Water Mass Tracer Research Articles

Measuring oxygen concentrations to improve the detection capabilities of an ocean circulation observation array

The North Atlantic meridional overturning circulation (MOC) may weaken or even collapse in response to anthropogenic climate forcing, with potentially nontrivial socioeconomic impacts. One currently implemented MOC observation system uses temperature and salinity (as well as other) observations along a zonal transect in the North Atlantic. The resulting MOC estimate has, however, a relatively low signal‐to‐noise ratio due to large internal variability and observation errors. Observations of hydrographic tracers that are mechanistically linked to MOC changes may increase the signal‐to‐noise ratio. A MOC slowdown is associated in model simulations with a shoaling of the boundary between North Atlantic Deep Water and Antarctic Bottom Water. This shoaling results in detectable trends in water mass tracers. Here we deploy a virtual observation array into a numerical model starting in model year 2006 to test whether observing the apparent oxygen utilization (AOU) in addition to the MOC estimate improves detection capabilities. Our detection method accounts for observation errors, autocorrelated variability, and uncertainty about the initial conditions. Neglecting the effects of observation errors and the uncertainty about the initial conditions results in artificially early detection times. The MOC signal alone enables reliable detection in roughly five decades. Adding AOU observations reduces this detection time by approximately 40%.

Distinguishing between terrestrial and autochthonous organic matter sources in marine environments using fluorescence spectroscopy

The optical properties of chromophoric dissolved organic matter (CDOM) are frequently used as tracers of water masses in bays and estuaries but present unique challenges in the ocean due to the small quantities of organic matter present and the similarities between spectra from coastal and offshore environments. Samples collected on trans-oceanic cruises in the Pacific and Atlantic oceans were used to investigate the optical characteristics of dissolved organic matter in waters with limited freshwater influence (salinity >30). Parallel Factor Analysis (PARAFAC) of fluorescence spectra revealed that coastal and oceanic dissolved organic matter (DOM) fluorescence could be separated into at least eight separate components: 4–5 humic-like and 3–5 protein-like signals. Two of the humic components were identified as representing terrestrial organic matter and their signals could be traced in the open ocean (Pacific and Atlantic) at levels of approximately 1.5% of riverine concentrations. An additional humic component, traditionally identified as the “marine” or “M” peak, was found to be both sourced from land and produced in the ocean. These results demonstrate that the supply, mixing and removal of terrestrial organic matter in oceanic waters can be observed with relatively simple measurement techniques, suggesting that fluorescence spectroscopy could play a useful role in future studies of the origin and fate of DOM in oceanic environments.

Persistently declining oxygen levels in the interior waters of the eastern subarctic Pacific

Fifty years of measurements at Ocean Station Papa (OSP, 50°N, 145°W) show trends in the interior waters of the subarctic Pacific that are both impacted by short term (few years to bi-decadal) atmospheric or ocean circulation oscillations and by persistent climate trends. Between 1956 and 2006, waters below the ocean mixed layer to a depth of at least 1000 m have been warming and losing oxygen. On density surfaces found in the depth range 100–400 m ( σ θ = 26.3–27.0), the ocean is warming at 0.005–0.012 °C y −1, whereas oxygen is declining at 0.39–0.70 μmol kg −1 y −1 or at an integrated rate of 123 mmol m −2 y −1 (decrease of 22% over 50 years). During this time, the hypoxic boundary (defined as 60 μmol O 2 kg −1) has shoaled from ∼400 to 300 m. In the Alaska Gyre, the 26.2 isopycnal occasionally ventilates, whereas at OSP 26.0 σ θ has not been seen at the ocean surface since 1971 as the upper ocean continues to stratify. To interpret the 50 year record at OSP, the isopycnal transport of oxygenated waters within the interior of the subarctic Pacific is assessed by using a slightly modified “NO” parameter [Broecker, W., 1974. “NO” a conservative water-mass tracer. Earth and Planetary Science Letters 23, 100–107]. The highest nitrate–oxygen signature in interior waters of the North Pacific is found in the Bering Sea Gyre, Western Subarctic Gyre and East Kamchatka Current region as a consequence of winter mixing to the ∼26.6 isopycnal. By mixing with low NO waters found in the subtropics and Okhotsk Sea, this signature is diluted as waters flow eastward across the Pacific. Evidence of low NO waters flowing north from California is seen along the coasts of British Columbia and SE Alaska. Oxygen in the subsurface waters of the Alaskan Gyre was supplied ∼60% by subarctic and 40% by subtropical waters during WOCE surveys, whereas such estimates are shown to periodically vary by 20% at OSP. Other features discernable in the OSP data include periods of increased ventilation of deeper isopycnals on an ∼18 year cycle and strong, short term (few month) variability caused by passing mesoscale eddies. The potential impacts of declining oxygen on coastal ecosystems are discussed.

