The impact of solution chemistry on Mytilus edulis calcite and aragonite

Geochimtco er Cormuchimico Acra Vol. 0 Pergamon Press Ltd 1980. Printed

44, pp. 1265 lo 1278 m Great Britain

0016-7037/80/0901-1265102.00/O

The impact of solution chemistry on Mytilus edulis calcite and aragonite ROBERT B. LORENS*and MICHAELL. BENDER Graduate School of Oceanography, University of Rhode Island, Kingston, RI 02881, U.S.A. (Received 11 July 1979; accepted in revised form 17 April 1980)

Abstract-Magnesium/calcium, Sr/Ca, and Na/Ca atom ratios were determined in the calcite and aragonite regions of Mytilus edulis shells which were grown in semi-artificial ‘seawater’ solutions having varying Mg/Ca, Sr/Ca, and Na/Ca ratios. These ratios were measured by instrumental neutron activation, atomic absorption, and electron microprobe analytical techniques. Strontium/calcium ratios in both calcite and aragonite were linearly proportional to solution Sr/Ca ratios. Magnesium/calcium ratios in calcite increased exponentially when solution Mg/Ca ratios were raised above the normal seawater ratio; whereas in aragonite, Mg/Ca ratios increased linearly with increases in solution Mg/Ca ratios. Sodium/calcium and sulfur/calcium ratios in calcite covaried with Mg/Oa solution ratios. Conversely, in aragonite, Na/Ca ratios varied linearly with solution Na/Ca ratios. Magnesium is known to inhibit calcite precipitation at its normal seawater concentration. We infer from the results of the work reported here that Myths edulis controls the Mg activity of the outer extrapallial fluid, thus facilitating the precipitation of calcitic shell. Increases in sulfur content suggest that changes in shell organic matrix content occur as a result of environmental stress. Certain increases in Mg content may also be correlated to stress. Sodium/calcium variations, and their absolute amounts in calcite and aragonite, are best explained by assuming that a substantial amount of Na is adsorbed on the calcium carbonate crystal surface. Strontium/calcium ratios show more promise than either Mg/Ca or Na/Ca ratios as seawater paleochemistry indicators, because the Sr/Ca distribution coefficients for both aragonite and calcite are independent of seawater Ca and Sr concentrations.

lNTRODUCTlON

IT IS WIDELYrecognized that biogenic calcium carbonates are commonly not in equilibrium with seawater, and that they have compositions very different from those predicted on the basis of inorganic distribution coefficients. While the reasons for these effects are not well understood, there are extensive observations bearing on the problem. Biogenic calcium carbonates are produced as either calcite, aragonite, or less commonly vaterite. The elemental composition of each polymorph can vary with each species, even though the calcified material is produced from a similar seawater environment. Mineralogy and composition are affected by both environmental and physiological factors. MILLIMAN(1974) reviews the effects of temperature, salinity, and external solution composition on calcified tissue mineralogy and chemical composition. WATABEand WILBUR (1960), MEENAKSHIet al. (1974), CRENSHAW(1975) and MITTERER(1978) cite the action of the organic matrix and the periostracum on crystal growth and form. SIMKISS(1965) suggests that calcifying organisms may alter the composition of the mineral-forming fluid relative to that of the external environment. Changes in growth rate, possibly due to changes in temperature or nutrition, may also modify biogenic carbonate chemistry (ZOLOTAREV,1975; PILKEYand GOODELL,1963; SWAN, 1956). In this work we studied some aspects of these controls by assessing the impact of solution chemistry on the mineralogy * Present address: Oceanographic Laboratories, Code 7008 U.S. Naval Oceanographic Office, N.S.T.L. Station, Bay St Louis, MS 39522, U.S.A.

and chemical composition of the shell of the blue mussel Myths edulis. The shell of Myths edulis is divided into the two layers, aragonite and calcite, illustrated in Fig. 1A. Just as in most other calcifying organisms, calcification occurs in an area that is physically isolated from the external environment. In molluscs, shell is precipitated from the extrapallial fluid which is physically separated from the external solution by the old shell, the mantle membrane, and the periostracum (WILBUR, 1972). The calcite and aragonite shell layers of Mytilus edulis are precipitated from two different solutions, separated at the pallial attachment line. WILBUR (1972) indicates that the mantle mediates the transfer of ions from seawater to the extrapallial fluid. The result of the mediation can be the fractionation of ions and the production of carbonates that are uncharacteristic of direct sea water precipitations. Animals that precipitate shells of aragonite or high-Mg calcite would not need to fractionate against Mg. Animals precipitating low-Mg calcite (less than 1 mol% Mg), might be expected to reduce the Mg content of their extrapallial fluid to facilitate the formation of low-Mg calcite. The alternative, that the extrapallial fluid is a concentrated Mg residue left after the precipitation of low-Mg calcite, is contradicted by the observation that the Mg/Na ratio of Myths edulis extrapallial fluid is less than the seawater ratio (LORENS,1978). The most direct way to test for a biological fractionation of ions is to sample the extrapallial fluid. BERNER(1975) shows that normal seawater Mg/Ca ratios inhibit the formation of calcite, the usual

1265

ROBERTB. LOKENSand MICHAEL L. BENDER

1266

PRISMATIC

LAYER,

CALCITE

PERIOSTRACUM

UTER ,NNER

EXTRAPALLIAL

PALLIAL

FLUID

:

MANTLE

1 MANTLE

ATTACHMENf OUTER

EXTRAPALLIAL

FLUID

A

Fig. 1. Cross section of Myrilus &u/is mantle and shell. inset shows sampling locations for calcite (cut out area) and location of microprobe section (broken line).

being either aragonite or high-Mg calcite (calcite with greater than lOmoloi, Mg). Measurements of MgjI?a ratios in the calcite producing outer extrapailiai fluid of ~Mytilus ederlis suggest a depletion of up to 370/;, relative to sea water, in the Mg contents of the fluid; however, the amount of depletion is not enough to account for the low Mg content of ~ytilus edulis calcite. Also, sampling problems make it difficult to relate these samples to the actual calcifying solutions (LORENS, 1978). Another way to test for a vital effect on the metal composition of biogenic calcium carbonate is to grow calcifying animals in a series of external solutions which have different metal compositions. The derivative of biogenic shell composition with respect to solution composition is then compared to what one would expect from a strictly inorganic precipitation. This approach was adopted, and this paper documents the response of ~yt~~us edulis skeletal chemistry to changes in solution Mg/Ca, &/Ca and Na/Ca ratios. The effects observed give insights into the importance of external solution composition, the significanoe of physiological ion fractionation, and the calcification process as a whole. The relationships between solution composition and shell composition demonstrated in this study also reveal the potentiaf of, and the problems associated with, the use of biogenic calcium carbonates as environmental record keepers. product

