Relationship between water-soluble cations and palaeosalinity

f3ecxWcs

et Camachimica

Acta. 1974. Vol. SE, pp. 667 to 576. Pergsmon has.

Printed In Nortbm

Ireband

Relationship between water-soluble cations and palaeosalinity DAVID ALAN SPEARS Department of Geology, University of Sheffield, Sheffield, Sl 3JD, England (Received 13 July 1973; accepted in rev&d form 9 October 1973) All&r&-Finely ground shale samples from 8 Carboniferous borehole sequence were sh8ken in water and the concentration of the water-soluble Ka, K, Ce and Mg determined. The marine ehales in the sequence were chsmcterized by low concentrations of Ne snd K, and high concentrations of Ca and Mg. The reverse situation ws8 found for the non-merine and brackish shales. The electrical conductivity of the water extract ~8s higher for the marine shales than for the non-marine/brackish shales. It is suggested that the water-soluble cations were present in the sediment at the time of deposition 8s exchangeable cations which were released into the pore water during diagenesis 8s some of the exchange sites were eliminated. INTRODUCTION

earlier paper, the variation in the concentrations of exchangeable Na, K, Ca and Mg through a sequence of Carboniferous shales ww described (SPEARS, 1973). The variation is apparently related to the palaeosalinity. During the course of thie work, it became apparent that water-soluble cationa were present. This paper describes these cations in the Carboniferous shales. Little published information is available on the concentration of water-eoluble cations in shales. Data on pore-water chemistry is virtually restricted to sandstones. Notable exceptions are provided by the work of KEEL (19638, b), WEAVER and BECK (1971) and VAN MOORT (1971).

IN AN

SAMPLES The samples analysed in this work are the s8me ss those previously encllysed for exohsngeable cations (SPEARB, 1973). They were obtained from a National Coal Board borehole (Grid Kef. 44/528168) from depths ranging from 947 ft to 971 ft. It can therefore be assumed that overburden pressums and heat flow have been the same for 8ll samples. All samples should therefore have reached the same degree of diagenetio alteration. The diameter of the core at thi8 depth wa8 6 in. (15.2 cm) and core recovery WBBgood. The appearance of the core suggested that contsminetion by drilling fluids was negligible. As 8 safeguard. scrmpleswere t&en from the interior of the core. The shales 8re impermeable and contamination in the middle of the core would not be expected. Sample depth in the core and Bample numbers are shown on Fig. 1. Also shown is the deposition81 environment bssed on the macro-fauna. This is a marine incursion into 8 dominantly non-merine coal-bearing sequence of Upper Cerboniferous (Westpheli8.u) age. METHODS A finely ground shale sample (200 mg) WBBpieced in 8 stoppered centrifuge tube and 20 ml of distilled water added. The tube WBBmechanically sheken for 10 min and then allowed to stand for 1 hr before centrifuging and decanting. Amdyses were made using a Perkin-Elmer 303 atomic absorption spectrometer. The results for Na, K, Ca end Mg sre given on Table 1. The Fithian snd Morris reference illite samples have been used to illustrate the pwoision (Table 1). These s8mples are similar to the present samples in mineralogy. age snd f8oies, but they differ in that they are from outcrop rather than cores. The electrical oonductivity of the water extmct ~8s also messured. This WBBdone using 8 Grit% conductivity bridge. The results obtained 8re given on Table 1 snd 8re plotted on Fig. 2. 567

568

DAVIDAIANSPEARS No and K ,

ppm IO00

-

Co and Mg

,

iwm

I500

t-40’ e---K l

950 F&h

[

c

Brackish

i Ma&e --iII ---[II Bmdrish ?

1 Fresh

Fig.

1. To show the variation of ~vater-soluble Na, K, Ca and Mg through the sequence. The depositional environment is based on the macro-fauna.

DISTRIBUTION

OP

WATER-SOLUBLE

CATIONS

THROUGH

THE

SEQUENCE

The variation in the concentrations of Na, K, Ca and Mg through the sequence is shown on Fig 1. Very cIearly there are major stratigraphic changes in the concentrations. The distribution of Ca is very similar to that of Mg ; both elements are present

Conductivity,

pV

/cm

Co tMg. m-equiv I IOOg

NoiK, m-equiv. / 100 g 0 k

i

i

too

150

200

enwronmont

Deposdionot

.

Brackish

r

Brackish

?