Air–water mass transfer and tracer gases in stormwater systems

Reaeration is a central quality parameter for the performance of environmental systems such as ponds receiving urban and road runoff. Tracer gases can be used to measure reaeration rates in these systems. The methods comprise injection of a volatile tracer into an environmental system and subsequently measurement of the emission of the volatile tracer. The physical basis of such methods is the existence of a constant ratio between the air-water mass transfer coefficient for oxygen and the corresponding mass transfer coefficient for the volatile tracer gas. This constant ratio is often not clearly defined in the literature due to difficulties in both experimental procedures and handling of data. In this study such methods are evaluated and an experimental procedure and a corresponding data processing procedure for a general and reliable determination of mass transfer rates are presented. Propane is selected as an example of an appropriate tracer gas and the ratio between the mass transfer coefficients of oxygen and propane is determined.

Atlantic Meridional Overturning Circulation During the Last Glacial Maximum

The circulation of the deep Atlantic Ocean during the height of the last ice age appears to have been quite different from today. We review observations implying that Atlantic meridional overturning circulation during the Last Glacial Maximum was neither extremely sluggish nor an enhanced version of present-day circulation. The distribution of the decay products of uranium in sediments is consistent with a residence time for deep waters in the Atlantic only slightly greater than today. However, evidence from multiple water-mass tracers supports a different distribution of deep-water properties, including density, which is dynamically linked to circulation.

Relating estimates of CaCO3 production, export, and dissolution in the water column to measurements of CaCO3 rain into sediment traps and dissolution on the sea floor: A revised global carbonate budget

The global CaCO3 budget is constrained by new estimates of standing stocks, fluxes through the water column, and dissolution in the water column and on the sea floor. Previous estimates of carbonate production and export are indistinguishable within a large range of values, 0.4–1.8 Gt PIC yr−1. Globally, excess alkalinity (TA*) and water mass tracers indicate dissolution of 1.0 Gt PIC yr−1 between 200 and 1500 m, suggesting production and export must at least equal this amount. Most water column dissolution occurs at high latitudes, and alkalinity fluxes from outer shelf and upper slope sediments (100–1500 m) only support 5–10% of the TA* inventory. Below 2000 m, the sinking flux of PIC (0.6 Gt PIC yr−1) is consistent with the rate of sea floor dissolution (0.4 Gt PIC yr−1) plus burial (0.1 Gt PIC yr−1). This rain rate constrains the export value to >1.6 Gt PIC yr−1. Satellite‐based estimates of standing stocks of CaCO3 indicate a decrease equatorward, which is opposite in trend to sediment trap fluxes. This observation may be explained by an equatorward decrease in sinking particle dissolution, systematic changes in PIC residence time with latitude, or satellite retrieval problems. Globally averaged euphotic zone standing stock (5.4 mmol m−2) and export estimates indicate PIC residence times of 5–18 days.

Caribbean sclerosponge radiocarbon measurements re-interpreted in terms of U/Th age models

Previously unpublished AMS radiocarbon measurements of a sclerosponge from tongue of the ocean (TOTO), Bahamas, as well as preliminary data from an investigation of the radiocarbon records of sclerosponges living at different depths in the adjacent Bahamas basin, Exuma Sound, are interpreted in terms of U-series age models. The data are compared to an existing Caribbean sclerosponge radiocarbon bomb curve measured using standard gas proportional beta counting and used to interpret a 210Pb age model. The Δ14C records from the sclerosponges illustrate a potential for use of radiocarbon both as a tracer of subsurface water masses or as an additional age constraint on recently sampled sclerosponges. By using an independent age model, this study lays the framework for utilizing sclerosponges from different locations in the tropics and subtropics and different depths within their wide depth range (0–250m) to constrain changes in production of subtropical underwater in the Atlantic Ocean. This framework is significant because the proxy approach is necessary to supplement the short and coarse time series being used to constrain variability in the formation of Caribbean subtropical underwater, the return flow of a shallow circulation cell responsible for nearly 10% of the heat transported poleward in the N. Atlantic.

