# Geochemical transect through a travertine mount: A detailed record of CO 2 -enriched fluid leakage from Late Pleistocene to present-day – Little Grand Wash fault (Utah, USA)

Emmanuelle Frery, Jean-Pierre Gratier, Nadine Ellouz-Zimmerman, Pierre Deschamps, Dominique Blamart, Bruno Hamelin, Rudy Swennen

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## To cite this version:

Emmanuelle Frery, Jean-Pierre Gratier, Nadine Ellouz-Zimmerman, Pierre Deschamps, Dominique Blamart, et al.. Geochemical transect through a travertine mount: A detailed record of CO 2 -enriched fluid leakage from Late Pleistocene to present-day – Little Grand Wash fault (Utah, USA). Quaternary International, 2017, 437, pp.98 - 106. 10.1016/j.quaint.2016.09.035. hal-01566241

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# Geochemical transect through a travertine mount: A detailed record of CO₂-enriched fluid leakage from Late Pleistocene to present-day – Little Grand Wash fault (Utah, USA)

Emanuelle Frery<sup>a, *</sup>, Jean-Pierre Gratier<sup>b, c</sup>, Nadine Ellouz-Zimmerman<sup>d</sup>, Pierre Deschamps<sup>e</sup>, Dominique Blamart<sup>f</sup>, Bruno Hamelin<sup>e</sup>, Rudy Swennen<sup>g</sup>

## ARTICLE INFO

Article history:

Available online xxx

Keywords:

Travertine

CO₂-enriched fluids

Leakage

U/Th dating

Oxygen and carbon stable isotopes

Vein growth rate

## ABSTRACT

Active and fossil endogenic travertine mounts scattered along the Little Grand Wash fault are studied as records of Quaternary CO₂-enriched fluid leakage. This study focusses on a particular area where a fossil mount formed in a near-surface setting by successive circulation/sealing episodes from Late Pleistocene to Mid-Holocene and where a modern surface travertine is still being formed by a CO₂-enriched fluid source. The fossil mount is composed of horizontal and vertical veins whereby the vertical veins recorded numerous cycles of circulation/sealing/dissolution events and were used as conduits for the CO₂-enriched fluid circulation from the depth to the surface or along sub-horizontal fractures where successive precipitation events are recorded. The modern travertine is being built at the surface by successive eruption of Crystal Geyser, an anthropic geyser active since the 1930's.

δ¹³C and δ¹⁸O signatures and U/Th datings, ranging from 11.5 ky till present-day allows calibrating in detail the CO₂ enriched fluid leakage along a single fault segment and in a post glacial context, as last glaciations in the study area took place 15 ky ago. The dataset shows a high decrease of the oxygen stable isotope values till about 6 ky, then the variations reflect a constant range until present-day. This tends to restrain the period of local increase of the meteoric water input in the aquifer that is sourcing the CO₂-enriched water.

The fossil travertine represents a 7 ky-long record of CO₂ leakage above a natural reservoir, from Late Pleistocene to Mid-Holocene. The flux of CO₂ leakage through time and the total escaping volume have been computed and appears to be low in comparison with an anthropogenic leak provoked, for instance, by a non-sealed well.

© 2016 Elsevier Ltd and INQUA. All rights reserved.

## 1. Introduction

Endogenic travertines are records of paleoclimatic and paleo- tectonic events (Altunel and Hancock, 1993; Hancock et al., 1999; Faccenna et al., 2008; Brogi et al., 2010; Capezzuoli et al., 2010; De Filippis et al., 2011; Gratier et al., 2012; Priewisch et al., 2014; Ricketts et al., 2014; Frery et al., 2015). Travertines are calcium

carbonate agglomerates known to be built under near ambient conditions in continental areas (Pentecost, 2005; Capezzuoli et al., 2014). Endogenic is use to define the origin of the CO₂-enriched fluids sourcing the agglomerate formation. Endogenic travertines are usually built along fault systems, where deep CO₂ and/or hot water are escaping from the depth to the surface (Janssen et al., 1999; Glover and Robertson, 2003; Crossey et al., 2006, 2009). Little Grand Wash present-day active travertine called Crystal Geyser is formed by cold water (~17 °C) with a heavy carbon isotope composition (>4‰, Kampman et al., 2009). The δ¹³C and δ¹⁸O

