calcium in the pacific ocean

Deep-Sea Research, 1973, Vol. 20, pp. 717 to 726. Pergamon Press. Primed in Great Britain.

Calcium in the Pacific Ocean

S. TSUNOGAI,* H. YAMAHATA, S. KUDO* and O. SA1TO*

(Received27 November 1972; in revised form 22 February 1973; accepted4 March 1973)

Abstract---Calcium in the Pacific water was extensively determined with an error less than 0.1%. The highest Ca/C1 ratio was found in the deep water of the North Pacific and low values in the warm surface waters. The difference corresponds to the dissolution of 0.15 mM of calcium carbonate. The rate of increase of calcium in the deep water is estimated to be 4 or 12 × 10 -5 mM/yr. The relation between the increase of calcium and that of alkalinity in the abyssal water can be regarded as the dissolution of calcium carbonate, but the increase of calcium measured in the upper deep water between 1 and 2 km in depth is slightly larger than that expected from alkalinity. The discrepancy between salinity from conductivity and that from ehlorinity may amount to 0-007~ in the deep water of the North Pacific.

I N T R O D U C T I O N

RECENT studies on calcium in the oceans revealed that deep waters contain more calcium relative to chlorinity than do surface waters (e.g. DITTMAR, 1884; CULKIN and Cox, 1966; RILEY and TONGUDAI, 1967; SAGI, 1969; TsLrNOGAI, NISmMURA and NAKAYA, 1968a; TSUNOGAI, YAMAZAKI and NISHIMURA, 1971; LYAKHIN, 1971). The variation in the calcium content is chiefly caused by the precipitation of calcium carbonate from the surface water and by its dissolution in the deep water. The calcium content in deep water should gradually increase with time.

According to Stommel's theory on the deep circulation of the world oceans (STOMMEL, 1958; STOMMEL and ARONS, 1960), the deep water of the North Pacific Ocean is the oldest of the deep waters of the world oceans. Therefore, the highest ratio of calcium to chlorinity (Ca/Cl) is expected there, except for those waters in coastal areas influenced by the mixing of river water which contains more calcium relative to chloride. The earlier determinations of calcium in Pacific water (THOMPSON a n d WRIGHT, 1930; MIYAKE, 1939; FUKAI and SHIOKAWA, 1955; SUGAWARA and KAWASAKI, 1958) failed to prove this because of their inaccuracy and the sparsity of data for calcium in deep water.

Calcium is a major constituent of sea water and thus the variation of Ca/C1 in sea water may be significant in the exact relationship between salinity and chlorinity. We determined the concentration of calcium in about 1800 samples collected in the Pacific Ocean so accurately that the discussion of these problems became possible.

SAMPLING AND ANALYSIS OF CALCIUM

Sea water samples were collected on cruises KH-68-4 (Nov. 1968-Mar. 1969, in the central Pacific), KH-70-1 (Feb.-Mar. 1970, in the western North Pacific) and KH-70-2 (Apr.-June 1970, in the North Pacific) of the R.V. Hakuh5 Maru of the

*Department of Chemistry, Faculty of Fisheries, Hokkaido University, Hakodate, Japan.

717

718 S. TSUNOGAI, n . YAMAHATA, S. K U D O a n d O . S A t l o

Ocean Research Institute, University of Tokyo (Fig. 1). The hydrographic data on salinity, temperature, nutrients, etc. are given in the cruise reports (HoRmE, 1970, 1971a,b). The samples were stored in polyethylene bottles for a few months to two years.

Since salinity sometimes changed during the storage of a sample due to evaporation of water, the salinity was redetermined by an Auto Lab inductive salinometer just before the determination of calcium in the laboratory. Calcium in each sea water sample was determined in triplicate using the method of TSUNOGAI, NISH1MURA and NAKAYA (1968b), with an accuracy which was better than 99.9°/~,. The standard solution of calcium was made by dissolving certain amounts of sodium chloride, magnesium sulfate, and strontium chloride and adding pure calcium carbonate dissolved in a slight excess of hydrochloric acid, and the pH was then adjusted to 8-2 by sodium hydroxide. The purity of the standard calcium carbonate was 99.80% (see: TSUNOGAI, YAMAZAKI and NISHIMURA, 1971). The composition of the standard solution was approximately equal to that of sea water and the ratios of Mg/Ca and Sr/Ca were 3.13 and 0.0194, respectively.

