Biotic Contributions to the Formation of Submarine Iron-rich Hydrothermal Crusts near the Panarea Island, Tyrrhenian

Introduction: Recent volcanism and associated hydrothermal activity of the Aeolian Arc (SE Tyrrhenian Sea, Italy) have been studied for the past 30 years [1,2]. The discovery of metalliferous hot spring deposits in the Pacific in the 1960’s stimulated exploration for this type of deposit in the Tyrrhenian Sea [3,4] because there they occur at much shallower depths. Hydrothermal systems in the Tyrrhenian Sea have produced a wide range of iron oxyhydroxide and iron sulfide deposits. These deposits are almost purely hydrothermal in origin, with negligible contribution from diagenetic processes. In addition, the Tyrrhenian submarine hot spring systems are a habitat for thermophilic chemosynthetic microbial communities. As such they are an excellent analog model-system for microbial communities associated with iron cycling in young geological formations, as well as for ancient geological formations, such as banded-iron formations (BIF). BIF formation is still a subject of debate, and new evidence suggests a polygenetic origin with possible microbial involvement [5]. Based on iron isotope fractionation, involvement of phototrophic iron oxidizers and possibly iron reducers has been proposed. Nonetheless, similar isotope values could be obtained through abiotic reactions as well [6]. Biogenic iron oxides represent a potential tool in the search for past and present life signatures on Earth and other planetary systems. This is of particular significance for Mars, because iron cycling may have been an important aspect of the planet’s geochemistry. Although the role of microbes in iron cycling and the formation of iron deposits has been the focus of a number of studies over the past two decades, it is still not fully understood whether they were simply beneficiaries of iron depositing processes, or whether they were actively “driving” iron deposition. More research is needed in order to fully assess the significance of microorganisms for iron depositing processes. The objective of our study is the integration of morphological evidence of microbial features with geochemical data in order to establish that microorganisms played a critical role in the formation of the described iron accumulations. Site description and materials: The samples used in this study were retrieved from a depth of 100215m in an area affected by hydrothermal activity, during a CNR (Consiglio Nazionale delle Ricerche) cruise in 1995. The samples consist mainly of iron oxyhydroxide deposits. The latter are of the fragmental-brecciated type, as well as of the encrusting type, and are intimately associated with vulcanoclastic sands and tuffaceous material. All samples are related to volcanic activity that started 0.2-0.1 Ma ago. Typical temperature ranges of hydrothermal active locations in the study area are around 100°C and somewhat above, with slightly acidic to circum-neutral pH (around 5.5 and 6) [7]. Unlike other hydrothermal iron-crusts from the study area that have been described as Manganese (Mn) enriched [7], our samples contain little or no Mn. Methodology: Light microscopy and Scanning Electron Microscopy (SEM) with EDS (x-ray spectra) were utilized for characterization of mineral assemblages and detection of biomorphic structures. Luminescence microscopy was utilized to visualize distribution of organic matter within crust-samples. Samples analyzed for Total Organic Carbon (TOC) and δC of organic matter were homogenized and acidified to remove inorganic carbon prior to analysis, with necessary precautions to prevent contamination. To determine TOC and δC we used a Costech Elemental Analyzer with zero-blank autosampler that was in-line with Finnigan Delta Plus XP mass-spectrometer.