Geo.Cro.2-3-61-KB.pdf 363 � Stjepko Golubić1, Crescenzo Violante2, Anđelka Plenković-Moraj3 and Tonći Grgasović4 AB STRA CT Travertines and calcareous tufa are porous deposits formed by interactions between ambient precipitation of calcium carbonate and resident organisms under different temperature regimes. The distinctions between travertine as ther- mal spring deposits and calcareous tufa (Kalktuff) as deposits in the springs and rivers at ambient temperatures are fl uid. Both represent end points in bio- and physico-chemical calcifi cation processes across a broad gradient of tem- perature, mineral composition and ion saturation levels. Ecological preferences of micro- and macroorganisms in travertine depositional systems result in the re-distribution of water fl ow, modifi cation of the landscape and its ecol- ogy. The resulting sedimentary structures include new environmental settings with different and diversifi ed biota. They also include different microenvironments of diagenesis with different timings of the processes involved. Condi- tions in modern ambient temperature travertines of the Plitvice system of lakes and waterfalls are compared with the similar, ancient system of Rocchetta a Volturno, in the central Apennines. Diagenetic alterations are described and illustrated starting with biologically identifi ed primary deposits. Keywor ds: carbonate, diagenesis, freshwater, karst, micrite, precipitation, sparite, tufa, travertine 1 Biological Science Center, Boston University, 5 Cummington Str., Boston, MA 02215, USA; (golubic@bu.edu) 2 Institute for Coastal Marine Environment, National Research Council, Calata Porta di Massa Porto di Napoli, 80133 Napoli, Italy; (crevio@gms01.geomare.na.cnr.it) 3 Department of Biology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, HR–10000 Zagreb, Croatia; (aplenk@zg.biol.pmf.hr) 4 Croatian Geological Survey, Sachsova 2, HR-10000 Zagreb, Croatia; (tgrgasovic@hgi-cgs.hr) Travertines and calcareous tufa deposits: an insight into diagenesis Geologia Croatica 61/2–3 363–378 4 Pls. Zagreb 2008 Geologia CroaticaGeologia Croatica � 1. INTRODUCTION The term travertine has been used for centuries for porous building stone, desirable for its structural quality especially for the construction of vaults and arches (PENTECOST, 2005). The term derives from the Latin lapis tiburtinus in reference to the town Tibur, today’s Tivoli, near Rome, Italy. The term was also used to designate inscriptions in stone of that origin. The porous stone of Tivoli is still being quarried today, most of it originating from thermal springs of the area. The use of the name in geology has often been associated strictly with thermal spring deposits, whereas its ambient water counterpart is usually referred to as calcareous tufa (German: Kalktuff, Croatian: sedra). This distinction is diffi cult to maintain, be- cause hydrothermal springs, range in temperature, and so do the processes of mineral precipitation. As thermal waters cool, carbonate precipitates change from predominantly aragonitic to calcitic and they become rapidly associated with a different microbiota of increasing diversity. Travertine as building sto- ne includes both hot spring and ambient temperature deposits. Thus, for ecological as well as historic reasons, we prefer to use the term travertine for a range of sedimentary processes along the gradient of different temperatures, as has been the practice in the studies of the ambient temperature system of the Plitvice Lakes (SRDOČ et al., 1985). Deposition of trav- ertines and calcareous tufa in the region occurred at different times during interglacial and postglacial periods and remained Geologia Croatica Geologia Croatica 61/2–3 364 preserved in different stages of development (GOLUBIĆ, 1969). A review of travertine and tufa deposits of the world, listing occurrences in 22 European countries, is given by FORD & PEDLEY (1996), and they are common in the karstic region of Croatia (e.g. PRIMC-HABDIJA et al., 2001; PAVLOVIĆ et al., 2002). Travertines and calcareous tufa are considered excellent indicators of past climate conditions (e.g. PEŃA et al., 2000), an interest that has been recently triggered by the concerns about climate change. Stable isotope and trace ele- ment analyses address these topics (LOJEN et al., 2004; KELE et al., 2006; ANZALONE et al., 2007). Diagenetic alterations of travertine deposits depend on microenvironmental infl u- ences and constitute a background relevant to the interpreta- tion of the isotopic record (SRDOČ et al., 1983; ANDREWS et al., 1997). This work studies the formation and early dia- genesis of mineral products in biologically infl uenced sedi- mentary processes by recent-to-fossil comparisons, with ex- amples from the Plitvice travertine depositional system and from fossil travertine deposits of the river Volturno, in the Central Apennines (GOLUBIC et al., 1993). 2. MATERIALS AND METHODS Live mosses, algae and cyanobacteria were collected in the cascades of the Plitvice Lakes system, and observed and iden- tifi ed using light microscopy. Their relation to carbonate de- posits was studied using Scanning Electron Microscopy (SEM, Jeolco, MBL, Woods Hole, MA) of critical point-dried spec- imens. Live materials were decalcifi ed in dilute hydrochloric acid and studied by light microscopy. Fossil deposits were col- lected in the Plitvice National Park and in fossil travertine de- posits in the valley of the river Volturno, Italy. Following mac- roscopic identifi cation at the site of deposition, samples were selected using an incident light dissecting microscope. Select- ed samples were analyzed by SEM with subsamples prepared as petrographic thin sections and studied by light microscopy (Zeiss Universal) using transmitted light, DIC illumination and cross-polarized illumination. Samples were studied and the results presented in order to illustrate the advancement of diagenetic alteration processes. 