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Keywords:

  • Calcareous tufa;
  • central Italy;
  • late Quaternary;
  • pollen analysis;
  • taphonomy;
  • travertine

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study Area
  5. Materials and Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgements
  10. References

A palynological survey has been carried out on two late Quaternary travertine s.l. deposits of central Italy at Serre di Rapolano and Bagnoli (Tuscany region). The principal aim was to improve the understanding of factors that affect the accumulation and preservation of pollen grains in thermal (i.e. travertine) and ambient temperature (i.e. calcareous tufa) terrestrial carbonates. For this purpose, 52 526 pollen grains belonging to 118 pollen taxa from local to extra-regional sources were analysed in 200 samples from different travertine and calcareous tufa lithofacies. Of these, 97 samples, generally from thermal travertines, were barren in palynomorphs. By contrast, pollen grains in the remaining samples were well-preserved and did not show differential preservation features. These observations suggest that the pH of the solutions from which travertine or tufa precipitate is not a limiting factor in causing corrosion and/or destruction of pollen grains. Rather, the barrenness recorded in many samples appears to reflect a paucity of pollen rather than post-depositional destruction. Results suggest that the depositional energy of the environment during the deposition of travertine plays an important role in controlling pollen concentration, as verified in both depressions (lower energy to higher pollen concentration) and slopes (higher energy to lower pollen concentration), respectively. Analysis of pollen data suggests that relatively high depositional temperatures and rates of carbonate deposition are also major limiting factors for efficient accumulation of pollen grains. Results of this study reveal that understanding pollen taphonomy processes in travertine and calcareous tufa is critical to developing accurate palaeoenvironmental and palaeoclimatic reconstructions.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study Area
  5. Materials and Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgements
  10. References

As demonstrated by several studies carried out on marine and continental successions in the Mediterranean area, fossil pollen grains can be a powerful tool not only for the study of environmental characteristics and changes that occurred during the Quaternary but also for stratigraphical purposes (e.g. Allen et al., 2000; Tzedakis, 2005; Joannin et al., 2008; Allen & Huntley, 2009; Bertini, 2010; Bertini et al., 2010; Sánchez Goñi & Harrison, 2010; Suc et al., 2010). In continental settings, rich pollen records, especially those from the organic-rich silty-clay strata of lacustrine sites, have provided detailed floristic and vegetational evidence of related climatic conditions (e.g. Follieri et al., 1988; Allen & Huntley, 2009). However, very few papers deal with pollen in carbonates. The few published papers concerning pollen in continental carbonates, commonly named ‘travertine’, actually refer to ambient temperature deposits – usually named ‘calcareous tufa’ or simply ‘tufa’ – which often precipitate at karstic spring emergences of caves or in river falls in limestone bedrock. Examples in Europe and the Middle East are from Belarus (Makhnach et al., 2004), England (Murton et al., 2001), Germany (Riezebos & Slotboom, 1984; Urban, 2007), Spain (Burjachs & Juliá, 1994; Schulte et al., 2008), Italy (Pazzaglia et al., 2012), Israel (Weinstein-Evron, 1987; Kronfeld et al., 1988) and Turkey (Vermoere et al., 1999).

Within this context, travertines s.l. (including the lower temperature tufas) are potentially very useful deposits for the study of palaeoenvironment and palaeoclimate because of their strong sensitivity to environmental changes, such as: ambient temperature, depositional temperature, humidity and CO2 partial pressure (Andrews, 2006). In fact, calcareous tufa and, more rarely travertine, have been used as climate proxies for the reconstruction of late Quaternary palaeoclimate, usually using the isotopic composition of carbon and oxygen of the carbonate anion (Andrews, 2006 and references therein; Sun & Liu, 2010; Kele et al., 2011). Drawbacks concerning the use of travertine for palaeofloristic and palaeovegetational reconstructions are mainly related to the commonly accepted idea that they are not optimal for pollen preservation. In fact, in the 1970s, Gray & Boucot (1975) remarked on how many palynologists attributed the scarcity of pollen in many calcareous deposits to their destruction because of the alkaline environment. In the same study, these authors also suggest that the more critical factor for the preservation of pollen is deposition under oxidizing conditions. Even if both parameters, especially acting together, can destroy the pollen eventually accumulated, the previous cited European sites provide evidence for good preservation of pollen in continental carbonates.

Although there are extensive reviews concerning the palynology of cave sediments (Bastin, 1978; Turner, 1985; Carrión, 1992; Carrión & Scott, 1999; Carrión et al., 1999a,b, 2003; Navarro et al., 2000), and in particular speleothems (e.g. McGarry & Caseldine, 2004), there is still poor knowledge about taphonomic processes of pollen accumulation in travertine and calcareous tufa. In particular, detailed studies that include pollen transportation mechanisms and preservation parameters in relation with the environment, internal facies and the external morphology of the depositional area, especially for travertine, are missing. To provide more information, the results of detailed palaeoclimatic and palaeoenvironmental surveys on two travertine and calcareous tufa deposits in central Italy, studied in the past through the use of pollen and stable isotopes (Ricci, 2011), have been used here to investigate the multiplicity of processes that occur between dehiscence and recovery of pollen grains in terrestrial carbonates.

