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7/25/2019 Geochemistry of Carbon Dioxide in Six Travertine-Depositing Waters of Italy
1/16
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
7/25/2019 Geochemistry of Carbon Dioxide in Six Travertine-Depositing Waters of Italy
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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
7/25/2019 Geochemistry of Carbon Dioxide in Six Travertine-Depositing Waters of Italy
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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
7/25/2019 Geochemistry of Carbon Dioxide in Six Travertine-Depositing Waters of Italy
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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
7/25/2019 Geochemistry of Carbon Dioxide in Six Travertine-Depositing Waters of Italy
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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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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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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
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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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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 ,
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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
( u n p u b l i s h e d )
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
( u n p u b l i s h e d )
0 . 64 30 P e n te c os t
( u n p u b l i s h e d )
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
( u n p u b l i s h e d )
0 .038 20 P e n te c os t
( u n p u b l i s h e d )
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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