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[1] Abrupt decreases of the Atlantic meridional overturning circulation (MOC) during the Late Pleistocene have been directly linked to catastrophic discharges of glacimarine freshwater, triggering disruption of northward marine heat transport and causing global climate changes. Here we provide measurements of excess sedimentary 231 Pa/ 230 Th from a high-accumulation sediment drift deposit in the NE Atlantic that record a sequence of sudden variations in the rate of MOC, associated deep ocean ventilation and surface-ocean climatology. The data series reveal a sequential decrease in the MOC rate at � 18.0 ka BP ago that coincides with only transient and localized freshwater inputs. This change represents a substantial, though not total, cessation in MOC that predates the major Heinrich (H1) meltwater event by at least 1,200 years. These results highlight the potential of targeted freshwater perturbations in promoting substantial MOC changes without a direct linking with catastrophic freshwater surges. Citation: Hall, I. R., S. B. Moran, R. Zahn, P. C. Knutz, C.-C. Shen, and R. L. Edwards (2006), Accelerated drawdown of meridional overturning in the late-glacial Atlantic triggered by transient pre-H event freshwater perturbation, Geophys. Res. Lett., 33, L16616, doi:10.1029/2006GL026239.

Reduced sea ice has made the Arctic Ocean more accessible for vessel traffic. In turn, the heightened interest to better understand rapidly changing sea ice dynamics, ecosystems, and related ocean processes in the Arctic Ocean has led to closer interactions with and the need to avoid potential conflicts between scientific researchers and Indigenous coastal communities. In particular, researchers need to minimize spatial and temporal overlap of science activities with subsistence hunts as the Arctic is essential to Indigenous communities for their food security and cultural heritage. In this regard, a Community and Environmental Compliance Standard Operating Procedure (CECSOP) was recently developed for the R/V Sikuliaq, which is owned by the National Science Foundation and operated by the University of Alaska Fairbanks College of Fisheries and Ocean Sciences and is part of the University-National Oceanographic Laboratory System. The CECSOP was developed with input and guidance from Alaska Indigenous community groups, state and federal agencies, and sea-going scientists. Here the document's basic principles and procedures are described, as well as its utility in helping guide constructive discussions and interactions between scientific users of R/V Sikuliaq and subsistence hunting communities when research and subsistence hunt activities have spatial and temporal overlap. The CECSOP is a “living” document and subject to future modifications and improvements. It may serve as a model for other scientific, commercial and industrial vessel operators to ensure best practices between subsistence hunting communities and vessel operators in the Arctic.

We present 34 profiles of radon-deficit from the ice-ocean boundary layer of the Beaufort Sea. Including these 34, there are presently 58 published radon-deficit estimates of k, the air-sea gas transfer velocity in the Arctic Ocean; 52 of these estimates were derived from water covered by 10% sea ice or more. The average value of k collected since 2011 is 4.0 ± 1.2 m d−1 This exceeds the quadratic wind speed prediction of weighted kws = 2.85 m d−1 with mean weighted wind speed of 6.4 m s−1. We show how ice cover changes the mixed-layer radon budget, and yields an “effective gas transfer velocity”. We use these 58 estimates to statistically evaluate the suitability of a wind speed parameterization for k, when the ocean surface is ice covered. Whereas the six profiles taken from the open ocean indicate a statistically good fit to wind speed parameterizations, the same parameterizations could not reproduce k from the sea ice zone. We conclude that techniques for estimating k in the open ocean cannot be similarly applied to determine k in the presence of sea ice. The magnitude of k through gaps in the ice may reach high values as ice cover increases, possibly as a result of focused turbulence dissipation at openings in the free surface. These 58 profiles are presently the most complete set of estimates of k across seasons and variable ice cover; as dissolved tracer budgets they reflect air-sea gas exchange with no impact from air-ice gas exchange.

The contributions of micro-, nano-, and picoplankton to particle export were estimated from measurements of size-fractionated particulate 234Th, organic carbon, and phytoplankton indicator pigments obtained during five cruises between 2010 and 2012 along Line P in the subarctic northeast Pacific Ocean. Sinking fluxes of particulate organic carbon (POC) and indicator pigments were calculated from 234Th–238U disequilibria and, during two cruises, measured by a sediment trap at Ocean Station Papa. POC fluxes at 100 m ranged from 0.65 to 7.95 mmol m−2 d−1, similar in magnitude to previous results at Line P. Microplankton pigments dominate indicator pigment fluxes (averaging 69 ± 19% of total pigment flux), while nanoplankton pigments comprised the majority of pigment standing stocks (averaging 64 ± 23% of total pigment standing stocks). Indicator pigment loss rates (the ratio of pigment export flux to pigment standing stocks) point to preferential export of larger microplankton relative to smaller nano- and picoplankton. However, indicator pigments do not quantitatively trace particle export resulting from zooplankton grazing, which may be an important pathway for the export of small phytoplankton. These results have important implications for understanding the magnitude and mechanisms controlling the biological pump at Line P in particular, and more generally in oligotrophic gyres and high-nutrient, low-chlorophyll (HNLC) regions where small phytoplankton represent a major component of the autotrophic community.

The Northeast USA is experiencing severe impacts of a changing climate, including increased winter temperatures and accelerated relative sea level rise (RSLR). The sediment-poor, organic-rich nature of many Southern New England salt marshes makes them particularly vulnerable to these changes. In order to assess how marsh accretion has changed over time, we returned to Narragansett Bay, RI where salt marsh vertical accretion rates were documented almost 30 years ago. Using radionuclide tracers (210Pb and 137Cs), we observe no significant change in overall accretion rates (0.27–0.69 cm year−1) compared to historical averages (0.24–0.60 cm year−1), but we document a shift in how these marshes maintain elevation. Organic matter now plays a smaller role in contributing to vertical accretion across all study sites, declining by 22 % on average. We attribute this reduction to potentially higher decomposition rates fueled by higher water temperature. Inorganic matter also contributes less to accretion (declining by 44 % on average at marshes located more internal to the estuary), likely due to diminishing sediment supply in this region. With organic and inorganic solids accounting for less of the total accretion, several of the marshes are experiencing symptoms of swelling, with water and porespace contributing more towards accretion compared to historical values. Accretion rates (0.27–0.45 cm year−1) at these organic-rich (>40 % sediment organic matter) marshes are predominantly lower than the current (30 years) rate of RSLR (0.41 ± 0.07 cm year−1). These results, combined with the increased rate of RSLR and the hardened shorelines inhibiting landward migration, call into question the long-term survivability of these marshes.

In this study we present dissolved and particulate 230Th and 232Th results, as well as particulate 234Th data, obtained as part of the GEOTRACES central Arctic Ocean sections GN04 (2015) and IPY11 (2007). Samples were analyzed following GEOTRACES methods and compared to previous results from 1991. We observe significant decreases in 230Th concentrations in the deep waters of the Nansen Basin. We ascribe this nonsteady state removal process to a variable release and scavenging of trace metals near an ultraslow spreading ridge. This finding demonstrates that hydrothermal scavenging in the deep‐sea may vary on annual time scales and highlights the importance of repeated GEOTRACES sections.

et al., Measurements of 129I carried out on sea ice samples collected in the central Arctic Ocean in 2007 revealed relatively high levels in the range of 100-1400×107 at L-1 that are comparable to levels measured in the surface mixed layer of the ocean at the same time. The 129I/127I ratio in sea ice is much greater than that in the underlying water, indicating that the 129I inventory in sea ice cannot be supported by direct uptake from seawater or by iodine volatilization from proximal (nearby) oceanic regimes. Instead, it is proposed that most of the 129I inventory in the sea ice is derived from direct atmospheric transport from European nuclear fuel reprocessing plants at Sellafield and Cap La Hague. This hypothesis is supported by back trajectory simulations indicating that volume elements of air originating in the Sellafield/La Hague regions would have been present at arctic sampling stations coincident with sampling collection., This work was partially funded by the Ministerio de Educación y Ciencia (MEC) of Spain (POL2006–00449), the prize ICREA Academia, funded by the Generalitat de Catalunya (PM), and the fellowship AP2006–03071 (PC-M). PM was supported in part by a Gledden Visiting Fellowship awarded by the Institute of Advanced Studies at The University of Western Australia.

