Linking process and product in terrestrial carbonates using a solution thermodynamic approach

Introduction Conclusions References


Introduction
Terrestrial carbonate deposits have outstanding potential to act as an archive of climatic change (Andrews, 2006) and/or changes in the behaviour of crustal fluids (Minissale, 2004).However, despite their high potential and the considerable research effort that has already been expended on them there remains considerable debate and confusion over how mineral-forming processes can be linked to the sedimentology and petrology of tufas, travertines and other terrestrial chemical deposits.Early efforts were based on straightforward applications of conventional carbonate petrographic schemes.The standard scheme of Dunham (1962) modified by Embury and Kloven (1972) works admirably for most freshwater limestones, but lacks precision on account of the numerous biological elements present and makes no attempt to link product to process.Considerable advances from these early efforts have been made for geothermal deposits by numerous authors (Chafetz and Folk, 1984;Folk and Chafetz, 1983;Carrara et al., 1998;Carrara, 1994;Folk, 1994;Folk et al., 1985) and this has been further developed by Guo and Riding (1998).Much of the current petrographic terminology applied to ambient temperature tufas has also developed upon the Folk and Dunham schemes (see Pentecost, 2005).However, to date no single petrographic scheme has proven sufficiently effective to become standard.
In contrast, a diverse group of field classification schemes have independently been developed specifically for freshwater carbonates (Stirn, 1964;Irion and Müller, 1968;Schneider et al., 1983;Pentecost and Lord, 1988) emphasising; the role of botanical Introduction

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Full elements (Pentecost and Lord, 1988); their distinct geomorphological expression within the landscape (Symoens et al., 1951;Golubić, 1969;Nicod, 1981;Szulc, 1983); or combinations of biological and geomorphological aspects (Buccino et al., 1978;Ferreri, 1985;D'Argenio and Ferreri, 1988;Golubić et al., 1993).Others (e.g.Julia, 1983;Golubić, 1969) have discussed a process-based approach frequently involving cyclic progression of the facies, and an important observation to arise from more recent classifications was the role played by the phytoherm (freshwater reef) in tufa accumulation (see discussion in Pedley, 1992Pedley, , 1990)).This work was drawn together by Pedley (1990) and Ford and Pedley (1997) who offered an integrated, rationalised scheme for classifying freshwater carbonates which embraced both laboratory and field scales.Modifications of these schemes are now available for tropical tufas (e.g.Carthew et al., 2006) and new controls on individual facies arise on continuing field (Drysdale et al., 2003b;Brasier et al., 2011) and laboratory-based research (Pedley and Rogerson, 2010a).
Alternative field classifications continue to evolve (e.g.Glover and Robertson, 2003;Peña et al., 2000, for tufas), but these schemes are yet to be updated to reflect recent developments in understanding biofilm processes (Decho, 2010).Moreover, there remains a lack of a single scheme that adequately discriminates both geothermal and meteoric-type deposits simultaneously.Attempts have also been made to bridge and update existing classification schemes to also encompass geothermal deposits (e.g.Jones and Renaut, 2010), but these remain focussed on products at the expense of knowledge about processes.In this regard, it is surprising that there has not yet been an attempt to classify these deposits on the basis of whether they are dominantly biomediated or physico-chemical deposits.In the light of new insights into these processes arising from experimental work (Pedley and Rogerson, 2010b;Pedley et al., 2009;Bissett et al., 2008a;Shiraishi et al., 2008b), this is a major shortcoming.
Ultimately, we are yet to achieve the goal of a universally effective classification for terrestrial deposits that adequately reflect their unique modes of origin.Understanding how processes can be read from products is a key step on that journey.Introduction

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Full Major stumbling blocks remain to achieving the key step of aligning process and product in these systems into a single systematic view.Most seriously, the independent development of terminology between ambient temperature deposits and thermal deposits maintains the division of these communities.Even the basic terminology distinguishing terrestrial carbonates is not without conflict as some workers (e.g.Pentecost, 2005;Pentecost and Viles, 1994) recommend that all freshwater deposits should be called travertines, with the caveat that the prefixes "meteogene" or "thermogene" be applied for deposits in which the original precipitating source waters are either cool, shallow circulation-meteoric or hot, deep circulation (thermogene, with temperatures typically 20-60 degrees centigrade at the point of emergence).Others (including the authors of this manuscript) recommend that travertine is reserved only for deposits in geothermal settings.The pressing need to see all terrestrial carbonate systems as related and not independent of each other is emphasised by the case of all "thermogene" or "travertine" systems, where vegetation colonisation occurs as waters cool towards ambient temperatures, resulting in deposits which are often indistinguishable from "meteogene" "tufas".While we argue that there is an advantage in retaining the terms "tufa" and "travertine" to distinguish the source of the ions precipitating from the system, we simultaneously recognise that the downside of a simple division in terms of temperature is that alkaline lakes (e.g.Mono Lake, Connell and Dreiss, 1995) may generate deposits significantly different to normal "tufa" regardless of being ambient in terms of temperature.
Here we attempt the first steps to overcome these stumbling blocks, building on the huge progress made in understanding the relationships between various types of deposits by previous workers outlined above, and propose a new direction in summarising precipitating system solution chemistry using simple thermodynamic concepts.Our thermodynamic approach is based on the assumption that physico-chemical forcing of precipitation will ultimately be a product of high Gibbs free energy in the solution the precipitation is derived from, as first proposed by Dandurand et al. (1982).Introduction