3. RESULTS AND DISCUSSION 3.1. The geomorphology of travertine deposits The main distinction between thermal and ambient water trav- ertine deposits is in the porosity. In thermal deposits, where precipitation is rapid, the porosity is produced by the trapped gas and by micrometer sized organisms embedded into the deposit. Additional microstructure is produced by crystal growth (PENTECOST, 1990). Ambient temperature systems attract, in addition to microorganisms, also mosses, insect larvae and plants, which shape the deposit leaving imprints and cavities of corresponding size. Both systems commonly produce a cal- careous deposit, characterized by an even distribution of wa- ter-fl ow and formation of pools behind carbonate barriers. Both systems result in differentiation between lotic (running water, rapids), and lentic (relatively still water), environmen- tal settings along streams and rivers. In thermal systems, car- bonate precipitation predominates, guided by differential de- gassing, evaporation and cooling, favouring precipitation whe re the water is in direct contact with the atmosphere. The micro- organisms are present but their contribution to the shapes of the deposits appears to be limited (PENTECOST, 1990 vs. CHAFETZ & FOLK, 1984 and CHAFETZ & GUIDRY, 1999). The diversity and size of organisms increases with the lower- ing of temperature to ambient levels, and their role in carbon- ate deposition increases toward the establishment of chemical equilibrium in surface waters (MERZ, 1992; FRANČIŠ- KOVIĆ-BILINSKI et al., 2004). In thermal systems, the pools range from millimetre to metre scale, arranged step-wise down the slope, supported by smooth semicircular carbonate walls, over which the water overfl ows in a thin, evenly distributed fi lm (GUO & RIDING, 1999, fi gs. 13, 17). In ambient tem- perature systems the formation of pools and barriers is less regular, but they are constructed by the same principle of dif- ferentiated carbonate deposition. The scale of participating organisms and of the resulting deposits is larger, and the tim- ing is extended and subject to changes in seasons and meteo- rological conditions. The valley (thal) of the river that deposits calcareous tufa is modifi ed by the selective preference of calcifying rheophilic algae and mosses for growth in rapids, which are also the sites of maximum exchange of gases with the atmosphere. The growth of organisms, combined with carbonate deposition, produces a porous rapidly hardened substrate, which tends to partition the water fl ow. This results in the lateral displacement and fan-like distribution of both the deposit and the fl ow of water, ultimately extending across the river, forming a dam with a pool of water behind. This development has a regula- tory effect as the water loses its erosional power, further pro- moting deposition and vertical growth of dams. The lakes grow larger by competition in barrier accretion in which the growth of lower barriers fl oods those upstream. Once formed, the pooled water bodies became effective traps of eroded ma- terial (GOLUBIĆ, 1967, 1969). Gradually, the river is trans- formed into a series of distinct lakes and waterfalls, and the initially inclined river bed is altered into a series of terraces. Similar systems developed during Pleistocene and early Holo- cene throughout southern Europe (e.g. PEŃA et al., 2000), but few remained active. Different stages in the progression of this development can be observed in the Plitvice system of alternating lakes and waterfalls (Plate 1, Fig. 1). Impressive waterfalls (Pl. 1, Fig. 2) are the exception rather than the rule in this development, which normally tends to subdivide the water passages and dis- tribute them in a net-like fashion over the entire barrier, a con- dition which supports the growth of higher plants and trees. Yet, the barriers may become unstable and collapse, changing the water passages abruptly. Massive waterfalls may be cre- ated by occasional erosional events or, more often, by humans, in order to guide the water to their water mills or to maintain waterfalls for a tourist attraction. The shallow fl at-bottomed ponds between smaller barri- ers, initially support rapid water exchange and create condi- tions in which carbonate accumulates in microbially coated grains and forms laminated nodules or oncoids (GOLUBIC Geologia CroaticaStjepko Golubić et al.: Travertines and calcareous tufa deposits: an insight into diagenesis 365 & FISCHER, 1975; PEDLEY, 1990; HÄGELE et al., 2006). Within the Plitvice system, oncoids are sporadically present along shallow fl at shorelines of Lake Kozjak, and also as large fi elds in slow fl owing widened areas of the river Slunjčica at Rastoke (Pl. 1, Fig. 3). They are dominated by cyanobacteria and microscopic algae. Such shallow ponds deepen over time through the vertical growth of barriers between them (Pl. 1, Fig. 4) and then accumulate fi ne carbonate sand, which sup- ports aquatic plants such as Myriophyllum spicatum L. with charophytes on the shallower pond margins. Some water passages elevate their bed while maintaining the fl ow on top of a carbonate ridge. These, so called “cat walk” deposits (German: Kalkrinnen) are often products of small, carbonate supersaturated springs that emerge on slopes (Pl. 1, Fig. 5). This represents another depositional feature that is similar in both ambient temperature and hot water deposits (see GUO & RIDING, 1999). Carbonate precipitates a certain distance from the spring as soon as the carbonate equilibrium shifts in favour of precipitation. There is a preference of moss growth and associated deposition of carbonate in the spray at the margins of such water fl ows. Consequently, the creek bed is elevated while the margins prevent lateral overfl ow. Over time, the “cat walk” fl attens in the upper ranges while it steep- ens distally. The lateral overfl ow and displacement occurs with a delay so that the deposits ultimately widen to produce semi- conical tufa structures called “crone” or “cranier” in French, and “Hangtuff” in German. The Plitvice system of lakes and waterfalls developed in a valley overlying the Triassic dolomites, which narrows down- stream into a canyon carved into Cretaceous limestone (GO- LUBIĆ & GRGASOVIĆ, 2007). The upper lakes are wider and the ponds occupy different levels. The fossil sites at Roc- chetta a Volturno occur on slopes of an even wider valley and developed, accordingly, in the form of an extended set of ter- races with steeper slopes and waterfall facies on their outer margins. Similar systems have been described in Antalia, Tur- key (GLOVER & ROBERTSON, 2003). 