geochemistry of carbon dioxide in six travertine-depositing waters of italy

7/25/2019 Geochemistry of Carbon Dioxide in Six Travertine-Depositing Waters of Italy

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J o u r n a l

o f

ydrology

E L S E V I E R

[ ]

Journ al o f Hyd rology 167 (1995) 263-278

G e o c h e m i s tr y o f c a r b o n d i o x i d e i n s ix t ra v e r t in e d e p o s i ti n g

w a t e r s o f I t a l y

Allan Pentecost

Division of Li fe Sciences, King s College London, Campden H il l Road, London W 8 7AH , U K

Received 30 Janu ary 1994; revision accepted 3 Aug ust 1994

Abstract

T h e c h e m i c a l c o m p o s i t i o n s o f s ix t r a v e r t in e - d e p o s i t in g h o t s p r in g w a t e rs i n I t a l y a r e

d e s c r ib e d w i t h e m p h a s i s o n t h e c a r b o n d i o x i d e sy s te m . A l l s p r in g s c o n t a i n e d h i g h c o n -

c e n t r a t io n s o f C O 2 ( > 2 0 m M 1 -1 ) w i t h e q u i l ib r i u m p a r t i a l p r e s su r e s w e ll a b o v e t h o s e w h i c h

c o u l d h a v e b e e n f o r m e d i n c o n t a c t w i t h a s o il a t m o s p h e r e . A f t e r s u r f a c in g , t h e C O 2 is r a p i d l y

l o s t t o t h e a t m o s p h e r e , w i t h e v a s i o n ra t e s c l o s e t o t h e s p r in g s r a n g i n g f r o m 0 . 4 5 - 4 . 4 1 m M m - 2

s - l . P a r t i a l p r e s s u r e s o f C O 2 s h o w e d a n e x p o n e n t i a l d e c l in e w i t h d i s t a n c e , w h i c h i s c o n s i s t e n t

w i t h t h e s t a ti c f il m m o d e l w h e r e t e m p e r a t u r e a n d t u r b u l e n c e a re c o n s t a n t . D o w n s t r e a m C O 2

t r a n s f e r c o e ff i ci e n ts , w h i c h r a n g e d f r o m 6 6 t o 3 60 c m h - l w e r e c o n s i s t e n t w i t h m o d e r a t e l y

t u r b u l e n t f l o w , h o w e v e r , t h e r e w a s n o c o r r e l a t i o n b e t w e e n t u r b u l e n c e , m e a s u r e d a s t h e m e a n

s h e a r s t re s s , a n d C O 2 e v a s i o n r a t e . T h e c h a n n e l s i n v e s t i g a t e d h a d a l l b e e n m o d i f i e d b y m a n a n d

m o s t p o s s e s s e d e v e n w i d t h s a n d g r a d i e n t s .

A l l w a t e r s b e c a m e i n c r ea s i n g ly s u p e r s a tu r a t e d w i t h a r a g o n i t e a n d c a lc i te d o w n s t r e a m a n d

b o t h o f t h e se m i n e r a l s w e r e p r e s e n t i n f r e sh t r a v e r t i n e d e p o s i t s. T h e s u p e r s a t u r a t i o n w a s d r i v e n

a l m o s t e x c l u si v e ly b y g a s e v a s io n . C o m p a r i s o n o f d a y t i m e a n d n i g h t t im e e v a s i o n r a t es d e m o n -

s t r a t e d t h a t p h o t o s y n t h e t i c a c t i v i t y w a s a n i n s i g n i fi c a n t s o u r c e o f C O 2 f l u x i n t h e r e a c h e s

i n v e s t i g a t e d . C a r b o n d i o x i d e e v a s i o n is t h e r e f o r e p r i m a r i l y r e sp o n s i b l e f o r th e s u p e r s a t u r a t i o n

a n d p r o b a b l y a l s o t h e d e p o s i t i o n o f t r av e r t i n e a t t h e s e s it es .

T h e C a C O 3 c o n t e n t o f th e t r a v e rt i n e s r a n g e d f r o m 9 1 .3 t o 9 6 .0 w t % w i t h 1 . 7 - 4 . 1 % C a S O 4 ,

t r a c e s o f o r g a n i c m a t t e r a n d a c i d - i n s o l u b l e m i n e r a l s .

1 I n t r o d u c t i o n

S o m e o f th e w o r l d ' s l a r g e s t t r a v e r t i n e d e p o s i t s o c c u r i n I t a l y , a n d m o s t a r e b e l i e v ed

t o h a v e b e e n f o r m e d f r o m h o t s p r in g s h i g h ly c h a r g e d w i t h c a l c iu m a n d c a r b o n

d i o x i d e . L a r g e n u m b e r s o f h o t s p r i n g s t h a t a r e h ig h l y c h a r g e d w i t h C O 2 o c c u r i n

0022-1694/95/$09.50 1995 - Elsevier Science B.V. All rights reserved

S S D I 0 0 2 2 - 1 6 9 4 ( 9 4 ) 0 2 5 9 6 - 7


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264 A. Pentecost / Journal of Hydrology 167 1995) 263-278

Italy (Waring, 1965; Barnes et al., 1978), but less than a quarter of these are known to

deposit travertine.

Most of the large travertine sites, which are extensively quarried, are now inactive,

but small travertine-depositing springs are widespread and a number are associated

with recent volcanic centres. Several sites are clustered around the Vican centre in the

Roman volcanic province, where fault-controlled springs deposit mounds and sheets

of travertine near Viterbo, Lazio.

There have been a number of geochemical studies on these deposits, but most

research has concentrated on inactive sites (Dall'Aglio and Tedesco, 1968;

Malesani and Vannuchi, 1975; Manfra et al., 1976). However, it is known

that levels of carbon dioxide are much higher than those in equilibrium

with soil atmospheres and probably linked to recent volcanic activity. Previous

investigations of water chemistry have tended to concentrate on trace, rather

than bulk constituents and little progress has been made in either identifying the

origins of the solutions or the chemical changes occurring on contact with the

atmosphere.

Travertine deposition is frequently associated with biological activity and many hot

springs possess a rich phototrophic bacterial flora. Phototrophic microbes can

remove dissolved carbon dioxide by photosynthetic uptake, resulting in the direct

precipitation of carbonates (Krumbein, 1979). Microbes can also provide a suitable

framework for crystal nucleation and accretion (Pentecost and Riding, 1986; Emeis

et al., 1987). Photosynthetic activity must be weighed against the direct transfer of

carbon dioxide from water to atmosphere, which is independent of biological activity.