Two independent methods were used to estimate particle export fluxes of metals in the Gulf of Maine and Scotian Shelf. The first method used measurements of particulate trace metals (Cd, Cu, Fe and Pb) and metal/ 234 Th ratios on large (>53 µm), rapidly sinking, particles together with prior estimates of 234 Th/ 238 U disequilibrium to quantify the particle export flux of these metals. The second method adopted a mass balance approach whereby measurements of the distribution of dissolved ( 234 Th/ 238 U method, indicating that metal transport through shelf/slope regimes can be reasonably characterized using relatively limited data sets. Specifically, these results indicate that ~40–60% of the Pb and Zn inputs to the Gulf of Maine and Scotian Shelf are scavenged and exported to the sediments, whereas only ~5–15% of the Ni and Cu inputs are removed by particle export, as would be anticipated based on their respective particle affinities.

[1] Particulate organic carbon (POC) export fluxes and net primary production (NPP) rates are used to assess seasonal patterns in the export ratio (e-ratio = POC export/NPP) in relation to proximity of the sea-ice edge near the shelf break of the eastern Bering Sea during 2008–2010. POC fluxes were relatively low in April (4.6 ± 1.6, trap, and 5.7 ± 4.3 mmol C m−2 d−1, 234Th-derived) and increased in May–early June (19.9 ± 13.3 and 17.0 ± 8.8 mmol C m−2 d−1). POC export reached a maximum in mid-June–mid-July (30.0 ± 12.6 and 48.1 ± 17.4 in 2009; 33.1 ± 27.6 and 57.0 ± 68.4 mmol C m−2 d−1 in 2010) and decreased by late July (13.1 ± 4.7 and 14.1 ± 8.0 mmol C m−2 d−1). NPP rates were relatively high and export fluxes low near the ice-edge in spring leading to e-ratios 1, which is attributed to a temporal decoupling, or offset, of spring NPP and export during summer. While these observations reveal a seasonal progression in POC export and the e-ratio, there is no direct relationship to sea-ice proximity. Furthermore, based on a water column-sediment 234Th budget, the off-shelf export of POC during spring-summer is estimated to be 24 ± 35 mmol C m−2 d−1, which represents an off-shelf e-ratio of 0.07 and 0.21 for contemporaneous seasonally averaged daily NPP and 0.17 and 0.52 for historical monthly averaged daily NPP. An implication is that off-shelf POC transport may represent a seasonal net sink for CO2 in this and other polar shelf regions.

Th abstract As part of the Bering Ecosystem Study-Bering Sea Integrated Ecosystem Research Program (BEST-BSIERP), distributions of 234 Th in the water column and sediments were measured at approximately 60 stations over the shelf and slope/oceanic regions of the eastern Bering Sea during spring and summer of 2009 and 2010. During this study period, the inventory of sediment excess 234 Th ( 234 Thxs) on the shelf was determined to be ∼1/3 of the total 234 Th deficit in the overlying water column; the average focusing factor (FFTh ¼sediment 234 Thxs/water column 234 Th deficit) was 0.3470.23. In addition, the export flux of 234 Th from the shelf to the slope/oceanic region was determined to be on average ∼30% of the total production of 234 Th throughout the shelf. Taken together, these results indicate that 234 Th and associated particles are largely retained on the shelf, and that shelf sediments represent a sink for particles. In contrast, the FFTh in the slope/oceanic region was higher, averaging 1.5271.34. The higher FFTh at the slope is attributed primarily to enhanced scavenging removal and sediment deposition of 234 Th associated with periods of high biogenic particle flux at the marginal ice zone during the spring sea- ice retreat. A more general conclusion is that the ocean margin of the eastern Bering Sea may serve as an accumulation area for particles and associated reactive chemicals.

Measurements of groundwater-dissolved inorganic nitrogen (nitrate + nitrite + ammonia) and phosphate concentrations were combined with recent, radium-based, submarine groundwater discharge (SGD) fluxes and prior estimates of SGD determined from Darcy’s Law, a hydrologic model, and total recharge to yield corresponding SGD nutrient fluxes to Ninigret, Point Judith, Quonochontaug, and Winnapaug ponds, located in southern Rhode Island. Results range from 80 to279 mmol N m−2 year−1 and 4 to 15 mmol P m−2 year−1 for Ninigret, 48 to 265 mmol N m−2 year−1 and 4 to 23 mmol P m−2 year−1 for Point Judith, 31 to 62 mmol N m−2 year−1 and 1 to 2 mmol P m−2 y−1 for Quonochontaug, and 668 to 1,586 mmol N m−2 year−1 and 29 to 70 mmol P m−2 year−1 for Winnapaug ponds, respectively. On a daily basis, the SGD supply of dissolved inorganic nitrogen and phosphorus is estimated to represent ∼1–6 % of the total amount of these nutrients in surface waters of Ninigret, Point Judith, and Quonochontaug ponds and up to 84 and 17 % for Winnapaug, respectively, which may reflect a greater SGD nutrient supply to this pond because of the proximity of fertilized golf courses. With regard to the total external input of these essential nutrients, SGD represents 29–45 % of dissolved inorganic nitrogen input to Ninigret, Point Judith, and Quonochontaug ponds and as much as 93 % for Winnapaug pond. For phosphorus, the contribution from SGD represents 59–85 % of the total external input for Ninigret, Point Judith, and Quonochontaug ponds and essentially all of the phosphorus input to Winnapaug pond. Estimated rates of primary productivity potentially supported by the average supply of dissolved inorganic nitrogen from SGD range from 10 g C m−2 year−1 for Ninigret, 13 g C m−2 year−1 for Point Judith, 4 g C m−2 year−1 for Quonochontaug, and as high as 84 g C m−2 y−1 for Winnapaug pond. The imputed SGD-derived rates of primary productivity represent 4–9 % of water column primary production for Ninigret, Point Judith, and Quonochontaug ponds, and 74 % for Winnapaug pond, a result that is reasonably comparable to several other coastal environments where estimates of SGD nutrient supply have been reported. The implication is that SGD represents an ecologically significant source of dissolved nutrients to the coastal salt ponds of southern Rhode Island and, by inference, other coastal systems.