Fig. 2. To show the variation in the electrical conductivity of the water extract through the sequence. The sum of the Na and K, and the sum of Ca and Mg am also shown.

Relationship

between water-soluble

569

cations and palaeosalinity

Table 1. Concentrations in ppm whole rock of water-soluble Ne, K, Ca and Mg, through the Carboniferous shale sequence. Also shown is the electrical conductivity of the water extract in micromhos/cm Sample number

Ne

K

Ca

(ppm)

(ppm)

Mg (ppm)

Conductivity

(ppm)

684 680 687 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 26 26 27 28 29 30 31 32 689 691

945 1025 955 1355 1210 1145 1370 930 1590 1000 760 1030 1050 1135 910 1290 1205 1230 1465 1755 390 650 465 215 255 555 26 21 1310 1550 775 1620 555 525 390 1065 1355

260 560 595 785 830 800 835 995 970 1040 870 820 985 725 84.5 835 705 1350 1075 730 71 57 72 18 28 13 22 17 31 325 220 390 165 180 13 310 335

1 1 1 7 1 1 20 1 10 1 1 1 1 1 1 3 1 1 6 11 755 605 600 630 600 500 570 475 460 14 185 9 745 640 795 1 1

2 2 3 12 9 11 13 12 15 14 13 11 12 8 10 11 8 14 14 9 950 830 860 995 1030 825 1590 1445 450 6 52 6 200 170 765 1 1

51.0 57.8 51.8 38.7 41.5 39.7 41.8 40.2 67.2 48.9 40.2 45.9 46.0 48.2 43.2 74.0 58.3 59.8 75.4 88.0 184.9 163.5 161.9 186.0 187.4 162.0 239.9 285.2 153.4 91.6 99.0 80-l 99.0 83.0 160.6 57.5 80.7

Fithian illite 10 determinations Standard deviation Morris illite 6 determinations Standard deviation

62

285

1245

580

7

12

150

17

650

1255

190

290

17

115

7

12

@mho+m)

DAVIDALANSPEARS

570

at very low concentrations in the upper part of the sequence, and both increase markedly between samples 17 and 18. This increase in Ca and Mg takes place in that part of the sequence where there is believed to have been a change in the palaeosalin&y. The high Ca and Mg concentration correspond to the marine env~onment, and the low concentrations to the fresh/brackish environment. It is also between samples 17 and 18 that major changes in the exchangeable cations occur (SPEARS, 1973). The water-soluble Na and K concentrations show the reverse behaviour to Ca and Mg. Concentrations are high in the upper part of the sequence and fall in the middle. Again, there is a sudden change between samples 17 and 18. At the base of the sequence, the variation in all four elements is more complicated. In part, this can be attributed to mineralogical changes. Samples 30 and 31 are limestones, and sample 28 is phosphate-rich. If the non-clay samples are excluded (28,30 and 31), then the samples 27, 29, 689 and 691 all have low concentrations of Ca and Mg and high concentrations of Ns and K. These values are comparable with those in the fresh~rackish samples higher in the sequence. Samples 689 and 691 are straightforward because they were deposited in a fresh-water environment prior to the marine incursion. Based on the macro-fauna and the exchangeable cations, it was previously suggested that samples 27 and 29 were deposited in an environment where the marine influence was reduced (SPEARS, 1973). The concentrations of water-soluble cations are consistent with this view. Sample 32 is marine on the fossil evidence and the water-soluble cations are comparable to those in the overlying marine shales. The major differences recorded in the concentrations of Na, K, Ca and Mg through the sequence can thus be related to the nature of the depositional environment and to the palaeosalinity in particular. THE RELATIO~S~ BETWEEN THE EATER-SOLUBLE CATIO~SS AND THE DEPOSIT~ONALENYEBOWMENT If the water-soluble cations are present in connate water, this could account for the agreement between chemistry and depositional environment. It is conceivable that the differences detected in the water-soluble cations are due to original salinity differences in the pore-water. Average values for the soluble cations in marine and non-ma~ne~brac~sh shales are presented on Table 2. Also shown on Table 2 are average values for river and sea-water. The total concentration of cations does not Table 2. Average water-soluble Na, K, Ca and Mg concentrations (ppm whole rock) for marine and non-marine/brackish ehales. Also shown are average values (ppm) for river and sea-water

~on-marinelbr~~3h (samples 087-17) Marine group (samples 18-25) Hiver water? Sea-water? * Standard

deviation

group. + KRAUSK~PF,

1967.