Temporal stability of the neodymium isotope signature of the Holocene to glacial North Atlantic

The neodymium isotopic composition of marine precipitates is increasingly recognized as a powerful tool for identifying changes in ocean circulation and mixing on million year to millennial timescales. Unlike nutrient proxies such as δ13C or Cd/Ca, Nd isotopes are not thought to be altered in any significant way by biological processes, and thus they can serve as a quasi‐conservative water mass tracer. However, the application of Nd isotopes in understanding the role of thermohaline circulation in rapid climate change is currently hindered by the lack of direct constraints on the signature of the North Atlantic end‐member through time. Here we present the first results of Nd isotopes measured in U‐Th‐dated deep‐sea corals from the New England seamounts in the northwest Atlantic Ocean. Our data are consistent with the conclusion that the Nd isotopic composition of North Atlantic deep and intermediate water has remained nearly constant through the last glacial cycle. The results address long‐standing concerns that there may have been significant changes in the Nd isotopic composition of the North Atlantic end‐member during this interval and substantiate the applicability of this novel tracer on millennial timescales for paleoceanography research.

Global gridded data set of the oxygen isotopic composition in seawater

We present a new 3‐dimensional 1° × 1° gridded data set for the annual mean seawater oxygen isotope ratio (δ18O) to use in oceanographic and paleoceanographic applications. It is constructed from a large set of observations made over the last 50 years combined with estimates from regional δ18O to salinity relationships in areas of sparse data. We use ocean fronts and water mass tracer concentrations to help define distinct water masses over which consistent local relationships are valid. The resulting data set compares well to the GEOSECS data (where available); however, in certain regions, particularly where sea ice is present, significant seasonality may bias the results. As an example application of this data set, we use the resulting surface δ18O as a boundary condition for isotope‐enabled GISS ModelE to yield a more realistic comparison to the isotopic composition of precipitation data, thus quantifying the ‘source effect’ of δ18O on the isotopic composition of precipitation.

Adiabatic reduction of circulation‐related CO2 air‐sea flux biases in a North Atlantic carbon‐cycle model

Physical transport processes of carbon, alkalinity, heat, and nutrients to a large extent control the partial pressure of CO2 at the sea surface and hence the oceanic carbon uptake. Using a state‐of‐the‐art biogeochemical model of the North Atlantic at eddy‐permitting resolution we show that biases in the simulated circulation generate errors in air‐sea fluxes of CO2 which are still larger than those associated with the considerable uncertainties in parameterizations of the air‐sea gas exchange. A semiprognostic correction method that adiabatically corrects the momentum equations while conserving water mass properties and tracers is shown to yield a more realistic description of the carbon fluxes into the North Atlantic at little additional computational cost. Owing to upper ocean flow patterns in better agreement with observations, simulated CO2 uptake in the corrected regional model is larger by 25% compared to the uncorrected model.

Mobile Voltammetric Laboratory for Ship-Board and Shore-Based Analyses of Dissolved Copper

Environmental Context. Determining concentrations of dissolved copper in seawater is of interest, because copper (i) affects the health of aquatic biota, and (ii) has the potential to act as a water mass tracer, a marker which can be used to follow the movement of water in oceans. In situ analyses of copper and other trace metals in seawater have become increasingly popular, because they circumvent potential contamination problems associated with sample collection, storage, and transport to off-site analysis locations. This paper describes the assembly of a low-cost, mobile laboratory utilizing adsorptive cathodic stripping voltammetry (AdCSV) for the real-time determination of total dissolved copper in surface waters. Abstract. The main advantages of using a mobile laboratory for the real-time analyses of copper in surface waters over classical discrete sampling include a reduction in the risk of sample contamination and significant savings in both time and money to the analyst. This paper outlines the development of a portable voltammetric laboratory, capable of deployment from ships or road vehicles, and an experimental method for the real time analysis of total dissolved copper in surface waters. It has been tested in estuarine and coastal waters to the west of Ireland and has proved suitable for the analysis of both fresh and marine waters. The system includes a trace metal-free sampling fish (for shipboard sampling), in-line filtration and UV-digestion units, and a voltammetric analyzer and electrode controlled by a portable PC. Analysis of samples is completed using adsorptive cathodic stripping voltammetry (AdCSV). Using this portable AdCSV laboratory, the high sample throughput – approximately three per hour with standard additions completed on every sample – means that for ship-board surveys good spatial coverage is possible without the need for time-consuming station stops. For van-based tidal surveys, the portable AdCSV laboratory is both straightforward and low in cost to deploy.