WOS:000391139900001; International audience; Deposition of continental mineral aerosols (dust) in the Eastern Tropical North Atlantic Ocean, between the coast of Africa and the Mid-Atlantic Ridge, was estimated using several strategies based on the measurement of aerosols, trace metals dissolved in seawater, particulate material filtered from the water column, particles collected by sediment traps and sediments. Most of the data used in this synthesis involve samples collected during US GEOTRACES expeditions in 2010 and 2011, although some results from the literature are also used. Dust deposition generated by a global model serves as a reference against which the results from each observational strategy are compared. Observation-based dust fluxes disagree with one another by as much as two orders of magnitude, although most of the methods produce results that are consistent with the reference model to within a factor of 5. The large range of estimates indicates that further work is needed to reduce uncertainties associated with each method before it can be applied routinely to map dust deposition to the ocean. Calculated dust deposition using observational strategies thought to have the smallest uncertainties is lower than the reference model by a factor of 2-5, suggesting that the model may overestimate dust deposition in our study area. This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

Seasonal patterns in the partitioning of phytoplankton carbon during receding sea ice conditions in the eastern Bering Sea water column are presented using rates of 14 C net primary productivity (NPP), phototrophic plankton carbon content, and POC export fluxes from shelf and slope waters in the spring (March 30–May 6) and summer (July 3–30) of 2008. At ice-covered and marginal ice zone (MIZ) stations on the inner and middle shelf in spring, NPP averaged 767 93 mmol C m � 2 d � 1 , and in ice-free

Spatial and seasonal variability in net primary production (NPP) is reported for the Eastern Bering Sea shelf in spring and summer 2008 and 2009. Euphotic zone integrated NPP in the spring, estimated from simulated in situ 14C-incubations, varied ∼30-fold across the northern and southern regions of the Inner, Middle, and Outer domains of the shelf. During spring, rates were lowest and similar in the North and South region of the Inner domain, while the highest rates were in the Southern region of the Outer domain as a result of extensive ice-edge phytoplankton blooms. Generally, phytoplankton integrated chlorophyll (Chl-a) standing stocks were dominated (>50%) by large (>5 μm) cells. With the exception of the southern Middle domain, integrated rates of NPP were consistently lower in summer than spring, while there was no consistent pattern in changes in integrated Chl-a. Conversely, phytoplankton growth rates (μ) were lower in spring (0.22±0.09 d−1) than summer (0.42±0.17 d−1), consistent with a seasonal shift from a biomass-controlled production system to a growth-rate controlled production system. Associated with this shift in control was a decrease in the size distribution of chlorophyll from ∼60% of the Chl-a>5 μm in the spring to ∼30% of the Chl-a>5 μm in the summer. Despite the widespread distribution of stations, these observations highlight the variable nature of NPP in the Bering Sea, which makes the estimation of seasonal or annual rates in any domain or across the entire shelf difficult, if not impossible, on direct observations alone. A vertically generalized productivity model (VGPM) was used to assimilate the more extensive underway dataset from two cruises to improve the spatial distribution of calculated NPP included in the regional estimates of NPP. The VGPM output captured ∼83% of the variance in measured 14C production, accurately estimated observed NPP (Model II regression slope±stdev.; 0.92±0.06), and allowed for a better constrained estimation of shelf-wide productivity due to higher data density in each region/domain. These results, when combined with published data on shelf-wide productivity suggest that the ecosystem response to climate change (whether an increase or decrease in productivity) would have to exceed a factor of two from mean conditions before being detectable from a comparable survey effort.

Pico-plankton and nano-plankton are generally thought to represent a negligible fraction of the total particulate organic carbon (POC) export flux in oligotrophic gyres due to their small size, slow individual sinking rates, and tight grazer control that leads to high rates of recycling in the euphotic zone. Based upon recent inverse modeling and network analysis however, it has been hypothesized that pico-plankton, including the cyanobacteria Synechococcus and Prochlorococcus, and nano-plankton contribute significantly to POC export, via formation and gravitational settling of aggregates and/or consumption of those aggregates by mesozooplankton, in proportion to their contribution to net primary production. This study presents total suspended particulate (>0.7 μm) and particle size-fractionated (10–20 μm, 20–53 μm, >53 μm) pigment concentrations from within and below the euphotic zone in the oligotrophic subtropical North Atlantic, collected using Niskin bottles and large volume in-situ pumps, respectively. Results show the indicator pigments for Synechococcus, Prochlorococcus and nano-eukaryotes are; (1) found at depths down to 500 m, and; (2) essentially constant, relative to the sum of all indicator pigments, across particle size fractions ranging from 10 μm to >53 μm. Based upon the presence of chlorophyll precursor and degradation pigments, and that in situ pumps do not effectively sample fecal pellets, it is concluded that these pigments were redistributed to deeper waters on larger, more rapidly sinking aggregates likely by gravitational settling and/or convective mixing. Using available pigment and ancillary data from these cruises, these Synechococcus, Prochlorococcus and nano-plankton derived aggregates are estimated to contribute 2–13% (5 ± 4%), 1–20% (5 ± 7%), and 6–43% (23 ± 14%) of the total sediment trap POC flux measured on the same cruises, respectively. Furthermore, nano-eukaryotes contribute equally to POC export and autotrophic biomass, while cyanobacteria contributions to POC export are one-tenth of their contribution to autotrophic biomass. These field observations provide direct evidence that pico- and nano-plankton represent a significant contribution to the total POC export via formation of aggregates in this oligotrophic ocean gyre. We suggest that aggregate formation and fate should be included in ecosystem models, particularly as oligotrophic regions are hypothesized to expand in areal extent with warming and increased stratification in the future.

In 1999, the large surface-rupturing earthquakes of Izmit and Duzce completed a 60-year cycle that included a westward migration of nine consecutive large earthquake failures (>50 km surface rupture), which started with the 1939 Erzincan earthquake in eastern Turkey. In this study, we focused on seismic cycles and seismic risk predictability along the North Anatolian Fault (NAF). Toward the west end of the NAF (26°E–32°E, i.e. Bolu), large earthquake frequency is measured from either historic earthquake catalogs, or geologic records from isolated outcrops and marine sediment cores from the Marmara Sea. In comparison, the eastern part of the NAF zone (32°E–42°E) is less well documented by palaeo-seismologic archives. Thus, the sediment records of lake basins located on the eastern NAF zone constitute a unique opportunity for testing a new palaeo-seismologic approach. To this end, we used a diverse array of complementary methods involving: (1) a 600-km transect of fault-related lakes, (2) sedimentologic observations on cores from six lakes, and (3) a comparison between records of catastrophic sediment transfers in lakes (i.e. radionuclide chronomarkers and erosion tracers) and historic earthquake reports. Our study indicates that lakes along the NAF are sensitive geologic recorders of large surface-rupturing earthquakes (surface-wave magnitude (M s) ≥ 6.9); smaller intensities are not recorded. The most responsive lake systems exhibit increases in sediment accumulation by a factor of >40 for a >3-m strike-slip displacement (M s ≥ 7). However, based on results from the 1939 Erzincan earthquake (M s = 7.8) chronostratigraphic marker, large surface-rupturing earthquakes are detected only by certain lake records and not by others. Matching multiple lake records along the NAF provides information both on the location of a surface rupture of a paleo-earthquake as well as its magnitude. Finally, the shallow lake basins along the NAF could potentially document cycles of large seismic events for at least the late Holocene.

Activities of dissolved, particulate, and sedimentary 210Pb were measured in the shelf-slope region of the Chukchi Sea. Samples were collected as part of the Shelf–Basin Interactions (SBI) Phase II process study (6 May–15 June, 2002) along three shelf–basin transects identified as West Hanna Shoal, East Hanna Shoal, and Barrow Canyon. Distributions of 210Pb and suspended particulate matter indicate efficient removal of 210Pb over the shelf by particle scavenging. Low 210Pb activities measured throughout the halocline of the Canada Basin are attributed to shelf scavenging and subsequent advective transport into the interior basin. Additionally, 210Pb inventories were used to construct a water-column-sediment budget of 210Pb and determine regions of particle export and deposition on the continental shelf and slope. Sediment focusing calculated with this 210Pb budget was observed throughout the shelf-slope region, particularly in shallow (∼100 m) shelf waters at Barrow Canyon. Despite elevated concentrations of suspended particulate matter in Barrow Canyon, the 210Pb budget does not indicate that sediment transport occurred from the West and East Hanna Shoals into Barrow Canyon.