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Full To drive rapid precipitation, high levels of excess dissolved calcium carbonate are necessary in the ambient water.Consequently, the primary partition between tufa-forming and non-tufa river systems should coincide with some threshold in calcite supersaturation.However, Pentecost (1992) showed that in the Yorkshire Dales (UK) no systematic relationship between precipitation and degree of calcium carbonate supersaturation was present.An additional problem of this view is that the activation energy for calcite is −48.1 kJ mol −1 (Inskeep and Bloom, 1985) whereas the onset of significant riverine calcium carbonate deposition is considerably lower (e.g.−7 kJ mol −1 , Dandurand et al., 1982) leaving a wide energetic gap between where spontaneous nucelation would be expected to form from thermodynamic principals and where precipitation is found in natural systems.It is therefore already abundantly clear that thermodynamic considerations are not sufficient to explain site-to-site differences at low supersaturation, that kinetic and "surface" conditions are responsible for managing the thermodynamic barrier to precipitation and other explanation of these additional conditions must be sought.
At higher supersaturation, physico-chemical precipitation (i.e. that regulated by classic thermodynamic considerations) occurs either due to ingassing of atmospheric CO 2(g) under hyperalkaline conditions due to the presence of excess OH − (aq) (Clark and Fontes, 1990;Andrews et al., 1997) or, more normally, due to outgassing of CO 2 to atmosphere due to the presence of excess HCO − 3 (aq) (Emeis et al., 1987).Rapid loss of CO 2(aq) primarily occurs when fresh spring waters come into contact with air depleted in CO 2 relative to aquifer air (Chafetz and Folk, 1984).Degassing-driven precipitation occurs according to the le Chatellier principle via;

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Full A consequence of this is that HCO − 3 (aq) rapidly decreases downstream from point sources of spring water, driving rapid precipitation close to the spring and decreasing rate of precipitation in a downstream direction (Chen et al., 2004;Lorah and Herman, 1988;Merz-Preiss and Riding, 1999).A complicating factor is that calcite does not precipitate directly from bicarbonate, making the pH-related conversion of HCO − 3 (aq) to CO 2− 3 (aq) potentially limiting of precipitation rate (Dreybrodt et al., 1997).This is demonstrated by the observation that precipitation can be slow immediately adjacent to the spring, but rapid a few 10's or 100's of meters downstream (Dandurand et al., 1982).Turbulent flow and active agitation increases the area of the air-water interface and therefore promotes gas exchange.Consequently, precipitation is considered to be enhanced at waterfalls and rapids by the same mechanism as occurs at springs (Chen et al., 2004).Carbon dioxide solubility is also strongly affected by temperature and evaporation will also promote increased supersaturation.Water at 0 • C is capable of dissolving about three times as much CO 2 as at 30 • C (Dramis et al., 1999) and as cold karst waters emerge its temperature increases, resulting in CO 2 out-gassing and enhancing supersaturation of the water (Lorah and Herman, 1988).Mixing and high surface area: volume promote warming, and consequently may potentially be enhanced at rapids and waterfalls.In temperate climates, slower downstream warming may result in reduced precipitation at proximal sites while warmer climates encourage more tufa formation closer to the spring (Drysdale et al., 2003a).High ambient air temperatures and low relative humidities will enhance levels of saturation, which is an important driver of precipitation in tropical tufa systems (Carthew et al., 2006;Drysdale and Gale, 1997).
The impact of temperature and humidity can be large enough to significantly alter the large-scale morphology of major tufa systems, with high rates of precipitation reported from tufa systems in Spain relative to otherwise similar systems in the UK (Pedley et al., 1996).
It should be noted, however, that the excess free energy provided by desorption of CO 2(aq) is ∼ − 5 kJ mol −1 , which is a full order of magnitude lower than the energy required for spontaneous nucleation.It may be the kinetics of the degassing processes Introduction

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Full rather than its impact on calcite saturation that is critical to initiating nucleation (Dandurand et al., 1982).If kinetics are dominant, different reaches and even different layers of water at a single reach of a freshwater systems may have very different saturation characteristics.Surface layer kinetic influences will ultimately be bound up with water flow structure, which regulate vertical ion transport rates.These authors are not aware of any specific study of flow structure in these settings.Bottom layer kinetic influences are fully understood, however.

Bottom layer kinetic influences; the diffusive boundary layer
At sites with flowing water, flow separation occurs at the water-carbonate interface, forming a boundary layer across which ions are exchanged primarily by diffusion (Zaihua et al., 1995;Liu and Dreybrodt, 1997;Dreybrodt and Buhmann, 1991 (Zaihua et al., 1995) showed that the consequence of this behaviour was that deposition rates in fast flowing water are higher than still water by a factor of four.This mechanism has also been investigated via numerical modelling, which suggests that it may be a fundamental control on the patterns of development of tufa barrages (Hammer et al., 2007(Hammer et al., , 2008;;Hammer, 2008;Veysey and Goldenfeld, 2008).Indeed, a degree of consensus is emerging from this work that "equilibrium" degassing at sites of enhanced flow has very little impact on the rate of precipitation, which is essentially driven by enhanced vertical ion flux, particularly where flows become unstable (Hammer et al., 2010).This consensus in the physical literature is yet to be fully recognised in the sedimentological literature.Introduction

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Biological factors
The mechanisms and state of knowledge of microbially induced and influenced calcite precipitation has recently been thoroughly reviewed (Dupraz et al., 2009).Consequently, we here aim to only summarise the most relevant parts of the very extensive knowledge concerning microbe-calcite interaction.

