their signal is well-integrated throughout coastal surface waters, these isotopes have been increasingly utilized to quantify SGD over the past several decades. In this study, a combined radium and salt balance is used to yield estimates of fresh and saline SGD to four of Rhode Island's coastal ponds (Winnapaug, Quonochontaug, Ninigret, and Point Judith Ponds). Fresh SGD rates determined for each pond range between 0 and 17 L m− 2 d− 1. These values largely fall in line with previous estimates made using hydrologic models. The uncertainty of saline SGD estimates for each pond is high; minimum and maximum saline SGD estimates differ by as much as three orders of magnitude. These results echo the discrepancy between two previous radium-based studies from this location. The variability of dissolved radium in the Rhode Island coastal aquifer plays a large role in the uncertainty of these saline SGD estimates. Groundwater dissolved radium activity is dependent upon many different factors, including groundwater salinity and redox conditions, sediment thorium and radium activities, and the amount of time to which the sediment has been exposed to saline groundwater. Due to the heterogeneous nature of glacial outwash sediments, none of these parameters are likely consistent throughout the aquifer in this location. Groundwater 226Ra and 228Ra activities in the coastal ponds vary significantly both laterally and with sediment depth. For example, groundwater samples collected in Ninigret Pond separated by a vertical distance of only 5 cm differ by as much as 3 × 103 dpm 100 L− 1 for both 226Ra and 228Ra. The results from this study indicate that a single groundwater sample or suite of samples may not accurately reflect the average radium activity of the groundwater actually discharging into each pond. Targeted sampling in areas where SGD is qualitatively observed can potentially remedy this problem.

Measurements of Pb isotopes in aerosols, precipitation, and size-fractionated particulate matter from the Gulf of Maine, Scotian Shelf and Labrador Sea are used to investigate the source of Pb. The (206)Pb/(207)Pb ratio in aerosols and precipitation collected at New Castle, NH suggests that anthropogenic Pb is a mixture of US and Canadian sources. (206)Pb/(207)Pb ratios in >53 microm particulate matter from the Gulf of Maine and Scotian Shelf slope waters are consistent with contaminant Pb inputs predominantly from US and Canadian sources, while in shelf waters (206)Pb/(207)Pb ratios in >0.4 microm and >53 microm particles are consistent with a mixture of US and Canadian sources, as well as Pb associated with resuspended surface sediment. (206)Pb/(207)Pb ratios in particulate matter (>0.4 microm, 10-53 microm, and >53 microm) from Labrador Sea surface waters range from 1.165 to 1.211 and are a mixture of Pb derived from ore compositions consistent with Broken Hill, Australia and southeast Missouri, US sources.

In order to assess the current state of net community production (NCP) in the southeastern Bering Sea, we measured the spatio-temporal distribution and controls on dissolved inorganic carbon (DIC) concentrations in spring and summer of 2008 across six shelf domains defined by differing biogeochemical characteristics. DIC concentrations were tightly coupled to salinity in spring and ranged from ~1900 μmoles kg−1 over the inner shelf to ~2400 μmoles kg−1 in the deeper waters of the Bering Sea. In summer, DIC concentrations were lower due to dilution from sea ice melt, terrestrial inputs, and primary production. Concentrations were found to be as low ~1800 μmoles kg−1 over the inner shelf. We found that DIC concentrations were drawn down 30–150 μmoles kg−1 in the upper 30 m of the water column due to primary production and calcium carbonate formation between the spring and summer occupations. Using the seasonal drawdown of DIC, estimated rates of NCP on the inner, middle, and outer shelf averaged 28 ± 9 mmoles C m−2 d−1. However, higher rates of NCP (40–47 mmoles C m−2 d−1) were observed in the "Green Belt" where the greatest confluence of nutrient-rich basin water and iron-rich shelf water occurs. We estimated that in 2008, total NCP across the shelf was on the order of ~96 Tg C yr−1. Due to the paucity of consistent, comparable productivity data, it is impossible at this time to quantify whether the system is becoming more or less productive. However, as changing climate continues to modify the character of the Bering Sea, we have shown that NCP can be an important indicator of how the ecosystem is functioning.

