Accepted Manuscript

Title: Preservation of original aragonite structure inDesmophyllum castellolense (Scleractinia) from the EoceneIgualada Basin (NE Spain)

Author: Aureli Alvarez Perez German Alvarez Perez

PII: S1871-174X(14)00017-1DOI: http://dx.doi.org/doi:10.1016/j.palwor.2014.04.001Reference: PALWOR 244

To appear in: Palaeoworld

Received date: 9-9-2012Revised date: 18-1-2014Accepted date: 5-4-2014

Please cite this article as: Perez, A.A., Perez, G.A.,Preservation of original aragonitestructure in Desmophyllum castellolense (Scleractinia) from the Eocene Igualada Basin(NE Spain), Palaeoworld (2014), http://dx.doi.org/10.1016/j.palwor.2014.04.001

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Preservation of original aragonite structure in Desmophyllum castellolense (Scleractinia) from

the Eocene Igualada Basin (NE Spain)

Aureli Álvarez Pérez a, German Álvarez Pérez b, *a Departament de Geologia, Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona),

Spain [E-mail address: aureli.alvarez@uab.cat].b Departament d’Estratigrafia, Paleontologia i Ciències Marines, Universitat de Barcelona, Martí i

Franquès s/n, 08028 Barcelona, Spain [E-mail address: galvarez@xtec.cat].

* Corresponding author.

Abstract

Scleractinian corals produce an aragonite skeleton that is usually converted to calcite by diagenetic

processes after death. However, examples that have preserved the original aragonite skeleton and its

original microstructure have been found in the Igualada Basin (NE Spain). The species

Desmophyllum castellolense is described and analyzed. X-ray diffraction was used to confirm the

skeletal aragonite and analyze the associated elements. The microstructure of the skeleton is

described using polished and thin sections with the aid of scanning electron microscopy (SEM) and

optical microscopy. The description of this species is taken from Álvarez Pérez (1993, 1997).

Keywords: Scleractinia; Desmophyllum castellolense; Eocene; Aragonite; Microstructure

1. Introduction

Scleractinian corals, with few exceptions, produce an aragonite exoskeleton by chemical

precipitation of ectoderm. This precipitation is influenced not only in form but also in composition

by environmental conditions (Slotarski et al., 2007; Cuif et al., 2011).

On the death of the coral, the fossilization process preserves the aragonite skeleton or

transforms it into calcite. Most of the Eocene corals were fossilized as calcite.

In this paper, we confirm the existence of a scleractinid coral, Desmophyllum castellolense, in

the Eocene Igualada Basin, which has preserved its original aragonite skeleton and its

microstructure.

This study offers new insights into Eocene corals, particularly those of the Igualada Basin.

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2. Stratigraphy and setting

The Eocene Igualada Basin is located in the eastern sector of the Ebro Forelan Basin bounded

by the Central and Eastern Pyrenees to the north and the Catalan Coastal Ranges to the southeast

(Fig. 1A).

[Figure 1 here]

The sediments in the Igualada Basin consist of alternating marine and continental deposits

extending from the Ilerdian to the Priabonian (Fig. 2).

Different depositional sequences have been identified in the Igualada Basin. The Paleocene–

Eocene deposits have been divided from bottom to top into the following formal lithostratigraphic

units:

- Mediona Formation. This is composed of red mudstone layers with widespread paleosoil (late

Thanetian) (Anadon et al., 1983).

- Orpí Formation. This is made up of limestone beds rich in miliolids (early and middle Ilerdian)

(Hottinger, 1962).

- Pontils Formation. This is constituted by red mudstone, siltstone, sandstone, conglomerate and

limestone of lacustrine facies (late Ilerdian, Cuisian and Lutetian) (Anadón and Feist, 1981).

- Santa Maria Group:

- Collbàs Formation. This consists of siltstone, sandstone and conglomerate in the lower part,

marl in the middle part, and reefal limestone in the upper part (early Bartonian) (Serra-Kiel et

al., 1997).

