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

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 (d 13 C) and oxygen (d 18 O) in the carbonate and organic carbon (d 13 C org ) were determined and compared with d 18 O of water and d 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 ( (Sayles & Curry, 1988). The d 13 C value of added DIC was )13AE6&, corresponding to the dissolution of CO 2 with d 13 C between )19AE5 and )23AE0& 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 d 13 C and decreasing d 18 O. The lack of covariation between d 13 C and d 18 O indicates that precipitation of calcite was not kinetically controlled for either of the elements. d 13 C and d 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.

Stable isotope composition of the Miocene Dinaride Lake System deduced from its endemic mollusc fauna

“…Despite the absence of a major river, we interpret the strongly negative oxygen values as expression of freshwater influx by springs or rivulets, lowering the elevated isotope level of the lake water. Modern rivers in the area have isotopically rather heavy d 18 O values of c. -8% and d 13 C DIC values of c. -12 to -13% (Lojen et al, 2004). These data fit excellently to the observed freshwater end-member data in the mollusc aragonite.…”

Equilibrium vs. kinetic fractionation of oxygen isotopes in two low-temperature travertine-depositing systems with differing hydrodynamic conditions at Baishuitai, Yunnan, SW China

“…As many studies have shown, isotopic equilibrium is rarely maintained under natural conditions during calcite precipitation (Gonfiantini et al, 1968;Friedman, 1970;Fouke et al, 2000;Lojen et al, 2004Lojen et al, , 2009Kele et al, 2008Kele et al, , 2011. In earth surface travertine depositing environments the situation is even more complex due to the intense CO 2 outgassing and high precipitation rates.…”

Current tufa sedimentation in a high discharge river: A comparison with other synchronous tufa records in the Iberian Range (Spain)

“…Therefore, coupled hydrochemical and sedimentological analyses are crucial to decipher the factors that control the variations in the tufa deposition's attributes through space and time. Moreover, the stable-isotope composition of the water and related sediment can record short-term climatic and hydrological changes on different time scales (i.e., seasonal, interannual and decadal); thus this type of analysis is useful to infer highresolution environmental changes (Lojen et al, 2004;Andrews, 2006;Osácar et al, 2013a, b).…”