3.2. The constructive role of aquatic mosses Aquatic mosses such as Cratoneurum commutatum (HEDW.) ROTH, Eucladium verticillatum (WITH.) BRUCH & SCHIM- PER, Bryum pseudotriquetrum (HEDW.) P. GAERTN., B. MEY & SCHERB. and Didymodon tophaceus (BRID.) JUR. are known as travertine-forming organisms. They thrive in the water spray in waterfalls where they become encrusted by car- bonate. By their growth preferences, the mosses guide the precipitation and modify the fl ow of water (Pl. 1, Fig. 6). How- ever, the carbonate encrustation takes place on older leafl ets and on the moss stems rather than on young actively growing leafl ets (Pl. 1, Fig. 7), which suggests that there is no direct relationship between moss photosynthesis and carbonate de- position. Instead, mosses are stimulated to grow at their tips so as to escape being cemented in carbonate. In contrast, the carbonate precipitate is associated with a biofi lm of microor- ganisms, which overgrows the moss with age. Some carbon- ate deposited in the moss cushions is allochthonous, i.e. pre- cipitated upstream and then trapped by the moss. Carbonate precipitation starts on crystallization nuclei provided by the microbial biofi lm of cyanobacteria, diatoms and microscopic chlorophytes, such as the desmid Oocardium stratum (Pl. 1, Fig. 10). Similar microbial biofi lms coat the branches and leaf litter trapped in the waterways (Pl. 1, Fig. 7). Encrusted by carbonate, the moss cushions form porous sponge-like carbonate sediment that retains the shape of the moss and fossilizes instantly, often incorporating other plant material (Pl. 1, Fig. 8). As the calcifi cation continues, the mos- ses respond by growing out of the deposit. In the race that en- sues between moss growth and calcifi cation, the latter is shap- ed and directed by the growth of mosses which, in turn, follow the water currents, cascades and spray. The porous spongy consistency of moss thalli provides the architectural frame- work for the travertine deposit. The environmental require- ments of rheophilic aquatic mosses promote their settlement in rapids and cascades, where they become encrusted by car- bonate. Such biologically controlled deposition is contrary to the pattern of inorganic deposition of particulate matter. In ad- dition, the presence of mosses in travertine deposits organizes rock porosity at scales ranging from mm-size spaces between moss leafl ets and stems to cm- and metre-scale cavities and caves formed under cascades and waterfalls. The environmen- tal consequence of moss participation is the formation of a voluminous travertine structure from relatively small amounts of carbonate mineral, which would otherwise form compact crusts, similar to cave stone (GOLUBIC, 1973). Due to the rheophily of aquatic mosses and microorganisms, carbonate accumulates in rapids rather than in pools and depressions where it is deposited as fi ne particulate matter (calcilutite). Fossil travertine of both thermal and ambient temperature sys- tems has been used in architecture since ancient times as seen in the ruins of Pompeii and in the Doric Greek temples of Magna Grecia, in present southern Italy (Pl. 1, Fig. 9). The landscape modifi cation by the combined activity of aquatic mosses, eukaryotic microorganisms such as Oocar- dium stratum (Pl. 1, Fig. 10), and calcifying cyanobacteria (Pl. 1, Fig. 11) has important ecological consequences, as new niches and opportunities for growth arise that did not previ- ously exist in the river. The water behind travertine dams ac- cumulates as lakes with their own communities of plankton, benthos and littoral, which are alien to fast fl owing rivers. Vaults of travertine that support, (and are shaped by) water- falls, overhang and enclose cave spaces beneath them (Pl. 1, Fig. 2), with new habitats provided with trickling water and a humidity-saturated atmosphere, able to support a luxurious microfl ora of cyanobacteria and algae specialized to photo- synthesize under dim illumination, e.g. Scytonema julianum (KÜTZING) MENEGHINI, S. myochrous (DILLW.) AGAR- DH (Pl. 1, Fig. 11) and Petalonema alatum BERKELEY (Pl. 1, Fig. 12). The spongy porous deposit becomes a home to bacteria as well as to numerous minute invertebrates that thrive on the organics produced in the system and are carried by wa- ters trickling through the porous deposit. These developments derive from the growth preferences and adaptive capacity of aquatic mosses (Pl. 2, Fig. 1), which are in that sense the real architects of cold water travertine deposits (ARENAS et al., 2000). Mosses fossilize well and identify past environmental Geologia Croatica Geologia Croatica 61/2–3 366 PLATE 1 Calcareous tufa formations and their microbiota 1 View of the two lowermost lakes of the Plitvice system of Korana River, Kaluđerovac and Novakovića Brod, separated by a low barrier. 2 Water falls down the tall travertine barrier in the upper parts of the Plitvice system, “Mali prštavac” waterfall. Note the overhanging tufa enclosing caves under the water arches. 3 Shallow ponds with high water exchange rates in a part of the water-fl ow network of the River Slunjčica, a tributary of the Korana (Rastoke village), supporting a “pavement” of oncoids. Insert: two of the oncoids, the right one cut open to show the laminated internal texture. Scale bar is 1 cm long. 4 A series of small tufa barriers with cascades, separating shallow ponds, lower lakes. 5 “Cat walk”- type calcareous deposit in a small carbonate super-saturated spring located on a hill slope. 