Occurrence and mechanism of precipitation of lacustrine carbonates

Abstract

Lake sediments have been attracting the growing interest from the various fields of geosciences for the last decades. This development of lacustrine sedimentology greatly owes to the fact that hydrocarbon source rocks have been found in the freshwater lacustrine deposits. Lacustrine study has also been stimulated by the recent growing interests and concern on the global climatic change and environmental crisis; nonglacial annual layers (varves) of lake sediments reflect the changing climatic conditions, providing high-resolution time series data. Lake sediments are also important as a sink of atmospheric CO₂. Profundal facies of northern temperate lakes are mostly composed of fine-grained calcite, forming nonglacial varve laminations. Lacustrine carbonates ranges in composition from low-Mg calcite to magnesite through high-Mg calcite, dolomite, hydromagnesite, nesquehonite, and even aragonite, reflecting the wide variations in salinity and Mg/Ca ratio of lake waters. Accumulation rate of lacustrine carbonates is estimated to be, at least, 1000mm per 1000years. This figure is similar to that of rapidly growing coral reef limestones. Precipitation of inorganic, lacustrine carbonates has been explained to be caused by a drastic increase of pH, which in turn, has been caused by a decrease in dissolved CO₂ and/or HCO₃^- through photosynthesis of blue green algea and diatoms. Thus the precipitation of inorganic carbonates in lake environments is strongly linked with bioactivities and is called as bio-induced carbonates.