# O/C Isotopic and EPR Signature of Marble from the Apuan Alps (Italy): A Critical Review
## 1 Introduction  
The use of the worldwide-famous Apuan Alps marble (AAM) as a dimensional and ornamental stone for outstanding sculptures, artefacts and cladding of buildings started with the Romans in the I century B.C. For two centuries, the use of marble from Luni (_Marmor Lunensis_), the Roman harbour from which the marble was shipped to the whole Empire, overrode all other marble across the Mediterranean area [1]. In the II century A.D., its use decreased, and it ceased in the V century with the fall of the Western Empire. In the XI century, the use of AAM started again and reached its fame in the Renaissance with [PERSON]'s outstanding artefacts [1, 2, 3, 4, 5, 6]. In archaeology and art-history, it is very important to define the type of marble, and the relative provenance, of a certain artefact or building for both knowledge and conservation purposes. Three different approaches can be used in determining the type of stone material:  
* Documental analysis carried out on commercial and historical text and archives that report the orders, origin and types of stone in the materials used. * Technical analysis including petrographic, mineralogical, geochemical and isotopic analyses allowing the attribution to a lithotype based on existing databases (DBs). * Expert evaluation carried out by professionals, based on their knowledge and regarding lithology, stone grain size, texture and warp. An attribution is reliable if more than one of these approaches point towards the same attribution. Where there is a lack of historical documents, defining the provenance of historical marble through only skilled expert judgment is not always an easy task; therefore, reliable technical analysis can help in the attribution. In the last several decades, the C and O isotopic ratio has emerged as a discriminating technical analysis suitable for the correct attribution of the provenance of marble [7; 8; 9; 10; 11; 12]. This analysis discriminates on the basis of the C/O isotopic ratio, a parameter that is established during the sedimentary process, is a function of the climate of that time and can be altered by metamorphism [11; 12]. As reported by the abovementioned authors, the factors in play are as follows:  
* The origin of the carbonate: chemical precipitate or organic mud. * The composition of water present during diagenesis and its later history. * The temperature of metamorphism and its thermal gradient. * Fractionation with pore waters and other mineral phases during metamorphism. A uniform isotopic composition can be obtained if the following criteria are met:  
* The protolith was deposited and underwent diagenesis in a uniform environment. * Isotopic equilibrium was attained and maintained during sedimentation or metamorphism. * The marble unit is homogeneous, preferably almost pure carbonate, and thick enough. * The metamorphic gradient was not too steep. The principal variations can be related to the following features:  
* The exchange between the carbonates and accessory silicates or oxides within the marble or near its contact with other formations. * A steep metamorphic gradient; a thermal gradient of 15 \({}^{\circ}\)C/km or less is not a problem, while a gradient towards 30 \({}^{\circ}\)C/km results into a great range in \(\xi\)18O values. * Significant weathering of the analysed sample. From these points, high constraints result for the correct attribution of a marble sample based solely on isotopic analysis; this implies the need for a deep knowledge of the geological history and context of the analysed materials before proposing reliable attribution. In the last decades, Electron Paramagnetic Resonance (EPR) investigations, exploiting the weak paramagnetic properties of marble, have also been profitably added to the pool of techniques able to aid in the determination of marble's origin [13; 14; 15].