Measurements of particle size-fractionated POC/ 234 Th ratios and 234 Th and POC fluxes were conducted using surface-tethered, free-floating, sediment traps and large-volume in-situ pumps during four cruises in 2004 and 2005 to the oligotrophic eastern Mediterranean Sea and the seasonally productive western Mediterranean and northwest Atlantic. Analysis of POC/ 234 Th ratios in sediment trap material and 10, 20, 53, 70, and 100 μm size-fractionated particles indicate, for most stations, decreasing ratios with depth, a weak dependence on particle size, and ratios that converge to ∼1–5 μmol dpm −1 below the euphotic zone (∼100–150 m) throughout the contrasting biogeochemical regimes. In the oligotrophic waters of the Aegean Sea, 234 Th and POC fluxes estimated using sediment traps were consistently higher than respective fluxes estimated from water-column 234 Th– 238 U disequilibrium, observations that are attributed to terrigenous particle scavenging of 234 Th. In the more productive western Mediterranean and northwest Atlantic, 234 Th and POC fluxes measured by sediment trap and 234 Th– 238 U disequilibrium agreed within a factor of 2–4 throughout the water column. An implication of these results is that estimates of POC export by sediment traps and 234 Th– 238 U disequilibrium can be biased differently because of differential settling speeds of POC and 234 Th-carrying particles.

Submarine groundwater discharge (SGD) to coastal southern Rhode Island was estimated from measurements of the naturally-occurring radioisotopes 226 Ra ( t 1/2 = 1600 y) and 228 Ra ( t 1/2 = 5.75 y). Surface water and porewater samples were collected quarterly in Winnapaug, Quonochontaug, Ninigret, Green Hill, and Pt. Judith–Potter Ponds, as well as nearly monthly in the surface water of Rhode Island Sound, from January 2002 to August 2003; additional porewater samples were collected in August 2005. Surface water activities ranged from 12–83 dpm 100 L − 1 (60 dpm = 1 Bq) and 21–256 dpm 100 L − 1 for 226 Ra and 228 Ra, respectively. Porewater 226 Ra activities ranged from 16–736 dpm 100 L − 1 (2002–2003) and 95–815 dpm 100 L − 1 (2005), while porewater 228 Ra activities ranged from 23–1265 dpm 100 L − 1 . Combining these data with a simple box model provided average 226 Ra-based submarine groundwater fluxes ranging from 11–159 L m − 2 d − 1 and average 228 Ra-derived fluxes of 15–259 L m − 2 d − 1 . Seasonal changes in Ra-derived SGD were apparent in all ponds as well as between ponds, with SGD values of 30–472 L m − 2 d − 1 (Winnapaug Pond), 6–20 L m − 2 d − 1 (Quonochontaug Pond), 36–273 L m − 2 d − 1 (Ninigret Pond), 29–76 L m − 2 d − 1 (Green Hill Pond), and 19–83 L m − 2 d − 1 (Pt. Judith–Potter Pond). These Ra-derived fluxes are up to two orders of magnitude higher than results predicted by a numerical model of groundwater flow, estimates of aquifer recharge for the study period, and values published in previous Ra-based SGD studies in Rhode Island. This disparity may result from differences in the type of flow (recirculated seawater versus fresh groundwater) determined using each technique, as well as variability in porewater Ra activity.

Seasonal changes in Th scavenging and particle aggregation were determined along two shelf-basin transects in the western Arctic Ocean during the spring (May–June) and summer (July–August) of 2002 and 2004. Measurements of dissolved and particulate 234 Th and 228 Th activities were used to quantify Th residence times and reversible rates of Th sorption and particle aggregation. Prior to the spring bloom in 2002, 234 Th and 228 Th residence times were equal and Th scavenging was concordant, indicating predominately steady-state conditions. In contrast, scavenging of 234 Th and 228 Th in the summer of 2002 and the spring and summer of 2004 was discordant, indicating a departure in scavenging rates from steady-state conditions during periods of seasonally high biological activity and particle export. Rates of particle aggregation and disaggregation were calculated using a one-dimensional reversible exchange model and 234 Th and 228 Th activities in small (1–53 μm) and large (>53 μm) particles. Maximum rates were determined coincident with the chlorophyll maximum (25–100 m) and increased by an order of magnitude between periods of low and high productivity. These Th measurements provide evidence that seasonally enhanced rates of particle aggregation might increase the magnitude of the particulate organic carbon (POC) flux in this Arctic regime.

Measurements of the naturally occurring radioisotopes 223 Ra ( t 1/2 = 11.4 days) and 224 Ra ( t 1/2 = 3.66 days) in southern Rhode Island salt ponds were combined with a simple model to obtain independent estimates of the age of these coastal waters. Surface water and porewater samples were collected quarterly in Winnapaug, Quonochontaug, Ninigret, Green Hill, and Pt. Judith-Potter Ponds, as well as nearly monthly in the surface water of Rhode Island Sound, beginning January 2002 through August 2003. Surface water activities ranged from 1–78 dpm 100 L − 1 and 5–885 dpm 100 L − 1 for 223 Ra and 224 Ra, respectively. Porewater radium activities ranged from 3 to 715 dpm 100 L − 1 for 223 Ra, and 57–4926 dpm 100 L − 1 for 224 Ra. Results indicate seasonally varying water mass ages for Ninigret (5–12 days), Winnapaug (2–6 days) and Pt. Judith-Potter Ponds (1–9 days) and, in contrast, relatively constant ages for Green Hill (5–7 days) and Quonochontaug Ponds (3–6 days).

Measurements of 234 Th/ 238 U disequilibria and particle size-fractionated (1, 10, 20, 53, 70, 100mm) organic C and 234 Th were made to constrain estimates of the export flux of particulate organic C (POC) from the surface waters of the Ligurian, Tyrrhenian and Aegean Seas in March–June 2004. POC exported from the surface waters (75–100 m depth) averaged 9.2 mmol m � 2 d � 1 in the Ligurian and Tyrrhenian Seas (2.370.5–14.973.0 mmol m � 2 d � 1 ) and 0.9 mmol m � 2 d � 1 in the Aegean Sea. These results are comparable to previous measurements of 234 Th-derived and sediment-trap POC fluxes from the upper 200 m in the Mediterranean Sea. Depth variations in the POC/ 234 Th ratio suggest two possible controls. First, decreasing POC/ 234 Th ratios with depth were attributed to preferential remineralization of organic C. Second, the occurrence of maxima or minima in the POC/ 234 Th ratio near the DCM suggests influence by phytoplankton dynamics. To assess the accuracy of these data, the empirical 234 Th-method was evaluated by quantifying the extent to which the 234 Thbased estimate of POC flux, PPOC, deviates from the true flux, FPOC, defined as the p-ratio (p-ratio ¼ PPOC/FPOC ¼ STh/ SPOC, where S ¼ particle sinking rate). Estimates of the p-ratio made using Stokes’ Law and the particle size distributions of organic C and 234 Th yield values ranging from 0.93–1.45. The proximity of the p-ratio to unity implies that differences in the sinking rates of POC- and 234 Th-carrying particles did not bias 234 Th-normalized POC fluxes by more than a factor of two.

