Source Water Masses Research Articles

Meridional and zonal changes in water properties along the continental slope off central and northern Chile

The Humboldt-09 cruise covered a narrow meridional band along the Chilean continental slope (44–23º S). Here we use physical and biochemical data from a long meridional section (4000 km) and three short zonal sections (100 km) to describe the distribution of the different water masses found in this region. Six water masses were identified: Subantarctic Water (SAAW), Summer Subantarctic Water (SSAW), Subtropical Water (STW), Equatorial Subsurface Water (ESSW), Antarctic Intermediate Water (AAIW), and Pacific Deep Water (PDW). For the first time, a novel set of source water mass properties (or water types) is introduced for SSAW, and nutrient and dissolved oxygen water types are proposed for all the water masses. Optimum multiparameter (OMP) analysis was used through an iterative process to obtain a sound definition of the water types that minimizes the residuals of the method. Both the classic OMP and the quasi-extended OMP models reproduced the data rather well. Finally, the spatial distribution of the different water masses was calculated with the quasi-extended OMP, which is not influenced by the respiration of organic matter. The distribution of the different water masses is presented over the meridional and zonal transects and in property-property diagrams. A smooth meridional transition from subantarctic to tropical and equatorial water masses is observed in this area. This transition takes place in surface, central, and intermediate waters over distances of the order of 1000 km. The meridional transition contrasts with the abrupt zonal changes found in the cross-slope direction, which are of comparable magnitude but over distances of the order of 100 km. Both AAIW and SAAW (fresh and well oxygenated) partially mix with the hypoxic ESSW and, therefore, play an important role in the ventilation of the southern part of the oxygen minimum zone.

Using Self-Organising Maps to investigate long-term changes in deep Adriatic water patterns

The paper attempts to document long-term changes in deep Adriatic water patterns by applying the Self-Organising Maps (SOM) method to temperature, salinity, dissolved-oxygen content, orthophosphate and total inorganic nitrogen profiles sampled at a single deep station in the South Adriatic Pit (SAP) over a half century (1957–2009). Seasonality observed in upper layers has been removed by the least-squares fitting of the annual and semi-annual sinusoidal functions. The sensitivity of the SOM to various parameter combinations reveals the importance of temperature, salinity and dissolved oxygen for mapping different water patterns, while nutrients have less influence on quality and applicability of SOM solutions to the extraction of characteristic SAP water profiles. The quality of fit obtained for different combination of the measured parameters introduced to a SOM suggests that the incomplete combinations of input parameters increase an imperfection in the applicability of SOMs to the dataset. Two modes of long-term changes in the SAP obtained by the SOM analyses are discussed with respect to the processes that drive the variability in the area, e.g., the Adriatic-Ionian Bimodal Oscillation: where the first mode is characterised by rapid changes in the transition of SAP water masses, observed before 1980s (less adoptable by the SOMs), and the second mode is characterised by steady transitions (better adoptable by the SOMs), observed in the 1990s and the 2000s. The SOM method is found to have certain advantages when compared to other methods that have previously been used to distinguish the Adriatic water masses, as it does not depend on predefinition of water mass sources and allows for gaps in series.

Thermocline circulation and ventilation of the Japan/East Sea, part II: A source water-mass mixing (SWAM) model