Early-Middle Pleistocene deep circulation in the western subtropical Atlantic: southern hemisphere modulation of the North Atlantic Ocean

Abstract Early-Middle Pleistocene climate and deep ocean hydrography have been reconstructed using oxygen and carbon isotope ratio measurements in planktonic and benthic foraminifera from a high-deposition-rate sedimentary succession recovered at the Bermuda Rise, in the northern Sargasso Sea (Ocean Drilling Program Leg 172, Site 1063). The site’s water depth makes it sensitive to changes in the balance between North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) in the deep North Atlantic, allowing the evaluation of perturbations in deep water production at different timescales. Millennial-scale fluctuations, superimposed on the longer period oscillations of orbital origin, occurred during all the observed climate states. The highest amplitude sub-Milankovitch fluctuations were mainly associated with interglacials, particularly after the intensification of the glacial regime at c. 900 ka. Using the benthic carbon isotope signal as a water mass tracer, and by comparing the δ 13 C record to a suite of drill sites in the North Atlantic and Pacific oceans, it has been possible to infer that the relative strength of AABW production varied through time, as did that of NADW. A scenario in which the two different source components of deep water can undergo dramatic changes in their circulation regime through time should be taken into consideration when evaluating the role of thermohaline circulation in global climate change.

Neodymium isotopic composition and rare earth element concentrations in the deep and intermediate Nordic Seas: Constraints on the Iceland Scotland Overflow Water signature

Neodymium isotopic composition and rare earth element concentrations were measured in seawater samples from eleven stations in the Nordic Seas. These data allow us to study how the Iceland Scotland Overflow Water (ISOW) acquires its neodymium signature in the modern ocean. The waters overflowing the Faroe Shetland channel are characterized by ɛNd = −8.2 ± 0.6, in good agreement with the only other data point, published 19 years ago. In the Greenland and Iceland Seas the water masses leading to the formation of the ISOW display lower neodymium isotopic composition, with ɛNd around −11 and −9, respectively. Since no water masses in the Nordic Seas are characterized by ɛNd > −8, the radiogenic signature of the ISOW likely reflects inputs from the highly radiogenic Norwegian Basin basaltic margins (Jan‐Mayen, Iceland, Faroe, with ɛNd ≈ +7). In addition to the neodymium isotopic composition, the rare earth element patterns suggest that these inputs occur via the remobilization (which includes resuspension and dissolution) of sediments deposited on the margins. Whereas the neodymium isotopic composition behaves conservatively in the oceans in the absence of lithogenic inputs, and can be used as a water mass tracer, these results emphasize the role of interactions, between sediments deposited on margins and seawater, in the acquisition of the neodymium isotopic composition of water masses. These results should allow a better use of this parameter to trace the present and the past circulation in the North Atlantic.

Tracing water masses with 129I in the western Nordic Seas in early spring 2002

The distribution of 129I was utilized for labeling water masses in three sections of the western Nordic Seas. An increase of the tracer in Polar Waters of the East Greenland Current was observed between the Fram Strait and 72°N section and attributed to either unaccounted Polar Waters and/or recirculation of cold and fresh Atlantic Waters from the West Spitzbergen Current. Recent convection homogenized 129I in upper 1000 m of the Greenland Sea, and similar concentrations were observed in dense waters of the Denmark Strait. The densest outflow waters were not found in either the Greenland Sea or the East Greenland Current at 72°N.

U and Th concentrations and isotope ratios in modern carbonates and waters from the Bahamas