Evasion of carbon dioxide to the atmosphere has been shown to increase with

turbulence (Dandurand et al., 1982; Herman and Lorah, 1987), leading to rise in

pH and carbonate ion activity. This in turn leads to calcite and aragonite

supersaturation, favouring the precipitation of these minerals. The rate of transfer

at these hot springs is unknown but assumed to be high because of the high CO2

partial pressures in these waters.

The aims of this investigation are threefold: first, to investigate the chemical

composition of travertine-depositing thermal waters, with emphasis on the CO2

system; second, to estimate CO2 evasion rates from in situ analyses and finally to

determine the travertine composition. The results are used to evaluate the significance

of gas evasion and photosynthetic activity in carbonate deposition.

2 M e t h o d s

2.1. Field sites

Four of the six sites were grouped around Viterbo in Lazio: Bagnaccio, Bullicame-

Dante, Bullicame-West and Le Zitelle. Two sites, Bagni san Fillipo and Bagni di

Vignone are situated further nor th (Figs. 1 and 2(a)-(c)). At all sites travertine is

being deposited in stream beds which have been altered by channelling to provide

bathing water. These modifications were in most cases an advantage for the


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A. Pentecost / Journal o f Hydrology 167 1995) 263 -278 265

J

/

T O S C A N A

R a d i c o f a n i , ,

t /

; r

,

/

J N Q

f - ' L a g o d i

Bo l s e n a

/

U M B R I A

= C a n i n o 1,

T u s c a n i a

I V i t e r b o

O r t e

L A Z I O

t o-~_ L a g o d i

2 0 k r n , ~ ~ V i c o

Fig. 1. Location of sites investigated. 1, Bagn accio;2, Bagn i san Fillipo; 3, Ba gni di V ignoni; 4, Bullicame-

Dante; 5, Bullicame-West;6, L e Zitelle.

e s t i m a t i o n o f C 0 2 t r a n s fe r , a s t h e y p r o v i d e d f a ir l y e v e n s t r e a m g r a d i e n t s, d e p t h s a n d

w i d t h s .

2 .2 . W a t e r s a m p l i n g a n d a n a ly s i s

S a m p l i n g w a s c a r r i e d o u t o n a s e ri es o f f ie ld tr ip s b e t w e e n F e b r u a r y 1 98 8 a n d A p r i l

1 9 93 . M o s t w a t e r s a m p l e s w e r e c o l l e c t e d in 1 30 m l g l as s b o t t l e s ( e x c l u d i n g S i, w h e r e

p o l y t h e n e w a s u s e d ) . W a t e r s a m p l e s w e r e c o ll e c t ed a t t h e s p ri n g s, a n d a t o n e o r m o r e

p o i n t s d o w n s t r e a m , a t d i s t an c e s o f 7 .8 - 1 1 1 m f r o m t h e s p ri n g s (T a b l e 1 ). G a s

a n a l y s e s (C O 2 , 0 2 , H 2 S ) w e r e p e r f o r m e d i m m e d i a t e l y a f t e r t h e s a m p l e s h a d c o o l e d ,

b u t m o s t r e m a i n i n g a n a l y s e s w e r e p e r f o r m e d i n t h e l a b o r a t o r y a f t e r p r e s e r v i n g w i th

e i t h e r 0.0 1 M H C 1 o r 0 . 0 0 1 % H g C1 2.

T h e p H w a s d e t e r m i n e d i n s it u u s i n g a C o r n i n g 1 20 p H m e t e r a n d g l as s e l e c t ro d e

c a l i b r a t e d w i t h N B S b u f f er s o f p H 4 .0 a n d 7 .4 . A c o o l e d w a t e r s a m p l e w a s t h e n

d i l u t e d f i v e f o l d w i t h C O 2 - f r e e d is t il le d w a t e r , a n d t h e p H r a i s e d im m e d i a t e l y to


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266

A. Pentecost / Journal o f Hydrology 167 1995) 2 63 -27 8

a) Bullicarne-Dante

~25rn

b) Bullicame-West

~ } pools

I

5 0 m 2 J

I

c) Le Z itelle

i

l i .

S t r a d a V a l o r e . . . . . . . . . . . . . . . . .

....... I 50rn ~ ~

Fig . 2 . De ta i l s o f wa te r c o u r se s a t t he V i t e r bo ho t sp r ings . (a ) B u l l i c a r ne - Da n te ; (b ) gu l l i c a m e - W e s t ; (c ) Le

Z i t e l le . S t a r s show pos i t i o ns o f t he sp r ings .

b e t w e e n 8 .2 a n d 8 . 8 w i t h CO 2 - f r e e N a O H t o p r e v e n t g a s tr a n s f e r t o t h e a t m o s p h e r e .

T h e s a m p l e w a s t h e n p l a c e d i n a fi x e d - v o l u m e t i t r a ti o n f la s k ( s ee E d m o n d , 1 9 70 ), a n d

t h e a l k a l i n it y d e t e r m i n e d p o t e n t i o m e t r i c a l l y u s i n g a m i c r o b u r r e t t e f ille d w i t h 1 .0 M

H C I . T h e t o t a l C O 2 w a s o b t a i n e d u s i n g a c o m p u t e r p r o g r a m w h i c h c a l c u la t e d t h e

c o r r e c t e n d p o i n t o f t h e s a m p l e b y i te r a t i o n ( P e n t e c o s t , 1 9 92 ). P r o g r a m i n p u t s w e r e in

s i t u a n d p o s t a l k a l i p H , s p r i n g a n d t i t r a t i o n t e m p e r a t u r e , n o r m a l i t y a n d v o l u m e o f

t i tr a n t , a n d s o l u t i o n s p e c if ic c o n d u c t i v i ty . A l k a l i n i ty c o r r e c t i o n s f o r o t h e r b a s e s

(bora te , s i l i ca t e and su lph ide ) were de te rmined for a l l s amples . A t l eas t f ive fo ld

d i l u t io n o f s a m p l e s w a s n e c e s s a r y f o r t h e s e w a t e r s t o p r e v e n t t h e CO 2 r e l e a s e d d u r i n g

t i t r a t i o n e s c a p i n g f r o m s o l u t i o n a n d f o r m i n g b u b b l e s i n t h e f l a s k . T o t a l s u l p h i d e ,

t h i o s u lp h a t e a n d o x y g e n w e r e d e t e r m i n e d b y t h e c o m b i n e d m e t h o d o f I n g v o r s e n a n d


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A. P entecost / Journal of Hydrology 167 199 5) 263-278

Table 1

Hy drologica l da ta re la t ing to ca rbon dioxide evasion ra tes

267

Characteristic Ba gn acci o-P aul a Springs (1)

A - B B - C C - D D - E 3 4 5 6

Discharge (1 s l) 3.7 3.7 3.5 3.1 3.2 5.4 3.7 4.4

System length (m) 7.6 12 11.4 32 48 78 23 68

M ean wi dth (m) 0.2 0.2 0.17 0.38 0.42 0.74 0.13 1.02

M ean de pth (era) 3.4 3.4 3.0 3.0 5.8 3.4 11.2 2.5

W ate r a rea (m 2) 1.5 2.4 1.9 12 20 58 3.0 69

M ean w ater 62.0 61.3 60.3 55,8 40.7 50 55 55

tempera ture C

M ean gra die nt 1.01 1.11 3.75 1,54 7.80 1.43 0.60 1.85

degrees

M ean she ar stress 60 65 190 80 780 80 115 80

( gc m - l s - l )

Bed charac tera S S S L H L S /R L/H

a Stream b ed characteristics: S , smooth; L, loose travertine crust with micro bial mats; R , rou gh w ith small

(1 cm) pro tuberan ces greater than 1 cm.