Photosynthesis and respiration
Consumption of CO 2 by photosynthesis results in enhanced pH, and consequently in increased proportion of dissolved inorganic carbon present as CO 2−  3 (aq) , close to the bodies of micro-and macrophytes as well as phototrophic microbes such as cyanobacteria.Consequently, when photosynthesis occurs in solutions at or near saturation with respect to CaCO 3 , calcite may precipitate as sheaths around algal filaments and coatings on plants (Pentecost, 1978(Pentecost, , 1987)).Respiration operates in the inverse sense, tending to increase dissolved CO 2 levels, reduce pH and inhibit precipitation.Consequently, tufa systems exist within a dynamic equilibrium where precipitation may be triggered or prevented by the balance of microbial metabolic processes (Visscher and Stolz, 2005;Decho, 2010).This balance will primarily reflect the state of health of the biofilm itself, as a growing community must, by definition, be consuming more carbon than it is releasing and vice versa.The impact of microbial metabolisms is well reflected in an investigation of chemical fluxes associated with calcite precipitating solutions, which indicates increased flux of Ca 2+ and HCO − 3 to biofilm-encrusted surfaces relative to non-encrusted surfaces in high light conditions, but the reverse under dark conditions (Shiraishi et al., 2008a).Curiously, biofilms are capable of modifying their microenvironment so that the pH of interstitial water remains constant regardless of changes in the macroenvironment (Bissett et al., 2008b), so this effect may actually be amplified under low ambient pH conditions.Certainly, influxes of low pH water, argued by Pentecost (1992) to be a potential explanation for the absence of tufa from certain stream in Yorkshire, would have to be rather persistent to alter the ability of biofilms to Introduction

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Full promote precipitation, and therefore be likely to be resolved (and therefore not cryptic) in the data he presents.
The degree to which photosynthesis alters the precipitation process is debated within the literature, however (Shiraishi et al., 2008a).Monitoring of hydrochemical conditions over 4 diurnal cycles at a single station in Davys Creek, Australia, revealed little impact.Atmospheric temperature was identified as the dominant control on the CO 2(aq) budget, implying that metabolic processes were minor of importance (Drysdale et al., 2003a).Conversely, studies of precipitation over diurnal cycles in China (Liu et al., 2008), Turkey (Bayari and Kurttas, 1995) and the UK (Spiro and Pentecost, 1991) indicate that macrophytes, algae and cyanobacteria play a dominant role in regulating precipitation rate.Laboratory studies indicate that precipitation to carbonate surfaces on the flow bed does not occur in the absence of biofilm (Shiraishi et al., 2008a, b;Pedley et al., 2009), and that photosynthesis may be critical to overcoming the energetic barrier to precipitation (Shiraishi et al., 2008a).This concept is given strong support by repeated observation of calcification of Charophyte bodies within watermasses otherwise incapable of precipitating calcite (Anadon et al., 2002;Eremin et al., 2007;Pentecost et al., 2006).However, these effects still await proper quantification.

Organic hydrogel templating
The state of knowledge of "templating" -i.e.precipitation of calcite onto organic hydrogels -has recently been thoroughly reviewed (Decho, 2010).For an in-depth overview of the literature on this subject, readers are commended to this review.For convenience, we provide a summary here.Templating is a well known, and widely used, means of accelerating, controlling and initiating mineral precipitation (Decho, 2010).
Although it has been demonstrated that the chelation of Ca 2+ ions to the natural organic gels formed by Extracellular Polymeric Saccarides (EPS) in biofilms (at least when bidentate) is generally an inhibiting factor in carbonate mineral precipitation in seawater (Kawaguchi and Decho, 2002), in vitro experiments have shown this may not be the case in a freshwater setting with low ionic strength (Pedley et al., 2009; Introduction

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Full   et al., 2008) and binding is more likely to be unidentate (Saunders et al., 2013).As EPS gels generally account for between 50 and 90 % of the total organic matter of a biofilm (Wingender et al., 1999), this means that metal-organic intermediaries may play a critical role in determining both rate and character of carbonate precipitate in terrestrial settings (Rogerson et al., 2010;Saunders et al., 2013).
As hinted above, the key process in regulating whether EPS promotes or impedes mineralisation is the immobilisation of calcium ions by anionic functional groups on EPS molecules (Decho, 2010).Understanding this process is still in its infancy, and highly depending on nanno-scale variations in EPS composition, which regulates the behaviour of "microdomains" which can have highly variable electrostatic properties.
Promotion of precipitation is most likely to be enhanced where divalent cation binding is unidentate (i.e.only one electron volt of charge in the cat ion is offset by the ligands binding it to the organic molecule).Unidentate binding will considerably reduce the activation energy barrier which usually limits spontaneous precipitation (Dittrich and Sibler, 2010), making a significant contribution to promoting mineralisation."Biologically influenced" precipitation arising from biopolymer templating is expected to vary from "normal" physic-chemical precipitation in terms of location, rate, morphology and chemistry and it is critical to understanding past system behaviour to determine whether "bio-influence" has been significant, or not.However, these processes remains understood only at descriptive levels and this remains a major target for the research community (Decho, 2010).

