IAPSO Standard Seawater Research Articles

Vacuum extraction of high-salinity water for the determination of oxygen and hydrogen isotopic compositions using cavity ring-down spectroscopy

Tracking the hydrogen and oxygen isotopic compositions of water samples can aid in understanding the water cycle. Compared to that of fresh water, analyzing the hydrogen and oxygen isotopic composition of high-salinity water is still a challenging task owing to the influence of dissolved salts. In this study, desalination was achieved by vacuum extraction, and the oxygen and hydrogen isotope ratios (δ18O and δ2H) were measured using cavity ring-down spectroscopy (CRDS). Different solutions of the individual salts (NaCl, KCl, MgCl2, and CaCl2) were prepared with deionized water (DI) having a known stable isotopic composition. After vacuum extraction, the maximum differences in isotopic composition was no more than 0.09‰ for δ18O and 0.30‰ for δ2H at 4 mol/L NaCl and KCl and 1 mol/L MgCl2 and CaCl2. The isotopic compositions of two high-salinity samples (IAPSO standard seawater and brine water) were determined by the proposed procedure as well as thermal conversion/elemental analyzer-isotope ratio mass spectrometry for comparison. The δ18O and δ2H results obtained using the two methods were consistent. The influence of isotope-salts was completely overcome by the proposed vacuum extraction desalination procedure. Thus, the method enables the rapid extraction of a large number of samples, regardless of the type and origin of the salt solutions, provided the concentrations of MgCl2 and CaCl2 are below 1.0 mol/L.

Trace element and Sr isotopic composition of bottom and near-surface oceanic water in the southern region of the Emperor Ridge

During the 86th voyage of the research vessel “Akademik M.A. Lavrentiev” (July–August 2019), near-bottom and near-surface water samples (at a depth of 10 m) were taken in the immediate vicinity and above the Koko, Jingu, Nintoku, and Suiko seamounts in the southern part of the Emperor seamount chain. Trace element concentrations (including REE) in these samples are higher than in the IAPSO Standard Atlantic Seawater. The distribution and mutual correlations of elements involved in biochemical processes indicate an active role of plankton in the accumulation of these elements, as well as an increase in its amount with increasing water temperature. 87Sr/86Sr ratios in water samples vary from 0.70891 to 0.70917. Moreover, near-bottom water, taken farther south of 44°N, is more radiogenic than near-surface. This difference can be explained by the dilution of the surface layer with meteoric water, because at the time of sample collection over the Koko, Jingu, and Nintoku seamounts it was raining daily, while no rainfall was observed over the Suiko seamount during the sample collection and the week before it. The rapid restoration of isotopic equilibrium indicates a high rate of vertical mixing of ocean water, which was indicated by the earlier studies of oceanic water chemical composition.

A High Precision Method for Calcium Determination in Seawater Using Ion Chromatography

Calcium (Ca2+) is an important major cation in seawater, which is closely related to the oceanic biogeochemistry cycle. Direct and accurate determination of Ca2+ concentration is required for a more comprehensive study of the carbonate system in seawater. Due to the high background concentration of Ca2+ in seawater and small variances of Ca2+ during CaCO3 precipitation and dissolution process, a precision of better than 0.1% (of approximately ±10 µmol/kg) is very much desired for carbonate chemistry related studies. In this study, a simple, non-toxic and labor-saving technique using ion chromatography (IC) has been developed to determine Ca2+ in seawater with an overall precision of better than 0.1%. This proposed method, using IAPSO standard seawater as a reference, can get a result within 15 min and only requires a small sample volume (~1 ml). The concentrations and flow rates of the eluent have been optimized to achieve the best chromatographic separation (20 mmol/L and 1.0 mL/min were selected in this study). Our work suggests that sample dilutions have no discernible effect on Ca2+ determinations. However, the measured Ca2+ concentration shows a linear decrease with the increasing Mg/Ca ratio in samples, which could be corrected by a derived formula to achieve high accuracy within 0.1%. This optimized method has been applied to the analysis of Ca2+ distribution in Southwest Indian Ocean and the laboratory study on the calcite precipitation in seawater.