The recently obtained high resolution conductivity-temperature-depth (CTD), Argo and bottle data combined with historical geochemical data are used in a source water-mass mixing (SWAM) model for the Japan/East Sea (JES) thermocline. The water-mass properties for resolving model source water types include two physical conservative tracers: potential temperature θ (°C) and salinity S (psu), a dynamical tracer: fN 2 (10 6 s −3) (where f is the Coriolis frequency and N 2 is the squared buoyancy frequency), dissolved oxygen, O 2 (μmol kg −1), and nutrient silicate, H 4 SiO 4 (μmol kg −1) and two conservative chemical tracers: initial phosphate, P O 4 0 (μmol kg −1) and NO (μmol kg −1). The mixing scheme comprises three source water masses: North Pacific Subtropical Water (NPSW) which is the only invasion water mass from the open North Pacific and two locally formed water masses, Tatar Strait Water (TSW) and Peter the Great Bay Water (PGBW). The SWAM model is performed on three neutral density surfaces σ N =25.8, 26.4 and 27.0 encompassing the thermocline from about 50 to 180 dbar. The model-derived mixing fraction provides a quantitative description of the source water masses. Results show that NPSW contributes to only about one third of the mixing ratio while PGBW mixing proportion is more than 50%. This implies that most of NPSW is actually transformed and renewed in the JES by winter convection and probable brine rejection when NPSW is considered as a sole input source and PGBW and TSW are the transformed end-members of NPSW. Also it means that what we see the JES recirculation is actually the dominant PGBW water recirculation rather than the NPSW. The high mixing fraction of PGBW explains why JES water content is highly ventilated and has a very high oxygen and renewal rate. It is found that the JES transport is contributed by NPSW for 0.72±0.13 and 1.11±0.16 Sv by PGBW and 0.27±0.05 Sv by TSW, respectively. A total annual mean transport with an error bar is thus 2.10±0.34 Sv, very close to the Tsushima Current transport at 2.3–2.4 Sv observed in the KTS with a possible error of ±0.5 Sv. Additional study was carried out on a year-to-year basis to examine the temporal variability of the water-mass property and mixing for the last three quarters of the last century from 1925 to 2000. It is found that NPSW has undergone no substantial variation in most of its properties except for silicate which decreases significantly and therefore retains its open ocean characteristics. PGBW has experienced strong increase in potential temperature, salinity and oxygen and decrease in silicate. The same situation occurred to TSW except for its less significant increase in oxygen. The increase in thermocline oxygen contrasts to the oxygen decrease in deep water found in previous studies and is most likely the consequence of shallow overturning circulation under a general warming trend since weakening of potential vorticity fN 2 is also detected. The temporal variability of the water-mass mixing fractions reveals a remarkable discovery—TSW was once the major contributor of the water-mass formation, mixing and transformation in the northern and western Japan Basin prior to the warming episode started in the 1950s with a high fraction of 70–80%. TSW contributed also significantly to the Ulleung and Yamato Basins at that time with a fraction of 30% and 40%, respectively, compared to 20% and 30% in 2000. It was only after the 1950 s that PGBW started to take over TSW becoming the major player in the JES thermocline circulation and ventilation till today consistent with the annual mean situation discussed above.

Thermocline circulation and ventilation of the East/Japan Sea, part I: Water-mass characteristics and transports

Abstract The East/Japan Sea (EJS) has unique water-mass characteristics in the western Pacific marginal seas due to limited exchange with the open North Pacific. The major inflow of source water mass is North Pacific Subtropical Water (NPSW) carried by the Kuroshio branching and Tsushima Current. The locally formed cold/fresh waters from the Tatar Strait and Russia coast by winter convection mix with NPSW contributing to water-mass transformation, especially during winter when upper isopycnal surfaces outcrop and thermocline is ventilated. The geographic limit of the Korea/Tsushima Strait (KTS) with a sill depth of about 120-140 dbar confines the inflow of lower NPSW, and so the EJS thermocline layer is somewhat truncated with a rather thin layer for about 100 dbar. This study uses high resolution conductivity-temperature-depth (CTD) and Argo data with a third decimal or higher accuracy for temperature and salinity obtained mainly by Research Institute of Oceanography, Seoul National University through domestic and international collaboration with several Korean ocean research institutes and Russia and USA partners since early 1990s. The basin covered data were divided into summer and winter half-year representing seasonal difference since most CTD surveys were conducted purposely in summer and winter. Analysis is made for the upper layer, about 50-70 dbar, from the surface to the upper main thermocline and the thermocline layer down to about 150-180 dbar south of the subpolar front (SPF). The lower thermocline is defined slightly below the sill depth of the KTS, considering the deepening of NPSW after passing through the KTS. The thermocline layer is encompassed by three selected neutral density surfaces σN=25.8, 26.4 and 27.0 with a distance of about 40 dbar between two neighboring surfaces. The core of thermocline is followed by the σN=25.8 surface characterized by a salinity maximum and a minimum of potential vorticity. Winter convection is discussed and compared with three other major convection sites of the world's oceans, the Gulf of Lions, Labrador Sea and Greenland Sea, showing some common and distinctive features, especially the extremely low salinity of the EJS. Water-mass properties on neutral density surfaces are analyzed with the water-mass Turner angle (WTu) and circulation and transport are deducted from geostrophic calculations. From the 15-year mean hydrography, a basin-wide net annual mean transport of about 2.10±0.29 Sv (1 Sv=106 m3 s-1) is estimated with summer and winter transports of 2.56±0.36 and 1.63±0.23 Sv, respectively. This transport is slightly less than the annual mean transport of the Tsushima Current at the KTS, 2.4 Sv from cable and 2.3 Sv from other direct current meter and geostrophic methods but matches the ±14% error bar of ±0.29 Sv adjusted by ±150 dbar from the reference level of 800 dbar. This error bar is close to the error of ±0.34 Sv determined from water-mass conservation residual in a separated study. Three mechanisms are discovered to explain the seasonal difference in the Tsushima Current transports: the stronger winter Ekman pumping, outcropping and southward crossing flow. During winter, the Tsushima Current branches are imposed under strong wind stress curl in the Ulleung Basin and Yamato Basin, showing a doubling Ekman downwelling transport, partly weakening the Tsushima Current flow in the eastern boundary. Meanwhile the thermocline isopycnal surfaces outcrop in winter, reducing volume transport due to reduced space and thickness. The southward currents in the southern Ulleung Basin and Yamato Basin are perpendicular to the Tsushima Current branches west of Japan, which weakens the eastern boundary current in winter.