6 Luxurious cushions of the moss Cratoneurum commutatum, growing in the water spray and participating in the even distribution of water along the barrier. Scale bar is ca 20 cm long. 7 A leaf of Alnus glutinosa trapped in the cascade and overgrown by globular Oocardium colonies. Scale bars in fi gs 7, 8 and 9 are 1 cm long. 8 Fossil calcareous tufa with cavities and incorporated branches of trees fallen into the lakes. 9 Doric temple in Paestum, southern Italy, built of ambient water travertine that preserves aquatic mosses. 10 Cells of Oocardium stratum inserted in the calcifi ed tubes, forming a single layer of cells on the surface of each colony. 11 Two species of Scytonema (Cyanobacteria). The smaller one on the upper left is calcifi ed S. julianum next to the larger uncalcifi ed S. myochrous below. 12 The nitrogen fi xing cyanobacterium Petalonema alatum forms soft gelatinous colonies in the moist cave entrances underneath the waterfalls. Note the diff erently pigmented nitrogen-fi xing heterocyst in the centre. Scale bars in fi gures 10, 11 and 12 are 10 μm long. Geologia CroaticaStjepko Golubić et al.: Travertines and calcareous tufa deposits: an insight into diagenesis 367 Geologia Croatica Geologia Croatica 61/2–3 368 changes such as seasonal rhythms or the lateral displacement of depositional regimes (Pl. 2, Fig. 2), often preserving the tax- onomic identities of ancient travertine builders (Pl. 2, Fig. 3). 3.3. The role of cyanobacteria and micro-algae A closer look at the moss leafl et surfaces reveals a biofi lm of cyanobacteria and diatoms in close association with calcite crystals (Pl. 2, Fig. 4). In addition to aquatic mosses, the oth- er most-important organisms involved in the formation of am- bient water travertine are cyanobacteria and eukaryotic algae, which form biofi lms and promote calcifi cation (Pl. 2, Figs. 5– 7). Without their contribution, precipitation of calcium carbo- nate would still occur, but the precipitate may not be retained in rapids, but transported downstream instead. Experiments with exposed copper grids that are toxic to micro organisms and do not support biofi lm development also remain uncalcifi ed (SRDOČ et al., 1985). Calcifi cation on exposed slides shows a positive linear correlation with the rates of periphyton growth (PRIMC-HABDIJA et al., 2001). Species-specifi c differences in the degree of calcifi cation and in the shape of the resulting calcium carbonate crystals have been documented in cases of freshwater (OBENLÜNESCHLOSS & SCHNEIDER, 1991) as well as marine cyanobacteria (GOLUBIC & CAMPBELL, 1981). Similar cases of biological crystal specifi city have been documented in ambient water travertines (e.g. GOLUBIC et al., 1993). While some cyanobacterial species calcify, other species remain uncalcifi ed, although growing under the same condi- tions and in close proximity to each other (Pl. 1, Fig. 11). In order to investigate the role of microphytes in the proc- ess of tufa deposition in the Plitvice Lakes National Park area, qualitative and quantitative analyses of periphyton were car- ried out between 1985 and 1990 (PLENKOVIĆ et al., 2002). The connection between carbonate deposition and periphyton was tested by experimental exposure of microscope slides. Glass slides were submerged in streaming water and exposed for 30, 60, and 90 days. Seven locations with different inten- sities of natural tufa deposition were chosen along the Plitvice Lake system. Temperature, pH and current were monitored simultaneously. Growth of periphyton was analyzed, and 191 species were determined with the highest diversity being shown by diatoms (139 species). The dominance of diatom species was observed at all locations (especially in winter), irrespec- tive of changes in physicochemical properties of the water and the intensity of CaCO3 precipitation. A particularly intensive precipitation of microcrystalline calcite was found in the pres- ence of cyanobacteria: Chamaesiphon incrustans GRUNOW, Homoeothrix varians GEITLER, Hydrococcus rivularis ME- NE GHINI, and diatoms: Achnanthes affi nis GRUNOW, A. microcephala GRUNOW, A. pyrenaica HUSTEDT, Synedra vaucheriae KÜTZING, Cymbella affi nis KÜTZING, C. mi- crocephala GRUNOW, Gomphonema olivaceum KÜTZING, G. olivaceum var. calcareum CLEVE, and G. parvulum GRU- NOW. In addition to physicochemical and geological factors, the qualitative composition and density of the microphyte community appeared to be of major importance in the forma- tion of tufa. However, recent massive developments of differ- ent benthic diatoms in the Plitvice Lake system associated with an increase in nutrients correlate instead with a decline in calcifi cation intensity, especially in smaller pools on top of the barriers (unpublished observation). Calcite precipitation by diatoms (Pl. 2, Fig. 6), is associ- ated more directly with cell products than their photosynthet- ic activity, which is less evident with cyanobacteria where the calcifi cation occurs on extracellular sheaths enveloping the cells (Pl. 2, Fig. 6, insert). Diatoms are ubiquitous epiphytes on mosses (Pl. 2, Fig. 7), but are themselves overgrown by smaller microorganisms such as bacteria (Pl. 2, Fig. 7, insert). Crystal growth takes place around gelatinous stalks produced by the cells of the diatom Gomphonema olivaceum var. cal- careum CLEVE, and are arranged in stromatolitic laminae through pulses of diatom growth (WINSBOROUGH & GOL- UBIC, 1987). This relationship between a microbial product and crystal nucleation is consistent with the concept of orga- nomineralization sensu TRICHET (1967) and DÉFARGE & TRICHET (1993). Initiation of calcite precipitation on moss leafl et surfaces without the presence of microbial epiphytes, but associated with organic templates of apparent microbial origin, has been recently described by TURNER & JONES (2005). This particular pattern of “dendritic” calcite precipi- tate has been described earlier and associated with a Schizo- thrix sp. in the Everglades, Florida (MERZ & ZANKL, 1993). We found the same crystallization pattern associated with the sheaths of the subaerial cyanobacterium Geitleria calcarea FRIEDMANN, which inhabits the semi-enclosed caves in the travertine barriers, beneath the waterfalls (Pl. 2, Fig. 8). The crystallization follows the mineral