Sample description and preparation

The samples analysed in this work are listed in Table 1. The first two are fossil shells composed of aragonite, a metastable phase of CaCO₃, while the others are skeletal fossils of organisms (coral, sponge and algae) and microbialites. All samples are made of CaCO₃, calcite or aragonite, with some silicates in the case of the microbialites. A calcite mineral rock of abiotic origin is also included for comparison. The estimated geological ages of the samples are also reported in Table 1 where the beginning and the end of each period/epoch are those established by the International Commission on Stratigraphy (ICS) deputed to the terrestrial stratigraphy on a global scale (Ogg et al. Reference Ogg, Ogg and Gradstein2008).

Table 1.

Studied samples listed in order of confidence of biological origin

In the last column, we report the types of micromorphology of each sample (DP, defined pattern; MM, massive micromorphology).

The fossil shells of two marine gastropod molluscs, already studied by Orofino et al. (Reference Orofino, Blanco, D'Elia, Licchelli, Fonti and Marzo2010), have been collected in clay deposits located at two different sites, about 30 km apart, in the Salento Peninsula, Italy. The Xenophora gastropod comes from a Pleistocene deposit and in particular, in relation to stratigrapy, from a whitish calcarenite with an upper greenish clayey–sandy interval, having an elevated paleontological interest for the abundance of the fossil associations with excellently preserved taxa.

The Ampullinopsis crassatina, that biostratigraphical indications allowed us to assign to the late Oligocene, was collected from a lignite clayey deposit, overlapped on a blanket of mineralized residual deposits (bauxitic residual deposits) resting on the local carbonatic basement.

The samples S/L, S1A and U2 are skeletal organisms (coral, sponge and algae, respectively) embedded in microbial carbonates such as sample S/L(M). They have been selected within three rock specimens that developed, in time, in two distinct palaeoecological conditions, characteristic of Alpe di Specie and Punta Grohmann carbonate outcrops, in the Dolomites, Italy.

The S/L and S1A samples were extracted from two different Alpe di Specie rock samples in which skeletal organisms (Tubiphytes, skeletal cyanobacteria, sphinctozoan and inozoan sponges, etc.) represent a minor component of the rock (usually <40%). The composition is dominated by the micritic fraction (about 60%), mainly represented by autochthonous micrite (microbialite), with subordinate amounts of micrite interpreted as detrital (allochthonous micrite) (Russo et al. Reference Russo, Neri, Mastandrea and Laghi1991; Tosti et al. Reference Tosti, Guido, Demasi, Mastandrea and Russo2011, Reference Tosti, Guido, Mastandrea, Demasi and Russo2012, Reference Tosti, Mastandrea, Guido, Demasi, Russo and Riding2014). The microbialites or autochthonous micrites, which may exhibit both dense microcrystalline (aphanitic) or peloidal microfabric, are sometimes organized in stromatolitic laminae or thrombolitic fabric.

The U2 was selected from a rock sample collected in the basinal section cropping out at the base of Punta Grohmann mountain, belonging to the Sasso Piatto Massif, in the province of Bolzano, Italy. The analysed sample belongs to the Punta Grohmann buildups. These buildups represent the last records of the bioconstructions that first appeared in the Late Pennsylvanian. They are characterized by subcentimetre skeletons (mainly calcified microbes, Tubiphytes and other problematica, small sponges) intimately associated with microbialites and cements. The organic-induced nature of microbialite was supposed on the base of micromorphological and biogeochemical evidence (Russo et al. Reference Russo, Neri, Mastandrea and Baracca1997; Tosti et al. Reference Tosti, Guido, Demasi, Mastandrea and Russo2011, Reference Tosti, Guido, Mastandrea, Demasi and Russo2012, Reference Tosti, Mastandrea, Guido, Demasi, Russo and Riding2014). Their biotic origin has been confirmed by Blanco et al. (Reference Blanco, Orofino, D'Elia, Fonti, Mastandrea, Guido and Russo2013, Reference Blanco, D'Elia, Orofino, Mancarella, Fonti, Mastandrea, Guido, Tosti and Russo2014) with independent methods.

The stromatolitic sample GE, Upper Jurassic in age (Tithonian, 151–146 Ma) was collected from Thüste Quarries, south of Hanover, Germany. In this area, stromatolites developed in stressed environments, probably represented by a lagoonal setting, with alternate deposition of oolitic limestone and evaporites (Jahnke & Ritzkowski Reference Jahnke and Ritzkowski1980).

In order to obtain fine particulate materials for SEM morphological analyses, the fossil shells (Xenophora and A. crassatina) and the mineral calcite were ground with a mechanical mortar grinder and then the size fraction between 20 and 50 µm was sieve selected for our investigation. The other specimens, instead, were collected from specific areas of the rock samples using a thin drill bit. For instance, in Fig. 1 the original rock sample is shown along with the areas where the powders, pertaining to S/L skeletal organism and S/L(M) microbialite, were picked up.

Fig. 1.

The rock sample where the skeletal grains of the coral (S/L) and microbialite S/L(M) particles have been picked up. The specific areas are shown by white rectangles.