Study Area

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study Area
  5. Materials and Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgements
  10. References

Among the many travertine and tufa deposits of central Italy, which are clearly related to the presence of a regional thermal anomaly and strong CO2 degassing (Minissale, 2004), travertine and calcareous tufa from two different Quaternary deposits were selected: Serre di Rapolano and Bagnoli (Fig. 1). The active Rapolano geothermal area [43°15′28·77″ N 11°37′01·63″ E, ca 350 m above sea-level (m a.s.l.); Fig. 1] is located in the eastern side of the Siena Basin, a tectonic depression developed during the Neogene collapse of the hinterland of the Northern Apennines (e.g. Martini & Sagri, 1993). Here, travertine is present at two sites: (i) at the Serre di Rapolano area, a topographic low with many active quarries from the Etruscan period, but now a dead area in terms of travertine deposition (Minissale et al., 2002); and (ii) at higher elevation around Rapolano town. In particular, at Rapolano, there are several active quarries, but also one active thermal spring at San Giovanni (38°C), depositing abundant travertine. Both the Rapolano and Serre di Rapolano deposits have been studied in detail in the past in terms of: (i) description of the several depositional facies present there (Carrara et al., 1998; Guo & Riding, 1998, 1999); (ii) anomalous isotopic composition of the precipitated travertine at the San Giovanni spring (Guo et al., 1996); and (iii) relation between the deposition of travertine and active tectonics (Brogi & Capezzuoli, 2009; Brogi et al., 2010).

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Figure 1. Location map of the Serre di Rapolano, Terme San Giovanni and Bagnoli terrestrial carbonate sites in Tuscany (central Italy).

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Bagnoli (43°26′38·59″ N 11°03′13·19″ E, ca 200 m a.s.l.; Fig. 1) is located about 50 km north-west of Rapolano, in the Val d'Elsa Basin and, like the Siena Basin, is another of the several NW–SE post-orogenic extensive structural Neogene depressions that characterizes the peri-Thyrrhenian sector of central Italy. During the Quaternary, the widespread erosional regime related to the general uplift of this area, was episodically interrupted by sporadic depositional events (Capezzuoli et al., 2007). The depositional events were characterized by calcareous sediments derived from the alteration of nearby Mesozoic carbonate sequences and were related to the circulation of lower temperature waters of mixed origin (meteoric and thermal) flowing into the travertine depositional area (Capezzuoli et al., 2007, 2008). In Bagnoli, there is a hypothermal spring at 23°C (Casagli et al., 1990) that still precipitates a small quantity of soft tufa on a slope. The whole deposit consists of alternating layers of travertine and tufa, suggesting a change in the chemical characteristics of the mother water over time, possibly related to different CO2 partial pressures.

Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study Area
  5. Materials and Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgements
  10. References

Pedley (2009) summarized the complex history of the terrestrial carbonate classification (e.g. D'Argenio & Ferreri, 1988; Pentecost, 1995; Fouke et al., 2000; Riding, 2002); and accordingly, in this study, the term travertine is used to describe the crystalline rock precipitated from relatively high temperature, strongly CO2-degassing waters, and the term calcareous tufa is limited to deposits formed from near ambient temperature waters, rich in macrophytes. Following Pedley (2009), however, calcareous tufa can also be present in a thermal system, as a distal facies of an original travertine, downstream from the thermal emergence. In this study, the classification of travertine and the relative lithofacies is according to Guo & Riding (1998) and Capezzuoli & Gandin (2007), whereas the classification of the palaeoenvironmental models for calcareous tufa is from Pedley (1990, 2009) and Capezzuoli & Gandin (2007).

Sites and sampling

Samples from Serre di Rapolano were collected from quarries located at Oliviera (Fig. 2A) and Le Querciolaie (Fig. 2B). Travertine was collected from vertical sections, at intervals of 20 to 30 cm, extracted as cores from the walls using a circular saw and/or a drill (Fig. 3). At Oliviera, the exposed travertine wall is about 30 m thick and includes two main carbonate successions (A and B) extending in time from 157 ± 15 to 24 ± 4 ka (Brogi et al., 2010) and separated by a relevant terrigenous (pedogenized) episode (Fig. 2A); in this quarry 113 samples were collected from six cores. At Le Querciolaie quarry, 51 samples were collected from a wall section of about 15 m (Fig. 2B), where several pedogenized strata mark repeated interruptions in the carbonate deposition between 60 ka and 30 ka (Bellucci, 2007).

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Figure 2. Oliviera (A) and Le Querciolaie (B) travertine quarries located at Serre di Rapolano. Vertical black bars represent the studied wall sections. At Oliviera (A) the travertine exposed is ca 30 m thick, and the terrigenous stratum underlined by the black and white dot line, separates the two main carbonate successions (named A and B in Brogi et al., 2010). Six cores (1 to 6) were collected at Oliviera. At Le Querciolaie (B), where the outcropping sampled travertine section is ca 15 m thick, one core was collected.

