Bajnóczi Bernadett, Horváth Zoltán, Demény Attila, Mindszenty Andrea
Institute for Geochemical Research, Hungarian Academy of Sciences, 1112, Budapest, Budaörsi út 45, Hungary.
Isotopes Environ Health Stud. 2006 Dec;42(4):335-50. doi: 10.1080/10256010600991045.
Calcrete nodules and concretions in unusually large amounts are embedded in the Quaternary clay-rich (Vertisol-type) 'red clay' soil-sedimentary complex at the pediment of the Mátra Mountains (Hungary). Stable isotope signatures were studied in nodules and septarian concretions, uncommon due to their several millimeter sized calcite crystals filling voids and fractures, to reveal their origin. The isotope composition of calcrete covers a wide range: delta18O=-5.9 to-10.4 per thousand and delta13C=-8.9 to-12.3 per thousand (vs. V-PDB). Isotope compositions support pedogenic (sensu stricto) and/or shallow groundwater origin for the calcrete nodules and concretions, the role of 'evolved' (isotopically modified) groundwaters in the formation of secondary carbonate was possibly subordinate. Late-stage, large, Mn-rich euhedral calcite crystals in concretions have the lowest delta13C values, which are interpreted as a result of larger contribution of isotopically light organic carbon due to decomposition of organic matter under reducing conditions. Precipitation of late calcite crystals in concretions occurred in early diagenetic environment after shallow burial of the 'red clay' paleovertisol.
在匈牙利马特劳山山前的第四纪富粘土（变性土类型）“红粘土”土壤沉积复合体中，嵌入了数量异常多的钙质结核和硬结核。对结核和晶洞状硬结核进行了稳定同位素特征研究，这些结核和硬结核并不常见，因为其几毫米大小的方解石晶体填充了孔隙和裂缝，以揭示它们的成因。钙质结核的同位素组成范围很广：δ18O = -5.9‰至-10.4‰，δ13C = -8.9‰至-12.3‰（相对于V-PDB）。同位素组成支持钙质结核和硬结核的成土（严格意义上）和/或浅层地下水成因，“演化”（同位素改性）地下水在次生碳酸盐形成中的作用可能较小。硬结核中晚期的、大的、富含锰的自形方解石晶体具有最低的δ13C值，这被解释为在还原条件下由于有机质分解，同位素较轻的有机碳贡献较大的结果。硬结核中晚期方解石晶体的沉淀发生在“红粘土”古变性土浅层埋藏后的早期成岩环境中。