lattice of calcite, but fa- vours sharp crystal edges over the plates, which are complet- ed later (Pl. 2, Fig. 9). Dominant assemblages of cyanobacteria and diatoms are present in different natural habitats along the system accord- ing to their specifi c adaptations and growth preferences. The exclusive domain of cyanobacteria is on travertine cones un- der the waterfalls, which receive the direct impact of falling water. These microorganisms are structurally supported by carbonate deposition in and around their extracellular sheaths and envelopes. The associated carbonate deposit is more com- pact than that forming around mosses, with a fi ne, microme- ter-scale porosity provided by the fi laments of cyanobacteria. The most common among these carbonate-encrusted cyano- bacteria are Phormidium incrustatum (NÄGELI) GOMONT. We have followed the early stages in encrustation of this organ- ism in the Plitvice travertines (Pl. 2, Fig. 6, insert and Fig. 10), as well as early and late diagenetic changes (see below). Other cyanobacteria that show different degrees of calci- fi cation in the Plitvice system are Schizothrix fasciculata (NÄ- GELI) GOMONT, and the moderately encrusted Phormidum favosum (BORY) GOMONT, P. uncinatum GOMONT and Hydrocoleum homoeotrichum KÜTZING, These organisms are common in rapidly fl owing channels over and around tra- vertine barriers. Similar habitats show frequent growth of hemispherical colonies of Rivularia haematites (DE CAN- DOLE) AGARDH, hardened by the dense zonal deposition of calcite grains. A specifi c difference in carbonate precipitates in this and other freshwater species of Rivularia has been doc- umented by OBENLÜNESCHLOSS & SCHNEIDER (1991). Geologia CroaticaStjepko Golubić et al.: Travertines and calcareous tufa deposits: an insight into diagenesis 369 High rates of calcifi cation, as well as a species-specifi c modifi - cation of precipitates, are shown by chlorophytes, especially by the desmid Oocardium stratum and the xanthophyte Vaucheria geminata (VAUCH.) DE CANDOLLE. These cases have been selected in the present study of diagenesis (see below). Although subject to numerous investigations, the deposi- tion of calcareous tufa in karstic waters remains incompletely explained. The problem stems largely from different interpre- tations of the biogenic vs. abiogenic components which are responsible for calcifi cation and the formation of travertines. To approach this question in a constructive fashion requires realization of the complexity of the process, and its sequential development. The environmental conditions such as pH, car- bonate chemistry and saturation levels of solutions change as a part of this process and so do the nature and proportion of biogenic vs. abiotic infl uences. An average carbon atom en- gaged in this process, passes through the phases of (a) carbon- ate chemistry, (b) nucleation, crystal organization and growth, and (c) accumulation and build-up of carbonate sediment. Each of these phases may be responding to a different balance of biogenic vs. abiotic forces, and they may infl uence each other sequentially (GOLUBIC, 1973; EMEIS et al., 1987). Carbonate deposition is a process at the intersection of in- organic and organic carbon cycling, which ranges in scale from molecular to global (GOLUBIC et al., 1979; GOLUBIC & SCHNEIDER, 1979). The supersaturation of karstic springs, which is the prerequisite for carbonate precipitation, is large- ly biogenic by enrichment with respiratory CO2 in soils. The precipitation of carbonate that follows the emergence of these waters is part of the adjustment of the preceding biogenic car- bonate dissolution. The extensive fossil tufa deposits of Roc- cheta a Volturno, used here in the current Recent-to-fossil com- parison, are no longer actively accumulating, since the slopes of the watershed have been deforested and the soils washed out (D’ARGENIO et al., 1995). The barriers of the Plitvice Lake system are still in the process of active accretion (ZWICK- ER & RUBINIĆ, 2005), within its densely forested watershed largely protected by its National Park status. A predominance of “abiotic” infl uences on carbonate pre- cipitation (e.g. CHEN et al., 2004), is expected at a relatively short distance from the spring, as part of the re-adjustment of excessive (respiratory) CO2 transported in the ground waters to the pCO2 of the atmosphere. It is promoted by increase of the air-water interface in waterfalls (ZHANG et al., 2001). In the hard calcite-saturated waters of the Plitvice Lakes system, locally strong aeration promotes considerable calcium carbon- ate precipitation. The predominance of biogenic infl uences on carbonate che mistry through microbial photosynthetic activity is ex- pected later in the water fl ow, after equilibrium with the at- mosphere has largely been reached (MERZ-PREISS & RID- ING, 1999; FRANČIŠKOVIĆ-BILINSKI et al., 2004). The microbial biogenic infl uences on crystal nucleation and con- struction of minerals are separate from the carbonate chem- istry (FREYTET & VERRECCHIA, 1998), and operate at a much fi ner scale than the sedimentary processes under the infl uences of aquatic mos ses, the latter producing the macro- architecture of travertine deposits. 