Anomalous salinity-density relations of seawater in the eastern central Atlantic

In the context of the recent oceanographic standard TEOS-10, the Thermodynamic Equation of Seawater – 2010, a new Reference-Composition Salinity Scale was introduced in 2008. For IAPSO Standard Seawater, values of this Reference Salinity are related to 1978 Practical Salinity by a definite factor, however, regional seawater samples may deviate from this equation. Over the past decade, various such salinity anomalies were investigated by direct density measurements and published in several studies. Typically, anomalies arise from non-dissociated solutes such as silicate which increase seawater density but not electrical conductivity. In contrast, negative density anomalies cannot be explained by added, electrically neutral salts, and their chemical or metrological origins remain elusive. In this paper, densities and Practical Salinities of seawater of the central eastern Atlantic Ocean in the region 18 °S to 33 °N, 22 °W to 12 °E are reported, measured between 2008 and 2013. Density anomalies in the range from 0 to –13 g m-3 were observed, consistent in magnitude with negative anomalies found by other authors elsewhere. No significant spatial or temporal trends of the anomalies could be observed.

Stability of stored salinity samples

AbstractOceanographers measure water sample salinity in a laboratory environment to calibrate conductivity sensors or as a sole measure of in situ salinity. Established practice compares instrument performance against IAPSO Standard Seawater standards. Secondary standards can be used to reduce costs associated with assessing performance. However, results from our studies show the use of a secondary standard can also track the potential impact of prolonged sample storage. Initial results from secondary standard samples, used to trace instrument drift, showed erratic salinities. Two storage experiments were conducted to determine the potential cause of these results, and establish stability of stored salinity samples. The cause was identified as the plastic insert used to seal the glass bottles that originated from a faulty batch. We recommend that, where salinity samples are stored for subsequent analysis (longer than 1 month in our storage conditions), a secondary standard is used to track instrument drift and sample storage as a matter of course.

Chinese Primary Standard Seawater: Stability checks and comparisons with IAPSO Standard Seawater

The authors give a brief introduction to the Chinese Primary Standard Seawater, with a description of its preparation procedures. IAPSO Standard Seawater (IAPSO SSW), was taken as a stable reference in the stability check of Chinese Primary Standard Seawater (CP SSW), and linear regression model as well as hypothesis testing were introduced into the analysis of check results; a demonstration check of CP SSW (batch number P8) achieved a positive conclusion. In comparisons of several batches of these two kinds of standard seawater on Practical Salinity, identical seawater samples from a homogeneous source were measured repeatedly. To evaluate the comparison results, performance criteria referred to as En numbers were adopted, the maximum of which was 0.42, indicating that no significant differences lay between these two kinds of SSWs when used to determine Practical Salinity. Measures taken to assure the reliability of measurement results are presented.

Simultaneous determinations of fluorine, chlorine, and sulfur in rock samples by ion chromatography combined with pyrohydrolysis

We present a simplified method for simultaneous determinations of fluorine (F), chlorine (Cl), and sulfur (S) contents and isotopic abundances in rock samples. After using pyrohydrolysis to extract F, Cl, and S from a sample and transfer it into a sodium hydroxide (NaOH) solution, concentrations of F, Cl, and S as sulfate are measured by ion chromatography. The method is applicable to a variety of rock materials, including peridotite, basalt, gabbro, andesite, rhyolite, sulfide minerals, and possibly to sea salt, with F, Cl, and S contents of several μg g-1 to several tens of weight percent in 40 to 250 mg of rock powder. F, Cl, and S contents obtained by 5-7 repeated analyses of each of nine geological reference materials were in good agreement with previously reported values. In certain reference materials, sulfite peaks were observed, indicating incomplete oxidation to sulfate during S extraction by pyrohydrolysis. However, total S contents in those samples analysed by ICP-AES were also in good agreement with previous reported values. The RSDs of F, Cl, and S contents in reference materials were <6%, except for samples with F content <10 μg g-1 (∼20%) or S content <∼20 μg g-1 (<20%). In addition, Cl isotope compositions of seawater (IAPSO Standard Seawater and Pacific Ocean water) from sample solutions in this study were identical within error to reported values, indicating that Cl isotopes do not fractionate during pyrohydrolysis. After small isotopic fractionation corrections, multiple isotope compositions of S (32S, 33S, 34S and 36S) in solutions of sulfide (IAEA-S-1, IAEA-S-2, and IAEA-S-3), IAPSO Standard Seawater, and gabbro (JGb-1), also agree well with their respective reported values. Thus, this method is applicable to obtain accurate Cl and S contents as well as their isotopic compositions from a single aliquot of sample.

