Estimating the Absolute Salinity of China Offshore Seawater Using Nutrients and Inorganic Carbon Data

In June 2009, the Intergovernmental Oceanographic Commission of UNESCO released the international thermodynamic equation of seawater – 2010 (TEOS-10 for short) (IOC et al, 2010) to define, describe and 10 calculate the thermodynamic properties of seawater. Compared to Equation of Ocean State-80 (EOS-80 for short), the most obvious change of TEOS-10 is taking Absolute Salinity as salinity argument, replacing the Practical Salinity used in ocean society for 30 years. Due to the lack of observation data, the applicability of Absolute Salinity algorithm in the offshore and semi-closed sea is not very clear to date. Based on the Marine Integrated Investigation and Evaluation Project of China Offshore, other relevant data together with Pa08 model, we obtain the magnitude, distribution characteristics 15 and formation mechanism of Absolute Salinity in China offshore. As the main composition anomaly relative to SSW, calcium carbonate, originating from terrestrial input of high calcium carbonate content and re-dissolution of sediment of China offshore, raises the Absolute Salinity Anomaly δSA as high as 0.20 g·kg and increases the Practical Salinity about 0.04 at most comparing to the chlorinity-based salinity. Moreover, both of them show obvious seasonal variation. Finally, relevant suggestions are proposed for the accurate measurement and expression of Absolute Salinity of the China offshore. 20


Introduction
Absolute Salinity which is traditionally defined as the mass fraction of dissolved material in seawater, replaces Practical Salinity as the salinity argument of TEOS-10 seawater standard for the thermodynamic properties of seawater are directly influenced by the mass of dissolved constituents whereas Practical Salinity depends only on conductivity. Since the https://doi.org/10.5194/os-2020-84 Preprint. Discussion started: 21 September 2020 c Author(s) 2020. CC BY 4.0 License. obvious problems in the correct presentation of time series and/or transects that begin near the coast and end well offshore (Wright, 2011). 50 Therefore, this paper firstly clarifies the definition, status and application Absolute Salinity, secondly, based on the measured data and related research results, calculate the magnitude, temporal and spatial distribution characteristics and formation mechanism of Absolute Salinity of the China offshore seawater, thirdly, analyze the Practical Salinity change caused by relative composition variation; finally, based on the above results, put forward relevant suggestions and future research directions for the accurate measurement and expression of absolute salinity of China offshore seawater. 55

2
Methods and data

Calculation of Absolute Salinity
According to definition, the Absolute Salinity of seawater is essentially based on adding up the mass of solute in a sea water sample, soln = 1 ∑ =1 (1) 60 Where, ci is the molar concentration of component i in seawater per kilogram, Mi is the molar mass of the component, and Nc is the number of species of component in seawater.
It's impractical to carry out a full chemical analysis for the seawater to get the Absolute Salinity regularly. While the primary and most demanding purpose of oceanographic salinity measurements is the calculation of seawater density to estimate significant ocean currents driven by sometimes tiny horizontal pressure gradients. In TEOS-10, Absolute Salinity 65 is defined as the density of seawater can be accurately calculated by the following equation.
Therefore, SA is also called density salinity in this equation.
It will occur A ≠ A soln when the salt concentration coefficient change with seawater components. The TEOS-10 have provided the conversion formula to get A soln . The SA in this paper refers to the density salinity unless otherwise specified. 70 https://doi.org/10.5194/os-2020-84 Preprint. Discussion started: 21 September 2020 c Author(s) 2020. CC BY 4.0 License.
In order to get SA, Millero (2008) defines a stoichiometric composition model (the Reference Composition or RC) based on the SSW, and specifies an algorithm to determine a consistent estimate of the mass fraction of dissolved material in a sample of arbitrary salinity with the RC R = PS • P , 2 < P < 42 In Eq. 3, the factor PS between the reference salinity of standard seawater and the practical salinity is (35.16504/35) 75 g· kg -1 , mainly due to original evaporation technique used by Sø rensen in 1900(Forch et al, 1902 lead to some volatile components of the dissolved material were missing in the mass calculation of the dissolved material in the sea water. For general seawater, it can be considered as the mixture of standard seawater concentrated/diluted with a small amount of other components. The calculation formula of Absolute Salinity (mass fraction of dissolved material) is as follows: At present there are three methods for determining Absolute Salinity Anomaly δSA. First, to obtain it by comparisons with direct density measurements performed in the laboratory (Millero et al, 2008;Wright et al, 2011). According to the density difference = − ( R , 25℃, 0 dbar) and the haline contraction coefficient which is 0.7519 for SSW, δSA is determined by This procedure is useful for laboratory studies or in situations where ocean water can be obtained from sampling bottles retrieved from certain depths.
Second, to estimate it from an additional correlation equation if chemical measurements of the most variable seawater constituents in the open ocean (carbonate system and macro-nutrients) are also available (Pawlowicz, et al, 2011 In which, the units of each component on the right are all mmol· kg -1 , ∆NTA = TA − 2.3 × P /35 is the standardized ∆TA, and ∆NDIC = DIC − 2.08 × P /35 is the standardized ΔDIC. Third, to calculate δSA by global Gouretski and Koltermann (2004) hydrographic atlas. Due to the lack of seawater component data, McDougall et al. carried out regression calculation on the practical salinity, density and silicate concentration data of 811 seawater samples worldwide, and found that δSA is directly related to S i (OH) 4 Take the effects of evaporation and rainfall on ocean salinity into consideration, Eq.7 can be simplified as: in which, = atlas atlas ⁄ , both the atlas and atlas are from the global Gouretski and Koltermann (2004) hydrographic atlas. 100 Eq.9 is adopted in GSW to calculate δSA with uncertainty in the ocean is less than 0.0047g· kg -1 . For the semi-closed Baltic sea, Feistel (2011) has fitted an empirical formula for calculating δSA which is mainly due to rivers bringing material of anomalous composition into the Baltic, it has been also incorporated into GSW algorithm library.