Synchronous Deglacial Overturning and Water Mass Source Changes

Understanding changes in ocean circulation during the last deglaciation is crucial to unraveling the dynamics of glacial-interglacial and millennial climate shifts. We used neodymium isotope measurements on postdepositional iron-manganese oxide coatings precipitated on planktonic foraminifera to reconstruct changes in the bottom water source of the deep western North Atlantic at the Bermuda Rise. Comparison of our deep water source record with overturning strength proxies shows that both the deep water mass source and the overturning rate shifted rapidly and synchronously during the last deglacial transition. In contrast, any freshwater perturbation caused by Heinrich event 1 could have only affected shallow overturning. These findings show how changes in upper-ocean overturning associated with millennial-scale events differ from those associated with whole-ocean deglacial climate events.

Simulation of Cavitation in Water Saturated Porous Media Considering Effects of Dissolved Air

An extension of a mathematical model for non-isothermal multiphase materials to consider the dissolution of air in liquid water and air mass sources during its desorption at lower water pressure is presented. The solid skeleton is assumed elasto-plastic; heat, water and air flows and water phase changes are taken into account. Physics of air dissolution and desaturation due to the air released from liquid water during cavitation in porous media are discussed. A numerical example where cavitation develops during shear band development in undrained water-saturated dense sands is solved with the developed model as discretized in space and time with the Finite Element Method.

Observation and origin of an interannual salinity anomaly in the Mozambique Channel

A positive salinity anomaly of 0.2 PSU was observed between 50 and 200 m over the years 2000–2001 across the Mozambique Channel at a section at 17°S which was repeated in 2003, 2005, 2006, and 2008. Meanwhile, a moored array is continued from 2003 to 2008. This anomaly was most distinct showing an interannual but nonseasonal variation. The possible origin of the anomaly is investigated using output from three ocean general circulation models (Estimating the Circulation and Climate of the Ocean, Ocean Circulation and Climate Advanced Modeling, and Parallel Ocean Program). The most probable mechanism for the salinity anomaly is the anomalous inflow of subtropical waters caused by a weakening of the northern part of the South Equatorial Current by weaker trade winds. This mechanism was found in all three numerical models. In addition, the numerical models indicate a possible salinization of one of the source water masses to the Mozambique Channel as an additional cause of the anomaly. The anomaly propagated southward into the Agulhas Current and northward along the African coast.

Siak River System — East-Sumatra: Characterisation of sources, estuarine processes, and discharge into the Malacca Strait

