FORMATION OF LAMINATE TRAVERTINES AT BAGNO-VIGNONE, ITALY

Travertine at Bagno Vignoni, Tuscany, is deposited from thermal CO2-ri ch water up-welling from an artificial pool, Bagno Grande, which conta ins a thick benthic mat of cyanobacteria. The travertine mound, over 4 0 m in height, consists of calcite feather crystals and micrite. Over much of the mound, but particularly in the lower path the travertine i s finely banded, with pale bands of feather crystals alternating with dark bands containing micrite. Deposition rate measurements showed tha t the bands were formed daily, with the slightly narrower and darker b ands formed at night. These nocturnal bands also contained wind-borne dust. It is postulated that the dust caused heterogeneous nucleation o f micrite and reduced the development of feather crystals. The deposit ion rate of travertine ranged from 127 to 348 mu m/day and was highest al the top of the mound. Water analysis conducted over 27 h demonstra ted a pronounced diurnal variation in the dissolved carbon dioxide of Bagno Grande, consistent with the photosynthetic activity of the benth ic cyanobacteria. This variation could be traced to the base of the tr avertine mound but was not alone responsible for the higher deposition rate during the day. Water temperature on the mound also increased du ring the day and would have aided CO2 evasion. Photosynthetic activity on the mound itself appeared to have far less influence on the carbon dioxide pur than Bagno Grande. The upper mound supported unlithified mats of the cyanobacteria Lyngbya (Phormidium) laminosum and Spirulina labyrinthiformis. These mats sometimes possessed fine travertine lami nae resulting from cyanobacterium phototaxis and may have been respons ible for some lamination in the upper mound. The CO2 flux measurements indicated that overall, atmospheric evasion of carbon dioxide was mor e significant to travertine deposition than photosynthetic activity.