Pawlowicz model for calculating conductivity based on seawater composition (hereinafter referred to as Pa08 105 model)
Given the exact ion composition of solution, the Pa08 model (Pawlowicz, 2008) can be used to calculate the conductivity of seawater. The simplified form is as follows: Where, C is the ions composition of the solution, Nc is the number of species of ions in solution, ̅ , and * are the 110 equivalent conductivity per mole of i th ion, valence and the corresponding chemical equivalent ion concentration.
Based on the SSW, the Pa08 model calculates the difference between the calculated conductivity κ 08 ( * ) and the measured conductivity κ( * ), Then assuming the ratio between the measured conductivity κ( * ) and the calculated conductivity κ 08 ( * ) does not 115 change when the SSW composition has only a small perturbation * relative to the reference seawater, that is, κ( * + * ) = κ 08 ( * + * ) • (1 + ) −1 Thus, the equivalent electrical conductivity ̅ of 18 kinds of ions in seawater at 25 ° C is calculated. When the temperature of the seawater sample θ ≠ 25 ° C, ignoring the influence of pressure on the conductivity, and the conductivity calculated from the seawater composition is revised using the following formula: 120

Observation data
The

Reference Salinity SR of the China offshore seawater
Based on the observation, the Practical Salinity SP of China offshore seawater diluted by the low salinity river runoff ranges from 12 and 34.5, the minimum of 12 occurs outside the gate of the Yangtze River runoff into the sea, the maximum of 34.5 appears in the Kuroshio way. By the way, an extreme minimum SP of 0.1 appears in the south branch of Yangtze River in the summer of 2006, since river salinity is beyond the scope of this paper, no further discussion will be made 135 here.
Based on Eq.3, SR of China offshore ranges from 12 to 34.66 g· kg -1 .

Absolute salinity Anomaly δSA of the China offshore seawater
Using Eq.6 in the section 2.1, the δSA of China offshore ranges from 0 to 0.2 g· kg -1 , the largest is one order higher than that of the open ocean. The ΔNTA item in Eq.6 contributes to δSA as high as 90%, so the largest δSA appears in the 140 northern Jiangsu shoals, the Yangtze River estuary, the Bohai Sea and the Pearl River estuary where the ΔNTA is high, as shown in Fig.2. Due to the lack of complete chemical analysis data, the following researches for different subjects have indicated that the positive ΔNTA in the above areas is caused by the input of high CaCO3 content rivers and the re-dissolution of sediments.
Based on the investigation of 13 cruises from April 2011 to February 2012, Qi Di (2013)  ] and ΔNTA is 132~250 umol·kg -1 and 245~480 umol· kg -1 respectively in the north branch of Yangtze River in dry period and transition period. In terms of magnitude, these results are approximately consistent with the following assumption of the Pa08 model in the region mentioned above, Other studies have also indicated that due to the accumulation of materials entering the sea from the old Yellow River and the ancient Yangtze River, the CaCO3 concentration of surface sediments on the seafloor near the coast of northern Jiangsu ranges from 2.8% to 10.5% (Qin, et al, 1989;Yang and Youn, 2007). The NDIC of the Yellow Sea near China has always been high, even when strong biological activity in spring reduces the surface ΔNTA, the sediment of PIC will resuspend and maintain the high dissolved CaCO3 of seawater through the solid-liquid balance (Hong, 2012;Zhang, 160 et al, 1995).
The above research results fully explain the following spatial and temporal distribution characteristics of A S  of China offshore.
Centered at 33.4°N and 121°E, δSA gradually decreases from the coast to the offshore in the northern Jiangsu shoal. In the central area, δSA is always greater than 0.05 g· kg -1 all the year round. The δSA of the bottom layer is always higher 165 than that of the surface layer except in summer when the carbonate-rich terrestrial input from the Huihe River system is higher than the dissolution CaCO3 of bottom sediments. So the maximum value of 0.15 g· kg -1 appears on the bottom layer in winter while the minimum value of 0.05 g· kg -1 appears in the surface layer in spring. In winter, due to the strong Yellow Sea Warm Current invading, the area where δSA greater than 0.05 g· kg -1 shrinks towards the shore where the maximum of δSA locates. 170 The largest δSA of the Bohai Sea appears at the estuary of carbonate-rich Yellow River, decreases outwards, is always greater than 0.05 g· kg -1 and rises to the maximum of 0.1 g· kg -1 on the bottom in winter. As a semi-closed shallow sea with low exchange with the open ocean, the δSA in the whole Bohai Sea is always larger than 0.02 g· kg -1 and the difference between δSA of the bottom and that of the surface is not obvious. https://doi.org/10.5194/os-2020-84 Preprint. Discussion started: 21 September 2020 c Author(s) 2020. CC BY 4.0 License.
As China's largest runoff into the sea, the Yangtze River is rich in freshwater and nutrients from land. At its gate to the 175 sea, the δSA is greater than 0.1 g· kg -1 all year round. However, due to the large consumption of phytoplankton, the nutrients decreases rapidly outside the gate, ΔNTA remains the primary contributor to the δSA. The surface coverage of the 0.05 g· kg -1 isocline varies with seasons and depths, reach to the maximum in summer and with little variation in other seasons.
The δSA greater than 0.05g· kg -1 also occurs at the mouth of the Pearl River and Minjiang River in summer (flood season), and less than 0.02g· kg -1 in other seasons. 180 In the rest area, the magnitude of δSA is below 0.005g· kg -1 , which is the same as the magnitude of the statistics uncertainty of the Absolute Salinity Anomaly in the open ocean, could be ignored.

