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Understanding the process of CO 2 degassing during precipitation of calcite from a Ca2+–HCO 3 − solution is crucial for interpreting isotope compositions in the calcite precipitates. Unlike diffusion-controlled outgassing, i.e., dissolved CO 2 escaping from the solution via diffusion, degassing caused by precipitation of calcite is accompanied by a large carbon isotope fractionation between CO 2 (g) and HCO 3 − due to breaking of C O bond, with an equilibrium fractionation of ca. −9‰ at ambient temperature. Such a magnitude of fractionation has a great influence on carbon isotope compositions (δ13C) of DIC (dissolved inorganic carbon) reservoir in the solution and thus on δ13C of calcite precipitated from it. However, knowledge on isotope fractionation is limited during precipitation-associated degassing of CO 2 in a supersaturated solution where rapid calcite precipitation drives CO 2 degassing out of isotopic equilibrium. Here we show the data of water chemistry and carbon isotope compositions of DIC and carbonate precipitates in a mountain stream at Baishuitai, China. Results from numerical models show there exist large degrees of carbon isotope disequilibrium during precipitation-associated degassing of CO 2 via HCO 3 − dehydration and dehydroxylation. Average carbon isotope fractionation between CO 2 (g) and HCO 3 − ( ε C O 2 (g) - HC O 3 - ) calculated from our dataset is about −20‰ which is much lower than the equilibrium value. Moreover, an inverse correlation between ε C O 2 (g) - HC O 3 - and precipitation rate was observed, indicating higher precipitation rates cause ε C O 2 (g) - HC O 3 - farther from equilibrium value. By compiling the data from this study and literatures, we infer that disequilibrium isotope fractionation of carbon between CO 2 (g) and HCO 3 − may be common during the growth of speleothem and travertine from a solution supersaturated with respect to calcite. The rate dependence of ε C O 2 (g) - HC O 3 - has special implications for speleothem archives from ventilated caves. As partial pressure of CO 2 in cave atmosphere evolves with ventilation, variable precipitation rates of calcite will cause inconstant degrees of disequilibrium isotope fractionation between CO 2 and DIC and thus perturb the time-series of speleothem's δ13C records for paleo-environmental reconstruction. [ABSTRACT FROM AUTHOR]

Biological carbon pumping (BCP) is a key process in which dissolved inorganic carbon in terrestrial aquatic ecosystems is utilized by aquatic autotrophs for photosynthesis and transformed into autochthonous organic matter (AOC). However, the mechanisms underlying BCP and the amount of generated AOC deposited effectively, are still poorly understood. Therefore, we conducted a systematic study combining modern hydrochemical monitoring and a sediment trap experiment in Fuxian Lake (Yunnan, SW China), the second-deepest plateau, oligotrophic freshwater lake in China. Temperature, pH, EC (electrical conductivity), DO (dissolved O2), [HCO3−], [Ca2+], SIc, partial CO2 (pCO2) pressure, and carbon isotopic compositions of HCO3− (δ13CDIC) in water from Fuxian Lake all displayed distinct seasonal and vertical variations. This was especially apparent in an inverse correlation between pCO2 and DO, indicating that variations of hydrochemistry in the lake water were mainly controlled by the metabolism of the aquatic phototrophs. Furthermore, the lowest C/N ratios and highest δ13Corg were recorded in the trap sediments. Analyses of the C/N ratio demonstrated that the proportions of AOC ranged from 30% to 100% of all OC, indicating that AOC was an important contributor to sedimentary organic matter (OC). It was calculated that the AOC flux in Fuxian Lake was 20.43 t C km−2 in 2017. Therefore, AOC produced by carbonate weathering and aquatic photosynthesis could potentially be a significant carbon sink and may have an important contribution to solving the lack of carbon sinks in the global carbon cycle. [ABSTRACT FROM AUTHOR]