EXPERIMENTAL METHODS Culturing Mytilus rdufis In this study, the shell chemistries of groups of Mytilus edulis were determined after they had been grown in sea water solutions having varying Na/Ca, Sr/Ca, and Mg/Ca ratios. Mussel specimens were collected on June 13, 1975 from a running seawater water-table in the Graduate Schoot of Oceanography tank farm. All the mussels were from the spring spat-fall and when collected were 3-10 mm tang. Twenty numbered mussels were placed in each of eight 5 I glass aquaria containing 4 1of filtered Narragan-

sett Bay water (S = IQ:‘&).The tanks were maintained at room temperature (22--24°C) and were constantly aerated. The mussels were fed from a culture of Isochrvsis which was maintained in standard F-2 culture media. Every second day the cells from approximately 6 1of phytoplankton culture were con~ntrated into 8 approximately equal cetl packs. This was done using a continuous feed centrifuge fitted with an 8 position rotor. Each pack of cells was resuspended in IO mi of culture media taken from the tank containing the mussels that were to be fed. Approximately 1 ml of this suspension was added to each mussel culture tank 3 times a day. On July I the length of each mussel was measured to 0.01 mm with a Vernier micrometer and the original Bay water in each tank replaced by a special seawater medium. These special media were prepared by mixing I part natural seawater with I part synthetic sotution. For each tank the synthetic solution was different and the result was a series of artificial ‘seawater’ solutions which had a range of Sr,Ca, Mg/Ca, and Na/Ca ratios (Table 1). Solution No. 1 wasacontrol cont~ning 100~; Narragansett Bay water. Since each artificial culture solution was 50% natural seawater, essential trace constituents were majntained at 50% of their natural levels. For each culture solution (Table 1) the Mg/Ca and Sr/Ca ratios were determined by flameless atomic absorption using the method of standard additions. The Na/Ca values were calculated from the compositions of the reagents used to make the culture media. The calcium concentration of each solution was 9.3 mM except for solution No. 2 which had a calcium concentration of 33 m&i. Sulfate and bicarbonate con~ntrations were 25.4 and 2.1 mM respectively. During the eight week experiTable 1. Metal to calcium mole ratios of solutions used to culture &fgriit~s edutis. Solution 1 was lOtJo/, Narragansett Bay water; solutions 2 through 8 were prepared as described in the text SOLUTION .l-~.--. 1 2 ;: 5 6 c:

Mg/Ca

Na/Ca

Sr/Cax103

5.1 0.75

46. 13. 50. 46. 43. 40. 34. 29.

8.4 1.4 1.2

53:: 6.4 8.4 10.0 13,6

6.8 12.4 12.1 4.5 4.7

1267

The impact of solution chemistry Table 2. Microprobe

analytical data: counting times, counting precisions, and material used as standards STANDARD

SAMPLE COUNTING PRECISION

INSTRUMENT CHANNEL

ELEMENT ANALYZED

1

Na

75 set

(6% when

Mg

75 set

when 8% when >I58 when

Ca

10 set (50,OOOcts)

s

75 set

~6% when S/Ca>10x10-3 35% when S/Ca= 1~10~~

Sr

75 set

(10% when Sr/Ca>1.5x10-3 >25% when Sr/Ca<1.0x10-3

3

COUNTING TIME

<4%

Na/Ca>9xlO-3

Diopsitejadite

0.5%

weekly. At the mussels were remeasured and

Analyticat procedure Ail concentration values are reported as atom ratios with respect to calcium. This approach eliminates errors and uncertainties associated with weighing small samples, monitoring neutron fluxes, making absolute standards, correcting for porosity effects on probe concentration determinations, and diluting samples. It also facilitates comparison of solution and solid compositions. The frozen mussels were prepared for analysis in the following manner. The shells were thawed one at a time. The bulk of the soft tissue was removed with tweezers; the remaining organic matter, including the periostracum, was removed by placing each shell in a 5.25% NaOCi solution (commercial bleach) for 30min at room tem~rature. The shells were then rinsed with deionized water, soaked for 30min in deionized water, rinsed again with deionized water, and then dried in an oven. Samples for instrumental neutron activation analysis (INAA) were taken by chipping off bits of newly grown calcite (grown during the experiment) from the tip of the shell as indicated in Fig. 1B. After INAA. these samples were dissolved for atomic absorption analysis (AAA). Approximately 1 mg of shell was analyzed by INAA for Na, Sr, Cl and Ca; details of the method were described by BENDERef af. (1975). After the irradiated sample had cooled it was dissolved for AAA in 1 ml of a solution containing 1OOOppm Cs, 1OOOppm La and 0.3 N HNO,. An aliquot was further diluted with the C-La solution and analyzed for Mg and Ca by AAA. Magnesium was determined with a heated graphite furnace system (Perkin Elmer HGA-2100) using 1 or 5~1 injections. Calcium was determined by flame. For both techniques the standard conditions described by Perkin Elmer were used. Standards contained 1OOOppm Cs, 1OOOppm La, 0.3 N HNO,, and Mg and Ca in roughly the same proportions as found in the shells. Thin sections for microprobe analysis were prepared commercially from lengthwise cross sections as illustrated by the dashed line in Fig. IB. These sections were carbon coated and analyzed as described below. Some sections were not suitable for analysis; they were either cracked or had embedded polishing grit. However, at least one thin section representing the mussels in each solution was suitable for analysis. A Jeoi JXA-SOA electron probe microanalyzer with Krise1 Control electron beam automation was used to measure

COMPOSITION

15.74% Ca 9.55% Mg

SOURCE Geophysical Laboratory

26.22% Si 1.71% Na

Mg/CaXxlO-3 Mg/Ca=5x10-3 Mg/Ca=ZxlO-3

ment period,rhe culture solutionswere changed

the end of the ex~riment frozen.