group

1187 (255)*

877 (172)

322 (234) 6.3 10,556

37 (25) 2.3 380

on sixteen samples non-rna~ne~br~~ah

4 (5)

591 (85) 15 400

11 (3)

1064 (291) 4.1 1272

group, eight samples marine

Relation&p between water-soluble cations and palaeoaalinity

571

increase from one group of shales to the other corresponding to the increase from river to sea-water. Possibly the concentration8 in the fresh/brackish shales could have been enhanced due to Donnan phenomena. In the fresh water (low electrolyte) environment, interpenetration of the Gouy layers on the clay minerals should occur at an early stage in compaction (BERNER, 1971, p. 111). Ion8 should therefore be retained within the sediment. On the other hand, in the marine (high electrolyte) environment, interpenetration of the Gouy layers should only occur in the final stages of compaction. Most of the ions present in the pore water should therefore escape. While this explanation could account for the comparable cation totals in the two groups of shales. it does not explain the cation proportions. In the marine shales, the proportions of the water-soluble cation8 should bear some resemblance to the proportions in sea-water, but this is not the ca8e. In the fresh/brackish shales, if extensive salt filtration had occurred, retention of divalent rather than univalent cation8 would be expected. It must be concluded that there is no simple relationship between the water-soluble cation8 now present and the composition of the original pore water. The lack of such a relationship suggests that diagenetic changes have been important. The relationship of the water-soluble cations to other aspects of the chemistry, including the exchangeable cations, has been investigated by calculating correlation coefficients between pairs of variables. Significant correlations between relevant variables are shown on a correlation matrix (Table 3). The correlation between the water-soluble cation8 follows from the relation8hipe shown on Fig. 1. (In the marine shales, Ca and Mg increase, Na and K decrease.) Thus correlation8 between Na and K and between Ca and Mg are positive, whereas cofielations between either Na or K against either Ca or Mg are all negative. The correlation8 involving exohangeable cation8 reflect the decrease in exchangeable Na, K and Ca, and increase in Mg in the marine shalea (SPEARS, 1973). Thus water-soluble Na and K correlate positively with exchangeable Na, K and Ca. Water-soluble Ca correlates negatively with exchangeable Na, K and Ca. Unfortunately, exchangeable Mg only shows a significant correlation with whole rock Ca. This lack of correlation is due to the small increase in Mg compared with the other cations, hence variation8 in clay abundance and minor carbonate are more important. In the earlier work (SPEARS,1973) this problem was partly overcome by using the ratio of exchangeable Mg to exchangeable Ca. This ratio shows a negative correlation with water-soluble Na and K and positive correlation with water-soluble Ca and Mg (Table 3). Also included on Table 3 are some whole rock variables. The correlation8 involving these generally reflect variations in clay abundance and do not provide any additional information on the origin of the water-soluble cations. The correlation matrix therefore illustrates the relationships between the water-soluble cations and their relationships with the exchangeable cations. The distribution of water-soluble and exchangeable cation8 through the sequence is very similar and is closely related to the palaeosalinity. These observations are interpreted in the following discussion to mean that palaeosalinity controls the exchangeable cations and that these ultimately give rise to the water-soluble cations. Diagenetic reactions must be involved, but before considering this aspect, the possibility of leea direct link8 between the water-soluble cations and sediment type must be eliminated.

DAVID ALAN SPEARS

572 Table

3. Correlation

matrix for whole rock, water-soluble and exohangeable cations in Carboniferous shales Whole rock

Na

K

Ca

Water-soluble Mg

Al

Na

K

Ca

Exchangeable Mg

Ne,

K

Ca

Mg Mgj Ca

- t_ greater than 95 per cent confidence level. 8 @ greater than 99 per cent confidence level. Excluded from the analysis are the non-shale samples numbers 28, 30, and 31.

