C and O stable isotope variability in recent freshwater carbonates (River Krka, Croatia)

Abstract

Abstract Three types of recent carbonate precipitates from the River Krka, Croatia, were analysed: (1) bulk tufa from four main cascades in a 34 km long section of the river flow through the Krka National Park; (2) a laminar stromatolite‐like incrustation formed in the tunnel of a hydroelectric power plant close to the lowest cascade; and (3) recent precipitates collected on artificial substrates during winter, spring and summer periods. Stable isotope compositions of carbon (δ 13 C) and oxygen (δ 18 O) in the carbonate and organic carbon (δ 13 C org ) were determined and compared with δ 18 O of water and δ 13 C of dissolved inorganic carbon (DIC). The source of DIC, which provides C for tufa precipitation, was determined from the slope of the line ([DIC]/[DIC 0 ]−1) vs. (δ 13 C‐DIC × ([DIC]/[DIC 0 ])) ( Sayles & Curry, 1988 ). The δ 13 C value of added DIC was −13·6‰, corresponding to the dissolution of CO 2 with δ 13 C between −19·5 and −23·0‰ Vienna Pee Dee Belemnite (VPDB). The observed difference between the measured and calculated equilibrium temperature of precipitation of bulk tufa barriers indicates that the higher the water temperature, the larger the error in the estimated temperature of precipitation. This implies that the climatic signals may be valid only in tufas precipitated at lower and relatively stable temperatures. The laminar crust comprising a continuous record of the last 40 years of precipitation shows a consistent trend of increasing δ 13 C and decreasing δ 18 O. The lack of covariation between δ 13 C and δ 18 O indicates that precipitation of calcite was not kinetically controlled for either of the elements. δ 13 C and δ 18 O of precipitates collected on different artificial substrates show that surface characteristics both of substrates and colonizing biota play an important role in C and O isotope fractionation during carbonate precipitation.