Extracellular enzyme activity
As of the date of submission, we are not aware of any study specific investigating the behaviour of extracellular enzyme activity in tufa systems.However, their role in regulating Ca 2+ dynamics in karst soils (Li et al., 2005(Li et al., , 2007) ) and associated with cyanobacterial activity in soda lakes (Kupriyanova et al., 2003) and stromatolites (Kupriyanova et al., 2007) is well established and they are anticipated to be material to the mechanisms of "bio-influence" outlined in the preceding section (Decho, 2010).It may be that Introduction

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Full synthesis of these enzymes are partly responsible for the ability of biofilm to regulate their internal pH (Bissett et al., 2008b), and they are therefore material to understanding why the lower limit of thermodynamic precipitation of calcite is so difficult to define (see Sect. 1.1.1).

Biofilms as bioreactors
In addition to providing a concentrated zone of photosynthesis, an abundance of nucleation sites and effective buffering of changes in external pH, they also provide a substrate in which bacteria with anaerobic metabolisms may flourish (Shiraishi et al., 2008c;Visscher and Stolz, 2005).These biofilm ecologies combine to create an "alkalinity engine" the regulation, operation and impact of which are only partially understood (Dupraz et al., 2009).In solutions buffered by bicarbonate, anaerobic metabolisms will either promote or impede precipitation, depending on the specific stoichiometry of the electron donation-absorption system (Visscher and Stolz, 2005), as summarised in Table 1.Given the range of chemistries possible in alkaline spring systems, all of these components of the alkalinity engine are possibly occurring within the sites we are interested in.However, the most likely anaerobic metabolism found in these systems is sulphate reduction.As this metabolism promotes generation of alkalinity, it can be assumed that non-aerobic degradation of organic matter favours mineralisation of calcite.

Methods and source of case study data
If we are to discriminate systems on the basis of whether biologically induced/influenced calcite precipitation is significant, and whether this precipitation is related to simple (exclusively microbial) or more complex ecologies (involving macrophytes) we first need to assess the magnitude of the chemical force promoting spontaneous nucleation of calcite.Introduction

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Full The Gibbs free energy of an ion pair in solution (e.g.Ca

2+
(aq) and CO 2− 3 (aq) ) can be straightforwardly estimated from its state of saturation (Dandurand et al., 1982) via: where G is the Gibbs Free Energy of the solution (in kJ), R is the ideal gas constant (8.314472 × 10 −3 kJ K −1 mol −1 ), T is the temperature (in Kelvin) and Ω is the saturation index with respect to the ion pair of interest.Saturation of a carbonate solution is generally calculated using a chemical model, such as PHREEQC.However, these models are generally developed for freshwater systems, and the high ionic strength of some travertine-depositing systems (which can be higher than seawater, Tiziano, 2011) complicates their use across the entire range of terrestrial carbonates.The use of different models, or indeed different generations of the same model, may also cause differences in derived parameters such as G.Moreover, different published works present different quantities of data regarding minor ions, making an approach based on model-based convergence of the solution charge balance partially dependent on the nature of the source.To circumvent these potential issues, here we estimate Ω values analytically.A further advantage of using this approach is that it makes the internal calculations underlying our approach explicit to any reader.
The fundamental relationship underlying our analytical approach is where γ 1 and γ 2 are the activity coefficients for Ca Activity coefficients γ 1 and γ 2 are calculated according to (Robinson and Stokes, 2002) log 10 γ 1 = (−4αI 0.5 )/(1 + βA Ca I 0.5 ) (4) where I is the ionic strength of the solution, A Ca and A CO 3 are the bond lengths of these ions in calcite (1.14 × 10 −10 and 1.36 × 10 −10 Angstroms, respectively) and α and β are coefficients calculated from and where q 0 is an electron volt (1.6021×10 −19 J), N A is Avogadro's number (6.023×10 23 ), ε 0 is the permittivity of free space (8.8542 × 10 −12 C 2 N −1 m −2 ) and ε T reflects thermal expansion and can be calculated from An additional complication of using existing field data is that the typical pH range for terrestrial carbonate producing systems is 8 to 9, meaning that most dissolved inorganic carbon (DIC) is present as bicarbonate.This means in most cases [CO 3 ] 2− (aq) must be estimated from the Henderson-Hasselbach-like control on the second ionisation coefficient (k 2 ) within the DIC system exerted by the pH, which varies with temperature and ionic strength (Patterson et al., 1984)

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Full Ultimately, we find that using this analytical approach G can be calculated from fieldderived measurements of temperature, pH, [Ca 2+  (aq) ], [HCO −  3 (aq) ] (or [CO 2−  3 (aq) ] where pH > 10) and ionic strength (i.e. the sum of all charge arising from dissolved ions), which is a dataset that can be reasonably created for any new system.
Although G is a fundamentally important parameter controlling the potential rate of reaction between the ions in solution and solid precipitates, it primarily represents the likelihood of the system to overcome the activation energy of the crystallisation reaction.It is therefore convenient to reduce it to a dimensionless index reflecting the likelihood of spontaneous nucleation of calcite.Experimentally-derived values for the activation energy of spontaneous calcite nucleation are 48.1 ± 4.3 kJ mol −1 (Inskeep and Bloom, 1985).For our final classification we will use the dimensionless "Spontaneous Nucelation Index" ("SNI") in preference to the dimensional G.It should be noted that as activation energy will be considerably reduced at any available solid surface, and particularly those surfaces that provide suitable templates for formation of calcite, nucleation in dirty and complex field settings will be considerably below this value, so rapid precipitation will straightforwardly occur at SNI values well below 1.
A potentially critically important, first-order parameter controlling the biological components the presence of which will affect the water: precipitate system is temperature.At high temperature (typically over 40 • C), macrophytes are excluded leaving only microbial processes to interact with mineral precipitation.At temperatures above ∼ 60 • C, biofilms will be dominated by thermophyles, diversity falls and the potential for biomediated precipitation will thus be altered.
We therefore propose that -to first-order -sites can be classified in a binary sense according to their SNI and water temperature, and our analysis will therefore focus primarily on the impact of high/low entropy (SNI) versus high/low enthalpy (T).