The IAPWS industrial formulation for the thermodynamic properties of seawater

In 2008, the International Association for the Properties of Water and Steam (IAPWS) adopted a standard formulation for the thermodynamic properties of seawater as a sum of contributions to the Gibbs free energy from pure water and from dissolved sea salt. For pure water, the IAPWS formulation for general and scientific use (IAPWS-95) was used. However, for industrial uses such as desalination and seawater power-plant cooling, it is likely to be more convenient to use the computationally simpler IAPWS formulation for industrial use (IAPWS-IF97), which is standard in the steam power industry. This paper documents this approach and gives formulas for calculating thermodynamic properties of seawater and steam (volume, enthalpy, isobaric heat capacity, etc.). The calculation of colligative properties (such as boiling and freezing points and osmotic pressure) is also described, as is the calculation of properties of two-phase states such as brine-vapor and brine-ice (sea ice). The computing speeds for these calculations are faster than those using IAPWS-95 by factors on the order of 100–200. The use of IAPWS-IF97 instead of IAPWS-95 for industrial seawater calculations is endorsed in IAPWS Advisory Note No. 5: Industrial Calculation of the Thermodynamic Properties of Seawater. This use is valid for IAPSO Standard Seawater with sea salt of the Reference Composition in specified regions inside the pressure, temperature, and salinity ranges of 0.3 kPa ≤ p ≤ 100 MPa, 261 K ≤ T ≤ 353 K, and 0 ≤ S ≤ 0.12 kg kg−1.

Direct chromatographic separation and quantification of calcium and magnesium in seawater and sediment porewaters

Direct analysis of Ca2+ and Mg2+ is required for accurate determination of metastable carbonate mineral phase saturation states (ΩCaCO3; ΩMgCO3) in seawater, sediment porewaters, and other high ionic strength brines. To this end, we have implemented a method using High Performance Chelation Ion Chromatography (HPCIC) in which metal ion complexation at the stationary phase renders separation efficiency insensitive to high ionic strength matrix effects common to other ion chromatography (IC) methods. This method, using direct automated on‐column injection, vastly increases sample throughput capacity in comparison to current titration methods. Calcium and magnesium ions in IAPSO standard seawater were selectively separated using a monolithic silica column (100 × 4.6 mm ID) activated with a covalently bonded iminodiacetic acid (IDA) chelator. The colored ion complexes resulting from post‐column reaction (PCR) of the ions with a metallochromic indicator, in this case 4‐(2‐pyridylazo)‐resorcinol (PAR), were detected spectrophotometricaly at 510 nm. Optimization of flow rate, eluent concentration, pH, and sample injection volume allowed baseline separation of Mg2+(0.05474 mol kg−1) and Ca2+ (0.01065 mol kg−1) in less than 8 min using 2 µL seawater sample injections. At a flow rate of 1 mL min−1, peak elutions occurred respectively at 4 and 5 min, using an eluent containing 0.1 M potassium chloride and 1 mM nitric acid adjusted to pH 2.5. Retention time variability below 0.5% for both metals following more than 200 injections indicates long‐term stability of the derivatized monolithic silica column. Method application to marine sediment porewaters is discussed.

Precise determination of triple Sr isotopes (δ87Sr and δ88Sr) using MC-ICP-MS

The non-traditional stable strontium (Sr) isotopes have received increasing attention recently as new geochemical tracers for studying Sr isotopic fractionation and source identification. This has been attributed to the advancement in multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), allows to determine precisely and simultaneously of the triple Sr isotopes. In this study, we applied a modified empirical external normalization (EEN) MC-ICPMS procedure for mass bias correction in Sr isotopic measurement using 92Zr/90Zr. High-purity Zr Standard was spiked into sample solutions and the degree of fractionation was calculated off-line using an exponential law. The long-term external reproducibility for NIST SRM 987 δ87Sr and δ88Sr was better than 0.040‰ and 0.018‰ (2SD), respectively. The IAPSO standard seawater was used as a secondary standard to validate the analytical protocol and the absolute ratios measured were 0.709161±0.000018 for 87Sr/86Sr, 0.177±0.021‰ for δ87Sr, and 0.370±0.026‰ for δ88Sr (2SD, n=7). These values are in good agreement with the literature data analyzed by thermal ionization mass spectrometry (TIMS) double spike technique. Rock standards, BHVO-2, BCR-2 and AGV-2 were also analyzed to validate the robustness of the methodology and showed identical results with literature data. Compared to previous 91Zr/90Zr correction, we obtained improved results based on 92Zr/90Zr, probably due to similar mass difference between 92Zr/90Zr and measured Sr isotopes. The new analytical protocol presented in this study not only improves the analytical precision but also increases sample efficiency by omitting the use of the standard-sample bracketing (SSB) procedure.