Interdisciplinary pollution studies were performed in the Siak River system in the Riau Province of East-Sumatra (Indonesia). Remote sensing investigations combined with in situ measurements in different seasons of the years 2004–2006 were focused on the identification of different sources of water masses in the tributaries and on the discharge into the estuary and Malacca Strait. Ship-borne measurements comprised the determination of the concentration and composition of optically active water constituents and water colour. Satellite data of different spectral and spatial resolution were implemented. Sources of different water masses such as humic substance dominated rivers, erosion areas of high suspended matter concentration and areas of limited bio-productivity are identified. Sources of humic substances are the rivers Tapung Kanan, Mandau, Siak Kecil, and Bukit Batu which are draining peatlands. The absorption coefficients of dissolved organic substances are partly (Siak Kecil, 51.8 m − 1 ) more than double of the currently admitted coefficients. The highest suspended matter concentrations near a channel between Siak and Siak Kecil leading to lowest transparency are caused by the estuarine turbidity maximum developing at the tidal front. The lowest Chlorophyll concentrations were measured between Pekanbaru and the channel, in and downstream of an industrial area. The concentration and distribution of water constituents are characterised by distinct regional patterns independent from the monsoon phases. The high absorption of dissolved organic humic substances originated from draining peatlands shifts the maximum of spectral reflectances to 700 nm leading to the extreme brownish to red-brownish water colour. High scattering of suspended particles increases the reflectance in the entire spectral range. The strong changes in the water colour enables satellite data of the visible spectral range to follow the distribution patterns of the Siak River discharge in the estuary which are strongly related to tidal phases. During low tide, the transport of Siak River water is directed into the Malacca Strait north-westwards and during high tide into Selat Panjang. Time series of daily available MODIS images have shown that the main transport direction in all seasons is the outflow into the Malacca Strait and further north-westwards following the large scale circulation.

Intense warming and salinification of intermediate waters of southern origin in the eastern subpolar North Atlantic in the 1990s to mid‐2000s

Recent thermohaline changes in the layer of intermediate waters (IW) advected into the eastern subpolar North Atlantic from lower latitudes are quantified using the data from the repeated transatlantic sections. Positive trends in temperature and salinity in the IW density class at ∼53°N (0.049°C/a and 0.0088/a, 1992–2002) and ∼60°N (0.044°C/a and 0.0085/a, 1997–2005) are derived. The unexpectedly high rates of the IW warming and salinification cannot be explained solely by the long‐term and recent decadal changes at the intermediate levels in the midlatitude North Atlantic and appear to be a consequence of the northward advance of the source water masses caused by the North Atlantic Oscillation‐induced contraction of the subpolar gyre.

Arctic vs. North Atlantic water mass exchanges in Fram Strait from Pb isotopes in sedimentsThis article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.

Surface sediment samples (n = 10), collected between Spitzbergen and Greenland, and two cores raised east (C04) and west (C16) from Fram Strait were analyzed for their chemical and isotopic (Pb) compositions to trace the source of sediments and water masses exchanging between the Arctic and the North Atlantic oceans. In surface sediments, variable major and trace element concentrations suggest variations in both the mineralogy (carbonate and quartz dilution of other silicate minerals) and source regions of detrital supplies, based on Th/Zr and, to a lesser extent, on Th/U ratios. Each core site shows specific but nearly constant Th/Zr ratios, indicating homogeneous source supplies. At both core sites, Pb concentrations and isotopic compositions display similar patterns: homogeneous low Pb and radiogenic crustal signals below 5–10 cm, contrasting with high Pb and less radiogenic anthropogenic inputs at core-tops. However, the differing pre-anthropogenic Pb isotopic ratios in C04 and C16 confirm the involvement of distinct source supplies east and west of Fram Strait. We suggest that this isotopic specificity is mainly owing to inputs of material carried from northwestern Europe by the North Atlantic water mass and from the Laptev Sea by the Transpolar Drift, respectively. Some material from the Greenland margin and possibly from the North Atlantic Ocean may reach this zone as well. Sediments from the western Arctic are not significantly transported into the Fram Strait area, suggesting that the Canadian and the Eurasian basins remained decoupled, at least during the time span of the cored sediments (∼2000 years).

The seasonal variability of the northern Benguela undercurrent and its relation to the oxygen budget on the shelf