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

An analysis of the 234 Th method for determining the export flux of particulate organic carbon ( P POC ) from the upper ocean using in situ pumps or water bottles shows that the accuracy of the method (the ratio of the experimental value of P POC divided by the true POC flux, F POC ), defined as the p -ratio, is equal to the mean settling velocity of particulate 234 Th divided by that of POC. Therefore, P POC is equivalent to the true POC flux ( F POC ) and the p -ratio is equal to unity if, and only if, POC and particulate 234 Th have the same mean sinking velocities. A simple particle settling model is discussed that invokes Stokes’ Law settling velocities, volume:surface area ( V :SA) fractionation of C: 234 Th and two assemblages of particles having different sizes and densities. The model is used to illustrate the ranges of parameter values that conform to values of the p -ratio sufficiently close to unity that the experimentally determined POC flux can be considered to be an accurate representation of the true POC flux. Despite the over-simplification of real systems implicit in the model, the results suggest that p -ratios 234 Th on particles is governed by the volume to surface area ( V :SA) ratio of the particles. This results in a greater proportion of 234 Th compared to C being associated with the smaller, more slowly settling particles and, as a result, normalization of the POC flux to the 234 Th flux provides an under-estimate of the former quantity. However, when the smaller particle assemblage dominates the vertical flux, as could occur in open ocean regimes having high aeolian inputs of dense, rapidly settling, inorganic particles, then the p -ratio could exceed unity ( p -ratio >1) resulting in an over-estimate of the POC flux using the 234 Th method. High levels of flocculation associated with phytoplankton blooms in productive regions of the ocean are likely to produce p -ratios approaching unity, because flocs would tend to preserve the V :SA partitioning of the original particle size distribution and thereby minimize differences in the mean settling speeds of POC and particulate 234 Th (Waite, A.W., Hill, P.S., 2006. Flocculation, phytoplankton and the accuracy of 234 Th-based estimates of the vertical flux of particulate organic carbon in the sea. Marine Chemistry in press). Selective sampling of the large particle fraction using, for example, 53-μm screens can produce a more accurate estimate of the true POC flux, but may not entirely shift the p -ratio to a value of unity.

As part of the 2002 Shelf-Basin Interactions (SBI) process study, measurements of the seasonal variation in the export flux of particulate organic carbon (POC) are reported for the upper waters of the Chukchi Sea. POC fluxes were quantified by determination of 234Th/238U disequilibrium and POC/234Th ratios in large ( > 53 μ m ) aggregates collected using in situ pumps. Samples were collected at 35 stations on two cruises, one in predominantly ice-coved conditions during the spring (May 6–June 15) and the other in predominantly open water during the summer (July 17–August 26). Enhanced particle export was observed in the shelf and slope waters, particularly within Barrow Canyon, and there was a marked increase in particle export at all stations during the summer (July–August) relative to the spring (May–June). 234Th-derived POC fluxes exhibit significant seasonal and spatial variability, averaging 2.9 ± 5.3 mmol C m - 2 d - 1 ( range = 0.031 – 22 mmol C m - 2 d - 1 ) in the spring and increasing ∼ 4 -fold in the summer to an average value of 10.5 ± 9.3 mmol C m - 2 d - 1 ( range = 0.79 – 39 mmol C m - 2 d - 1 ) . The fraction of primary production exported from the upper waters increases from ∼ 15 % in the spring to ∼ 32 % in the summer. By comparison, DOC accumulation associated with net community production represented ∼ 6 % of primary production ( ∼ 2 mmol C m - 2 d - 1 ) . The majority of shelf and slope stations indicate a close agreement between POC export and benthic C respiration in the spring, whereas there is an imbalance between POC export and benthic respiration in the summer. The implication is that up to ∼ 20 % of summer production ( ∼ 6 ± 7 mmol C m - 2 d - 1 ) may be seasonally exported off-shelf in this productive shelf/slope region of the Arctic Ocean.

Activities of 231Pa and 230Th in surface sediments from the Arctic Ocean and Greenland–Norwegian Seas are used to examine the redistribution of these water-column produced tracers between the low productivity interior basins and high particle flux marginal areas. Sediment 231Paxs/230Thxs ratios throughout the Canadian and Eurasian Basins and the high particle flux Chukchi slope region are essentially invariant (average = 0.063 ± 0.014, N = 39) and significantly below the water column production ratio (0.093). 231Paxs/230Thxs ratios are also below the production ratio in the seasonally high productivity Greenland–Norwegian Sea basins (average = 0.082 ± 0.024, N = 13), though they are ∼ 30% higher than the interior Arctic. The low sediment 231Paxs/230Thxs ratios are attributed to the net export of ∼ 39% of dissolved 231Pa produced in intermediate/deep waters, as opposed to ∼ 10% of 230Th, through Fram Strait. The negligible 230Th export, combined with the reported low 230Thxs inventory of basin sediments and high 230Thxs inventory in Chukchi slope sediments, points to significant boundary scavenging of 230Th in the Arctic. The invariant 231Paxs/230Thxs ratios further indicate that scavenging of 230Th is comparable to 231Pa (i.e., fractionation to a similar degree between the two nuclides) in both the interior basins and margins. Thus, while significant boundary scavenging of both 231Pa and 230Th occurs in the Arctic, there is a distinct lack of basin-margin fractionation and a sizable export of 231Pa. There is also likely to be export of other similarly particle-reactive radionuclides (e.g., 10Be, Pu isotopes) out of the Arctic.

The magnitude of the flux of biogenic particulate organic carbon (POC) exported from the surface waters of the world ocean and remineralized at depth is critical to constraining models of the global carbon cycle, yet remains controversial. The use of upper ocean sediment traps is still one of the primary tools for determining this export flux, although trap fluxes have been shown to vary significantly because of hydrodynamic and sample collection biases. Over the past decade, 234 Th increasingly has been used as a tracer to estimate POC export from the euphotic zone by multiplying the depth-integrated 234 Th flux by the POC/ 234 Th ratio of sinking particles. The accuracy of this technique is highly dependent on the natural variability in the POC/ 234 Th ratio and 234 Th flux, yet the significance of this variability to estimates of POC export remains uncertain. Based on an analysis of new 234 Th and POC data from the Labrador Sea and a review of 25 previous independent field studies, we report that POC export fluxes can vary 2-10 times or more solely because of variability in the POC/ 234 Th ratio and procedures used to estimate the 234 Th flux. Recommended improvements include studies of the biological, chemical, and physical mechanisms controlling 234 Th-organic matter interactions in seawater; detailed comparisons of POC/ 234 Th ratios in size-fractionated and sediment trap material; increased spatial and temporal sampling density of 234 Th; and more standardized procedures to calculate the 234 Th export flux.