Metrological traceability of oceanographic salinity measurement results

Abstract. Consistency of observed oceanographic salinity data is discussed with respect to contemporary metrological concepts. The claimed small uncertainty of salinity measurement results traceable to the conductivity ratio of a certified IAPSO Standard Seawater reference is not metrologically justified if results are compared on climatic time scales. This applies in particular to Practical Salinity SP, Reference Salinity SR, and the latest estimates of Absolute Salinity using the TEOS-10 formalism. On climate time scales an additional contribution to the uncertainty that is related to unknown property changes of the reference material must be accounted for. Moreover, when any of these measured or calculated quantity values is used to estimate Absolute Salinity of a seawater sample under investigation, another uncertainty contribution is required to quantify the accuracy of the equations relating the actually measured quantity to the Absolute Salinity. Without accounting for these additional uncertainties, such results cannot be used to estimate Absolute Salinity with respect to the International System of Units (SI), i.e. to the unit chosen for the mass fraction of dissolved material in the sample, which is "g kg−1". From a metrological point of view, such deficiencies in the calculations involving other quantities will produce SI-incompatible results. We outline how these problems can be overcome by linking salinity to primary SI measurement standards.

Absolute Salinity, ''Density Salinity'' and the Reference-Composition Salinity Scale: present and future use in the seawater standard TEOS-10

Abstract. Salinity plays a key role in the determination of the thermodynamic properties of seawater and the new TEOS-101 standard provides a consistent and effective approach to dealing with relationships between salinity and these thermodynamic properties. However, there are a number of practical issues that arise in the application of TEOS-10, both in terms of accuracy and scope, including its use in the reduction of field data and in numerical models. First, in the TEOS-10 formulation for IAPSO Standard Seawater, the Gibbs function takes the Reference Salinity as its salinity argument, denoted SR, which provides a measure of the mass fraction of dissolved material in solution based on the Reference Composition approximation for Standard Seawater. We discuss uncertainties in both the Reference Composition and the Reference-Composition Salinity Scale on which Reference Salinity is reported. The Reference Composition provides a much-needed fixed benchmark but modified reference states will inevitably be required to improve the representation of Standard Seawater for some studies. However, the Reference-Composition Salinity Scale should remain unaltered to provide a stable representation of salinity for use with the TEOS-10 Gibbs function and in climate change detection studies. Second, when composition anomalies are present in seawater, no single salinity variable can fully represent the influence of dissolved material on the thermodynamic properties of seawater. We consider three distinct representations of salinity that have been used in previous studies and discuss the connections and distinctions between them. One of these variables provides the most accurate representation of density possible as well as improvements over Reference Salinity for the determination of other thermodynamic properties. It is referred to as "Density Salinity" and is represented by the symbol SAdens; it stands out as the most appropriate representation of salinity for use in dynamical physical oceanography. The other two salinity variables provide alternative measures of the mass fraction of dissolved material in seawater. "Solution Salinity", denoted SAsoln, is the most obvious extension of Reference Salinity to allow for composition anomalies; it provides a direct estimate of the mass fraction of dissolved material in solution. "Added-Mass Salinity", denoted SAadd, is motivated by a method used to report laboratory experiments; it represents the component of dissolved material added to Standard Seawater in terms of the mass of material before it enters solution. We also discuss a constructed conservative variable referred to as "Preformed Salinity", denoted S∗, which will be useful in process-oriented numerical modelling studies. Finally, a conceptual framework for the incorporation of composition anomalies in numerical models is presented that builds from studies in which composition anomalies are simply ignored up to studies in which the influences of composition anomalies are accounted for using the results of biogeochemical models. 1TEOS-10: international Thermodynamic Equation of Seawater 2010, http://www.teos-10.org/.

A model for predicting changes in the electrical conductivity, practical salinity, and absolute salinity of seawater due to variations in relative chemical composition