Compilation of field data
We do not pretend that the compilation of field data presented here is in any way comprehensive, and simultaneously recognise the advantage to future classification

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Full schemes if it were more comprehensive.We therefore link this paper to an online resource to which further data can be added, calculations are performed using a single set of equations and calculated parameters added to our database (URL will be inserted here).The field data used in the subsequent analyses are summarised in Table 2, which shows the wide range of compositions of near neutral-alkaline spring waters occurring in nature.To simplify this disparate data, a number of sub-populations are classified in terms of region of occurrence and water temperature (Table 2).Variance within these sub-populations remains quite high, with calcium concentration in cold Turkish springs varying by nearly 300 % of the mean (Table 2).Ionic strength, which is only partially dependant on the position of a location relative to source, is the most constant; ionic strength in the water at Plitvice varies only by about 10 % of the mean despite this being a relatively large dataset (n = 65).Even with the simplification provided by regional grouping, the data remain markedly unstructured in terms of the relationships between key parameters such as calcium and carbonate concentrations (Fig. 1a) or temperature and ionic strength (Fig. 1b), all of which vary independently.
Comparison of water temperature and the SNI (Fig. 2) does not result in more structure within the data, but it does result in better clustering of the data than individual ion concentrations, as sites can differ between very high carbonate ion but low calcium ion concentration (e.g.Mono Lake) and vice versa (many Italian springs).The thermodynamic approach therefore provides a suitable simplification of the data for further analysis.

Towards linking process to product
The first-order analysis of sites based on entropy versus enthalpy proposed above is shown in Fig. 3.There is a natural clustering of data in the region of the origin, reflecting the relatively high abundance of data from "meteogene" sources within non-tectonic karstic regions.Nevertheless, our collection of field data does encompass the majority of entropy: enthalpy space reasonable to expect from natural systems.We first report

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Full a case study from each quartile of the diagram, then highlight some key features of the transition zones between (e.g. between "hot" and "cold" systems) and on the edges (e.g. at the lower limit of significant carbonate production) of these quartiles.

"Hot, non-biomediated" systems with high enthalpy and entropy
Example: La Zitelle, Italy; Terme San Giovanni, Italy Informally -"Super-Travertine" In these systems, Gibbs free energy is high enough for only moderate assistance to be required for precipitation and biota are restricted to thermophyle microbes by high water temperature.Precipitation will be rapid and without material control from biological activity.
In high temperature thermal sites, calcium carbonate saturation levels are too extreme for any macrophytes or thermophile microbial biofilm development (Fig. 4).Such extreme hydrothermal waters degas rapidly immediately upon resurgence and this triggers a very rapid precipitation of calcite which nucleates onto the substrate, which provides surfaces with reduced activation energy further permitting the reaction (Fig. 4a,  d, e).Such spontaneous crystalline precipitates typically consist of palisade stands of sparite crystals with their C axes oriented normal to substrate and with individuals from 1-60 mm long (Fig. 4c).Clusters of calcite rosettes are also common.Crystal length appears to be controlled by water depth suggesting that standing pools best encourage this development.However, macroscopic rhombic calcite crystals also nucleate rapidly on the bottom of fast-flowing gutters to form complex fanning twins with overall botryoidal morphologies (Fig. 4b).When followed away from resurgence points these fabrics progressively give way to microbial dominated travertine and ultimately to ambient temperature paludal tufa laminites with dense macrophyte associations (Fig. 4d).

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"Hot, biomediated" systems with high enthalpy and low entropy
For example: Acqua Bora, Italy Informally -"Travertine" This is the largest category of precipitates associated with hydrothermal processes.In these systems, Gibbs free energy is too low to allow rapid spontaneous nucleation, so that precipitation is dependent on some mechanism allowing the activation energy to be reduced.However, biota are restricted by high water temperature meaning that despite low SNI, precipitation as a result of physical/chemical processes may still dominate.
Deposits may be confined to the immediate vicinity of the resurgence point and these may take the form of whaleback ridges, pinnacles or mounds.In addition, low angle travertine sheets often extend well beyond these resurgence points (see details of Italian examples in Capezzuole and Gandin, 2005).Laminae dominate the deposits and commonly are arranged into discrete bundles each separated by a truncation surface.These truncations may be karstified and buried by palaeosols indicating prolonged breaks in the depositional history.Individual laminae within bundles commonly show bubble, bushy or microlaminar fabrics (Fig. 5a, b).Shrubby laminae (not all of microbial origin) are ubiquitous (Fig. 5c).All macrophytes and many algae are excluded.Typically the deposits are extremely well cemented and generally are all well laminated at centimetre to millimetre scale.The deposits may form terracettes with narrow rimstone pools and microgours on low angle margins around resurgences but on steeper slopes (Fig. 5d) the faster flowing waters may create ripple developments.