The seasonal variations in the advection and mixing of water masses in the northern Benguela were studied in relation to the oxygen minimum zone over the Namibian shelf. The used data set consists of hydrographic and current measurements from an oceanographic mooring 20 nm off Walvis Bay, monthly CTD transects from the Namibian 23°S monitoring line and recent large-scale hydrographic surveys. The current time series showed an intermittent southward continuation of the Angola Current (AC) through the Angola–Benguela frontal zone (ABFZ) into the northern Benguela, commonly known as poleward undercurrent. In austral summer hypoxic, nutrient rich South Atlantic central water (SACW) from the Angola Gyre is transported into the northern Benguela, whereas during the winter season the oxygen rich Eastern SACW (ESACW) spreads northward. The water mass analysis reveals a mixing between both water masses in the northern Benguela between the ABFZ and the Lüderitz upwelling cell (27°S). The oxygen balance over the Namibian shelf depends to a high extent on the water mass composition of the upper central water layer, controlled by the large scale and local circulation. The deviation of the measured oxygen concentration from its mixing concentration, calculated with the source water mass properties, was used to quantify the oxygen consumption. A new local definition SACW was derived to exclude biogeochemical processes, taking place in the Angola Gyre. The oxygen deficit in the northern Benguela central water amounted to about 60–80 μmol l −1 at the shelf edge and increased up to 150 μmol l −1 on the shelf, due to local oxygen consumption. In the austral summer anoxic bottom waters are observed at the central Namibian shelf, which correlate to an SACW fraction >55%. Periods with high SACW fraction in the water mass composition were congruent with hydrogen sulphide events detected by remote sensing.

Decadal ventilation and mixing of Indian Ocean waters

Chlorofluorocarbon (CFC) and hydrographic data from the World Ocean Circulation Experiment (WOCE) Indian Ocean expedition are used to evaluate contributions to decadal ventilation of water masses. At a given density, CFC-derived ages increase and concentrations decrease from the south to north, with lowest concentrations and oldest ages in Bay of Bengal. Average ages for thermocline water are 0–40 years, and for intermediate water they are less than 10 years to more than 40 years. As compared with the marginal seas or throughflow, the most significant source of CFCs for the Indian Ocean south of 12°N is the Southern Hemisphere. A simple calculation is used to show this is the case even at intermediate levels due to differences in gas solubilities and mixing of Antarctic Intermediate Water and Red Sea Water. Bottom water in the Australia–Antarctic Basin is higher in CFC concentrations than that to the west in the Enderby Basin, due to the shorter distance of this water to the Adelie Land coast and Ross Sea sources. However, by 40°S, CFC concentrations in the bottom water of the Crozet Basin originating from the Weddell Sea are similar to those in the South Australia Basin. Independent observations, which show that bottom water undergoes elevated mixing between the Australia–Antarctic Basin and before entering the subtropics, are consistent with high CFC dilutions (3–14-fold) and a substantial concentration decrease (factor of 5) south to north of the Southeast Indian Ridge. CFC-bearing bottom waters with ages 30 years or more are transported into the subtropical South Indian Ocean by three western boundary currents, and highest concentrations are observed in the westernmost current. During WOCE, CFC-bearing bottom water reaches to about 30°S in the Perth Basin, and to 20°S in the Mascarene Basin. Comparing subtropical bottom water-CFC concentrations with those of the South Pacific and Atlantic oceans, at comparable latitudes, Indian Ocean bottom water-CFC concentrations are lower, consistent with its high dissipation rates from tidal mixing and current fluctuations as shown elsewhere. Thus, the generally high dilutions and low CFC concentrations in bottom water of the Indian Ocean are due to distance to the water mass source regions and the relative effectiveness of mixing. While it is not surprising that at thermocline, intermediate, and bottom levels, the significant ventilation sources on decadal time scales are all from the south, the CFCs show how local sources and mixing within the ocean affect the ventilation.

Last global extinction in the deep sea during the mid‐Pleistocene climate transition

Twenty percent (19 genera, 95 species) of cosmopolitan, deep‐sea (500–4000 m), benthic foraminiferal species became extinct during the late Pliocene–Middle Pleistocene (3–0.12 Ma), with the peak of extinctions (76 species) occurring during the mid‐Pleistocene Climate Transition (MPT, 1.2–0.55 Ma). One whole family (Stilostomellidae, 30 species) was wiped out, and a second (Pleurostomellidae, 29 species) was decimated with just one species possibly surviving through to the present. Our studies at 21 deep‐sea core sites show widespread pulsed declines in abundance and diversity of the extinction group species during more extreme glacials, with partial interglacial recoveries. These declines started in the late Pliocene in southern sourced deep water masses (Antarctic Bottom Water, Circumpolar Deep Water) and extending into intermediate waters (Antarctic Intermediate Water, North Atlantic Deep Water) in the MPT, with the youngest declines in sites farthest downstream from high‐latitude source areas for intermediate waters. We infer that the unusual apertural types that were targeted by this extinction period were adaptations for a specific kind of food source and that it was probably the demise of this microbial food that resulted in the foraminiferal extinctions. We hypothesize that it may have been increased cold and oxygenation of the southern sourced deep water masses that impacted on this deep water microbial food source during major late Pliocene and Early Pleistocene glacials when Antarctic ice was substantially expanded. The food source in intermediate water was not impacted until major glacials in the MPT when there were significant expansion of polar sea ice in both hemispheres and major changes in the source areas, temperature, and oxygenation of global intermediate waters.

