Palaeoclimatic records from stable isotopes in riverine tufas: Synthesis and review

Traditionally, fresh water carbonate research has focused on the sedimentology and palaeontology of ancient lacustrine deposits. Lithofacies in such low-energy deposits are typically fine-grained, developed uniformly in a generally concentric distribution (‘bulls-eye’ pattern) and are predictable even when preserved imperfectly. In contrast, because of their local lithofacies and palaeontological complexities, fluvial carbonates were either delegated to a status of ‘minor geomorphological features’ or barely considered prior to the 1970s. This viewpoint was based on the depositional record of fluvial and spring-fed fresh water carbonates, which were considered to be restricted generally to localized karstic areas. Such deposits are often preserved as scattered patches of ambient temperature tufa. Occasionally, however, in active tectonic areas, localized travertine deposits are also developed from deeply circulating hydrothermal waters. With a few exceptions (for example, basins with high subsidence rates or in arid climate zones), these fresh water carbonates are prone to erosion from continuing river incision and thus may not be preserved in the geological record. A partial record of fluvial and spring-deposited carbonates is often preserved in Quaternary deposits, but the record in older deposits is typically fragmentary and often diagenetically modified. Yet once their unique facies architecture (and specialized nomenclature) is understood, these carbonates provide an important record of past sedimentological cycles of great value in palaeoenvironmental landscape modelling. The emphasis of modern research is to acquire information that explains how active systems function. In this respect, tufas reveal much of how carbonate precipitation is a shared product of physico-chemical and microbiological biomediation processes. Likewise, travertines not only show an intimate interrelation with active tectonism but also hold great potential as monitors of past volcanic carbon dioxide emissions. In addition, both tufas and travertines contain palynological records that can be used as proxy indicators of climate change. Perhaps no other field of sedimentology has witnessed more developments and applications over such a brief period of study.

https://onlinelibrary.wiley.com/doi/epdf/10.1111/sed.12075

Decoding tufa and travertine (fresh water carbonates) in the sedimentary record: The state of the art

Stable isotope geochemical study of Pamukkale travertines: New evidences of low-temperature non-equilibrium calcite-water fractionation

Carbonate build-ups in lacustrine, hydrothermal and fluvial settings: comparing depositional geometry, fabric types and geochemical signature

Abstract A field experiment was performed at four sites in Slovakia and Poland in order to identify factors that influence the growth rate and textures of modern tufas. Two pairs of tablets were placed at each study point, every pair consisting of a limestone tablet and a copper tablet. One pair at each site was changed every three or four months, while the second pair was left for approximately 14 months. Each tablet was weighed before placing and after removal to find the amount of the tufa growth. Tufa growth rate was found to depend on SIcalc. of parent water, though deposition of tufa on limestone tablets was substantially faster than on copper tablets. This result indicates that micro-organisms are essential for more efficient growth of tufa. Tufa growth rate was higher in fast-flowing water than in nearby sluggish flow settings. Fast-growing tufa has crystalline texture or consists of highly encrusted algal filaments. The latter texture is due to faster growth of micro-organisms, forced by rapid crystallization of calcite on their cells. The slow-growing tufa exhibits mainly micritic textures. Clotted micrite with numerous diatoms forms mostly in winter, while encrusted algal filaments are typical for spring and summer growth.

Factors controlling growth of modern tufa: results of a field experiment

Three types of recent carbonate precipitates from the River Krka, Croatia, were analysed: (1) bulk tufa from four main cascades in a 34 km long section of the river flow through the Krka National Park; (2) a laminar stromatolite-like incrustation formed in the tunnel of a hydroelectric power plant close to the lowest cascade; and (3) recent precipitates collected on artificial substrates during winter, spring and summer periods. Stable isotope compositions of carbon (d 13 C) and oxygen (d 18 O) in the carbonate and organic carbon (d 13 Corg) were determined and compared with d 18 O of water and d 13 C of dissolved inorganic carbon (DIC). The source of DIC, which provides C for tufa precipitation, was determined from the slope of the line ([DIC]/[DIC0])1) vs. (d 13 C-DIC · ([DIC]/[DIC0])) (Sayles & Curry, 1988). The d 13 C value of added DIC was )13AE6&, corresponding to the dissolution of CO2 with d 13 C between )19AE5 and )23AE0& Vienna Pee Dee Belemnite (VPDB). The observed difference between the measured and calculated equilibrium temperature of precipitation of bulk tufa barriers indicates that the higher the water temperature, the larger the error in the estimated temperature of precipitation. This implies that the climatic signals may be valid only in tufas precipitated at lower and relatively stable temperatures. The laminar crust comprising a continuous record of the last 40 years of precipitation shows a consistent trend of increasing d 13 C and decreasing d 18 O. The lack of covariation between d 13 C and d 18 O indicates that precipitation of calcite was not kinetically controlled for either of the elements. d 13 C and d 18 O of precipitates collected on different artificial substrates show that surface characteristics both of substrates and colonizing biota play an important role in C and O isotope fractionation during carbonate precipitation.

Wolfgang Papesch

Papers

C and O stable isotope variability in recent freshwater carbonates (River Krka, Croatia)