MATERIAL

Apatite

38.62% Ca 0.148% S

U.S.G.S.

Sr-Glass

25.01% Si

Geophysical

39.32% Sr

Laboratory

the Ca, Mg, Sr, Na and S contents of the mussel shell sections. The instrument was operated at 15 keV, with a 0.01 PA specimen current and a 6 pm electron beam diameter. Sample concentrations were determined from standards by comparing sample and standard count rates. The count rates were corrected for current drift, background, and deadtime by the computer controller. Since a well defined, homogeneous, calcium carbonate matrix standard containing all the elements of interest was not available, the’standards as specified in Table 2 were used. Comparisons were made between the results from microprobe analysis and from INAA and AAA (reported later in this paper). ZAF corrections were made using the program MAGIC IV written by COLBY(1975); three oxygens were specified and carbon was calculated by difference based on a CaC03 formula. The counting procedure used for each sample is outlined in Table 2, along with the counting precision associated with each element. Some sample burning occurred, especially in calcite.

EXPERIMENTAL

RESULTS

Mytilus edulis in culture solutions with normal or higher than normal Mg/Ca ratios (solution numbers 1, 4, 5, 6 and 7) appeared healthy and cleared the water within an hour when fed an algal suspension. The growth rates of these mussels, from 1 to 4mm/g-wk, were similar to the 2.7mm,%wk rate reported for Mytilus edu~is by Stromgren (1976). Mytilus edulis in solution 2 took longer than an hour to visibly clear the water of algae, while the mussels in solution 3 did not feed well and their water was green with algae much of the time. Accordingly, mussel growth rates in these later 2 solutions were much reduced; only 7 out of the 20 mussels in solution 2 grew sufficiently for sampling, and only two mussels from solution 3 grew sufficiently. Mussels in solution 8, the highest Mg/Ca ratio solution, were severely affected by the changed environment. For several weeks these mussels remained completely relaxed, unable to close their valves or withdraw their foot between their valves when touched. EventuaIly they adapted to their new environment, but insufficient growth occurred for INAA or AAA sampling. Suf-

ROBERTB. LORENSand

1268

MICHAELL. BENDER

Table 3. Shell chemistry data determined by INAA and AAA, including growth rate, for animals growing sufficiently for analyses. Growth rate data for animals not analyzed are reported in LORENS(1978) SAMPLE1 NO.

INITIAL

GROWTH RATE (mm/8wk)

M9/$a (IO

6.8 7.4 8.0 8.5 9.4

3.8 3.9 1.3 0.9 2.4

6.6 6.2 4.9 8.2 6.7

0.94 1.10 1.19 1.11 1.18

12.6 6.9 10.4 9.9 10.6

7 2 n.d. 17 9 6

9.3 9.4 9.4 10.1

0.9 2.8

6.6 6.5 5.2 6.9

1.15 1.03 0.97 1.03

9.3 10.1 10.9 9.4

n.d. n.d. n.d. 7

6.2

1.01

11.9

3

4.9 5.8

1.10 1.00

9.9 10.0

::i

1.03 1.26 1.09

11.4 10.2 10.9

4 7 4

1.11 1.06 1.02 1.02 1.10

12.9 12.3 11.2 12.4 9.2

LENGTH (mm)

l-l l-2 1-3 l-4 l-5 l-6 l-7 l-8 l-9 l-10

11.3

3.0 1.2 2.7

ATOM

RATIOS

IN THE

NEW

CALCITE Cl/

a

(10Y)

)

l-11

11.6

1.8

l-12 1-13 1-14 l-15

11.0 11.1 12.3 11.2

2.9 2.6 1.0 1.9

1-16 l-17

12.2 13.0 12.7 12.4 15.5

1.2 2.8 2.4

7.2 7.2 8.5 8.4 9.7

1.3 1.1

2-9 2-10 2-15

1.5 1.0 1.2

2.4 6.4 3.1 2.7 *

0.17 0.44 0.21

4.5 7.1 4.6

0.20 0.22

4.1 4.8

7 7 n.d. n.d. 8

2-16 2-17

11.6 12.2

1.3 1.0

2.0 1.8

0.21 0.18

3.8 4.1

11 n.d.