Benthic foraminiferal fauna turnover at 2.1 Ma in the northern South China Sea

Quantitative analysis of benthic foraminifera from ODP Site 1146 in the northern South China Sea (SCS) shows that abundance of Bulimina alazanensis, sometimes up to about 90%, decreased gradually since 3.2 Ma, especially at 2.1 Ma. Abundance of other benthic foraminiferal species, Globobulimina subglobosa and Cibicidoides wuellerstorfi, increased after 2.1 Ma. Comparison with changes in oxygen and carbon isotopes of planktonic and benthic foraminifera shows that high abundance values of B. alazanensis corresponded with lower values of oxygen isotope, but for carbon isotope, high values of the species were consistent with heavier carbon isotope of benthic foraminifera and lighter carbon isotope of planktonic foraminifera, respectively, and vice versa. Considering factors such as uplift of Bashi Strait, expansion of the North Hemisphere Glaciation, strengthening of East Asian winter monsoon and variations in oxygen and carbon isotope of foraminifera, changes of B. alazanensis in ODP Site 1146 suggest that the source of deep water masses of the northern South China Sea changed from the warm Pacific deep water with high oxygen content to Pacific Intermediate water with low oxygen content at 2.1 Ma. In addition, the strengthened East Asian winter monsoon resulted in increased primary productivity, high nutrient and suboxic bottom water. Variations in species of B. alazanensis seemed to be unable to tolerate environmental stress induced by deep water masses and productivity changes.

Oxygen isotope composition of phosphate in organic compounds: Isotope effects of extraction methods

Organophosphorus (P org) is an important phosphorus (P) source in aquatic environments and P org cycling is an integral part of the overall biogeochemical cycle of P. It has been recently demonstrated that the 18O/ 16O ratio of PO 4 3 - ( δ 18O P) is a useful tracer of reactions and processes controlling the biogeochemical cycling of inorganic phosphate (P i) in natural waters. The goal of this work was to develop methods that can recover the δ 18O value of organically bound PO 4 3 - moiety groups in P org ( δ 18 O P org ). This work compares the oxygen (O) isotope effects of four different P i extraction methods: dry combustion, acid hydrolysis, UV radiation (UVR) and enzyme-catalyzed hydrolysis. The dry combustion method for releasing PO 4 3 - from P org promotes extensive oxygen isotope exchange between phosphate and atmospheric O 2 and thus, erases the original δ 18 O P org value. Both acid and enzyme-catalyzed hydrolysis of P org incorporate oxygen atoms from water into extracted P i and thus δ 18O values of P i extracted using these two methods only partially retain the original δ 18 O P org value. The oxygen isotope effect on P i extracted from P org using UVR methods shows that the original δ 18 O P org values of phosphomonoesters are retained in extracted PO 4 3 - moieties, and the oxygen isotope effects on P i extracted from phosphodiesters (e.g., due to water incorporation) are small. Thus, δ 18O values of PO 4 3 - moieties extracted from P org may be applied to trace P org sources and also to study water-mass sources and mixing processes.

Possible source of advected water mass and residence times in the multi‐structured Sea of Japan using rare earth elements

The Sea of Japan is a small, fast‐acting model of global oceanic circulation. Observations from 1998 and 2001 are analyzed to clarify its circulation. New bottom water formed in early 2001 off Vladivostok was observed later that year near the southeastern edge of the Japan Basin, implying a velocity of 2.3–3.9 cm s−1. Tb–Yb patterns of rare earth elements distinguish two classes of water volume: inactive old/well‐mixed water and active relatively young water, frequently affected by the subduction of the surface water originating from Tsushima Warm Current (TWC). The proportion of TWC to old water and each residence time are: 50–75% and ∼15 years (200–400 m), and 35–80% and ∼38 years (600–1500 m). Using six strata, an improved estimate of the average residence time of Japan Sea Proper Water is >86 years; this includes the contributions from active and inactive strata.