Site key: 1, Bagnaccio, A represents spring orifice, with B, C, etc. consecutively downstream; 3, Bagni di

Vigno ni; 4, Bullicame-Dan te; 5, BuUicam e W est; 6, Le Zitelle.

J o r g e n s e n ( 1 97 9 ). A t s o m e s i t es , t h e p r e s e n c e o f o x y g e n w a s t e s t e d q u a l i t a t i v e l y u s i n g

t h e s e n s it i v e r e s a z u r i n m e t h o d ( C a l d w e l l e t a l ., 1 9 84 ). M o s t o f t h e r e m a i n i n g c a t i o n s

w e r e d e t e r m i n e d u s i n g a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t r y . C h l o r i d e a n d s u l p h a t e

w e r e d e t e r m i n e d b y a r g e n t o m e t r y a n d t u r b i d i m e t r y r e s p e c t i v e ly ( G r e e n b e r g , 1 98 5) .

2 . 3 . M i n e r a l s a t u r a t i o n q u o t i e n t s, p C 0 2 a n d tr a v e r ti n e c o m p o s i t i o n

T h e s a t u r a t i o n q u o t i e n t s f~a, f~c a n d 9tg o f a r a g o n i t e , c a l c i t e a n d g y p s u m ,

r e s p ec t iv e l y , w e r e d e t e r m i n e d u s i n g W A T E Q ( T r u e s d e l l a n d J o n e s , 1 97 4) .

Ca)s cO 3)s

~ a, g = ( C a ) m ( C O 3 ) m

W h e r e s r e f e rs t o t h e s a m p l e a n d m t h e i o n a c t iv i t y p r o d u c t f o r th e m i n e r a l ( a r a g o n i t e ,

c a l c it e ) a t t h e m e a s u r e d t e m p e r a t u r e a n d p r e s s u r e . W h e n f~ = 1 t h e w a t e r s a m p l e i s a t

e q u i l i b r i u m ( i.e . s a t u r a t e d w i t h t h e m i n e r a l ) , v a l u e s a b o v e u n i t y i n d i c a t e s u p e r -

s a t u r a t i o n . T h e s a m e p r o g r a m o u t p u t s t h e e q u i l i b r i u m C O 2 p a r t i a l p r e ss u r e s o f

t h e s e s o l u t i o n s , p l u s th e ' a q u e o u s ' ( t h a t i s, u n i o n i s e d ) C O 2 c o n c e n t r a t i o n .

T r a v e r t in e c a r b o n a n d o x y g e n s t a b l e i s o t o p e c o m p o s i t i o n w a s d e t e r m i n e d a n d

X - r a y d i f f r ac t i o n u s e d t o c o n f i r m t h e m i n e r a l o g y .

2 . 4 . C a r b o n d i o x i d e f l u x a n d t r a n s fe r c o e f fi c ie n t s

T h e c a r b o n d i o x i d e f lu x w a s e s t im a t e d f r o m t h e d i f f er e n ce b e t w e e n t h e a n a l y t i c a l

c o n c e n t r a t i o n s o f c a r b o n d i o x i d e b e t w e e n t w o f i xe d p o i n t s , a l l o w a n c e b e i n g m a d e f o r


6/16

268 A . P e n t e c o s t / J o u r n a l o f H y d r o l og y 1 67 1 9 9 5 ) 2 6 3 - 2 7 8

a n y C a C O 3 p r e c i p i t a te d in b e tw e e n . R a t e s c o u l d n o t b e c o r r e c t e d f o r p h o t o s y n t h e s i s

d i r e c t l y , b u t s a m p l e s w e r e t a k e n a t m i d d a y a n d m i d n i g h t t o p r o v i d e a n e s t i m a t e o f

b i o l o g i c a l a c t iv i t y , i .e . b e t w e e n p h o t o s y n t h e s i s w h i c h r e s u l ts i n n e t r e m o v a l o f C O 2

f r o m t h e w a t e r i n t h e d a y , a n d r e s p i r a t io n w h i c h p r o v i d e s a s m a l l n e t i n p u t o f C O 2

i n t o t h e w a t e r a t n i g h t . A t m o s t s ite s, s a m p l e s w e r e t a k e n o n l y in t h e u p p e r r e a c h e s o f

t h e s tr e am s , b u t a t B a g n a c c i o , a s e q u en c e o f d o w n s t r e a m m e a s u r e m e n t s w a s m a d e .

T h e s u rf a ce a r e a o f w a t e r e x p o s e d b e t w e e n t h e t w o p o i n t s w a s d e t e r m i n e d b y

m e a s u r i n g t h e a v e r a g e s t r e a m w i d t h , u s i n g t e n e q u a l l y s p a c e d p o i n t s . D i s c h a r g e

w a s e s t i m a t e d a l o n g a s h o r t l e n g th b y c a l c u l a ti n g t h e c r o s s - s e c ti o n a l a re a o f fl o w

a n d t h e f l o w r a t e u s i n g f iv e t i m e d t r a n s i t s o f s u r f a c e - f l o a t i n g p a p e r d i s c s. A s m a l l

c o r r e c t i o n w a s m a d e f o r d r a g ( M o r i s a w a , 1 9 68 ). T h e f lu x is e x p r e s s e d a s m M o l e C O 2

m - 2 s - 1 . T h e C O 2 t r a n s f e r c o e f f ic i e n ts w e r e c a l c u l a t e d u s i n g t h e s t a t ic f i lm m o d e l

( G i s l a s o n , 1 9 8 9 ) w h e r e t h e f l u x , F i s e x p r e s s i b l e a s

F = k C s -

Cw)

= k[(Pco2K ) - Cw ]

w h e r e k is t h e t r a n s f e r c o e f f ic i e n t, Cs is t h e c o n c e n t r a t i o n o f g a s a t t h e f il m t o p , C w is

t h e c o n c e n t r a t i o n o f g a s in w a t e r b e l o w t h e f ilm , P c % is th e p a r t i a l p r e s s u r e o f C O 2 i n

a i r a b o v e t h e f il m a n d K s is H e n r y ' s L a w c o n s t a n t . T h e f il m th i c k n es s , z is g i v e n b y

z = D / F

w h e r e D is t h e c o e f fi c ie n t o f d i f f u s io n o f C O 2 in w a t e r , o b t a i n e d f r o m

B r o e c k e r a n d P e n g ( 1 9 7 4 ) . F o r t h e c a l c u l a t i o n s , P c % h a s b e e n t a k e n a s i t s

a t m o s p h e r i c v a l u e , 1 0 -3 .4 4 b a r a n d C w a s t h e b u l k c o n c e n t r a t i o n o f C O 2 ( a q ) .