- Igualada Formation. This has been divided into two units:

1) “Castellolí Deltaic Complex” is composed of conglomerate, sandstone and azoic marl

(middle and late Bartonian) (Serra-Kiel et al., 1997).

2) “Castellolí Marl and Limestone” is constituted by limestone with mud supported beds

rich in oysters, packstone and grainstone that was made up of larger foraminifers, and

locally, coral and coralline-algae build-ups interbedded with marl that contain larger

foraminifers, bryozoans and molluscs (early Priabonian) (Costa et al., 2009, 2013).

- La Tossa Formation. This is constituted by reefal limestone. In the other parts of the basin,

this unit contains marl, sandstone and conglomerate of deltaic facies interbedded with reefal

limestone (middle Priabonian) (Cascella and Dinarès-Turell, 2009; Costa et al., 2009, 2013).

- Artés Formation. This is composed of a variety of facies: siliciclastic continental and marine

materials, carbonate shallow-shelf beds, stromatolitic facies and evaporites (anhydrite, gypsum,

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halite and potash) (Taberner et al., 2002; Serra-Kiel et al., 2003a, b) (late Priabonian) (Anadón et

al., 1992).

[Figure 2 here]

3. Materials

The samples studied in this work were collected from the “Castellolí Marl and Limestone” in

the proximity of the following dwellings: “Can Manyoses”, “Can Francolí”, “Casa Nova” and “Can

Llucià” to the NE of Castellolí (Fig. 1B).

Most specimens underwent considerable pressure with broken septa (Fig. 3G).

The holotype of Desmophyllum castellonense is housed in the Geological Museum of the

Diocesan Seminar of Barcelona (MGSB) and is catalogued as N-52.081 (Fig. 3A, B).

The thin sections (P) and the samples studied belong to the G. Álvarez Pérez collection (GA).

[Figure 3 here]

4. Methods

4.1. The microscopy used

4.1.1. Located at Universitat Autònoma de Barcelona (UAB)

a) Scanning electron microscopy (SEM) JEOL, type 5SM-6300, equipped with EDAX to detect

the elements present in the sample.

b) Diffractometer of X-ray, type INCAR-OXFORD.

4.1.2. Located at Scientific and Technological Centres of the Universitat de Barcelona (CCiTUB)

c) Scanning electron microscopy (SEM) JEOL, type JSM-6510, equipped with EDS (energy

dispersive spectrometry).

d) Scanning electron microscopy (SEM) JEOL, type JSM-7100F, equipped with EDS.

4.2. Samples studied

Three samples of Desmophyllum castellonense were placed in acrylic resin to obtain polished

and thin sections. Polished sections were treated in 5% of HCl for 5 seconds before analysis using

spectrometer EDAX.

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4.3. Comparison of the results

The diffractogram of Desmophyllum castellolense was compared with that of the Cereiphyllia

tenuis, Eocene coral fossilized in calcite and with that of Acropora cervicornis, Recent coral with

an aragonite skeleton.

4.4. Identification of microstructural elements

Identification and subsequent description of microstructural elements were based on the works

of Alloiteau (1957), Wells (1967), Russo (1976), Lazier et al. (1999), Sorauf (1999), Risk et al.

(2002) and Cuif et al. (2011).

5. Fossilization

5.1. Coral in life

In life, the form and composition of the coral skeleton depend on the coral itself and on

environmental factors such as pH, microbial invasion, hydrolysis and oxidation. The skeleton is

perforated and degraded by numerous organisms (worms, sponges, bryozoans and algae etc.) (Cuif,

2010; Cuif et al., 2011).

In life, skeletal structures are protected by tissue and remain stable; but, after death, bioerosion

and chemical dissolution modify the shape and composition of the skeleton.

5.2. After coral’s death

5.2.1. Fossilization of the skeleton

After coral’s death, its aragonite skeleton:

- remains stable for the whole process of fossilization with the result that the skeletal

microstructures are preserved totally or partially (Fig. 3C).

- may be transformed into calcite as a result of the action of external diagenetic agents.

