Геоложки анализ на три мраморни фрагмента със средновековни български надписи от IX век
Geological analysis of three marble fragments with medieval Bulgarian inscriptions from the 9th century

Summary/Abstract: Three marble fragments bearing medieval Bulgarian inscriptions from the 9th century, discovered at different locations in Shumen Province, Northeastern Bulgaria, were investigated. One of them has long been known to preserve part of the text of the 30-year peace treaty between Bulgaria and Byzantium. Epigraphic analysis suggests that the inscriptions are likely related and represent parts of the same text, referring to the same peace treaty. The aim of the present study is to assess whether the three fragments may represent parts of a single originally monolithic column. To address this question, an integrated analytical approach was applied, including isotopic-geochemical, mineralogical-petrographic, cathodoluminescence, and geochemical analyses. The isotopic data show a tight clustering of δ¹³C (+2 to +3‰) and δ¹⁸O (–2 to –1‰) values within a narrow field on the δ¹³C–δ¹⁸O diagram, indicating a common provenance and overlapping with the isotopic fields of several Eastern Mediterranean marble sources, including Proconnesos, Thasos 1 and 2, Paros 1 and 2, Hymettos, and Carrara. The mineralogical-petrographic characteristics of the investigated marble fragments are identical in all samples. The marbles are white to slightly greyish-white, medium-grained, calcitic in composition, with evenly distributed graphite bands along their length and a characteristic strong H₂S odor upon crushing and grinding. Their microstructural parameters (heteroblastic fabric with a maximum grain size – MGS of 2–3 mm) exclude several other potential sources with isotopic overlap, including Carrara, Hymettos, Thasos (Aliki), the Parian lychnites, as well as Bulgarian deposits from the Topolovgrad and Ilindentsi areas and North Macedonian marbles from the Sivec region. Cathodoluminescence images reveal a uniform, weak dark-blue luminescence – a diagnostic feature of a limited number of sources, including Proconnesian marbles. Geochemical results further support this provenance through low total rare earth element concentrations (ΣREE), similar PAAS-normalized REE distribution patterns with moderate to weak negative Ce anomalies, and ΣREE-Y ratio trends consistent with published data for Proconnesian marbles. High Sr concentrations and low Fe and Mn contents, together with their diagnostic ratios, fall within narrow ranges characteristic of Proconnesian signatures. The combined isotopic, petrographic, cathodoluminescence, and geochemical evidence demonstrates that all three columns originate from the Proconnesos quarries on Marmara Island. The nearly identical macroscopic appearance, the geometry of the graphite bands, the microstructural parameters, and the relatively homogeneous composition of the three inscribed marble fragments allow the hypothesis that the material derives from a single marble block—an unlikely scenario in the case of independent extraction events within the large and heterogeneous Marmara deposit. When considered together with the epigraphic context, these results support the conclusion that the three fragments most probably belonged to one originally monolithic column that was subsequently divided into separate pieces, each of which appears to have followed a distinct historical trajectory.

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