<footnote><sup>a</sup> CSIRO, Kensington, WA, USA</footnote>

<footnote><sup>b</sup> ISTerre, Université Grenoble Alpes, 38041, Grenoble, France</footnote>

<footnote><sup>c</sup> ISTerre, CNRS, 38041, Grenoble, France</footnote>

<footnote><sup>d</sup> IFPEN, Rueil-Malmaison, France</footnote>

<footnote><sup>e</sup> Aix-Marseille University, CNRS-IRD, CEREGE UM34, Europole Mediterraneen de l Arbois, BP80, 13545, Aix-en-Provence, France</footnote>

<footnote><sup>f</sup> LSCE, Gif sur Yvette, France</footnote>

<footnote><sup>g</sup> KU Leuven, Celestijnenlaan 200E, 3001, Heverlee, Belgium</footnote>

<footnote>* Corresponding author.
E-mail address: emanuelle.frery@csiro.au (E. Frery).</footnote>
signature of Green River fossil travertine mounts also demonstrate an endogenic origin of the fluids (Heath, 2004; Dockrill and Shipton, 2010). U/Th dating of these mounts calcium carbonate precipitations range from 400 ky to present-day (Burnside, 2010).

Such endogenic travertines located along Little Grand Wash and Salt Wash faults have been demonstrated to be formed episodically (Shipton et al., 2004; Dockrill, 2005; Burnside, 2010; Dockrill and Shipton, 2010; Kampman et al., 2012; Burnside et al., 2013; Frery et al., 2015). Kampman et al. (2013) showed that the CO₂ pulses can be related to glacial cycles.

In this contribution a detailed study of δ¹³C- and δ¹⁸O-signatures recorded along a travertine mount section is presented. U/Th dating and a part of the stable isotope measurements along this profile have been published in Frery et al. (2015) in order to build a conceptual model of episodic fluid circulation along an active fault. The record lasts from Late Pleistocene to present-day, providing a unique detailed record of this period of time that deserves to be closely analysed regarding i) the structure of the dated travertine veins; ii) the significance of the data regarding the Quaternary glaciation episodes on the Northwest part of the Colorado Plateau as well as iii) the calibration of CO₂ leakage during this period of time.

Results from more than 300 stable isotope analyses of the carbonate veins forming the travertine are analysed through time, based on 14 U/ThMeasurements. Stable isotope measurements are shown for the whole dated period, from the thin section scale to the

outcrop scale and on present-day active travertine forming close to the studied outcrop. Vertical and horizontal veins are studied regarding their petrography and their isotopic signature evolution through time. The results are then discussed in the perspective of the Late Holocene post-glacial cycle in the region and as potential proxy to calibrate the CO₂ paleo-leaky volumes along a particular segment of the fault during this period of time.

## 2. Location and sampling

The study focused on the so-called T1 fossil travertine mount, which is located in the footwall of a normal fault called Little Grand Wash (Fig. 1A and B). The mount is partially eroded. An accurate chronology has been obtained through 14 radiometric datings (U/Th) of 9 travertine veins sampled along a 10 m-thick profile (Fig. 1C). All samples were cleaned using a chemical pre-treatment to remove possible contamination. ²³⁴U/²³⁰Th ages were then measured with a TIMS mass spectrometer (CEREGE-Aix en Provence). The accuracy of the datings (from 11.469 to 4.559 ky) ranges from 27 to 9 years.

362 calcium carbonate powders (~1 mg each) were then collected using a dental drill on polished sections of 5 dated veins and 6 undated veins. These analyses were performed with an optima micromass spectrometer (LSCE, Gif sur Yvette, France). The accuracy on the measurements is ±0.04‰ and ±0.06‰ respectively for δ¹³C and δ¹⁸O. The results are reported relative to

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Geological setting and sampling: (A) Location of the Little Grand Wash and Salt Wash faults within the Colorado Plateau. (B) Simplified geological map of Little Grand Wash fault. (C) Picture of the studied travertine mount and location of the sampling (modified from Condon, 1997; Dockrill, 2005 and Frery et al., 2015).</figcaption></figure>
V-PDB and are given in the classical delta notation (Coplen et al., 1983).