3-13 3-7

10.9 9.6

1.5 2.0

4.3 3.8

n.d. 1.14

13.4 12.8

4-l 4-2 4-3 4-4 4-5

7.9 8.8

3.1 2.7 2.6 2.3 2.8

4.6 6.0 5.1 6.3 6.1

0.80 0.90 0.77 0.91 0.86

10.8

7

9.1 7.7 10.3

n.d. n.d. n.d. 7

4-6 4-7 4-8 4-9

10.5

2.0

9.4 12.0

4.5 0.3 2.2 4.4

5.2 5.7

0.81 0.97 *

9.7 11.1

615 6.2

0.86 0.84

10.9 11.5

3 4

4.0

6.6

0.97

11.6

4

1.2 2.2 3.2 3.9

6.3 5.6 5.2 7.1

1.01 0.78 0.81 0.82

10.0

5 n.d. 7 3

2.4

10.3 9.4

3.6 1.3 2.8

6.0 12.6 5.9 5.5 5.9

0.81

4-18 4-19 4-20

11.2 11.5 11.8 12.6 14.8

5-l 5-2 5-3 5-4 5-6 5-J 5-8 5-9 5-10

11.0 11.0 11.5 11.1 12.6 11.1 12.7 11.4 12.2

0.9 1.7 2.3 3.2 1.6 3.7 2.9 3.7 3.7

10.0 8.0 8.6 13.9 7.1 7.4 6.9 8.2 6.1

1.90 1.69 1.59 1.72 1.51 1.76 1.51 1.45 1.45

10.6 11.8 10.0 11.6 11.3 12.3 11.7 12.8 12.2

5-11 5-12 5-14 5-15

11.2 10.7 11.3 11.6

Z:? 2.2 2.3

7.0 8.7 11.0 7.5

1.52 1.63 1.60 1.50

10.4 10.6 11.0 12.5

n.d. n.d. 4

5-16 5-17 5-18 5-19 5-20

11.6 13.1 12.6 14.4 14.0

4.2 2.0 2.9 0.7 3.0

14.6

1.51

14.0

8.0 7.9 *

1.51 1.60 *

11.3 13.4 *

6 n.d. 5 *

11.9

1.52

12.5

4

l-18 l-19 l-20 2-2 2-3

4-10 4-11 4-12 4-13 4-14 4-15 4-16 4-17

i:; 9.0

11.1 11.4

11.1 10.0 11.6 11.9 11.8

2.0 1.3

1.2

l

* 5.8 6.0 5.0 6.1

1.15 0.86 0.94 0.90

10.1

l

9.8 12.4 10.7

10.3 10.9 9.3

n.d. 8

10 8

5 5 *

4 n.d. 5 ‘Y 10 n.d. n.d. 8 7 7 n.d. n.d. 5

11

1269

The impact of solution chemistry Table 3 (contd.) SAMPLE NO. 6-l 6-2 6-3 6-4 6-5 6-7 6-8 6-9 6-10 6-12 6-13 6-14 6-15 6-16 6-17 6-18 6-19 6-20 7-l 7-3 7-4 7-6 ::; 7-9 7-11 7-12 7-13 7-14 7-17 7-18

INITIAL LENGTH (mm) 12.7 13.4 11.9 13.2 7.7 12.2 11.7 12.6 11.8 11.0 11.7 13.9 12.6 12.3 11.8 12.3 13.1 13.5 8.1 8.4 t:; 7.8 11.5 9.3 12.0 12.4 11.0 10.5 13.6 14.1

GROWTH ATOM RATIOS IN THE NEW CALCITE Na/Ca Cl/Na RATE Sr/sa Mg/Ca (mm/8wk) (103) (103) (102) (10 ) 1.7 1.4 1.1 2.4 2.7 2.2 1.9 1.2 1.8 1.6 ;:: 2.5 2.4 1.5 1.9 1.9 1.7 2.1 i:: 2.4 1.7 1.2 1.7 2.1 ::; 2.9 0.9 1.2

8.2 13.3 31.1 41.2 11.2 23.6 30.1 11.4 17.5 18.8 12.4 29.5 17.2 27.7 15.0 9.5 19.4 14.7

1.44 1.54 1.94 1.88 1.37 1.56 1.77 1.40 1.35 1.73 1.38 n.d. n.d. 1.67 1.48 1.45 1.50 1.68

11.4 11.7 15.0 15.5 12.5 14.1 14.1 12.4 9.9 15.1 9.7 13.7 13.6 14.7 13.1 12.5 13.7 12.1

57.0 44.3 51.4 36.8 50.6 39.3 71.7 39.6 63.8 58.9 38.6 54.7 60.0

0.62 0.70 0.66 0.61 0.64 0.62 0.69 0.65 0.80 0.67 0.62 0.66 0.70

16.3 14.4 16.5 14.6 15.3 12.0 18.8 12.3 16.8 15.7 16.6 14.9 14.8

n.d. 8 n.d. 5 6" n.d. n.d. 3 n.d. n.d. n.d. 1 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. : 4 5 5

* Sample not analyzed. 1Number denotes: solution number-shell number. ’ Not detectable.

for microprobe analysis was available. In all, only one mussel died (from solution 2) before they were all dispatched at the end of the experiment. Results of determinations for Mg/Ca, Sr/Ca, Na/Ca, and Cl/Na atom ratios in newly precipitated Mytilus edulis calcite are listed in Table 3. Also listed in Table 3 is the observed overall growth rate for each mussel that grew sufficiently for analysis. Growth rates of less than 0.9 mm/8-wks did not provide sufficient material for sampling. Plots of shell chemistry versus observed growth rate in the different solutions do not result in any significant correlations; these plots (except for sea water, Fig. 2) are not included here, but are available from the senior author or in LORENS(1978). Over 1000 spots in both new and old calcite and aragonite regions were analyzed by microprobe. This data set is tabulated in L~RENS(1978) and is summarized here in Tables 4 and 5. Maps of two sections are presented in Figs 5 and 6. Maps of sections from the other solutions are in L~RENS(1978) and are available from the senior author. In each map the indicated zones of new and old calcite, new and old aragonite, and the transition zone between new and old calcite are inferred from spot chemical compositions, Shell chemistry is compared to solution chemistry in Figs 3,4, 8 and 9. Strontium/calcium, MgJCa, and Na/Ca ratios in calcite as a function of solution chemistry are given, respectively, in Figs 3, 4 and 8. The Sr and Na data shown are the means determined from ficient growth

"0

1.5-

;1.00 ‘;

(I\

'lh,,

*0.5 -

01

0

GROWTH

2 RATE,

Fig. 2. Calcite composition

4 mm/WPJEEKS

versus growth rate of Mytilus

edulis in natural sea water. Each bar represents data from a

single mussel; the length of the bar indicates the uncertainty due to counting statistics.

1270

ROBERT B. LORENSand MICHAELL. BENDER

Table 4. Shell chemistry data determined by microprobe for calcite regions (one cr uncertainty). An asterisk indicates that the point is not plotted in Figs 4 and 5 _ __..____ ______.__._~~~ __ ~~~. ~ SAMPLE