Measurements of the naturally occurring radionuclides, 210Po, 210Pb, 228Ra and 226Ra were conducted on water and sediment samples collected during the Arctic Ocean Section (1994) and Beaufort Sea (1995) expeditions and are used to assess circulation and shelf/slope scavenging in the Western Arctic Ocean. 228Ra concentrations are higher over the continental shelves of the Chukchi and Beaufort Seas compared to interior waters, consistent with a shelf source for 228Ra. However, 228Ra/226Ra ratios for Pacific-origin water have a lower end member value (ca. 1.0) compared to literature values of 3.6 for end member 228Ra/226Ra ratios for Atlantic-origin water passing across the shelf of the Laptev Sea. The shift during the 1990s in the position of the axis of the Trans-Polar Drift and the consequent change in the pathways for the flow of freshwater from the Russian continental shelves is reflected in a shift of elevated 228Ra concentrations in surface water from the Lomonosov Ridge to the Mendeleyev Ridge. 210Pb water column distributions in the Eurasian Basin are similar to those measured in the North Atlantic and indicate scavenging residence times of 15–20 yr. 210Pb concentrations are lower in intermediate waters of the Makarov and Canada Basins, because of enhanced scavenging at the continental margin. The lowest 210Pb concentrations (

231Pa and 230Th concentrations were determined in filtered seawater and suspended particulate matter collected from the Labrador Sea and the Equatorial and South Atlantic to constrain their application as tracers of intermediate/deep water age and Atlantic thermohaline circulation. Distributions of total 231Pa and 230Th indicate the influence of recently formed North Atlantic Deep Water, as evidenced by nearly invariant concentrations below ∼1000 m in the Labrador Sea and increasing 231Pa and 230Th concentrations as deep waters progress southward from northern source regions. Application of a scavenging–mixing model to both tracer distributions indicates an intermediate/deep water age of 12 yr in the Labrador Sea and a ∼30–140 yr transit time to the low-latitude stations. We attribute a striking increase in total 230Th in the Labrador Sea from 1993 to 1999 to aging of intermediate waters as a consequence of the cessation of deep convection in the Labrador Sea since 1993. The temporal change in the 230Th age of these waters is consistent with the 6 yr time interval between the observations. The average particulate 231Pa/230Th activity ratio in the Labrador Sea and low-latitude deep waters is 0.057±0.003, significantly below the 231Pa/230Th production ratio (0.093) and in agreement with excess 231Pa/230Th ratios in Holocene sediments (0.060±0.004) and sediment trap material (0.034±0.012) from the Atlantic and model simulations. This observation is consistent with the southward transport of deep water strongly attenuating boundary scavenging in the Atlantic. A latitudinal dependence in particle fractionation of these tracers is also evident, with elevated fractionation factors (FTh/Pa) observed near the Equator and South Atlantic gyre (∼11) compared to low values in the Labrador Sea (∼3) and Southern Ocean (∼2). There also exists a depth dependence in FTh/Pa, characterized by low values in surface waters, a broad mid-depth maximum, and decreasing values towards the sea-floor. The latitudinal and depth variations in FTh/Pa are suggested to reflect differences in the chemical composition of marine particles.

High-precision measurements of 231 Pa and 230 Th in filtered seawater and suspended particulate matter samples are reported for the Equatorial and South Atlantic. Distributions of 231 Pa and 230 Th clearly indicate the influence of advection, as evidenced by departures from scavenging models that predict a linear increase with depth for these tracers. Application of a scavenging-mixing model implies a deep water transit time of ∼60-100 years from the northern source water regions. The average particulate 231 Pa/ 230 Th activity ratio is 0.0498 ± 0.0160, a factor of ∼2 lower than the 231 Pa/ 230 Th production ratio of 0.093 and in agreement with reported excess 231 Pa/ 230 Th ratios of 0.06 ± 0.004 in Holocene sediments north of 50°S in the Atlantic. These water column data further suggest that lateral eddy diffusive transport combined with enhanced scavenging in high-particle flux marginal regions (boundary scavenging) is weakly expressed in the Atlantic. Particle fractionation of these tracers is also indicated by the elevated fractionation factors of F Th/Pa = 4.32-24.04 (ave. = 9.97 ± 4.98) compared to values of ∼1-4 in the Southern Ocean.

Concentrations of a suite of trace elements (Al, Ag, As, Cd, Co, Cr, Cu, Fe, Ni, Pb, Sr, V, Zn) were measured in aerosol and precipitation samples collected at a coastal site in New Castle, NH, from August 1996, through July 1997. Metal concentrations in aerosol and precipitation exhibit a high degree of temporal variability over the annual cycle, varying by approximately one order of magnitude or less for aerosol metals and by ∼2–3 orders of magnitude in precipitation. Estimates of the total annual atmospheric deposition of metals to the Gulf of Maine range from ∼10 3 kg yr −1 for Ag, ∼10 4 –10 5 kg yr −1 for the majority of metals, and ∼10 6 kg yr −1 for the crustal elements Al and Fe.

Radionuclide measurements have been conducted on sediment, seawater and biota samples collected in Chernaya Bay, on the southern coast of Novaya Zemlya, the site of two underwater nuclear tests conducted in the 1950s. 239,240Pu levels in sediments from the central region of Chernaya Bay exceed concentrations of 15,000 Bq/kg, and are among the highest ever reported for the marine environment. It is estimated that approximately 11 TBq of 239,240Pu from the tests has been retained in the sediments of Chernaya Bay. Plutonium from Chernaya Bay is distinguished by 240Pu/239Pu atom ratios of 0.03 that are much lower than ratios of 0.18 typical of global fallout. High levels of 137Cs (Bq/kg) and 60Co (Bq/kg) were also measured in surface sediments in the central regions of Chernaya Bay near the presumed epicentre of the explosions. Applications of a biodi!usion model to excess 210Pb sediment depth pro"les indicate that the distribution of 239,240Pu is governed mainly by sediment mixing in this low sedimentation rate ((0.1 cm/yr) regime and, as a result, most of the 239,240Pu has been retained in the upper 20 cm of the sediment column. Elevated levels of 239,240Pu measured in Macoma (104 Bq/kg), Fucus (15 Bq/kg) and polychaete (1292 Bq/kg) from Chernaya Bay, indicate that 239,240Pu levels in the benthos are comparatively high and that signi"cant uptake has occurred in the food chain. Although levels of 239,240Pu in bottom water from Chernaya Bay are high (4.2 Bq/m3), restricted exchange over the fjord sill limits the present rates of 239,240Pu transport from

This overview compares and contrasts trends in the magnitude of the downward Particulate Organic Carbon (POC) flux with observations on the vertical profiles of biogeochemical parameters in the NE subarctic Pacific. Samples were collected at Ocean Station Papa (OSP, 50°N, 145°W), between 18–22 May 1996, on pelagic stocks/rate processes, biogenic particle fluxes (drifting sediment traps, 100–1000 m), and vertical profiles of biogeochemical parameters from MULVFS (Multiple Unit Large Volume Filtration System) pumps (0–1000 m). Evidence from thorium disequilibria, along with observations on the relative partitioning of particles between the 1–53 μm and >53 μm classes in the 50 m mixed layer, indicate that there was little particle aggregation within the mixed layer, in contrast to the 50–100 m depth stratum where particle aggregation predominated. Vertical profiles of thorium/uranium also provided evidence of particle decomposition occuring at depths ca. 150 m; heterotrophic bacteria and mesozooplankton were likely responsible for most of this POC utilisation. A water column carbon balance indicated that the POC lost from sinking particles was the predominant source of carbon for bacteria, but was insufficient to meet their demands over the upper 1000 m. While, the vertical gradients of most parameters were greatest just below the mixed layer, there was evidence of sub-surface increases in microbial viability/growth rates at depths of 200–600 m. The C:N ratios of particles intercepted by free-drifting and deep-moored traps increased only slightly with depth, suggesting rapid sedimentation even though this region is dominated by small cells/grazers, and the upper water column is characterised by long particle residence times (>15 d), a fast turnover of POC (2 d) and a low but constant downward POC flux.