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Figure 3. Sampling phases of travertine at the Oliviera quarry.

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At Terme San Giovanni, located 3 km north-west of Serre di Rapolano and 2 km south of Rapolano, pollen samples were collected in a 2 m deep thermal pool (Fig. 4A), close to the fissure-ridge, which is currently the active travertine-depositing site in the area. At present, most of the water is diverted from the natural emergence to a building for balneotherapy. In this place, two samples, one from the water column and one from the soft travertine at the bottom of the pool were collected, respectively, plus one sample of the moss present close to the edges.

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Figure 4. Terme San Giovanni thermal pond. (A) Location of thermal water (*), soft travertine (o) and moss (□) samples, respectively. (B) Summary pollen data expressed as median concentration values. AP, arboreal plants; NAP, non-arboreal plants.

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At Bagnoli, 36 samples (including both travertine and tufa) were collected from a section cropping out 50 m from the actively depositing hypothermal spring. Travertine and calcareous tufa alternations (Fig. 5) are sequenced in three main depositional units (lower, middle and upper) testifying to the intermittent activity of the thermal spring during the time interval including the warming event within the Late Glacial at ca 15 ka (Capezzuoli et al., 2010).

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Figure 5. Calcareous tufa and low thermal travertine alternations at Bagnoli. (A) An unconformity separates travertine of the lower depositional unit from calcareous tufa of the middle deposition unit (Capezzuoli et al., 2008). (B) Calcareous tufa of the lower depositional unit with more compact travertine strata.

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Laboratory procedure for pollen samples

All of the collected samples were processed for palynological analyses. About 100 g of each sample (including the soft travertine sample from the Terme San Giovanni pond) was dissolved by three different chemical attacks: HCl to dissolve calcium carbonate; HF to dissolve the eventual silicates; and HCl in a warm bath to dissolve fluorosilicates. Pollen grains were then released from the sediment by using a heavy liquid (ZnCl2) and, finally, the residue was washed in an ultrasonic bath and preserved in glycerine. A Lycopodium tablet was added to each sample for the pollen concentration computation at the beginning of the treatment according to the marker method of Matthews (1969).

The 50 litres of water collected from the thermal pool at Terme San Giovanni was stored in three plastic tanks after the sampling date in December 2009. After allowing one week for suspended material to settle, approximately two-thirds of the content of each tank was syphoned from just below the water surface. The residual material was extracted from the remaining volume by washing in an ultrasonic bath, and then was processed for pollen analysis following standard procedures, except that the reaction time with HF was reduced and the separation of pollen from the sediment using ZnCl2 was omitted. Finally, the residue was washed again in the ultrasonic bath and preserved in glycerine. Pollen concentration was calculated by using the volumetric method of Cour (1974).

The moss sample (21·8 g), taken 1 m from the thermal pool of Terme San Giovanni in December 2009, was also treated. After the addition of a Lycopodium tablet for the concentration count, the sample was processed for pollen analysis, in conformity with the following procedure: successive addition of HCl, HF and KOH. Pollen was then separated from the sediment by using ZnCl2 and, after washing in the ultrasonic bath, was preserved in glycerine.

Pollen data set

Pollen data are presented here as concentration values expressed in grains g−1. Median concentration values are used to reduce the effect of outliers due to skewed distribution of data. Pollen concentration curves are traced in both detailed and summary pollen diagrams; in the latter, taxa were organized in five informal groups: (i) total arboreal (AP) and non-arboreal (NAP) taxa; (ii) arboreal taxa excluding Pinus; (iii) Pinus; (iv) shrubby plus herbaceous taxa excluding aquatic and wetland taxa; and (v) aquatic and wetland herbaceous taxa (for example, Cyperaceae, Potamogeton and Sparganium). Pollen spectra percentages of total AP, with respect to the sum of AP and NAP, are also summarized.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study Area
  5. Materials and Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgements
  10. References

Sedimentological evidence

The Oliviera quarry

Six cores (1 to 6) were collected throughout the Oliviera travertine (Figs 2A, 6 and 7):

  • Core 1 (Fig. 6) includes the basal part of succession A (Brogi et al., 2010). Two metres of porous, pedogenized travertine, with reddish veins, from the base, are followed by more compact travertine characterized by dominant dark micritic layers intercalated, sometimes, by light-coloured layers. Bubbles, and sometimes reeds and paper-thin rafts, are present as well as vegetal macroremains (stem and leaf).
  • Core 2 (Fig. 6) extends up to the terrigenous stratum separating succession A from succession B. Brecciated levels, bubbles, paper-thin rafts, reeds and leaf imprint are present. In the upper part of the core, pedogenized and karstified travertine, including terrigenous levels, was recognized. Light, sub-horizontal layers, are more frequent than in the underlain portion (core 1).
  • Core 3 (Fig. 7) covers the basal part of succession B, just above the clay interstratum. Thick light-coloured travertine with crystalline crusts oriented according to the slope is widespread. Microbial mats, bubbles, paper-thin rafts, pisoids, shrubs, reed and macroremains (leaf and gland imprints) were also observed. An unconformity marked by a terrigenous level limits the last 80 cm of the core; the latter is characterized by dark (sometimes brecciated) micritic travertine.
  • Core 4 (Fig. 7), from succession B, is marked by the presence of thin, light-grey films related to the carbonate mud modification. Reeds and shells of continental invertebrates are also present.
  • Core 5 (Fig. 7), from succession B, is characterized by alternations of sub-horizontal light and darker micritic layers. Bubbles, paper-thin rafts, reeds, pisoids, shrubs, bones (of little vertebrates), intraclasts and gastropods were also recognized.
  • Core 6 (Fig. 7), from succession B, is dominated by a dark travertine, with terrigenous, pedogenized and karstified features. A palaeosol, around 20 cm from the top, marks the end of the travertine deposition.
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Figure 6. Oliviera quarry: cores 1 and 2 from the succession A (Fig. 2A). Charts to the right of the logs show the pollen percentage values of the arboreal plants (AP).