• Calcites precipitated from the solutions with identical chemical and isotopic compositions have different δ 13 C values. • The DBL thickness affects the pH and the δ 13 C value of HCO − 3 at the mineral-solution interface. • Our finding offers a mechanistic interpretation on the variability of the ε calcite / DIC in riverine or cave environments. It is often assumed that the carbon isotope composition (δ 13 C) of carbonate minerals records that of the dissolved inorganic carbon (DIC) species with insignificant disequilibrium effect. However, results from field observations and laboratory experiments have shown that the δ 13 C difference between calcite and solution can vary up to 3‰ even under a similar set of solution composition and temperature, raising uncertainties on the δ 13 C's paleoclimate and paleoecology implications. One likely cause is the variable calcite precipitation rates and pH values induced by different thicknesses of the stagnant liquid layer between solid and well mixed bulk solution (i.e., diffusion boundary layer, DBL). To test this hypothesis, we selected a well-studied natural travertine deposit (Baishuitai, SW China) which consists of meter-scale travertine terraces. Calcite crystals at both the pool bottom and the rim of the terraces are inorganically precipitated from solutions identical in chemical, isotopic composition and temperature, but different only in hydrodynamics. This difference results in thicker DBL for water at the pool-bottom than for water flowing across the rim. We found that the δ 13 C and Mg/Ca ratios are higher while Sr/Ca ratios are lower for the pool-bottom calcites than for the rim calcites. By applying a mass transfer model, we quantitatively link the differences in carbon isotope and elemental compositions of the abiogenic calcites to the different hydrodynamic conditions. The inverse variation in Mg/Ca and Sr/Ca ratios in calcites arises from the different precipitation rates between at the pool-bottom and at the rim, while the consistently lower δ 13 C for calcites at the rim is due to their higher pH at the solid-solution interface than for calcites precipitated at the pool-bottom. In contrast, calcite precipitation rate has little influence on carbon isotope fractionation between calcite and HCO − 3 . Our results demonstrate the role of DBL thickness in governing the δ 13 C of HCO − 3 at mineral surface, which can assist to interpret the variable δ 13 C values of calcites in riverine or cave environments. [ABSTRACT FROM AUTHOR]

δ13C and D14C measurements on dissolved inorganic carbon (DIC), particulate organic carbon (POC) and aquatic plants from a karst spring and two spring-fed ponds in Laqiao, Maolan Township, Libo County, southeastern Guizhou of China in January, July and October of 2013 have been carried out to understand the roles of aquatic photosynthesis through DIC uptake in surface karst waters. The mean D14C and δ13C values of DIC for the spring, midstream pond (MP) and downstream pond (DP) are −26 ± 36‰ and −13 ± 2‰, 6 ± 56‰ and −12 ± 3‰, and 0 ± 64‰ and −9 ± 2‰, respectively. The carbon source for the DIC is mainly from biogenic CO 2 rather than the dissolution of limestone rock as the D14C and δ13C of limestone are about −1000‰ and 2‰, respectively. The enrichment trend of D14C DIC and δ13C DIC from the spring to the DP indicates CO 2 exchange between atmospheric CO 2 and DIC, because D14C and δ13C values of atmospheric CO 2 are ca. 50‰ and −8‰, respectively. The average D14C POC values in the spring, MP and DP were −325‰, −123‰ and −158‰, respectively, which are all lower than these of the DIC in each reservoir. The lower D14C values of the POC may be caused by older soil carbon from surface runoff and dust fall. More aquatic algae were formed through photosynthesis in the stream ponds, especially in summer, shown by strongly increased D14C POC and evidence of growth in EDS/SEM analyses. Furthermore, the D14C values of the submerged aquatic plants range from −153‰ to −26‰, reflecting that the aquatic plants used DIC for photosynthesis. The D14C value of an emergent plant which uses atmospheric CO 2 during photosynthesis is 52.5 ± 0.3‰, equivalent to the atmospheric D14C. Seasonal variations of D14C DIC and δ13C DIC are influenced by soil CO 2 input, primary productivity in the ponds, and CO 2 exchange; hydrochemical condition show lower D14C values but higher δ13C values in cold/dry season, and vice versa in summer rainy season. A simple mass balance calculation indicates ~90% of carbon for the spring DIC is from biogenic CO 2 , with higher contribution in summer due to higher productivity. Although this simple calculation may overestimate the biogenic CO 2 , it indicates that organic decomposition is a major carbon source for DIC in the karst hydrological system. The results of the present study have implications for 14C dating on aquatic plant remains, regional and perhaps global carbon budgets, and the different behaviors of 13C and 14C in karst systems. • The DIC carbon source of the karst spring is mainly from biogenic CO2. • 13C and 14C tracing C source and transformation including photosynthesis CO 2 uptake • Aquatic plants uptake DIC CO 2 may cause an older 14C age. • Biological pump effect in terrestrial water can be an important natural carbon sink. [ABSTRACT FROM AUTHOR]