Calcium carbonate powders were also sampled from thin sections for δ¹³C and δ¹⁸O analyses at the Friedrich-Alexander-University (Germany). Carbonate powders were reacted with 100% phosphoric acid at 75 °C using a Kiel III carbonate preparation line, connected online to a ThermoFinnigan 252 massspectrometer. All values were reported in permil relative to V-PDB by assigning a δ¹³C value of +1.95‰ and a δ¹⁸O value of -2.20‰ to NBS19. Reproducibility was checked by replicate analyses of laboratory standards and is better than 0.04‰.

Additional δ¹³C and δ¹⁸O analyses from the active Crystal Geyser travertine (location see Fig. 1) were acquired using the same technique and added to the dataset in order to get a range of current δ¹³C and δ¹⁸O values. Note that the CO₂-enriched water supplying the active travertine relates to the Glen Ruby #1, an oil well drilled in 1936 that was not sealed after been exploited for a few years. Thus the samples coming from this active travertine cannot be older than 80 years.

**3. Results**

**3.1. Field observations**

The fossil travertine mount reveals a complex internal structure, composed of calcium carbonate veins capped by conglomerates, i.e. pieces of the Curtis Formation outcropping in the hanging wall of the fault and a highly altered surface travertine (Fig. 1C).

Two main kinds of calcium carbonate veins can be distinguished thanks to their different orientation and internal structure, e.g. horizontal and vertical veins. The horizontal or sub horizontal veins (Fig. 2A–D) are characterized by their homogenous white colour and their subhorizontal parallel thin layering, parallel to the vein orientation. The vein growth direction is most of the time oriented from top to bottom and they are formed by calcium carbonate. The vein thickness is variable, from few centimetres to ten of centimetre thick.

The observed vertical veins (Fig. 2D, E, G) are formed by a series of calcium carbonate precipitation as well as other constituents such as micrite. "They are characterized by primary parallel

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Field observations along the fossil travertine mount outcrop (Fig. 1C). Note the difference between the horizontal veins (A, B, C and F) and the vertical veins (D, E and G). Growth direction illustrated by the red arrows. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
precipitations, growing from the edges to the centre of the veines. Other events, such as micritisation, are also recorded, erasing this primary orientation."

## 3.2. $\delta^{13}C$ and $\delta^{18}O$ signature evolution from Late Pleistocene to present-day

$\delta^{13}C$ and $\delta^{18}O$ signatures of the carbonate veins sampled along the T1 profile and Crystal Geyser travertine mount are plotted in function of the recorded precipitation age obtained by U/Th dating of the travertine veins in Fig. 3. From Late Pleistocene till present-day, a general trend of both stable isotope values decreasing with time is observed (Fig. 3). The values can roughly be clustered into two groups. Group I values were deposited between 12 and 6 ky. It is characterized by $\delta^{18}O$ values decreasing from -11.5 to -13.5‰ and $\delta^{13}C$ from +5.5 to +5‰. The decrease of both isotopic signatures through time is apparent and both $\delta^{13}C$ and $\delta^{18}O$ variations are closely correlated. Group II datings range from 6 ky till present-day. $\delta^{18}O$ and $\delta^{13}C$ values are characterized by a rather constant variation range, respectively in the range -13.5 and -12.6‰ and +4.4 and +5‰. The dataset is more scattered and the correlation between the two isotopic signatures is less obvious than in the case of group I.

Both nature and origin of the CO₂-enriched fluids are quite stable through the recorded precipitation occurrences, as attested by the narrow range of variation of all the recorded isotopic values

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Evolution of $\delta^{13}C$ and $\delta^{18}O$ signature (‰,V-PDB) in function of U/Th dating from Late Pleistocene till present-day, data coming from travertine T1 and present day Crystal Geyser travertine. Two groups of different signature I and II are found. Group I: $\delta^{13}C$ and $\delta^{18}O$ values decrease with time, Group II: $\delta^{13}C$ and $\delta^{18}O$ values are more or less stabilized.</figcaption></figure>

(Fig. 3), from Late Pleistocene till present-day. However, a gap in the record appears between the youngest dated vein (4.559 ± 0.023 ky) of the fossil travertine T1 and the active travertine built by Crystal Geyser source. The youngest dated vein is located close by the top of the outcrop, only capped by altered layered surface travertine, consisting of material difficult to precisely date.