The short residence times of Th and Pa in seawater make them very responsive to changes in the ocean environment. We use a new multi-ion-counting technique to make Th and Pa isotope measurements in seawaters from a near-shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs (the Bahamas). An unusual feature of the Bahamas setting is the shallow depth of water residing on the bank tops. These waters have significantly lower 232Th/ 230Th (∼10,000) than those immediately adjacent to the banks (24,000–31,000) and a ( 231Pa/ 230Th) near the production ratio (∼0.1). The change in 232Th/ 230Th and ( 231Pa/ 230Th) on the bank tops is explained by almost quantitative removal of Th and Pa by scavenging, and their replacement with a mixture of 230Th and 231Pa alpha-recoiled from the underlying carbonates, together with Th from dust dissolution. Analysis of a water profile in the Tongue of the Ocean, which separates the Great and Little Bahama Banks, allows us to trace the movement of bank-top water to depth. A distinct minimum in both 232Th/ 230Th (∼13,000) and ( 231Pa/ 230Th) (∼0.5) is observed at ∼430 m and is interpreted to reflect density cascading of bank-top water with entrained carbonate sediment. These results suggest that Th and Pa can be used as water-mass tracers in near-shore environments. Uranium concentration measurements on the same waters demonstrate that U is conservative across a range in salinity of 2 psu, with a concentration of 3.33 ppb (at a salinity of 35). The incorporation of U and Th isotopes into marine carbonates has also been assessed by analyzing carbonate samples from the same location as these Bahamas waters. Such incorporation is critical for U-Th geochronology. U isotope analyses demonstrate that seawater δ 234U averages 146.6 and does not vary by more than 2.5%o, and that carbonates capture this value. Additional high precision measurements (≈±1%o) on modern carbonates confirm that all oceans have identical δ 234U. Modern marine carbonates are shown to have 232Th/ 230Th ratios that reflect the local seawater in which they formed.

Behaviour of the optical properties of coloured dissolved organic matter under conservative mixing

The optical properties of coloured dissolved organic matter (CDOM) can be used, in some environments, to trace water masses and provide information about the dynamics of the dissolved organic fraction in natural waters. This work presents the results from a modelling exercise, laboratory experiment and field data, which describe the variations in the optical properties of CDOM during mixing. The exponential slope coefficient ( S) is frequently used to characterise different CDOM pools; however, its behaviour during conservative mixing of two different CDOM types is often misunderstood. Identification of a theoretical conservative mixing line allows the rapid identification of non-conservative processes (e.g. in situ production, flocculation and degradation) acting on the pool during mixing. The results suggest that some of the patterns reported in the literature could purely be a result of conservative mixing rather than a product of non-conservative processes.

The Behavior of U- and Th-series Nuclides in Groundwater

Research Article| January 01, 2003 The Behavior of U- and Th-series Nuclides in Groundwater Donald Porcelli; Donald Porcelli Department of Earth Sciences, University of Oxford Parks Rd,. Oxford, OX1 3PR, United Kingdom, don.porcelli@earth.ox.ac.uk Search for other works by this author on: GSW Google Scholar Peter W. Swarzenski Peter W. Swarzenski Coastal Marine Geology Program, US Geological Survey, St. Petersburg, Florida, 33701, USA, pswarzen@usgs.gov Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2003) 52 (1): 317–361. https://doi.org/10.2113/0520317 Article history first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Donald Porcelli, Peter W. Swarzenski; The Behavior of U- and Th-series Nuclides in Groundwater. Reviews in Mineralogy and Geochemistry 2003;; 52 (1): 317–361. doi: https://doi.org/10.2113/0520317 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Groundwater has long been an active area of research driven by its importance both as a societal resource and as a component in the global hydrological cycle. Key issues in groundwater research include inferring rates of transport of chemical constituents, determining the ages of groundwater, and tracing water masses using chemical fingerprints. While information on the trace elements pertinent to these topics can be obtained from aquifer tests using experimentally introduced tracers, and from laboratory experiments on aquifer materials, these studies are necessarily limited in time and space. Regional studies of aquifers can focus on greater scales and time periods,... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Trace metal distributions off the Antarctic Peninsula in the Weddell Sea

Dissolved trace metals (Ag, Al, Cd, Co, Cu, Fe, Ni, Pb, and Zn), inorganic nutrients (PO4, H4SiO4), and chlorophyll a were measured at 19 stations along a surface water transect from the Antarctic Peninsula into the Weddell Sea. The range of concentrations of metals and nutrients measured along the western rim of the Weddell Sea were consistent with previous results from the Southern Ocean. Metal levels measured in the Weddell Sea showed two distinct patterns: (1) metals (Al, Co, and Pb) that generally had lower levels in the Weddell Sea; and (2) dissolved constituents (Ag, Cd, Cu, Fe, Ni, Zn, PO4, H4SiO4) that showed an enrichment in the Weddell Sea, as compared to other ocean basins. While this dichotomy suggested that high metal concentrations may result from natural processes, the impact of anthropogenic processes on metal levels in Antarctic waters is also evident. A comparison of the stable lead isotopic composition reported for surface waters of the Weddell Sea and our Southern Hemisphere aerosol samples suggested that the cycling of Pb in those waters has been influenced by industrial Pb from South America. The importance of biological activity on the cycling of bioactive metals in the Weddell Sea was suggested by the inverse relationship between chl a concentrations and trace metal residence time. A strong linear relationship between Cd and PO4 was observed, as in other oceans. The Cd/PO4 ratio along the western rim of the Weddell Sea was consistent with previous ratios reported for the northern part of the Weddell Sea and the Antarctic Circumpolar Current. However, those ratios were significantly higher than the previously reported Cd/PO4 ratio for the southern part of the Weddell Sea, suggesting that Cd/PO4 ratios within the same oceanographic basin are susceptible to temporal and spatial variability. We also showed that the Ag/Cu ratio could potentially be used as a geochemical tracer of different water masses in the world ocean. The Ag/Cu ratio in the Weddell Sea was essentially the same as the ratio reported for the Pacific Ocean, suggesting that Weddell Sea surface waters may influence the composition of trace metals in subsurface waters of the Pacific.

