TYPOMORPHISM OF THE CHEMICAL COMPOSITION OF BIOGENIC CALCIUM CARBONATES WITHIN METHANE ANOMALIES ON THE BLACK SEA BOTTOM

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This work explores typomorphism of the chemical composition of biogenic calcium carbonates in shells within areas of methane anomalies formed by deep fluids in Black Sea shelf bottom sediments.

Sampling was conducted at the Pradniprovska structure, northwestern Black Sea shelf. Mytilus galloprovincialis shells were selected from 53 samples of bottom sediments to assay chemical composition in biogenic shell carbonates as a record of the mineral formation environment of benthic organisms.

Compositional data were obtained using spectrometric analysis from a Thermo Scientific ICP-AES with a CID or CMOS detector operating in the range of 167-820 nm and based on the dissolution of substances in a mixture of nitric, hydrochloric, and hydrofluoric acids at ≥ 125 °C. Quantitative characteristics of 23 elements in each sample were obtained.

The results were as follows: a) chemical elements in shells are distributed unevenly, and this unevenness is due to two reasons – the formation of biogenic carbonate of shells and the ability of carbonates to isomorphically capture some elements from the mineral-forming environment; b) quantitatively, three groups of elements can be distinguished: macroelements, mesoelements, and microelements.

The macroelements (Na, S, Mg, Sr, Fe, Si, Mn, P, K) that we classify as synergetic characterize the mineral formation environment. Among them, Na, Mg, Sr, Fe, Mn, K are isomorphically part of the structure of calcite or aragonite, and non-structural impurities captured during the process of biogenic carbonate agglutination (Si, S,P) are clearly distinguished.

The mesoelements(Ba, Al, As, Zn, Cu, Ti) include two subgroups. One (As, Zn, Cu) associated with sulfide mineral formation and paragenetic to biogenic calcite, characterizes the participation of typical crustal elements in mineral formation. The other connects with the agglutination of clay minerals in sediments(Al, Ti) or the isomorphism of the biogenic formation of calcium carbonate (Ba).

The microelements (Ni, Tl, Pb, Cr, V, Mo, Co, Se) are infrequent and unevenly distributed. The main group (Ni, Cr, Co, V)is probably due to the influence of the deep fluids (mantle elements).

These groups are typomorphic signs of biogenic carbonate formation since they were recovered in zones influenced by deep-seafluids, and, therefore, they serve as search indicators for such areas on the Black Sea shelf.