Abstract. Salinity determination in seawater has been carried out for almost 30 years using the Practical Salinity Scale 1978. However, the numerical value of so-called practical salinity, computed from electrical conductivity, differs slightly from the true or absolute salinity, defined as the mass of dissolved solids per unit mass of seawater. The difference arises because more recent knowledge about the composition of seawater is not reflected in the definition of practical salinity, which was chosen to maintain historical continuity with previous measures, and because of spatial and temporal variations in the relative composition of seawater. Accounting for these spatial variations in density calculations requires the calculation of a correction factor δSA, which is known to range from 0 to 0.03 g kg−1 in the world oceans. Here a mathematical model relating compositional perturbations to δSA is developed, by combining a chemical model for the composition of seawater with a mathematical model for predicting the conductivity of multi-component aqueous solutions. Model calculations for this estimate of δSA, denoted δSRsoln, generally agree with estimates of δSA based on fits to direct density measurements, denoted δSRdens, and show that biogeochemical perturbations affect conductivity only weakly. However, small systematic differences between model and density-based estimates remain. These may arise for several reasons, including uncertainty about the biogeochemical processes involved in the increase in Total Alkalinity in the North Pacific, uncertainty in the carbon content of IAPSO standard seawater, and uncertainty about the haline contraction coefficient for the constituents involved in biogeochemical processes. This model may then be important in constraining these processes, as well as in future efforts to improve parameterizations for δSA.

Consistency of practical salinity measurements traceable to primary conductivity standards: Euromet project 918

Measurements of practical salinity are based on conductivity measurements, which are traceable to the conductivity of a defined KCl reference solution. The conductivity of this reference solution must be traceable to the SI in order to state a reliable uncertainty for practical salinity measurements and to guarantee their comparability. Currently, the conductivity ratios from various old standard seawater batches are compared instead. However this method is inadequate on a long time-scale, which is important for oceanography studies, because it lacks a time-invariant reference. Therefore the equivalence of conductivity measurements traceable to primary conductivity standards has been investigated in Euromet project 918. Several European metrology institutes measured the conductivity of a 5 S/m KCl solution and the practical salinity of IAPSO standard seawater. The study estimates relative standard uncertainties to be of the order of 10−4 to 10−3 for such measurements.

Development of micro-fluidic nitrate-selective sensor based on doped-polypyrrole nanowires

Abstract This paper presents a flow-through, nitrate-selective, electrochemical sensor developed by electropolymerization (doping) of polypyrrole nanowire (PPy-NW) electrodes. The flow-through micro-fluidic platform enabled a realistic and controlled testing environment. Cyclic voltammetry, amperometry and flow-through analysis were performed to evaluate the sensor performance for determination of nitrate ions in two sets of calibration solutions (DI water and IAPSO standard seawater). The sensor response was linear over the range from 10 μM (0.14 ppm Nitrate-N) to 1 mM (14 ppm Nitrate-N), with a sensitivity of 1.17–1.65 nA/μM and detection limit of 4.5 ± 1 μM or 0.063 ± 0.014 ppm Nitrate-N. The electrochemically doped PPy-NW electrodes show improved selectivity for chloride, sulfate, phosphate and perchlorate ions, without the loss from interfering ions.

The composition of Standard Seawater and the definition of the Reference-Composition Salinity Scale

Fundamental determinations of the physical properties of seawater have previously been made for Atlantic surface waters, referred to as “Standard Seawater”. In this paper a Reference Composition consisting of the major components of Atlantic surface seawater is determined using these earlier analytical measurements. The stoichiometry of sea salt introduced here is thus based on the most accurate prior determination of the composition, adjusted to achieve charge balance and making use of the 2005 atomic weights. Reference Seawater is defined as any seawater that has the Reference Composition and a new Reference-Composition Salinity S R is defined to provide the best available estimate of the Absolute Salinity of both Reference Seawater and the Standard Seawater that was used in the measurements of the physical properties. From a practical point of view, the value of S R can be related to the Practical Salinity S by S R = ( 35.16504 / 35 ) g kg - 1 × S . Reference Seawater that has been “normalized” to a Practical Salinity of 35 has a Reference-Composition Salinity of exactly S R=35.16504 g kg −1. The new independent salinity variable S R is intended to be used as the concentration variable for future thermodynamic functions of seawater, as an SI-based extension of Practical Salinity, as a reference for natural seawater composition anomalies, as the currently best estimate for Absolute Salinity of IAPSO Standard Seawater, and as a theoretical model for the electrolyte mixture “seawater”.

IAPSO Standard Seawater: Definition of the Uncertainty in the Calibration Procedure, and Stability of Recent Batches

Abstract Standard seawater (SSW) has been employed by oceanographers as a reference material in the determination of salinity for over a century. In all that time, this is the first study to determine the uncertainty of the SSW manufacturing process. SSW is calibrated in reference to carefully prepared solutions of potassium chloride (KCl). All uncertainties in the preparation and measurement of KCl solutions and of new SSW are calculated. The expanded uncertainty of the SSW conductivity ratio is found to be 1 × 10−5, based on a coverage factor of 2, at the time of manufacture. There is no discernible “within batch” variability. No significant variability of quality within or between batches of KCl is found. Measurement of SSW “offsets” from the label conductivity ratio as long as 5 yr after the SSW batch manufacture are reported, and no significant change in label conductivity ratio for SSW batches P130 through P144 outside the expanded uncertainty of 1 × 10−5 is found. This last result is in contrast to some other studies, and herein are suggestions as to why this may be the case.