## 3.3. Successive precipitation events recorded in studied vertical veins

Several vertical veins have been sampled and studied at thin-section scale. These veins reflect numerous precipitation events, cross-cutting each other. For instance, at least 9 precipitation events can be sorted within a same vertical vein, as illustrated in detail in Fig. 4. The nature of the different types of mineralization, such as elongated thin aragonite or euhedral calcite crystals, indicate different mineralization processes and cycles of precipitation. The range in $\delta^{18}O$ and $\delta^{13}C$ variations between the different events recorded (Fig. 4C) is narrow, less than 1‰ for both isotopes. Based on this observation it is not unlikely that even if several cementation increments are observed, their isotopic signatures reflecting the CO₂-enriched fluid nature and origin, remain relatively similar. Detailed analysis of vertical vein systems showed a succession of precipitation events. Vertical veins can record several events of CO₂-enriched fluid precipitation, dissolution and new precipitation events and thus does not necessarily represent an archive of the precipitation that occurred at a given time.

## 3.4. Continuous dated geochemical records from studied horizontal veins

Studied horizontal veins are formed by discrete travertine successions providing a better record of the successive precipitations between two dated samples. The thickest carbonate veins of the outcrop (veins 42 and 44 located in Fig. 1) were studied in detail (Fig. 5) with a punctual isotopic analysis each 2–5 mm and a U/Th dating every 100–150 mm. Such horizontal travertine veins grow from top to bottom and thus lifted the rocks above them. This is explained by the effect of crystallization force (Gratier et al., 2012).

From the U/Th dating, a linear vein growth rate has been estimated from 0.08 to 0.71 mm/yr. However, the growth rate is variable along the vein trajectory and its value is highly dependent on the interval of time considered to make the interpolation. Nevertheless, a continuous rising of the growth rate is noticeable during the entire period of time recorded in both veins, from 11.469 to 5.851 ky (Fig. 5).

The oldest vein (44), formed in the period 11.469 ± 0.015 till to 9.175 ± 0.014 ky, shows two patterns with regard to its stable isotope signature, e.g.: i) for the period between 11.469 ± 0.015 till 10.053 ± 0.012 ky, where both $\delta^{13}C$ and $\delta^{18}O$ decrease and ii) from 10.053 ± 0.012 till 9.175 ± 0.014 ky, the correlation between the two isotope ratios is less obvious and the average value remains rather constant. The other vein (42), which dates from 6.830 ± 0.014 till 5.851 ± 0.010 ky, does not reflect any correlation between $\delta^{18}O$ and $\delta^{13}C$ and the average values remain rather constant.

Two millennial evolutionary trends were thus observed: 1) decrease and close correlation of stable isotope ratios or 2) linear behaviour with time and without a close correlation in $\delta^{18}O$ and $\delta^{13}C$ ratio variations. In parallel, a near constant growth rate (0.1–0.2 mm/y) is observed from 11.5 to 6 ky, then an increase of this growth rate up to 0.7 mm/y since 6 ky.

Both horizontal veins are continuously laminated, with laminations occurring parallel to the vein opening plane.
<figure><img src="image_6.png" /></figure>