Particulate organic carbon (POC) data collected by small-volume (∼1–2 l) bottle filtration and large-volume (∼100–600 l) methods are compared for samples from the central Arctic, Equatorial Pacific, Equatorial and South Atlantic, Gulf of Maine, and Narragansett Bay. Small-volume samples were collected using Niskin® and Go-Flo® bottles and large-volume samples were collected using in situ pumps and large-volume bottle filtration. Results indicate that small-volume bottle POC data are often greater than large-volume results, by as much as 2–4 times, in regions with low POC concentration (

Poretics® glass fiber filters of 0.7 μm nominal pore size (GF-75) were evaluated for determination of particulate organic carbon and nitrogen in seawater and freshwater. Carbon and nitrogen blanks determined by CHN analysis of precombusted 25 mm filters were, respectively, 0.8 ± 0.3 and 0.02 ± 0.01 μmol for Poretics® GF-75 filters, compared to 1.7 ± 0.7 and 0.19 ± 0.12 μmol for Whatman® GF/F filters. These results suggest that GF-75 filters are characterized by lower blanks and better analytical precision for determination of carbon and nitrogen than GF/F filters. Comparable POC and PON concentrations were obtained using GF-75 and GF/F filters on paired analyses of samples from the Gulf of Maine, Narragansett Bay, and a freshwater creek. Depth profiles of POC and PON determined using GF-75 filters are presented for the Gulf of Maine and the Canada Basin, Arctic Ocean. Results reported in this study demonstrate that the Poretics® GF-75 filter is a suitable, cost effective, alternative to the widely used Whatman® GF/F filter.

Measurements of dissolved and particulate230Th and232Th by thermal ionization mass spectrometry were made on samples collected from the Labrador Sea and the Denmark Strait and Iceland-Scotland overflow waters. The large-scale feature of low and invariant230Th evident in deep waters of the Labrador Sea and previously observed in the deep northern Atlantic can be reproduced using a reversible scavenging model that includes the effect of ventilation. Advective transport also must be important in the redistribution of230Th in other regions of the Atlantic and for other long-lived tracers, such as231Pa,26Al and10Be.

234 Th (t1/2 = 24.1 days) was used as a tracer to investigate scavenging and particle export in the upper ocean along a transect from the Chukchi Sea to the North Pole during the 1994 Arctic Ocean Section (August–September). Large volume water samples (200–6001) were collected using in situ pumps for the determination of dissolved and particulate 234 Th activity and POC and PON concentrations. The lowest total 234 Th/ 238 U activity ratio (0.14) was observed in the Chukchi Sea, consistent with enhanced scavenging in these particle-rich shelf waters. 234 Th/ 238 U disequilibrium in the interior Arctic is attributed to local biological production and the associated particle export. Estimates of the export flux of POC and PON from the upper 20–30 m were made using the 234 Th, POC and PON results. POC export ranged from 38 mmol C m −2 d −1 on the Chukchi Shelf to 0.3–7 mmol C m −2 d −1 (mean=3±2 mmol C m −2 d −1 ) within the ice-covered interior Arctic Ocean. PON export ranged from 4.8 mmol N m −2 d −1 on the Chukchi Shelf to 0.05-1.1 mmol N m −2 d −1 (mean = 0.36 ±0.39 mmol N m −2 d −1 ) in the central Arctic Ocean.

An intercomparison was conducted between 14 different cross-flow filtration (CFF) systems. Each CFF membrane had a 1000 nominal molecular weight cut-off, and five different manufacturers' membranes were tested. The goal of this exercise was to examine whether the different CFF systems were behaving in a well-defined and operationally reproducible manner in marine applications. Surface seawater from Woods Hole and mid-depth waters from Hawaii were prefiltered ( 100 μM. The variation in OC blanks between CFF systems, even of a single type, suggested that cleaning and handling protocols were critical. One of the primary features of this intercomparison was the large range in apparent retention characteristics of the different CFF systems when used on natural seawater samples. In both settings, the quantity of colloidal material retained by the CFF systems followed the order: Amicon > Filtron ≥ Osmonics > Membrex. For example, in the Hawaii sample, the Amicon CFF system retained on average 43% colloidal OC, while the Filtron, Osmonics and Membrex retained < 4%. Variations within a factor of 2–5 were found within a single membrane type. Other results from this intercomparison suggested that these same relative retention characteristics hold for Fe and Al, organic nitrogen and organic phosphorus, optical properties, and colloid standards. Time-series samples of permeate showed a generally increasing OC concentration with time or concentration factor which must be taken into account when interpreting CFF data. We recommend that considerable care be taken in quantifying CFF blanks and in assessing the CFF cut-off for each system prior to use in marine applications. Time-series sampling and the use of standard molecules in controlled experiments are encouraged in order to further our understanding of the behavior of natural compound assemblages during CFF processing.

Seawater samples (1–2 liters) were collected from the Norwegian Sea and Denmark Strait and analyzed for 230Th and 232Th using highly sensitive thermal ionization mass spectrometry (TIMS). Depth profiles of dissolved 230Th and 232Th are characterized by surface water minima (

The transpolar drift is strongly enriched in 228Ra accumulated on the wide Arctic shelves with subsequent rapid offshore transport. We present new data of Polarstern expeditions to the central Arctic and to the Kara and Laptev seas. Because 226Ra activities in Pacific waters are 30% higher than in Atlantic waters, we correct 226Ra for the Pacific admixture when normalizing 228Ra with 226Ra. The use of 228Ra decay as age marker critically depends on the constancy in space and time of the source activity, a condition that has not yet adequately been tested. While 228Ra decays during transit over the central basin, ingrowth of 228Th could provide an alternative age marker. The high 228Th/228Ra activity ratio (AR = 0.8–1.0) in the central basins is incompatible with a mixing model based on horizontal eddy diffusion. An advective model predicts that 228Th grows to an equilibrium AR, the value of which depends on the scavenging regime. The low AR over the Lomonosov Ridge (AR = 0.5) can be due to either rapid transport (minimum age without scavenging 1.1 year) or enhanced scavenging. Suspended particulate matter load (derived from beam transmission and particulate 234Th) and total 234Th depletion data show that scavenging, although extremely low in the central Arctic, is enhanced over the Lomonosov Ridge, making an age of 3 years more likely. The combined data of 228Ra decay and 228Th ingrowth confirm the existence of a recirculating gyre in the surface water of the eastern Eurasian Basin with a river water residence time of at least 3 years.

Kinetic experiments were conducted using batch systems to investigate the removal of dissolved Al and 234Th tracer by dead Phaeodactylum tricomutrnm diatoms in seawater. Experiments were conducted at constant temperature (2°C), pH (7.8), and salinity (30 psu), using realistic oceanic concentrations of dissolved Al (50 nM), and 1, 2.5, 5, and 10 mg/L suspensions of dead diatoms in ultrafiltered (

Vertical profiles of dissolved aluminum (A1) at five stations in the Weddell Sea are presented. Concentrations of dissolved A1 in the Weddell Sea range from 1 to 5 nM, considerably lower than A1 levels in the Atlantic, typically in the 10–40 nM range, and similar to levels of 0.06–6 nM reported for the Pacific. The slight maximum in dissolved A1 (4–5 nM) in the surface waters is suggested to be derived primarily from the partial dissolution of atmospheric dust. Minimum concentrations of dissolved A1 (1–1.6 nM) are observed at intermediate depths (1500–3000m). The slight increase in dissolved A1 of about 2 nM with proximity to the sediment-water interface implies that sediments are not a significant source of dissolved A1 to the deep waters. The combination of the low atmospheric input and the seasonally high rate of primary productivity that characterize the Southern Ocean can account for the low concentrations of dissolved A1 in the Weddell Sea. The results reported in this study suggest that the low A1 levels in the deep North Pacific can be explained by advection of low-A1 source waters originating in the Weddell Sea.