2 .5 . M e a n s h e a r s t r e s s

M e a n s h e a r s t r e s s c a n b e e s t i m a t e d f r o m

r = p g R S

w h e r e p i s t h e d e n s i t y o f w a t e r , g is th e a c c e l e r a t i o n o w i n g t o g r a v i t y , R i s t h e

h y d r a u l i c r a d iu s o f th e s t r e a m b e d a n d S is t h e s t r e a m g r a d i e n t , o b t a i n e d b y l e ve ll in g

( R i c h a r d s , 1 9 8 2) . T h i s r e l a t i o n s h i p s t r i c t ly a p p l ie s t o s t r e a m s e c t i o n s w h e r e t h e r e i s n o

a c c e l e r a t i o n , i.e . w i t h o u t c a s c a d e s . T h e s e w e r e a b s e n t i n a l l o f th e s t r e a m s e c t i o n s

i n v e s t i g a t e d .

3 Resu l t s

3 .1 . S i t e h y d r o l o g y

S o m e p e r t i n e n t h y d r o l o g i c a l c h a r a c te r i s ti c s a r e s h o w n i n T a b l e 1. D i s c h a r g e a t a ll

s p r i n g s i s l es s t h a n 5 1 s - l , a n d t h e c h a n n e l s l e a d i n g f r o m t h e s p r i n g s r a r e l y e x c e e d e d I

m i n w i d t h a n d 1 0 c m i n d e p th . M o s t s t r e a m s s e e p e d i n t o t h e g r o u n d a f t e r a s h o r t

d i s ta n c e a n d o n l y a t L e Z i t el le d i d t h e y f lo w i n t o a s e c o n d - o r d e r s t r e a m , a t w h i c h

p o i n t t r a v e r t in e d e p o s i t i o n c e a s ed . S p r i n g w a t e r t e m p e r a t u r e s v a r i e d f r o m s it e t o s i te ,


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269

b u t t h e V i t e r b o g r o u p ( B a g n a c c i o , B u l li c a m e a n d L e Z i te l le ) a ll h a d t e m p e r a t u r e s c l o s e

t o 6 2 C . G r a d i e n t s g e n e r a l ly w e r e s li g h t a l o n g t h e m a n - m a d e c h a n n e l s . A t B a g n o

V i g n o n i , h o w e v e r , t h e s t r e a m f l o w e d d o w n a s t e e p g r a d i e n t b e f o r e f in a l ly c a s c a d i n g

o v e r t h e m a i n t r a v e r t in e m o u n d . F l o w r a t e s ra r e l y e x c e e d e d 1 m s - I a t a n y o f t h e site s.

T h e a v e r a g e s h e a r s t r es s e s ( T a b l e 1 ) r e f le c t d i f fe r e n c e s i n t h e g r a d i e n t a n d s h o w s t h a t

B a g n a c c io A - B , B - C , D - E , B u l l ic a m e - D a n t e a n d L e Z it el le a ll h a v e a b o u t t h e sa m e

m e a n s h e a r s t re s s ( 4 0 - 8 0 g c m - 1 s - l ) . C h a n n e l b e d c h a r a c t e r is t i c s w e r e v a r i a b l e a n d a t a ll

s it es co n s i s te d o f tr a v e r ti n e . T h e d e p o s i ts w e r e o f t e n s m o o t h a n d h a r d , a s a t B a g n a c c i o

a n d B a g n i d i V i g n o n i , b u t a t L e Z it e ll e , t h e m a i n c h a n n e l p o s s e s s e d a f e w d e e p p o o l s w i t h

j a g g e d , u p s t r e a m - d i r e c t e d c o n c r e t i o n s b u t t h e s e d id n o t a p p e a r t o g i ve ri se t o h i g h s h e a r

s tr e ss e s (s e e T a b l e 1 ). T h e h i g h e s t s h e a r s t re s se s w e r e o b t a i n e d a t B a g n i d i V i g n o n i w h e r e

t h e w a t e r s d e s c e n d e d t h e s t ee p , u p p e r s e c t i o n o f t h e t r a v e r t in e m o u n d .

3 .2 . W a t e r c h e m i s t r y

A l l s p ri n g w a t e r s c o n t a i n e d h i g h c o n c e n t r a t i o n s o f c a r b o n d i o x i d e , c a l c iu m ,

m a g n e s i u m a n d s u l p h a t e , w i t h l o w e r le v el s o f s o d i u m a n d c h l o r i d e ( T a b l e 2) .

S p r i n g w a t e r p H w a s c l o s e t o 6 .4 a n d a ll w a t e r s c o n t a i n e d le ss t h a n 0 .2 m M o x y g e n

a n d t o t a l s u l p h i d e .

T h e p C O 2 o f th e e m e r g i n g w a t e r s w a s h ig h a n d o f t e n a p p r o a c h e d a t m o s p h e r i c

p r e s s u r e . A l l w a t e r s a r o s e a p p a r e n t l y s u p e r s a t u r a t e d w i t h r e s p e c t t o b o t h a r a g o n i t e

a n d c a l ci te a n d b o t h m i n e r a l s o c c u r r e d i n th e a s s o c i a te d t r a v e r ti n e s . S u p e r s a t u r a t i o n

w a s p a r t i c u l a r l y h i g h a t t h e B a g n i s a n F i l l i p o s p r in g , w h i c h a l s o h a d t h e l o w e s t p C O 2 .

T h i s w a s p r o b a b l y b e c a u s e t h e w a t e r s w e r e c o n d u c t e d o n t o t h e t r a v e r t i n e f r o m a

l a r g e p i p e , w h i c h c o n n e c t e d w i t h t h e s o u r c e a b o u t 2 0 0 m d i s t a n t , p e r m i t t i n g s o m e

d e g a s s in g w i t h i n t h e p ip e . F o r t h is r e a s o n , C O 2 f lu x m e a s u r e m e n t s w e r e n o t m a d e a t

t h i s si te . T h r e e o f t h e s p r i n g w a t e r s w e r e a ls o c l o s e t o g y p s u m s a t u r a t i o n ( T a b l e 2 ).