REGION

I-18

new trans old

2-16

"W transl tram2 Old

N

Na/Cax103 Sr/Cax103 llg/Ca~10~ WaxlO Y.5+2.8 8.7f2.6 15.lf4.7

1.0+0.2 1.510.5 1.1co.3

10 40

O.JfO.2 0.9f0.5 0.4+3.2 1.4+0.3

6.1'1.4

9

0.7+0.4

5.iii.5

6.511.3

2-17

old*

3-10

new trans old

11 18 18

9.5ti.4 9.9%?.8 lS.ii2.3

i.VO.2 1.5iO.2

5.9+1.4 14.0t4.7 6.411.7

q.gi1,; 6.214.3 2.5ic.7

4-16

new trans old

17 12 23

8.qt1.4 e.o+1.4 14.2~3.1

0.7'0.1 1.1+0.3 l.OiO.2

ij.6iZ.l 19.7i5.5 8.ltl.l

2.90 9 4.7:2:9

5-14

new tram old

tk? 41

11.3t1.7 9.1+2.2 15.3i2.5

1.4kO.2 1.510.4 1.2f0.3

14.913.1 27.-'40. 8.9-2.8

3.6-1.4 8.3'4.1 2.3to.9

6-u 5-15

oId* Old*

2 4

10.821.4 13.&.8

1.7Q.i I.9iO.2

new

19 14 29

9.912.4 11.6f4.5 13.2+3.1

0.9t0.2 1.120.3 1.4%.2

%.?14. 75.230. 8.6i3.1

37 new 14 tram1 trans? 12 trans3* 5 trans4* 3 old 44

14.m.2 18.8k1.4 19.5Q.3 19.7+1.0 8.9f1.7 15.014.2

0.et0.3 1.5t0.2 1.310.2 o.e+o.4 1.4to.1 1.4ro.3

104.%3. 10.9‘fl.Z 7.6k1.9 128.%2. 37.fll. 9.0+2.5

7-18

tram

old 8-18

__._

_~.___

all the mussels grown, and are determined by INAA and AAA. The Mg data in calcite, Fig. 4, shows data determined by microprobe and also determined by AAA. The microprobe data for all elements is summarized in Tables 4 and 5; plots of the Sr and Na data vs solution composition are in LORENS (1978) and show the same features illustrated in Figs 3 and 8.

6.8;0 1 4.iit1:1

1.7?0.6

1.8'0.2 2.9’o.c

9.29.4 10.1r2.0 2.7+1.2 z,.52;,4 5.8Q.8 4.0*2.0 4.Q1.5 8.2i3.4 2.1ro.9

Some blocks of spots within a region have chemistries different from other blocks of spots within the same region; these areas are indicated in the map figures and separate means are listed in Tables 4 and 5. Aragonite shell Sr/Ca, Mg/Ca, and Na/Ca ratios as determined by microprobe are plotted against solution

Table 5. Shell chemistry data determined by microprobe for aragonite regions (one a unccrtaintyl ~._ ~. .~~_~ ~__ ~Na/Cax103

Sr/Cax103

~~/CW.103

5/Cax103

l-18

new old

7 14

20.0f1.9 22.5+3.0

2.2*0.4 1.7to.6

2.01‘0.5 1.5t0.6

2.0+0.4 1.7io.5

2-15

new old

::

13.&.0 25. IQ.0

0.6-(0.2 1.3t0.2

0.4ro.4 0.5t0.3

l.fzO.3 1.5t0.3

SAMPLE REGION

N

-.

2-16

old

19

24.7t4.0

1.4to.z

0.7*o.fi

1.4*0.4

2-17

new old

18 10

12.5?1.6 23.9f1.6

0.5to,3 0.820.3

0.3t0.3 1.3t0.4

1.4t0.4 1.3f0.4

3-10

new old

20 17

22.0’1.5 24.0%. 1

1.8fO.4 1.5t0.2

2.150.5 0.7+0.2

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Sr/Ca x lo3 IN SOLUTION Fig. 3. Sr/Ca ratios of new calcite determined by INAA (k I a) vs solution Sr/Ca ratios. The dashed line is the line, constrained to pass through the origin, which best fits the data.

1271

The impact of solution chemistry 140

OLD

t-

TRANSITION

NEW

t

20

0

0

5

10

0 Mg/Ca

10 5 IN SOLUTION

0

5

10

Fig. 4. MgCa ratios of each shell region (+ 1 u) vs Mg/Ca solution ratios. Filled circles are derived from microprobe analysis of the sections listed in Table 4. Open circles are derived from AAA analysis of individual shells (see also L,ORENS and BENDER, 1977). The open circles are placed in the iransifi5n zone section for comparison purposes o&y.

Fig. 5. Cross section of mussel grown in natural sea water (shell Nos l-18) with superimposed contours of Metal/Ca ratios determined by microprobe. Each dot records the site of one microprobe analysis. The different regions are identified by the codes: NC for new calcite, TZC for transition zone calcite, OC for old calcite, and A for aragonite.

TLG-1

1213

The impact of solution chemistry

I Y--Y%-+ S/C8

x 10’

I

4

6

I

I

12

IN CALCITE

Fig. 7. Mg/Ca ratios of each calcite region vs S/Ca ratios of each calcite region ( + 1 u) as determined by microprobe.

if

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‘51

IO50

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_

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0

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Fig. 8. Na/Ca ratios of new calcite determined by INAA (+ 1 u) vs: A, solution Na/Ca ratios, and B, solution Mg/Ca ratios. Points with no uncertainty indicated are single determinations; other values are means derived from 5 to 19 determinations as listed in Table 3.

30_

ATOM

RATIOS

IN CULTURE

N&C8

SOLUTIONS

Fig. 9. Sr/Ca, Mg/Ca, and Na/Ca atom ratios in new aragonite as determined by microprobe (+ 1 a) vs their respective solution ratios. The solid line in the Na/Ca graph is the Na/Ca ratio predicted by WHITE (1975).

1274

ROBERTB. LOKWG

chemistry ratios in Fig. 9. The relation and S in calcite is illustrated in Fig. 7.

between

and

Mg

DISCUSSION The data in this paper provides several tools for studying skeletal chemistry controls. First, one can examine the relationship between growth rate and skeletal chemistry. Second, from the dependence of skeletal chemistry on external solution chemistry, one can evaluate the effect of biological ion fractionation on shell chemistry. Third, temporal variations in skeletal chemistry are revealed by the microprobe analyses and give additional insights into the significance of skeletal mineralogy and chemistry. Aragonite and calcite shell compositions respond differently to changes in solution chemistry and will be discussed separately. For the sake of discussion, the sections analyzed by microprobe are divided into five zones. Old calcite and old aragonite refer to parts of the shell precipitated before the animals were captured. The transition zone refers to the first new calcite grown in the culture solutions; it is generally characterized by having the highest Mg/Ca ratio in the shell (e.g. Figs 4 and 5). New calcite is the calcite grown in culture after the transition zone was deposited; new aragonite is the aragonite grown in culture. Mineralogy and solution composition are the major determinants of the trace metal composition of biogenic calcium carbonate; temperature and salinity have a secondary role (MILLIMAN, 1974). Compared to calcite, aragonite more readily accepts larger cations, such as Sr2+. due to its more openly spaced crystal lattice and greater Ca-0 bond distances. [The higher density of aragonite, as compared to calcite, results from more efficient packing of the carbonate units in aragonite (Lippmann, 197311. Ions with radii smaller than the radius of Ca’+, such as Mg2+. are more readily substituted into the calcite lattice. The relationship between solution composition and mineral composition is expressed through the use of distribution coefficients defined by the equation:

where D is the distribution coefficient relating the metal (Me) to the calcium concentration ratio found in the solution and in the calcium carbonate polymorph. Presumably. if the distribution coefficient is known from inorganic experiments, then one can predict the metal to calcium ratio of either the solid or the solution, if a value for one is known. Similarly, when applied to biogenic calcite where the solid phase is known and sea water is used as the solution phase, a difference in D from the expected value can be attributed to a difference in extrapallial fluid composition relative to sea water composition. A major problem, however, is that to do this one must be certain that the distribution coefficient used correctly describes

