Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 Introduction to tufas and speleothems H. MARTYN PEDLEY* & MIKE ROGERSON Department of Geography, University of Hull, Cottingham Road, Hull, HU6 7RX, UK *Corresponding author (e-mail: [email protected]) Ambient temperature freshwater carbonates On a regional scale the occurrences of tufas and precipitate as surface deposits within karstic speleothems are both controlled by water table fluc- stream, lake and swamp environments (tufas) and tuations. Typically, tufa deposition is associated in subterranean situations (speleothems), where with predominantly high water tables and although they line vadose caves and fracture systems. tufas enjoy global distributions from the tropics Although physico-chemical mineral precipitation to polar regions, they are most effective as bio- contributes significantly to both kinds of deposit, constructors where spring fed streams are not there is a clear spatial association between the subjected to spate conditions. Similarly, tufa devel- development of tufa deposits and the occurrence opments are severely impaired by fluctuating water of microbial biofilms. This fact, and the recent dis- tables associated with increasingly arid climatic covery that the occurrence of certain heterotrophic cycles. Limitation on surface carbonate precipi- bacteria promote precipitation onto the surface of tation is consequently derived from the necessity stalactites (Cacchio et al. 2004), strongly implicates for biofilm development combined with the equal a degree of microbial influence in the calcite pre- necessity of adequate supply of dissolved calcium cipitation process, regardless of the environmental and carbon, which must be present at least in part context. To add to the inherent complexity of as carbonate. The latter requirement of sufficient 2þ 22 these systems, there is considerable interplay Ca (aq) and CO3 (aq) ionic activity demands that between biological and physical processes to con- these ions are not lost from solution before resur- sider. Water velocity and turbulence will strongly gence, making it likely that tufas will develop best affect biofilm colonization and may damage the where caves are flooded, thereby minimizing the community, thereby affecting carbonate precipi- distribution of the subterranean vadose environment tation rates, in addition to regulating important where speleothems develop most abundantly. kinetic limitations on precipitation via modifi- Curiously, these elevated tables are frequently cations of the calcium ion delivery rate. Exchange encouraged by the tufa growth itself as a conse- of CO2 gas at the air–water interface is an important quence of the valley bottom ponding and back flood- conditioner for precipitation in vadose systems but ing caused by barrage development. Conversely, as will also occur within surface systems as a conse- lower water tables become established and the quence of photosynthesis. It is only by considering subterranean vadose environment becomes more karst hydrological systems holistically that these important, speleothems will become established. processes can be untangled. Conceptually, the occurrence of abundant tufa or Tufas and speleothems share the same soil- speleothem deposition simply reflects the position derived meteoric water supply, represent zones of of a hydrochemical ‘knick-point’, which occurs deposition of calcium ions chemically eroded from when sufficient CO2 has been lost from solution for the same geological sources and produce laminated carbonate ions to become abundant, for example deposits which are superficially similar. In passing when the thermodynamic gradient promoting pre- from cave environments via resurgences (Fig. 1) cipitation (Gibbs Free Energy) exceeds the barrier into surface waterways, individual packages of presented by the activation energy. This knick-point water pass down an interconnected hydrological may occur either above or below ground depending system at any point in which the conditions necess- on the height of the water table. ary for calcite precipitation may be achieved. As part of the same hydrochemical system, tufas Within the deposits that this precipitation creates, it and speleothems offer an inseparable duo when is apparent that there is a progressive gradation exploring the climatic archive, and will reflect the from massive, laminated speleothems fabrics into same processes within the catchment. Much stromatolitic, biofilm dominated tufas fabrics. In palaeo-environmental information in tufas and fact, speleothems and tufa represent two end speleothems can be extracted from geochemical members within a continuum of freshwater carbon- time series created from these deposits. However, ate reflecting different balances of physico- one of the greatest obstacles to collective use of chemically and biologically controlled precipitation. these materials in ‘climate’ reconstruction is the From:PEDLEY,H.M.&ROGERSON, M. (eds) Tufas and Speleothems: Unravelling the Microbial and Physical Controls. Geological Society, London, Special Publications, 336, 1–5. DOI: 10.1144/SP336.1 0305-8719/10/$15.00 # The Geological Society of London 2010. 2 Downloaded from http://sp.lyellcollection.org/ M. PEDLEY & M. ROGERSON byguestonSeptember28,2021 Fig. 1. A schematic diagram of terrestrial carbonate precipitation occurring within an interlinked system. This continuum exhibits dominantly physico-chemical processes at one extreme (speleothems, on the left of diagram, and travertines, on the right) and dominantly biological processes at the other (oncoid and lake carbonates). Tufa systems typically fall within the middle of this continuum, exhibiting both physical and microbial control. When calcite saturation no longer allows tufa framework development, oncoids typically are developed (top right, showing cross section of oncoid with stromatolitic structure as inset). All these deposits properly should be considered as part of the same depositional system within karst environments, with individual occurrences related to the position of the hydrochemical ‘knick-point’, that is, when precipitation activation energy is breached by local biological influences. Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 INTRODUCTION TO TUFAS AND SPELEOTHEMS 3 oversimplified understanding of the actual fresh- France, Germany, Italy, Poland, Slovenia, Spain, water carbonate precipitation process and how it is Switzerland and the UK. It also attracted specialists controlled by the ever-changing environment. The from Israel, Turkey and Canada. The remit of the traditional view that tufas and speleothems are meeting was to bring tufa and speleothems research- formed entirely by physico-chemical precipitation ers together, and especially sedimentologists and processes provides the foundation for this research, geochemists, in order to demonstrate that these but is unlikely to be sustainable indefinitely in the increasingly separate research areas were essen- face of increasing evidence for biological influence tially dealing with a single carbonate precipitate on the precipitation process. continuum. A one day fieldtrip to the Caerwys This newly perceived consideration that both fluvial barrage tufas of North Wales provided chemical and microbiological processes are further stimulus to informal discussion and gave involved in carbonate precipitation (in both marine an excellent opportunity for liaison between tufa systems as well as the freshwater environments con- and speleothems specialists. sidered in this volume) has resulted in the develop- The articles herein reflect the work of 63 spe- ment of a new interface discipline which bridges cialists (principally bacteriologists microbiologists, both the physical and biological sciences. Its remit hydro- and geochemists and sedimentologists) is to view and interpret the planet, in space and based mainly in academic institutions. The remit time, as an integrated system with life as an essential of this volume is to develop a better understanding driver within all terrestrial and oceanic environ- of the biological and chemical influences on carbon- mental systems. This interface discipline has ate precipitation associated principally with ambient variously been referred to as ‘Biogeology’, ‘Bio- temperature freshwater carbonates. Many processes geoscience’ and ‘Geomicrobiology’ though it is are common to both surface and subterranean situ- most frequently referred to as ‘Geobiology’ ations yet a research dichotomy has developed (Noffke 2005), which is our preference. One of the over the past 30 years. Tufa researchers have paramount requirements for future freshwater car- tended in the past to be carbonate sedimentologists bonate research in this context is to build a new or biostratigraphers interested in using facies foundation for understanding carbonate precipi- models in order to characterise the flow dynamics tation processes based on a holistic approach deriv- and proxy-datable stratigraphy. In contrast, spe- ing data from all relevant physical, chemical and leothems research has primarily been the domain biological processes acting within a system. of the geochemist. There is now a need to draw Microbial processes fundamentally underpin many the two research themes together. large scale carbonate depositional systems. Conse- The following thirteen chapters of this volume, quently, the present need within geosciences is for dealing principally with carbonate precipitate detailed assessments