3 .3 . C 0 2 f l u x a n d t r a v e r t in e d e p o s it io n

A t B a g n a c c i o , t h e f a ll i n T D I C ( t o ta l d is s o lv e d i n o r g a n i c c a r b o n ) a n d C O 2 ( a q ) is

c l e a rl y d e m o n s t r a t e d ( F ig . 3 ) . T h e r a t e o f fa ll fo r b o t h is n o n - l i n e a r w i t h d i s ta n c e , b u t

f o r C O 2 ( a q ) a l o g a r i t h m i c f a ll i s e v i d e n t ( F ig . 4 ). A s C O 2 e v a s i o n p r o c e e d e d , p H r o s e

f r o m a r o u n d 6 . 4 t o 7 . 4 . O x y g e n i n v a d e d t h e w a t e r w i t h a n a p p a r e n t l y l i n e a r r a t e o f

u p t a k e w i t h d i s t a n c e a n d a t t h e l o w e s t s i t e , 6 5 m d i s t a n t f r o m t h e s o u r c e , o x y g e n

s a t u r a t i o n h a d r e a c h e d 7 6 % . T h e m e a n o x y g e n i m p o r t r a te w a s e st im a t e d a s 30 # M

m - 2 s - I . T h e c a r b o n d i o x i d e f l u x d if f e r e d v e r y l i tt le b e t w e e n d a y a n d n i g h t ( T a b l e 3 ).

T r a v e r t i n e d e p o s i t i o n i n t h e c h a n n e l s w a s m o n i t o r e d b y m e a s u r i n g d i s s o l v e d

c a lc i um . R a t e s o f C O 2 d e p o s i t i o n ( in t o C a C O 3 ) w e r e lo w w h e n c o m p a r e d w i t h

d e g a s s i n g , t h e l a r g e s t a m o u n t C a C O 3 p r e c i p i t a t e d b e t w e e n t h e h i g h e s t a n d l o w e s t

s it es a t B a g n a c c i o b e i n g 0 . 3 m M l - l . A t t h e o t h e r V i t e r b o s p r in g s a n d a t B a g n i d i

V i g n o n i , th e d if f e re n c e s b e t w e e n m i d d a y a n d m i d n i g h t m e a s u r e m e n t s w e r e a g a in

s m a ll a n d c a l c iu m c a r b o n a t e d e p o s i t i o n a m o u n t e d t o a m a x i m u m o f 0 .3 m M 1-1.

A t B u l l ic a m e - D a n t e a n d L e Z i te l le t h e m a x i m u m C O 2 ' l o ss ' t o C a C O 3 d e p o s i t i o n w a s

0 . 7 6 m M 1-1 a n d 1 .5 m M 1 l , r e s p e c t iv e l y .


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A. P entecost / Journal o f Hydrology 167 1995) 263-278

Table 2

Chemical composition of spring waters

Determinand Site

2 3 4 5 6

t(c) 64.9 47.0 33.2 55.5 56.3 62.9

pH 6.32 6.72 6.75 6.30 6.54 6.32

TDICa(mM) 31.2 36.4 21.9 28.5 21.6 23.0

HCO 3 (mM) 17.4 27.8 16.6 15.8 14.6 12.8

pCO2(atm) 0.881 0.395 0.177 0.705 0.384 0.632

log pCO2 -0.055 -0.403 -0.752 -0.152 -0.416 -0.199

CO2 (aq) (mM) 13.8 8.25 4.96 12.7 6.80 10.2

Ca (mM) 14.3 18.0 18.0 14.3 14.0 14.2

Mg (mM) 6.91 8.13 8.55 5.65 5.75 4.9

Sr ( M) 92 - - - 98

Na (mM) 2.00 5.00 2.85 3.30 2.80 1.9

K (mM) 0.21 0.05 0.60 0.85 0.85 0.08

CI (mM) 2.00 0.40 1.57 0.25 0.40 0.43

SO4 (raM) 12.9 15.6 11.3 11.4 11.4 9.7

total S ( M) 32 39 34 76 82 112

02 (mM)b 0.01 + 0.26 0.02 0.22 +

9tcc 5.70 14.8 7.22 3.94 6.46 3.98

f~a 4.02 10.0 4.82 2.74 4.51 2.82

f~g 0.86 1.23 - 0.87 - -

a TDIC Total dissolved inorganic carbon.

b + indicates waters resazurin-positive.

c ~ c a g saturation ratios for calcite, aragonite and gypsum.

Sites: 1, Bagnaccio, Paula Springs; 2, Bagni san Fillipo, top spring; 3, Bagni di Vignoni; 4, Bullicame-Dante;

5, Bullicame-West;6, Le Zitelle.

Ca rb on dioxide flux betwe en sites was variabl e (Table 3), with a range of 0.45-4.41

mM CO2 m -2 s -1 The highest flux was obt ain ed at Bagnaccio, just b elow the springs,

and the lowest were obt ain ed at Le Zitelle, in the upper, slow-flowing chann el. On

average, day tim e rates of degassing (1.8 mM CO2 m -2 s -l ) were the same as nigh tti me

rates, A significant posit ive correlati on was obt ained between the f lux and both the

mea n pCO2 (P < 0.01) and the temper ature (P < 0.1), but no correlat ion was found

between the f lux and stream gradient. Although there was some tendency for s i tes

with higher shear stresses to have larger CO2 fluxes, there was no significant

correlati on between these variates .

The car bo n dioxide tr ansfer coefficients rang ed from 66 to 360 cm h -1 ( Table 3)

with estimated f i lm thicknesses in the range 0.1- 0.4 cm.

3 .4 . T r a v e r t i n e c o m p o s i t i o n

The major travertine consti tuents are l is ted in Table 4. In a ll samples, the deposits

conta ined more than 90% ca lc ium carbona te but with s ignificant amoun ts of gypsum,

which amo un ted to 4.1% at Bullicame West. The Viterbo travertines normall y con-

sis ted of mixtures of aragonite and calcite and only at Le Zite lle was aragonite the


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A. Pentecost/Journal of Hydrology 167 1995) 263-278

271

7 5

7 0

6 5

2 0

15

10

o

o

0~0 ~ 8

T D IC m M / l ~

0 1 0 2 0 3 0 4 0 5 0

m d o w n s t r e a m

150

1 0 0

50

Fig. 3. Dow nstream chemical changes at Paula spring, Bagnaccio on 8 April 1993. (a) Oxygen; (b) pH; (c)

total dissolved inorganic carbon; (d) CO2 (aq). Full l ines denote midday measurements; broken l ines,

midnight.

p r e d o m i n a n t m i n e ra l . T r a c e s o f a r a g o n i t e w e r e f o u n d a t B u l l i c a m e - W e s t , a n d a t

B u l l i c a m e D a n t e t h e t r a v e r t i n e c o n s i s t e d e n t i r e l y o f c a lc i te .