The latest batch-to-batch difference table of standard seawater and its application to the WOCE onetime sections

An updated batch-to-batch difference table of IAPSO standard seawater (SSW) up to P145 is proposed. The batch-to-batch difference table is based on several recent SSW comparison experiments, including the experiments conducted independently at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and Woods Hole Institute of Oceanography (WHOI) at about the same time using the same procedure. Proposed batch-to-batch differences range from 1.2 × ×× ×× 10 -3 to -1.9 × ×× ×× 10 -3 with refer- ence to the average of those from P91 to P102. Batch-to-batch differences from P29 to P145 with reference to the recent batches and this average over every 5 years since 1960 are also presented, together with standard deviation. This reveals that incon- sistency among batches has improved since 1980s. In particular, the standard devia- tion was 0.3 × 10 -3 in this decade, which is about one-half the value reported previ- ously and almost equal to the modern measurement precision (0.2 × 10 -3 ) and is within- batch difference (<0.3 × 10 -3 ). Proposed batch-to-batch differences were applied to the observational results of the WOCE hydrographic onetime section (WHP onetime) in the Indian Ocean. Average absolute salinity differences at 14 crossover points in the Indian Ocean were slightly larger, from 1.2 × 10 -3 to 1.5 × ×× ×× 10 -3 , when the batch-to- batch difference table was applied; however, when results from the Indian, Pacific, and Atlantic Oceans were combined, application of the batch-to-batch difference ta- ble yielded statistically acceptable salinity differences. The table was also applied to WHP sections P1 and P17 (revisited about 10 years after the original observations during the WOCE period) and sections I1, I7, and I8 (visited twice by different re- search vessels in the same year). In all cases, the table corrected unrealistically large salinity changes in space and time. The results suggest that the application of the batch-to-batch table to well-controlled salinity data such as WOCE datasets would be effective in making the datasets more consistent in space and time.

Standard seawater comparison up to P129

Batch to batch differences of IAPSO standard seawater (SSW) batches P103 to P129 were examined. Several comparison experiments, in which 2–23 batches of SSW were compared with each other, were carried out during the period from 1991 to 1997. Batch to batch differences for the KCl-labeled batches, P91 through P129 except for P113, P115, P117, P125 and P126, ranged from −2.2×10 −3 to 1.2×10 −3 in salinity with an average of −0.1×10 −3 in salinity. Within-batch differences were observed to be <0.3×10 −3 in salinity for recent batches except a few for which the differences were up to 0.6–0.9×10 −3 in salinity. The KCl-labeled batches generally agreed to within 1.0×10 −3 in salinity with the assumed reference. Applications of batch to batch differences of SSW for World Ocean Circulation Experiment (WOCE) expedition salinity data led to a significant reduction in salinity variance at crossovers both at the regional scale of the Philippine Sea and the basin scale of the Pacific Ocean and Atlantic Ocean. To obtain accuracy better than 2×10 −3 in salinity, within-batch differences were also considered before the standardization for several ampoules of SSW. It is prudent that we continue the monitoring of batch to batch differences of SSW by comparing new with previous batches, so that one may correct both future salinity data and the historical salinity data.

An Evaluation of Some Recent Batches of IAPSO Standard Seawater

Abstract The authors examine changes in the salinity of the International Association for the Physical Sciences of the Ocean standard seawater (SSW) as used in seven cruises between 1991 and 1997. Ten batches of SSW were used during this time—several more than once—such that a clear demonstration of the effect of aging over months and years on SSW salinity can be made. Thus, the authors demonstrate that simple “offsets” intended to “correct” SSW salinity changes are inappropriate. Interest in intercruise salinity compatibility is high, as scientists attempt to reconcile section measurements made during the World Ocean Circulation Experiment among experiments. SSW salinity changes are one source of salinity differences of O(0.001) between sections. Herein, the authors provide a demonstration of how SSW measurements can be collated to generate a batch-by-batch history of SSW salinity evolution for more accurate sample salinity evaluation.