THE OCCURRENCE OF CALCAREOUS TUFA (METEOGENE TRAVERTINE) ON THE IZHORA PLATEAU, NORTHWESTERN RUSSIA

Freshwater carbonate deposits known as travertine and tufa are found in a wide variety of geographic and climatic settings, but their deposition is typically confined to interglacial periods. Bound by the Baltic-Ladoga glint, the Izhora Plateau is an upland region south of the Gulf of Finland and marks one of the northernmost localities (59.6° N) of such deposits. There, calcareous tufa facies associated with lacustrine, paludal, and riverine settings are common within spring-fed, linear depressions that direct the modern course of the Izhora River. This association led M.Yu. Nikitin to hypothesize a neotectonic control on local tufa deposition, perhaps associated with the retreat of the Scandinavian Ice Sheet. Initial palynological and malacofaunal studies constrained the most active phase of deposition between the Boreal and Atlantic stages of the early to middle Holocene. Subsequent U-Th and ¹⁴C dating of microdetrital tufa and underlying peat corroborated this time frame, but a precise geochronological framework is still lacking. To elucidate the origin and development of tufa deposits, we obtained stable isotope (C and O) data across a 2-meter section of microdetrital (lacustrine) tufa. Carbon isotope values range from -7 to -9 (per mil V-PDB), consistent with biomediated precipitation from meteogene (primarily soil-sourced CO₂) fluids. Oxygen isotope values range from -13 to -11 (per mil V-PDB) and increased with time, consistent with climatic warming from ~9.5–7 ka. Active formation of calcareous tufa today is limited to several small streams and artificial ponds, but these sites provide a modern analogue to interpret isotopic trends. Temperature and d18O values of spring waters do not vary seasonally, but reflect annual air temperature and precipitation, making this site suitable for paleoclimate reconstructions. We argue that while tufa deposition occurred during the wettest interval of the Holocene, climatic changes did not contribute significantly to its decline. Postglacial reactivation of faults likely influenced the hydrogeological situation on the plateau and may even have enhanced the calcite saturation state of spring waters by providing an endogenic source of CO₂. This factor partially explains the mid-Holocene decline of tufa/travertine deposits, especially at high latitudes.