T h e s t a b l e i s o to p i c c o m p o s i t i o n s s h o w t h a t a l l o f th e t r a v e r t i n e s w e r e e n r i c h e d w i t h

13C a n d 1 8 0 ( T a b l e 1 ) i n r e l a t i o n t o th e P D B a n d S M O W s t a n d a r d s .

4 D i s c u s s i o n

4.1. The dissolved carbon dioxide and its origin

C a r b o n d i o x i d e - - r ic h d i s ch a r g e s a r e f o u n d t h r o u g h o u t t h e w e s t e rn l im b


10/16

272

A. Pentecost / Journal of Hydrology 167 1995) 2 63-2 78

1.0

0.1

p C O 2 ( a t m )

' ~ ' 8 ~ o

0.01

0 50 100 150

m . d o w n s t r e a m

Fig. 4. Log (CO2 {aq}) as a funct ion of distance from springs, Paula springs, Bagnaccio. Full line, midday;

broken line, midnighL

of the Italian peninsula and are associated with all of the recent Italian volcanic

centres.

Numerous analyses of thermal waters may be found in Waring (1965) but

measurements of dissolved carbon dioxide have rarely been undertaken on

travertine-depositing thermal springs. Some information is available from Japan

(Kitano, 1963), Italy (Malesani and Vannuchi, 1975), Wyoming (Friedman, 1970)

and Bolivia (Risacher and Eugster, 1979). These data are listed with some previously

unpublished informat ion in Table 5(a). Carbon dioxide levels from tectonically active

regions fall in the range 12.7-67 mM 1 l with equilibrium partial pressures of

0.22-1.0 atm. These partial pressures greatly exceed those of the soil atmosphere

which lie in the range 0.01-0.1 atm (Atkinson and Smith, 1976).

Table 3

Carbon dioxide flux and transfer coefficients

Site Flux mM CO 2 (m -2 s-1) Transfer coefficient k

(cmh -1)

Day rate Night rate

Day Night

Bagnaccio A-B 4.4 2.2 246 122

Bagnaccio B-C 3.5 3.4 274 275

Bagnaccio C-D 3.0 1.4 347 183

Bagnaccio D-E 0.9 0.7 249 203

Bagni di Vignone 1.3 1.7 187 360

Bull icame-Dante 0.5 0.5 93 66

Bullicame-West 2.4 3.6 109 157

Le Zitelle 0.5 0.5 80 70


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A. Pentecost / Journal o f Hydrology 167 1995) 263-278

Table 4

Co mp osition o f fresh travertines

273

Determinand Si te

2 3 4 5 6

Ca CO 3 (% ) 94.1 96.0 92.4 96.0 91.3 95.4

CaSO 4 (% ) 3.4 2.3 3.3 3.1 4.1 1.7

SrCO 3 (% ) 1.6 1.4 1.4 0.3 2.4 1.6

Organic ma t te ra (% ) 0.48 0.14 0.80 0.37 1.84 0.94

Acid-insoluble (% ) 0.15 0.08 1.73 0.11 0.32 0.24

minerals

M g (ppm ) 985 1900 4460 4690 1380 920

Sr (pp m ) 9600 8100 4000 1510 14200 9340

K (pp m) 15 - 35 39 -

N a (ppm ) 440 57 730 480 55 24

Fe (pp m) 135 136 230 145 136 57

M n (ppm) 10 7 4 34 102 146 14

P (ppm) 43 8 82 - -

Mine ra logyb A + C A + C C C C + A(tr ) A + C(tr)

~13C

PD B 6.29 6.50 4.30 5.91 5.92 4.93

618 0 SM O W 18.71 19.90 20.71 19.56 19.28 17.62

a Includes some sulph ur

b A, ara gon ite; C, calcite; tr , trace.

Sites: 1, Bagn accio; 2, Bagni san F illip o; 3, Bagni di Vignoni; 4, Bullicame-Dante; 5, Bullicame-West; 6, Le

Zitelle.

A c a r b o n d i o x i d e s o u r c e a d d i t i o n a l t o t h e s o i l i s n e c e s s a r y t o p r o d u c e s u c h h i g h

p r e s s u r e s a t t h e e a r t h ' s s u rf a ce . L i m e s t o n e a n d / o r o r g a n i c m a t t e r d e c a r b o n a t i o n i n

a r e a s o f r e g i o n a l o r c o n t a c t m e t a m o r p h i s m h a s o f t e n b e e n su g g e s t e d to a c c o u n t f o r

t h e a d d i t i o n a l c a r b o n d i o x i d e ( H u r l e y e t a l . , 1 9 6 6; B a r n e s e t a l . , 1 98 4; C a t h e l i n e a u e t

a l ., 1 9 89 ; D e i n e s , 1 99 2 ). N o d e c a r b o n a t i o n p r o c e s s e s h o w e v e r h a v e y e t b e e n u n e q u i -

v o c a l l y i d e n t i f ie d w i t h a n y g e o t h e r m a l a r e a . T h e I t a l i a n v o l c a n o e s a n d a s s o c i a t e d

g e o t h e r m a l f ie l d s o c c u r i n a c o m p l e x t e c t o n i c r e g i o n w h e r e c r u s t a l t e n s i o n , d e e p

f a u l t i n g a n d s u b d u c t i o n a r e a l l i n p r o g r e s s ( C h e s t e r , 1 9 85 ) t h o u g h i t i s n o t p o s s i b l e

a t p r e s e n t t o i d e n t i f y w h i c h C O 2 - e v o l v i n g p r o c e s s i s a s s o c i a t e d w i t h t h e h o t s p r in g s

i n v e s t i g a te d . T h e i s o t o p i c c o m p o s i t i o n o f th e t r a v e r t in e s a n d t h e i r a s s o c i a t e d w a t e r s

( P a n i c h i a n d T o n g i o r g i , 1 97 6; M a n f r a e t a l . , 19 7 6) t o g e t h e r w i t h t h e r e s u l t s o b t a i n e d

h e r e ( T a b l e 4 ) i n d i c a t e a h e a v i e r s o u r c e o f c a r b o n w h i c h c o u l d b e p r o v i d e d b y l i m e -

s t o n e d e c a r b o n a t i o n a t h i g h te m p e r a t u r e s .

