FTIR spectroscopic features of the pteridosperm Ruflorinia orlandoi and host rock (Springhill Formation, Lower Cretaceous, Argentina)

Highlights

• Foliar remains from the Lower Cretaceous of Patagonia are chemically analyzed.

• A spectroscopic study of Ruflorinia orlandoi fronds and host rock is performed.

• FTIR data of three samples forms are evaluated for the first time through PCA.

• PCA shows aromatic compounds as constituents of cuticle biomacropolymers.

• Host rock is chemically similar to the compressions and liptinite macerals.

Abstract

Ruflorinia orlandoi (Pteridospermophyta) fronds are chemically analyzed for the first time by semi-quantitative Fourier transform infrared (FTIR) spectroscopy. This analysis allows the chemical characterization of the mesophyll and cuticle revealing the functional groups preserved in different frond parts (pinnae and rachis). The specimens collected in the Springhill Formation at the Río Correntoso locality (Lower Cretaceous, Santa Cruz province, Argentina) are compressions with very well-preserved cuticular features. The R. orlandoi remains (pinnae and rachis) are spectroscopically analyzed into two samples: compressions (Cp) and cuticles (Ct). Additionally, a third sample form from the host rock and named associated coal (V) is spectroscopically analyzed. Semi-quantitative data derived from Cp, Ct, and V spectra are evaluated by principal component analysis. The results indicate that Cp samples have a similar chemical composition whereas Ct samples show a greater variability. The latter could be related to intraspecific variability of foliar characters (e.g., trichomes and cuticular striations). Furthermore, Ct samples exhibit high contents of aromatic carbon groups suggesting a distinctive composition, likely including cutin/cutan biomacropolymers and/or phenolic compounds. Considering each specimen, the rachis shows a higher aromatic carbon content than pinnae as a consequence of the presence of more lignified tissues in the former. The V samples have the lowest relative intensity of aliphatic groups. On the other hand, the functional-group composition of R. orlandoi remains and V samples are compared with kerogen types and coal macerals showing a general chemical composition similar to type II kerogen. The latter is related to cuticles, spores, pollen grains, and resins.

Introduction

The Springhill Formation is a well-known hydrocarbon source in the subsurface of southern Argentina and Chile. Despite the fact that the Springhill Formation deposits have a wide distribution area, the outcrops in the Santa Cruz province, Argentina, are discontinuous and comprise few locations (Carrizo and Del Fueyo, 2015). Among these, the Río Correntoso locality is characterized by having abundant plant fossil remains mainly as compressions with well-preserved cuticles. They consist of fronds, simple to pinnate leaves, leafy branches showing different degrees of branching, and scale-like leaves belonging to Pteridospermophyta, Bennetittales, Ginkgophyta, and Coniferales (Baldoni, 1977; Carrizo et al., 2014; Carrizo and Del Fueyo, 2015; Carrizo et al., 2019a; Carrizo et al., 2019b; Lafuente Diaz et al., 2019).

Cuticular studies have been especially important for the identification of different plant groups, such as the case of Pteridopspermophyta through the establishing of the first true pteridosperms known from the Springhill Formation, Ruflorinia orlandoi (Carrizo et al., 2014). Moreover, two new records including Ruflorinia sp. and Pteridosperma sp. indet. emphasize the importance of the Mesozoic ferns-like taxa in this unit (Carrizo and Del Fueyo, 2015). Ruflorinia is a foliage genus which was established by Archangelsky (1963) based on plant fossil remains from the Anfiteatro de Ticó Formation (Baqueró Group, Lower Cretaceous) in the Santa Cruz province, Argentina. From this unit, three taxa were erected: Ruflorinia sierra Archangelsky, R. pilifera Archangelsky, and R. papillosa Villar de Seoane (Archangelsky, 1963; Archangelsky, 1964; Villar de Seoane, 2000). Additionally, the cupulate organ Ktalenia circularis Archangelsky and R. sierra foliage were found to represent parts of the same frond system, thus establishing Ruflorinia as a seed fern taxon belonging to Caytoniales (Archangelsky, 1963; Taylor and Archangelsky, 1985).

On the other hand, the cuticular studies have also been focused on the chemical analysis of diagenetically altered compounds preserved in the plant fossils (Holloway, 1982; Nip et al., 1986; Tegelaar et al., 1991; Kerp, 1990; van Bergen et al., 1994; among others). Thus, a wide variety of chemical analysis techniques have been applied to plant fossil studies, including Fourier transform infrared (FTIR) spectroscopy. Over recent decades, the FTIR spectroscopy has been employed to characterize the chemical composition (presence of functional groups) of plant fossil remains mainly from coalified compressions of Devonian, Pennsylvanian, and Triassic from Canada, Czech Republic, Spain, and Brazil, among others (e.g., Pšenicka et al., 2005; D'Angelo, 2006; Zodrow et al., 2009; D'Angelo et al., 2011; Zodrow et al., 2012; Matsumura et al., 2016; Vajda et al., 2017; D'Angelo and Zodrow, 2018; D'Angelo, 2019; Jardine et al., 2019). Plant fossil compressions from Argentina have also been studied using FTIR spectroscopy analyses, such as the cases of the Dicroidium flora from the Triassic of the Mendoza province (central-western Argentina), where Johnstonia coriacea Walkom, Johnstonia stelzneriana Frenguelli, and Dicroidium odontopteroides Gothan, and some other taxa were spectroscopically studied (e.g., D'Angelo, 2006; Zodrow et al., 2009; D'Angelo et al., 2011; D'Angelo and Zodrow, 2018). Recently, the FTIR spectroscopy analyses have been focused on plant fossil remains from the Lower Cretaceous of the Santa Cruz province including the podocarp Squamastrobus tigrensis Archangelsky and Del Fueyo, and the bennettitalean Ptilophyllum micropapillosum Lafuente Diaz, Carrizo, and Del Fueyo (Lafuente Diaz et al., 2018; Lafuente Diaz et al., 2019). In this contribution, different frond parts (pinnae and rachis) of the pteridosperm Ruflorinia orlandoi Carrizo and Del Fueyo from the Springhill Formation are chemically analyzed by semi-quantitative Fourier transform infrared (FTIR) spectroscopy, in order to obtain the first chemical characterization of the mesophyll and cuticle in leaves of this Patagonian taxon. In addition, the associated coal extracted from the host rock is spectroscopically analyzed.