Large-volume sampling of 234Th was conducted to estimate particulate organic carbon (POC) export in conjunction with drifting sediment trap deployments in the northern Barents Sea in July 2003 and May 2005. 234Th-derived POC fluxes averaged 42.3 ± 39.7 mmol C m− 2 d− 1 in 2003 and 47.1 ± 30.6 mmol C m− 2 d− 1 in 2005. Sediment trap POC fluxes averaged 13.1 ± 8.2 mmol C m− 2 d− 1 in 2003 and 17.3 ± 11.4 mmol C m− 2 d− 1 in 2005, but better reflected the transient bloom conditions that were observed at each station within a season. Although 234Th fluxes agreed within a factor 2 at most stations and depths sampled, sediment trap POC fluxes were lower than large-volume POC flux estimates at almost every station. This may represent an under-collection of POC by the drifting sediment traps or, conversely, an over-collection of POC by the large-volume sampling of 234Th. It is hypothesized that the offset between the two methods is partly due to the presence of the prymnesiophyte Phaeocystis pouchetii, which potentially causes a large variation in > 53-μm POC/234Th ratios. Due to the large proportion of dissolved carbon or mucilage released by P. pouchetii, and because it is thought that P. pouchetii does not contribute significantly to the vertical export of biogenic matter in the Barents Sea, the application of large-volume sampling of 234Th may yield relatively high, and possibly inaccurate POC/234Th ratios. Hence, POC fluxes derived from 234Th sampling may be inappropriate and drifting sediment traps might be a more reliable method to measure the vertical export of biogenic matter in regions that have recurrent P. pouchetii blooms, such as the Barents Sea.

Large-volume sampling of 234Th and drifting sediment trap deployments were conducted as part of the 2004 Western Arctic Shelf–Basin Interactions (SBI) spring (May 15–June 23) and summer (July 17–August 26) process cruises in the Chukchi Sea. Measurements of 234Th and particulate organic carbon (POC) export fluxes were obtained at five stations during the spring cruise and four stations during the summer cruise along Barrow Canyon (BC) and along a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. 234Th and POC fluxes obtained with in situ pumps and drifting sediment traps agreed to within a factor of 2 for 70% of the measurements. POC export fluxes measured with in situ pumps at 50 m along BC were similar in spring and summer (average = 14.0 ± 8.0 mmol C m− 2 day− 1 and 16.5 ± 6.5 mmol C m− 2 day− 1, respectively), but increased from spring to summer at the EHS transect (average = 1.9 ± 1.1 mmol C m− 2 day− 1 and 19.5 ± 3.3 mmol C m− 2 day− 1, respectively). POC fluxes measured with sediment traps at 50 m along BC were also similar in both seasons (31.3 ± 9.3 mmol C m− 2 day− 1 and 29.1 ± 14.2 mmol C m− 2 day− 1, respectively), but were approximately twice as high as POC fluxes measured with in situ pumps. Sediment trap POC fluxes measured along the EHS transect also increased from spring to summer (3.0 ± 1.9 mmol C m− 2 day− 1 and 13.0 ± 6.4 mmol C m− 2 day− 1, respectively), and these fluxes were similar to the POC fluxes obtained with in situ pumps. Discrepancies in POC export fluxes measured using in situ pumps and sediment traps may be reasonably explained by differences in the estimated POC/234Th ratios that arise from differences between the techniques, such as time-scale of measurement and size and composition of the collected particles. Despite this variability, in situ pump and sediment trap-derived POC fluxes were only significantly different at a highly productive station in BC during the spring.

As part of the Western Arctic Shelf–Basin Interactions (SBI) project, the production and fate of organic carbon and nitrogen from the Chukchi and Beaufort Sea shelves were investigated during spring (5 May–15 June) and summer (15 July–25 August) cruises in 2002. Seasonal observations of suspended particulate organic carbon (POC) and nitrogen (PON) and large-particle (453mm) size class suggest that there was a large accumulation of carbon (C) and nitrogen (N) between spring and summer in the surface mixed layer due to high phytoplankton productivity. Considerable organic matter appeared to be transported from the shelf into the Arctic Ocean basin in an elevated POC and PON layer at the top of the upper halocline. Seasonal changes in the molar carbon:nitrogen (C:N) ratio of the suspended particulate organic matter (POM) pool reflect a change in the quality of the organic material that was present and presumably being exported to the sediment and to Arctic Ocean waters adjacent to the Chukchi and Beaufort Sea shelves. In spring, low particulate C:N ratios (o6; i.e., N rich) were observed in nitrate-replete surface waters. By the summer, localized high particulate C:N ratios (49; i.e., N-poor) were observed in nitrate-depleted surface waters. Low POC and inorganic nutrient concentrations observed in the surface layer suggest that rates of primary, new and export production are low in the Canada Basin region of the Arctic Ocean. r 2005 Elsevier Ltd. All rights reserved.