**Fig. 4.** Details of different vein generations: example of a thin section from travertine T1, little Grand Wash fault (localized Fig. 1C). A. Records of mineralization episodes at the thin-section scale. The colours reflect the different diagenetic events. The nature of these events is specified in parentheses (v vertical carbonate vein; h horizontal carbonate vein; s: sandstone; b: micrites). The arrows give the mineral growth direction and senses of each event. The order of apparition of each event is in the legend: 1 for the oldest and 9 for the youngest. Detailed of the sealing relative chronology may be summarized as follows: a) The vertical fibrous vein v1 had been sealed by elongated thin aragonite phases (picture B 1). This vein is almost fully erased by the other mineralization stages. b) v1 is crossed by a horizontal thin vein with centripetal growth (h1) filled with a euhedral-calcite mineralization with an open space in the middle (picture B 2). c) Then, there is another horizontal vein (h2) of aragonite including micritic laminations (b1) and altered aragonite crystals (picture B 3). d) Other centripetal growth nearly vertical veins with subhorizontal mineralization (v2) crosscut the system h1/b1. At the middle, micrite (b2) and sand infill (s1) are present. This sandstone may originate from the Green River that is located near the outcrop. e) This is cut by other later horizontal veins (h3). These veins are composed of compact aragonite with top-to-bottom growth direction (4). f) The last vertical veins (v3) cross-cut the entire thin-section. The associated precipitations consist of euhedral centripetal growth calcium carbonate with an empty space in the middle (picture B 5). The cathodoluminescence study shows an orange colour, reflecting a difference in composition between this vein and the other phases which are non-luminescent (picture B 6). B. Pictures of the different mineralisation events observed on the thin-section localised on A – the mineral growth direction is indicated with red arrows. 1. A piece of vertical vein v1 with associated horizontal aragonite fibres is crossed by a horizontal vein h1 with vertical fibres – 2. Thin horizontal vein h1 with centripetal growth characteristics. – 3. Detail of the horizontal veins h2 top to bottom growth: altered aragonite minerals with micritic laminations b1. – 4. Compact aragonite phases in the horizontal top to bottom growth vein h3. – 5 & 6. Respectively natural light and cathodoluminescence image of the last mineralization event with the centripetal growth of vertical thin vein v3 crossing the micritic event b2. The veins v3 are clearly orange. C. δ¹³C and δ¹⁸O (‰, V-PDB) from the veins observed on the thin section (A). The nature of each phase is specified in brackets (v vertical carbonate vein; h horizontal carbonate vein; s: sandstone; b: micrites). The main part of the samples, excepted 1 (v1) and 4 (cv2), have similar δ¹³C and δ¹⁸O signature, with a variation range, Δ δ¹³C and δ¹⁸O, inferior to 1‰. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Evolution of the stable isotopes signature (‰ V-PDB) and mineral growth rate in function of U/Th dating illustrated by two carbonate veins named 42 and 44 located in Fig. 1C. The growth rate is near constant from 11,5 to 6 ky then increase since 6 ky. In parallel, one can see two millennial geochemical evolutionary trends: 1) decrease and strong correlation of δ¹³C and δ¹⁸O or 2) linear behaviour with time and a possible opposed δ¹⁸O and δ¹³C ratio evolution.</figcaption></figure>

**4. Discussion**

**4.1. A detailed geochemical record of a post glacial interval**

Green River fault leakage has been studied over the past 400,000 years on 40 separated travertine mounds by Kampman

et al. (2012). This present study completes this regional work with a detailed dataset focussed on the record of Late Pleistocene to present-day CO₂-enriched fluid flow. The studied T1 travertine mount records carbonate precipitations from 11.469 ± 0.015 ky ago up to 4.559 ± 0.023 ky ago, thus archiving up to 7 ky of precipitation history at this location.
During this period, travertine precipitation was not constant through time: vein sizes are variable and some periods of time are marked by a high frequency of sealing whereas other periods are marked by a lack of record (Figs. 2 and 5).

From 11.469 ± 0.015 till 10.053 ± 0.012 ky, δ¹³C and δ¹⁸O values decrease with a high covariant variation. This covariant change observed at the vein scale possibly indicates kinetically-driven vein precipitation (Kele et al., 2011; Kampman et al., 2013). Kampman et al. (2012) demonstrated a causal relationship between CO₂-pulsing and vein opening by isotopic analyses (C, O, Sr ratio and Ba/Ca and Sr/Ca elemental ratio). The opening of the veins is linked with a pulse of CO₂ in the system followed by a degassing step contemporaneous to the vein growth. The pulse of CO₂ is marked by a decrease in δ¹³C-values and the degassing by a rise in δ¹³C. Our data tends to record a pulse of CO₂ in the system. This variation in δ¹³C and δ¹⁸O correlation is due to isotope fractionation between C present as CO₂ (aq) and HCO₃⁻ in the spring waters and carbon in CaCO₃. The extreme enrichment in δ¹³C in the veins is then a result of Rayleigh fractionation during degassing of the fluid (Kampman et al., 2012). The vein samples enriched in heavier δ¹³C compared to δ¹³C composition of the present-day Crystal Geyser water (<1.5‰ VPDB, Heath, 2004) may be due to rapid CO₂ degassing (Fouke et al., 2000; Fouke, 2011).