4. DIAGENESIS IN TRAVERTINE DEPOSITS Diagenesis of travertine deposits starts early and proceeds un- evenly. It is initiated contemporaneously with travertine bar- rier building, as calcifi cation instantly fossilizes the biological structure. Further diagenesis is conditioned by the very poros- ity of the deposit. As water seeps through the travertine, the gravitational fi eld determines its passage, so that some con- duits are in frequent use, while other parts of the porous sedi- ment are avoided and remain unaffected. As in soils, the in- terstitial waters in travertine barriers collect respirational CO2 and may dissolve some of the carbonate deposit, which sub- sequently precipitates when equilibrium with atmospheric pCO2 is re-established (GOLUBIĆ, 1969, 1973). The process is similar to the deposition of stalactites in caves: it produces a coating of compact crystalline carbonate over travertine that becomes superfi cially indistinguishable from cavestone (Pl. 1, Fig. 8, smooth surfaces). At a microscopic scale, minute details in the travertine deposit become dissolved or re-ar- ranged, gradually losing original texture and biogenic signa- ture. As part of the same process, other places within the trav- ertine structure remain unchanged. This condition permits comparison between different stages of the diagenetic process, which have been arrested and preserved by the changes of water passage. Preserved thalli of interglacial and early Holocene Cra- toneurum commutatum remain preserved and recognizable in the caves of the waterfall facies of the Roccheta a Volturno deposits (Pl. 2, Fig. 11). Similarly, the stromatolitic laminae of alternating chironomid housings and zones of Oocardium stratum, testify to seasonal changes on the travertine slopes at the same fossil site (Pl. 2, Fig. 12), as discussed by GOLUBIC et al. (1993, 1995), and documented by stable isotope meas- urements (ANZALONE et al. 2007). 4.1. Diagenesis of the micrite deposit of Phormidium incrustatum (cyanobacteria) Diagenetic alterations in sites exposed to direct water impact show considerable variation, as shown in calcifi ed colonies of the cyanobacterium Phormidium incrustatum (Pl. 3, Fig. 1). When active, fi laments of P. incrustatum are composed of sheathed trichomes, about 4 μm in diameter. The trichomes are surrounded by a fi rm sheath of extracellular polymers (EPS), which are the locus of intensive calcifi cation. Fila- ments are often arranged parallel to each other, forming up- right bundles that jointly resist the water impact. The bundles build colonies in the form of lens-shaped cushions, two to three cm in diameter, slightly inclined in the direction of wa- ter fl ow (Pl. 3, Fig. 2). In petrographic thin section under un- polarized and polarized transmitted light, the outlines of these fi laments appear dark due to the densely arranged grains of micritic calcite (GOLUBIC et al., 1993, pl. 2, fi gs. 5, 8). In sections perpendicular to the direction of bundles, the fi la- ments seen in cross section are similarly outlined by primary micrite, whereas the spaces between bundles are fi lled with sparry calcite (Pl. 3, Fig. 3). Fractured samples along the bun- dles as seen by SEM, (Pl. 3, Fig. 4), reveal uniformly distrib- uted calcifi ed tubes of empty sheaths preserved as micritic linings. Geologia Croatica Geologia Croatica 61/2–3 370 PLATE 2 Aquatic mosses and biofi lms, the main architects of calcareous tufa 1 Cratoneurum commutatum dominates the cascades of the Plitvice system. Scale bar is ca 5 cm long. 2 Fossil moss, Bryum pseudotriquetrum, forming distinct, apparently seasonal laminae on the travertine slopes at Rocchetta a Volturno, central Apennines. 3 Well preserved fossil moss plantlets of Eucladium verticillatum near calcareous waterfalls at Tivoli, Italy. Scale bar is 1 cm long. 4 Critical-point dried moss plantlets under Scanning Electron Microscope (SEM). Note the dense overgrowth of microorganisms forming an epiphytic biofi lm. Scale bar is 100 μm long. 5 The tip of a moss leafl et coated by microbial biofi lms dominated by Leptolyngbya (cyanobacteria), and sessile eukaryotic green algae and diatoms. Scale bar is 10 μm long. 6 Detail of the biofi lm, dominated by pinnate diatoms and associated carbonate precipitates. Insert: heavily calcifi ed tube formed by the cyanobacterium Phormidium incrustatum. Scale bar is 10 μm long, also for the insert. 7 Tip of a diff erent moss leafl et densely covered by Synedra (Bacillariophyta), Insert: heterotrophic bacteria over- growing the diatom. Scale bar is 50 μm for the leafl et and 20 µm for the insert. 8 Calcifi ed sheaths of the cyanobacterium Geitleria calcarea and G. fl oridana growing subaerially in the semi- enclosed caves under the waterfalls. 9 Detail of the sheaths shown in fi g. 8, illustrating distinctive fi brous growth of the calcite crystals. Scale bar is 1 μm long. 10 Early growth stages of Phormidium incrustatum (Cyanobacteria) with heavily calcifi ed sheaths (see insert in fi g. 6). Scale bar is 50 μm long. 11 Fossil lithifi ed moss (Cratoneurum commutatum) comprising a cave ceiling in the waterfall facies of Roccketta a Volturno. The details are suffi ciently well preserved to identify the fossil organism. 12 Interglacial stromatolitic travertine on the slope facies of Rocchetta a Volturno showing seasonal deposition of Oocardium stratum, alternating with housings of chironomid larvae. Oocardium settles every June, following chironomid maturation. Geologia CroaticaStjepko Golubić et al.: Travertines and calcareous tufa deposits: an insight into diagenesis 371 Geologia Croatica Geologia Croatica 61/2–3 372 PLATE 3 Diagenesis of a calcareous deposit of Phormidium incrustatum 1 Calcifi ed cushions of Phormidium populations which dominate areas with a strong impact of falling water. The fl ow of water and orientation of the cyanobacterial fi laments are toward the observer. Scale bar is 5 cm long. 2 Section through the cushions of Phormidium in side view; the direction of water fl ow is from left to right. Scale bar is 1 cm long. 3 Petrographic thin section perpendicular to the fi lament orientation. Note dark micritic outlines of cyanobacterial sheaths and large sparitic fi ll of the interfi lament spaces. Scale bar is 20 μm long. 4 Longitudinal fracture through a bundle of Phormidium fi laments preserved as calcifi ed tubes. Scale bar is 20 μm long. 5 Early stages of diagenesis of Phormidium travertine. Note that the recrystallization starts in the interior of the bundles and progresses outward. Peripheral fi lament imprints are perfectly preserved whereas the interior appears as a solid carbonate block. Scale bar is 100 μm long. 6–8 Advanced diagenetic alteration of Phormidium travertine. Scale bars are 50 μm long 6 Petrographic thin section through an altered bundle of Phormidium. Note the pseudostromatolitic lamination stained by bands of concentrated iron precipitate alternating with clear zones. 