TYPOMORPHISM OF THE CHEMICAL COMPOSITION OF BIOGENIC CARBONATES WITHIN THE PRADNIPROV METHANE ANOMALY ON THE BLACK SEA SHELF
Authors: Dikol O. S. 
Published: February 2024

Problem Statement and Purpose. The aim of the article is to study the typomorphism of the chemical composition of biogenic calcium carbonates within methane anomalies formed by deep fluids in the bottom sediments of the Black Sea shelf.
Gas chromatography was performed at the «Pradniprovska» study area. The results of the site`s bottom sediments analysis confirmed the content of pure methane and its homologues in the samples. Due to the presence of methane homologues, oxygen and carbon isotope analysis of the sediment shells was performed (it is known that the most stable information about the chemical composition of the mineral formation environment can be stored in biogenic carbonates of benthic organisms’ shells). The values obtained from the analysis had a small spread and did not show any visible variability.
Data & Methods. Data on the chemical composition of benthic organisms’ shells can be obtained using inductively coupled plasma atomic emission spectrometry.
This method requires large samples by weight, so only the shells of the Mytilus mollusks were taken for the study due to their relatively large size. The gas-geochemical studies were based on the methodology of the NDL‑3 laboratory. The extracted gas was stored in glass tubes under a saturated saline solution, which ensured minimal losses from gas dissolution in the seal fluid, followed by studying it using a «Tsvet‑400» gas chromatograph. The separation of hydrocarbon gases was carried out on a chromatographic column filled with aluminum oxide. The detector is flame ionization. Calibration was carried out before the start of each cycle with calibration gas mixtures of standard composition.
The spectrometric analysis was performed on a Thermo Scientific inductively coupled plasma atomic emission spectrometer with a CID or CMOS detector operating in the range of 167–820 nm.
The method is based on the dissolution of substances in a mixture of nitric, hydrochloric and hydrofluoric acids at a temperature of at least 125 °C, followed by the determination of elements in solutions by inductively coupled plasma atomic emission. To consider matrix effects and improve measurement accuracy, calibration using internal standard lines is used.
Results. The analysis of the obtained material allowed the author to draw the following conclusions: a) chemical elements in shells are unevenly distributed, and this unevenness is determined by two main reasons – the peculiarity of the formation of biogenic carbonate of shells and the ability of the carbonate to isomorphically capture a number of elements from the mineral-forming environment; b) according to the quantitative characteristics, three groups of elements can be distinguished: macroelements, mesoelements and trace elements. The first group of elements (macroelements) includes Na, S, Mg, Sr, Fe, Si, Mn, P, K. Among them, one can clearly distinguish elements that are isomorphically included in the structure of calcite or aragonite (Na, Mg, Sr, Fe, Mn, K) and elements captured as non-structural impurities in the agglutination process during the growth of biogenic carbonate (Si, S and P).
The second group (mesoelements) includes Ba, Al, As, Zn, Cu, Ti, among which two groups can also be distinguished. One is associated with the formation of sulfide minerals paragenetic to biogenic calcite (As, Zn, Cu), and the other with agglutination of clay minerals in sediments (Al, Ti), or a feature of isomorphism in the biogenic formation of calcium carbonate (Ba). The third group (trace elements) is associated with elements of low content and uneven distribution (Ni, Mo, Pb, Se). The main group here is Ni, Cr, Co, V. These groups, taking into account the fact that they were found in the areas of penetration of deep-sea fluids into the sedimentary strata, allows to consider them as typomorphic signs of biogenic carbonate formation in the areas of fluid influence, and, consequently, as a search sign of such areas on the Black Sea shelf.