The ratio Ca/CI was calculated by dividing the measured concentration of calcium (g/l. at 20°C) by the density calculated from salinity and by dividing S/1.80655, where

• ~ , 06 • . . . . !

/ • • . "%\ ~

',.'~ : : oN \ < . ~ , . -

4¢-.'. ' ' :' l , ~o I , (

- - * • -60

I ~ ! I / / _ 7 0 / / :, 1 20 1 50"E 180 150'W 120 90

Fig. 1. Locations where sea water samples were collected from about 25 depths at each station.

Calcium in the Pacific Ocean 719

S is salinity in the laboratory and 1 "80655 is a conversion factor of salinity to chlorinity (UNESCO, 1966; Cox, Ct~rdN and RILEY, 1967). The concentration of calcium (g/kg) in sea water was obtained by multiplication of the Ca/CI ratio by chlorinity converted from salinity which was determined aboard shortly after sampling.

DISTRIBUTION OF CALCIUM IN THE PACIFIC OCEAN

Vertical profiles of the ratio, Ca/C1 (Figs. 2 and 3 which were selected from 73 profiles in the Pacific Ocean as examples) clearly show that the deep waters contain more calcium than the surface waters, although the difference in Ca/CI between the deep and the surface is small in the Antarctic Ocean (TstmOGAI, YAMAZAKI and NISHXMtmA, 1971). These profiles, however, often have maxima in a layer at about 2-4 km in depth and the Ca/CI ratio is not large in water near the bottom or in deeps such as the Tonga Trench.

The cross-section of Ca/CI along 170°W (Fig. 4) is compared with that of salinity (Fig. 5). Water masses can be designated by the cross-section of salinity. High salinity waters in the surface of the tropical and subtropical Pacific contain the least calcium relative to chlorinity (Ca/C1 of less than 2105 × 10-5). The Ca/C1 ratio (around 2115 × 10 -5) of the Antarctic Intermediate Water of low salinity does not change as it flows northward. On the other hand, the Ca/C1 in the Pacific Deep Water designated by salinity around 34"7%o gradually increases from the South to the North Pacific.

2110 CalCt x ,

x

2 km

6

I!

I0

2130 35 SaLinity '

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

Fig. 2. Vertical distributions oftheratio of calcium to chlorinity shown by the crosses in the unit of 10 -5 and salinity in the unit of %0 at Sta. 26 of the KH-68-4 cruise, 23°11'S, 169°58'W.

720 S. TSUNOGAI, H. YAMAHATA, S. KUDO and O. SAITO

3/..0 0

I km

2110 CalCt 2130 34.5 Salinity

I

X X

X

X

X

X

X

X

X

Fig. 3. Vertical distributions of the ratio of calcium to chlorinity shown by the crosses in the unit of 10 -5 and salinity in the unit of %o at Sta. 7 of the KH-70-2 cruise, 33°03'N, 169°59'W.

30 'N 0 30 *S 60 0 \ ~ ~ ,.~ '20~ 60

4-- km

5

Fig. 4. Distribution of the ratio of calcium to chlorinity shown in (Ca/Ci × 105--2100) in a vertical section along 170°W.


, /. 0 20 "N , 0 , 20 *S /.0 SO 0 t . ~ ~ M _ " ~ ' , ~ r . . E ~ , ~ L ~ - J _ O . L _ ~ tLr ---J

Fig. 5. Distribution of salinity in %o in the same section as Fig. 4.

The highest value of Ca/Cl (max. 2134 × 10 -5) was found in the deep water at 2-4 km in depth in mid-latitudes of the North Pacific. The distribution of Ca/C1 in the deep water seems to be inversely correlated with that of 814C (Table 1) which was determined by BIEN, RAKESTRAW and SUESS (1965). This fact makes possible a rough estimation of Ca/CI from 814C in the deep water.

Table 1. Mean values of the calcium/chlorinity ratio, 814C and alkalinity/chlorinity in the various waters of the Pacific Ocean.

Ca/CI × 105 814C* Alk/CI × 10st (~oo) (meq/g of CI)

Surface 30°S-30°N 2104 Deep 70°S-40°S 2119 -- 185

40°S-0°S 2124 --200 0°N-30°N 2128 --220

30°N-50°N 2132 --235 Mean of the whole

Pacific Ocean 2123 of. Indian Ocean, Deep + 70°S-40°S 2116 - 155

125"5 128"7

131'1

*Cited from BraN, RAKESTRAW and SuvAs (1965). tCalculated from the data published in HogmE (1970) and HORmE (1971b). ~,Cited from Tslmoom, YAMAZAKI and NlSmMURA (1972).

