Geochemistry of Quaternary travertines in the region north of Rome (Italy): structural, hydrologic and paleoclimatic implications

摘要

In the Tyrrhenian region of central Italy, late Quaternary fossil travertines are widespread along tow major regional structures: the Tiber Valley and the Ancona-Anzio line. The origin and transport of spring water from which travertines precipitate are elucidated by chemical and isotopic studies of the travertines and associated thermal springs and gas vents. These are consistent differences in the geochemical and isotopic signatures of thermal spring waters, gas vents and present and fossil travertines between east and west of the Tiber Valley. West of the Tiber Valley, δ~(13)C of CO_2 discharged from gas vents and δ~(13)C of fossil travertines are higher than those to the east. To the west the travertines have higher strontium contents, and gases emitted from vents have higher ~3He/~4He ratios and lower N_2 contents, than to the east. Fossil travertines to the west have characteristics typical of thermogene (thermal spring) origin, whereas those to the east have meteogene (low-temperature) characteristics (including abundant plant casts and organic impurities). The regional geochemical differences in travertines and fluid compositions across the Tiber Valley are interpreted with a model of regional fluid flow. The regional Mesozoic limestone aquifer is recharged in the main axis of the Apennine chain, and the groundwater flows westward and is discharged at springs. The travertine-precipitating waters east of the Tiber Valley have shallower flow paths than those to the west. Because of the comparatively short fluid flow paths and low (normal) heat flow, the groundwaters to the east of the Tiber Valley are cold and have CO_2 isotopic signatures, indicating a significant biogenic contribution acquired from soils in the recharge area and limited deeply derived CO_2. In contrast, spring waters west of the Tiber Valley have been conductively heated during transit in these high heat-flow areas and have incorporated a comparatively large quantity of CO_2 derived from decarbonation of limestone. The elevated strontium content of the thermal spring water west of the Tiber Valley is attributed to deep circulation and dissolution of a Triassic evaporite unit that is stratigraphically beneath the Mesozoic limestone. U-series age dates of fossil travertines indicate three main periods of travertine formation (ka): 220-240, 120-140 and 60-70. Based on the regional flow model correlating travertine deposition at thermal springs and precipitation in the recharge area, we suggest that pluvial activity was enhanced during these periods. Our study suggests that travertines preserve a valuable record of paleofluid composition and paleoprecipitation and are thus useful for reconstructing paleohydrology and paleoclimate.