MICHAEL L. BENIZK

the situation to which it is applied (see LORENS, 1978). In the present paper, shell versus solution relationships are evaluated in terms of both natural and experimental distribution coefficients; the object is to separate biological and inorganic influences leading to the observed metal composition of the biogenic calcium carbonates grown in this study, Relath

he~een

growth ruts’ und skeletal composition

Our results do not support previous conclusions (ZOLOTAREV, 1975; PILKEY and GOODELL, 1963; SWAN, 1956) that the skeletal chemistry of molluscs is related to growth rate. No significant trends are evident when either Sr/Ca, Mg/Ca or Na/Ca ratios of calcite grown in this study are compared to mussel growth rates. Plots of calcite shell chemistry versus growth rate for mussels from each solution are given in LORENS (1978); here. only plots for those mussels grown in natural sea water are presented (Fig. 2). BUKCHARDT and FRITY (1977) performed an expcriment similar to that reported here on the uptake of Sr by aragonitic snails; they also found no variation in Sr content that could be attributed to tither growth rate or temperature. At the same time. we cannot rule out the possibility that certain environmental variables causes cation ratios and growth rates to covary sympathetically.

The Sr:‘C’a ratios of newly precipitated calcite increase linearly with increases in Sr/Ca solution ratios (Fig. 3). The distribution coefficient equals 0.13 _t 0.01, as calculated from the slope of a best fit line through the new shell data. Experiments by KATZ et al. (1972) indicate that, for an inorganic precipitation, the distribution coefficient for Sr in calcite should be 0.05 (25°C). Such a difference could result from a biological fractionation effect; however, a study of cave limestones, and the solutions from which they precipitate, yields distribution coefficients similar to that found in Mytilus edulis calcite (HOLLAND et al.. 1964). The difference between D found for Mytilus edulis and cave limestone. and D found by Katz is probably due to the precipitation rate dependence of D, rather than a difference between the Sr content of sea water and the Sr content of the extrapallial fluid. LORENS(1978) finds that Dsl-for calcite is a function of calcite precipitation rate. At an estimated rate of biogenie calcite precipitation, the measured inorganic distribution coefficient is 0.09.-0.12, so no fractionation anomaly is implied for the biological distribution coefficients. Mugnesium in culcite The Mg/Ca ratio of new calcite increases exponentially with the Mg/Ca ratio in the culture solution (Fig. 4). Some of these data are reported in LORENS and BENDER (1977). They conclude that Myths edulis normally excludes Mg from the outer extrapallial fluid. When the Mg content of the experimental cul-

The impact of solution chemistry ture solution is raised above the normal Mg content of seawater, the biological Mg exclusion mechanism becomes saturated and is no longer able to effectively control outer extrapallial fluid Mg concentrations. The result is a nonlinear increase in the Mg content of the outer extrapallial fluid, which is reflected by a similar increase in the Mg content of Mytilus edulis calcite. The microprobe data reported in this paper support the conclusion summarized above and provide additional evidence that biological processes are at work influencing the Mg content of Mytilus edulis calcite. In the shells analyzed by microprobe, the very first growth of tank calcite is a transition zone characterized by Mg/Ca shell ratios that are substantially greater than those found in either old or subsequently grown new calcite (Figs 4 and 5). An exception occurs in mussels grown in solution 8 (Fig. 6), where the Mg/Ca ratio of parts of the transition zone are substantially lower than in the new calcite; this will be discussed later. We speculate that the increase in the Mg/Ca ratio in the transition zone reflects a temporary deterioration of the Mg exclusion mechanism resulting from the stress of capture and the adjustment to a new environment. Eventually, as indicated by the lower Mg/Ca ratio in new calcite, the organisms acclimate to their new environment and recover at least part of their ability to discriminate against Mg. If a temporary breakdown in the Mg exclusion mechanism is a generalized response to stress, it may provide a current or historical indicator of pollution or natural disasters.* As mentioned in the last paragraph, parts of the transition zone (TZC-1 and TZC-2, Fig. 6) of the mussel grown in solution 8 are unique in that they have a lower Mg/Ca ratio than the adjacent new calcite; these areas also have Sr/Ca and Na/Ca ratios higher than those in the new calcite. These three features are in fact diagnostic of aragonite, and aragonite may have precipitated, rather than calcite, in this region of the shell. If the pallial attachment had advanced to near the tip of the shell, the region TZC-2 (Fig. 6) could have been precipitated by the aragonitic precipitating mantle. However, we suspect that this is not the case, because calcitic region TZC4 separates region TZC-2 from the aragonitic region (NA and OA), and because of the difference in chemistries between the new aragonite layer (NA) and the anomalous aragonite layer (TZC-2). The alternative, that aragonite has precipitated in a normally calcitic area *We should note here that kinetic factors as well as solution composition are thought to influence the Mg content of calcite. (See discussions by LAFONer al., 1978). In this study it is not possible to elucidate the impact of kinetics on the Mg content of shell calcite. However, in a gross sense, it appears that in this study kinetics plays a secondary role to solution composition in causing changes in shell calcite. Major changes in shell Mg chemistry occur sympathetically with solution chemistry changes, yet shell growth rates are much the same in each solution and no correlations are found within each solution between Mg content and growth rate.