Contrast to the δSA calculated by GSW
By GSW function library and the corresponding climate silicate and practical salinity data, the δSA of China offshore ranges from 0 to 0.015 g· kg -1 , the maximum appears at the bottom near to the Xisha Islands and the Dongsha Islands and 185 the rest is below 0.002 g· kg -1 . It's one order of magnitude less than that in the 3.2 section, distribution characteristics are also significantly different. These differences mainly come from the following aspects: (1) CaCO3 is the main relative composition anomaly of China offshore seawater and the primary contributor to the δSA.
(2) Silicates of the observation is 100μmol·kg -1 higher than climatological data of the GSW function library in the mouth of the Yangtze River, Minjiang River and Pearl River at most. 190

Practical Salinity change δSP caused by CaCO3 dissolution
The nitrite and phosphate is temporarily not considered in the calculation for their concentration ranges from 0 to 0.01 mmol· kg -1 and 0 to 0.005 mmol· kg -1 respectively in the existing observation which are much smaller than those items in Eq. 6 and Eq.14 above.
First, based on observation, ΔN[Ca 2 + ] , ΔN[HCO3 -] and ΔN[CO3 2-] are derived respectively by Eq.14 and carbonate 195 equilibria; then, the conductivity change δC25°C corresponding to these ions change at 25°C by Pa08 is calculated; then, δC25°C is converted to the conductivity change δC at seawater sample temperature by Eq.13; finally, δSP is calculated using Eq.15, in which Cobs is conductivity of CTD reading, Sp is the SSW practical salinity corresponding to Cobs. P = P ( obs ) − P ( obs − ) variations can be as high as 0.03.

Conclusion and analysis
The proposal and implementation of the concept of SA in TEOS-10 is to accurately quantify the total mass of inorganic substance dissolved in sea water, ensures the density and related quantities are accurately represented by Gibbs function 210 and corrects errors caused by the measuring the properties of seawater such as chloride and conductivity to get the salinity.
In this paper, based on the observation and chemical composition model Pa08, the magnitude, distribution characteristics and formation mechanism of Absolute Salinity in China offshore are obtained: 1) The Absolute Salinity SA ranges from 12 to 34.66 g· kg -1 , in which SR ranges from 12 to 34.66 g· kg -1 and the Absolute Salinity Anomaly δSA ranges from 0 to 0.20 g· kg -1 ; 4) Under the combined effects of different water system dynamics, terrestrial input, marine biological activities, and redissolution of marine sediments, the δSA in China offshore seasonal variations are obvious, and the maximum can be as 220 high as 0.05g· kg -1 ; the difference between the surface layer and the bottom layer is also up to 0.1g· kg -1 ; 5) The practical salinity will change 0.04 at most due to relative composition variation of sea water.
With the limited observations, this paper only lists the magnitude and distribution characteristics of δSA in China offshore from 2006 to 2007. At present, we have collated the long-term series of seawater composition data to continue the study on δSA changes and get an empirical formula to calculate it. The current researches are only based on the existing seawater composition data, and the exact influence of other composition is still very clear. Therefore, it is necessary to carry complete chemical analysis for the main components of seawater or the density measurement in the following estuaries of the Yangtze River, Pearl River, Minjiang River, and the semi-closed Bohai Sea. 235