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Figure 7. Oliviera quarry: cores 3 to 6 from succession B (Fig. 2A). Charts to the right of the logs show the pollen percentage values of the arboreal plants (AP). For the legend see Fig. 6.

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The Le Querciolaie quarry

The core (Fig. 8) is 13·5 m thick and consists of an alternation of dark-coloured and light-coloured travertine. In the two dark basal intervals, a pedogenized and karstified travertine is present; it shows evidence of terrigenous fillings (karstic silt). The crystalline crusts are concentrated mainly between the 5·5 to 2·7 m and 2·4 to 1·8 m intervals, respectively. Among the many lithofacies present (i.e. bubbles, pisoids, paper-thin rafts and shrubs), reeds are absent.

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Figure 8. Le Querciolaie quarry. Chart to the right of the log shows the pollen percentage values of the arboreal plants (AP).

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The Bagnoli site

The succession at Bagnoli (Fig. 9) consists of three tabular units separated by erosional surfaces. Each unit consists of two main portions: calcareous tufa and travertine (Capezzuoli et al., 2008). The former (calcareous tufa) appears as bundles of micritic carbonate layers while travertine is well-stratified with shrubs, paper-thin rafts and reeds (Capezzuoli et al., 2008).

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Figure 9. Bagnoli succession. Charts to the right of the logs show the pollen percentage values of the arboreal plants (AP).

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In summary, the study of both high temperature (at Serre di Rapolano) and low temperature (at Bagnoli) travertine has allowed the documentation of different lithofacies primarily associated with the depression (DDS) and slope (SDS) depositional systems (Fig. 10) of Guo & Riding (1998). In the DDS, marshy facies, with dark massive travertine (DDS-a; Fig. 11A) and lighter laminated bacterial/cyanobacterial travertine (DDS-b; Fig. 11B to D), as wells as: shrubs, bubbles, pisoids, reeds, paper-thin rafts and lithoclasts travertine were recognized. In SDS, the prevalent facies was associated with laminated crystalline travertine (crystalline crusts; Fig. 11E).

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Figure 10. Schematic profile of the geothermal facies (modified from Guido & Campbell, 2012) identified in the travertine deposits of Serre di Rapolano and Bagnoli: SDS, slope deposition system; DDS, depression deposition system.

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Figure 11. Travertine lithofacies at Serre di Rapolano and Bagnoli. (A) Dark massive marsh facies of the depression depositional system (Oliviera quarry). (B), (C) and (D) Light laminated bacterial/cyanobacterial travertine of the depression depositional facies [(B) at Oliviera quarry; (C) and (D) shrubs and paper-thin rafts at Bagnoli, respectively]. (E) Crystalline crusts of slope facies in the Oliviera quarry.

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The poorly layered, porous/chalky, marly calcareous tufa at Bagnoli (Fig. 12) was interpreted as a phytohermal/phytoclastic facies deposited in a palustrine/lacustrine environment (Capezzuoli et al., 2008).

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Figure 12. (A) Palaeoenvironmental reconstruction of the Bagnoli spring emergence area (modified from Capezzuoli et al., 2008) where arrows indicate the potential mechanisms of pollen transport. (B) Photographs of the phytohermal/phytoclastic calcareous tufa of Bagnoli; the trowel used for scale is 23 cm long.

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Palynology

A total of 52 526 pollen grains belonging to 118 pollen taxa, specifically: 77 herbs, 29 trees and 12 shrubs, from both regional (for example, upland taxa) and local (for example, principally herbs) sources were recognized by the analysis of 200 terrestrial carbonate samples and are shown in Fig. 13; in samples where pollen was found, the concentration varied from about 100 to more than 300 grains g−1.

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Figure 13. List of pollen taxa and median concentrations: (A) for calcareous tufa of Bagnoli; (B) for depression depositional system (DDS) travertine of Bagnoli; (C) for DDS travertine of Le Querciolaie quarry; (D) for DDS travertine of the Oliviera quarry; (E) for slope depositional system (SDS) travertine of the Oliviera quarry.

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Relations between the pollen assemblages and the sedimentary context, associated with both travertine (at Serre di Rapolano and Bagnoli) and calcareous tufa (at Bagnoli) are examined and summarized below as two different case studies.