Laboratory and field studies were conducted to investigate the significance of resuspended sediments as a source of dissolved Al to the deep northwest Atlantic. Sediment resuspension experiments demonstrate the effect on dissolved Al concentration (initially 11 nM) of adding natural suspended sediments (ca. 0.1–10 mg/L) to seawater. The concentration of dissolved Al increased by the resuspension of sediments; for example, addition of 0.15 mg/L sediments caused dissolved Al to increase by 10 nM. Distributions of dissolved and leachable particulate Al off the tail of the Grand Banks, near the highenergy western boundary current, show elevated levels in the near-bottom waters. We suggest that resuspended sediments associated with nepheloid layers along the western boundary of the North Atlantic are a source of dissolved Al. Strong western boundary currents provide the energy to resuspend and maintain intense nepheloid layers of sediments. Continued resuspension and deposition of sediments within the nepheloid layer promotes the release of Al from sediments to the overlying water. The Al-rich terrigenous sediments that predominate along the deep boundary of the Denmark Strait, Labrador Sea, Newfoundland and off Nova Scotia constitute a potentially significant source of dissolved Al. Release of Al from resuspended sediments associated with nepheloid layers at a more northern location (e.g., Denmark Strait) may contribute to the near-linear increase in dissolved Al with depth observed in the deep northwest Atlantic.

[1] Particulate organic carbon (POC) export fluxes were estimated in the shelf-slope region of the Chukchi Sea using measurements of 234Th−238U disequilibria and the POC/234Th ratio in large (>53-μm) particles. These export fluxes were used in conjunction with rates of primary productivity and benthic carbon respiration to construct a POC budget for this shelf-slope region. Samples were collected along a series of shelf-basin transects in the spring (May–June) and summer (July–August) of 2004. These stations were previously occupied during the ice covered (spring) and open water (summer) seasons of 2002, allowing for an interannual comparison of export flux. In contrast to 2002, when open water POC fluxes were significantly higher than in the ice-covered period, POC export fluxes in 2004 were similar during the spring (average = 19.7 ± 24.8 mmol C m−2 d−1) and summer (average = 20.0 ± 14.5 mmol C m−2 d−1). The high POC fluxes measured during the spring are attributed to a plankton bloom, as evidenced by exceptionally high rates of primary productivity (average = 124.4 ± 88.1 mmol C m−2 d−1). The shelf-slope budget of particulate organic carbon indicates that 10–20% of primary productivity was exported below 50 m but was not consumed during benthic carbon respiration or burial and oxidation in underlying sediments. Furthermore, a water column−sediment budget of 234Th indicates that particulate material is retained in shelf sediments on a seasonal basis.

Since the 1960s, thorium isotopes occupy a special place in the oceanographer’s toolbox as tracers for determining rates and mechanisms of oceanic scavenging, particle dynamics, and carbon fluxes. Due to their unique and constant production rates from soluble parent nuclides of uranium and radium, their disequilibrium can be used to calculate rates and time scales of sinking particles. In addition, by ratio-ing particulate 234 Th (as well, in principle, other Th-nuclides) to carbon (and other elements), and linking this ratio to the parent–daughter disequilibrium in the water column, it is possible to calculate fluxes of carbon and other elements. Most of these applications are possible with little knowledge of the dissolved chemical properties of thorium, other than its oxidation state (IV) and tendency to strongly sorb to surfaces, i.e., its bparticle- or surface-activityQ. However, the use of any tracer is hindered by a lack of knowledge of its chemical properties. Recent observations in the variability of carbon to 234 Th ratios in different particle types, as well as of associations of Th(IV) with various marine organic biomolecules has led to the need for a review of current knowledge and what future endeavors should be taken to understand the marine chemistry of thorium. D 2005 Elsevier B.V. All rights reserved.

Iodine 129 discharged from nuclear fuel reprocessing plants in France and the United Kingdom is transported into the Nordic Seas on timescales of 3 - 5 years. Tracer I-129 is subsequently injected into intermediate waters that overflow the sills between Greenland, Iceland, and Scotland and ventilate the North Atlantic Deep Waters ( NADW). During the early 1990s, discharges of I-129 increased by 600%, resulting in a large, well- resolved tracer " front'' whose passage through the Nordic Seas is presently being observed by a time series of I-129 measurements on the WOCE ( AR7W) section in the Labrador Sea. The highest I-129 levels were measured below depths of 3000 m in Denmark Strait Overflow Water ( DSOW). These levels increased by about 300% between 1997 and 2001 to values > 40 x 10(7) atoms/ L, indicating that the leading edge of the tracer " front'' from the early 1990s was being observed entering the NADW. Using a simple mixing/ advection model, I-129 and CFC 11 results were used to calculate transit times of 0.4 - 2.6 years for the flow of DSOW from formation regions in the Nordic Seas to the Labrador Sea. Measurements on samples collected in the Irminger Sea in 2001 gave transit times of 0.3 years for core DSOW immediately south of Denmark Strait and about 2 years for locations off the southern tip of Greenland. An increase in DSOW transit times observed in the Labrador Sea between 1999 and 2001 probably reflects a more general weakening in the subpolar gyre of the North Atlantic during the 1990s.

The goal of this proposal was to develop new sampling and analytical techniques which could be used to understand the transformation rates and mechanisms of exchange between dissolved, colloidal, small particle and large particle size classes of organic carbon in ocean margins. To meet this goal, the authors focused on the development of cross-flow filtration (CFF) for the isolation of colloidal material from the dissolved phase in seawater. In addition, they tested and optimized high sensitivity techniques for the measurement of thorium isotopes using thermal ionization mass spectrometry (TIMS) and carbon isotopes using accelerator mass spectrometry (AMS) and stable C mass spectrometric techniques. In this final report, they highlight some of these new sampling and analytical developments as well as preliminary results from the first DOE cruise this past April. The report is broken down into 4 sections, namely (1) colloidal sampling strategies, (2) TIMS analytical developments, (3) carbon isotopic measurements and (4) results from the R/V Columbus Iselin cruise. For more detailed discussion of the findings, they have included as an appendix to this final report manuscripts which have been published or will be submitted during this funding cycle.

Dissolved and acetic acid leachable particulate Al ([Al]HAc) concentrations have been measured as a time series during a spring phytoplankton bloom in Bedford Basin off Nova Scotia, Canada. Two factors were observed to influence Al levels: the spring bloom and river runoff. Changes in dissolved Al and [Al]HAc associated with intense biological activity resulted in a 55–65 n m (40%) decrease in dissolved Al and a coincident similar increase in [Al]HAc over 15–20 days. The intensity of Al removal is apparently a function of biogenic particle loads. A pulse in river runoff caused dissolved Al to increase roughly 10 fold (up to 1130 n m ) and [Al]HAc by nearly 4 fold (up to 175 n m ) in the surface 5 m. These data indicate that biological activity can significantly alter concentrations of Al in coastal waters and distributions among dissolved and particulate forms. In the presence of strong sources, such as river runoff, these effects can be obscured.