This transformation into calcite may occur:

- by dissolution: the dead coral is buried in situ by sediment deposited by external agents. This

sediment becomes compact as a result of diagenetic processes. As the aragonite coral skeleton

is dissolved, an empty space takes the outer form of the coral. This space remains empty or is

subsequently filled with calcite crystals. The skeletal microstructures are not preserved (Fig.

3E).

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- by neomorphism: the transformation of aragonite into calcite as a result of polymorphism in

the two minerals. The changes occur in the structure of the crystal passing from the

orthorhombic structure of aragonite to the trigonal structure of calcite. In this diagenetic

process, the molecules of aragonite are transformed one by one into the molecules of calcite.

This transformation is governed by the interaction of pressure and temperature as shown in the

diagram of equilibrium phases (Fig. 4). The microstructure of the coral is preserved totally or

partially (Giménez and Taberner, 1997).

[Figure 4 here]

5.2.2. The cavities and the gaps

The cavities of the corals and the gaps between them are filled with:

- micritic sediment of calcite and some celestine:

- calcite crystals (Fig. 6A).

- celestine crystals (Fig. 6B).

After studying the geographical distribution of sulphur, oxygen and strontium isotopes

Taberner et al. (2002) deduced that:

- calcite results from the partial dissolution of limestones and marls and from dissolution of

aragonite skeletons of the corals. Because of the pressure and the temperature of the

environment, calcium carbonate precipitates as calcite (Risk et al., 2002).

- celestine precipitated after the mixing of meteoric waters due to the dissolution of evaporites

rich in (SO42-) and marine fluids relative rich in (Sr2+) (Taberner et al., 2002).

Meteoric waters from the dissolution of the “Puig Aguilera” and “El Catarro” reefs transport

small ions (Sr2+) released when aragonite skeletons are dissolved.

6. Mineralogical analysis

6.1. Analysis with X-ray Diffraction

Qualitative analysis by means of X-ray Diffraction of a mixture of minerals enabled us to

ascertain whether each coral had spectres of minerals forming part of the mixture (van Meerssche

and Feneau-Dupont, 1973; Gay, 1977; Pitarch et al., 2011a, b). Accordingly, the diffractrogram was

compared with that of calcite (Fig. 5A) and that of aragonite (Fig. 5B).

[Figure 5 here]

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The minerals are identified in the diffractograms, with the maximum intensity as the reference

point (de Jong, 1967). Calcite corresponds to the angle 2θ = 29.399 (Fig. 5A) and aragonite to the

angle 2θ = 26.223 (Fig. 5B) (Barraud, 1960; Ducros, 1971; Klein and Hurlbut, 1997).

Three samples were selected for mineralogical analysis by X-ray Diffraction. The first sample

was the Acropora cervicornis, a Recent coral with an aragonite skeleton. The second sample was

Cereiphyllia tenuis, an Eocene coral, with a calcite skeleton. The third sample was Desmophyllum

castellolense, an Eocene coral, which we assume to be an aragonite skeleton.

In the diffractogram of Acropora cervicornis (Fig. 5C), the line of maximum intensity

coincides with that of aragonite. The corresponding line for calcite shows a relatively low intensity.

Aragonite is therefore the dominant phase in the sample.

In the diffractogram of Cereiphyllia tenuis (Fig. 5D), the line of maximum intensity coincides

with that of calcite. The line of aragonite is very weak, which suggests a residual presence in the

sample.

The diffractogram of Desmophyllum castellolense (Fig. 5E) shows the line of aragonite.

Nevertheless, the calcite line is prominent. Aragonite proceeds from the coral skeleton and calcite

from the sediment. It was not possible to remove the sediment in the preparation of the sample for

the X-ray study.

6.2. Energy Dispersive Spectroscopy (EDAX)

Scanning electron microscopy (SEM) equipped with EDAX (spectrograph of disperse energy)

enabled us to detect the elements present in the sample. Thus, the presence of carbonates and other

minerals was confirmed in the sample. Celestine was confirmed by detecting strontium (Sr), sulphur

(S) and oxygen (O) in the sample (Fig. 5F).