Iodine-129 Concentrations in Marginal Seas of the North Pacific and Pacific-influenced Waters of the Arctic Ocean

Water sampling during the 1993 IV Russian–US Joint Expedition to the Bering and Chukchi Seas (BERPAC) indicates that Pacific Ocean burdens of the long-lived radionuclide 129I are relatively low in the Pacific-influenced Arctic, particularly compared to high latitude waters influenced by the North Atlantic. These low concentrations occur despite the presence of potential submerged anthropogenic sources in the East Sea (Sea of Japan), and in the northwest Pacific Ocean, east of the Kamchatka Peninsula. The concentration of 129I entering the Arctic Ocean through Bering Strait, ∼0.7×10 8 atoms kg −1, is only slightly higher than observed in deep Pacific waters. Similar concentrations (0.44– 0.76×10 8 atoms kg −1 ) measured in Long Strait indicate no significant transfer of 129I eastward into the Chukchi Sea in the Siberian Coastal Current from the Siberian marginal seas to the west. However, the concentrations reported here are more than an order of magnitude higher than the Bering Strait input concentration estimated ( 1.0×10 6 atoms kg −1 ) from bomb fallout mass balances, which supports other existing evidence for a significant atmospheric deposition term for this radionuclide in surface ocean waters. Near-bottom water samples collected in productive waters of the Bering and Chukchi Seas also suggest that sediment regeneration may locally elevate 129I concentrations, and impact its utility as a water mass tracer. As part of this study, two deep 129I profiles were also measured in the East Sea in 1993–1994. The near-surface concentration of 129I ranged from 0.12 to 0.31×10 8 atoms kg −1 . The 129I concentration showed a steady decrease with depth, although because of active deep water ventilation, the entire 3000 m water column exceeded natural concentrations of the radionuclide. Atom ratios of 129I/ 137Cs in the East Sea also suggest an excess of 129I above bomb fallout estimates, also possibly resulting from atmospheric deposition ultimately originating from nuclear facilities.

Tracing water masses with particle trajectories in an isopycnic-coordinate model of the global ocean

Offline particle trajectories are obtained for a quasi-global isopycnic-coordinate OGCM using an analytical method, adapted for use with online time-integrated isopycnal and diapycnal mass fluxes. The method is highly efficient, allowing the calculation of large ensembles of such trajectories. These ensembles can be used to establish pathways and transformations associated with the global circulation of water masses on timescales which are well in excess of any feasible model integration length. The method is here used to investigate the important, yet poorly observed, transformation of North Atlantic Deep Water (NADW) through slow spreading, upwelling and diapycnal mixing (defined when and where density decreases below a threshold value). A fundamental problem arises through unsteadiness in the thickness of NADW layers (due to various model flaws and/or intrinsic variability). Particles converge on gridboxes where layers inflate during the online time-integration period. Depending on the degree of layer inflation, only a fraction of NADW particles can be diagnosed as transformed at some point along their respective trajectories. However, the unsteadiness of layer thickness decreases during a 50-year spin-up, implying fewer converged trajectories and an increased fraction of transformed NADW. Using trajectories to trace NADW southward across the equatorial Atlantic, with mass fluxes from years 10, 30 and 50 of model spin-up, the transformed percentage (of NADW exported from the North Atlantic) increases from 17–18% (with fluxes from years 10 and 30) to 41% (using year 50 fluxes). In the latter case, about 30% of the NADW upwells south of 30°S after 500–1000 years. Most of the remaining 70% upwells in the South and North Pacific after 1000–2500 years.