The weighted mean of Ca/CI in the Pacific Ocean (2123 × 10 -5) is slightly larger than the Ca/CI ratios in the deep water of the Antarctic Ocean. These are probably larger than those of the Atlantic waters (by considering the 8"C values). Thus, the Ca/C1 ratio of (2115-2120) × 10 -5 seems to be a probable mean of Ca/CI in the world oceans, as predicted by TstmoGAI, YAMAZAKI and NISmMORA (1971).

722 s. TSUNOGAI, H. YAMAHATA, S. KUDO and O. SAITO

THE RATE OF INCREASE OF C A L C I U M IN DEEP WATER

Calcium in the deep water of the Pacific Ocean is undersaturated with respect to calcium carbonate (BERNER, 1965; LI, TAKAHASHI and BROECra~R, 1969) and thus the calcium content of the deep water is to be increased by the dissolution of particulate calcium carbonates. The rate of increase of calcium is estimated by the following two methods.

(a) The closed system model

The deep water in the Pacific Ocean has only one source (KNAuSS, 1962). The Pacific Deep Water comes from the Antarctic Ocean whereas the deep water of the Atlantic Ocean is mixed with water from the Weddell Sea. The northward speed and the transport rate of the Pacific Deep Water is estimated to be 0.05 to 0.1 cm/sec and 15 to 25 × 106 m~/sec, respectively (KNAuSS, 1962).

We assume a steady northward flow of this deep water along 170°W. The roughly proportional distribution of Ca/CI to 314C of the water (Table 1) supports this as- sumption because the effects of dilution by shallower water and of the flow in different directions and speeds on the variations of Ca/CI and 3uC values are largely cancelled out. The rate of increase of calcium is simply estimated by dividing the difference of Ca/CI between two stations by the lapse of time calculated from the 31~C decrease. The differences between the Pacific Deep Water in the North Pacific and the Antarc- tic Deep Water are 13 × 10 -5 for Ca/CI and 50~o for 314C which corresponds to the elapsed time of 510 years. Thus, the rate of increase of calcium in the deep water turns out to be 12 × 10 -5 mM/yr (M : moles/l.). On the other hand, the rate can also be estimated in the same way from the alkalinity increase, because the increase in alkalinity is chiefly caused by the dissolution of calcium carbonates. The rate of increase of calcium is estimated to be 11 × 10 -~ mM/yr from the increase of 11 meq/1, in alkalinity between these two regions. This is in good agreement with that estimated directly from the concentration of calcium.

(b) The one-dimensional diffusion and advection model

In the North Pacific Ocean, the vertical distribution of a conservative property may be assumed to be governed by a balance between vertical diffusion and vertical advection (see: MUNK, 1966 ; CRAIG, 1969; TSUNOGAI, 1972a). For a non-conservative element such as calcium, the balance in the deep water can be expressed by the following equation,

~C ~2C ~C 8t - - O ~ - - W E q - J , (1)

where C is the concentration of calcium, t is the time and Z is the depth. In the above equation, the vertical eddy diffusivity (D), the vertical advection velocity (I4 0 positive downward and the rate of increase of calcium from the particulate flux (aT) are assumed to be constant regardless of the depth in the deep water.

In a steady-state condition, the solution of equation (1) is given by

W J C = cl + c2exp(+ ~ Z ) -- ~ Z , (2)

where el and c~ are constants and can be determined from the vertical profile of the


constituent. The numerical values of W/D and D are --1.2 km -x (TSUNOGAI, 1972a) and 1.2 cm~/sec (TStrNO~AI, 1972b), respectively, in the mid-latitudes of the North Pacific Ocean. Equation (2) was applied to the vertical distribution of calcium at Sta. 7 of the KH-70-2 cruise, 33°N, 170°W (Fig. 6) and a rate obtained for J of 4 × 10 -5 mM/yr. This corresponds to 0.15 moles CaCOs/m2/yr or 1.8 gC/m2/yr in a water column between 1 and 5 km depth. The rate of increase of carbon by the dissolution of calcium carbonate is about one-third of that by the oxidation of organic matter (TsuNOGAI, 1972a) using the same model. The rate might be estimated from the distribution of alkalinity, but a reliable rate could not be obtained because of the large random error in the alkalinity determination.