The marine shales do contain more pyrite (SPEARS, 1964). At outcrop, pyrite decomposes relatively rapidly giving rise to acid solutions. It is conceivable that pyrite partially decomposed in the extraction procedure, and this is responsible for the differences recorded between the shale types. The same differences were recorded, however, when the dissaggregation and the extraction were made in a nitrogen atmosphere. Additional evidence that pyrite is not responsible is provided by the occurrence of this mineral elsewhere in the sequence. [Sulphur percentages in samples 6 and 689 are I.93 and 149, respectively, (SPEARS 1964, Table l).] In these samples there is no evidence that the water-soluble cations are influenced by the presence of pyrite. Pyrite is not the only diagenetic mineral to occur in the sequence, but it is the only one which occura uniformly within one group of shales. Other diagenetic minerals occur sporadically through the sequence and can therefore be eliminated as a control on the water-soluble cations. It might be suggested that the differences in the water-soluble cations between the marine and non-rna~ne~br~~sh shales is due not to palaeosali~ty but to some other aspect of the environment, such as pH or Eh. There is not, however, any supporting evidence for this. The evidence provided by the diagenetic iron minerals, for example, indicates similar Eh-pH conditions during early diagenesis in the different shales. DLAQENETIC HISTORY OF THE SHALES

The water-solubIe cations now present in the shales do not bear a simple relationship to those present in the original pore water. Presumably the original composition has been greatly modified by diagenetic changes, but in such a manner that there is

Relationship between water-soluble cations and palasosalinity

673

still a vertical variation through the sequence corresponding to the depositional changes. This variation ha8 not been destroyed by the movement of pore fluid8 The exchangeable cations vary through the either before or after compaction. sequence in a similar manner, but in this case the cations now present can be directly related to those originally present. A satisfactory explanation for the origin of the water-soluble cation8 has to take all the above factors into account. Probable diagenetic change8 involving the clay minerals must also be considered. The clay minerals in the Carboniferous shales are, in order of abundance, illite, kaolin& and chlorite. Treatment with ethylene glycol shows that the illite contains a small proportion of montmorillonite layers. The diagenetic formation of such a clay assemblage has been investigated by a number of writers. Notable contributions have been made by BURST (1959), DUNOYER DE SEGONZAC (1964) and more recently by PERRY and HOWER (1970), WEAVER and BECK (1971) and VAN MOORT (1971) These authors have recorded change8 in claymineralogy and various aspects of the chemistry as a function of the depth of burial (increasing temperature). There is substantial evidence that as the diagenetic alteration increases there is a progressive increase in the number of illite layer8 at the expense of montmorillonite layers in mixed-layer ill&-montmorillonites. It is therefore a reasonable assumption that the illitc in the Carboniferous shales originally contained a higher proportion of montmorillonite layer8 than it does now. The conversion of these layers during diagenesis wouldleadto a reductionin cationexchange ca.pacity. Theproportionsof exchangeable cations in these shales are comparable with modern sediments, which suggest that K does not preferentially replace any particular cation during diagenesis, but rather the replacement is controlled by the accessibility of suitable Sites. The displaced cations could constitute an important part of the water-soluble fraction. It is worth noting that the disaggrcgation technique used in this work will not reproduce the original grain size. Fracturing of original grains will be inevitable. Thus sites become available for exchange which may have been inaccessable during diagenesis. This is one way in which the original proportions of exchangeable cation8 could survive diagenesis. The water-soluble cations in the shales could be derived in part from the convereion of montmorillonite layers during diagenesis. Other exchange sites destroyed during diagenesis would also contribute. Colloidal material could also be important. Thus organic colloids are involved in reaction8 at all stages in diagenesis (coalification) not only in coals but also in sediments (TEICHMOLLER and TEICHMULLER, 1968). If the water-soluble cations were originally present on exchange sites, which were destroyed during diagenesis, then this would explain the relationship8 which now exist between the water-soluble and exchangeable cations. The positive relationships between water-soluble and exchangeable Na, K and Mg can be accounted for. It is more difficult, however, to explain the negative relationship between water-soluble and exchangeable Ca. It is possible that the water-soluble Ca is now high due to the former presence of shell debris in the marine shales. There is very little, if any, calcium carbonate now present in these marine shales. Furthermore, in the Gulf Coast sediments, the decrease in whole rock Ca values was attributed to solution of calcium carbonate (PERRY and HOWER, 1970). If the above explanation for the origin of the water-soluble cations is correct,