During three icebreaker cruises in the Arctic Ocean under different sea-ice conditions in 2002, undisturbed benthic surface sediments were collected and assayed for the presence of a short-lived (t1/2=53 d), particle-reactive cosmogenic radionuclide, 7Be, that is solely derived from atmospheric deposition. Under largely ice-covered conditions in May–June 2002, we did not detect this radionuclide in benthic surface sediments, despite significant inventories present in ice-rafted snow on the overlying sea ice (mean=86.8 Bq m−2±32.0 SD; n = 9 ). During the July–August 2002 Shelf–Basin Interactions (SBI) cruise aboard the USCGC Healy and during a simultaneous cruise of the CCGS Sir Wilfrid Laurier on the Bering and Chukchi Shelf, which occupied the same general region following retreat and dissolution of Arctic ice cover, the 7Be present in this snow as well as surface deposition on to the sea ice-free water surface was detected in many benthic surface sediments, including some as deep as 945 m in Barrow Canyon. Inventories of 7Be in sediments were as high (∼60 Bq m−2) as the entire decay-corrected inventory present earlier in some snow samples collected on the sea-ice cover. Other deposition indicators such as the inventories of sediment chlorophyll, sediment oxygen respiration rates and 234Th-derived export fluxes also showed post-ice melt particle deposition and vertical transport, but in most cases the 7Be deposition was not tightly correlated with these other indicators, suggesting that 7Be sedimentation may not be controlled by the same processes. Our observations indicate that materials in sea ice, including contaminants, particulate organic, and mineral matter originating from atmospheric deposition or entrained in continental shelf sediments and rafted onto sea ice, can be rapidly transported to depth. The re-distribution of these materials as sea-ice drifts and eventually melts has the potential for impacting Arctic Ocean biogeochemical cycles and contaminant concentrations in areas of the Arctic remote from the original point of deposition.

The vertical sinking flux of particulate Al, Fe, Pb, and Ba from the upper 250 m of the Labrador Sea has been estimated from measurements of 234 Th/ 238 U disequilibrium and the respective metal/ 234 Th ratios in >53 μm size particles. 234 Th-derived particulate metal fluxes include in situ scavenged metals, labile lithogenic metals, and metals derived from external input (e.g., atmospheric supply). In contrast to the POC/ 234 Th ratio, particle size-fractionated (0.4–10 μm, 10–53 μm, and >53 μm) Al/ 234 Th, Fe/ 234 Th and Pb/ 234 Th, and Ba/ 234 Th ratios generally increase with depth and exhibit no systematic change with particle diameter. Sinking fluxes of particulate Al (2.47–22.3 μmol m −2 d −1 ), Fe (2.69–16.3 μmol m −2 d −1 ), Pb (2.85–70 nmol m −2 d −1 ), and Ba (0.13–2.1 μmol m −2 d −1 ) at 50 m (base of the euphotic zone) and 100 m (base of the mixed layer) are largely within the range of previous sediment trap results from other ocean basins. Estimates of the upper ocean residence time of Al (0.07–0.28 yr) and Pb (0.8–2.9 yr) are short compared to previously reported values. The settling rate of >53 μm particles calculated from the 234 Th data ranges from 14 to 38 m d −1 .

We report concentrations of 231Pa, 230Th, and 232Th measured in unfiltered seawater samples from three stations occupied during the 1994 Arctic Ocean Section; at the western edge of the Canada Basin, in the Makarov Basin, and in the Amundsen Basin at the North Pole. The North Pole data agree well with previously published profiles from the Amundsen Basin. 230Th and 231Pa concentrations in the Canada and Makarov Basins are intermediate between those reported previously for the Beaufort Sea and the Alpha Ridge, reinforcing the observation that scavenging rates are variable in the western and central Arctic and can be as rapid as in the Eurasian Basin and other oceans. Thorium-232 data indicate that relatively high rates of scavenging under the permanent ice cover are driven at least in part by offshore transport of detrital particulate material. A comparison of water column 230Th 231Pa ratios throughout the Arctic suggests that there may be little basin-scale fractionation of these two isotopes in this ocean, contrary to expectations based on the relatively large area of the continental shelves and their contrasts with the deep basins. Models of shelf–basin exchange and 230Th/231Pa scavenging, incorporating available constraints, support this suggestion and point to the need for more data from the Arctic shelves, where the imprint of Th/Pa fractionation, if any, is expected be most pronounced.