T h e r m a l s p r i n g s o c c u r r i n g i n t e c t o n i c a l l y q u i e t r e g i o n s ( T a b l e 5 b ) h a v e l o w e q u i l i -

b r i u m p C O 2 v a l u e s t h a t f a ll w i t h i n t h e r a n g e o f r e g i o n a l so i l p C O 2 a n d r e p r e s e n t

d e e p - f l o w i n g m e t e o r i c w a t e r s u n l i k e l y t o h a v e c o n t a c t e d t h e r m a l l y g e n e r a t e d C O 2

s o u r c e s .

4 .2 . G a s f l u x

T h e c a r b o n d i o x i d e e v a s i o n r a t e s ( T a b l e 3 ) w e r e f o u n d t o b e h i g h i n a l l c a s es ,


12/16

274 A. Pentecost / Journal o f Hydrology 167 1995) 2 63 -27 8

T a b l e 5

C a r b o n d i o x i d e l e v el s in t r a v e r t i n e - d e p o s i t i n g th e r m a l s p r i n g s

S i t e / c o u n t r y T D I C ( m M 1 -1 ) p C O 2 t ( C ) R e f e r e n c e

( a t m )

a) Tectonically/volcanically active regtons

H e b e r H o t S p r s 2 1 .0

U t a h

J a p a n 1 2 . 7 - 6 7

K a r l o v y - V a r y 5 4 . 5

C z e c h R e p u b l i c

M a m m o t h H o t S p r in g s 1 9 .6 - 20 .1

W y o m i n g

P a s t o s G r a n d e s c . 1 5.3

B o l iv i a

R a p o l a n o T e r m e 6 0 .2

I t a ly

S t N e c ta i r e , 65 . 2

F r a n c e

b) Tectonically quiet regions

B a t h S p a 4 . 38

E n g l a n d

B or m io , I t a ly 6 . 07

L a g u n a G r a n d e 4 .0 4

M e x i c o

M a t l o c k B a t h 4 .3 0

E n g l a n d

0 . 40 45 P e n te c os t

( u n p u b l i s h e d )

0 . 6 6 - 1 .0 2 2 6 - 1 0 0 K i t a n o

(1963)

0 . 906 57 P e n te c os t


0 . 3 1 - 0 . 3 3 7 3 a F r i e d m a n

(1970)

0 . 22 37 a R i sa c h e r a n d

Eugs t e r ( 1979)

0 . 50 26 P e n te c os t


0 . 64 30 P e n te c os t


0 .0 58 4 6 a E d m u n d s a n d

M i le s ( 1991)

0 . 056 38 a De C a p i t a n i

e t a l . (1974)

0 . 012 28 P e n te c os t


0 .038 20 P e n te c os t


a

C a l c u l a t e d f r o m d a t a i n t e x t .

r e f le c t in g t h e h i g h p a r t i a l p r e s s u r e s o f t h e g a s i n s o l u t i o n . T h e r e is l it tl e i n f o r m a t i o n

o n r a t es o f c a r b o n d i o x id e e v a s i o n f r o m f r e s h w a t e r s o r h o t - s p r i n g s . I n a s t u d y o f g a s

t r a n s p o r t i n a G e r m a n t r a v e r t i n e - d e p o s i t i n g h il l s t r e a m , a r a t e o f 8 # M m - 2 s - 1 w a s

e s t i m a t e d ( U s d o w s k i e t a l. , 1 9 79 ). A c o n s i d e r a b l y h i g h e r r a t e o f 12 0 # M m - 2 s 1 w a s

r e p o r t e d f r o m t h e M o n t e z u m a W e l l s o u r c e ( C o l e a n d B a c h e l d e r , 1 96 9). N e i t h e r o f

t h e se r a t es a p p r o a c h e s t h o s e o b t a i n e d h e r e ( T a b l e 3) d u e t o t h e m u c h h i g h e r p C O 2

a n d t e m p e r a t u r e s o f th e i s su i ng t h e r m a l w a t e r s .

A d e t a il e d i n v e s ti g a t io n o f s o m e c o ld a n d w a r m t r a v e r t i n e - d e p o s i t in g s t r e a m s o f

V i r g i n i a in d i c a t e d a p o s i t i v e r e l a t i o n s h i p b e t w e e n m e a n s h e a r s tr e s s a n d d e g a s s i n g

( H o f f e r - F r e n c h a n d H e r m a n , 1 98 9) w i th m a x i m u m C O 2 e v a s io n ra t es o f a r o u n d 1

/.tM k g - l s - 1 o n t h e c a s c a d e s w h e r e s h e a r i n g s t re s s e s a p p r o a c h e d 2 k g c m -1 s - 1 . T h e s e

r a te s c o m p a r e w i t h a m e a n e v a s i o n r a te o f 6 # M k g -1 s 1 f o r B a g n a c c i o w i t h s h e a r in g

s t re s s e s o f a b o u t 0 .1 k g c m 1 s -1 ( T a b l e 1 ). T h e l a c k o f c o r r e l a t i o n b e t w e e n d e g a s s i n g

r a t e a n d t h e m e a n s h e a r s tr e ss w a s u n e x p e c t e d a t th e I t a l i a n s it es b u t m i g h t b e

e x p l a i n e d b y t h e l a c k o f e x t r e m e d i ff e r en c e s i n t u r b u l e n c e ( i. e. c a s c a d e s ) d o w n t h e

w a t e r c o u r s e s i n v e s t i g a t e d .

T h e f lu x o f c a r b o n d i o x id e a c r o s s th e a i r - w a t e r i n t e r f ac e h a s b e e n s h o w n , f o r m o s t


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275

conditions of flow to be consistent with the static film model (Usdowski and Hoefs,

1990) and is given by F = kAC (see above). The t ransfer coefficient k is dependent

upon temperature and turbulence and where these factors are constant, the flux is

dependent solely upon the concent ration difference of carbon dioxide across the film.

A water sample subject to gas evasion under these conditions will undergo an

exponential decline in dissolved gas with time, providing there are no limitations

imposed by chemical reactions involving the gas. Carbon dioxide evasion at

Bagnaccio showed such a decline with distance despite the fact that temperature

varied downstream. A similar decline is also apparent in the data of Usdowski et

al. (1979) but these examples must be regarded as the exception rather than the rule.

At Bagnaccio the exponential decline must be attribu ted to the comparat ively simple

and even form of the artificial channel. The weak positive correlat ion between evasion

rate and temperature follows directly from the reduced solubility and increased

diffusivity of carbon dioxide with increasing temperature. Values for the transfer

coefficient and estimated film thickness are similar to those obtained for a meltwater

stream investigated by Gislason (1989) who found that they were indicative of

turbulent flow. Coefficients in excess of 100 cm h -l fall within the breaking-bubble

regime at the sea surface (Broecker and Siems, 1984), but bubble breaking was rarely

observed at the ho t springs. This suggests that the hot-spring waters were somewhat

less turbulent than that predicted by the Broecker and Siems (1984) model, but this is

accounted by the increasing magnitude of k with water temperature. Another

contributory factor, which would be difficult to demonstrate, is that the concen-

tration of carbon dioxide at the film surface is probably higher than the mean

atmospheric value owing to the large amounts of gas being discharged into the

overlying atmosphere.

