Origin of the gross morphology and internal texture of tufas of Shirokawa Town, Ehime Prefecture, southwest Japan

Abstract

Gross morphology and internal texture of tufa deposits in Shirokawa Town, Ehime Prefecture, vary due to local environments (including flow condition, substrate topography, and associated biota) within the tufa-bearing stream. Variations in gross morphologies of the tufa deposits include 1) tufa encrusting boulders, 2) plane slope, 3) mound, 4) terraces, and 5) cascade. These types are basically related to variations in the original topography although they share similarities in a local environment of strong water flow which is an essential condition for active tufa deposition. Laminated internal texture, the most striking feature of the Shirokawa tufa, normally develops in tufas colonized by cyanobacteria inhabiting the surface. Regular lamination is most commonly developed along the narrow water passage of the lower stream where strong water flow continues throughout the year. The laminated tufas consist of repetition of dense lighter-colored laminae and porous darker-colored laminae, which correspond to the summer and winter deposits, respectively. The lighter laminae are characterized by thick calcite encrustation on upward-growing filamentous cyanobacteria. The darker laminae also mainly consist of calcite encrustation of cyanobacterial filaments, but the porosity largely remains. The annual rhythm seen in the laminated texture can be explained by seasonal variation in abiotic calcite precipitation rate (large in summer and small in winter) which increases with water temperature, Ca²⁺ contents, and flow strength. Because of the well-defined chronological constrains, continuous deposits of laminated tufas can be excellent material for reconstructing terrestrial climates.