"Cold, non-biomediated" systems with low enthalpy and high entropy
For examples: Mono Lake, USA; Brook Bottom, Derbyshire

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Full Informally -"Super tufa" In these systems, Gibbs free energy is sufficiently high for only moderate assistance being required for precipitation.Biota may be abundant and diverse, but extreme composition of the water may also be limiting via toxicity.Regardless, abiotic mineral precipitation is dominant and sufficiently rapid for biological influence to be minor and therefore masked in resulting deposits.
Extremely high alkalinity in these environments leads to continuous pelagic lime mud precipitation which often results in a milky to pale turquoise colour to the water even during cooler periods (Fig. 6).Aquatic organisms are generally excluded from such waters due to the extremely high precipitation rates which choke gill-breathing organisms and deeply encrust vegetation in precipitates after only a few hours.The host waters, however, may not be toxic and extensive marginal vegetation may develop throughout the system (Pedley, 2000).Biofilms are thin and discontinuous and mainly located on the narrow spill-over points of rimstone pools in the paludal areas and in broader botryoidal areas of barrage growth across the outflowing watercourses (Fig. 6a).Precipitates are rapid with extensive development of rimstone pools each forming broad terracettes separated by narrow sinuous crested micro-dams.Pool depths in the paludal areas are generally a few centimetres deep and are infilled with lime muds (Fig. 6b, c).These muds are generated by precipitation within the pool areas and may be massive or finely laminated.Lithification generally is absent in the pool deposits and only weak within the rimstone dam areas.At faster flowing sites small transverse barrages of weakly lithified lime mud laminites develop into broad, vertical to overhanging botryoidal walls which impound small, deep pools floored with unlithified lime muds.These muds are typified by bizarre, micro-crystalline calcite growth-forms typified by imperfectly formed crystals, some with baroque faces, by hollow microspheres and by truncated skeletal crystal growth-forms.Tufa towers may grow under similar conditions within alkaline lakes (e.g.Mono Lake, USA).These vertical structures also exhibit rapid growth fabrics with occasional microbial laminations but are essentially inorganic carbonate precipitates.Introduction

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"Cold, biomediated" systems with low enthalpy and entropy
For example: Plitvice, Croatia Informally -"Tufa" These are the classic, karstic tufa systems and are the most widespread form of terrestrial carbonate.Gibbs free energy is too low to allow spontaneous nucleation, so that precipitation is dependent on some mechanism allowing the activation energy to be reduced.Biota are abundant and diverse, permitting the full range of biomediation mechanisms to be active.
These deposits (Fig. 7) are extensively developed in association with phytoherm framestones in fluviatile perched springline, paludal and lacustrine settings (Pedley, 1990).Biofilms are developed on all available surfaces and all actively precipitate thick calcium carbonate laminae often composed of alternations of spar and micrite cements and commonly developed into stromatolitic growths (Fig. 7a).Micritic precipitates with a characteristic laminar or peloidal texture may also dominate the system (Fig. 7b, c).A diverse range of invertebrates, vertebrates and vegetation are always associated with these sites.Precipitates rapidly lithify and progressively entomb earlier biomedated areas.Typically, they agrade and prograde into lithofacies scale deposits (Fig. 7c) which bury macrophytes and are capable of infilling valleys.These impound bodies of water from pond to lake scale (Fig. 7d) within which further pelagic precipitation occurs and regularly laminated lake floor lime muds accumulate which may be associated with sapropels.

Interfacial systems with temperature in the region 30-40 • C
These deposits show overlapping similarities with travertine and tufa systems, and are likely to show interfingering of facies stratigraphically at individual sites.The most important impact of this in terms of product is that aquatic macrophytes are likely to be

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Full excluded by the high temperature, but may periodically establish when/where the temperature threshold (ca.30 • C) is reached.Microbiota may be abundant and diverse depending on local ecological factors, but the composition of biofilms will differ from systems developing at lower temperatures.Diatoms grow best at temperatures < 35 and photosynthetic elements of biofilms above this temperature will be dominated by cyanobacteria (Blanchard et al., 1996) meaning that the metabolic functioning of biofilms will alter through the 30-40 • C temperature range.These deposits manifest in an abundance of thermophile and ambient temperature microbial colonisers associated with scattered, small cushion-shaped phytoherms (often single species of grass) throughout (Fig. 4a, b).There is also a marked tendency for laminite development, partially arising from the flow conditions of these sites which are typically found on the outermost gently inclined parts of travertine sheets and distal margins of thermal fissures (Fig. 4a).Here, the virtually ambient temperature of the outflowing former-hydrothermal waters is sufficiently low to permit scattered macrophyte colonisation.Figure 4b shows a profile view of the colonisers in Cava Oliviera quarry.
Note the non-preservation of roots and the thick accumulation of laminites around the vegetation.In some instances over 100 mm of laminate accumulation has occurred during the lifetime of individual plants, which attests to high entropy in these cases.