Travertine case study
The Serre di Rapolano thermal travertine

The Oliviera succession (Fig. 2A) represents the best natural ‘laboratory’ for the present pilot study because of the quite high number of samples collected from the different lithofacies present there (DDS-a, DDS-b and SDS; Fig. 11A, B and E) and the relatively high number of pollen taxa discovered (Table 1; Fig. 13). According to the median concentration values (Fig. 14A), DDS-a samples have higher pollen concentrations (96 grains g−1) than DDS-b and especially SDS samples. In both DDS-a and DDS-b samples, group 4, including herbaceous taxa (for example, Poaceae and Asteraceae) with the exception of the aquatic and wetland taxa (see group 5), has the higher concentration (up to 38 grains g−1 in DDS-a). It is followed, in descending order, by groups 2 (up to 18 grains g−1 in DDS-a) and 3 (up to 5 grains g−1 in DDS-a) (Fig. 14A) which, together, include the arboreal taxa, and then by group 5 (up to 4 grains g−1 in DDS-a) with aquatic and wetland taxa, such as Cyperaceae, Sparganiaceae (including Sparganium), Phragmites, Myriophyllum, Potamogeton, Alismataceae and Typha latifolia. In DDS-a, Cyperaceae prevails whereas in DDS-b (as well as in SDS) Sparganium is more represented. With respect to DDS, SDS samples (Fig. 14A) show a lower total pollen content (11 grains g−1) with: (i) concentration of arboreal taxa excluding Pinus (for example, deciduous Quercus and Olea of group 2) higher than that of shrubs and herbs (group 4); and (ii) a much lower concentration of herbaceous aquatic and wetland taxa (group 5).

Table 1. Summary of pollen data sets from: high (Oliviera and Le Querciolaie) and low (Bagnoli) thermal travertine. AP, arboreal plants; NAP, non-arboreal plants
 Pollen samples (rich versus barren)Number of samples for lithofacies DDS-a; DDS-b; SDS (rich versus barren)Pollen taxa (AP versus NAP)Pollen concentrations (grains g−1)
Oliviera113 (65/48)73; 28; 12 (41/32; 14/14; 10/2)111 (39/72)4 to 7631∙5
Le Querciolaie51 (17/34)34; 11; 6 (17/17; -/11; -/6)80 (32/48)7 to 1358
Bagnoli21 (14/7)–; 21; – (–; 14/7; –)70 (25/45)22 to 796
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Figure 14. Median of pollen concentration values of high [(A) Oliviera quarry; (B) Le Querciolaie quarry] and low [(C) Bagnoli] thermal travertines, respectively. AP: arboreal plants; NAP: non-arboreal plants.

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At Le Querciolaie (Fig. 2B), a high number of barren samples are found; they refer especially to DDS-b and SDS facies (Table 1). The samples rich in pollen are from the marshy facies of the dark massive travertine (DDS-a). Pollen concentrations of herbaceous plants (dominated by Asteraceae) are higher than that of trees, as also observed in the Oliviera DDS facies, despite the overall lower concentration values (Fig. 14B). At Le Querciolaie, in fact, the total pollen concentration value for DDS-a is about 28 grains g−1, whereas the other vegetal groups are less than 10 grains g−1 (Fig. 14B). Cyperaceae is the dominant taxon of group 5 here (Fig. 13) as in DDS-a of Oliviera.

The soft young travertine sample from the bottom of the pond at the active spring of Terme San Giovanni shows a pollen concentration value of 41 grains g−1 and a good presence of herbs of group 4 (mainly Chenopodiaceae; Fig. 4B). In the thermal water sample, where carbonate starts to precipitate, the concentration of pollen reaches 416 grains l−1; again the herbaceous taxa of group 4 reach the higher concentration values. Hedera, also present in high values in the soft young travertine, reaches a peak concentration value here. In the moss sample, concentration values reach 49 000 grains g−1, and arboreal taxa such as deciduous Quercus, Pinus and Olea have higher values with respect to the herbaceous taxa.

The Bagnoli low thermal travertine

Palynological data (Table 1; Fig. 14C) from laminated bacterial/cyanobacterial low thermal travertines of marshy facies (DDS-b) (Figs 5, 11C and D) present a total pollen concentration value of about 90 grains g−1, comparable with that of the thermal Oliviera travertine DDS-a facies. Median values of AP of group 2 (for example, deciduous Quercus and Carpinus) and NAP of group 4 (for example, Asteraceae Asteroideae and Poaceae) are similar (ca 40 grains g−1). Pinus is well-represented whereas pollen concentrations of aquatic and wetland taxa (mainly Cyperaceae; group 5) are extremely low (0·26 grains g−1).

Calcareous tufa case study

As mentioned previously, at Bagnoli (Fig. 5) tufas are also present intercalated with travertine. Only one sample is barren in palynomorphs; pollen concentration values range from about 8 grains g−1 up to 330 grains g−1 (Table 2). The total pollen concentration of calcareous tufa (as median values) is about 110 grains g−1 (Fig. 15), with NAP concentrations of group 4 (for example, Asteraceae Cichorioideae) slightly higher than AP of group 2 (ca 40 grains g−1). Pinus is present with concentrations of about 20 grains g−1. Concentrations of the herbaceous aquatic and wetland taxa of group 5 (Cyperaceae prevalently) are very low (0·5 grains g−1).