Foreign elements were not detected.

7. Description

Desmophyllum castellolense G. Álvarez Pérez, 1993

7.1. External morphology

Coral: solitary, from cone-like to cylindrical form (Fig. 3A). The young individuals are firmly fixed

to the substrate by a basal plate (Fig. 3K). Individual adults are free.

Calices: elliptical (Fig. 3B) or circular (Fig. 3D).

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Septa are hexamerally arranged in five complete cycles and one incomplete sixth cycle. Between 96

and 129 septa were counted. Their length and thickness depend on the series to which they belong

(Fig. 3D, G). Septa of the two first cycles are equal and are prominent in the calice. The upper part

of the septa is rounded owing to its fan-like arrangement of trabeculae that form the septum (Fig.

3H, I). The edges of the septa are smooth with a rhopaloid inner part (Fig. 3F). The lateral faces of

the septa of the two first cycles are smooth (Fig. 3I), whereas those of the remaining ones are

covered with small spiniform teeth (Fig. 3H, I).

Pseudo-columella: formed by the union of the inner edge of septa (Fig. 3D, I).

Costae: visible in the whole extension of the coral, well developed near the calice. They are a

continuation of the septa (costosepta) (Fig. 3A, J, K).

Wall: septothecal (Fig. 3D).

Dissepiments: lamellae in the lower part of the coral. These lamellae are between adjacent septa

(Figs. 3B, C, 7D).

Exotheca: finely granulated lamina that covers the outer space of the coral (Figs. 3J, 7E).

The samples studied do not exceed 5 cm in height. Calical diameter varies between 0.5 cm and 2.5

cm (Álvarez Pérez, 1993, 1997).

7.2. Microstructure

7.2.1. Microstructure of basic elements

The elementary components of skeletal coral are the aragonite crystals grouped in the following

structures:

Bundles of fibres (sclerodermites): constituted by a centre of calcification surrounded by aragonite

fibrous crystals (Fig. 6C, D).

Line of bundles (trabecula): formed by several bundles welded together (Fig. 6D).

Fibre layers (stereome): constituted by acicular aragonite crystals that are parallel without the

centre of calcification (Fig. 6E, F) (Risk et al., 2002; Cuif et al., 2011).

[Figure 6 here]

7.2.2. Microstructure of structural elements

Basal plate: The planula larva makes a basal plate from separate seed crystallites on which

aragonite fibre bundles nucleate (Fig. 3K) (Sorauf, 1999; Lazier et al., 1999; Cuif et al., 2011).

Septa: formed by:

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- the first structure formed by the union in the same plane of line of bundles (trabecula) that extend

upwards in the form of a fan, giving the septum a rounded appearance. This type of growth is

observed in the lateral face of the septum in the form of concentric rings (Fig. 3H, I). The dark

median line septum indicates that it is occupied by the base of the bundles, leaving no empty

spaces. The empty spaces in the median line of the septum may be occupied by calcite sediment

(Fig. 6A) or by celestite sediment (Fig. 6B).

- the second structure formed by the layers of fibrous crystals joined laterally to the previous

structure. These layers increase the thickness of the septum, giving it greater consistency. They

cover the septum and new bundles are added vertically (Fig. 7A).

Costae: a continuation of the septa formed by bundles joined to the outer edge of the septum and

occasionally covered with layers of exotheca (Fig. 7B).

Wall: constituted by bundles attached laterally to the neighbouring septa joining them together (Fig.

7B).

Endotheca: the layers of fibrous crystals of the septa could be extended to cover the inner part of the

wall, giving rise to endotheca (Fig. 7C).

Exotheca: formed by layers of fibrous crystals covering the costae and the outer part of the wall

(Fig. 7E).

Dissepiments: sheets formed by bundles covered with layers of fibrous crystals (Fig. 7D).

Pseudo-columella: formed in the centre of the calice by the union of some septa (Fig. 7F) (Sorauf,

1999; Lazier et al., 1999; Risk et al., 2002; Cuif et al., 2011).

