Earliest Steps of Diagenesis in Living Scleractinian Corals: Evidence from Ultrastructural Pattern and Raman Spectroscopy

Abstract

The precise understanding of diagenetic pathways occurring during fossilization of biomineralized skeletons is of critical importance for various related fields in both paleontology and sedimentology. In particular, this may have fundamental implications in paleoenvironmental studies for interpreting geochemical proxies extracted from these skeletons.The effects of the first diagenetic processes on both mineral and organic compounds present in skeletons of scleractinian corals were investigated in two different coral colonies (Lobophyllia corymbosa and Leptastraea purpurea) by combining mapping of skeletal ultrastructural patterns by SEM and in situ analysis by Raman microspectroscopy. The two elementary ultrastructural features typical of scleractinian corals, the centers of calcification and the sets of aragonite fibers, have been characterized in the uppermost parts (i.e. the ''living zone'') of both coral colonies. Diagenetic features affecting the skeleton itself and also represented by fibrous diagenetic cements have been analyzed in the oldest basal parts of each colony. All Raman spectra display several vibrational bands typical of aragonite (but no calcite). Additionally, several Raman bands indicate the presence of organic compounds, some of which likely characterize proteinous components, and provide the first in situ chemical record of the skeletal organic matrices. Tentative assignments to CH, CH 2 , CH 3 , NH 2 , CHO, CNC, and SO groups are proposed. In the ''living zone,'' some of these bands are different in the two coral species. In Raman spectra measured in the oldest skeletal parts of both colonies, these bands decrease markedly in number and diversity or even disappear, indicating that significant decay of intraskeletal organic matrices does occur in just a few years. Therefore, early diagenesis in these skeletons results from the interaction between the degradation of organic matrices and the diagenetic processes affecting the skeletal aragonite submicron-size crystals.

Intra-skeletal calcite in a live-collected Porites sp.: Impact on environmental proxies and potential formation process

“…Additionally, COCs are rich in organics that, besides the sub-micronic size of their aragonite crystals, makes them very quickly degradable (Perrin and Smith, 2007a;Perrin and Smith, 2007b). COCs are therefore the first part of the skeleton to be dissolved (Ayling et al, 2006;Gvirtzman et al, 1973;James, 1974;Perrin and Smith, 2007b;Rabier et al, 2008). In our study, some of the COCs are replaced by calcite and this intra-skeletal calcite was also observed in the first 5 mm of the colony that corresponds to the living tissue part.…”

“…This would favor the COC preferential dissolution, as suggested by Allison et al (2005b). Additionally, COCs are rich in organics that, besides the sub-micronic size of their aragonite crystals, makes them very quickly degradable (Perrin and Smith, 2007a;Perrin and Smith, 2007b). COCs are therefore the first part of the skeleton to be dissolved (Ayling et al, 2006;Gvirtzman et al, 1973;James, 1974;Perrin and Smith, 2007b;Rabier et al, 2008).…”

Reconstructing skeletal fiber arrangement and growth mode in the coral <i>Porites lutea</i> (Cnidaria, Scleractinia): a confocal Raman microscopy study

“…The CRM images (Fig. 3b) show the typical fan-like fibers (Pratz, 1882;Oglivie, 1896) for scleractinian coral skeletons using PLM (Fig. 3c).…”

“…To demonstrate what kind of chemical information can be obtained, we analyzed the spectra of some areas showing increased fluorescence in more detail (longer integration time) and compared the spectra to other studies using Raman spectroscopy on biominerals. Spectral lines not attributed to aragonite were detected ( Jolivet et al, 2008;Zhang et al, 2011), Amid III (1190-1310cm −1 , Jolivet et al, 2008Zhang et al, 2011), CH bands (2850-3080 cm −1 , Perrin and Smith, 2007;Jolivet et al, 2008) and OH-groups (3200-3400 cm −1 , Jolivet et al, 2008). Recent studies used Raman spectroscopy to characterize organic matrix in biominerals (Kaczorowska et al, 2003;Perrin and Smith, 2007;Jolivet et al, 2008; Nehrke and Nouet, 2011;), but coral Raman spectroscopic analyses covering a spectral region between 100-3200 cm −1 are quite rare.…”

“…Pratz, 1882;von Heider, 1886;von Koch, 1887;Oglivie, 1896). The first biomineralization concepts emphasize the resemblance of microstructural elements as purely inorganic crystals.…”