Transport of South Atlantic water through the passages south of Guadeloupe and across 16°N, 2000–2004

CTD, vessel-mounted ADCP and LADCP measurements in the Caribbean passages south of Guadeloupe (three repeats) and along 16°N (five repeats) were carried out between December 2000 and July 2004. The CTD data were used to calculate the contribution of South Atlantic water (SAW) in the upper 1200 m between the isopycnals σ θ = 24.5 and 27.6. Northern and southern source water masses are defined and an isopycnal mixing approach is applied. The SAW fractions are then combined with the ADCP flow field to calculate the transport of SAW into the Caribbean and across 16°N. The SAW inflow into the Caribbean through the passages south of Guadeloupe ranges from 7.6 to 11.6 Sv, which is 50–75% of the total inflow. The mean (9.1±2.2 Sv) is in the range of previous estimates. Ambiguities in the northern and southern source water masses of the salinity maximum water permitted us only to calculate the contribution of SAW from the eastern source in this water mass. We estimated the additional SAW transport by the western source to be of the order of 1.9±0.7 Sv. The calculation of the SAW transport across 16°N was hampered by the presence of several anticyclonic rings from the North Brazil Current (NBC) retroflection region, some of the rings were subsurface intensified. Provided that the rings observed at 16°N are typical rings and that all rings which are annually produced in the NBC retroflection area (6.5–8.5 per year) reach 16°N, the SAW ring transport across 16°N is calculated to 5.3±0.7 Sv. From the 5 repeats at 16°N, only two showed a net northward flow, suggesting that the mean northward SAW transport is dominated by ring advection. The joint SAW transports of the Caribbean inflow (9.1 Sv) and the flow across 16°N (5.3 Sv) sum up to 14.4 Sv. The transport increases to 16.3 Sv if the additional SAW transport from the western source of SMW (1.9±0.7 Sv) is included. These transport estimates and the following implications depend strongly on the assumption that the surface water in the Caribbean inflow is of South Atlantic origin. The transport estimates are, however, in the range of the inverse model calculations for the net cross-hemispheric flow. About 30–40% of this transport is intermediate water from the South Atlantic, presumably supporting studies which found the contributions of intermediate and upper warm water to be of a comparable magnitude. For the upper warm water ( σ θ < 27.1 ), the Caribbean inflow seems to be the major path (7.9±1.6 Sv), the ring induced transport across 16°N is about 30% of that value. The intermediate water transport across 16°N was calculated to be 2.3–3.6 Sv, the inflow into the Caribbean is slightly smaller (1.5–2.4 Sv).

Variability of dense water formation in the Ross Sea

This paper presents results from a model study of the interannual variability of high salinity shelf water (HSSW) properties in the Ross Sea. Salinity and potential temperature of HSSW formed in the western Ross Sea show oscillatory behaviour at periods of 5–6 and 9 years superimposed on long-term fluctuations. While the shorter oscillations are induced by wind variability, variability on the scale of decades appears to be related to air temperature fluctuations. At least part of the strong decrease of HSSW salinities deduced from observations for the period 1963–2000 is shown to be an aliasing artefact due to an undersampling of the periodic signal. While sea ice formation is responsible for the yearly salinity increase that triggers the formation of HSSW, interannual variability of net freezing rates hardly affects changes in the properties of the resulting water mass. Instead, results from model experiments indicate that the interannual variability of dense water characteristics is predominantly controlled by variations in the shelf inflow through a sub-surface salinity and a deep temperature signal. The origin of the variability of inflow characteristics to the Ross Sea continental shelf can be traced into the Amundsen and Bellingshausen Seas. The temperature anomalies are induced at the continental shelf break in the western Bellingshausen Sea by fluctuations of the meridional transport of circumpolar deep water with the eastern cell of the Ross Gyre. In the Amundsen Sea, upwelling due to a persistently cyclonic wind field carries the signal into the surface mixed layer, leading to fluctuations of the vertical heat flux, anomalies of brine release near the sea ice edge, and consequently to a sub-surface salinity anomaly. With the westward flowing coastal current, both the sub-surface salinity and deep temperature signals are advected onto the Ross Sea continental shelf. Convection carries the signal of salinity variability into the deep ocean, where it interacts with modified circumpolar deep water upwelled onto the continental shelf as the second source water mass of HSSW. Sea ice formation on the Ross Sea continental shelf thus drives the vertical propagation of the signal rather than determining the signal itself.