[Figure 7 here]

8. Conclusions

Most Eocene corals of the Eocene Igualada Basin are fossilized in calcite.

Some corals have preserved the aragonite skeleton, as is the case with Desmophyllum

castellolense. This has allowed us to study their microstructure made up of bundles (sclerodermites

and trabecula) and layers of fibrous crystals (stereome).

Acknowledgements

We express our gratitude to Dr. Pere Busquets Buezo, professor of the University of Barcelona

(UB), for his constructive comments and to Dr. Javier García Veigas, researcher of the Scientific

and Technological Centres of the University of Barcelona (CCiTUB), for his technical help. We

also thank the help of the reviewers for improving this paper.

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Figures

Figure 1. (A) Geological location of the Igualada Basin, modified after Serra-Kiel et al. (2003b).

(B) Location of the Castellolí outcrop.

Figure 2. Lithostratigraphic units of the Igualada Basin

Figure 3. Desmophyllum castellolense. (A, B) holotype (MGSB N-52.081), (A) lateral view, (B)

calice view. (C) (GA 5974, P 568), aragonite skeleton, “co” for pseudo-columella, “d” for

dissepiment, “s” for septum. (D) (GA 5740), “co” for pseudo-columella, “w” for wall. (E) (GA

6441, P 565), “m” for micritic sediment, “sc” for aragonite septa replaced by calcite crystals. (F)

(GA 4383), Rhopaloid inner edge of the septum, “s” for septum. (G) (GA 4210), broken septa. (H)

(GA 5722.2), septal spiniform teeth face. (I) (GA 8043.3), “co” for columella, “s” for septal smooth

face. (J) (GA 5986), granulated exotheca. (K) (GA 12241), “bp” for basal plate.

Figure 4. Diagram of Pressure-Temperature of calcite and aragonite.

Figure 5. (A) Calcite diffractogram. (B) Aragonite diffractogram. (C) Diffractogram of Acropora

cervicornis, Recent coral with aragonite skeleton. (D) Diffractogram of Cereiphyllia tenuis with

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calcite skeleton. (E) Diffractogram of Desmophyllum castellolense, Eocene coral of the Igualada

Basin. (F) Desmophyllum castellolense (GA 4210), spectogram of low energy (EDAX).

Figure 6. Desmophyllum castellolense. (A) (GA 4210), “ca” for calcite, “s” for septum, “ml” for

median line of calcite. (B) (GA 5117), “ce” for celestite, “s” for septum, “ml” for median line of

celestite. (C) (GA 4254), bundle (sclerodermite), “a” for aragonite crystals. (D) (GA 4254), line of

bundles (trabecula), “b” for bundle (sclerodermite). (E) (GA 4254), fibre layers (stereome). (F) (GA

4254), enlarged part of (E), parallel crystals of aragonite are observed.

Figure 7. Desmophyllum castellolense (GA 5974, P 568). (A) Microstructure of a septum, “lb” for

layer of bundles (stereome), “ml” for median line of calcite, “t” for line of bundles (trabecula). (B)

Microstructure, “ca” for calcite, “cs” for costa, “ml” for median line of calcite, “s” for septum, “w”

for wall. (C) Microstructure, “e” for endotheca, “s” for septum. (D) Microstructure, “d” for

dissepiment, “s” for septum. (E) Microstructure of the exotheca. (F) Microstructure, “co” for

pseudo-columella, “s” for septum with dark median line.

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Figure 1

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Stage Group Formation Interval

EOCENE

PRIABONIAN

ARTÉS

SANTA MARÍA

LA TOSSA DELTAIC REEFAL

COMPLEX

IGUALADA

CASTELLOLÍ MARL AND LIMESTONE

CASTELLOLÍ DELTAIC COMPLEX

BARTONIAN COLLBÁS

LUTECIAN

PONTILS

CUISIAN

ORPÍ ILERDIAN

PALEOCENE THANETIAN MEDIONA

Lithostratigraphic units of the Igualada Basin

Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7