The distribution of size-fractionated particulate trace metals (Al, Fe, Mn, Pb, Cu, V, and Co) was investigated in the Gulf of Maine–Scotian Shelf and Labrador Sea by collection of particulate matter using water bottles (Go-flo) and large volume in-situ pumps (Challenger Oceanic Systems and Services). Trace metal procedural filter blanks for Poretics membrane filters (0.4- and 10 μm pore size) and Nitex screens (53 μm mesh size) were sufficiently low that metal concentrations could be measured reliably. These results validate the use of Challenger Oceanics Systems and Services in-situ pumps for collection of particulate trace metals (Al, Fe, Mn, Pb, Cu, V, and Co) in shelf, slope, and open ocean Atlantic waters. In the Gulf of Maine–Scotian Shelf and Labrador Sea, trace metal concentrations per volume filtered generally decrease with increasing particle size for all metals. In the upper 250 m of slope waters of the Gulf of Maine–Scotian Shelf and in the Labrador Sea, trace metal concentrations in all particle sizes are lower than in shelf waters. Higher particulate metal concentrations in shelf waters are consistent with an increase in the supply of these trace metals with proximity to continental sources. In addition, an increase in particulate trace metal concentrations in shelf waters with depth is attributed to an input from resuspended sediment.

We have developed techniques by sector-field inductively coupled plasma mass spectrometry (ICP-MS) for measuring the isotopic composition and concentration of uranium and thorium, focusing on the rare isotopes, 230Th and 234U. These isotopes have been widely used as tracers in earth sciences, e.g., chronology, paleoclimatology, archeology, hydrology, geochemistry, and oceanography. Measurements made on reference materials demonstrate that the analytical precision approximates counting statistics and that the accuracy of the measurement is within error of accepted values. Routine measurement times are 20 min for U and 10 min for Th. The sensitivities (ions counted/atoms introduced) are 2–3‰ for U and 1.5–2‰ for Th. Samples of 10–40 ng of 238U (0.5–2.0 pg of 234U) give measurement precisions of 1–2‰ (2σ) for δ234U and U concentration ([U]). Only 0.4 pg of 230Th are needed to achieve [230Th] and 230Th/232Th data with errors less than 5‰ even for cases where 230Th/232Th is 10−5 or less. Our ICP-MS data, including uranium standards, thorium standards, 238U–234U–230Th–232Th dating of speleothems and 230Th–232Th in oceanic particulates, replicates measurements made by thermal ionization mass spectrometry (TIMS). Compared to TIMS, the ICP-MS method allows smaller sample size and higher sample throughput due to higher sensitivity, fewer sample preparation steps and shorter measurement times. However, mass biases, intensity biases, spectral interferences and instrumental blanks are significant and must be addressed.

The large discharge of radioactivity into the northwest Pacific Ocean from the 2011 Fukushima Dai-ichi nuclear reactor accident has generated considerable concern about the spread of this material across the ocean to North America. We report here the first systematic study to our knowledge of the transport of the Fukushima marine radioactivity signal to the eastern North Pacific. Time series measurements of (134)Cs and (137)Cs in seawater revealed the initial arrival of the Fukushima signal by ocean current transport at a location 1,500 km west of British Columbia, Canada, in June 2012, about 1.3 y after the accident. By June 2013, the Fukushima signal had spread onto the Canadian continental shelf, and by February 2014, it had increased to a value of 2 Bq/m(3) throughout the upper 150 m of the water column, resulting in an overall doubling of the fallout background from atmospheric nuclear weapons tests. Ocean circulation model estimates that are in reasonable agreement with our measured values indicate that future total levels of (137)Cs (Fukushima-derived plus fallout (137)Cs) off the North American coast will likely attain maximum values in the 3-5 Bq/m(3) range by 2015-2016 before declining to levels closer to the fallout background of about 1 Bq/m(3) by 2021. The increase in (137)Cs levels in the eastern North Pacific from Fukushima inputs will probably return eastern North Pacific concentrations to the fallout levels that prevailed during the 1980s but does not represent a threat to human health or the environment.