7 The same frame in cross polarized transmitted light shows the uniform optical orientation of the entire column as a single crystal. 8 Large monocrystalline palisades originated from bundles of Phormidium fi laments. Fig. 8 represents further development of the stage shown in fi g. 5 above. Geologia CroaticaStjepko Golubić et al.: Travertines and calcareous tufa deposits: an insight into diagenesis 373 Geologia Croatica Geologia Croatica 61/2–3 374 PLATE 4 Calcareous deposition by Oocardium stratum and Vaucheria geminata 1 Detail of a micro-reef produced by Oocardium showing monocrystalline calcareous tubules that branch with each cell division of the desmid. Scale bar is 50 μm long. 2 Petrographic thin-section through an upright colony of Oocardium in cross polarized light. Note that crystal orientation is the same for each clonal population of the alga. 3 Similarly fractured upright colony seen by SEM. Scale bars in fi gs. 2 and 3 are 100 μm long. 4 Detail of the rim of the Ocardium calcareous tube following hypochlorite treatment to remove organic components. Note the orientation of the cleavage pattern is consistent with a single calcite crystal and the cylindrical imprint of the organic tubule. Scale bar is 5 μm long. 5 Critical-point dried clonal colony of Ocardium stratum showing cells in the state of division inserted into calcareous tubes. Scale bar is 30 μm long. 6 Early diagenetic alteration of an Oocardium calcite grain. Note the euhedral outline of the grain, while its interior contains the original tubules. Scale bar is 50 μm long. 7–9 Three advanced stages in diagenetic alteration of an Oocardium colony. Scale bars in these fi gures are 1 mm long. Note the gradual increase in the average size of calcite grains, while their upward divergence remains preserved. 10–15 Calcifi ed fi laments of Vaucheria geminata; Fig. 10 – SEM view of the calcifi ed network of Vaucheria fi laments; Fig. 11 – A fractured group of heavily calcifi ed fi laments; Fig. 12 – A single fi lament of Vaucheria with the residue of the cell wall visible inside the calcifi ed tube. Note the radial arrangement of microspar grains encrusting the fi lament. Scale in Figs. 10–12 is 100 μm long; Fig. 13 – Petrographic thin-section of diagenetical- ly altered Vaucheria fi laments; Fig. 14 – SEM of broken Vaucheria travertine obliquely cutting the calcareous tubes; Fig. 15 – Petrographic thin-section of a calcifi ed Vaucheria fi lament in cross section. Scale bars in Figs. 13–15 are 50 μm long. Geologia CroaticaStjepko Golubić et al.: Travertines and calcareous tufa deposits: an insight into diagenesis 375 Geologia Croatica Geologia Croatica 61/2–3 376 Diagenetic changes of Phormidium incrustatum deposits are accompanied by gradual recrystallization, within which the size of calcite grains increases. This process usually starts in the centres of fi lament bundles and spreads toward the bun- dle periphery. This stage is shown in Pl. 3, Fig. 5, where the left side of the image contains bundles unaffected by recrys- tallization, whereas two bundles to the right are cleaved and show compact interiors, while the periphery of each bundle preserves individual tubules of the original biogenic imprint. In later stages of diagenesis, observed in those parts of the same sample that were affected by serving as a water conduit, the original biogenic signature is completely erased (Pl. 3, Figs 6–8). In plain transmitted light, a petrographic thin section along the bundle shows pseudostromatolitic structure with rhythmic lamination of iron enrichment lines (Pl. 3, Fig. 6). The same section in cross-polarized light (Pl. 3, Fig.7) reveals that the entire pseudostromatolite is comprised of a single crystal grain. The diagenetically altered Phormidium incrus- tatum bundles as shown by SEM (Pl. 3, Fig. 8), are converted into large calcitic palisades with external outlines of the orig- inal bundles smoothed but still recognizable. This observed diagenetic sequence is invariably trends towards an increase in the average carbonate grain size, where- as the preserved micritic carbonate identifi es the parts of the original mineralogy. There was no evidence of secondary mic- ritization in this process. 4.2. Diagenesis of primary sparite of Oocardium stratum (Chlorophyceae) Diagenesis of fossil Oocardium tufa has been reported and dis- cussed by GOLUBIC et al. (1993). The present study compares the process with other biogenically modifi ed primary precipi- tates. The desmid Oocardium stratum is a common constituent of ambient temperature travertines. Its life cycle, growth pat- terns and ecology have been studied since the 1930s (WALL- NER, 1933, 1934). In the Karstic region of Croatia, it was fi rst observed by Ivo Pevalek, an early explorer of the Plitvice sys- tem, and later found in the travertine deposits of the river Krka (PAVLETIĆ & GOLUBIĆ, 1956). Its taxonomic status was subsequently clarifi ed by GOLUBIĆ & MARČENKO, (1958). The characteristic calcifi ed light green colonies (Pl. 1, Fig. 7) of Oocardium form contiguous microscopic reefs (GOLUBIC et al., 1993, pl. 3). The extracellular polymers produced by this unicellular green alga form a ring around the cell, which starts calcifying by organizing a single calcite crystal that conforms to the shape of the organic ring. As the production of the poly- mer continues, the cell is lifted from the substrate, carried by a calcifi ed tube. As the cell divides, the tube branches (Pl. 4, Fig. 1), while continuing the orientation of the monocrystal- line lattice. Ultimately, the entire clonal colony produces a single calcite spar crystal, as evident by cross-polarized light microscopy (Pl. 4, Fig. 2) compared with SEM images (Pl. 4, Fig. 3). The combined biological and mineralogical charac- teristic of the mineral is illustrated in Pl. 4, Fig. 4, showing uniform orientation of calcite cleavage lines, combined with the round shape of the organic tubule. An entire clonal colony with its mineral product is shown in the SEM of a critical- point-dried preparation (Pl. 4, Fig. 5). Diagenetic alteration of these primary spars starts by