Current tufa sedimentation in a changing-slope valley: The River Añamaza (Iberian Range, NE Spain)

• Six-month monitoring of modern carbonate fluvial sedimentation (2007–2010)
• Analysis of factors controlling hydrochemical and sedimentological variations
• Hydrochemistry and tufa distribution are linked to river slope and springs location.
• The local flow conditions and biological substrate control facies deposition rates.
• Seasonal deposition rates and isotope values linked to temperature and discharge.

A three-year study of modern carbonate sedimentation was conducted through analysis of sedimentological and hydrochemical parameters measured every six months at 10 sites along a high-slope river in northeastern Spain (River Añamaza). Three stretches of the river were characterised. The dominant water inputs from the upstream karstic springs, primarily from the Jurassic rock aquifer, determined the SO4–HCO3–Ca composition of the river water. From this area, decreasing trends in alkalinity, calcium and total dissolved inorganic carbon occurred downstream in both the warm and cool periods as a result of calcite precipitation. Tufa thickness variations were consistent with such hydrochemical evolution. Deposition rates increased downstream, primarily where the gradient is steeper (middle stretch), and subsequently decreased at the downstream gently sloped stretch. Therefore, the slope along the river and the distance from the main upstream springs conditioned the spatial distribution of tufa deposits by determining the chemical characteristics of the water.The monitored sites represent four primary fluvial subenvironments with distinct sedimentary facies. The highest carbonate deposition rates were measured in fast-flow conditions (stromatolites, facies A). Deposits with calcite-coated algae and mosses (facies C) formed in cascades and small jumps with very fast-flowing water exhibited lower deposition rates and typical erosion. Lower rates corresponded to slow flow conditions in which thin stromatolite crusts and/or filamentous and calcareous algae and tufa fragments (facies B) accumulated. In slow-flowing water areas near springs (upstream stretch) calcite was absent, linked to the higher pCO2 of water; microbial films and aquatic flora composed the biofacies (facies D) in such subenvironment. Therefore, tufa deposition rates in each fluvial subenvironment were controlled by the amount of CO2 outgassing linked to local flow conditions, the biological substrate type and the chemical attributes of the water along each stretch.A six-month pattern of deposition rates, best marked in the steeper stretch, was recorded from thickness measurements (spring + summer periods = 4.30 mm; autumn + winter periods = 1.22 mm). Parallel variations were recorded using PWP (Plummer, Wigley, Parkhurst) and calcite mass balance calculations. Multivariate statistical analyses (HCA and PCA) clearly separated hydrochemical and sedimentological attributes in samples from warm and cool periods. Exceptions to this pattern were related to changes in the water discharge; high discharge events caused the dilution of the chemical composition and erosion of the tufa deposits. Therefore, temperature-dependent factors (i.e., physico-chemical and biological processes) coupled with discharge changes controlled the seasonal variations in deposition rates. The stable-isotope composition of calcite and river water also reflected the discharge effects. However, the seasonal variation signature of temperature was preserved in the δ18O of sediment.