1275

due to an overload of Mg, is suggested, but not confirmed, and certainly requires further experimental attention. Sulfur in calcite The S/Ca ratio in Mytilus edulis calcite is highly variable, probably reflecting variability in the abundance of organic matrix in the skeletons. Culture solution sulfur (as sulphate) concentrations are constant and therefore not the cause of variability. Sea salt contamination is also ruled out as causing variability in S/Ca ratios, because S/Ca ratios do not correlate with Na/Ca ratios. We favor the idea that S is associated with the organic phase. According to MILLIMAN(1974), the organic matter content of Mytilus edulis is 1.5-5.5 wtp/O of the total shell. The organic matrix, which consists of a polysaccharide protein complex, contains sulfur as ester sulphates attached to sugar moieties (HUNT, 1970). The S content of Mytilus edulis organic matrix is unknown, but Mercenaria mercenaria matrix contains almost 10% by weight ester sulfates (CRENSHAW,1972b). Using these approximations one can calculate a S/Ca range of 4 x 10e3 to 17 x 10e3; this is similar to what is found here (Fig. 7). There is a rough correlation between Mg/Ca and S/Ca ratios in Mytilus edulis calcite (Fig. 7). For two reasons it is unlikely that this correlation reflects binding of Mg on to organic S04. First, there is not sufficient Mg to account for a S-Mg stoichiometric relationship (60-85% of the shell Mg is found in solid solution with calcite; LXJRENSand BENDER, 1977). Second, studies of the metal binding characteristics of calcium carbonate associated organic matrices show them to be almost completely specific for Ca*+ (see CRENSHAW,1972b, on Mercenaria mercenaria organic matrix and W~C~DWARD and DAVIDSON,1968, on porcine costal cartilage). Since Mg inhibits calcite nucleation and crystal growth (PYTKOWICZ, 1965; BERNER, 1975) Mytilus edulis may compensate for increased Mg levels in the calcite forming extrapallial fluid by secreting additional sulfate bearing organic matrix, thereby providing additional nucleation sites and higher Ca concentrations at these nucleation sites. CRENSHAW(1975) and WC~~DWARDand DAVIDK~N(1968) suggest that Ca-binding sulfate units of the organic matrix sequester Ca in localized spots, then release the Ca into solution. This causes a localized supersaturation sufficient to seed calcite crystal growth. If the organic matrix is acting as a nucleating agent in this manner, the degree of supersaturation required for nucleation may be controlled in part by the degree of sulfonation of the organic matrix, and in part by the quantity of organic matrix. Once crystal growth is initiated, it can be maintained by higher than normal carbonate concentrations. Sodium in calcite

WHITE (1975) has studied the inorganic coprecipita-

1176

ROBERT B. LORENS and MICHAELL. BENDER

tion of Na into calcite. His data predict that calcites precipitated from solutions which have Na/Ca ratios similar to seawater will have Na/Ca ratios in calcite of 0.6 x 10e3 at a pH = 7.8, and 1.0 x lo-3 at a pH = 8.8. This amount is substantially less than what is observed in Mytilus edulis calcite. The Na/Ca ratio of the new calcite produced by animals grown in natural seawater, solution 1, is (10.6 + 1.5) x 10m3. Earlier studies by the authors using the INAA technique find low Mg calcite planktonic foraminifera to have ratios of (6.5 & 1.5) x low3 (LORENSet al., 1977; BENDERet al., 1975). This is lower than in Mytilus edulis, but still above White’s prediction. This data might suggest that Na is concentrated in the calcite producing extrapallial fluid. However, this is not a likely explanation, since a tremendous influx of Na would be required to obtain the observed ratios by equilibrium precipitation; according to White’s results, a NaiCa ratio greater than 500 would be required. It is difficult to account for the decrease in shell Na that occurs with increasing solution Na (Fig. 8A) and its direct relation with solution Mg (Fig. 8B). WHITE (1975) found that Mg can compete with Na for lattice positions, causing a depletion in Na when the Mg content of calcite is high; it is a small effect and in the opposite direction to that observed here for Mytilus rdulis, where the highest Na and highest Mg shell contents are found together. Our favored explanation assumes that a significant fraction of the total Na is adsorbed onto the calcite crystal surface. TRAVIS(1968) has studied the organization of the Mytilus edulis prismatic layer: she finds that individual calcite crystals are surrounded by an organic sheath much like the brick and mortar arrangement of a brick wall. The actual crystal surface area of a whole shell is then much more than its external surface area. From Travis’ measurements, the dimensions of an average crystal are 125 x 860 x 860 A, with an approximately 20 A thick organic sheath between the calcite crystals. Using a calcite density of 2.7g/cm3, one can calculate that each average crystal contains 2.5 x lo-” moles of Ca. WHITE’S (1975) data suggest that the bulk crystal has a Na/Ca = 1 x 10m3, which gives 2.5 x lo-” moles Na in the crystal. For Mytilus rdulis, the observed Na/Ca = 10 x 10e3, which gives a total of 25 x IO-” moles Na. The difference, 23.5 -+ 10e21 moles Na. according to this explanation. is adsorbed at the crystal surface. A surface area of 1.9 x lo-” cm2 is calculated for the average crystal. MOLLERand SASTRI(1973), from both experimental evidence and theoretical calculations, report that there are 8.3 x lo-” moles of Ca-exchangeable sites per cm2 of calcite surface. If this last number is analogous to the number of potential adsorption sites on the crystal surface. then surface-adsorbed Na would occupy only a little over lo”,, of these sites. Earlier in this section the increase in organic matrix content of these shells was discussed. The excess or-

ganic matrix found in the calcite produced from the high Mg concentration solution might be used to form additional crystal chambers of smaller than normal dimensions, thus affording tighter control of the mineralization process. By analogy, a bricklayer would use more mortar to build a wall, using the same spacing between bricks, if the bricks were of smaller dimension. If the calcite crystals are smaller, they will have a higher specific surface area, and will therefore adsorb more Na ions. This model can explain the relationship between shell Na and solution Mg. We speculate that the excess Mg causes Mytilus edulis to exert more control on the mineralization process, which results in smaller calcite crystals of higher specific surface area, which then allows a higher Na content of the whole calcite shell. The microprobe data (for example, Fig. 5) shows that. in both the new calcite and old calcite regions, the highest Na/Ca ratios are on the outer side of the shell, and they decrease going toward the inner shell side. The opposite behavior is discerned for the Mg/Ca ratios. The cause of these gradients is unknown. Strontium, mngnesium, and sodium in aragonite.