This study recorded δ¹⁸O highly decrease in the older samples from 11.5 to 10.5 ky and then keep clearly decreasing up to 6 ky. This result aligns with the Green River travertine mounts evolution trend (Kampman et al., 2013) that reflects a mean general decrease of around 1‰ for the last 15 ky. This detailed U/Th, δ¹⁸O, δ¹³C record suggests that the main decrease period observed in δ¹⁸O signature is taking place at the end of the Pleistocene, in a short period of time that ended less than 9 ky after the last local glaciation that took place 15 ky ago (Marchetti et al., 2011). Then, from 6 ky, the δ¹⁸O values remain quite constant until present-day. δ¹⁸O recorded in the travertine is function of the water temperature and δ¹⁸O as well as other environmental factors, such as evaporation (Andrews, 2006). Present-day chemical and isotopic analyses of Crystal Geyser bubbling water suggested a mix origin: meteoritic water mixed with groundwater from the Jurassic Navajo regional aquifer with the input of deep CO₂ from a paleozoic aquifer (Kampman et al., 2009). The water δ¹⁸O values are quite constant, with a smooth 0.3‰ oscillation (from -14.4 to -14.1‰ SMOW, Heath, 2004). Study of other active bubbling springs along the Little Grand Wash fault and Salt Wash fault showed a similar fluid origin with a δ¹⁸O values are quite constant, with less that 1‰ variation that could be directly explained by the water temperature range, 16.7 to 17.7 °C, from one spring to another (Heath et al., 2009).

The Quaternary δ¹⁸O values decrease in Green River area had been interpreted as a potential post glacial increase of meteoric water in the system (Kampman et al., 2012). This study dataset shows that, in this hypothesis, the post-glacial increase of meteoric water in the system is rapid until 6 ky and then do not evolve considerably until present-day. The immediate post-glacial period (11.5 till 6 ky) is also marked by a slow grow rate (0.08 mm/y) of the vein (Fig. 5). Following this period, the vein grow rate rises up to 0.71 mm/y at least between 6,1 and 5,8 ky (no data are available later) and the δ¹³C and δ¹⁸O variations stay constant, from 6 ky till present-day (Fig. 3).

## 4.2. CO₂ leakage from Late Pleistocene to Mid Holocene

The total volume of CO₂ leakage at the surface can be estimated from the volume of calcium carbonate precipitated in the travertines.

At the Crystal Geyser outlet, based on hydrologic mass balance modelling of the present-day CO₂ water composition, some authors (Heath, 2004; Shipton et al., 2005) estimated the proportion of the total CO₂ flux accounting for travertine mount formation. They estimated that the volume of CO₂ leakage recorded in the calcium carbonate precipitations represents between 6.6% (Heath, 2004) and 10% (Shipton et al., 2005) of the total dissolved CO₂ in the water sample, which is only a minor part of the total leakage. Based on these estimations, a minimal value of the CO₂ leakage can be estimated from the T1 record, as the CO₂ escaping as a free phase is not taking into account in the chemical models based on the water sampling and that the travertine outcrop has been partially eroded.

In the case of the T1 fossil travertine mount studied here (Fig. 1C), the total mass of precipitated CO₂ recorded in the outcrop over time was calculated considering the calcium carbonate precipitation equation (Eq. (1)):

$$m_{CO_2}^{\text{precipitated}} = \rho_{CaCO_3} \cdot V_{CaCO_3} \frac{M_{CO_2}}{M_{CaCO_3}} \quad (1)$$

The molar mass ratio of carbon dioxide and calcium carbonate and the volume density of the calcium carbonate are considered as constant parameters:

$$\frac{M_{CO_2}}{M_{CaCO_3}} = \frac{44 \text{ g.mol}^{-1}}{100, 1 \text{ g.mol}^{-1}}$$

$$\rho_{CaCO_3} = 2, 7.10^3 \text{ kg.m}^{-3}$$

Eq. (1) can therefore be simplified so that the mass of precipitated CO₂ is a direct function of the volume of calcium carbonate precipitated:

$$m_{CO_2}^{\text{precipitated}} = 119 \cdot V_{CaCO_3} \quad (2)$$

In order to calculate the volume of calcium carbonate ($V_{CaCO_3}$) a 5 m long rectangular section along the travertine with a 1 m² surface base was considered (Fig. 1C). The dated veins represent 45 ± 10% of the total block volume of the travertine mount T1. The veins are considered to be rectangular parallelepipeds with thicknesses as measured in the field and the initial surface area of the travertines being extrapolated to its initial value before erosion. This initial value has been extrapolated from present-day measurements of travertine T1 surface calculation, based on aerial photos (Dockrill, 2005) and our structural analysis. Details of the CO₂ volume and leakage flow computations are given in Table 1.