The long-lived uranium decay products 230Th and 231Pa are widely used as quantitative tracers of adsorption to sinking particles (scavenging) in the ocean by exploiting the principles of radioactive disequilibria. Because of their preservation in the Pleistocene sediment record and through largely untested assumptions about their chemical behavior in the water column, the two radionuclides have also been used as proxies for a variety of chemical fluxes in the past ocean. This includes the vertical flux of particulate matter to the seafloor, the lateral flux of insoluble elements to continental margins (boundary scavenging), and the southward flux of water out of the deep North Atlantic. In a section of unprecedented vertical and zonal resolution, the distributions of 230Th and 231Pa across the North Atlantic shed light on the marine cycling of these radionuclides and further inform their use as tracers of chemical flux. Enhanced scavenging intensities are observed in benthic layers of resuspended sediments on the eastern and western margins and in a hydrothermal plume emanating from the Mid-Atlantic Ridge. Boundary scavenging is clearly expressed in the water column along a transect between Mauritania and Cape Verde which is used to quantify a bias in sediment fluxes calculated using 230Th-normalization and to demonstrate enhanced 231Pa removal from the deep North Atlantic by this mechanism. The influence of deep ocean ventilation that leads to the southward export of 231Pa is apparent. The 231Pa/230Th ratio, however, predominantly reflects spatial variability in scavenging intensity, complicating its applicability as a proxy for the Atlantic meridional overturning circulation.

The Arctic Ocean is an important sink for atmospheric carbon dioxide (CO2) with a recent estimate suggesting that the region accounts for as much as 15 % of the global uptake of CO2. The western Arctic Ocean, in particular is a strong ocean sink for CO2, especially in the Chukchi Sea during the open water season when rates of primary production can reach as high as 150 g C m−2. The Arctic marine carbon cycle, the exchange of CO2 between the ocean and atmosphere, and the fate of carbon fixed by marine phytoplankton appear particularly sensitive to environmental changes, including sea ice loss, warming temperatures, changes in the timing and location of primary production, changes in ocean circulation and freshwater inputs, and even the impacts of ocean acidification. In the near term, further sea ice loss and other environmental changes are expected to cause a limited net increase in primary production in Arctic surface waters. However, recent studies suggest that these enhanced rates of primary production could be short lived or not occur at all, as warming surface waters and increases in freshwater runoff and sea ice melt enhance stratification and limit mixing of nutrient-rich waters into the euphotic zone. Here, we provide a review of the current state of knowledge that exists about the rates of primary production in the western Arctic as well as the fate of organic carbon fixed by primary produces and role that these processes play in ocean acidification in the region.

The naturally occurring radionuclide 234Th (t1/2 = 24.1 days) was used to examine the organic carbon cycle in the Gulf of Maine. A seasonal study (March, June, and September 1995) was conducted in the central Gulf of Maine in Wilkinson and Jordan Basins, and a regional survey, which included the Scotian Shelf, was conducted during August-September 1997. Particulate organic carbon (POC) export (particulate export production) was estimated from a three-dimensional steady state model of 234Th flux combined with measurements of the POC/234Th ratio on >53-μm particles. The POC export for this region was seasonally variable; average values ranged from 15 to 34 mmol C m−2 d−1, between 11% and 25% of the regionally integrated primary production. The Gulf of Maine was a net source (to the Mid-Atlantic Bight) of dissolved organic carbon (2.4 mmol C m−2 d−1) amounting to ∼2% of carbon uptake rates. Organic carbon burial in the sediments was a minor fraction of the primary production, averaging 1.6 mmol C m−2 d−1. Though only a fraction of total export production was buried in the sediments, these estimates close the budget for organic carbon in the Gulf of Maine. An implication is that off-shelf export may not be as important as previously estimated in this shelf region.