As in calcite, the Sr/Ca ratio of Mytilus edulis aragonite increases linearly with increases in the Sr/Ca ratio of the solution. A distribution coefficient of 0.23 f 0.04 is calculated from the slope of the data in Fig. 9; this is similar to the value of 0.237 + 0.027 found by BUCHARDTand FRITZ (1977). Inorganic experiments, on the other hand, have given a distribution coefficient of 1.15at 20°C for aragonite (KINSMAN and HOLLAND, 1969). In addition, aragonitic ooids, aggregates, algaes, and coelenterates are reported to give distribution coefficients ranging from 1.0 to 1.3 (MILLIMAN,1974). We are at a loss to explain why molluscs reflect one distribution coefficient whereas other organisms and inorganic aragonite reflect another. Sr exclusion by molluscs is probably not the answer. For one thing BUCHARDTand FRITZ (1977) changed the Sr/Ca ratio in their solutions over 6 orders of magnitude and still found a linear relationship between the shell and solution compositions. If a biological fractionation process is at work depleting the Sr content of the aragonite-producing extrapallial fluid, it reacts quite differently to an excess in Sr concentration than does the Mg-excluding process in the calcite-secreting mantle. For another, one doubts that molluscs should work to exclude Sr from their extrapallial fluid, since Sr in the amounts present in both fresh and saline environments does not affect the precipitation of aragonite. In contrast to the response of calcite, the Mg/Ca ratio in Myths edulis aragonite increases linearly with increases in Mg/Ca solution ratios (Fig. 9). From the slope of the line a distribution coefficient of 3.5 x 10m4 is calculated. Aragonitic ooids reflect a distribution coefficient of 4 x lob4 (MILLIMAN,1974). We know of no published experimental work on the

1217

The impact of solution chemistry laboratory coprecipitation of Mg in aragonite. From the inner extrapallial fluid analysis that CRENSHAW (1972a) reports, there does not appear to be any significant Mg fractionation process occurring in the aragonite-precipitating mantle of Mytilus edulis. It is interesting to note that as pelecypods grow, the function of the individual cells comprising the mantle changes. KENNEDY et al. (1969) suggest that initially mantle cells secrete periostracum, then as the animal grows they produce outer shell layer, become part of the pallial attachment, and then produce inner shell layer. If this is correct, then mantle cells which at one time actively regulate Mg passage into the calciteproducing extrapallial fluid eventually cease to regulate Mg passage into the aragonite-producing extrapallial fluid. WHITE’S(1975) data predict Na/Ca = 12.9 x 10m3 for the inorganic precipitation of aragonite from sea water (Fig. 9); Myths edulis Na/Ca ratios are 50% above this prediction. The difference between observed and predicted Na/Ca ratios is less than what is found for calcite as discussed earlier. If a biological fractionation effect is responsible for the difference in observed and predicted values, then either the Na activity must be enhanced, or the Ca activity depressed in the inner extrapallial fluid. CRENSHAW’S(1972a) measurements of the aragonite-producing fluid indicate little change in Na concentrations. He finds calcium concentrations to acutally increase by about 25% relative to sea water, but this is due to binding with organic molecules which could decrease the calcium solution activity. However, this change does not appear to be sufficient to explain the Mytilus edulis Na/Ca ratio in terms of White’s distribution coefficient. The difference might be explained by a nonequilibrium effect similar to that shown for Sr in calcite (LORENS, 1978); however, data for Na are not available. CONCLUSION Two questions asked about calcifying organisms are: what controls the minera!ogy of the precipitate, and what controls the minor element composition of the precipitated calcium carbonate? In this report we have attempted to answer these questions in part by studying the effect of solution composition on shell chemistry and mineralogy. Myths edulis precipitates both calcite and aragonite, each from a distinctly different extrapallial fluid. Magnesium, which in high concentrations inhibits the growth of calcite, appears to be extensively regulated by Mytilus edulis so as to minimize the Mg concentration of the calcite producing extrapallial fluid. When stressed with higher than normal external Mg concentrations, Mytilus edulis responds to the increase in extrapallial fluid Mg by increasing the amount of organic matrix (as indicated by sulfur) associated with the calcite. Previous reports (see KENNEDY et al., 1969; for review) support the view that the

organic matrix is a prime factor determining whether calcite or aragonite is precipitated. Our results give no independent information on this question; however, they do suggest that magnesium concentration could be a critical factor determining shell mineralogy, since Mytilus edulis may have produced aragonite in the prismatic layer when magnesium concentrations in the calcite-producing extrapallial fluid became very high. In contrast to the calcite-secreting mantle cells, aragonite-secreting mantle cells do not appear to regulate magnesium. Increases in aragonite Mg are directly proportional to increases in the external solution Mg concentration. The Sr/Ca ratio of Mytilus edulis aragonite and calcite increases linearly with increases in solution Sr/Ca ratios. No evidence for significant biological fractionation of Sr in the calcite-producing extrapallial fluid is found. Differences in distribution coefficients suggest that Sr may be discriminated against in the aragoniteproducing extrapallial fluid. Although Na is found in Mytilus edulis aragonite and calcite at levels higher than predicted by available inorganic distribution coefficient data, we doubt that Na is biologically concentrated. Sodium in aragonite increases linearly with increases in solution Na concentration. In contrast, Na in calcite increases with increases in solution Mg concentration, not Na concentration. The relationship for calcite may be explained if a significant fraction of the Na exists adsorbed to the crystal surface, rather than in the bulk crystal lattice. The utility of biogenic carbonate to paleo sea water composition enthusiasts appears to be limited by the ion regulating capability of calcifying animals. Changes in metal composition due to changes in growth rate appear to be minimal. A study of Mg using calcite secreting organisms is not recommended; aragonite provide a more direct relationship between Mg in solution and Mg in calcium carbonate. What appears to be a significant adsorption effect will make a study of Na in samples difficult, since variations in Na content could be expected as crystal dimensions change. Strontium has the most straightforward relationship between solution and solid; strontium concentrations in both aragonite and calcite are directly proportional to solution Sr concentrations. Acknowledgements-We would like to acknowledge H. SIGURDSSON and C. NIELSONwho taught R.B.L. how to use their electron microprobe. We also thank A. N. SASTRYfor the use of his animal culture facilities. Financial aid was provided by NSF (grant No. DES 37125).

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ROBERTB. LORENSand MICHAELL. BEN~XH

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