The total CO₂ leakage flow, inferred from the U/Th dated veins (Fig. 6), shows an interesting evolution through this studied period of time (11.5 ky till 4 ky): the temporal variation in CO₂ flow is episodic with 4 mean periods of fluid flow and vein sealing that last about half of the total period duration (Fig. 6). Consequently, even if the total mass of CO₂ emissions is difficult to determine, a new minimum value of CO₂ leakage in natural conditions is calibrated. The total natural CO₂ leakage flow inferred from the travertine mass is low, less than 0.9 tonnes per year. For comparison, the present-day monitoring of Crystal Geyser allow to calibrate a high seepage rate of 11.000 tonnes/year (Gouveia et al., 2005; Brogen et al., 2006). This
<caption>**Table 1**</caption>

Volume and flux of CO₂ leakage recorded in the travertine mound from Late Pleistocene to present-day. The total CO₂ volume is extrapolated from the travertine mount volume and two hypotheses on this fraction percentage of the total leaky CO₂: 6.6% and 10% (Heath, 2004; Shipton et al., 2005).

<table><thead><tr><th rowspan="2"></th><th rowspan="2">Veins fraction (% ±5%)</th><th rowspan="2">Fraction of the travertine mount (% ±5%)</th><th rowspan="2">Veins thickness (mm ± 5%)</th><th rowspan="2">Mass CO₂ precipitated (T ± 30%)</th><th colspan="2">Mass total of CO₂ leakage (T ± 30%)</th><th rowspan="2">Growth rate (mm/y ± 10%)</th><th rowspan="2">Leakage time-lapse (y ± 15%)</th><th colspan="2">Flux of CO₂ leakage (T/ky ± 40%)</th></tr><tr><th>6.6%</th><th>10%</th><th>6.6%</th><th>10%</th></tr></thead><tbody><tr><td>A1</td><td>15</td><td>6,8</td><td>203</td><td>48</td><td>730</td><td>482</td><td>0,1</td><td>2025</td><td>361</td><td>238</td></tr><tr><td>A10</td><td>15</td><td>6,8</td><td>203</td><td>48</td><td>730</td><td>482</td><td>0,1</td><td>2025</td><td>361</td><td>238</td></tr><tr><td>A17</td><td>15</td><td>6,8</td><td>203</td><td>48</td><td>730</td><td>482</td><td>0,1</td><td>2025</td><td>361</td><td>238</td></tr><tr><td>39</td><td>4</td><td>1,8</td><td>54</td><td>13</td><td>195</td><td>129</td><td>0,7</td><td>77</td><td>2524</td><td>1666</td></tr><tr><td>42</td><td>20</td><td>9,0</td><td>270</td><td>64</td><td>974</td><td>643</td><td>0,2–0,7</td><td>988</td><td>985</td><td>650</td></tr><tr><td>44</td><td>23</td><td>10,4</td><td>311</td><td>74</td><td>1120</td><td>739</td><td>0,08–0,1</td><td>2282</td><td>491</td><td>324</td></tr><tr><td>46</td><td>6</td><td>2,7</td><td>81</td><td>19</td><td>292</td><td>193</td><td>0,4</td><td>203</td><td>1442</td><td>952</td></tr><tr><td>41</td><td>1</td><td>0,5</td><td>50</td><td>0</td><td>3</td><td>2</td><td>0,7</td><td>71</td><td>38</td><td>25</td></tr><tr><td>13</td><td>1</td><td>0,5</td><td>80</td><td>0</td><td>4</td><td>3</td><td>0,7</td><td>114</td><td>38</td><td>25</td></tr><tr><td>Tot dated veins</td><td>100</td><td>45</td><td>1350</td><td>321</td><td>4868</td><td>3213</td><td>0,4</td><td>7285</td><td>668</td><td>441</td></tr><tr><td>Tot undated veins</td><td></td><td>15</td><td>600</td><td>143</td><td>2164</td><td>1428</td><td>0,4</td><td>7285</td><td>297</td><td>196</td></tr><tr><td>Tot travertine veins</td><td></td><td>60</td><td>1800</td><td>428</td><td>6491</td><td>4284</td><td>0,4</td><td>7285</td><td>891</td><td>588</td></tr><tr><td>Tot carbonate travertine</td><td></td><td>70</td><td>2100</td><td>500</td><td>7573</td><td>4998</td><td>0,4</td><td>7285</td><td>1039</td><td>686</td></tr></tbody></table>