While carbon dioxide was lost from the hot waters, oxygen was absorbed. At

Bagnaccio, the rate of uptake was consistent with the transfer model of Tsivoglou

and Neal (1976) where uptake is a function of stream gradient and transit time.

4 3 Chemical composition and travertine formation

All of the springwaters are of the Ca -H CO 3- SO 4 type with high levels of

magnesium and total dissolved solids ranging from 3.1 to 3.9 g 1 1 (Table 2). The

low salinities and chloride levels show that the waters are non-marine in origin.

Chemistries of the Viterbo springs are markedly similar to each other, suggesting a

common source. High levels o f sulphate are probab ly the result of gypsum/anhydr ite

dissolution as sediments containing these minerals are widespread in central Italy.

The dissolution o f CaSO 4 and CaCO 3 may lead to subsurface precipitation of CaCO3

through the common ion effect (Freeze and Cherry, 1979). Some evidence of

subsurface deposition is apparent at Bagnaccio (R. L. Folk, personal com-

munication, 1991), and the spring waters appear to rise slightly supersaturated with

respect to both calcite and aragonite, though close to gypsum saturation (Table 2).

The gross composition of these waters is similar to other t ravertine-depositing springs

in central Italy (Malesani and Vannuchi, 1975).

Experimental work has shown tha t a ragonite is precipitated in preference to calcite


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A. Pen tecost /Jou rnal of Hydrology 167 1995) 263-278

a t t e m p e r a t u r e s e x c e e d i n g a b o u t 4 0 C ( L i p p m a n n , 1 97 3) . B e c a u s e m o s t o f t h e

t h e r m a l s p r in g s h a v e t e m p e r a t u r e s e x c e e d i n g 4 5 C , t h e o c c u r r e n c e o f a r a g o n i t e w a s

n o t s u r p r is i n g ( T a b l e 4 ), th o u g h i ts a b s e n c e a t B u l l ic a m e - D a n t e ( t 4 0 - 5 5 . 5 C ) w a s .

T h e c o m p a r a t i v e l y h i g h l ev e ls o f S r i n t h e t r a v e r t in e ( T a b l e 4 ) m a y b e a t t r i b u t e d t o i ts

h i g h l e v e l i n th e w a t e r a n d t h e l o w p a r t i t i o n c o e f f ic i e n t f o r S r i n a r a g o n i t e ( C i p r i a n i e t

a l. , 1 9 7 7 ). A l t h o u g h r e l a t iv e l y h i g h M g l ev e l s a r e p r e s e n t t o h i g h l ev e l s i n t h e w a t e r ,

h i g h m a g n e s i a n c a l ci te w a s n o t d e t e c t e d b y X - r a y d i f f ra c t i o n i n th e t r a v e r ti n e . T h e

o c c u r r e n c e o f g y p s u m ( T a b l e 4) p r o b a b l y r e su l te d f r o m e v a p o r a t i o n o f t he

C a S O 4 - s a t u r a t e d w a t e r .

T h e d e p o s i t i o n o f t ra v e r t i n e f r o m t h e r m a l s p r in g s h a s b e e n o b s e r v e d o n n u m e r o u s

o c c a s i o n s a n d is t h e s u b j e c t o f m a n y i n v e s t ig a t io n s . T w o v i e w s a r e c o m m o n l y

e x p r e s s e d c o n c e r n i n g i t s f o r m a t i o n , t h o u g h t h e y a r e n o t m u t u a l l y e x c l u s i v e

( P e n t e c o s t , 1 9 9 0 ). O n e c o n s i d e r s t h a t d e p o s i t i o n i s t h e r e s u l t o f C O 2 e v a s i o n , w h i c h

r a is e s p H a n d l ea d s t o r e a d j u s t m e n t o f th e d i s s o lv e d c a r b o n a t e e q u i li b r iu m .

S u b s e q u e n t l y , i n c r e a s i n g c a r b o n a t e s u p e r s a t u r a t i o n a n d p r e c i p i t a t i o n o c c u r . T h e

o t h e r v i e w e m p h a s i z e s t h e r o l e o f b i o l o g ic a l p ro c e s s e s in c o n t r o l l i n g c a r b o n a t e

p r e c i p i t a ti o n , e i th e r t h r o u g h t h e r e m o v a l o f C O 2 v ia p h o t o s y n t h e s i s a n d / o r a

c a t a l y t ic ef fe c t o p e r a t i n g a t t h e o r g a n i s m s u r fa c e . B e c a u s e o r g a n is m s a r e a b u n d a n t

a t a l l o f t h e s e s p r i n g s t h e i r p o s s i b l e e f f e c ts c a n n o t b e i g n o r e d .

I f p h o t o s y n t h e s i s i s s i g n if ic a n t i n r e m o v i n g C O 2 f r o m t h e w a t e r , t h e n a m a r k e d

d i f fe r e n c e i n t h e b u l k T D I C a n d t h e e v a s io n r a t e s h o u l d b e a p p a r e n t b e t w e e n

m e a s u r e m e n t s c a r r ie d o u t a t m i d d a y a n d m i d n i g h t. N o e v id e n c e w a s f o u n d h e r e t o

s u p p o r t s i g n if ic a n t p h o t o s y n t h e t i c a c t iv i ty , a n d i t m u s t b e c o n c l u d e d t h a t g a s e v a s i o n

is t h e m a j o r d r i v i n g f o r c e l e a d i n g t o c a r b o n a t e s u p e r s a t u r a t i o n a t a l l o f t h e s e si te s.

W h e t h e r o r g a n is m s p r o c e e d t o c a ta l y se c a r b o n a t e d e p o s i t io n f r o m a w a t e r t h a t h a s

b e c o m e h i g h ly s u p e r s a t u r a t e d a s a re s u l t o f C O 2 e v a s i o n is a n o t h e r m a t t e r a n d i s

b e y o n d t h e s c o p e o f t h is s t u d y .

cknowledgements

I w i s h to t h a n k D r . T . P J o n e s o f th e I n s t i tu t e f o r G e o l o g y , U n i v e r s i ty o f T u b i n g e n

f o r t h e s t a b le i s o t o p e a n a l y s e s a n d P r o f e s s o r R . L . F o l k f o r a s si s ta n c e w i t h s o m e o f t h e

i n s i t u a n a l y s e s .

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geochemistry of carbon dioxide in six travertine-depositing waters of italy

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