Table 2. Summary of pollen data sets from the calcareous tufa of Bagnoli. AP, arboreal plants; NAP, non-arboreal plants
LocationPollen samples (rich versus barren)LithofaciesPollen taxa (AP versus NAP)Pollen concentrations (grains g−1)
Bagnoli15 (14/1)Phytohermal/Phytoclastic facies70 (23/47)8∙4 to 331∙1
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Figure 15. Median of pollen concentration values from calcareous tufa at Bagnoli. AP: arboreal plants; NAP: non-arboreal plants.

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study Area
  5. Materials and Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgements
  10. References

Previous palynological studies on the two terrestrial carbonates of Serre di Rapolano and Bagnoli yielded an indispensable data set for reconstruction of the main vegetational changes and the associated palaeoclimate inferences stratigraphically framed in the latest Quaternary by 14C and U/Th datings (Ricci, 2011). Specifically, the recognized changes in vegetal assemblages were associated with the glacial/interglacial phases since ca 130 ka, including the Eemian, the last interglacial (Figs 6 to 9 and 16). Nevertheless, taphonomical and methodological aspects related to the accumulation of pollen in travertine pools and slopes, need to be further developed and revised carefully in order to exclude major biases that could affect the pollen record of carbonate deposits.

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Figure 16. Stratigraphical location of Oliviera, Le Querciolaie and Bagnoli successions and evolution of Greenland temperature and European temperate forests (37°N) over the last climatic cycle, methane (CH4) concentration, precession index, obliquity and ice volume variations. (From Sánchez Goñi et al., 2008 modified).

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In the present survey, 97 of the 200 carbonate samples, generally from high thermal travertines (DDS-a, DDS-b and SDS facies; Table 1; Fig. 10), are barren in palynomorphs. The barrenness is less frequent in the ambient temperature deposits (Table 2). Changes in pollen assemblages were mostly associated with glacial/interglacial phases, the development of which was independent from the lithofacies and the depositional system considered (Ricci, 2011); for instance, at the Oliviera quarry, similar phases of increments in thermophilous arboreal taxa (including the Mediterranean ones), have been recorded in correspondence with different depositional systems, i.e. DDS in core 1 and SDS in core 3. Although, from another point of view, the same depositional system can be associated with different climate conditions, for example DDS developed during both interglacials (core 1), as well as glacials, as indicated by the large spread of herbaceous plants including steppe taxa in core 4.

In most of the 103 rich samples, pollen grains are usually well-preserved and do not show differential preservation states (Fig. 17). In fact, in only a few samples, mostly from the pedogenized horizons of the Oliviera quarry (Figs 2A and 6), have degradation phenomena been observed among the scanty pollen grains (Fig. 18); they are probably related to the presence of oxygenated conditions, which is the more critical factor for the preservation of pollen (Havinga, 1964; Tschudy & Scott, 1969; Williams et al., 1996). Such oxygenated conditions, that favour chemical alterations, could have developed during wet–dry cycles (Havinga, 1967). The wet–dry cycles can also be associated with mechanical damage of pollen exine during desiccation events (Campbell & Campbell, 1994). In the case of calcareous deposits, many palynologists, as remarked by Gray & Boucot (1975), attribute the scarcity of pollen to its destruction because of the presence of an alkaline environment. Nevertheless, data presented in this study from both travertine and calcareous tufa, as well those from previous studies on calcareous tufa (Weinstein-Evron, 1987; Vermoere et al., 1999; Murton et al., 2001), provide evidence that alkalinity (pH > 7) in such deposits was not high enough to cause degradation/corrosion phenomena in the pollen exine. Moreover, the fact that in such deposits there are samples either with quite well-preserved grains or, oppositely, totally barren, seems to suggest that barrenness is more often due to an original lack of pollen in the sediments (or fast carbonate precipitation rate), rather than to its successive destruction under oxidizing conditions.

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Figure 17. Photographs of pollen at the optical microscope: (A) Plantago (calcareous tufa – Bagnoli); (B) Ericaceae (calcareous tufa – Bagnoli); (C) Knautia (DDS-b travertine – Bagnoli); (D) Zelkova (DDS-a travertine – Oliviera quarry); (E) Cyperaceae (DDS-a travertine – Oliviera quarry); (F) Tilia (DDS-a travertine – Oliviera quarry).

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Figure 18. Pollen grains from pedogenized travertines (Oliviera quarry) with evident phenomena of degradation and corrosion.