geysers is indeed being sourced from an abandoned well that crosscuts the regional seal and that was not totally closed after its production. This CO₂ flux is 10.000 times greater than the flux computed from the travertine mount that was formed by a natural CO₂ leakage along the fault. This highlights the preponderant role of the episodic sealing of the fault that controls the fluid flow (Frery et al., 2015). For instance, considering natural flow (computed in this study), the release of the 11 million tons of CO₂ stored in the Sleipner field, in the North Sea (Hermanrud et al., 2009; Chadwick et al., 2011) would take more than 10 My, whereas it would take only 1000 years if an open hole drilled through the reservoir were allowed to leak permanently without being sealed.

**5. Conclusion**

The studied endogenic travertine, located in the footwall of Little Grand Wash Fault, along the NW part of the Colorado Plateau, recorded CO₂-enriched fluid leakage from Late Pleistocene till Mid

Holocene. The detailed δ¹³C and δ¹⁸O dataset along T1 travertine mount profile represents a unique archive of the CO₂-enriched fluids evolution along a single segment of a fault. The record corresponds to a period of time following the last glacial event, which locally took place 15 ky ago.

Until 6 ky ago, the record is dominated by CO₂ pulses, rapid decrease of the δ¹⁸O values and a low growth rate of the travertine veins. The decrease of the δ¹⁸O values can indicate an increase of the meteoritic water concentration in the system (Kampman et al., 2012). Following that period, the δ¹⁸O values tend to equilibrium and episodes of lower amplitude CO₂ pulses occurred in correlation with an increase in growth rate. These episodes can be linked with the fault tectonics as supported by the development of different vertical and horizontal veins. The vertical veins record several cycles of sealing/dissolution/sealing, acting as conduit for CO₂-enriched fluids input in the system as described in Frery et al. (2015) from the same outcrop. Note that this kind of episodic leakage through the fault is also well described in a tectonically

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Evolution of cumulative leaky CO₂ flux in function of time from the Late Pleistocene till the Mid Holocene, computed from U/Th dated aragonite veins of travertine T1, Little Grand Wash Fault. The flux has been computed from the volume estimation of each vein averaged over the vein thickness. The veins are located in Fig. 1C and the modelling results are shown Table 1.</figcaption></figure>
active context from fissure ridge travertine mounts near Pamukkale (Turkey) (Brogi et al., 2014) where the vertical veins emplacement changed through time, migrating towards the fault hanging wall.

The travertine mount has also been used as a proxy to evaluate the rate of CO₂ leakage from Late Pleistocene to Mid Holocene. The total natural CO₂ leakage flow inferred from the travertine mass is low, less than 0.9 tonnes per year. A comparison with CO₂ leakage along an open hole near by the studied travertine mount that has been left open shows that anthropogenic CO₂ flux is 10.000 times greater than natural CO₂ leakage along the fault. This highlights the preponderant role of the sealing of the faults that controls the fluid flow. It illustrates that open holes seem to heal much more slowly than natural faults.

## Acknowledgements

The authors would like to acknowledge Laura Crossey as well as an anonymous reviewer for their constructive comments that helped to improve this manuscript. We also would like to thanks Herman Nijs who made the thin sections, Prof. M. Joachimsky (University of Erlangen, Germany) for his support on the stable isotope measurement and IFPEN for the financial support.

## Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.quaint.2016.09.035.

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## Web site:

Carbon Dioxide Information Analysis Center: http://cdiac.ornl.gov/.