The grains thus obtained were comparable in size to those derived from sieve selection as derived by SEM observation. Double-stick and electrically conductive adhesive carbon tabs were pressed to the surface of pin stub mounts of aluminium allowing the adhesion of diced silicon wafers (5 × 5 mm²). Afterwards the particulate materials were placed in small glass vessel containing ethyl alcohol and introduced in an ultrasonic unit (Elma Transsonic 890/H) for about 30 s to separate any agglomerates of grains. Using a pipette, a drop of this suspension was added to the silicon matrix. After the evaporation of the alcohol, the grains of the different samples remained attached to the matrix and then, since specimens viewed using a conventional SEM need to be coated with electrically conductive material, we sputtered the stubs with a nanometric gold layer using an Emitech K550X Sputter Coater. Finally, the various stubs were placed inside the SEM chamber in high vacuum mode and morphologically analysed with the secondary electron detector.

The mineral composition, reported in Table 1, has been determined using both the IR spectroscopy and the energy dispersive X-ray elemental analysis performed in our laboratory on all samples (for details see Blanco et al. Reference Blanco, D'Elia, Orofino, Mancarella, Fonti, Mastandrea, Guido, Tosti and Russo2014).

SEM analysis and conclusions

The morphological analysis has been done using a SEM JEOL JSM 6480LV, equipped with an iXRF Systems EDS Sirius SD spectrometer for the elemental composition.

In Figs 2 and 3, typical SEM images of particles of the two fossil shells are reported, while in Fig. 4 the micrograph of mineral calcite grains is shown. It is evident the well-defined crystal pattern of the shells particles compared with the massive micromorphology of the mineral crystals imaged at the same scale.

Fig. 2.

SEM image of the Xenophora particles.

Fig. 3.

SEM image of the Ampullinopsis crassatina particles.

Fig. 4.

SEM image of abiotic calcite mineral particles.

Similar morphologies at micrometric scale are observable in Fig. 5(a) and from Figs 6 to 9 for the skeletal organisms (samples S/L, S1A and U2), microbialites and stromatolites (samples S/L(M) and GE, respectively). Except for the case of S1A (sponge), in which all the particles have the same crystalline structure (see Fig. 6 and last column of Table 1), in the other samples some grains exhibit a well-defined pattern (labelled DP in Table 1), while others show a massive micromorphology (labelled MM) as the abiogenic calcite crystals. As an example, we can refer to the difference between panels (a) and (b) in Fig. 5 (S/L sample) and the comparison between Figs 5(b) and 4 (calcite) that indicates an analogous arrangement.

Fig. 5.

SEM images of the coral particles (samples S/L). Some particles show a well-defined pattern (a) while others a massive micromorphology (b).

Fig. 6.

SEM image of the sponge particles (sample S1A).

Furthermore, we note that within those biogenic samples showing a well-defined pattern, as evident, the biogenic samples can be divided in two categories: those having a ‘laminar’ pattern (coral and sponge, Figs 5(a) and 6) similar to those of the fossil shells and those having a granular pattern of small crystals (algae, microbialites and stromatolites, Figs 7–9). Even if such differences could be ascribed to a diverse degree of complexity of the involved forms of life, we tentatively speculate that they can be linked to the diverse type of biomineralization. In fact, microorganisms are remarkably adept at forming mineral phase and this process can occur in two different ways. The first involves mineral precipitation in the open environment, without any apparent control by the cell over the mineral product. This process was defined by Lowenstam (Reference Lowenstam1981) as ‘biologically induced biomineralization’, with mineral forming only as a by-product of the cells metabolic activity or through its interaction with the surrounding aqueous environment (Konhauser Reference Konhauser2007). By contrast, the second way, ‘biologically controlled biomineralization’, is completely regulated, allowing the organisms to precipitate mineral that serve some physiological purpose. This process is specifically designed to form minerals through the development of intracellular (within the cytoplasm) or epicellular (on the cell wall) organic matrices, into which specific ions are actively introduced and their concentrations controlled in order to achieve the right mineral saturation states. Since the mineralization site is isolated from outside the cell by a barrier through which ions cannot freely diffuse, minerals can form anyway, in spite of thermodynamically unfavourable external conditions (Konhauser Reference Konhauser2007).

Fig. 7.

SEM image of the algae particles (sample U2).

Fig. 8.

SEM image of the microbialite particles (sample S/L(M)).

Fig. 9.

SEM image of the stromatolites particles (sample GE).

To investigate the distribution of these different crystal patterns of particles, we have morphologically examined more than 50 grains of each sample and we have summarized the results of this analysis in the last column of Table 1.

It is worthwhile to note that these results cannot be conclusive although they give precise indications on the main differences between the crystal micromorphologies of biogenic and abiogenic carbonates. This means that, in order to reach meaningful conclusions, we need to analyse the morphology of other samples as well as a statistically significant number of particles. Furthermore, special attention should be given to diagenesis (Tucker & Wright Reference Tucker and Wright1990), namely the chemical and physical post-depositional transformation processes that alter the original chemistry and microstructure of the carbonate minerals, producing a uniform final product difficult to discriminate between biotic versus abiotic origin. However, we think that the effort towards this line of research may possibly provide a method useful for discriminating the origin of Martian carbonates with the aim to characterize the surface and near-subsurface of the red planet in search for extraterrestrial life.