574

DAVIDALANSPEARS

then cations must have been released at an advanced stage of compaction otherwise the chemistry would be dominated by the connate water. A favourable combination of pressure and temperature could be the explanation. Analyses of water-soluble cations reported in the literature (WEAVER and BECK, 1971; VAN MOORT,1971)differ from those in this paper in that Na is the major cation followed by K and with smaller amounts of Mg and Ca. Certainly, the samples studied by Weaver and Beck are much less compacted than the Carboniferous shales, and it could be that pressure-temperature conditions during diagenesis were very different. In the work of K~HNEL (1963a, b) finely ground shale samples from the High Silesian Coal Measures were extracted with water and the electrical conductivity measured. Water-sofuble ions and exchangeable cations were also determined, but the main emphasis of the work was on conductivity measurements following the discovery that marine shales were characterized by higher values. This palaeosalinity method is reported to have proved of value in locating marine horizons in a dominantly non-marine sequence. The increase in the conductivity was thought to be due to a higher salt content and especially sodium chloride. The electrical conductivity of the water extract from the present shales was measured following Kiihnel. The results are given on Table 1 and show graphically on Fig. 2. There is an impressive increase in the electrical conductivity in the marine part of the sequence. Also plotted on Fig. 2 are the cation concentrations, and clearly the increase in conductivity is due to Ca and Mg rather than Na. Confirmation of higher conductivities in the water extract from marine shales does suggest, however, that the results obtained in this work may be reproduced in other sequences. CONCLUSIONS The concentrations of water-soluble Na, K, Ca and Mg vary in a significant manner through a borehole sequence of Carboniferous shales. In the marine shales the concentrations of Ca and Mg are high and Na and K are low, whereas in the nonmarine and brackish shales the reverse is true. The electrical conductivity of the water extract from the marine shales is higher than that from the non-marine and brackish water shales. This variation in conductivity is identical to that noted by K-EL (1963a, b) in the High Silesian Coal Measures. The water-soluble cations in the Carboniferous shales are believed to be related to the exchangeable cations and both are thought to be controlled by palaeosalinity. It is proposed that the water-soluble cations originally occupied exchange sites which were eliminated during diagenesis. The diagenetic conversion of some montmorillonite layers in mixed-layer mica-montmorillonites could be important and so too could the modification of inorganic and organic oolloids. Ac&o&edgenaents-The analytical assistance of Mrs. S. M. RHODESis gratefully acknowledged. The mthor would also like to thank Dr. C. D. CURTISand Dr. R. K-EL for the helpful discussions in the course of this work. REFERENCES R. A. (1971) Principlea of C?wnicaE Sedimentology, 240 pp. McGraw-Hill. R~X-JT J. F. (1969) Post-d&genetic clay mineral environmental relationships in the Gulf Coast Eocene. In Claya and Clay Minerals, 6th Nat. Cmf., pp. 327-341. Pergamon Press.

BERNER

Relationship between water-soluble cations and palaeoselinity

575

DUNOYEB DE SE~ONZAC G. (1964) Les 8rgiles du Cretace sup&ieur dens le basin de Douale (Cameroun): probleme de diagenese. Bull. Surv. Cart. Qwl., Ala. Lorr. 17, 267-310. KRAUSKOPF K. B. (1967) I?&cduc&n to Ueochemislq, 721 pp. McGraw-Hill. K&ZNEL R. (1963s) New method of identification of marine eediments in the High Silesian Coal Measures (in Czech). Sb. Ved. Prac. Vyeokej Skoly Tech. 0, 227-204. KNEEL R. (1963b) DBtermination du milieu de formation des roches argilleusea d’sp& leur conductiviti Blectrique des extraits aquatiques. Int. Stockholm, Vol. II, pp. 295-304. Pergemon Press. VAN MOORT J. C. (1971) A comparative study of the diagenetic alteration of clay minerals in Mesozoic Shales from Papua, New Guinea, and in Tertiary shales from Louisiana, U.S.A. C&y8 Ckzy Miwal19, l-20. PERRY E. and HOVER J. (1970) Burial disgenesis in Gulf Coast pelitic sediments. Clay8 Clay Mineral 18,165-177. SPEAXI D. A. (1964) The major element geochemistry of the Mansfield Marine Band in the Westphalian of Yorkshire. Cfeochim.Co.wwchim. Acta 28, 1697-1696. SPEARS D. A. (1973) Relationship between exchangeable cations and palaeosalinity. Geochim. Coemachim. Actu 87, 77-86. TEICEMULLER M. snd TEIC nhlULLER R. (1968) Diagenesis of CO81(coahfication). Jn .&agene& in Sediments, (editors G. Larsen and G. V. Chilingar), pp. 391-415. Elsevier. WEAVER C. E. and BECK K. C. (1971) Clay water diagenesis during burial: how mud becomes gneiss. (feel. Sot. Amer. Spec. Paper X34,96 pp.