2 km

, , o s ,

I t |

1 t !

Ix I

Fig. 6. Vertical distribution of calcium in the same station as Fig. 2. The solid curve shows the distribution of calcium calculated from the equation (2) f o r J = 4 × 10 -s mM/yr and the dashed

curve was obtained by neglecting the d value.

The rate estimated for the one-dimensional model is about one-third of that estimated above with the model for a closed system. One of the causes of the discrepancy seems to be derived from analytical errors in the determination of calcium. In the one- dimensional model, the analytical error is so serious that the uncertainty of our results may be by a factor of 2 or more.

THE RELATION BETWEEN THE INCREASE OF CALCIUM

AND THAT OF ALKALINITY IN THE DEEP WATER

When one mole of calcium carbonate is dissolved or precipitated in water, two equivalents of alkalinity in the water increases or decreases, respectively. Thus, the relation can be expressed by

724 S. TSUNOGAI, H. YAMAHATA, S. KUDO and O. SArro

X -= -¥o-~ ~Ix

and Y =- Yo + 2Ax, (3)

where X is the concentration of calcium in mM and Y alkalinity in meq/l., X0 and Yo are those at the original condition and Ax is the amount of dissolved calcium carbonate in mM. From the relation 0) , we get

2 X - - Y = 2 X o - - Yo. (4)

Since the effects of Ax on chlorinity (Ci) and density (p) is small in the usual sea water, the equation (4) can be written as

c - : 2 L : - 2 x 0 - r0 CI • p CI • p - (meq/g of CI). (5)

We calculated the (" value for the Pacific water by using alkalinity (HoRmE, 1970, 1971b) and our data, and the results are plotted (Fig. 7). If the source of calcium in the water were only the dissolution of calcium carbonate, the C value must be constant. However, the high C values occur in the water between I and 2 km depth especially in the North Pacific Ocean (Fig. 7). This fact suggests that there are sources of calcium other than the dissolution of calcium carbonates. One possibility might be the dissolution or ion exchange of silicate materials in the water. The contribution from this source is probably larger in the North than in the South Pacific because of the larger proportion of the land area. This then may explain why the high C values exist in the North Pacific Ocean. However, the systematic error in the determination of alkalinity may somewhat contribute to the values in the northern North Pacific, because the samples from the northern stations at more than 30°N were collected on a different cruise from the others.

Fig. 7.

30 "N 0 30 *S 60

Distribution of the C values indicated in the unit of 10 -~ meq/g of chlorinity in the same section as Fig. 4,


EFFECT OF THE D I S S O L U T I O N OF C A L C I U M C A R B O N A T E ON THE R E L A T I O N

B E T W E E N E L E C T R I C A L C O N D U C T I V I T Y A N D C H L O R I N I T Y

Our results on the determination of calcium show that the ratios of Ca/CI range from 2101 × 10 -s in the surface water of the tropical and subtropical Pacific Ocean to 2133 × 10 -5 in the deep water of the North Pacific Ocean. The difference corresponds to the dissolution of about 0.15 mM calcium carbonate. PARK 0964) estimated an increase in conductivity of 0.014% from the dissolution of 0-1 mM calcium carbonate for deep water. Therefore, a maximum discrepancy of 0"007~oo in salinity can be found in the difference between the salinity derived from conductivity and that from chlorinity, if the increase of calcium in the deep water is only due to the dissolution of calcium carbonate.

Cox, CULKIN and RILEY (1967) have determined conductivity and chlorinity accurately, and their difference in salinity as determined by conductivity and by chlorinity is higher by 0.004~oo on the average for the deep water of the world oceans than for the surface water. Among their deep water samples, however, there were few from the North Pacific Ocean. Thus, most of the discrepancy between the conductivity and chlorinity ratios of the sample when compared with the standard is caused by the variation of Ca/CI from sample to sample. When salinity is measured by using Copen- hagen Standard Sea Water which is Atlantic Surface Water, the salinity of the North Pacific Deep Water appears to be higher by 0.007%° than salinity derived from chlorinity.

Acknowledgements--We are indebted to Professor M. N I s ~ t r ~ for critical evaluation of the manu- script. We are also grateful to Professor Y. HoRmE and staff members of R.V. Hakuh5 Maru for collecting the samples.

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calcium in the pacific ocean

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