Interfacial systems with moderate SNI
In close analogy with temperature, systems which are marginal in terms of SNI will present mixed petrologies and geomorphologies from the high and low entropy type systems.Again in close analogy with high enthalpy systems, high entropy systems will inevitably evolve into low entropy systems in time and space.However, the SNI range this transition will occur at is not well determined by our data, due to lack of specifically collected field petrographic evidence, beyond a general indication it will lie at SNI ∼ 0.2.This transition is an interesting target for future field research.
In terms of product, biomediated and physico-chemical type-products will interfinger both in time and space in these interfacial systems, in the same manner that high Introduction

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Full and low enthalpy products interfinger in time and space in thermal interfacial systems.Again, there is a lack of field data concerning the diagnosis of these entropic transition zones.

Defining the thermodynamic lower boundary of terrestrial carbonate precipitation
In these marginal systems where there is insufficient Gibbs Free Energy for calcite to nucleate spontaneously, precipitation will largely if not exclusively be biomediated.Consequently, at high temperatures where waters are too hot for macrophyte colonisation (although they may be extensively colonised by thermophillic microbial mats) and thus biomediation is limited, significant precipitates and petrographic data are sparse.
Any carbonate precipitates will be thin and slow to develop.Typically, thin calcite rafts floating on the surface of resurgence pools are the only tangible precipitates (Fig. 8a).
Progressively as the precipitate slowly develops these rafts become too heavy to be held on the meniscus and they sink to produce chaotic pseudo-floe calcite sheet debris.At low temperature, an extensive development of biofilm will coat available substrates (Fig. 9a, b) but precipitates will tend to be restricted to thin discontinuous micritic laminae.Generally, in cooler humid climatic situations these developments are localised to resurgence points, the resurgences sometimes being identified by a very restricted patch of thin tufa in its immediate vicinity (Fig. 9c).Commonly the carbonate precipitates are not visible in the field, but can be viewed by binocular microscope as whitish films or particulate micrite on and within the biofilm surface.A diverse range of biota is associated with these sites.A common type of development in the Yorkshire dales and in Belgium is associated with "Cron"-type sites (Pentecost, 2005) where discontinuous biofilms develop in the capilliary zone on the surface of grass or moss cushions.The micrite precipitates may lithify into thin delicate sheets or may constantly wash or fall from these vertical surfaces and develop into peripheral detrital lime mud deposits.In flowing waters there is a common tendency for marginal systems to be associated with thin calcite films precipitated in association with epiphyllous algae on the under surface Introduction

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Full of river weed such as Potomogetum.Superficial oncoids in which a few thin skins of carbonate are wrapped around suitable sized clastic nuclei are another common association of sub-tufa environments (Fig. 9a, b).However, we do not find that it is possible to draw a horizontal line on Fig. 3 dividing sites precipitating significant and insignificant carbonate.This echos a previous assessment of surface water in the Yorkshire Dales (UK), where no spatial relationship between water chemistry and precipitate occurrence could be identified (Pentecost, 1992) and other efforts in which precipitation was simultaneously present and absent in hydrochemically similar sites (Hagele et al., 2006;Ledger et al., 2008).Thus, we do not find the failure of thermodynamics to determine this boundary a surprising result; as SNI decreases and biomediation becomes more important to carbonate creation, small-scale variations in SNI will inevitably become immaterial.Clearly another direction is needed to determine this boundary.

Towards a new paradigm for defining the lower limit on carbonate precipitation at low SNI
As biofilms play a demonstrable role in promoting calcite precipitation (Bissett et al., 2008a;Shiraishi et al., 2008a, b;Pedley et al., 2009;Rogerson et al., 2008) and typically demonstrate more a tendency to alter the macroenvironment rather than be altered by it (Rogerson et al., 2010;Bissett et al., 2008b) it is clear that a nonthermodynamic paradigm for the process determining the lower limit of terrestrial carbonate precipitation will arise from better understanding of microbial processes.This will require a fundamental re-assessment of how fieldwork in these systems is performed, as even rather basic microbiological information is very seldom collected for terrestrial carbonate systems.Using a case study from Lincolnshire (UK) (Fig. 10), we propose a potential way forward.

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Full In this case study, we emphasise the failure of thermodynamic assessments to determine the lower boundary of significant terrestrial carbonate production, and simultaneously attempt to link that boundary to a relatively simple assessment of on-site microbial activity.Although probably not significant players in biomediation of precipitate, local maxima in the suspended bacterial count will reflect locally enhanced productivity.This close association of higher biomass and diversity at tufa sites relative to non-tufa sites is not our suggestion, but a known feature of these systems (Marks et al., 2006).
To incorporate a simple microbiological measurement to the classification framework we propose here, we use suspended bacterial count as a proxy for the ecological energy flux (Marks et al., 2006) in the same way that water chemistry measurements traditionally act as a proxy for the gradient of thermodynamic forcing (see Sect. 2).