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It is reasonable to think that pollen reaches the depositional area of carbonate accumulation almost exclusively by wind (air-borne) and/or by running water (water-borne), assuming that the role of animal-borne and insect-borne pollen transport is negligible. In general, because that the ratio between water-borne and air-borne inputs depends on the relation between the size of the hydrological catchment and other factors, such as the depositional surface area, the morphology and the vegetation near the catchment (Brown et al., 2007), it is plausible to suppose that in travertine systems, where the carbonate precipitates close to the emergence (negligible catchment area for thermal water), the water-borne pollen input probably is not the prevalent one. In contrast, the water-borne pollen component is potentially higher in the palustrine/lacustrine calcareous tufa, such as at Bagnoli, because of enhanced transport by streams and rivers (Fig. 12A). The pollen produced by local vegetation (principally by anemophilous taxa) within a few metres of the sampling site is particularly abundant in the DDS-a and DDS-b travertine of Serre di Rapolano and Bagnoli. It principally belongs to cosmopolitan herbaceous plants, such as Asteraceae Cichorioideae and Poaceae (group 4, Fig. 14A to C). In contrast, aquatic and wetland taxa indicative of local edaphic conditions, such as Cyperaceae, Sparganiaceae (including Sparganium), Phragmites, Myriophyllum, Potamogeton, Alismataceae and Typha latifolia, always stay in low quantities (group 5, Fig. 14A to C). Unfavourable environmental and edaphic conditions for the growth and expansion of such taxa are also indicated by the pollen record of both the young unconsolidated travertine and the thermal water samples from the thermal pool of Terme San Giovanni, as well as the nearby moss (group 5, Fig. 4B). Among the DDS travertine of Serre di Rapolano, it is in the DDS-a facies that wetland and aquatic herbs reach their relatively higher values (Fig. 14A and B), suggesting that the dark massive travertine used to precipitate in the pools, quite deep, and persisted throughout the year (Fig. 10). Lighter laminated bacterial/cyanobacterial travertine of DDS-b facies instead was probably related to thin sheets of water in smaller ponds (Fig. 10). The Cyperaceae record testifies to the presence of freshwater depositional areas subjected to significant fluctuations of the water levels. The relative increase in marshy plants, such as Sparganium and Typha, and also Potamogeton and Myriophyllum, attests to the presence of phases of deepening of the water column. Deciduous Quercus, always abundant (in Bagnoli and SDS travertine of Oliviera quarry median concentrations are higher than 20 grains g−1; Fig. 13), as well as other temperate broad-leaved deciduous plus some sclerophyll forest taxa (for example, Carpinus, Ostrya type, Quercus ilex and Olea) contributed, together with Pinus (particularly abundant in the DDS-a travertine of Le Querciolaie and in the calcareous tufa of Bagnoli; Fig. 13), to both local and extra-local pollen (from vegetation within a few hundred metres) components. The presence in all lithofacies of high altitude taxa, such as Picea, Abies, Betula and Fagus, especially during phases interpreted as corresponding to warm conditions (Ricci, 2011), indicates that these taxa also contribute to the regional pollen component. The very scanty occurrence of Cedrus in the latest Quaternary deposits of Bagnoli (Fig. 13) also seems to indicate a long-distance, transported pollen input, from extra-regional air masses coming from southern Europe (Magri & Parra, 2002; Magri, 2012).

The overall pollen concentration pattern from all the analysed Tuscan sites is generally lower than 7631 grains g−1 (Fig. 19) which is the peak value in the Serre di Rapolano high thermal travertine. Concentration values are in the same order of magnitude as those from other Pleistocene European carbonate deposits shown in Fig. 19: for example, calcareous tufa of Abric Romanì (ca 100 grains g−1 with a maximum value of 3016 grains g−1; Burjachs & Juliá, 1994), Buckinghamshire (ranging from 3·3 to 10·6 grains g−1; Murton et al., 2001) and River Aguas (generally lower than 500 grains g−1 with maximum values around 2000 grains g−1; Schulte et al., 2008). In any case, the pollen concentrations in terrestrial carbonates appear to be significantly lower than those usually recorded in terrigenous sediments, where they can reach (for example, in lacustrine deposits) millions of grains g−1 (for example, at Valle di Castiglione: Follieri et al., 1988; Accesa lake: Drescher-Schneider et al., 2007). Such different behaviour is possibly related to the different order of sediment deposition rates between carbonates and terrigenous sediments, specifically: an inverse relation between pollen concentration and accumulation rates is inferred. A fast carbonate deposition rate from parent water, with a mean value of about 200 mm yr−1 for travertine (with high rates, up to 1 mm yr−1; Pentecost, 2005; Viles & Pentecost, 2007) and about 5 mm yr−1 for calcareous tufa (Pentecost, 2005), is in fact observed worldwide. On the contrary, lower accumulation rates have been reported for lacustrine (pollen-rich) environments (for example, 0·32 mm yr−1, Valle di Castiglione, Follieri et al., 1988). Moreover, the progressive decrease in pollen median concentration values, from ambient temperature palustrine/lacustrine deposits (calcareous tufa of Bagnoli) versus thermal deposits (travertine of Bagnoli and Serre di Rapolano) is evident in Fig. 20. Higher pollen concentrations in travertine deposits from the low thermal depression of Bagnoli, and lower pollen concentrations in travertine from the high thermal slope (SDS) of Serre di Rapolano were recorded (Fig. 20). A similar behaviour is also confirmed by simply considering the concentration trend of pollen groups 2 to 4 (Fig. 20).