Profiles of particulate and dissolved 234 Th ( t 1/2 =24.1 d) were collected during the Canadian JGOFS program (February, May, and August 1996 and February 1997) along a transect from 125°W to 145°W at 50°N (Ocean Station Papa – OSP). Persistent features included a deficit of 234 Th relative to its parent 238 U in the upper ∼75 m and occasionally an excess of total 234 Th (particulate+dissolved) at ∼150–200 m attributed to redissolution of the particulate-bound nuclide. To calculate POC export fluxes, a steady-state model of 234 Th export was combined with measurements of the POC/ 234 Th ratio determined on particles >1-μm collected by in situ filtration. Along the transect, e-ratios (POC export/primary production) calculated for the 0.1% light level were similar during February 1997 and May 1996, 0.06–0.08 (avg. POC flux =2.8–7.1 mmol C m −2 d −1 ). During August 1996, e-ratio s reached a maximum, averaging 0.13 (avg. POC flux=7.6 mmol C m −2 d −1 ). Particulate 234 Th fluxes measured by drifting cylindrical sediment traps were 3-fold greater than the flux calculated for the water column 234 Th deficit. Finally, to study changes in 234 Th and POC export on short time-scales, some stations were sampled twice within 36 hours. During February 1997, 234 Th increased nearly 50% from 60–100 m in a 36 h period. Hydrographic data suggest that this change was due to advective processes.

The export flux of particulate organic carbon and particle scavenging rates were calculated using a 234Th-based approach along a transect through the mid-Atlantic from 35°S to 10°N during May and June of 1996. The median residence time of dissolved 234Th along the transect was 135 days, with intense scavenging from 5°N to 6° N ( τ d =86±17 days). Vertical profiles and depth-integrated samples (surface-100 m) of dissolved ( 53- μm ) 234 Th activities were collected to estimate export fluxes of particulate organic carbon (POC). A steady-state model of the 234Th activity balance in the upper 100 m was combined with measurements of the POC/234Th ratio on >53-μm particles. POC export ranged from 1.5 mmol C m −2 d −1 in the central South Atlantic to 31 mmol C m −2 d −1 at 2.5°S. Export production along the Atlantic equator (5°S–5°N, 25°E–5°W) is estimated to be 0.22 Gt C yr−1. The 234Th-derived POC export estimates reported here and from other studies are comparable with model simulations of export production from the Princeton General Circulation Model (GCM) for the Atlantic Ocean.

A two-dimensional transport and scavenging model has been developed and applied to a limited set of 238U–234Th disequilibria data in order to examine the relative significance of horizontal versus vertical removal of chemicals in coastal waters. During an intense scavenging episode in September 1993 (>95% 238U–234Th disequilibrium), vertical scavenging was found to be more important than horizontal transport in both Inner and Outer Casco Bay, Gulf of Maine. However, in May 1994 the two-dimensional model suggested that onshore horizontal dispersion of 234Th was substantial. Recognition of this horizontal flux required us to increase the net vertical scavenging flux in Inner Casco Bay by a factor of three over that obtained based only on the local 238U–234Th disequilibrium. The radionuclide (210Pbxs, 234Thxs, 7Be) record of the underlying sediments provided supporting evidence for onshore horizontal transport of chemicals. The highest sedimentary inventories for all three radionuclides were found at the stations nearest to the coast. As anticipated from their relative particle-affinities, the “regional boundary-scavenging” indicator 7Be/234Thxs was highest at the coastal boundary. The application of the two-dimensional 234Th-based transport model to assess the distributional fate of other chemicals was demonstrated for Casco Bay using simultaneously measured polycyclic aromatic hydrocarbons (PAHs). Based on limited PAH data, the model results suggest that about half of the pyrene and benzo[a]pyrene introduced to Portland Harbor, ME may be settling locally and that the remainder is exported to offshore locations. The approach introduced here, coupling information on particle-mediated vertical scavenging, chemical phase distribution, and tide-induced horizontal dispersion, should provide a useful mechanistic framework for elucidating quantitatively the dispersal of a wide range of geochemically and environmentally important chemicals in the coastal ocean.

Size-fractionated seawater samples were collected from the Gulf of Maine to determine the fraction (fc/d) of total dissolved ( ∼ 1–10 mg L−1), such as in near-shore and estuarine waters.

Concentrations of Cd, Cr, Cu, Ni and Pb were determined in filtered water, suspended particulate matter, and bottom sediments from a ∼2000 km section of the Ob and Irtysh Rivers. Dissolved Cd, Cr, Cu and Ni concentrations are similar to, or higher than, results from other Russian Arctic and large world river-estuaries. Concentrations of Cd, Cr, Cu, Ni and Pb in suspended particulate matter are generally comparable to results from other Russian Arctic and large world rivers and estuaries. Comparison of trace metal ratios in crustal material and suspended particulate matter and bottom sediment suggests that the source of Cr, Cu and Ni is continental weathering. Particulate Cd and Pb are elevated relative to their crustal abundance, suggesting a source of these metals to the Ob-Irtysh in addition to continental weathering.