re- crystallization of the external parts, from where it proceeds toward the interior. The exterior of altered spars (Pl. 4, Fig. 6) gains a rhombohedral morphology typical of pure calcite, whereas the interior still preserves the tubular imprint of the original Oocardium tubules. The same has been shown in pet- rographic thin sections (GOLUBIC et al., 1993, pl. 4, fi gs. 4–10). Advanced stages of diagenetic recrystallization are il- lustrated in Pl. 4, Figs. 7–9. Note that these three fi gures are at the same low magnifi cation. The recrystallization resulted fi rst in the complete loss of the internal tubular structure, reta ining only “clonal” crystalline identities in the form of wedge -shaped grains (Pl. 4, Fig.7). The next step resulted in re-organization of the grains toward a radial symmetry with pseudostroma- tolitic zonation (Pl. 4, Fig. 8). This stage has certain morpho- logical properties similar to those shown in the diagenetic se- quence of Phormidium incrustatum (compare with Pl. 3, Fig. 5) suggesting convergent diagenetic modifi cation starting from different original precipitates. This convergence is further sup- ported by the next stage (Pl. 4, Fig. 9). 4.3. Diagenesis of microsparites of Vaucheria geminata (Xanthophyceae) Large colonies of the fi lamentous xanthophyte (Heterocontae) Vaucheria geminata are regularly calcifi ed, forming large spon- gy travertine deposits that compare in volume with mosses (Pl. 4, Fig. 10). The organism is characterized by tubular fi la- ments without cross walls which occasionally branch. Repro- ductive organs are carried on short side branches. In travertine depositing systems, the cell walls of Vaucheria fi laments are rapidly covered by calcite crystals, which continue to grow outward until they reach a fairly uniform size, intermediate between Phormidium micrite and Oocardium sparite (Pl. 4, Figs. 11, 12). The petrographic thin section images (Pl. 4, Figs. 13, 15) agree well with the SEM images. This type of encrus- tation apparently results from competing growth of synchro- nously seeded calcite grains, in which only the initial nucle- ation is under the control of the cell wall composition (Pl. 4, Fig. 14), whereas further growth of crystals continues under supersaturation of interstitial waters, competing for space. The average size of grains gradually increases from the cell wall surface outward (Pl. 4, Figs. 12, 15). By comparing different stages in diagenetic alteration, we again observed the same pattern of grain size increase following recrystallization as in other examples, although the initial stages are species-specif- ic and quite different from each other. 5. CONCLUSIONS The formation of thermal and ambient water travertines have the following properties in common: even distribution of wa- ter and modifi cation of the fl ow into pools separated by walls of carbonate deposit. In the ambient temperature tufa of the Plitvice system, carbonate precipitation shows differences spe- cifi c to particular microorganisms, whereas the travertine bar- riers are constructed by mosses. Diagenetic alteration of pri- mary precipitates starts contemporaneously with carbonate Geologia CroaticaStjepko Golubić et al.: Travertines and calcareous tufa deposits: an insight into diagenesis 377 deposition, but depends on the later access of water. In the po- rous calcareous tufa, diagenesis is uneven, and primary pre- cipitates remain locally preserved. Diagenetic changes are derived from recrystallization of primary precipitates and from secondary coating of the struc- tures by cave-stone type deposits. Diagenetic recrystallization leads convergently toward an increase in the average carbon- ate grain size. No evidence of secondary micritization proc- esses was found. The present study has identifi ed several active environ- ments within the Plitvice travertine system that correspond to lithofacies assemblages described from Quaternary travertines in the central Apennines (FERRERI, 1985; D’ARGENIO & FERRERI, 1987), reviewed by GOLUBIC et al. (1993). Litho- facies assemblages of lentic environments described as (1) Calcareous sands with travertine intercalations, are observed in the Plitvice system in smaller deeper ponds separated by barriers that are arranged in terraces between larger upper lakes. (2) Phytoclastic calcarenite and phytohermal travertine with phanerogam fragments characterize shallow lentic con- ditions (quiet waters). Within this lithofacies assemblage, we have identifi ed an oncolitic setting (Pl. 1, Fig. 3). A further four lithofacies associations refer to habitats and biota in lotic (fast fl owing) environments with an increasing slope angle, as a consequence of accretion of travertine barriers. The most conspicuous feature is the stromatolitic type of lamination, which characterizes (3) Phytoclastic packstone-grainstone and stromatolitic travertine organized in lensoid and tabular bod- ies. Within this lithofacies, we have studied modern Oocar- dium in association with “bibliolitic” travertine, (layered car- bonate-encrustged phanerogame leaf litter), on the edges of Cratoneurum facies. In the Plitvice system this environment is localized in parts of the barrier with gradual slope. (4) Phyto- clastic rudstone with microhermal and stromatolitic travertine with moss cushions and (5) Micro-phytohermal and stroma- tolitic travertine with grasses are the two most widespread en- vironments dominating on all the barriers of the Plitvice sys- tem. Within the above environmental context, the present study has uncovered the taxonomic identity of one of the fos- sil deposits reported by GOLUBIC et al. (1993, p. 236, pl. 2, fi g. 8) as Vaucheria geminata (Pl. 4, Figs. 10–15). The sequences of the travertine lithofacies combinations described above, when superimposed in a vertical sedimen- tary section do not refl ect a change in climatic conditions. In- stead, they are consequence of the lateral displacement of wa- terways and sedimentary environments, which are part of the normal depositional processes in travertines. 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