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Figure 19. Diagram of total pollen concentration values (grains g−1) of carbonates from Serre di Rapolano and Bagnoli. Bibliographic data on calcareous tufa are also reported for comparison. Numbers above bars express maximum pollen concentrations values (*).

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Figure 20. Palynological concentration of median values for the studied deposits: (A) Bagnoli calcareous tufa; (B) depression depositional system (DDS) Bagnoli travertine; (C) DDS Serre di Rapolano travertine (Le Querciolaie and Oliviera quarries are grouped together); (D) slope depositional system (SDS) Serre di Rapolano travertine (Oliviera quarry). In the lower part of the figure, the main characteristics of deposits are summarized.

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The relative energy of the depositional system (stream speed, steep slope, etc.) also plays an important role in controlling pollen accumulation, as summarized in Fig. 21. In a high energy environment, such as that associated with SDS facies (Fig. 21A), small amounts of pollen can be trapped (for example, small sinking areas on the slope) because pollen generally flows away with the water. In contrast, in a quiet depositional system where the energy is low, such as a depression (Fig. 21B), a larger amount of pollen grains can be trapped, either coming from plants surrounding the pool, or more distal, or both.

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Figure 21. Schematic reconstruction of (A) slope and (B) depression (thermal water pool) environments in a generic travertine system. The arrows indicate the water-borne pollen component whereas the dotted lines represent the air-borne component.

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Imaging a continuum of characteristics from ambient temperature deposits (tufa) to thermal deposits (travertine) according to Pedley (2009), it is worthy of note that the main change in pollen concentration does not occur at the passage between calcareous tufa and travertine (see Bagnoli data, Fig. 20), but rather from lower thermal to higher thermal travertine (Bagnoli and Serre di Rapolano travertine, Fig. 20). Relatively high deposition temperatures and associated quick carbonate precipitation processes seem to be the main limiting factors for greater pollen concentration values.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study Area
  5. Materials and Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgements
  10. References

The main factors that have affected the taphonomic history of pollen grains during the time between their dispersion and their recovery in terrestrial carbonates have been examined through the study of two late Quaternary Tuscan travertine and calcareous tufa deposits from central Italy. The following evidence of major interest for a better comprehension of pollen record response to palaeoenvironmental and palaeoclimatic events can be summarized.

  1. Despite the significant number of barren samples and the generally low pollen concentrations (from few units to some hundreds of grains g−1) that force treatment of a large quantity of sediment (ca 80 to 100 g), well-preserved pollen can be recovered in the different sedimentary carbonate facies.
  2. The presence of well-preserved pollen grains in both travertine and tufa deposits excludes that an alkaline environment is per se responsible for the destruction or corrosion phenomena. The frequent barren samples are probably not only a function of oxygenation as a consequence of desiccated phases due to climatic or tectonic causes, but also a consequence of a fast precipitation rate of travertine in the slope depositional facies.
  3. In slope depositional areas, especially if steep, the energy of the water is a further parameter for the low accumulation rate of pollen because the pollen is diverted towards more distal flat areas.
  4. Different pollen inputs and components (for example, local to extra-regional, water-borne and air-borne) are well-represented in the different lithofacies; changes in pollen assemblages are therefore coherent with both palaeoenvironmental and palaeoclimatic changes.
  5. Lithofacies and depositional systems are independent (especially in travertine) of palaeoclimatic changes, as indicated by pollen assemblages.
  6. The median pollen concentration values show a progressive increase passing from travertine to ambient temperature palustrine/lacustrine calcareous tufa.
  7. In travertine, the higher pollen concentrations are evident in the low thermal depression areas whereas the lower concentrations are evident in high thermal slope areas.
  8. The reduction in pollen concentration from low to higher thermal travertine provides evidence that higher temperatures and associated quicker processes of carbonate deposition are the limiting factors for pollen accumulation.
  9. The best type of terrestrial carbonate deposit for palaeoclimatic reconstruction using pollen is calcareous tufa, followed by low thermal travertine and, finally, by high thermal travertine.

The results of this pilot research are a very promising starting point for further insights and more extensive comparisons between pollen records, lithofacies and eventual δ13C and δ18O isotopic data, in order to solve the main taphonomic issues involved in terrestrial carbonate sedimentation.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Study Area
  5. Materials and Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgements
  10. References

We acknowledge Andrea Brogi, Enrico Capezzuoli, Francesco Bellucci, Orlando Vaselli and Anna Gandin, for their participation during the field observations and the sampling. The authors also wish to thank Elda Russo Ermolli, for fruitful comments and the detailed review made on the manuscript. A special thanks to Enrico Capezzuoli, who also kindly encouraged this work and freely contributed ideas and discussions. The editorial assistant Linda Pickett was helpful in improving the manuscript. Ex 60% MIUR (Ministry of Education, University and Research) and CNR grants supported this research.

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  2. Abstract
  3. Introduction
  4. Study Area
  5. Materials and Methods
  6. Results
  7. Discussion
  8. Conclusions
  9. Acknowledgements
  10. References
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