Glacial water mass geometry and the distribution of δ13C of ΣCO2 in the western Atlantic Ocean

Oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa Cibicidoides and Planulina from 25 new piston cores, gravity cores, and multicores from the Brazil margin. The cores span water depths from about 400 to 3000 m and intersect the major water masses in this region. These new data fill a critical gap in the South Atlantic Ocean and provide the motivation for updating the classic glacial western Atlantic δ13C transect of Duplessy et al. (1988). The distribution of δ13C of ΣCO2 requires the presence of three distinct water masses in the glacial Atlantic Ocean: a shallow (∼1000 m), southern source water mass with an end‐member δ13C value of about 0.3–0.5‰ VPDB, a middepth (∼1500 m), northern source water mass with an end‐member value of about 1.5‰, and a deep (&gt;2000 m), southern source water with an end‐member value of less than −0.2‰, and perhaps as low as the −0.9‰ values observed in the South Atlantic sector of the Southern Ocean (Ninnemann and Charles, 2002). The origins of the water masses are supported by the meridional gradients in benthic foraminiferal δ18O. A revised glacial section of deep water δ13C documents the positions and gradients among these end‐member intermediate and deep water masses. The large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation.

Circulation in the Southern Ocean during the Paleogene inferred from neodymium isotopes

Long-term records of neodymium (Nd) isotopes from sedimentary archives can be influenced by both changes in water mass mixing and continental weathering. Results of Nd isotopic analyses of fossil fish teeth from ODP Site 689 (Maud Rise, Southern Ocean) provide a long, continuous, high-resolution marine sediment Nd isotope record (expressed in ɛ Nd units). Correlation of down core secular variations between the ɛ Nd record, δ 13C values from benthic foraminifera, and clay mineral assemblages demonstrates that long-term variability of Nd isotope ratios reflect changes in ocean circulation, and that only minor fluctuations in ɛ Nd values are associated with changes in continental weathering on Antarctica. Nonradiogenic ɛ Nd values at Site 689 during the middle Eocene require the contribution of an end member with ɛ Nd<−9.5. Southern Ocean deep water may have been too radiogenic in the middle Eocene ( ɛ Nd=−8.5), though this end member may not be fully characterized. A possible source of deep water outside of the Southern Ocean in the middle Eocene is the Tethys Sea ( ɛ Nd=−9.3 to −9.8). The presence of Warm Saline Deep Water (WSDW) on Maud Rise is consistent with the Nd isotope results. The onset of more radiogenic ɛ Nd values at ∼40.8 Ma coincides with other changes at Site 689 which are consistent with a switch from a warm bottom water mass in the middle Eocene to a colder bottom water mass in the late middle Eocene. A rapid shift to radiogenic ɛ Nd values beginning at 37 Ma is best explained by the opening of Drake Passage. The shift coincides with increases in phytoplankton production throughout the Atlantic sector of the Southern Ocean that document the development of upwelling cells presumably related to more effective latitudinal circulation. After the Eocene/Oligocene boundary when large-scale ice sheets developed on Antarctica, Southern Ocean sourced water masses, such as Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW), had a greater influence on the hydrography of the study area. An early Oligocene trend to nonradiogenic compositions resulted in similar values to the modern ɛ Nd values of these water masses. The modern ɛ Nd values of AAIW and AABW reflect a significant contribution of North Atlantic Deep Water (NADW), thus decreasing ɛ Nd values in the early Oligocene may have resulted from the export of Northern Component Water (NCW, similar to modern NADW). During the late Oligocene and early Miocene, the long-term trends of the record follow benthic δ 13C values. Variability in the Nd isotope record most likely reflects fluctuations in ocean circulation arising from changes in the relative contributions of different end member water masses to the Southern Ocean. An interval where ɛ Nd values and δ 13C values are not correlated may reflect the influence of a short-lived weathering event on the ɛ Nd record. Early Miocene ɛ Nd values resemble those of modern Southern Ocean water masses, indicating a shift toward present-day patterns of ocean circulation.