Deciphering the origin of dubiofossils from the Pennsylvanian of the Paraná Basin, Brazil

Abstract

Abstract. Minerals are the fundamental record of abiotic processes over time, while biominerals are one of the most common records of life due to their easy preservation and abundance. However, distinguishing between biominerals and abiotic minerals is challenging due to the superimposition and repetition of geologic processes and the interference of ubiquitous and diverse life on Earth's surface and crust. Mineral dubiofossils, being potential outcomes of both abiotic and biotic environments, emerge as valuable entities that can contribute significantly to the understanding of this issue, facilitating the testing and refinement of biogenicity criteria. The aim of this contribution is to decipher the origin and history of branched mineralized structures that were previously considered mineral dubiofossils from the Pennsylvanian of the Paraná Basin, Brazil. While this material has different forms and refers to biological aspects, it is challenging to associate it with any known fossil group due to the overlapping geological processes occurring in a transitional deposit of Rio do Sul Formation (Itararé Group of the Paraná Basin), particularly in close proximity to a sill from the Serra Geral Group (Lower Cretaceous), which has undergone thermal effects. Given the absence of attributes essential for supporting the initial hypotheses proposing the material as a potential set of sponge spicules or a result of contact metamorphism in Pennsylvanian turbidites, the objects are now investigated as mineral dubiofossils. To address this challenge, we have developed a descriptive protocol for dubiofossils, building upon prior research in the field. This protocol evaluates the following aspects: (1) morphology, texture, and structure; (2) relationship with the matrix; (3) composition; and (4) context. This is done by assessing indigeneity and syngenicity and comparing the specimens with abiotic and biotic products. Applying this protocol to our samples revealed a wide range of morphologies with internal organization, predominantly composed of calcite with impurities such as iron, magnesium, aluminum, and oxygen. The inferred indigeneity suggests the presence of these minerals concurrently with or prior to the intrusion of the sill. Extensive comparisons were made between the studied samples and a broad spectrum of abiotic minerals, as well as controlled, induced, and influenced biominerals from similar contexts. These comparative analyses encompassed sponge spicules; sea urchin and algae skeletons; minerals induced or influenced by fungi, bacteria, and microbial mats; and inorganic pre- and synsedimentary–eodiagenetic minerals like evaporites, springs, and other precipitates, and mesodiagenetic–metamorphic crystals. Despite this comprehensive analysis, no hypothesis emerged as significantly more likely than others. The comparative analysis did allow us to exclude the possibility of the samples being controlled biominerals due to their patternless diversity of morphologies, as well as purely thermometamorphic in origin due to their branched elongated forms. The occurrence of these structures suggests a complex history: a syn-depositional or eodiagenetic origin of some carbonate or sulfate (gypsum, ikaite, dolomite, calcite, aragonite, siderite), potentially associated with the presence of microbial mats, which may have served as templates for mineralization and mediated mineral growth. Mesodiagenesis could have further modified the occurrence through processes such as mineral stabilization, agglutination, aging, and growth. However, the primary agent responsible for the formation of the dubiofossil was the Cretaceous intrusion, which dissolved and replaced the initial minerals, resulting in the precipitation of calcite. Throughout these steps, a combination of physical–chemical and biological reactions, influenced by intrinsic matrix characteristics, organic matter content, and distance from the intrusive body, may have contributed to the heightened morphological complexity observed, thus corroborating the origin of the material becomes even more challenging. Consequently, both the hypotheses pertaining to the formation of biotic and abiotic sulfates and carbonates remain plausible explanations, hence sustaining the classification of the material as a dubiofossil. This material illustrates how dubiofossils can be a result of a complex history and overlapping geological processes. It also highlights the difficulty in differentiating biominerals from abiotic minerals due to the scarcity of biogenicity arguments.