A 1-kDa (1 kDa = 1000 daltons) Osmonics spiral-wound cross-flow filtration (CFF) membrane has been evaluated for sampling Cd, Cu and Ni in 1-kDa fractions of prefiltered seawater. Size-fractionated samples were collected as part of a CFF intercomparison experiment (Buesseler et al., 1996-this issue) and in the Gulf of Maine. Mass-balance results averaged 93 ± 6% (Cd), 81 ± 19% (Cu), and 99 ± 18% (Ni) for the intercomparison experiment and 56 ± 17% (Cd), 119 ± 38% (Cu), and 91 ± 6% (Ni) for the Gulf of Maine samples. Differences in mass balance between these studies may be related, in part, to removal of sub-μm particles and associated trace metals during prefiltration. An Amicon CFF system showed higher (2–12 ×) colloidal concentrations for Cu and Ni compared to Filtron and Osmonics systems; all CFF systems showed similar Cd, Cu and Ni mass-balance results. A model based on particle impaction with a filter shows the potential for adsorption of sub-μm particles by a 1-kDa Osmonics CFF membrane and conventional (Nuclepore) filters.

[1] A series of 223Ra (t1/2 = 11.4 d) and 224Ra (t1/2 = 3.66 d) measurements made in the Mid-Atlantic Bight yield eddy diffusivity (K) estimates of 1.2 ± 0.3 × 102 m2 s−1 and 1.4 ± 0.2 × 102 m2 s−1, respectively. These results fall in line with previous studies from multiple locations throughout the ocean, in which 224Ra-based eddy diffusivities invariably exceed those determined using 223Ra. Such a pattern conflicts with the Fickian model for eddy diffusivity, in which K is constant. Moreover, this trend runs counter to the length scale-dependent view of eddy diffusion, which suggests that K values estimated using 223Ra should exceed those of 224Ra, because the length scale of the former is greater than that of the latter. A finite mixing-length model based on the concept of differential diffusion is used to provide an explanation for this discrepancy.

[1] Time series measurements of the nuclear fuel reprocessing tracers, 129I and 137Cs, and ventilation tracer, CFC-11, were used to determine circulation time scales for Atlantic Water (AW) in the Arctic Ocean. Measurements in surface water are consistent with an advection model and transit times from the North Sea of 1–4 years to the Barents Sea, 3–6 years to the Kara Sea, and 9–12 years to the North Pole. Tracer ages for intermediate waters do not support an advection model because they vary with time. The tracer relationships were employed to determine transit time distributions (TTD), using inverse Gaussian functions, that implicitly incorporate the effects of mixing in simulating fluid flow. Tracer concentrations were used to constrain the ratio of the width, Δ, of the TTD to the mean age, Γ, a measure of the relative importance of mixing, giving values for most intermediate water regimes of Δ/Γ ≤ 1. These results are in contrast to values, Δ/Γ 1 typical of the North Atlantic, indicating the greater relative importance of advection compared to mixing in the Arctic Ocean. Mean ages for AW increase from 12–20 years in the southern Makarov Basin to 30–40 years near the Arctic outflow north of Greenland. A decline in Δ/Γ with distance along the direction of AW boundary flow from the Makarov into the Canada Basin may indicate relatively stronger mixing in regimes upstream of the Makarov Basin but also implies that TTDs may differ from inverse Gaussian functions for flow in the Arctic domain.

Particle-reactive, naturally occurring radionuclides are useful tracers of the sinking flux of organic matter from the surface to the deep ocean. Since the Joint Global Ocean Flux Study (JGOFS) began in 1987, the disequilibrium between (234)Th and its parent (238)U has become widely used as a technique to measure particle export fluxes from surface ocean waters. Another radionuclide pair, (210)Po and (210)Pb, can be used for the same purpose but has not been as widely adopted due to difficulty with accurately constraining the (210)Po/(210)Pb radiochemical balance in the ocean and because of the more time-consuming radiochemical procedures. Direct comparison of particle flux estimated in different ocean regions using these short-lived radionuclides is important in evaluating their utility and accuracy as tracers of particle flux. In this paper, we present paired (234)Th/(238)U and (210)Po/(210)Pb data from oligotrophic surface waters of the subtropical Northwest Atlantic and discuss their advantages and limitations. Vertical profiles of total and particle size-fractionated (210)Po and (234)Th activities, together with particulate organic carbon (POC) concentrations, were measured during three seasons at the Bermuda Atlantic Time-series Study (BATS) site. Both (210)Po and (234)Th reasonably predict sinking POC flux caught in sediment traps, and each tracer provides unique information about the magnitude and efficiency of the ocean's biological pump.

