Current tufa sedimentation in a high discharge river: A comparison with other synchronous tufa records in the Iberian Range (Spain)
Abstract The results from sedimentological, isotopic and hydrochemical analyses of current tufa sedimentation conducted in a high-discharge river (Ebron River, northeastern Spain; 1.49 m 3 /s) through six-month monitoring over 3.5 years are discussed in terms of the factors that control local carbonate deposition through space and time, and compared with results from other synchronous tufa records in the same climatic domain. The findings allow for discerning the influence of the riverbed slope, hydrochemistry, discharge and groundwater inputs on tufa attributes and assess the significance of tufa as archives of certain climatic events on a regional scale. In the Ebron River, the dominant upstream karstic springs from a Jurassic-rock aquifer determined the river's HCO 3 –Ca composition. Two river stretches were differentiated according to localised increments in both pCO 2 , resulting from additional groundwater inputs, and SO 4 content, influenced by evaporite-bearing units. The variations in tufa's thickness through space were strongly controlled by CO 2 -rich springs and local slope variations. The monitored sites represent four primary subenvironments with distinct sedimentary facies, whose attributes suggest that 1) the tufa deposition rates in each fluvial subenvironment are mainly controlled by the CO 2 -outgassing intensity linked to local flow conditions and the biological substrate type, and 2) stromatolites represent the thickest and most complete record. The six-month variations in tufa thickness and calculated calcite mass in the Ebron River were controlled by temperature-dependent physico-chemical and biological parameters, coupled with high-discharge events that provoked tufa erosion. The smaller deposition of the Ebron River compared to two other synchronous tufa records in the Iberian Range is linked to 1) the absence of long areas of increased slope, 2) the occurrence of significant CO 2 -rich groundwater springs in the middle reach, and 3) the higher discharge and water depth. Certain high-discharge events were recorded as lower deposition rates concurrent in the three rivers. Moreover, anomalous water temperatures calculated from the calcite δ 18 O in the three rivers for a coincident time span support a regional anomaly in the precipitation δ 18 O. Thus, the evolution of the short-term tufa deposition rates and calcite δ 18 O composition through time can detect regional climate and hydrology changes and therefore can be robust criteria for correlation in the geological record.