Background information on sites
Annual rainfall in Lincolnshire is 577 mm, and average summer and winter air temperatures are 18 and 5 • C, respectively.The first location, Stainton le Vale (UK Grid reference TF17622 BNG93804; Fig. 10), lies on the Cretaceous chalk Wold hills and aquifer water emerges from two springs positioned a few meters apart at the base of a quartzarenite bed that occurs within the Chalk.Limited carbonate precipitation occurs along the stream banks and onto the biofilm-encrusted carbonate surfaces of a small (1.5 m high) tufa cascade (Fig. 11).Three sites have been studied from this valley; S1 is upstream near the springs where there is no geomorphologically significant carbonate forming, S2 is adjacent to the waterfall where crystalline patches of older carbonate outcrop at the water surface/stream bank interface but is barely producing a significant "tufa" product, S3 is downstream where the stream emerges from woods and flows through cultivated land, and no deposition is evident.The second location is at Waddingham, which lies on the eastern side of the Lincolnshire Edge Jurassic

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Full limestone hills (UK Grid Reference SK96457 BNG95311; Fig. 10).Again three sites were studied, one of which shows discontinuous oncoid development (W2) and two which are respectively immediately upstream and immediately downstream of the oncoid site (W1 and W3) and produce no significant precipitation.All six sites are included in preceding figures as "Lincolnshire sites".
The precipitate at site S2 is a friable, porous deposit adhering to the outer surface of siliciclastic stream bank muds.The precipitate is closely associated with organic matter, moss, fungi, heterotrophic bacteria and cyanobacteria from stream waters and surrounding vegetation.Microscopic grains cemented into the carbonate fabric enable small scale aggradational seasonal laminae to be seen (Fig. 11).The sheet-like morphology of both precipitate and biofilm laminae consist of alternative clotted micrite and spar calcite.The degree of lithification, scarcity of detrital grains and association with organic components together identify this deposit as a tufa according to the terminology first proposed by Pedley (1994).Counter-intuitively, calcium (up to 490 mg L −1 ; Table 3) and bicarbonate (up to 880 mg L −1 ; Table 3) concentrations are all higher at Waddingham than at Stainton (up to 260 mg L −1 and up to 460 mg L −1 ; Table 3), presumably reflecting the source water being derived from friable limestone rather than the less solution-prone well lithified Chalk.Both pH (7.5-8.3) and magnesium concentrations (Table 3) were similar in both streams.It is therefore completely unsustainable to assume the tufa/sub-tufa boundary is controlled by the thermodynamic forcing of source waters; the simple water chemistry approach we otherwise advocate in this paper would indicate that Waddingham was more likely to be above the tufa to sub-tufa boundary than is Stainton.There must, therefore, be another mechanism dominating these systems which is not reflected within the water analyses.

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Full On the basis of our laboratory-derived understanding of these systems, the difference then must arise from microbiological effects.Suspended bacterial abundance (Table 3; established via a simple cell-counting procedure (Hobbie et al., 1977) where cells in 50 mL of formaldehyde-fixed water were stained with Acridine orange, drawn through a filter membrane with 0.22 µm pore size and counted under an epifluorescent microscope) strongly differs between the S2 site where tufa is forming (22 617 cells L −1 ) and both other sites at Stainton (3490 cells L −1 and 3134 cells L −1 for S1 and S3, respectively) and all locations at Waddingham (1325 cells L −1 , 1228 cells L −1 and 1131 cells L −1 respectively for stations W1-W3).
Although we do not know if these suspended cells themselves are inducing precipitation, it is likely that they at least reflect a more diverse and abundant benthic ecology at the S2 site than at any of the other sites studied in this case study.The difference in microbiological activity at sites precipitating significant calcite and those not doing so has previously been reported (Hagele et al., 2006;Marks et al., 2006;Ledger et al., 2008), but requires significant further field-based research before it is understood at the fundamental biogeochemical level.A fundamental re-assessment of well-known systems is therefore required, and testing the concept that suspended cells can be used as a proxy for enhanced biologically induced (Lowenstam, 1981) precipitation may be a fruitful initial avenue to explore.

Conclusions
Binary analysis of the entropy (Spontaneous Nucleation Index) and enthalpy (water temperature) of terrestrial carbonate-producing systems appears to be a fruitful avenue to pursue with the goal of a single, unified process-oriented classification of these systems in mind.Both parameters are material to regulating whether biological influences are key in the precipitation process.As understanding past precipitation environments are critically dependent on establishing whether precipitation was via metalorganic intermediaries or direct from solution, determining which quartile of entropy : enthalpy

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Full space a precipitate reflects will underpin all subsequent investigation of that material and its environmental significance.
We do not currently have the information available to determine the SNI level at which non-biological precipitation begins to dominate, other than that this level appears to be in the region of 0.2.This is likely to be a fruitful avenue of research to pursue, as once determined different precipitation mechanics can securely be invoke to understanding the origin of a precipitate determined by whether the product appears to indicate conditions above or below this critical value.
We also do not have the information available to understand the lower limit of SNI at which significant precipitation takes place.Indeed, we find it is unlikely any such division can be determined, but that precipitation dynamics at this level is regulated by biology rather than purely physical processes.We emphasise this problem with a new case study.We also propose a simple field-based means of further investigation of the role of biology in regulating this system, and show that at sites of precipitation suspended cell count is up to an order of magnitude higher than adjacent reaches of the same river.This is also likely to be a fruitful avenue for future research.Full  et al., 1980).

Supplementary material related to this article is available
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) respectively, squared brackets indicate concentrations (in moles) and k sp is the saturation coefficient of calcite, which is dependent on temperature and calculated from (Plummer and Busenberg, 1982) k sp = 10 { −171.9065−(0.077993T )+( 2839.31T )+(71.595(log 10 T ))} (3Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 4.1 Lincolnshire case study; demonstrating the coincidence of high microbial activity and precipitation of significant carbonate in "borderline" systems Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Table 2 .
Summary of field data.