The Canadian Basin of the Arctic Ocean, largely ice covered and isolated from deep contact with the more dynamic Eurasian Basin by the Lomonosov Ridge, has historically been considered an area of low productivity and particle flux and sluggish circulation. High-sensitivity mass-spectrometric measurements of the naturally occurring radionuclides protactinium-231 and thorium-230 in the deep Canada Basin and on the adjacent shelf indicate high particle fluxes and scavenging rates in this region. The thorium-232 data suggest that offshore advection of particulate material from the shelves contributes to scavenging of reactive materials in areas of permanent ice cover.

[1] The Arctic Ocean and adjacent continental shelf seas such as the Chukchi and Beaufort Seas are particularly sensitive to long-term change and low-frequency modes of atmosphere-ocean-sea-ice forcing. The cold, low salinity surface waters of the Canada Basin of the Arctic Ocean are undersaturated with respect to CO2 in the atmosphere and the region has the potential to take up atmospheric CO2, although presently suppressed by sea-ice cover. Undersaturated seawater CO2 conditions of the Arctic Ocean are maintained by export of water with low dissolved inorganic carbon content and modified by intense seasonal shelf primary production. Sea-ice extent and volume in the Arctic Ocean has decreased over the last few decades, and we estimate that the Arctic Ocean sink for CO2 has tripled over the last 3 decades (24 Tg yr 1 to 66 Tg yr 1 ) due to sea-ice retreat with future sea-ice melting enhancing air-to-sea CO2 flux by 28% per decade. Citation: Bates, N. R., S. B. Moran, D. A. Hansell, and J. T. Mathis (2006), An increasing CO2 sink in the Arctic Ocean due to sea-ice loss, Geophys. Res. Lett., 33, L23609, doi:10.1029/2006GL027028.

The naturally occurring radionuclide 226Ra (t1/2 = 1600 years) was used as a tracer to determine ground water input to Point Judith, Potter, Green Hill and Ninigret ponds in southern Rhode Island. Measurements of 226Ra activity were made in samples collected from salt ponds, pore waters, sediments, and local ground water wells during June-August, 1997. These results were combined with a simple box model to derive ground water input fluxes of 0.1-0.3 cm3 cm-2 d-1 (2-5 x 10(7) L d-1), which are comparable to previous estimates of ground water input to these ponds.

RECENT observations of abundant, nonliving, submicrometre particles in the upper ocean1–3 and new measurements of 'dissolved9organic carbon4–7 have fuelled speculation concerning the role of colloidal matter in ocean chemistry and biology. Colloids may act as reactive intermediates in the marine geochemistry of trace metals8–12, and a biologically labile pool of colloidal matter would affect models of ocean carbon cycling13–16. Here we report the use of naturally occurring 234Th as an in situ tracer to estimate the residence time of colloidal matter in the surface waters near Bermuda. The 234Th activity of colloidal matter (size range 10,000 nominal molecular weight to 0.2 μm) is similar to that of small particles (0.2–53 μm). Modelling of our results indicates a mean residence time of colloidal 234Th with respect to aggregation into small particles of 10 days, which is roughly the same as for small-particle 234Th, yet a factor of ∼6 less than for the dissolved pool. These results suggest that, more generally, macromolecular colloidal matter has a short residence time and hence a rapid turnover rate in the upper open ocean.

A bloom of Chaetoceros gracilis was initiated in a mesocosm (the Dalhousie Universtiy Aquatron Tower Tank), and the concentrations of Al, Cu, Cu-complexing capacity, and Cd were followed throughout the course of the bloom and subsequent crash. Whereas the total Cu concentration remained constant over the course of the experiment, the proportion of organically bound C cu and the Cu-complexing capacity increased during the growth of the bloom, and remained constant after the population crash. Both Al and Cd were removed from solution during the late stages of the bloom and crash; whereas the al was essentially swept from the surface layer, the Cd was only reduced to about half of the starting concentration. The concentration of all three elements was regulated by events occuring in the phytoplankton bloom sequence.